,radio-,activity ,,cambridge ,,physical ,,series4 ,,general ,,editors3--,f4 ,h4 ,,neville1 ,m4,a41 ,f4,r4,s4 ,,and ,w4 ,c4 ,d4 ,,whetham1 ,m4,a41 ,f4,r4,s4 ,,radio-,,activity ,,cambridge ,,university ,,press ,,warehouse ,c4 ,f4 ,,clay1 ,,manager4 ,london3 ,,fetter ,,lane1 ,e4,c4 ,glasgow3 #ej1 ,,wellington ,,street4 ,,also ,london3 ,h4 ,k4 ,,lewis1 #acf1 ,,gower ,,street1 ,w4,c4 ,leipzig3 ,f4 ,a4 ,,brockhaus4 ,new ,york3 ,,the ,,macmillan ,,company4 ,bombay and ,calcutta3 ,,macmillan ,,and ,,co41 ,,ltd4 ,all ,rights reserved4 ,,radio-,,activity ,,by ,e4 ,,rutherford1 ,d4,sc41 ,f4,r4,s41 ,f4,r4,s4,c4 ,,macdonald ,,professor ,,of ,,physics1 ,mc,g,i,l,l ,,university1 ,,montreal ,,second ,,edition ,,cambridge ,,at ,,the ,,university ,,press #aije ,first ,edition #aijd ,second ,edition #aije ,j4 ,j4 ,,thomson ,a ,,tribute ,,of ,,my ,,respect ,,and ,,admiration ,,preface ,,to ,,the ,,first ,,edition4 ,in this work1 ,i have endeavoured to give a complete and connected account1 from a physical standpoint1 of the properties possessed by the naturally radio-active bodies4 ,although the subject is comparatively a new one1 our knowledge of the properties of the radio-active substances has advanced with great rapidity1 and there is now a very large amount of information on the subject scattered throughout the various scientific journals4 ,the phenomena exhibited by the radio-active bodies are extremely complicated1 and some form of theory is essential in order to connect in an intelligible manner the mass of experimental facts that have now been accumulated4 ,i have found the theory that the atoms of the radio-active bodies are undergoing spontaneous disintegration extremely serviceable1 not only in correlating the known phenomena1 but also in suggesting new lines of research4 ,the interpretation of the results has1 to a large extent1 been based on the disintegration theory1 and the logical deductions to be drawn from the application of the theory to radio-active phenomena have also been considered4 ,the rapid advance of our knowledge of radio-activity has been dependent on the information already gained by research into the electric properties of gases4 ,the action possessed by the radiations from radio-active bodies of producing charged carriers or ions in the gas1 has formed the basis of an accurate quantitative method of examination of the properties of the radiations and of radio-active processes1 and also allows us to determine with considerable certainty the order of magnitude of the different quantities involved4 ,for these reasons1 it has been thought advisable to give a brief account of the electric properties of gases1 to the extent that is necessary for the interpretation of the results of measurements in radio-activity by the electric method4 ,the chapter on the ionization theory of gases was written before the publication of ,j4 ,j4 ,thomson's recent book on 8,conduction of ,electricity through ,gases10 in which the whole subject is treated in a complete and connected manner4 ,a short chapter has been added1 in which an account is given of the methods of measurement which1 in the experience of the writer and others1 are most suitable for accurate work in radio-activity4 ,it is hoped that such an account may be of some service to those who may wish to obtain a practical acquaintance with the methods employed in radio-active measurements4 ,my thanks are due to ,mr ,w4 ,c4 ,dampier ,whetham1 ,f4,r4,s41 one of the editors of the ,cambridge ,physical ,series1 for many valuable suggestions1 and for the great care and trouble he has taken in revising the proof sheets4 ,i am also much indebted to my wife and ,miss ,h4 ,brooks for their kind assistance in correcting the proofs1 and to ,mr ,r4 ,k4 ,mc,clung for revising the index4 ,e4 ,r4 ,,macdonald ,,physics ,,building1 ,,montreal1 ,february1 #aijd4 ,,preface ,,to ,,the ,,second ,,edition4 ,i feel that some apology is due to my readers for bringing out at such an early date a new edition which includes so much new material1 and in which the rearrangement is so extensive as to constitute almost a new work4 ,though only a year has passed since the book first made its appearance1 the researches that have been carried out in that time have been too numerous and of too important a character to permit the publishing of a mere reprint1 unless the author were to relinquish his purpose of presenting the subject as it stands at the present moment4 ,the three new chapters which have been added possibly constitute the most important change in the work4 ,these chapters include a detailed account of the theory of successive changes and of its application to the analysis of the series of transformations which occur in radium1 thorium1 and actinium4 ,the disintegration theory1 which was put forward in the first edition as an explanation of radio-active phenomena1 has in these later researches proved to be a most powerful and valuable method of analysing the connection between the series of substances which arise from the transformation of the radio-elements4 ,it has disclosed the origin of radium1 of polonium and radio-tellurium1 and of radio-lead1 and now binds together in one coherent whole the large mass of apparently heterogeneous experimental facts in radio-activity which have been accumulating since #ahif4 ,the theory has received a remarkable measure of verification in the past year1 and1 in many cases1 has offered a quantitative as well as a qualitative explanation of the connection between the various properties exhibited by the radio-active bodies4 ,in the light of this evidence1 radio-activity may claim to have assumed the position of an independent subject1 though one with close affinities to physics on the one hand and to chemistry on the other4 ,the present edition includes a large amount of new material relating to the nature and properties of the radiations and the emanations4 ,in the limits of this book1 it would have been found impossible1 even had it been thought desirable1 to include more than a brief sketch of the physiological effects of the rays4 ,the literature on this subject is already large1 and is increasing rapidly4 ,for reasons of space1 ,i have not been able to refer more than briefly to the mass of papers that have appeared dealing with the examination of various spring and well waters1 sediments1 and soils1 for the presence of radio-active matter4 ,in order to make the book more self-contained1 a short account has been given in ,chapter ,,ii of the magnetic field produced by an ion in motion1 of the action of an external magnetic and electric field upon it1 and of the determination of the velocity and mass of the particles constituting the cathode stream4 ,two appendices have been added1 one giving an account of some work upon the rays which was completed too late for inclusion in the subject matter of the book1 and the other containing a brief summary of what is known in regard to the chemical constitution of the various radio-active minerals1 the localities in which they are found1 and their probable geologic age4 ,for the preparation of the latter1 ,i am indebted to my friend ,dr ,boltwood of ,new ,haven1 who1 in the course of his researches1 has had occasion to analyse most of these minerals in order to determine their content of uranium and radium4 ,i hope that this account of radio-active minerals will prove of value to those who are endeavouring to elucidate the connection between the various radio-active substances and the inactive products which arise from their transformation4 ,for the convenience of those who have read the first edition1 a list of the sections and chapters which contain the most important additions and alterations is added below the table of contents4 ,the writing of a complete account of a subject like radio-activity1 in which so much new work is constantly appearing1 has been a matter of no little difficulty4 ,among other things it has involved a continuous revision of the work while the volume was passing through the press4 ,i wish to express my thanks to my colleague ,professor ,harkness for the care and trouble he has taken in revising the proofs and for many useful suggestions2 also to ,mr ,r4 ,k4 ,mc,clung for his assistance in correcting some of the proofs and in preparing the index4 ,e4 ,r4 ,,mcgill ,,university1 ,,montreal1 #i ,may1 #aije4 ,transcriber's ,note3 ,the plain text version of this ebook includes complex mathematical formulas in ,te,x notation4 ,these formulas are enclosed between double dollar signs4 ,the ,,html version includes the same formulas rendered in their original form4 ,,errata4 ,transcriber's ,note3 ,these corrections have been applied to the text in the book4 page #dh1 line #bd section #bah should read section #bhd #gg1 last line #bfc #bgj #abc1 #eth line from bottom #bed #bfa #abd1 #ajth #bdf #bec #aea1 line #c #bbh #bbi #aef1 #acth line from bottom #bfa #bfh #bjj1 line #i #bdf #bec #baf1 line #c #bfj #bfg #ahd1 at the top of #eth column of table the letter should be inserted4 ,,table ,,of ,,contents4 ,i4 ,radio-active ,substances #a ,,ii4 ,ionization ,theory of ,gases #ca ,,iii4 ,methods of ,measurement #hb ,,iv4 ,nature of the ,radiations #ajh ,v4 ,properties of the ,radiations #bja ,,vi4 ,continuous ,production of ,radio-active ,matter #bah ,,vii4 ,radio-active ,emanations #bch ,,viii4 ,excited ,radio-activity #bie ,,ix4 ,theory of ,successive ,changes #cbe ,x4 ,transformation ,products of ,uranium1 ,thorium and ,actinium #cdf ,,xi4 ,transformation ,products of ,radium #cga ,,xii4 ,rate of ,emission of ,energy #dah ,,xiii4 ,radio-active ,processes #dcg ,,xiv4 ,radio-activity of the ,atmosphere and of ,ordinary ,materials #eja ,appendix ,a4 ,properties of the ,rays #edc ,appendix ,b4 ,radio-active ,minerals #eed ,index #eei ,plate 7,fig4 #df;,a3 ,spectrum of ,radium ,bromide7 to face p4 #bjf 99 99 99 99 99 ,for the convenience of the reader1 the sections and chapters which contain mostly new matter1 or have been either partly or wholly rewritten1 are appended below4 ,chap4 ,i4 ,sections #ah1 #bj-bc4 ,,ii4 #dh-eb4 ,,iii4 #fi4 ,,iv4 #hc-he1 #ib1 #ic1 #ajc1 #ajd1 #ajf-ajh1 #aaa1 #aab4 ,v4 #aae1 #aag1 #aai1 #abb4 ,,vii4 #aga-agc4 ,,viii4 #ahb-ahd1 #aij4 ,,ix-,,xiv4 ,mostly rewritten4 ,,abbreviations ,,of ,,references ,,to ,,some ,,of ,,the ,,journals4 ,ber4 d4 deutsch4 ,chem4 ,ges4 ,berichte der deutschen chemischen ,gesellschaft4 ,berlin4 ,c4 ,r4 ,comptes ,rendus des ,sances de l',acadmie des ,sciences4 ,paris4 ,chem4 ,news4 ,chemical ,news4 ,london4 ,drude's ,annal4 ,annalen der ,physik4 ,leipzig4 ,phil4 ,mag4 ,philosophical ,magazine and ,journal of ,science4 ,london4 ,phil4 ,trans4 ,philosophical ,transactions of the ,royal ,society of ,london4 ,phys4 ,rev4 ,physical ,review4 ,new ,york4 ,phys4 ,zeit4 ,physikalische ,zeitschrift4 ,proc4 ,camb4 ,phil4 ,soc4 ,proceedings of the ,cambridge ,philosophical ,society4 ,cambridge4 ,proc4 ,roy4 ,soc4 ,proceedings of the ,royal ,society of ,london4 ,thses-,paris4 ,thses prsentes la ,facult des ,sciences de l',universit de ,paris4 ,wied4 ,annal4 ,annalen der ,physik4 ,leipzig4 ,,chapter ,i4 ,,radio-,,active ,,substances4 #a4 ,introduction4 ,the close of the old and the beginning of the new century have been marked by a very rapid increase of our knowledge of that most important but comparatively little known subject--the connection between electricity and matter4 ,no study has been more fruitful in surprises to the investigator1 both from the remarkable nature of the phenomena exhibited and from the laws controlling them4 ,the more the subject is examined1 the more complex must we suppose the constitution of matter in order to explain the remarkable effects observed4 ,while the experimental results have led to the view that the constitution of the atom itself is very complex1 at the same time they have confirmed the old theory of the discontinuous or atomic structure of matter4 ,the study of the radio-active substances and of the discharge of electricity through gases has supplied very strong experimental evidence in support of the fundamental ideas of the existing atomic theory4 ,it has also indicated that the atom itself is not the smallest unit of matter1 but is a complicated structure made up of a number of smaller bodies4 ,a great impetus to the study of this subject was initially given by the experiments of ,lenard on the cathode rays1 and by ,rntgen's discovery of the ,x rays4 ,an examination of the conductivity imparted to a gas by the ,x rays led to a clear view of the mechanism of the transport of electricity through gases by means of charged ions4 ,this ionization theory of gases has been shown to afford a satisfactory explanation not only of the passage of electricity through flames and vapours1 but also of the complicated phenomena observed when a discharge of electricity passes through a vacuum tube4 ,at the same time1 a further study of the cathode rays showed that they consisted of a stream of material particles1 projected with great velocity1 and possessing an apparent mass small compared with that of the hydrogen atom4 ,the connection between the cathode and ,rntgen rays and the nature of the latter were also elucidated4 ,much of this admirable experimental work on the nature of the electric discharge has been done by ,professor ,j4 ,j4 ,thomson and his students in the ,cavendish ,laboratory1 ,cambridge4 ,an examination of natural substances1 in order to see if they gave out dark radiations similar to ,x rays1 led to the discovery of the radio-active bodies which possess the property of spontaneously emitting radiations1 invisible to the eye1 but readily detected by their action on photographic plates and their power of discharging electrified bodies4 ,a detailed study of the radio-active bodies has revealed many new and surprising phenomena which have thrown much light1 not only on the nature of the radiations themselves1 but also on the processes occurring in those substances4 ,notwithstanding the complex nature of the phenomena1 the knowledge of the subject has advanced with great rapidity1 and a large amount of experimental data has now been accumulated4 ,in order to explain the phenomena of radio-activity1 ,rutherford and ,soddy have advanced a theory which regards the atoms of the radio-active elements as suffering spontaneous disintegration1 and giving rise to a series of radio-active substances which differ in chemical properties from the parent elements4 ,the radiations accompany the breaking-up of the atoms1 and afford a comparative measure of the rate at which the disintegration takes place4 ,this theory is found to account in a satisfactory way for all the known facts of radio-activity1 and welds a mass of disconnected facts into one homogeneous whole4 ,on this view1 the continuous emission of energy from the active bodies is derived from the internal energy inherent in the atom1 and does not in any way contradict the law of the conservation of energy4 ,at the same time1 however1 it indicates that an enormous store of latent energy is resident in the radio-atoms themselves4 ,this store of energy has not been observed previously1 on account of the impossibility of breaking up into simpler forms the atoms of the elements by the action of the chemical or physical forces at our command4 ,on this theory we are witnessing in the radio-active bodies a veritable transformation of matter4 ,this process of disintegration was investigated1 not by direct chemical methods1 but by means of the property possessed by the radio-active bodies of giving out specific types of radiation4 ,except in the case of a very active element like radium1 the process of disintegration takes place so slowly1 that hundreds if not thousands of years would be required before the amount transformed would come within the range of detection of the balance or the spectroscope4 ,in radium1 however1 the process of disintegration takes place at such a rate that it should be possible within a limited space of time to obtain definite chemical evidence on this question4 ,the recent discovery that helium can be obtained from radium adds strong confirmation to the theory2 for helium was indicated as a probable disintegration product of the radio-active elements before this experimental evidence was forthcoming4 ,several products of the transformation of the radio-active bodies have already been examined1 and the further study of these substances promises to open up new and important fields of chemical enquiry4 ,in this book the experimental facts of radio-activity and the connection between them are interpreted on the disintegration theory4 ,many of the phenomena observed can be investigated in a quantitative manner1 and prominence has been given to work of this character1 for the agreement of any theory with the facts1 which it attempts to explain1 must ultimately depend upon the results of accurate measurement4 ,the value of any working theory depends upon the number of experimental facts it serves to correlate1 and upon its power of suggesting new lines of work4 ,in these respects the disintegration theory1 whether or not it may ultimately be proved to be correct1 has already been justified by its results4 #b4 ,radio-active ,substances4 ,the term 8radio-active0 is now generally applied to a class of substances1 such as uranium1 thorium1 radium1 and their compounds1 which possess the property of spontaneously emitting radiations capable of passing through plates of metal and other substances opaque to ordinary light4 ,the characteristic property of these radiations1 besides their penetrating power1 is their action on a photographic plate and their power of discharging electrified bodies4 ,in addition1 a strongly radio-active body like radium is able to cause marked phosphorescence and fluorescence on some substances placed near it4 ,in the above respects the radiations possess properties analogous to ,rntgen rays1 but it will be shown that1 for the major part of the radiations emitted1 the resemblance is only superficial4 ,the most remarkable property of the radio-active bodies is their power of radiating energy spontaneously and continuously at a constant rate1 without1 as far as is known1 the action upon them of any external exciting cause4 ,the phenomena at first sight appear to be in direct contradiction to the law of conservation of energy1 since no obvious change with time occurs in the radiating material4 ,the phenomena appear still more remarkable when it is considered that the radio-active bodies must have been steadily radiating energy since the time of their formation in the earth's crust4 ,immediately after ,rntgen's discovery of the production of ,x rays1 several physicists were led to examine if any natural bodies possessed the property of giving out radiations which could penetrate metals and other substances opaque to light4 ,as the production of ,x rays seemed to be connected in some way with cathode rays1 which cause strong fluorescent and phosphorescent effects on various bodies1 the substances first examined were those that were phosphorescent when exposed to light4 ,the first observation in this direction was made by ,niewenglowski#a1 who found that sulphide of calcium exposed to the sun's rays gave out some rays which were able to pass through black paper4 ,a little later a similar result was recorded by ,h4 ,becquerel#b for a special calcium sulphide preparation1 and by ,troost#c for a specimen of hexagonal blend4 ,these results were confirmed and extended in a later paper by ,arnold#d4 ,no satisfactory explanations of these somewhat doubtful results have yet been given1 except on the view that the black paper was transparent to some of the light waves4 ,at the same time ,le ,bon#e showed that1 by the action of sunlight on certain bodies1 a radiation was given out1 invisible to the eye1 but active with regard to a photographic plate4 ,these results have been the subject of much discussion2 but there seems to be little doubt that the effects are due to short ultra-violet light waves1 capable of passing through certain substances opaque to ordinary light4 ,these effects1 while interesting in themselves1 are quite distinct in character from those shown by the radio-active bodies which will now be considered4 #c4 ,uranium4 ,the first important discovery in the subject of radio-activity was made in ,february1 #ahif1 by ,m4 ,henri ,becquerel#f1 who found that a uranium salt1 the double sulphate of uranium and potassium1 emitted some rays which gave an impression on a photographic plate enveloped in black paper4 ,these rays were also able to pass through thin plates of metals and other substances opaque to light4 ,the impressions on the plate could not have been due to vapours given off by the substances1 since the same effect was produced whether the salt was placed directly on the black paper or on a thin plate of glass lying upon it4 ,becquerel found later that all the compounds of uranium as well as the metal itself possessed the same property1 and1 although the amount of action varied slightly for the different compounds1 the effects in all cases were comparable4 ,it was at first natural to suppose that the emission of these rays was in some way connected with the power of phosphorescence1 but later observations showed that there was no connection whatever between them4 ,the uranic salts are phosphorescent1 while the uranous salts are not4 ,the uranic salts1 when exposed to ultra-violet light in the phosphoroscope1 give a phosphorescent light lasting about #ja seconds4 ,when the salts are dissolved in water1 the duration is still less4 ,the amount of action on the photographic plate does not depend on the particular compound of uranium employed1 but only on the amount of uranium present in the compound4 ,the non-phosphorescent are equally active with the phosphorescent compounds4 ,the amount of radiation given out is unaltered if the active body be kept continuously in darkness4 ,the rays are given out by solutions1 and by crystals which have been deposited from solutions in the dark and never exposed to light4 ,this shows that the radiation cannot be due in any way to the gradual emission of energy stored up in the crystal in consequence of exposure to a source of light4 #d4 ,the power of giving out penetrating rays thus seems to be a specific property of the element uranium1 since it is exhibited by the metal as well as by all its compounds4 ,these radiations from uranium are persistent1 and1 as far as observations have yet gone1 are unchanged1 either in intensity or character1 with lapse of time4 ,observations to test the constancy of the radiations for long periods of time have been made by ,becquerel4 ,samples of uranic and uranous salts have been kept in a double box of thick lead1 and the whole has been preserved from exposure to light4 ,by a simple arrangement1 a photographic plate can be introduced in a definite position above the uranium salts1 which are covered with a layer of black paper4 ,the plate is exposed at intervals for #dh hours1 and the impression on the plate compared4 ,no perceptible weakening of the radiation has been observed over a period of four years4 ,mme ,curie#g has made determinations of the activity of uranium over a space of five years by an electric method described later1 but found no appreciable variation during that period4 ,since the uranium is thus continuously radiating energy from itself1 without any known source of excitation1 the question arises whether any known agent is able to affect the rate of its emission4 ,no alteration was observed when the body was exposed to ultra-violet light or to ultra-red light or to ,x rays4 ,becquerel states that the double sulphate of uranium and potassium showed a slight increase of action when exposed to the arc light and to sparks1 but he considers that the feeble effect observed was another action superimposed on the constant radiation from uranium4 ,the intensity of the uranium radiation is not affected by a variation of temperature between #bjj ,c4 and the temperature of liquid air4 ,this question is discussed in more detail later4 #e4 ,in addition to these actions on a photographic plate1 ,becquerel showed that uranium rays1 like ,rntgen rays1 possess the important property of discharging both positively and negatively electrified bodies4 ,these results were confirmed and extended by ,lord ,kelvin1 ,smolan and ,beattie#h4 ,the writer made a detailed comparison#i of the nature of the discharge produced by uranium with that produced by ,rntgen rays1 and showed that the discharging property of uranium is due to the production of charged ions by the radiation throughout the volume of the gas4 ,the property has been made the basis of a qualitative and quantitative examination of the radiations from all radio-active bodies1 and is discussed in detail in chapter ,,ii4 ,the radiations from uranium are thus analogous1 as regards their photographic and electrical actions1 to ,rntgen rays1 but1 compared with the rays from an ordinary ,x ray tube1 these actions are extremely feeble4 ,while with ,rntgen rays a strong impression is produced on a photographic plate in a few minutes or even seconds1 several days' exposure to the uranium rays is required to produce a well-marked action1 even though the uranium compound1 enveloped in black paper1 is placed close to the plate4 ,the discharging action1 while very easily measurable by suitable methods1 is also small compared with that produced by ,x rays from an ordinary tube4 #f4 ,the rays from uranium show no evidence of direct reflection1 refraction1 or polarization#aj4 ,while there is no direct reflection of the rays1 there is apparently a diffuse reflection produced where the rays strike a solid obstacle4 ,this is due in reality to a secondary radiation set up when the primary rays impinge upon matter4 ,the presence of this secondary radiation at first gave rise to the erroneous view that the rays could be reflected and refracted like ordinary light4 ,the absence of reflection1 refraction1 or polarization in the penetrating rays from uranium necessarily follows in the light of our present knowledge of the rays4 ,it is now known that the uranium rays1 mainly responsible for the photographic action1 are deviable by a magnetic field1 and are similar in all respects to cathode rays1 i4e4 the rays are composed of small particles projected at great velocities4 ,the absence of the ordinary properties of transverse light waves is thus to be expected4 #g4 ,the rays from uranium are complex in character1 and1 in addition to the penetrating deviable rays1 there is also given off a radiation very readily absorbed by passing through thin layers of metal foil1 or by traversing a few centimetres of air4 ,the photographic action due to these rays is very feeble in comparison with that of the penetrating rays1 although the discharge of electrified bodies is mainly caused by them4 ,besides these two types of rays1 some rays are emitted which are of an extremely penetrating character and are non-deviable by a magnetic field4 ,these rays are difficult to detect photographically1 but can readily be examined by the electric method4 #h4 ,the question naturally arose whether the property of spontaneously giving out penetrating radiations was confined to uranium and its compounds1 or whether it was exhibited to any appreciable extent by other substances4 ,by the electrical method1 with an electrometer of ordinary sensitiveness1 any body which possesses an activity of the order of of that of uranium can be detected4 ,with an electroscope of special construction1 such as has been designed by ,c4 ,t4 ,r4 ,wilson for his experiments on the natural ionization of air1 a substance of activity and probably of that of uranium can be detected4 ,if an active body like uranium be mixed with an inactive body1 the resulting activity in the mixture is generally considerably less than that due to the active substance alone4 ,this is due to the absorption of the radiation by the inactive matter present4 ,the amount of decrease largely depends on the thickness of the layer from which the activity is determined4 ,mme ,curie made a detailed examination by the electrical method of the great majority of known substances1 including the very rare elements1 to see if they possessed any activity4 ,in cases where it was possible1 several compounds of the elements were examined4 ,with the exception of thorium and phosphorus1 none of the other substances possessed an activity even of the order of of uranium4 ,the ionization of the gas by phosphorus does not1 however1 seem to be due to a penetrating radiation like that found in the case of uranium1 but rather to a chemical action taking place at its surface4 ,the compounds of phosphorus do not show any activity1 and in this respect differ from uranium and the other active bodies4 ,le ,bon#aa has also observed that quinine sulphate1 if heated and then allowed to cool1 possesses for a short time the property of discharging both positively and negatively electrified bodies4 ,it is necessary1 however1 to draw a sharp line of distinction between phenomena of this kind and those exhibited by the naturally radio-active bodies4 ,while both1 under special conditions1 possess the property of ionizing the gas1 the laws controlling the phenomena are quite distinct in the two cases4 ,for example1 only one compound of quinine shows the property1 and that compound only when it has been subjected to a preliminary heating4 ,the action of phosphorus depends on the nature of the gas1 and varies with temperature4 ,on the other hand1 the activity of the naturally radio-active bodies is spontaneous and permanent4 ,it is exhibited by all compounds1 and is not1 as far as is yet known1 altered by change in the chemical or physical conditions4 #i4 ,the discharging and photographic action alone cannot be taken as a criterion as to whether a substance is radio-active or not4 ,it is necessary in addition to examine the radiations1 and to test whether the actions take place through appreciable thicknesses of all kinds of matter opaque to ordinary light4 ,for example1 a body giving out short waves of ultra-violet light can be made to behave in many respects like a radio-active body4 ,as ,lenard#ab has shown1 short waves of ultra-violet light will ionize the gas in their path1 and will be absorbed rapidly in the gas4 ,they will produce strong photographic action1 and may pass through some substances opaque to ordinary light4 ,the similarity to a radio-active body is thus fairly complete as regards these properties4 ,on the other hand1 the emission of these light waves1 unlike that of the radiations from an active body1 will depend largely on the molecular state of the compound1 or on temperature and other physical conditions4 ,but the great point of distinction lies in the nature of the radiations from the bodies in question4 ,in one case the radiations behave as transverse waves1 obeying the usual laws of light waves1 while in the case of a naturally active body1 they consist for the most part of a continuous flight of material particles projected from the substance with great velocity4 ,before any substance can be called 8radio-active0 in the sense in which the term is used to describe the properties of the natural radio-active elements1 it is thus necessary to make a close examination of its radiation2 for it is unadvisable to extend the use of the term 8radio-active0 to substances which do not possess the characteristic radiating properties of the radio-active elements which we have described1 and the active products which can be obtained from them4 ,some of the pseudo-active bodies will however be considered later in chapter ,,ix4 #aj4 ,thorium4 ,in the course of an examination of a large number of substances1 ,schmidt#ac found that thorium1 its compounds1 and the minerals containing thorium1 possessed properties similar to those of uranium4 ,the same discovery was made independently by ,mme ,curie#ad4 ,the rays from thorium compounds1 like those from uranium1 possess the properties of discharging electrified bodies and acting on a photographic plate4 ,under the same conditions the discharging action of the rays is about equal in amount to that of uranium1 but the photographic effect is distinctly weaker4 ,the radiations from thorium are more complicated than those from uranium4 ,it was early observed by several experimenters that the radiation from thorium compounds1 especially the oxide1 when tested by the electrified method1 was very variable and uncertain4 ,a detailed investigation of the radiations from thorium under various conditions was made by ,owens#ae4 ,he showed that thorium oxide1 especially in thick layers1 was able to produce conductivity in the gas when covered with a large thickness of paper1 and that the amount of this conductivity could be greatly varied by blowing a current of air over the gas4 ,in the course of an examination#af of this action of the air current1 the writer showed that thorium compounds gave out a material emanation made up of very small particles themselves radio-active4 ,the emanation behaves like a radio-active gas2 it diffuses rapidly through porous substances like paper1 and is carried away by a current of air4 ,the evidence of the existence of the emanation and its properties1 is considered in detail later in chapter ,,viii4 ,in addition to giving out an emanation1 thorium behaves like uranium in emitting three types of radiation1 each of which is similar in properties to the corresponding radiation from uranium4 #aa4 ,radio-active minerals4 ,mme ,curie has examined the radio-activity of a large number of minerals containing uranium and thorium4 ,the electrical method was used1 and the current measured between two parallel plates #h cms4 in diameter and #c cms4 apart1 when one plate was covered with a uniform layer of the active matter4 ,the following numbers give the order of the saturation current obtained in amperes4 ,pitchblende from ,johanngeorgenstadt #hc #aj ,joachimsthal #gj ,pzibran #fe ,cornwall #af ,cleveite #ad ,chalcolite #eb ,autunite #bg ,thorite from #jc to #ad ,orangite #bj ,monazite #je ,xenotine #jjc ,aeschynite #jg ,fergusonite #jd ,samarskite #aa ,niobite #jc ,carnotite #fb ,some activity is to be expected in these minerals1 since they all contain either thorium or uranium or a mixture of both4 ,an examination of the action of the uranium compounds with the same apparatus and under the same conditions led to the following results3 ,uranium 7containing a little carbon7 #bc #aj amperes ,black oxide of uranium #bf ,green #ah ,acid uranic hydrate #jf ,uranate of sodium #ab ,uranate of potassium #ab ,uranate of ammonia #ac ,uranous sulphate #jg ,sulphate of uranium and potassium #jg ,acetate #jg ,phosphate of copper and uranium #ji ,oxysulphide of uranium #ab ,the interesting point in connection with these results is that some specimens of pitchblende have four times the activity of the metal uranium2 chalcolite1 the crystallized phosphate of copper and uranium1 is twice as active as uranium2 and autunite1 a phosphate of calcium and uranium1 is as active as uranium4 ,from the previous considerations1 none of the substances should have shown as much activity as uranium or thorium4 ,in order to be sure that the large activity was not due to the particular chemical combination1 ,mme ,curie prepared chalcolite artificially1 starting with pure products4 ,this artificial chalcolite had the activity to be expected from its composition1 viz4 about #jd of the activity of the uranium4 ,the natural mineral chalcolite is thus five times as active as the artificial mineral4 ,it thus seemed probable that the large activity of some of these minerals1 compared with uranium and thorium1 was due to the presence of small quantities of some very active substance1 which was different from the known bodies thorium and uranium4 ,this supposition was completely verified by the work of ,m4 and ,mme ,curie1 who were able to separate from pitchblende by purely chemical methods two active bodies1 one of which in the pure state is over a million times more active than the metal uranium4 ,this important discovery was due entirely to the property of radio-activity possessed by the new bodies4 ,the only guide in their separation was the activity of the products obtained4 ,in this respect the discovery of these bodies is quite analogous to the discovery of rare elements by the methods of spectrum analysis4 ,the method employed in the separation consisted in examining the relative activity of the products after chemical treatment4 ,in this way it was seen whether the radio-activity was confined to one or another of the products1 or divided between both1 and in what ratio such division occurred4 ,the activity of the specimens thus served as a basis of rough qualitative and quantitative analysis1 analogous in some respects to the indication of the spectroscope4 ,to obtain comparative data it was necessary to test all the products in the dry state4 ,the chief difficulty lay in the fact that pitchblende is a very complex mineral1 and contains in varying quantities nearly all the known metals4 #ab4 ,radium4 ,the analysis of pitchblende by chemical methods1 using the procedure sketched above1 led to the discovery of two very active bodies1 polonium and radium4 ,the name polonium was given to the first substance discovered by ,mme ,curie in honour of the country of her birth4 ,the name radium was a very happy inspiration of the discoverers1 for this substance in the pure state possesses the property of radio-activity to an astonishing degree4 ,radium is extracted from pitchblende by the process used to separate barium1 to which radium is very closely allied in chemical properties#ag4 ,after the removal of other substances1 the radium remains behind mixed with barium4 ,it can1 however1 be partially separated from the latter by the difference in solubility of the chlorides in water1 alcohol1 or hydrochloric acid4 ,the chloride of radium is less soluble than that of barium1 and can be separated from it by the method of fractional crystallization4 ,after a large number of precipitations1 the radium can be freed almost completely from the barium4 ,both polonium and radium exist in infinitesimal quantities in pitchblende4 ,in order to obtain a few decigrammes of very active radium1 it is necessary to use several tons of pitchblende1 or the residues obtained from the treatment of uranium minerals4 ,it is thus obvious that the expense and labour involved in preparation of a minute quantity of radium are very great4 ,m4 and ,mme ,curie were indebted for their first working material to the ,austrian government1 who generously presented them with a ton of the treated residue of uranium materials from the ,state manufactory of ,joachimsthal in ,bohemia4 ,with the assistance of the ,academy of ,science and other societies in ,france1 funds were given to carry out the laborious work of separation4 ,later the ,curies were presented with a ton of residues from the treatment of pitchblende by the ,socit ,centrale de ,produits ,chimiques of ,paris4 ,the generous assistance afforded in this important work is a welcome sign of the active interest taken in these countries in the furthering of purely scientific research4 ,the rough concentration and separation of the residues was performed in the chemical works1 and there followed a large amount of labour in purification and concentration4 ,in this manner1 the ,curies were able to obtain a small quantity of radium which was enormously active compared with uranium4 ,no definite results have yet been given on the activity of pure radium1 but the ,curies estimate that it is about one million times that of uranium1 and may possibly be still higher4 ,the difficulty of making a numerical estimate for such an intensely active body is very great4 ,in the electric method1 the activities are compared by noting the relative strength of the maximum or saturation current between two parallel plates1 on one of which the active substance is spread4 ,on account of the intense ionization of the gas between the plates1 it is not possible to reach the saturation current unless very high voltages are applied4 ,approximate comparisons can be made by the use of metal screens to cut down the intensity of the radiations1 if the proportion of the radiation transmitted by such a screen has been determined by direct experiment on impure material of easily measurable activity4 ,the value of the activity of radium compared with that of uranium will however vary to some extent according to which of the three types of rays is taken as a basis of comparison4 ,it is thus difficult to control the final stages of the purification of radium by measurements of its activity alone4 ,moreover the activity of radium immediately after its preparation is only about one-fourth of its final value2 it gradually rises to a maximum after the radium salt has been kept in the dry state for about a month4 ,for control experiments in purification1 it is advisable to measure the initial rather than the final activity4 ,mme ,curie has utilized the coloration of the crystals of radiferous barium as a means of controlling the final process of purification4 ,the crystals of salts of radium and barium deposited from acid solutions are indistinguishable by the eye4 ,the crystals of radiferous barium are at first colourless1 but1 in the course of a few hours1 become yellow1 passing to orange and sometimes to a beautiful rose colour4 ,the rapidity of this coloration depends on the amount of barium present4 ,pure radium crystals do not colour1 or at any rate not as rapidly as those containing barium4 ,the coloration is a maximum for a definite proportion of radium1 and this fact can be utilized as a means of testing the amount of barium present4 ,when the crystals are dissolved in water the coloration disappears4 ,giesel#ah has observed that pure radium bromide gives a beautiful carmine colour to the ,bunsen flame4 ,if barium be present in any quantity1 only the green colour due to barium is observed1 and a spectroscopic examination shows only the barium lines4 ,this carmine coloration of the ,bunsen flame is a good indication of the purity of the radium4 ,since the preliminary announcement of the discovery of radium1 ,giesel#ai has devoted a great deal of attention to the separation of radium1 polonium and other active bodies from pitchblende4 ,he was indebted for his working material to the firm of ,p4 de ,haen1 of ,hanover1 who presented him with a ton of pitchblende residues4 ,using the method of fractional crystallization of the bromide instead of the chloride1 he has been able to prepare considerable quantities of pure radium4 ,by this means the labour of final purification of radium has been much reduced4 ,he states that six or eight crystallizations with the bromide are sufficient to free the radium almost completely from the barium4 #ac4 ,spectrum of radium4 ,it was of great importance to settle as soon as possible whether radium was in reality modified barium or a new element with a definite spectrum4 ,for this purpose the ,curies prepared some specimens of radium chloride1 and submitted them for examination of their spectrum to ,demaray1 an authority on that subject4 ,the first specimen of radium chloride examined by ,demaray#bj was not very active1 but showed1 besides the lines due to barium1 a very strong new line in the ultra-violet4 ,in another sample of greater activity1 the line was still stronger and others also appeared1 while the intensity of the new lines was comparable with those present due to barium4 ,with a still more active specimen which was probably nearly pure1 only three strong lines of barium appeared1 while the new spectrum was very bright4 ,the following table shows the wave-length of the new lines observed for radium4 ,the wave lengths are expressed in ngstrm units and the intensity of each ray is denoted by a number1 the ray of maximum intensity being #af4 ,wave length ,intensity ,wave length ,intensity #dhbfc #aj #dfjjc #c #dgbfi #e #decce #i #dfiif #c #ddcfa #f #dfiba #g #dcdjf #ab #dfhcj #ad #chadg #af #dfdai #d #cfdif #ab ,the lines are all sharply defined1 and three or four of them have an intensity comparable with any known lines of other substances4 ,there are also present in the spectrum two strong nebulous bands4 ,in the visible part of the spectrum1 which has not been photographed1 the only noticeable ray has a wave length #effe1 which is1 however1 very feeble compared with that of wave length #dhbfc4 ,the general aspect of the spectrum is similar to that of the alkaline earths2 it is known that these metals have strong lines accompanied by nebulous bands4 ,the principal line due to radium can be distinguished in impure radium of activity #ej times that of uranium4 ,by the electrical method it is easy to distinguish the presence of radium in a body which has an activity only of uranium4 ,with a more sensitive electrometer of the activity of uranium could be observed4 ,for the detection of radium1 the examination of the radio-activity is thus a process nearly a million times more sensitive than spectrum analysis4 ,later observations on the spectrum of radium have been made by ,runge#ba1 ,exner and ,haschek#bb1 with specimens of radium prepared by ,giesel4 ,crookes#bc has photographed the spectrum of radium in the ultra-violet1 while ,runge and ,precht#bd1 using a highly purified sample of radium1 observed a number of new lines in the spark spectrum4 ,it has been mentioned already that the bromide of radium gives a characteristic pure carmine-red coloration to the ,bunsen flame4 ,the flame spectrum shows two broad bright bands in the orange-red1 not observed in ,demaray's spectrum4 ,in addition there is a line in the blue-green and two feeble lines in the violet4 #ad4 ,atomic weight of radium4 ,mme ,curie has made successive determinations of the atomic weight of the new element with specimens of steadily increasing purity4 ,in the first observation the radium was largely mixed with barium1 and the atomic weight obtained was the same as that of barium1 #acge4 ,in successive observations with specimens of increasing purity the atomic weights of the mixture were #adf and #age4 ,the final value obtained recently was #bbe1 which may be taken as the atomic weight of radium on the assumption that it is divalent4 ,in these experiments about #ja gram of pure radium chloride was obtained by successive fractionations4 ,the difficulty involved in preparing a quantity of pure radium chloride large enough to test the atomic weight may be gauged from the fact that only a few centigrams of fairly pure radium1 or a few decigrams of less concentrated material1 are obtained from the treatment of about #b tons of the mineral from which it is derived4 ,runge and ,precht#be have examined the spectrum of radium in a magnetic field1 and have shown the existence of series analogous to those observed for calcium1 barium1 and strontium4 ,these series are connected with the atomic weights of the elements in question1 and ,runge and ,precht have calculated by these means that the atomic weight of radium should be #beh--;a number considerably greater than the number #bbe obtained by ,mme ,curie by means of chemical analysis4 ,marshall ,watts#bf1 on the other hand1 using another relation between the lines of the spectrum1 deduced the value obtained by ,mme ,curie4 ,runge#bg has criticised the method of deduction employed by ,marshall ,watts on the ground that the lines used for comparison in the different spectra were not homologous4 ,considering that the number found by ,mme ,curie agrees with that required by the periodic system1 it is advisable in the present state of our knowledge to accept the experimental number rather than the one deduced by ,runge and ,precht from spectroscopic evidence4 ,there is no doubt that radium is a new element possessing remarkable physical properties4 ,the detection and separation of this substance1 existing in such minute proportions in pitchblende1 has been due entirely to the characteristic property we are considering1 and is the first notable triumph of the study of radio-activity4 ,as we shall see later1 the property of radio-activity can be used1 not only as a means of chemical research1 but also as an extraordinarily delicate method of detecting chemical changes of a very special kind4 #ae4 ,radiations from radium4 ,on account of its enormous activity1 the radiations from radium are very intense3 a screen of zinc sulphide1 brought near a few centigrams of radium bromide1 is lighted up quite brightly in a dark room1 while brilliant fluorescence is produced on a screen of platino-barium cyanide4 ,an electroscope brought near the radium salt is discharged almost instantly1 while a photographic plate is immediately affected4 ,at a distance of one metre1 a day's exposure to the radium rays would produce a strong impression4 ,the radiations from radium are analogous to those of uranium1 and consist of three types of rays3 easily absorbed1 penetrating1 and very penetrating4 ,radium also gives rise to an emanation similar to that of thorium1 but with a very much slower rate of decay4 ,the radium emanation retains its activity for several weeks1 while that of thorium lasts only a few minutes4 ,the emanation obtained from a few centigrams of radium illuminates a screen of zinc sulphide with great brilliancy4 ,the very penetrating rays of radium are able to light up an ,x ray screen in a dark room1 after passage through several centimetres of lead and several inches of iron4 ,as in the case of uranium or thorium1 the photographic action is mainly due to the penetrating or cathodic rays4 ,the radiographs obtained with radium are very similar to those obtained with ,x rays1 but lack the sharpness and detail of the latter4 ,the rays are unequally absorbed by different kinds of matter1 the absorption varying approximately as the density4 ,in photographs of the hand the bones do not stand out as in ,x ray photographs4 ,curie and ,laborde have shown that the compounds of radium possess the remarkable property of always keeping their temperature several degrees above the temperature of the surrounding air4 ,each gram of radium radiates an amount of energy corresponding to #ajj gram-calories per hour4 ,this and other properties of radium are discussed in detail in chapters ,v and ,,xii4 #af4 ,compounds of radium4 ,when first prepared in the solid state1 all the salts of radium--the chloride1 bromide1 acetate1 sulphate1 and carbonate--are very similar in appearance to the corresponding salts of barium1 but in time they gradually become coloured4 ,in chemical properties the salts of radium are practically the same as those of barium1 with the exception that the chloride and bromide of radium are less soluble in water than the corresponding salts of barium4 ,all the salts of radium are naturally phosphorescent4 ,the phosphorescence of impure radium preparations is in some cases very marked4 ,all the radium salts possess the property of causing rapid colorations of the glass vessel which contains them4 ,for feebly active material the colour is usually violet1 for more active material a yellowish-brown1 and finally black4 #ag4 ,actinium4 ,the discovery of radium in pitchblende gave a great impetus to the chemical examination of uranium residues1 and a systematic search early led to the detection of several new radio-active bodies4 ,although these show distinctive radio-active properties1 so far none of them have been purified sufficiently to give a definite spectrum as in the case of radium4 ,one of the most interesting and important of these substances was discovered by ,debierne#bh while working up the uranium residues1 obtained by ,m4 and ,mme ,curie from the ,austrian government1 and was called by him actinium4 ,this active substance is precipitated with the iron group1 and appears to be very closely allied in chemical properties to thorium1 though it is many thousand times more active4 ,it is very difficult to separate from thorium and the rare earths4 ,debierne has made use of the following methods for partial separation3 7#a7 ,precipitation in hot solutions1 slightly acidulated with hydrochloric acid1 by excess of hyposulphite of soda4 ,the active matter is present almost entirely in the precipitate4 7#b7 ,action of hydrofluoric acid upon the hydrates freshly precipitated1 and held in suspension in water4 ,the portion dissolved is only slightly active4 ,by this method titanium may be separated4 7#c7 ,precipitation of neutral nitrate solutions by oxygenated water4 ,the precipitate carries down the active body4 7#d7 ,precipitation of insoluble sulphates4 ,if barium sulphate1 for example1 is precipitated in the solution containing the active body1 the barium carries down the active matter4 ,the thorium and actinium are freed from the barium by conversion of the sulphate into the chloride and precipitation by ammonia4 ,in this way ,debierne has obtained a substance comparable in activity with radium4 ,the separation1 which is difficult and laborious1 has not yet been carried far enough to bring out any new lines in the spectrum4 #ah4 ,after the initial announcement of the discovery of actinium1 several years elapsed before any definite results upon it were published by ,debierne4 ,in the meantime1 ,giesel#bi had independently obtained a radio-active substance from pitchblende which seemed similar in many respects to the actinium of ,debierne4 ,the active substance belongs to the group of cerium earths and is precipitated with them4 ,by a succession of chemical operations1 the active substance is separated mixed with lanthanum4 ,while intensely active in comparison with thorium1 the new active substance closely resembles it in radio-active properties1 although1 from the method of separation thorium cannot be present except in minute quantity4 ,giesel early observed that the substance gave off a radio-active emanation4 ,on account of the intensity of the emanation it emits1 he termed it the 8emanating substance40 ,recently this name has been changed to 8emanium10 and under this title preparations of the active substance prepared by ,giesel have been placed on the market4 ,giesel found that the activity of this substance was permanent and seemed to increase during the six months' interval after separation4 ,in this respect it is similar to radium compounds1 for the activity of radium1 measured by the electric method1 increases in the course of a month's interval to four times its initial value at separation4 ,there can be no doubt that the 8actinium0 of ,debierne and the 8emanium0 of ,giesel contain the same radio-active constituent1 for recent work#cj has shown that they exhibit identical radio-active properties4 ,each gives out easily absorbed and penetrating rays1 and emits a characteristic emanation of which the rate of decay is the same for both substances4 ,the rate of decay of the emanation is the simplest method of distinguishing actinium from thorium1 which it resembles so closely in radio-active as well as in chemical properties4 ,the emanation of actinium loses its radiating power far more rapidly than that of thorium1 the time taken for the activity to fall to half value being in the two cases #cg seconds and #eb seconds respectively4 ,the rapid and continuous emission of this short-lived emanation is the most striking radio-active property possessed by actinium4 ,in still air1 the radio-active effects of this emanation are confined to a distance of a few centimetres from the active material1 as it is only able to diffuse a short distance through the air before losing its radiating power4 ,with very active preparations of actinium1 the material appears to be surrounded by a luminous haze produced by the emanation4 ,the radiations produce strong luminosity in some substances1 for example1 zinc sulphide1 willemite and platinocyanide of barium4 ,the luminosity is especially marked on screens of zinc sulphide4 ,much of this effect is due to the emanation1 for1 on gently blowing a current of air over the substance1 the luminosity is displaced at once in the direction of the current4 ,with a zinc sulphide screen1 actinium shows the phenomena of 8scintillations0 to an even more marked degree than radium itself4 ,the preparations of emanium are in some cases luminous1 and a spectroscopic examination of this light has shown a number of bright lines#ca4 ,the distinctive character of the emanation of actinium1 as well as of the other radio-active products to which it gives rise1 coupled with the permanence of its activity1 renders it very probable that actinium will prove to be a new radio-active element of very great activity4 ,although very active preparations of actinium have been obtained1 it has not yet been found possible to free it from impurities4 ,consequently1 no definite observations have been made on its chemical properties1 and no new spectrum lines have been observed4 ,a more complete discussion of the radio-active and other properties of actinium is given in later chapters4 #ai4 ,polonium4 ,polonium was the first of the active substances obtained from pitchblende4 ,it has been investigated in detail by its discoverer ,mme ,curie#cb4 ,the pitchblende was dissolved in acid and sulphuretted hydrogen added4 ,the precipitated sulphides contained an active substance1 which1 after separation of impurities1 was found associated with bismuth4 ,this active substance1 which has been named polonium1 is so closely allied in chemical properties to bismuth that it has so far been found impossible to effect a complete separation4 ,partial separation of polonium can be made by successive fractionations based on one of the following modes of procedure3 7#a7 ,sublimation in a vacuum4 ,the active sulphide is more volatile than that of bismuth4 ,it is deposited as a black substance at those parts of the tube1 where the temperature is between #bej and #cjj ,c4 ,in this way polonium of activity #gjj times that of uranium was obtained4 7#b7 ,precipitation of nitric acid solutions by water4 ,the precipitated sub-nitrate is much more active than the part that remains in solution4 7#c7 ,precipitation by sulphuretted hydrogen in a very acid hydrochloric acid solution4 ,the precipitated sulphides are much more active than the salt which remains in solution4 ,for concentration of the active substance ,mme ,curie#cc has made use of method 7#b74 ,the process is1 however1 very slow and tedious1 and is made still more complicated by the tendency to form precipitates insoluble either in strong or weak acids4 ,after a large number of fractionations1 a small quantity of matter was obtained1 enormously active compared with uranium4 ,on examination of the substance spectroscopically1 only the bismuth lines were observed4 ,a spectroscopic examination of the active bismuth by ,demaray and by ,runge and ,exner has led to the discovery of no new lines4 ,on the other hand ,sir ,william ,crookes#cd states that he found one new line in the ultra-violet1 while ,berndt#ce1 working with polonium of activity #cjj1 observed a large number of new lines in the ultra-violet4 ,these results await further confirmation4 ,the polonium prepared by ,mme ,curie differs from the other radio-active bodies in several particulars4 ,in the first place the radiations include only very easily absorbable rays4 ,the two penetrating types of radiation given out by uranium1 thorium1 and radium are absent4 ,in the second place the activity does not remain constant1 but diminishes continuously with the time4 ,mme ,curie states that different preparations of polonium had somewhat different rates of decay4 ,in some cases1 the activity fell to half value in about six months1 and in others1 about half value in eleven months4 #bj4 ,the gradual diminution of the activity of polonium with time seemed at first sight to differentiate it from such substances as uranium and radium1 the activity of which appeared fairly permanent4 ,this difference in behaviour is1 however1 one of degree rather than of kind4 ,we shall show later that there is present in pitchblende a number of radio-active substances1 the activity of which is not permanent4 ,the time taken for these bodies to lose half of their activity varies in different cases from a few seconds to several hundreds of years4 ,in fact1 this gradual loss of activity is an essential feature of our theory of regarding the phenomena of radio-activity4 ,no radio-active substance1 left to itself1 can continue to radiate indefinitely2 it must ultimately lose its activity4 ,in the case of bodies like uranium and radium1 the loss of activity is so slow that no sensible alteration has been observed over a period of several years1 but it can be deduced theoretically that the activity of radium will eventually decrease to half value in a period of about #ajjj years1 while in the case of a feebly radio-active substance like uranium1 more than a #ajj million years must elapse before the diminution of the activity becomes appreciable4 ,it may be of interest here to consider briefly the suggestions advanced at various times to account for the temporary character of the activity of polonium4 ,its association with bismuth led to the view that polonium was not a new active substance1 but merely radio-active bismuth1 that is1 bismuth which in some way had been made active by admixture with radio-active bodies4 ,it was known that a body placed in the vicinity of thorium or radium became temporarily active4 ,the same action was supposed to take place when inactive matter was in solution with active matter4 ,the non-active matter was supposed to acquire activity by 8induction10 as it was called1 in consequence of its intimate contact with the active material4 ,there is no proof1 however1 that such is the case4 ,the evidence points rather to the conclusion that the activity is due1 not to an alteration of the inactive body itself1 but to an admixture with it of a very small quantity of intensely active matter4 ,this active matter is present in pitchblende and is separated with the bismuth but differs from it in chemical properties4 ,the subject cannot be considered with advantage at this stage1 but will be discussed later in detail in chapter ,,xi4 ,it will there be shown that polonium1 that is1 the radio-active constituent mixed with the bismuth1 is a distinct chemical substance1 which is allied in chemical properties to bismuth1 but possesses some distinct analytical properties which allow of a partial separation from it4 ,the polonium1 if obtained in a pure state1 should initially be several hundred times as active as pure radium4 ,this activity1 however1 is not permanent2 it decays with the time1 falling to half value in about six months4 ,the absence of any new lines in the spectrum of radio-active bismuth is to be expected1 for1 even in the most active bismuth prepared1 the active matter exists in a very small proportion4 #ba4 ,the discussion of the nature of polonium was renewed by the discovery of ,marckwald#cf that a substance similar to polonium can be separated from pitchblende2 the activity of this substance1 he stated1 did not decay appreciably with the time4 ,the method of separation from the bismuth chloride solution1 obtained from uranium residues1 was very simple4 ,a rod of bismuth or antimony1 dipped in the active solution1 rapidly became coated with a black deposit which was intensely active4 ,this process was continued until the whole of the activity was removed from the solution4 ,the active deposit gave out only easily absorbed rays1 and in that respect resembled the polonium of ,mme ,curie4 ,the active substance was found to consist mainly of tellurium1 and for this reason ,marckwald gave it the name of radio-tellurium4 ,in later work1 however1 ,marckwald#cg has shown that the active constituent has no connection with tellurium1 but can always be separated completely from it by a simple chemical process4 ,in order to obtain a large amount of the active substance1 #bjjj kilos4 of pitchblende were worked up4 ,this yielded #f kilos4 of bismuth oxychloride1 and from this was separated #ae grams of radio-tellurium4 ,the tellurium present was precipitated from a hydrochloric acid solution by hydrazine hydrochloride4 ,the precipitated tellurium still showed some activity1 but this was removed by repeating the process4 ,the active matter then remained in the filtrate1 and1 after evaporation1 the addition of a few drops of stannous chloride caused a small quantity of a dark precipitate which was intensely active4 ,this was collected on a filter and weighed only #d milligrams4 ,when plates of copper1 tin or bismuth were dipped into an hydrochloric acid solution of this active substance1 the plates were found to be covered with a very finely divided deposit4 ,these plates were intensely active1 and produced marked photographic and phosphorescent action4 ,as an illustration of the enormous activity of this deposit1 ,marckwald stated that a precipitate of milligram on a copper plate1 #d square centimetres in area1 illuminated a zinc sulphide screen so brightly that it could be seen by an audience of several hundred people4 ,the active substance of ,marckwald is very closely allied in chemical and radio-active properties to the polonium of ,mme ,curie4 ,both active substances are separated with bismuth and both give out only easily absorbed rays4 ,the penetrating rays1 such as are given out by uranium1 radium or thorium1 are completely absent4 ,there has been a considerable amount of discussion as to whether the active substance obtained by ,marckwald is identical with that present in the polonium of ,mme ,curie4 ,marckwald stated that his active substance did not sensibly diminish in activity in the course of six months1 but it is doubtful whether the method of measurement used was sufficiently precise4 ,the writer has found that radio-tellurium of moderate activity1 prepared after ,marckwald's method and sold by ,dr ,sthamer of ,hamburg1 undoubtedly loses its activity with time4 ,the radio-tellurium is obtained in the form of a thin radio-active deposit on a polished bismuth rod or plate4 ,a bismuth rod was found to have lost half its activity in about #aej days1 and a similar result has been recorded by other observers4 ,the two substances are thus similar in both radio-active and chemical properties1 and there can be no reasonable doubt that the active constituent present in each case is the same4 ,the evidence is discussed in detail in chapter ,,xi and it will there be shown that the active substance present in the radio-tellurium of ,marckwald is a slow transformation product of radium4 #bb4 ,radio-active lead4 ,several observers early noticed that the lead separated from pitchblende showed strong radio-active properties1 but considerable difference of opinion was expressed in regard to the permanence of its activity4 ,elster and ,geitel#ch found that lead sulphate obtained from pitchblende was very active1 but they considered that the activity was probably due to an admixture of radium or polonium with the lead1 and1 by suitable chemical treatment1 the lead sulphate was obtained in an inactive state4 ,giesel#ci also separated some radio-active lead but found that its activity diminished with the time4 ,on the other hand1 ,hofmann and ,strauss#dj obtained lead from pitchblende whose activity seemed fairly permanent4 ,they state that the radio-active lead resembled ordinary lead in most of its reactions1 but showed differences in the behaviour of the sulphide and sulphate4 ,the sulphate was found to be strongly phosphorescent4 ,these results of ,hofmann and ,strauss were subjected at the time of their publication to considerable criticism1 and there is no doubt that the lead itself is not radio-active but contains a small quantity of radio-active matter which is separated with it4 ,in later work#da1 it has been shown that radio-lead contains several radio-active constituents which can be removed temporarily from it by suitable chemical methods4 ,there can be no doubt that the lead separated from pitchblende by certain methods does show considerable activity and that this activity is fairly permanent4 ,the radio-active changes occurring in radio-lead are complicated and cannot be discussed with advantage at this stage1 but will be considered in detail in chapter ,,xi4 ,it will there be shown that the primary constituent present in lead is a slow transformation product of radium4 ,this substance then slowly changes into the active constituent present in polonium1 which gives out only easily absorbed rays4 ,this polonium can be separated temporarily from the lead by suitable chemical methods1 but the radio-lead still continues to produce polonium1 so that a fresh supply may be obtained from it1 provided an interval of several months is allowed to elapse4 ,it will be calculated later that in all probability the radio-lead would lose half of its activity in an interval of #dj years4 ,the constituent present in radio-lead has not yet been separated1 but it will be shown that1 in the pure state1 it should have an activity considerably greater than that of radium itself4 ,sufficient attention has not yet been paid to this substance1 for1 separated in a pure state1 it should be as useful scientifically as radium4 ,in addition1 since it is the parent of polonium1 it should be possible to obtain from it at any time a supply of very active polonium1 in the same way that a supply of the radium emanation can be obtained at intervals from radium4 ,hofmann and ,strauss have observed a peculiar action of the cathode rays on the active lead sulphate separated by them4 ,they state that the activity diminishes with time1 but is recovered by exposure of the lead for a short time to the action of cathode rays4 ,no such action is shown by the active lead sulphide4 ,this effect is due most probably to the action of the cathode rays in causing a strong phosphorescence of the lead sulphate and has nothing to do with the radio-activity proper of the substance4 #bc4 ,is thorium a radio-active element8 ,the similarity of the chemical properties of actinium and thorium has led to the suggestion at different times that the activity of thorium is not due to thorium itself1 but to the presence of a slight trace of actinium4 ,in view of the difference in the rate of decay of the emanations of thorium and actinium1 this position is not tenable4 ,if the activity of thorium were due to actinium1 the two emanations1 as well as the other products obtained from these substances1 should have identical rates of decay4 ,since there is not the slightest evidence that the rate of decay of activity of the various products can be altered by chemical or physical agencies1 we may conclude with confidence that whatever radio-active substance is responsible for the activity of thorium1 it certainly is not actinium4 ,this difference in the rate of decay of the active products is of far more weight in deciding the question whether two bodies contain the same radio-active constituent than differences in chemical behaviour1 for it is quite probable that the active material in each case may exist only in minute quantity in the matter under examination1 and1 under such conditions1 a direct chemical examination in the first place is of little value4 ,recent work of ,hofmann and ,zerban and of ,baskerville1 however1 certainly tends to show that the element thorium is itself non-radio-active1 and that the radio-activity observed in ordinary thorium compounds is due to the admixture with it of an unknown radio-active element4 ,hofmann and ,zerban#db made a systematic examination of the radio-activity of thorium obtained from different mineral sources4 ,they found generally that thorium1 obtained from minerals containing a large percentage of uranium1 were more active than those obtained from minerals nearly free from uranium4 ,this indicates that the radio-activity observed in thorium may possibly be due to a transformation product of uranium which is closely allied chemically to thorium and is always separated with it4 ,a small quantity of thorium obtained from the mineral gadolinite was found by ,hofmann to be almost inactive1 whether tested by the electric or by the photographic method4 ,later ,baskerville and ,zerban#dc found that thorium obtained from a ,brazilian mineral was practically devoid of activity4 ,in this connection the recent work of ,baskerville on the complexity of ordinary thorium is of interest4 ,by special chemical methods1 he succeeded in separating two new and distinct substances from thorium1 which he has named carolinium and berzelium4 ,both of these substances are strongly radio-active1 and it thus seems probable that the active constituent observed in ordinary thorium may be due to one of these elements4 ,if1 as we have suggested1 thorium itself is not active1 it is certainly a matter of surprise that ordinary commercial thorium and the purest chemical preparations show about the same activity4 ,such a result indicates that the methods of purification have not removed any of the radio-active constituent originally present4 ,whatever the radio-active constituent in thorium may ultimately prove to be1 it is undoubtedly not radium nor actinium nor any of the known radio-active substances4 ,in later chapters1 the radio-activity of thorium will1 for simplicity1 be discussed on the assumption that thorium is itself a radio-active element4 ,the analysis of the changes which occur will thus not refer to thorium itself but to the primary radio-active substance usually found associated with it4 ,the conclusions to be drawn from an examination of the radio-active processes are for the most part independent of whether thorium is itself radio-active or whether the radio-activity is due to an unknown element4 ,if thorium is not radio-active itself1 it is not possible to draw any conclusions upon the question of the duration of the primary radio-activity associated with it4 ,such a deduction cannot be made until the quantity of the radio-active element present in thorium has been definitely determined4 #bd4 ,if elements heavier than uranium exist1 it is probable that they will be radio-active4 ,the extreme delicacy of radio-activity as a means of chemical analysis would enable such elements to be recognized even if present in infinitesimal quantities4 ,it is probable that considerably more than the three or four radio-elements at present recognized exist in minute quantity1 and that the number at present known will be augmented in the future4 ,in the first stage of the search1 a purely chemical examination is of little value1 for it is not probable that the new element should exist in sufficient quantity to be detected by chemical or spectroscopic analysis4 ,the main criteria of importance are the existence or absence of distinctive radiations or emanations1 and the permanence of the radio-activity4 ,the discovery of a radio-active emanation with a rate of decay different from those already known would afford strong evidence that a new radio-active body was present4 ,the presence of either thorium or radium in matter can very readily be detected by observing the rate of decay of the emanations given out by them4 ,when once the existence of a new radio-element has been inferred by an examination of its radio-active properties1 chemical methods of separation can be devised1 the radiating or emanating property being used as a guide in qualitative and quantitative analysis4 ,footnote #a3 ,niewenglowski1 ,c4 ,r4 #abb1 p4 #che1 #ahif4 ,footnote #b3 ,becquerel1 ,c4 ,r4 #abb1 p4 #eei1 #ahif4 ,footnote #c3 ,troost1 ,c4 ,r4 #abb1 p4 #efd1 #ahif4 ,footnote #d3 ,arnold1 ,annal4 d4 ,phys4 #fa1 p4 #caf1 #ahig4 ,footnote #e3 ,le ,bon1 ,c4 ,r4 #abb1 pp4 #ahh1 #bcc1 #chf1 #dfb1 #ahif4 ,footnote #f3 ,becquerel1 ,c4 ,r4 #abb1 pp4 #dbj1 #eja1 #eei1 #fhi1 #gfb1 #ajhf1 #ahif4 ,footnote #g3 ,mme ,curie1 ,thse prsente la ,facult des ,sciences de ,paris1 #aijc4 ,footnote #h3 ,nature1 #ef1 #ahig2 ,phil4 ,mag4 #dc1 p4 #dah1 #ahig2 #de1 p4 #bgg1 #ahih4 ,footnote #i3 ,rutherford1 ,phil4 ,mag4 ,jan4 #ahii4 ,footnote #aj3 ,ibid4 ,footnote #aa3 ,le ,bon1 ,c4 ,r4 #acj1 p4 #hia1 #aijj4 ,footnote #ab3 ,lenard1 ,annal4 d4 ,phys4 #a1 p4 #dih2 #c1 p4 #bih1 #aijj4 ,footnote #ac3 ,schmidt1 ,annal4 d4 ,phys4 #fe1 p4 #ada1 #ahih4 ,footnote #ad3 ,mme ,curie1 ,c4 ,r4 #abf1 p4 #aaja1 #ahih4 ,footnote #ae3 ,owens1 ,phil4 ,mag4 ,oct4 #ahii4 ,footnote #af3 ,rutherford1 ,phil4 ,mag4 ,jan4 #aijj4 ,footnote #ag3 ,m4 and ,mme ,curie and ,g4 ,bemont1 ,c4 ,r4 #abg1 p4 #abae1 #ahih4 ,footnote #ah3 ,giesel1 ,phys4 ,zeit4 #c1 ,no4 #bd1 p4 #egh1 #aijb4 ,footnote #ai3 ,giesel1 ,annal4 d4 ,phys4 #fi1 p4 #ia1 #ahij4 ,ber4 d4 ,d4 ,chem4 ,ges4 p4 #cfjh1 #aijb4 ,footnote #bj3 ,demaray1 ,c4 ,r4 #abg1 p4 #abah1 #ahih2 #abi1 p4 #gaf1 #ahii2 #aca1 p4 #beh1 #aijj4 ,footnote #ba3 ,runge1 ,astrophys4 ,journal1 p4 #a1 #aijj4 ,annal4 d4 ,phys4 ,no4 #aj1 p4 #djg1 #aijc4 ,footnote #bb3 ,exner and ,haschek1 ,wien4 ,ber4 ,july #d1 #aija4 ,footnote #bc3 ,crookes1 ,proc4 ,roy4 ,soc4 #gb1 p4 #bie1 #aijd4 ,footnote #bd3 ,runge and ,precht1 ,annal4 d4 ,phys4 ,,xiv4 #b1 p4 #dah1 #aijd4 ,footnote #be3 ,runge and ,precht1 ,phil4 ,mag4 ,april1 #aijc4 ,footnote #bf3 ,watts1 ,phil4 ,mag4 ,july1 #aijc2 ,august1 #aijd4 ,footnote #bg3 ,runge1 ,phil4 ,mag4 ,december1 #aijc4 ,footnote #bh3 ,debierne1 ,c4 ,r4 #abi1 p4 #eic1 #ahii2 #acj1 p4 #bjf1 #aijj4 ,footnote #bi3 ,giesel1 ,ber4 d4 ,d4 ,chem4 ,ges4 p4 #cfjh1 #aijb2 p4 #cdb1 #aijc4 ,footnote #cj3 ,debierne1 ,c4 ,r4 #aci1 p4 #ech1 #aijd4 ,miss ,brooks1 ,phil4 ,mag4 ,sept4 #aijd4 ,giesel1 ,phys4 ,zeit4 #e1 p4 #hbb1 #aijd4 ,jahrbuch4 d4 ,radioaktivitt1 no4 #d1 p4 #cde1 #aijd4 ,footnote #ca3 ,giesel1 ,ber4 d4 ,d4 ,chem4 ,ges4 #cg1 p4 #afif1 #aijd2 ,hartmann1 ,phys4 ,zeit4 #e1 ,no4 #ah1 p4 #egj1 #aijd4 ,footnote #cb3 ,mme ,curie1 ,c4 ,r4 #abg1 p4 #age1 #ahih4 ,footnote #cc3 ,mme ,curie1 ,thse1 ,paris1 #aijc4 ,footnote #cd3 ,crookes1 ,proc4 ,roy4 ,soc4 ,may1 #aijj4 ,footnote #ce3 ,berndt1 ,phys4 ,zeit4 #b1 p4 #ahj1 #aijj4 ,footnote #cf3 ,marckwald1 ,phys4 ,zeit4 #d1 ,no4 #a b1 p4 #ea4 ,footnote #cg3 ,marckwald1 ,ber4 d4 ,d4 ,chem4 ,ges4 p4 #bffb1 ,no4 #ab1 #aijc4 ,footnote #ch3 ,elster and ,geitel1 ,annal4 d4 ,phys4 #fi1 p4 #hc1 #ahii4 ,footnote #ci3 ,giesel1 ,ber4 d4 ,d4 ,chem4 ,ges4 p4 #cgge1 #aija4 ,footnote #dj3 ,hofmann and ,strauss1 ,ber4 d4 ,d4 ,chem4 ,ges4 p4 #cjce1 #aija4 ,footnote #da3 ,hofmann1 ,gonder and ,wlfl1 ,annal4 d4 ,phys4 ,no4 #ac1 p4 #fae1 #aijd4 ,footnote #db3 ,hofmann and ,zerban1 ,ber4 d4 ,d4 ,chem4 ,ges4 ,no4 #ab1 p4 #cjic1 #aijc4 ,footnote #dc3 ,baskerville and ,zerban1 ,amer4 ,chem4 ,soc4 #bf1 p4 #afdb1 #aijd4 ,,chapter ,,ii4 ,,ionization ,,theory ,,of ,,gases4 #be4 ,ionization of gases by radiation4 ,the most important property possessed by the radiations from radio-active bodies is their power of discharging bodies whether positively or negatively electrified4 ,as this property has been made the basis of a method for an accurate quantitative analysis and comparison of the radiations1 the variation of the rate of discharge under different conditions and the processes underlying it will be considered in some detail4 ,in order to explain the similar discharging power of ,rntgen rays1 the theory#dd has been put forward that the rays produce positively and negatively charged carriers throughout the volume of the gas surrounding the charged body1 and that the rate of production is proportional to the intensity of the radiation4 ,these carriers1 or ions#de as they have been termed1 move with a uniform velocity through the gas under a constant electric field1 and their velocity varies directly as the strength of the field4 ,illustration3 ,fig4 #a4 ,suppose we have a gas between two metal plates ,a and ,b 7,fig4 #a7 exposed to the radiation1 and that the plates are kept at a constant difference of potential4 ,a definite number of ions will be produced per second by the radiation1 and the number produced will depend in general upon the nature and pressure of the gas4 ,in the electric field the positive ions travel towards the negative plate1 and the negative ions towards the positive1 and consequently a current will pass through the gas4 ,some of the ions will also recombine1 the rate of recombination being proportional to the square of the number present4 ,for a given intensity of radiation1 the current passing through the gas will increase at first with the potential difference between the plates1 but it will reach a limit when all the ions are removed by the electric field before any recombination occurs4 ,this theory accounts also for all the characteristic properties of gases made conducting by the rays from active substances1 though there are certain differences observed between the conductivity phenomena produced by active substances and by ,x rays4 ,these differences are for the most part the result of unequal absorption of the two types of rays4 ,unlike ,rntgen rays1 a large proportion of the radiation from active bodies consists of rays which are absorbed in their passage through a few centimetres of air4 ,the ionization of the gas is thus not uniform1 but falls off rapidly with increase of distance from the active substance4 #bf4 ,variation of the current with voltage4 ,suppose that a layer of radio-active matter is spread uniformly on the lower of two horizontal plates ,a and ,b 7,fig4 #a74 ,the lower plate ,a is connected with one pole of a battery of cells the other pole of which is connected with earth4 ,the plate ,b is connected with one pair of quadrants of an electrometer1 the other pair being connected with earth4 ,the current#df between the plates1 determined by the rate of movement of the electrometer needle1 is observed at first to increase rapidly with the voltage1 then more slowly1 finally reaching a value which increases very slightly with a large increase in the voltage4 ,this1 as we have indicated1 is simply explained on the ionization theory4 ,the radiation produces ions at a constant rate1 and1 before the electric field is applied1 the number per unit volume increases until the rate of production of fresh ions is exactly balanced by the recombination of the ions already produced4 ,on application of a small electric field1 the positive ions travel to the negative electrode and the negative to the positive4 ,since the velocity of the ions between the plates is directly proportional to the strength of the electric field1 in a weak field the ions take so long to travel between the electrodes that most of them recombine on the way4 ,the current observed is consequently small4 ,with increase of the voltage there is an increase of speed of the ions and a smaller number recombine4 ,the current consequently increases1 and will reach a maximum value when the electric field is sufficiently strong to remove all the ions before appreciable recombination has occurred4 ,the value of the current will then remain constant even though the voltage is largely increased4 ,this maximum current will be called the 8saturation0 current1 and the value of the potential difference required to give this maximum current1 the 8saturation ,p4,d40#dg ,the general shape of the current-voltage curve is shown in ,fig4 #b1 where the ordinates represent current and the abscissae volts4 ,illustration3 ,fig4 #b4 ,although the variation of the current with voltage depends only on the velocity of the ions and their rate of recombination1 the full mathematical analysis is intricate1 and the equations1 expressing the relation between current and voltage1 are only integrable for the case of uniform ionization4 ,the question is complicated by the inequality in the velocity of the ions and by the disturbance of the potential gradient between the plates by the movement of the ions4 ,j4 ,j4 ,thomson#dh has worked out the case for uniform production of ions between two parallel plates1 and has found that the relation between the current i and the potential difference ,v applied is expressed by ,ai ,bi ,v where ,a and ,b are constants for a definite intensity of radiation and a definite distance between the plates4 ,illustration3 ,fig4 #c4 ,in certain cases of unsymmetrical ionization1 which arise in the study of the radiations from active bodies1 the relation between current and voltage is very different from that expressed by the above equation4 ,some of these cases will be considered in section #dg4 #bg4 ,the general shape of the current-voltage curves for gases exposed to the radiations from active bodies is shown in ,fig4 #c4 ,this curve was obtained for #de grams of impure radium chloride1 of activity #ajjj times that of uranium1 spread over an area of #cc sq4 cms4 on the lower of two large parallel plates1 #de cms4 apart4 ,the maximum value of the current observed1 which is taken as #ajj1 was #ab #aj amperes1 the current for low voltages was nearly proportional to the voltage1 and about #fjj volts between the plates was required to ensure approximate saturation4 ,in dealing with slightly active bodies like uranium or thorium1 approximate saturation is obtained for much lower voltages4 ,tables ,i4 and ,,ii4 show the results for the current between two parallel plates distant #je cms4 and #be cms4 apart respectively1 when one plate was covered with a thin uniform layer of uranium oxide4 ,,table ,i4 #je cms4 apart ,volts ,current #abe #ah #be #cf #e #ee #a #fg #b #gb #d #gi #h #he #af #hh #ajj #id #cce #ajj ,,table ,,ii4 #be cms4 apart ,volts ,current #e #gc #a #ad #b #bg #d #dg #h #fd #af #gc #cge #ha #aab #ij #cge #ig #hjj #ajj ,the results are shown graphically in ,fig4 #d4 ,illustration3 ,fig4 #d4 ,from the above tables it is seen that the current at first increases nearly in proportion to the voltage4 ,there is no evidence of complete saturation1 although the current increases very slowly for large increases of voltage4 ,for example1 in ,table ,i4 a change of voltage from #abe to #be volts increases the current from #ah to #cf of the maximum1 while a change of voltage from #ajj to #cce volts increases the current only #f4 ,the variation of the current per volt 7assumed uniform between the range of voltages considered7 is thus about #ejjj times greater for the former change4 ,taking into consideration the early part of the curves1 the current does not reach a practical maximum as soon as would be expected on the simple ionization theory4 ,it seems probable that the slow increase with the large voltages is due either to an action of the electric field on the rate of production of ions1 or to the difficulty of removing the ions produced near the surface of the uranium before recombination4 ,it is possible that the presence of a strong electric field may assist in the separation of ions which otherwise would not initially escape from the sphere of one another's attraction4 ,from the data obtained by ,townsend for the conditions of production of fresh ions at low pressures by the movement of ions through the gas1 it seems that the increase of current cannot be ascribed to an action of the moving ions in the further ionization of the gas4 #bh4 ,the equation expressing the relation between the current and the voltage is very complicated even in the case of a uniform rate of production of ions between the plates4 ,an approximate theory1 which is of utility in interpreting the experimental results1 can however be simply deduced if the disturbance of the potential gradient is disregarded1 and the ionization assumed uniform between the plates4 ,suppose that the ions are produced at a constant rate q per cubic centimetre per second in the gas between parallel plates distant l cms4 from each other4 ,when no electric field is applied1 the number ,n present per c4c41 when there is equilibrium between the rates of production and recombination1 is given by q ,n1 where is a constant4 ,if a small potential difference ,v is applied1 which gives only a small fraction of the maximum current1 and consequently has not much effect on the value of ,n1 the current i per sq4 cm4 of the plate1 is given by ,neu,v i ----- l where u is the sum of the velocity of the ions for unit potential gradient1 and e is the charge carried by an ion4 u,v ---- l is the velocity of the ions in the electric field of strength ,v --- l ,the number of ions produced per second in a prism of length l and unit area of cross-section is ql4 ,the maximum or saturation current ,i per sq4 cm4 of the plate is obtained when all of these ions are removed to the electrodes before any recombination has occurred4 ,thus ,i q 4 l 4 e1 and fraci,i frac,nu,vql#b fracu,vl#bsqrtqalpha ,this equation expresses the fact previously noted that1 for small voltages1 the current i is proportional to ,v4 ,let i_/,i 1 then ,v frac rho l#b sqrt qalpha u ,now the greater the value of ,v required to obtain a given value of 7supposed small compared with unity71 the greater the potential required to produce saturation4 ,it thus follows from the equation that3 7#a7 ,for a given intensity of radiation1 the saturation ,p4,d4 increases with the distance between the plates4 ,in the equation1 for small values of 1 ,v varies as l4 ,this is found to be the case for uniform ionization1 but it only holds approximately for non-uniform ionization4 7#b7 ,for a given distance between the plates1 the saturation ,p4,d4 is greater1 the greater the intensity of ionization between the plates4 ,this is found to be the case for the ionization produced by radio-active substances4 ,with a very active substance like radium1 the ionization produced is so intense that very large voltages are required to produce approximate saturation4 ,on the other hand1 only a fraction of a volt per cm4 is necessary to produce saturation in a gas where the ionization is very slight1 for example1 in the case of the natural ionization observed in a closed vessel1 where no radio-active substances are present4 ,for a given intensity of radiation1 the saturation ,p4,d4 decreases rapidly with the lowering of the pressure of the gas4 ,this is due to two causes operating in the same direction1 viz4 a decrease in the intensity of the ionization and an increase in the velocity of the ions4 ,the ionization varies directly as the pressure1 while the velocity varies inversely as the pressure4 ,this will obviously have the effect of causing more rapid saturation1 since the rate of recombination is slower and the time taken for the ions to travel between the electrodes is less4 ,the saturation curves observed for the gases hydrogen and carbon dioxide#di are very similar in shape to those obtained for air4 ,for a given intensity of radiation1 saturation is more readily obtained in hydrogen than in air1 since the ionization is less than in air while the velocity of the ions is greater4 ,carbon dioxide on the other hand requires a greater ,p4,d4 to produce saturation than does air1 since the ionization is more intense and the velocity of the ions less than in air4 #bi4 ,townsend#ej has shown that1 for low pressures1 the variation of the current with the voltage is very different from that observed at atmospheric pressure4 ,if the increase of current with the voltage is determined for gases1 exposed to ,rntgen rays1 at a pressure of about #a mm4 of mercury1 it is found that for small voltages the ordinary saturation curve is obtained2 but when the voltage applied increases beyond a certain value1 depending on the pressure and nature of the gas and the distance between the electrodes1 the current commences to increase slowly at first but very rapidly as the voltage is raised to the sparking value4 ,the general shape of the current curve is shown in ,fig4 #e4 ,illustration3 ,fig4 #e4 ,the portion ,,oab of the curve corresponds to the ordinary saturation curve4 ,at the point ,b the current commences to increase4 ,this increase of current has been shown to be due to the action of the negative ions at low pressures in producing fresh ions by collision with the molecules in their path4 ,the increase of current is not observed in air at a pressure above #cj mms4 until the ,p4,d4 is increased nearly to the value required to produce a spark4 ,this production of ions by collision is considered in more detail in section #da4 #cj4 ,rate of recombination of the ions4 ,a gas ionized by the radiation preserves its conducting power for some time after it is removed from the presence of the active body4 ,a current of air blown over an active body will thus discharge an electrified body some distance away4 ,the duration of this after conductivity can be examined very conveniently in an apparatus similar to that shown in ,fig4 #f4 ,illustration3 ,fig4 #f4 ,a dry current of air or any other gas is passed at a constant rate through a long metal tube ,,tl4 ,after passing through a quantity of cotton-wool to remove dust particles1 the current of air passes over a vessel ,t containing a radio-active body such as uranium1 which does not give off a radio-active emanation4 ,by means of insulated electrodes ,a and ,b1 charged to a suitable potential1 the current between the tube and one of these electrodes can be tested at various points along the tube4 ,a gauze screen1 placed over the cross-section of the tube at ,d1 serves to prevent any direct action of the electric field in abstracting ions from the neighbourhood of ,t4 ,if the electric field is sufficiently strong1 all the ions travel in to the electrodes at ,a1 and no current is observed at the electrode ,b4 ,if the current is observed successively at different distances along the tube1 all the electrodes except the one under consideration being connected to earth1 it is found that the current diminishes with the distance from the active body4 ,if the tube is of fairly wide bore1 the loss of the ions due to diffusion is small1 and the decrease in conductivity of the gas is due to recombination of the ions alone4 ,on the ionization theory1 the number dn of ions per unit volume which recombine in the time dt is proportional to the square of the number present4 ,thus dn ---- n1 dt where is a constant4 ,integrating this equation1 #a #a --- --- t1 n ,n if ,n is the initial number of ions1 and n the number after a time t4 ,the experimental results obtained#ea have been shown to agree very well with this equation4 ,in an experiment similar to that illustrated in ,fig4 #f1 using uranium oxide as a source of ionization1 it was found that half the number of ions present in the gas recombined in #bd seconds1 and that at the end of #h seconds one-fourth of the ions were still uncombined4 ,since the rate of recombination is proportional to the square of the number present1 the time taken for half of the ions present in the gas to recombine decreases very rapidly with the intensity of the ionization4 ,if radium is used1 the ionization is so intense that the rate of recombination is extremely rapid4 ,it is on account of this rapidity of recombination that large voltages are necessary to produce saturation in the gases exposed to very active preparations of radium4 ,the value of 1 which may be termed the coefficient of recombination1 has been determined in absolute measure by ,townsend#eb1 ,mc,clung#ec and ,langevin#ed by different experimental methods but with very concordant results4 ,suppose1 for example1 with the apparatus of ,fig4 #f1 the time ,t1 taken for half the ions to recombine after passing by the electrode ,a1 has been determined experimentally4 ,then #a --- ,t1 ,n where ,n is the number of ions per c4c4 present at ,a4 ,if the saturation current i is determined at the electrode ,a1 i ,n,ve1 where e is the charge on an ion and ,v is the volume of uniformly ionized gas carried by the electrode ,a per second4 ,then ,ve ----- 4 i,t ,the following table shows the value of obtained for different gases4 ,value of 4 ,gas ,townsend ,mc,clung ,langevin ,air #cdbj e #cchd e #cbjj e ,carbon #cejj e #cdib e #cdjj e ,dioxide ,hydrogen #cjbj e ,the latest determination of the value of e 7see section #cf7 is #cd #aj ,e4,s4 units2 thus #aa #aj4 ,using this value1 it can readily be shown from the equation of recombination that1 if #aj ions are present per c4c41 half of them recombine in about #ji sec4 and #ii in #ij secs4 ,mc,clung 7loc4 cit47 showed that the value of was approximately independent of the pressure between #abe and three atmospheres4 ,in later observations1 ,langevin has found that the value of decreases rapidly when the pressure is lowered below the limits used by ,mc,clung4 #ca4 ,in experiments on recombination it is essential that the gas should be free from dust or other suspended particles4 ,in dusty air1 the rate of recombination is much more rapid than in dust-free air1 as the ions diffuse rapidly to the comparatively large dust particles distributed throughout the gas4 ,the effect of the suspension of small particles in a conducting gas is very well illustrated by an experiment of ,owens#ee4 ,if tobacco smoke is blown between two parallel plates as in ,fig4 #a1 the current at once diminishes to a small fraction of its former value1 although a ,p4,d4 is applied sufficient to produce saturation under ordinary conditions4 ,a much larger voltage is then necessary to produce saturation4 ,if the smoke particles are removed by a stream of air1 the current returns at once to its original value4 #cb4 ,mobility of the ions4 ,determinations of the mobility of the ions1 i4e4 the velocity of the ions under a potential gradient of #a volt per cm41 have been made by ,rutherford#ef1 ,zeleny#eg1 and ,langevin#eh for gases exposed to ,rntgen rays4 ,although widely different methods have been employed1 the results have been very concordant1 and fully support the view that the ions move with a velocity proportional to the strength of the field4 ,on the application of an electric field1 the ions almost instantly attain the velocity corresponding to the field and then move with a uniform speed4 ,zeleny#ei first drew attention to the fact that the positive and negative ions had different velocities4 ,the velocity of the negative ion is always greater than that of the positive1 and varies with the amount of water vapour present in the gas4 ,the results1 previously discussed1 of the variation of the current with voltage and of the rate of recombination of the ions do not of themselves imply that the ions produced in gases by the radiations from active bodies are of the same size as those produced by ,rntgen rays under similar conditions4 ,they merely show that the conductivity under various conditions can be satisfactorily explained by the view that charged ions are produced throughout the volume of the gas4 ,the same general relations would be observed if the ions differed considerably in size and velocity from those produced by ,rntgen rays4 ,the most satisfactory method of determining whether the ions are identical in the two cases is to determine the velocity of the ions under similar conditions4 ,in order to compare the velocity of the ions#fj1 the writer has used an apparatus similar to that shown in ,fig4 #f on p4 #dj4 ,the ions were carried with a rapid constant stream of air past the charged electrode ,a1 and the conductivity of the gas tested immediately afterwards at an electrode ,b1 which was placed close to ,a4 ,the insulated electrodes ,a and ,b were fixed centrally in the metal tube ,l1 which was connected with earth4 ,for convenience of calculation1 it is assumed that the electric field between the cylinders is the same as if the cylinders were infinitely long4 ,let a and b be the radii of the electrode ,a1 and of the tube ,l respectively1 and let ,v potential of ,a4 ,the electromotive intensity ,x 7without regard to sign7 at a distance r from the centre of the tube is given by ,x frac ,v r loge frac b a ,let u and u be the velocities of the positive and negative ions for a potential gradient of #a volt per cm4 ,if the velocity is proportional to the electric force at any point1 the distance dr traversed by the negative ion in the time dt is given by dr ,xu dt1 or dt frac loge frac ba r dr ,vu#b ,let r be the greatest distance measured from the axis of the tube from which the negative ion can just reach the electrode ,a in the time t taken for the air to pass along the electrode4 ,then t frac r#bb a#b #b ,vu#b loge frac ba ,if be the ratio of the number of the negative ions that reach the electrode ,a to the total number passing by1 then rho#b frac r#bb a#b b#b a#b ,therefore u#b frac rho#b 7b#b a#b7 loge frac ba #b ,vt ,similarly the ratio of the number of positive ions that give up their charge to the external cylinder to the total number of positive ions is given by u#a frac rho#a 7b#b a#b7 loge frac ba #b ,vt ,in the above equations it is assumed that the current of air is uniform over the cross-section of the tube1 and that the ions are uniformly distributed over the cross-section2 also1 that the movement of the ions does not appreciably disturb the electric field4 ,since the value of t can be calculated from the velocity of the current of air and the length of the electrode1 the values of the velocities of the ions under unit potential gradient can at once be determined4 ,the equation 7#a7 shows that is proportional to ,v1--i4e4 that the rate of discharge of the electrode ,a varies directly as the potential of ,a1 provided that the value of ,v is not large enough to remove all the ions from the gas as it passes by the electrode4 ,this was found experimentally to be the case4 ,in the comparison of the velocities1 the potential ,v was adjusted to such a value that was about one half1 when uranium oxide was placed in the tube at ,l4 ,the active substance was then removed1 and an aluminium cylinder substituted for the brass tube4 ,x rays were allowed to fall on the centre of this aluminium cylinder1 and the strength of the rays adjusted to give about the same conductivity to the gas as the uranium had done4 ,under these conditions the value of was found to be the same as for the first experiment4 ,this experiment shows conclusively that the ions produced by ,rntgen rays and by uranium move with the same velocity and are probably identical in all respects4 ,the method described above is not very suitable for an accurate determination of the velocities1 but gave values for the positive ions of about #ad cms4 per second per volt per centimetre1 and slightly greater values for the negative ions4 #cc4 ,the most accurate determinations of the mobility of the ions produced by ,rntgen rays have been made by ,zeleny#fa and ,langevin#fb4 ,zeleny used a method similar in principle to that explained above4 ,his results are shown in the following table1 where ,k is the mobility of the positive ion and ,k that of the negative ion4 ,gas ,k ,k ,k_/,k ,temperature ,air1 dry #acf #ahg #acge #ace ,c4 moist #acg #aea #aaj #ad ,oxygen1 dry #acf #ahj #acb #ag moist #abi #aeb #aah #af ,carbon dioxide1 #jgf #jha #ajg #age dry #jha #jge #jiae #ag moist ,hydrogen1 dry #fgj #gie #aae #bj moist #ecj #efj #aje #bj ,langevin determined the velocity of the ions by a direct method in which the time taken for the ion to travel over a known distance was observed4 ,the following table shows the comparative values obtained for air and carbon dioxide4 ,air ,k ,air ,k ,air ,,co ,k ,,co ,k ,,co ,k_/,k ,k_/,k ,direct #adj #agj #abb #jhf #jij #aje method 7,langevin7 ,current of #acf #ahg #acge #jgf #jha #ajg gas 7,zeleny7 ,these results show that for all gases except ,,co1 there is a marked increase in the velocity of the negative ion with the dryness of the gas1 and that1 even in moist gases1 the velocity of the negative ions is always greater than that of the positive ions4 ,the velocity of the positive ion is not much affected by the presence of moisture in the gas4 ,the velocity of the ions varies inversely as the pressure of the gas4 ,this has been shown by ,rutherford#fc for the negative ions produced by ultra-violet light falling on a negatively charged surface1 and later by ,langevin#fd for both the positive and negative ions produced by ,rntgen rays4 ,langevin has shown that the velocity of the positive ion increases more slowly with the diminution of pressure than that of the negative ion4 ,it appears as if the negative ion1 especially at pressures of about #aj mm4 of mercury1 begins to diminish in size4 #cd4 ,condensation experiments4 ,some experiments will now be described which have verified in a direct way the theory that the conductivity produced in gases by the various types of radiation is due to the production of charged ions throughout the volume of the gas4 ,under certain conditions1 the ions form nuclei for the condensation of water1 and this property allows us to show the presence of the individual ions in the gas1 and also to count the number present4 ,it has long been known that1 if air saturated with water-vapour be suddenly expanded1 a cloud of small globules of water is formed4 ,these drops are formed round the dust particles present in the gas1 which act as nuclei for the condensation of water around them4 ,the experiments of ,r4 von ,helmholtz and ,richarz#fe had shown that chemical reactions1 for example the combustion of flames1 taking place in the neighbourhood1 affected the condensation of a steam-jet4 ,lenard showed that a similar action was produced when ultra-violet light fell on a negatively charged zinc surface placed near the steam-jet4 ,these results suggested that the presence of electric charges in the gas facilitated condensation4 ,a very complete study of the conditions of condensation of water on nuclei has been made by ,c4 ,t4 ,r4 ,wilson#ff4 ,an apparatus was constructed which allowed a very sudden expansion of the air over a wide range of pressure4 ,the amount of condensation was observed in a small glass vessel4 ,a beam of light was passed into the apparatus which allowed the drops formed to be readily observed by the eye4 ,preliminary small expansions caused a condensation of the water round the dust nuclei present in the air4 ,these dust nuclei were removed by allowing the drops to settle4 ,after a number of successive small expansions1 the air was completely freed from dust1 so that no condensation was produced4 ,let v initial volume of the gas in the vessel1 v volume after expansion4 ,if v_/v #abe no condensation is produced in dust-free air4 ,if however v_/v #abe and #ach1 a few drops appear4 ,this number is roughly constant until v_/v #ach1 when the number suddenly increases and a very dense cloud of fine drops is produced4 ,if the radiation from an ,x ray tube or a radio-active substance is now passed into the condensation vessel1 a new series of phenomena is observed4 ,as before1 if v_/v #abe no drops are formed1 but if v_/v #abe there is a sudden production of a cloud4 ,the water drops of which this cloud is formed are finer and more numerous the greater the intensity of the rays4 ,the point at which condensation begins is very marked1 and a slight variation of the amount of expansion causes either a dense cloud or no cloud at all4 ,it now remains to be shown that the formation of a cloud by the action of the rays is due to the productions of ions in the gas4 ,if the expansion vessel is provided with two parallel plates between which an electric field can be applied1 it is seen that the number of drops1 formed by the expansion with the rays acting1 decreases with increase of the electric field4 ,the stronger the field the smaller the number of drops formed4 ,this result is to be expected if the ions are the centres of condensation2 for in a strong electric field the ions are carried at once to the electrodes1 and thus disappear from the gas4 ,if no electric field is acting1 a cloud can be produced some time after the rays have been cut off2 but if a strong electric field is applied1 under the same conditions1 no cloud is formed4 ,this is in agreement with experiments showing the time required for the ions to disappear by recombination4 ,in addition it can be shown that each one of the fine drops carries an electric charge and can be made to move in a strong uniform electric field4 ,the small number of drops produced without the action of the rays when v_/v #abe is due to a very slight natural ionization of the gas4 ,that this ionization exists has been clearly shown by electrical methods 7section #bhd74 ,the evidence is thus complete that the ions themselves serve as centres for the condensation of water around them4 ,these experiments show conclusively that the passage of electricity through a gas is due to the presence of charged ions distributed throughout the volume of the gas1 and verify in a remarkable way the hypothesis of the discontinuous structure of the electric charges carried by matter4 ,this property of the ions of acting as nuclei of condensation gives a very delicate method of detecting the presence of ions in the gas4 ,if only an ion or two is present per c4c41 their presence after expansion is at once observed by the drops formed4 ,in this way the ionization due to a small quantity of uranium held a yard away from the condensation vessel is at once made manifest4 #ce4 ,difference between the positive and negative ions4 ,in the course of experiments to determine the charge carried by an ion1 ,j4 ,j4 ,thomson#fg observed that the cloud formed under the influence of ,x rays increased in density when the expansion was about #aca1 and suggested in explanation that the positive and negative ions had different condensation points4 ,illustration3 ,fig4 #g4 ,this difference in behaviour of the positive and negative ions was investigated in detail by ,c4 ,t4 ,r4 ,wilson#fh in the following way4 ,x rays were made to pass in a narrow beam on either side of a plate ,,ab 7,fig4 #g7 dividing the condensation vessel into two equal parts4 ,the opposite poles of a battery of cells were connected with two parallel plates ,c and ,d1 placed symmetrically with regard to ,a4 ,the middle point of the battery and the plate ,a were connected with earth4 ,if the plate ,c is positively charged1 the ions in the space ,,ca at a short distance from ,a are all negative in sign4 ,those to the right are all positive4 ,it was found that condensation occurred only for the negative ions in ,,ac when v_/v #abe but did not occur in ,,ad for the positive ions until v_/v #aca4 ,thus the negative acts more readily than the positive ion as a centre of condensation4 ,the greater effect of the negative ion in causing condensation has been suggested as an explanation of the positive charge always observed in the upper atmosphere4 ,the negative ions under certain conditions become centres for the formation of small drops of water and are removed to the earth by the action of gravity1 while the positive ions remain suspended4 ,with the apparatus described above1 it has been shown that the positive and negative ions are equal in number4 ,if the expansion is large enough to ensure condensation on both ions1 the drops formed on the right and left of the vessel in ,fig4 #g are equal in number and fall at the same rate1 i4e4 are equal in size4 ,since the ions are produced in equal numbers from a gas electrically neutral1 this experiment shows that the charges on positive and negative ions are equal in value but opposite in sign4 #cf4 ,charge carried by an ion4 ,for a known sudden expansion of a gas saturated with water vapour1 the amount of water precipitated on the ions can be calculated readily4 ,the size of the drops can be determined by observing the rate at which the cloud settles under the action of gravity4 ,from ,stokes' equation1 the terminal velocity u of a small sphere of radius r and density d falling through a gas of which the coefficient of viscosity is is given by #b dgr u -------- #i where g is the acceleration due to gravity4 ,the radius of the drop and consequently the weight of water in each drop can thus be determined4 ,since the total weight of water precipitated is known1 the number of drops present is obtained at once4 ,this method has been used by ,j4 ,j4 ,thomson#fi to determine the charge carried by an ion4 ,if the expansion exceeds the value #aca1 both positive and negative ions become centres of condensation4 ,from the rate of fall it can be shown that approximately the drops are all of the same size4 ,the condensation vessel was similar to that employed by ,c4 ,t4 ,r4 ,wilson4 ,two parallel horizontal plates were fitted in the vessel and the radiation from an ,x ray tube or radio-active substance ionized the gas between them4 ,a difference of potential ,v1 small compared with that required to saturate the gas1 was applied between the parallel plates distant l cms4 from each other4 ,the small current i through the gas is given 7section #bh7 by ,nu,ve i ------ l where ,n number of ions present in the gas1 e charge on each ion1 u sum of the velocities of the positive and negative ions4 ,since the value of ,n is the same as the number of drops1 and the velocity u is known1 the value of e can be determined4 ,in his last determination ,j4 ,j4 ,thomson found that e #cd #aj electrostatic units4 ,a very concordant value1 namely1 #ca #aj1 has been obtained by ,h4 ,a4 ,wilson#gj1 by using a modified method of counting the drops4 ,a check on the size of the drops1 determined by their rate of fall1 was made by observing the rate at which the drops moved in a strong electric field1 arranged so as to act with or against gravity4 ,j4 ,j4 ,thomson found that the charge on the ions produced in hydrogen and oxygen is the same4 ,this shows that the nature of the ionization in gases is distinct from that occurring in the electrolysis of solutions where the oxygen ion always carries twice the charge of the hydrogen ion4 #cg4 ,diffusion of the ions4 ,early experiments with ionized gases showed that the conductivity was removed from the gas by passage through a finely divided substance like cotton-wool1 or by bubbling through water4 ,this loss of conductivity is due to the fact that the ions in passing through narrow spaces diffuse to the sides of the boundary1 to which they either adhere or give up their charge4 ,a direct determination of the coefficient of diffusion of the ions produced in gases by ,rntgen rays or by the rays from active substances has been made by ,townsend#ga4 ,the general method employed was to pass a stream of ionized gas through a diffusion vessel made up of a number of fine metal tubes arranged in parallel4 ,some of the ions in their passage through the tubes diffuse to the sides1 the proportion being greater the slower the motion of the gas and the narrower the tube4 ,observations were made of the conductivity of the gas before and after passage through the tubes4 ,in this way1 correcting if necessary for the recombination during the time taken to pass through the tubes1 the proportion ,r of either positive or negative ions which are abstracted can be deduced4 ,the value of ,r can be expressed mathematically by the following equation in terms of ,k1 the coefficient of diffusion of the ions into the gas with which they are mixed#gb1 ,r #d 7cdot#aie;e frac#c;cdot#ff,,kza#b,v cdot#jbdc;e frac#bb;cdot#c,,kza#b,v c47 where a radius of the tube1 ,z length of the tube1 ,v mean velocity of the gas in the tube4 ,only the first two terms of the series need be taken into account when narrow tubes are used4 ,in this equation ,r1 ,v1 and a are determined experimentally1 and ,k can thus be deduced4 ,the following table shows the results obtained by ,townsend when ,x rays were used4 ,almost identical results were obtained later1 when the radiations from active substances replaced the ,x rays4 ,coefficients of diffusion of ions into gases4 ,gas ,k ,k ,mean ,ratio for for value of ions ions of ,k values of ,k ,air1 dry #jbh #jdc #jcdg #aed moist #jcb #jce #jcce #aji ,oxygen1 dry #jbe #jcif #jcbc #aeh moist #jbhh #jceh #jcbc #abd ,carbonic acid1 #jbc #jbf #jbde #aac dry #jbde #jbee #jbe #ajd moist ,hydrogen1 dry #abc #aij #aef #aed moist #abh #adb #ace #aaa ,the moist gases were saturated with water vapour at a temperature of #ae ,c4 ,it is seen that the negative ion in all cases diffuses faster than the positive4 ,theory shows that the coefficients of diffusion should be directly proportional to the velocities of the ions1 so that this result is in agreement with the observations on the greater velocity of the negative ion4 ,this difference in the rate of diffusion of the ions at once explains an interesting experimental result4 ,if ionized gases are blown through a metal tube1 the tube gains a negative charge while the gas itself retains a positive charge4 ,the number of positive and negative ions present in the gas is originally the same1 but1 in consequence of the more rapid diffusion of the negative ions1 more of the negative ions than of the positive give up their charges to the tube4 ,the tube consequently gains a negative and the gas a positive charge4 #ch4 ,a very important result can be deduced at once when the velocities and coefficients of diffusion of the ions are known4 ,townsend 7loc4 cit47 has shown that the equation of their motion is expressed by the formula #a dp --- pu ---- n,xe1 ,k dx where e is the charge on an ion1 n number of ions per c4c41 p their partial pressure1 and u is the velocity due to the electric force ,x in the direction of the axis of x4 ,when a steady state is reached1 dp n,xe,k ---- #j and u ----1 dx p ,let ,n be the number of molecules in a cubic centimetre of gas at the pressure ,p and at the temperature #ae ,c41 for which the values of u and ,k have been determined4 ,then ,n_/,p may be substituted for n_/p1 and1 since ,p at atmospheric pressure is #aj1 #c #aju ,ne ---------- electrostatic units1 ,k where u is the velocity for #a volt 7i4e4 ,e4 ,s4 unit7 per cm4 ,it is known that one absolute electromagnetic unit of electricity in passing through water liberates #abc c4c4 of hydrogen at a temperature of #ae ,c4 and standard pressure4 ,the number of atoms in this volume is #bdf,n1 and1 if e is the charge on the hydrogen atom in the electrolysis of water1 #bdf ,ne #c #aj ,e4 ,s4 units1 ,ne #abb #aj ,e4 ,s4 units4 e u ,thus --- #bdf #aj ---- e ,k ,for example1 substituting the values of u and ,k determined in moist air for the positive ion1 e #bdf #acg --- ----- ----- #ajd4 e #ajj #jcb ,values of this ratio1 not very different from unity1 are obtained for the positive and negative ions of the gases hydrogen1 oxygen1 and carbon dioxide4 ,taking into consideration the uncertainty in the experimental values of u and ,k1 these results indicate that the charge carried by an ion in all gases is the same and is equal to that carried by the hydrogen ion in the electrolysis of liquids4 #ci4 ,number of the ions4 ,we have seen that1 from experimental data1 ,townsend has found that ,n1 the number of molecules present in #a c4c4 of gas at #ae ,c4 and standard pressure1 is given by ,ne #abb #aj4 ,now e1 the charge on an ion1 is equal to #cd #aj ,e4 ,s4 units2 thus ,n #cf #aj4 ,if ,i is the saturation current through a gas1 and q the total rate of production of ions in the gas1 ,i q --- 4 e ,the saturation current through air was found to be #ab #aj amperes1 i4e4 #cf ,e4,s4 units1 for parallel plates #de cms4 apart1 when #de gramme of radium of activity #ajjj times that of uranium was spread over an area of #cc sq4 cms4 of the lower plate4 ,this corresponds to a production of about #aj ions per second4 ,assuming1 for the purpose of illustration1 that the ionization was uniform between the plates1 the volume of air acted on by the rays was about #adh c4c41 and the number of ions produced per c4c4 per second about #g #aj4 ,since ,n #cf #aj1 we see that1 if one molecule produces two ions1 the proportion of the gas ionized per second is about #aj of the whole4 ,for uranium the fraction is about #aj1 and for pure radium1 of activity one million times that of uranium1 about #aj4 ,thus even in the case of pure radium1 only about one molecule of gas is acted on per second in every #ajj millions4 ,the electrical methods are so delicate that the production of one ion per cubic centimetre per second can be detected readily4 ,this corresponds to the ionization of about one molecule in every #aj present in the gas4 #dj4 ,size and nature of the ions4 ,an approximate estimate of the mass of an ion1 compared with the mass of the molecule of the gas in which it is produced1 can be made from the known data of the coefficient ,k of inter-diffusion of the ions into gases4 ,the value of ,k for the positive ions in moist carbon dioxide has been shown to be #jbde1 while the value of ,k for the inter-diffusion of carbon dioxide with air is #ad4 ,the value of ,k for different gases is approximately inversely proportional to the square root of the products of the masses of the molecules of the two inter-diffusing gases2 thus1 the positive ion in carbon dioxide behaves as if its mass were large compared with that of the molecule4 ,similar results hold for the negative as well as for the positive ion1 and for other gases besides carbon dioxide4 ,this has led to the view that the ion consists of a charged centre surrounded by a cluster of molecules travelling with it1 which are kept in position round the charged nucleus by electrical forces4 ,a rough estimate shows that this cluster consists of about #cj molecules of the gas4 ,this idea is supported by the variation in velocity1 i4e4 the variation of the size of the negative ion1 in the presence of water vapour2 for the negative ion undoubtedly has a greater mass in moist than in dry gases4 ,at the same time it is possible that the apparently large size of the ion1 as determined by diffusion methods1 may be in part a result of the charge carried by the ion4 ,the presence of a charge on a moving body would increase the frequency of collision with the molecules of the gas1 and consequently diminish the rate of diffusion4 ,the ion on this view may not actually be of greater size than the molecule from which it is produced4 ,the negative and positive ions certainly differ in size1 and this difference becomes very pronounced for low pressures of the gas4 ,at atmospheric pressure1 the negative ion1 produced by the action of ultra-violet light on a negatively charged body1 is of the same size as the ion produced by ,x rays1 but at low pressures ,j4 ,j4 ,thomson has shown that it is identical with the corpuscle or electron1 which has an apparent mass of about of the mass of the hydrogen atom4 ,a similar result has been shown by ,townsend to hold for the negative ion produced by ,x rays at a low pressure4 ,it appears that the negative ion at low pressure sheds its attendant cluster4 ,the result of ,langevin1 that the velocity of the negative ion increases more rapidly with the diminution of pressure than that of the positive ion1 shows that this process of removal of the cluster is quite appreciable at a pressure of #aj mms4 of mercury4 ,we must suppose that the process of ionization in gases consists in a removal of a negative corpuscle or electron from the molecule of the gas4 ,at atmospheric pressure this corpuscle immediately becomes the centre of an aggregation of molecules which moves with it and is the negative ion4 ,after removal of the negative ion the molecule retains a positive charge1 and probably also becomes the centre of a cluster of new molecules4 ,the terms electron and ion as used in this work may therefore be defined as follows3-- ,the electron or corpuscle is the body of smallest mass yet known to science4 ,it carries a negative charge of value #cd #aj electrostatic units4 ,its presence has only been detected when in rapid motion1 when1 for speeds up to about #aj cms4 a second1 it has an apparent mass m given by e_/m #ahf #aj electromagnetic units4 ,this apparent mass increases with the speed as the velocity of light is approached 7see section #hb74 ,the ions which are produced in gases at ordinary pressure have an apparent size1 as determined from their rates of diffusion1 large compared with the molecule of the gas in which they are produced4 ,the negative ion consists of an electron with a cluster of molecules attached to and moving with it2 the positive ion consists of a molecule from which an electron has been expelled1 with a cluster of molecules attached4 ,at low pressures under the action of an electric field the electron does not form a cluster4 ,the positive ion is always atomic in size1 even at low pressures of the gas4 ,each of the ions carries a charge of value #cd #aj electrostatic units4 #da4 ,ions produced by collision4 ,the greater part of the radiation from the radio-active bodies consists of a stream of charged particles travelling with great velocity4 ,in this radiation1 the particles1 which cause most of the ionization observed in the gas1 consist of positively charged bodies projected with a velocity about one-tenth the velocity of light4 ,the rays consist of negatively charged particles1 which are identical with the cathode rays generated in a vacuum tube1 and travel with a speed about one-half the velocity of light 7chapter ,,iv474 ,each of these projected particles1 in virtue of its great kinetic energy1 sets free a large number of ions by collision with the gas molecules in its path4 ,no definite experimental evidence has yet been obtained of the number of ions produced by a single particle1 or of the way in which the ionization varies with the speed1 but there is no doubt that each projected body gives rise to many thousand ions in its path before its energy of motion is destroyed4 ,it has already been mentioned 7section #bi7 that at low pressures ions moving under the action of an electric field are able to produce fresh ions by collision with the molecules of the gas4 ,at low pressures the negative ion is identical with the electron set free in a vacuum tube1 or emitted by a radio-active substance4 ,the mean free path of the ion is inversely proportional to the pressure of the gas4 ,consequently1 if an ion moves in an electric field1 the velocity acquired between collisions increases with diminution of the pressure4 ,townsend has shown that fresh ions are occasionally produced by collision when the negative ion moves freely between two points differing in potential by #aj volts4 ,if the difference be about ,v #bj volts1 fresh ions arise at each collision#gc4 ,now the energy ,w1 acquired by an ion of charge e moving freely between two points at a difference of potential ,v1 is given by ,w ,ve4 ,taking ,v #bj volts ,e4 ,s4 units1 and e #cd #aj1 the energy ,w required in the case of a negative ion to produce an ion by collision is given by ,w #bc #aj ergs4 ,the velocity u acquired by the ion of mass m just before a collision is given by #a --- mu ,ve1 #b and u sqrtfrac#b,vem ,now e_/m #ahf #aj electromagnetic units for the electron at slow speeds 7section #hb74 ,taking ,v #bj volts1 we find that u #bg #aj cms4 per sec4 ,this velocity is very great compared with the velocity of agitation of the molecules of the gas4 ,in a weak electric field1 the negative ions only produce ions by collision4 ,the positive ion1 whose mass is at least #ajjj times greater than the electron1 does not acquire a sufficient velocity to generate ions by collision until an electric field is applied nearly sufficient to cause a spark through the gas4 ,an estimate of the energy required for the production of an ion by ,x rays has been made by ,rutherford and ,mc,clung4 ,the energy of the rays was measured by their heating effect1 and the total number of ions produced determined4 ,on the assumption that all the energy of the rays is used up in producing ions1 it was found that ,v #age volts--a value considerably greater than that observed by ,townsend from data of ionization by collision4 ,the ionization in the two cases1 however1 is produced under very different conditions1 and it is impossible to estimate how much of the energy of the rays is dissipated in the form of heat4 #db4 ,variations are found in the saturation current through gases1 exposed to the radiations from active bodies1 when the pressure and nature of the gas and the distance between the electrodes are varied4 ,some cases which are of special importance in measurements will now be considered4 ,with unscreened active material the ionization of the gas is1 to a large extent1 due to the rays1 which are absorbed in their passage through a few centimetres of air4 ,in consequence of this rapid absorption1 the ionization decreases rapidly from the surface of the active body1 and this gives rise to conductivity phenomena different in character from those observed with ,rntgen rays1 where the ionization is in most cases uniform4 #dc4 ,variation of the current with distance between the plates4 ,it has been found experimentally#gd that the intensity of the ionization1 due to a large plane surface of active matter1 falls off approximately in an exponential law with the distance from the plate4 ,on the assumption that the rate of production of ions at any point is a measure of the intensity ,i of the radiation1 the value of ,i at that point is given by frac ii #a e- x where is a constant1 x the distance from the plate1 and ,i the intensity of the radiation at the surface of the plate4 ,while the exponential law1 in some cases1 approximately represents the variation of the ionization with distance1 in others the divergence from it is wide4 ,the ionization1 due to a plane surface of polonium1 for example1 falls off more rapidly than the exponential law indicates4 ,the rays from an active substance like radium are highly complex2 the law of variation of the ionization due to them is by no means simple and depends upon a variety of conditions4 ,the distribution of ionization is quite different according as a thick layer or a very thick film of radio-active matter is employed4 ,the question is fully considered at the end of chapter ,,iv41 but for simplicity1 the exponential law is assumed in the following calculations4 ,consider two parallel plates placed as in ,fig4 #a1 one of which is covered with a uniform layer of radio-active matter4 ,if the distance d between the plates is small compared with the dimensions of the plates1 the ionization near the centre of the plates will be sensibly uniform over any plane parallel to the plates and lying between them4 ,if q be the rate of production of ions at any distance x and q that at the surface1 then q qe-x4 ,the saturation current i per unit area is given by i intd qe' dx 1 where e is the charge on an ion1 qe' intd e- x dx fracqe' 7#a - e- d7 hence1 when d is small1 i4e4 when the ionization between the plates is nearly constant1 i qed4 ,the current is thus proportional to the distance between the plates4 ,when d is large1 the saturation current i is equal to qe_/1 and is independent of further increase in the value of d4 ,in such a case the radiation is completely absorbed in producing ions between the plates1 and frac ii #a e- d ,for example1 in the case of a thin layer of uranium oxide spread over a large plate1 the ionization is mostly produced by rays the intensity of which is reduced to half value in passing through #dc mms4 of air1 i4e4 the value of is #af4 ,the following table is an example of the variation of i with the distance between the plates4 ,distance ,saturation ,current #be mms4 #cb #e #ee #ge #gb #aj #he #abe #if #ae #ajj ,thus the increase of current for equal increments of distance between the plates decreases rapidly with the distance traversed by the radiation4 ,the distance of #ae mms4 was not sufficient to completely absorb all the radiation1 so that the current had not reached its limiting value4 ,when more than one type of radiation is present1 the saturation current between parallel plates is given by i ,a 7#a e d7 ,a#a 7#a e-#a d7 c4 where ,a1 ,a are constants1 and 1 the absorption constants of the radiations in the gas4 ,since the radiations are unequally absorbed in different gases1 the variation of current with distance depends on the nature of the gas between the plates4 #dd4 ,variation of the current with pressure4 ,the rate of production of ions by the radiations from active substances is directly proportional to the pressure of the gas4 ,the absorption of the radiation in the gas also varies directly as the pressure4 ,the latter result necessarily follows if the energy required to produce an ion is independent of the pressure4 ,in cases where the ionization is uniform between two parallel plates1 the current will vary directly as the pressure2 when however the ionization is not uniform1 on account of the absorption of the radiation in the gas1 the current does not decrease directly as the pressure until the pressure is reduced so far that the ionization is sensibly uniform4 ,consider the variation with pressure of the saturation current i between two large parallel plates1 one of which is covered with a uniform layer of active matter4 ,let absorption constant of the radiation in the gas for unit pressure4 ,for a pressure p1 the intensity ,i at any point x is given by frac ,i ,i e- p #a x ,the saturation current i is thus proportional to intd p,i dx intd p,ie-p#a x dx frac,i#a 7,i ep,a#a;d7 ,if r be the ratio of the saturation currents for the pressures p and p1 r frac #a e-p#aa d #a e-p#ba d ,the ratio is thus dependent on the distance d between the plates and the absorption of the radiation by the gas4 ,the difference in the shape of the pressure-current curves#ge is well illustrated in ,fig4 #h1 where curves are given for hydrogen1 air1 and carbonic acid for plates #ce cms4 apart4 ,illustration3 ,fig4 #h4 ,for the purpose of comparison1 the current at atmospheric pressure and temperature in each case is taken as unity4 ,the actual value of the current was greatest in carbonic acid and least in hydrogen4 ,in hydrogen1 where the absorption is small1 the current over the whole range is nearly proportional to the pressure4 ,in carbonic acid1 where the absorption is large1 the current diminishes at first slowly with the pressure1 but is nearly proportional to it below the pressure of #bce mms4 of mercury4 ,the curve for air occupies an intermediate position4 ,in cases where the distance between the plates is large1 the saturation current will remain constant with diminution of pressure until the absorption is so reduced that the radiation reaches the other plate4 ,an interesting result follows from the rapid absorption of radiation by the gas4 ,if the current is observed between two fixed parallel plates1 distant d and d respectively from a large plane surface of active matter1 the current at first increases with diminution of pressure1 passes through a maximum value1 and then diminishes4 ,in such an experimental case the lower plate through which the radiations pass is made either of open gauze or of thin metal foil to allow the radiation to pass through readily4 ,the saturation current i is obviously proportional to intd#a;d#b p,ie-p#a d 1 i4e4 to frac,i#a 7e-p#a d#a e-p#a d#b7 ,this is a function of the pressure1 and is a maximum when loge fracd#a;d#b p#a 7d#b d#a7 ,for example1 if the active matter is uranium1 p #af for the rays at atmospheric pressure4 ,if d #c1 and d #a1 the saturation current reaches a maximum when the pressure is reduced to about of an atmosphere4 ,this result has been verified experimentally4 #de4 ,conductivity of different gases when acted on by the rays4 ,for a given intensity of radiation1 the rate of production of ions in a gas varies for different gases and increases with the density of the gas4 ,strutt#gf has made a very complete examination of the relative conductivity of gases exposed to the different types of rays emitted by active substances4 ,to avoid correction for any difference of absorption of the radiation in the various gases1 the pressure of the gas was always reduced until the ionization was directly proportional to the pressure1 when1 as we have seen above1 the ionization must everywhere be uniform throughout the gas4 ,for each type of rays1 the ionization of air is taken as unity4 ,the currents through the gases were determined at different pressures1 and were reduced to a common pressure by assuming that the ionization was proportional to the pressure4 ,with unscreened active material1 the ionization is almost entirely due to rays4 ,when the active substance is covered with a layer of aluminium #ja cm4 in thickness1 the ionization is mainly due to the or cathodic rays1 and when covered with #a cm4 of lead1 the ionization is solely due to the or very penetrating rays4 ,experiments on the rays of radium were made by observing the rate of discharge of a special gold-leaf electroscope filled with the gas under examination and exposed to the action of the rays4 ,the following table gives the relative conductivities of gases exposed to various kinds of ionizing radiations4 ,gas ,relative ,rntgen ,density rays rays rays rays ,hydrogen #jjfic #jbbf #jaeg #jafi #jaad ,air #ajj #ajj #ajj #ajj #ajj ,oxygen #aaa #aaf #aba #aag #aci ,carbon dioxide #aec #aed #aeg #aec #afj ,cyanogen #ahf #aid #ahf #aga #aje ,sulphur dioxide #bai #bjd #bca #bac #gig ,chloroform #dcb #ddd #dhi #dhh #cai ,methyl iodide #eje #cea #eah #dhj #gbj ,carbon #eca #ecd #ehc #efg #dec tetrachloride ,with the exception of hydrogen1 it will be seen that the ionization of gases is approximately proportional to their density for the 1 1 rays of radium4 ,the results obtained by ,strutt for ,rntgen rays are quite different2 for example1 the relative conductivity produced by them in methyl iodide was more than #ad times as great as that due to the rays of radium4 ,the relative conductivities of gases exposed to ,x rays has been recently re-examined by ,mc,clung#gg and ,eve#gh1 who have found that the conductivity depends upon the penetrating power of the ,x rays employed4 ,the results obtained by them will be discussed later 7section #ajg74 ,this difference of conductivity in gases is due to unequal absorptions of the radiations4 ,the writer has shown#gi that the total number of ions produced by the rays for uranium1 when completely absorbed by different gases1 is not very different4 ,the following results were obtained3 ,gas ,total ,ionization ,air #ajj ,hydrogen #ie ,oxygen #ajf ,carbonic acid #if ,hydrochloric acid gas #ajb ,ammonia #aja ,the numbers1 though only approximate in character1 seem to show that the energy required to produce an ion is probably not very different for the various gases4 ,assuming that the energy required to produce an ion in different gases is about the same1 it follows that the relative conductivities are proportional to the relative absorption of the radiations4 ,a similar result has been found by ,mc,lennan for cathode rays4 ,he proved that the ionization was directly proportional to the absorption of the rays in the gas1 thus showing that the same energy is required to produce an ion in all the gases examined4 #df4 ,potential ,gradient4 ,the normal potential gradient between two charged electrodes is always disturbed when the gas is ionized in the space between them4 ,if the gas is uniformly ionized between two parallel plates1 ,child and ,zeleny have shown that there is a sudden drop of potential near the surface of both plates1 and that the electric field is sensibly uniform for the intermediate space between them4 ,the disturbance of the potential gradient depends upon the difference of potential applied1 and is different at the surface of the two plates4 ,in most measurements of radio-activity the material is spread over one plate only4 ,in such a case the ionization is to a large extent confined to the volume of the air close to the active plate4 ,the potential gradient in such a case is shown in ,fig4 #i4 ,the dotted line shows the variation of potential at any point between the plates when no ionization is produced between the plates2 curve ,a for weak ionization1 such as is produced by uranium1 curve ,b for the intense ionization produced by a very active substance4 ,in both cases the potential gradient is least near the active plate1 and greatest near the opposite plate4 ,for very intense ionization it is very small near the active surface4 ,the potential gradient varies slightly according as the active plate is charged positively or negatively4 ,illustration3 ,fig4 #i4 #dg4 ,variation of current with voltage for surface ionization4 ,some very interesting results1 giving the variation of the current with voltage1 are observed when the ionization is intense1 and confined to the space near the surface of one of two parallel plates between which the current is measured4 ,the theory of this subject has been worked out independently by ,child#hj and ,rutherford#ha4 ,let ,v be the potential difference between two parallel plates at a distance d apart4 ,suppose that the ionization is confined to a thin layer near the surface of the plate ,a 7see ,fig4 #a7 which is charged positively4 ,when the electric field is acting1 there is a distribution of positive ions between the plates ,a and ,b4 ,let n number of positive ions per unit volume at a distance x from the plate ,a1 ,k mobility of the positive ions1 e charge on an ion4 ,the current i per square centimetre through the gas is constant for all values of x1 and is given by i#a ,k#an#a;e fracd,vdx ,by ,poisson's equation frac d#b ,v da#b #dpi n#a e ,then i#a frac ,k#a #dpi frac d,v dx frac d#b ,v dx#b ,integrating 7frac d,v dx7#b frac #hpi i#ax ,k#a ,a where ,a is a constant4 ,now ,a is equal to the value of d,v ---- dx when x #j4 ,by making the ionization very intense1 the value of d,v ---- dx can be made extremely small4 ,putting ,a #j1 we see that frac d,v dx pm sqrt frac #hpi i#a x ,k#a ,this gives the potential gradient between the plates for different values of x4 ,integrating between the limits #j and d1 ,v pm frac#bc sqrt frac #hpi i#a ,k#a d rac#cb or i#a frac #i,v#b #cbpi d#c ,k#a ,if i is the value of the current when the electric field is reversed1 and ,k the velocity of the negative ion1 i#b frac #i,v#b #cb pi d#c ,k#b and i ,k ---- ---- 4 i ,k ,the current in the two directions is thus directly proportional to the velocities of the positive and negative ions4 ,the current should vary directly as the square of the potential difference applied1 and inversely as the cube of the distance between the plates4 ,the theoretical condition of surface ionization cannot be fulfilled by the ionization due to active substances1 as the ionization extends some centimetres from the active plate4 ,if1 however1 the distance between the plates is large compared with the distance over which the ionization extends1 the results will be in rough agreement with the theory4 ,using an active preparation of radium1 the writer has made some experiments on the variation of current with voltage between parallel plates distant about #aj cms4 from each other#hb4 ,the results showed 7#a7 ,that the current through the gas for small voltages increased more rapidly than the potential difference applied1 but not as rapidly as the square of that potential difference4 7#b7 ,the current through the gas depended on the direction of the electric field2 the current was always smaller when the active plate was charged positively on account of the smaller mobility of the positive ion4 ,the difference between i and i was greatest when the gas was dry1 which is the condition for the greatest difference between the velocities of the ions4 ,an interesting result follows from the above theory4 ,for given values of ,v and d1 the current cannot exceed a certain definite value1 however much the ionization may be increased4 ,in a similar way1 when an active preparation of radium is used as a source of surface ionization1 it is found that1 for a given voltage and distance between the plates1 the current does not increase beyond a certain value however much the activity of the material is increased4 #dh4 ,magnetic field produced by an ion in motion4 ,it will be shown later that the two most important kinds of rays emitted by radio-active substances consist of electrified particles1 spontaneously projected with great velocity4 ,the easily absorbed rays1 known as rays1 are positively electrified atoms of matter2 the penetrating rays1 known as rays1 carry a negative charge1 and have been found to be identical with the cathode rays produced by the electric discharge in a vacuum tube4 ,the methods adopted to determine the character of these rays are very similar to those first used by ,j4 ,j4 ,thomson to show that the cathode rays consisted of a stream of negatively electrified particles projected with great velocity4 ,the proof that the cathode rays were corpuscular in character1 and consisted of charged particles whose mass was very small compared with that of the hydrogen atom1 marked an important epoch in physical science3 for it not only opened up new and fertile fields of research1 but also profoundly modified our previous conceptions of the constitution of matter4 ,a brief account will accordingly be given of the effects produced by a moving charged body1 and also of some of the experimental methods which have been used to determine the mass and velocity of the particles of the cathode stream#hc4 ,consider an ion of radius a1 carrying a charge of electricity e1 and moving with a velocity u1 small compared with the velocity of light4 ,in consequence of the motion1 a magnetic field is set up around the charged ion1 which is carried with it4 ,the charged ion in motion constitutes a current element of magnitude eu1 and the magnetic field ,h at any point distant r from the sphere is given by eu sin ,h --------- r where is the angle the radius vector makes with the direction of motion4 ,the lines of magnetic force are circles around the axis of motion4 ,when the ion is moving with a velocity small compared with the velocity of light1 the lines of electric force are nearly radial1 but as the speed of light is approached1 they tend to leave the axis of motion and to bend towards the equator4 ,when the speed of the body is very close to that of light1 the magnetic and electric field is concentrated to a large extent in the equatorial plane4 ,the presence of a magnetic field around the moving body implies that magnetic energy is stored up in the medium surrounding it4 ,the amount of this energy can be calculated very simply for slow speeds4 ,in a magnetic field of strength ,h1 the magnetic energy stored up in unit volume of the medium of unit permeability is given by ,h ---- #h ,integrating the value of this expression over the region exterior to a sphere of radius a1 the total magnetic energy due to the motion of the charged body is given by intainfty frac,h#bhpi d7vol7 frace#b u#bhpi int#bpi intpi intainfty fracsin#b thetar#d r sin theta dphi rdtheta dr frace#b u#bd intpi intainfty frac7#a;cos#b theta7r#b sin theta dtheta 4 dr frace#b u#bc intainfty fracdrr#b frace#b u#bc;a ,the magnetic energy1 due to the motion1 is analogous to kinetic energy1 for it depends upon the square of the velocity of the body4 ,in consequence of the charge carried by the ion1 additional kinetic energy is associated with it4 ,if the velocity of the ion is changed1 electric and magnetic forces are set up tending to stop the change of motion1 and more work is done during the change than if the ion were uncharged4 ,the ordinary kinetic energy of the body is #a --- mu #b ,in consequence of its charge1 the kinetic energy associated with it is increased by eu ------ #c;a ,it thus behaves as if it possessed a mass m m where m is the electrical mass1 with the value #b;e ---- #c;a ,we have so far only considered the electrical mass of a charged ion moving with a velocity small compared with that of light4 ,as the speed of light is approached1 the magnetic energy can no longer be expressed by the equation already given4 ,the general values of the electrical mass of a charged body for speed were first worked out by ,j4 ,j4 ,thomson#hd in #ahhg4 ,a more complete examination was made in #ahhi by ,heaviside#he1 while ,searle#hf worked out the case for a charged ellipsoid4 ,recently1 the question was again attacked by ,abraham#hg4 ,slightly different expressions for the variation of electrical mass with speed have been obtained1 depending upon the conditions assumed for the distribution of the electricity on the sphere4 ,the expression found by ,abraham1 which has been utilized by ,kaufmann to show that the mass of the electron is electromagnetic in origin1 is given later in section #hb4 ,all the calculations agree in showing that the electrical mass is practically constant for slow speeds1 but increases as the speed of light is approached1 and is theoretically infinite when the speed of light is reached4 ,the nearer the velocity of light is approached1 the greater is the resisting force to a change of motion4 ,an infinite force would be required to make an electron actually attain the velocity of light1 so that1 according to the present theory1 it would be impossible for an electron to move faster than light1 i4e4 faster than an electromagnetic disturbance travels in the ether4 ,the importance of these deductions lies in the fact that an electric charge in motion1 quite independently of any material nucleus1 possesses an apparent mass in virtue of its motion1 and that this mass is a function of the speed4 ,indeed1 we shall see later 7see section #hb7 that the apparent mass of the particles constituting the cathode stream can be explained in virtue of their charge1 without the necessity of assuming a material body in which the charge is distributed4 ,this has led to the suggestion that all mass may be electrical in origin1 and due purely to electricity in motion4 #di4 ,action of a magnetic field on a moving ion4 ,let us consider the case of an ion of mass m carrying a charge e and moving freely with a velocity u4 ,if u is small compared with the velocity of light1 the ion in motion corresponds to a current element of magnitude eu4 ,if the ion moves in an external magnetic field of strength ,h1 it is acted on by a force at right angles both to the direction of motion1 and to that of the magnetic force and equal in magnitude to ,heu sin 1 where is the angle between the direction of the magnetic force and the direction of motion4 ,since the force due to the magnetic field is always perpendicular to the direction of motion1 it has no effect upon the velocity of the particle1 but can only alter the direction of its path4 ,if is the radius of curvature of the path of the ion1 the force along the normal is equal to mu -----1 and this is balanced by the force ,heu sin 4 ,if ---1 #b i4e4 if the ion is moving at right angles to the direction of the magnetic field mu ,heu ----- or m ,h ---- u e ,since u is constant1 is also constant1 i4e4 the particle describes a circular orbit of radius 4 ,the radius of the circular orbit is thus directly proportional to u1 and inversely proportional to ,h4 ,if the ion is moving at an angle with the direction of the magnetic field1 it describes a curve which is compounded of a motion of a particle of velocity u sin perpendicular to the field and u cos in the direction of the field4 ,the former describes a circular orbit of radius 1 given by m ,h --- u sin 2 e the latter is unaffected by the magnetic field and moves uniformly in the direction of the magnetic field with a velocity u cos 4 ,the motion of the particle is in consequence a helix1 traced on a cylinder of radius mu sin ----------1 e,h whose axis is in the direction of the magnetic field4 ,thus an ion projected obliquely to the direction of a uniform magnetic field always moves in a helix whose axis is parallel to the lines of magnetic force#hh4 #ej4 ,determination of e_/m for the cathode stream4 ,the cathode rays1 first observed by ,varley1 were investigated in detail by ,crookes4 ,these rays are projected from the cathode in a vacuum tube at low pressure4 ,they travel in straight lines1 and are readily deflected by a magnet1 and produce strong luminosity in a variety of substances placed in their path4 ,the rays are deflected by a magnetic field in the same direction as would be expected for a negatively charged particle projected from the cathode4 ,in order to explain the peculiar properties of these rays ,crookes supposed that they consisted of negatively electrified particles1 moving with great velocity and constituting1 as he appropriately termed it1 8a new or fourth state of matter40 ,the nature of these rays was for twenty years a subject of much controversy1 for while some upheld their material character1 others considered that they were a special form of wave motion in the ether4 ,perrin and ,j4 ,j4 ,thomson showed that the rays always carried with them a negative charge1 while ,lenard made the important discovery that the rays passed through thin metal foil and other substances opaque to ordinary light4 ,using this property1 he sent the rays through a thin window and examined the properties of the rays outside the vacuum tube in which they were produced4 ,the absorption of the rays by matter was shown to be nearly proportional to the density over a very wide range1 and to be independent of its chemical constitution4 ,the nature of these rays was successfully demonstrated by ,j4 ,j4 ,thomson#hi in #ahig4 ,if the rays consisted of negatively electrified particles1 they should be deflected in their passage through an electric as well as through a magnetic field4 ,such an experiment had been tried by ,hertz1 but with negative results4 ,j4 ,j4 ,thomson1 however1 found that the rays were deflected by an electric field in the direction to be expected for a negatively charged particle1 and showed that the failure of ,hertz to detect the same was due to the masking of the electric field by the strong ionization produced in the gas by the cathode stream4 ,this effect was got rid of by reducing the pressure of the gas in the tube4 ,the experimental arrangement used for the electric deflection of the rays is shown in ,fig4 #aj4 ,the cathode rays are generated at the cathode ,c1 and a narrow pencil of rays is obtained by passing the rays through a perforated disc ,,ab4 ,the rays then passed midway between two parallel insulated plates ,d and ,e1 d centimetres apart1 and maintained at a constant difference of potential ,v4 ,the point of incidence of the pencil of rays was marked by a luminous patch produced on a phosphorescent screen placed at ,,pp4 ,the particle carrying a negative charge e in passing between the charged plates1 is acted on by a force ,xe directed towards the positive plate1 where ,x1 the strength of the electric field1 is given by ,v ---- 4 d ,illustration3 ,fig4 #aj4 ,the application of the electric field thus causes the luminous patch to move in the direction of the positive plate4 ,if now a uniform magnetic field is applied at the plates ,d and ,e1 perpendicular to the pencil of rays1 and parallel to the plane of the plates1 and in such a direction that the electric and magnetic forces are opposed to one another1 the patch of light can be brought back to its undisturbed position by adjusting the strength of the magnetic field4 ,if ,h is the strength of the magnetic field1 the force on the particle due to the magnetic field is ,heu1 and when a balance is obtained ,heu ,xe1 or ,x u --- 7#a74 ,h ,now if the magnetic field ,h is acting alone1 the curvature of the path of the rays between the plates can be deduced from the deflection of the luminous patch4 ,but we have seen that mu ,h ----- 7#b74 e ,from equations 7#a7 and 7#b71 the value of u and e_/m for the particle can be determined4 ,the velocity u is not constant1 but depends upon the potential difference between the electrodes1 and this in turn depends upon the pressure and nature of the residual gas in the tube4 ,by altering these factors1 the cathode particles may be made to acquire velocities varying between about #aj and #aj cms4 per second4 ,this velocity is enormous compared with that which can be impressed ordinarily upon matter by mechanical means4 ,on the other hand1 the value of e_/m for the particles is sensibly constant for different velocities4 ,as a result of a series of experiments the mean value e_/m #gg #aj was obtained4 ,the value of e_/m is independent of the nature or pressure of the gas in the vacuum tube and independent of the metal used as cathode4 ,a similar value of e_/m was obtained by ,lenard#ij and others4 ,kaufmann#ia and ,simon#ib used a different method to determine the value of e_/m4 ,the potential difference ,v between the terminals of the tube was measured4 ,the work done on the charged particle in moving from one end of the tube to the other is ,ve1 and this must be equal to the kinetic energy #a -- mu #b acquired by the moving particle4 ,thus e u --- ---- 7#c74 m #b,v ,by combination of this equation with 7#b7 obtained by measurement of the magnetic deflexion1 both u and e_/m can be determined4 ,simon found by this method that e --- #ahfe #aj4 m ,it will be seen later 7section #hb7 that a similar value was deduced by ,kaufmann for the electrons projected from radium4 ,these results1 which have been based on the effect of a magnetic and electric field on a moving ion1 were confirmed by ,weichert1 who determined by a direct method the time required for the particle to traverse a known distance4 ,the particles which make up the cathode stream were termed 8corpuscles0 by ,j4 ,j4 ,thomson4 ,the name 8electron10 first employed by ,johnstone ,stoney1 has also been applied to them and has come into general use#ic4 ,the methods above described do not give the mass of the electron1 but only the ratio of the charge to the mass4 ,a direct comparison can1 however1 be made between the ratio e_/m for the electron and the corresponding value for the hydrogen atoms set free in the electrolysis of water4 ,each of the hydrogen atoms is supposed to carry a charge e1 and it is known that #if1jjj coulombs of electricity1 or1 in round numbers1 #aj electromagnetic units of quantity are required to liberate one gram of hydrogen4 ,if ,n is the number of atoms in one gram of hydrogen1 then ,ne #aj4 ,but if m is the mass of a hydrogen atom1 then ,nm #a4 ,dividing one by the other e_/m #aj4 ,we have seen already that a gaseous ion carries the same charge as a hydrogen atom1 while indirect evidence shows that the electron carries the same charge as an ion1 and consequently the same charge as the atom of hydrogen4 ,hence we may conclude that the apparent mass of the electron is only about of the mass of the hydrogen atom4 ,the electron thus behaves as the smallest body known to science4 ,in later experiments ,j4 ,j4 ,thomson showed that the negative ions set free at low pressures by an incandescent carbon filament1 and also the negative ions liberated from a zinc plate exposed to the action of ultra-violet light1 had the same value for e_/m as the electrons produced in a vacuum tube4 ,it thus seemed probable that the electron was a constituent of all matter4 ,this view received strong support from measurements of quite a different character4 ,zeeman in #ahig found that the lines of the spectrum from a source of light exposed in a strong magnetic field were displaced and doubled4 ,later work has shown that the lines in some cases are trebled1 in others sextupled1 while1 in a few cases1 the multiplication is still greater4 ,these results received a general explanation on the radiation theories previously advanced by ,lorenz and ,larmor4 ,the radiation1 emitted from any source1 was supposed to result from the orbital or oscillatory motion of the charged parts constituting the atom4 ,since a moving ion is acted on by an external magnetic field1 the motion of the charged ions is disturbed when the source of light is exposed between the poles of a strong magnet4 ,there results a small change in the period of the emitted light1 and a bright line in the spectrum is1 in consequence1 displaced by the action of the magnetic field4 ,according to theory1 the small change in the wave-length of the emitted light depends upon the strength of the magnetic field and on the ratio e_/m of the charge carried by the ion to its mass4 ,by comparison of the theory with the experimental results1 it was deduced that the moving ion carried a negative charge1 and that the value of e_/m was about #aj4 ,the charged ion1 responsible for the radiation from a luminous body1 is thus identical with the electron set free in a vacuum tube4 ,it thus seems reasonable to suppose that the atoms of all bodies are complex and are built up1 in part at least1 of electrons1 whose apparent mass is very small compared with that of the hydrogen atom4 ,the properties of such disembodied charges has been examined mathematically among others by ,larmor1 who sees in this conception the ultimate basis of a theory of matter4 ,j4 ,j4 ,thomson and ,lord ,kelvin have investigated mathematically certain arrangements of a number of electrons which are stable for small disturbances4 ,this question will be discussed more in detail in section #bgj4 #ea4 ,canal rays4 ,if a discharge is passed through a vacuum tube provided with a perforated cathode1 within certain limits of pressure1 luminous streams are observed to pass through the holes and to emerge on the side of the cathode remote from the anode4 ,these rays were first observed by ,goldstein#id and were called by him the 8,canal-strahlen40 ,these rays travel in straight lines and produce phosphorescence in various substances4 ,wien#ie showed that the canal rays were deflected by strong magnetic and electric fields1 but the amount of deflection was very small compared with that of the cathode rays under similar conditions4 ,the deflection was found to be opposite in direction to the cathode rays1 and this indicates that the canal rays consist of positive ions4 ,wien determined their velocity and the ratio e_/m1 by measuring the amount of their magnetic and electric deflection4 ,the value of e_/m was found to be variable1 depending upon the gas in the tube1 but the maximum value observed was #aj4 ,this shows that the positive ion1 in no case1 has a mass less than that of the hydrogen atom4 ,it seems probable that the canal rays consist of positive ions1 derived either from the gas or the electrodes1 which travel towards the cathode1 and have sufficient velocity to pass through the holes of the cathode and to appear in the gas beyond4 ,it is remarkable that1 so far1 no case has been observed where the carrier of a positive charge has an apparent mass less than that of the hydrogen atom4 ,positive electricity always appears to be associated with bodies atomic in size4 ,we have seen that the process of ionization in gases is supposed to consist of the expulsion of an electron from the atom4 ,the corresponding positive charge remains behind on the atom and travels with it4 ,this difference between positive and negative electricity appears to be fundamental1 and no explanation of it has1 as yet1 been forthcoming4 #eb4 ,radiation of energy4 ,if an electron moves uniformly in a straight line with constant velocity1 the magnetic field1 which travels with it1 remains constant1 and there is no loss of energy from it by radiation4 ,if1 however1 its motion is hastened or retarded1 the magnetic field is altered1 and there results a loss of energy from the electron in the form of electromagnetic radiation4 ,the rate of loss of energy from an accelerated electron was first calculated by ,larmor#if and shown to be #b;e ---- 7acceleration71 #c,v where e is the charge on the electron in electromagnetic units1 and ,v the velocity of light4 ,any alteration in the velocity of a moving charge is thus always accompanied by a radiation of energy from it4 ,since the electron1 set free in a vacuum tube1 increases in velocity in passing through the electric field1 energy must be radiated from it during its passage from cathode to anode4 ,it can1 however1 readily be calculated that1 in ordinary cases1 this loss of energy is small compared with the kinetic energy acquired by the electron in passing through the electric field4 ,an electron moving in a circular orbit is a powerful radiator of energy1 since it is constantly accelerated towards the centre4 ,an electron moving in an orbit of radius equal to the radius of an atom 7about #aj cms47 would lose most of its kinetic energy of motion in a small fraction of a second1 even though its velocity was originally nearly equal to the velocity of light4 ,if1 however1 a number of electrons are arranged at equal angular intervals on the circumference of a circle and move with constant velocity round the ring1 the radiation of energy is much less than for a single electron1 and rapidly diminishes with an increase in the number of electrons round the ring4 ,this result1 obtained by ,j4 ,j4 ,thomson1 will be discussed in more detail later when the stability of systems composed of rotating electrons is under consideration4 ,since the radiation of energy is proportional to the square of the acceleration1 the proportion of the total energy radiated depends upon the suddenness with which an electron is started or stopped4 ,now some of the cathode ray particles are stopped abruptly when they impinge on the metal cathode1 and1 in consequence1 give up a fraction of their kinetic energy in the form of electromagnetic radiation4 ,stokes and ,weichert suggested that this radiation constituted the ,x rays1 which are known to have their origin at the surface on which the cathode rays impinge4 ,the mathematical theory has been worked out by ,j4 ,j4 ,thomson#ig4 ,if the motion of an electron is suddenly arrested1 a thin spherical pulse in which the magnetic and electric forces are very intense travels out from the point of impact with the velocity of light4 ,the more suddenly the electron is stopped1 the thinner and more intense is the pulse4 ,on this view the ,x rays are not corpuscular like the cathode rays1 which produce them1 but consist of transverse disturbances in the ether1 akin in some respects to light waves of short wave-length4 ,the rays are thus made up of a number of pulses1 which are non-periodic in character1 and which follow one another at irregular intervals4 ,on this theory of the nature of the ,x rays1 the absence of direct deflection1 refraction1 or polarization is to be expected1 if the thickness of the pulse is small compared with the diameter of an atom4 ,it also explains the non-deflection of the path of the rays by a magnetic or electric field4 ,the intensity of the electric and magnetic force in the pulse is so great that it is able to cause a removal of an electron from some of the atoms of the gas1 over which the pulse passes1 and thus causes the ionization observed4 ,the cathode rays produce ,x rays1 and these in turn give rise to a secondary radiation whenever they impinge on a solid body4 ,this secondary radiation is emitted equally in all directions1 and consists partly of a radiation of the ,x ray type and also of electrons projected with considerable velocity4 ,this secondary radiation gives rise to a tertiary radiation and so on4 ,barkla#ih has shown that the secondary radiation emitted from a gas through which the rays pass consists in part of scattered ,x rays of about the same penetrating power as the primary rays as well as some easily absorbed rays4 ,part of the cathode rays is diffusely reflected on striking the cathode4 ,these scattered rays consist in part of electrons of the same speed as in the primary beam1 but also include some others of much less velocity4 ,the amount of diffuse reflection depends upon the nature of the cathode and the angle of incidence of the rays4 ,we shall see later 7chapter ,,iv47 that similar effects are produced when the rays from radio-active substances impinge upon solid bodies4 99 99 99 99 99 ,in this chapter an account of the ionization theory of gases has been given to the extent that is necessary for the interpretation of the measurements of radio-activity by the electric method4 ,it would be out of place here to discuss the development of that theory in detail1 to explain the passage of electricity through flames and vapours1 the discharge of electricity from hot bodies1 and the very complicated phenomena observed in the passage of electricity through a vacuum tube4 ,for further information on this important subject1 the reader is referred to ,j4 ,j4 ,thomson's ,conduction of ,electricity through ,gases1 in which the whole subject is treated in a full and complete manner4 ,a simple account of the effect of moving charges and the electronic theory of matter was given by the same author in the ,silliman ,lectures of ,yale ,university and published under the title ,electricity and ,matter 7,scribner1 ,new ,york1 #aijd74 ,footnote #dd3 ,j4 ,j4 ,thomson and ,rutherford1 ,phil4 ,mag4 ,nov4 #ahif4 ,footnote #de3 ,the word ion has now been generally adopted in the literature of the subject4 ,in using this word1 it is not assumed that the ions in gases are the same as the corresponding ions in the electrolysis of solutions4 ,footnote #df3 ,a minute current is observed between the plates even if no radio-active matter be present4 ,this has been found to be due mainly to a slight natural radio-activity of the matter composing them4 7,see chapter ,,xiv47 ,footnote #dg3 ,this nomenclature has arisen from the similarity of the shape of the current-voltage curves to the magnetization curves for iron4 ,since1 on the ionization theory1 the maximum current is a result of the removal of all the ions from the gas1 before recombination occurs1 the terms are not very suitable4 ,they have however now come into general use and will be retained throughout this work4 ,footnote #dh3 ,j4 ,j4 ,thomson1 ,phil4 ,mag4 #dg1 p4 #bec1 #ahii2 ,conduction of ,electricity through ,gases1 p4 #gc1 #aijc4 ,footnote #di3 ,rutherford1 ,phil4 ,mag4 ,jan4 #ahii4 ,footnote #ej3 ,townsend1 ,phil4 ,mag4 ,feb4 #aija4 ,footnote #ea3 ,rutherford1 ,phil4 ,mag4 ,nov4 #ahig1 p4 #add1 ,jan4 #ahii4 ,footnote #eb3 ,townsend1 ,phil4 ,trans4 ,a1 p4 #aeg1 #ahii4 ,footnote #ec3 ,mc,clung1 ,phil4 ,mag4 ,march1 #aijb4 ,footnote #ed3 ,langevin1 ,thse prsente la ,facult des ,sciences1 p4 #aea1 ,paris1 #aijb4 ,footnote #ee3 ,owens1 ,phil4 ,mag4 ,oct4 #ahii4 ,footnote #ef3 ,rutherford1 ,phil4 ,mag4 p4 #dbi1 ,nov4 #ahig4 ,footnote #eg3 ,zeleny1 ,phil4 ,trans4 ,a1 p4 #aic1 #aija4 ,footnote #eh3 ,langevin1 ,c4 ,r4 #acd1 p4 #fdf1 #aijb4 ,footnote #ei3 ,zeleny1 ,phil4 ,mag4 ,july1 #ahih4 ,footnote #fj3 ,rutherford1 ,phil4 ,mag4 ,feb4 #ahii4 ,footnote #fa3 ,zeleny1 ,phil4 ,trans4 #aie1 p4 #aic1 #aijj4 ,footnote #fb3 ,langevin1 ,c4 ,r4 #acd1 p4 #fdf1 #aijb1 and ,thesis1 p4 #aia1 #aijb4 ,footnote #fc3 ,rutherford1 ,proc4 ,camb4 ,phil4 ,soc4 #i1 p4 #daj1 #ahih4 ,footnote #fd3 ,langevin1 ,thesis1 p4 #aij1 #aijb4 ,footnote #fe3 ,helmholtz and ,richarz1 ,annal4 d4 ,phys4 #dj1 p4 #afa1 #ahij4 ,footnote #ff3 ,wilson1 ,phil4 ,trans4 p4 #bfe1 #ahig2 p4 #djc1 #ahii2 p4 #bhi1 #aijj4 ,footnote #fg3 ,thomson1 ,phil4 ,mag4 p4 #ebh1 ,dec4 #ahih4 ,footnote #fh3 ,wilson1 ,phil4 ,trans4 ,a1 #aic1 p4 #bhi1 #ahii4 ,footnote #fi3 ,thomson1 ,phil4 ,mag4 p4 #ebh1 ,dec4 #ahih1 and ,march1 #aijc4 ,conduction of ,electricity through ,gases1 ,camb4 ,univ4 ,press1 #aijc1 p4 #aba4 ,footnote #gj3 ,wilson1 ,phil4 ,mag4 ,april1 #aijc4 ,footnote #ga3 ,townsend1 ,phil4 ,trans4 ,a1 p4 #abi1 #ahii4 ,footnote #gb3 ,townsend1 loc4 cit4 p4 #aci4 ,footnote #gc3 ,some difference of opinion has been expressed as to the value of ,v required to produce ions at each collision4 ,townsend considers it to be about #bj volts2 ,langevin #fj volts and ,stark about #ej volts4 ,footnote #gd3 ,rutherford1 ,phil4 ,mag4 ,jan4 #ahii4 ,footnote #ge3 ,rutherford1 ,phil4 ,mag4 ,jan4 #ahii4 ,footnote #gf3 ,strutt1 ,phil4 ,trans4 ,a1 p4 #ejg1 #aija and ,proc4 ,roy4 ,soc4 p4 #bjh1 #aijc4 ,footnote #gg3 ,mc,clung1 ,phil4 ,mag4 ,sept4 #aijd4 ,footnote #gh3 ,eve1 ,phil4 ,mag4 ,dec4 #aijd4 ,footnote #gi3 ,rutherford1 ,phil4 ,mag4 p4 #acg1 ,jan4 #ahii4 ,footnote #hj3 ,child1 ,phys4 ,rev4 ,vol4 #ab1 #aija4 ,footnote #ha3 ,rutherford1 ,phil4 ,mag4 p4 #baj1 ,august1 #aija2 ,phys4 ,rev4 ,vol4 #ac1 #aija4 ,footnote #hb3 ,rutherford1 ,phil4 ,mag4 ,aug4 #aija4 ,footnote #hc3 ,a simple and excellent account of the effects produced by the motion of a charged ion and also of the electronic theory of matter was given by ,sir ,oliver ,lodge in #aijc in a paper entitled 8,electrons0 7,proceedings of the ,institution of ,electrical ,engineers1 ,part #aei1 ,vol4 #cb1 #aijc74 ,see also ,j4 ,j4 ,thomson's ,electricity and ,matter 7,scribner1 ,new ,york1 #aijd74 ,footnote #hd3 ,j4 ,j4 ,thomson1 ,phil4 ,mag4 ,april1 #ahhg4 ,footnote #he3 ,heaviside1 ,collected ,papers1 ,vol4 ,,ii4 p4 #ead4 ,footnote #hf3 ,searle1 ,phil4 ,mag4 ,oct4 #ahig4 ,footnote #hg3 ,abraham1 ,phys4 ,zeit4 #d1 ,no4 #a b1 p4 #eg1 #aijb4 ,footnote #hh3 ,a full account of the path described by a moving ion under various conditions is given by ,j4 ,j4 ,thomson1 ,conduction of ,electricity in ,gases 7,camb4 ,univ4 ,press1 #aijc71 pp4 #gi-ij4 ,footnote #hi3 ,j4 ,j4 ,thomson1 ,phil4 ,mag4 p4 #bic1 #ahig4 ,footnote #ij3 ,lenard1 ,annal4 d4 ,phys4 #fd1 p4 #bgi1 #ahih4 ,footnote #ia3 ,kaufmann1 ,annal4 d4 ,phys4 #fa1 p4 #edd2 #fb1 p4 #eif1 #ahig2 #fe1 p4 #dca1 #ahih4 ,footnote #ib3 ,simon1 ,annal4 d4 ,phys4 #fi1 p4 #ehi1 #ahii4 ,footnote #ic3 ,a complete discussion of the various methods employed to measure the velocity and mass of electrons and also of the theory on which they are based will be found in ,j4 ,j4 ,thomson's ,conduction of ,electricity through ,gases4 ,footnote #id3 ,goldstein1 ,berlin ,sitzber4 #ci1 p4 #fia1 #ahif2 ,annal4 d4 ,phys4 #fd1 p4 #de1 #ahih4 ,footnote #ie3 ,wien1 ,annal4 d4 ,phys4 #fe1 p4 #ddj1 #ahih4 ,footnote #if3 ,larmor1 ,phil4 ,mag4 #dd1 p4 #eic1 #ahig4 ,footnote #ig3 ,j4 ,j4 ,thomson1 ,phil4 ,mag4 ,feb4 #ahig4 ,footnote #ih3 ,barkla1 ,phil4 ,mag4 ,june1 #aijc4 ,,chapter ,,iii4 ,,methods ,,of ,,measurement4 #ec4 ,methods of ,measurement4 ,three general methods have been employed for examination of the radiations from radio-active bodies1 depending on 7#a7 ,the action of the rays on a photographic plate4 7#b7 ,the ionizing action of the rays on the surrounding gas4 7#c7 ,the fluorescence produced by the rays on a screen of platinocyanide of barium1 zinc sulphide1 or similar substance4 ,the third method is very restricted in its application1 and can only be employed for intensely active substances like radium or polonium4 ,the photographic method has been used very widely1 especially in the earlier development of the subject1 but has gradually been displaced by the electrical method1 as a quantitative determination of the radiations became more and more necessary4 ,in certain directions1 however1 it possesses distinct advantages over the electrical method4 ,for example1 it has proved a very valuable means of investigating the curvature of the path of the rays1 when deflected by a magnetic or electric field1 and has allowed us to determine the constants of these rays with considerable accuracy4 ,on the other hand1 as a general method of study of the radiations1 it is open to many objections4 ,a day's exposure is generally required to produce an appreciable darkening of the sensitive film when exposed to a weak source of radiation like uranium and thorium4 ,it cannot1 in consequence1 be employed to investigate the radiations of those active products which rapidly lose their activity4 ,moreover1 ,w4 ,j4 ,russell has shown that the darkening of a photographic plate can be produced by many agents which do not give out rays like those of the radio-active bodies4 ,this darkening of the plate is produced under the most varied conditions1 and very special precautions are necessary when long exposures to a weak source of radiation are required4 ,the main objection to the photographic method1 however1 lies in the fact that the radiations which produce the strongest electrical effect are very weak photographically4 ,for example1 ,soddy#ii has shown that the photographic action of uranium is due almost entirely to the more penetrating rays1 and that the easily absorbed rays produce in comparison very little effect4 ,speaking generally1 the penetrating rays are the most active photographically1 and1 under ordinary conditions1 the action on the plate is almost entirely due to them4 ,most of the energy radiated from active bodies is in the form of easily absorbed rays which are comparatively inactive photographically4 ,these rays are difficult to study by the photographic method1 as the layer of black paper which1 in many cases1 is required in order to absorb the phosphorescent light from active substances1 cuts off at the same time most of the rays under examination4 ,these easily absorbed rays will be shown to play a far more important part in the processes occurring in radio-active bodies than the penetrating rays which are more active photographically4 ,the electrical method1 on the other hand1 offers a rapid and accurate method of quantitatively examining the radiations4 ,it can be used as a means of measurement of all the types of radiation emitted1 excluding light waves1 and is capable of accurate measurement over an extremely wide range4 ,with proper precautions it can be used to measure effects produced by radiations of extremely small intensity4 #ed4 ,electrical ,methods4 ,the electrical methods employed in studying radio-activity are all based on the property of the radiation in question of ionizing the gas1 i4e4 of producing positively and negatively charged carriers throughout the volume of the gas4 ,the discussion of the application of the ionization theory of gases to measurements of radio-activity has been given in the last chapter4 ,it has been shown there that the essential condition to be fulfilled for comparative measurements of the intensity of the radiations is that the electrical field shall in all cases be strong enough to obtain the maximum or saturation current through the gas4 ,the electric field required to produce practical saturation varies with the intensity of the ionization and consequently with the activity of the preparations to be examined4 ,for preparations which have an activity not more than #ejj times that of uranium1 under ordinary conditions1 a field of #ajj volts per cm4 is sufficient to produce a practical saturation current4 ,for very active samples of radium1 it is often impossible to obtain conveniently a high enough electromotive force to give even approximate saturation4 ,under such conditions comparative measurement can be made by measuring the current under diminished pressure of the gas1 when saturation is more readily obtained4 ,the method to be employed in the measurement of this ionization current depends largely on the intensity of the current to be measured4 ,if some very active radium is spread on the lower of two insulated plates as in ,fig4 #a1 and a saturating electric field applied1 the current may readily be measured by a sensitive galvanometer of high resistance4 ,for example1 a weight of #de gr4 of radium chloride of activity #ajjj times that of uranium oxide1 spread over a plate of area #cc sq4 cms41 gave a maximum current of #aa #aj amperes when the plates were #de cms4 apart4 ,in this case the difference of potential to be applied to produce practical saturation was about #fjj volts4 ,since most of the ionization is due to rays which are absorbed in passing through a few centimetres of air1 the current is not much increased by widening the distance between the two plates4 ,in cases where the current is not quite large enough for direct deflection1 the current may be determined by connecting the upper insulated plate with a well insulated condenser4 ,after charging for a definite time1 say one or more minutes1 the condenser is discharged through the galvanometer1 and the current can readily be deduced4 #ee4 ,in most cases1 however1 when dealing with less active substances like uranium or thorium1 or with small amounts of active material1 it is necessary to employ methods for measuring much smaller currents than can be detected conveniently by an ordinary galvanometer4 ,the most convenient apparatus to employ for this purpose is one of the numerous types of quadrant electrometer or an electroscope of special design4 ,for many observations1 especially where the activity of the two substances is to be compared under constant conditions1 an electroscope offers a very certain and easy method of measurement4 ,as an example of a simple apparatus of this kind1 a brief description will be given of the electroscope used by ,m4 and ,mme ,curie in many of their earlier observations4 ,illustration3 ,fig4 #aa4 ,the connections are clearly seen from ,fig4 #aa4 ,the active material is placed on a plate laid on top of the fixed circular plate ,p1 connected with the case of the instrument and with earth4 ,the upper insulated plate ,p is connected with the insulated gold-leaf system ,,ll4 ,s is an insulating support and ,l the gold-leaf4 ,the system is first charged to a suitable potential by means of the rod ,c4 ,the rate of movement of the gold-leaf is observed by means of a microscope4 ,in comparisons of the activity of two specimens1 the time taken by the gold leaf to pass over a certain number of divisions of the micrometer scale in the eye-piece is observed4 ,since the capacity of the charged system is constant1 the average rate of movement of the gold-leaf is directly proportional to the ionization current between ,p and ,p1 i4e4 to the intensity of the radiation emitted by the active substance4 ,unless very active material is being examined1 the difference of potential between ,p and ,p can easily be made sufficient to produce saturation4 ,when necessary1 a correction can be made for the rate of leak when no active material is present4 ,in order to avoid external disturbances1 the plates ,,pp and the rod ,c are surrounded by metal cylinders1 ,e and ,f1 connected with earth4 #ef4 ,a modified form of the gold-leaf electroscope can be used to determine extraordinarily minute currents with accuracy1 and can be employed in cases where a sensitive electrometer is unable to detect the current4 ,a special type of electroscope has been used by ,elster and ,geitel1 in their experiments on the natural ionization of the atmosphere4 ,a very convenient type of electroscope to measure the current due to minute ionization of the gas is shown in ,fig4 #ab4 ,illustration3 ,fig4 #ab4 ,this type of instrument was first used by ,c4 ,t4 ,r4 ,wilson#ajj in his experiments of the natural ionization of air in closed vessels4 ,a brass cylindrical vessel is taken of about #a litre capacity4 ,the gold-leaf system1 consisting of a narrow strip of gold-leaf ,l attached to a flat rod ,r1 is insulated inside the vessel by the small sulphur bead or piece of amber ,s1 supported from the rod ,p4 ,in a dry atmosphere a clean sulphur bead or piece of amber is almost a perfect insulator4 ,the system is charged by a light bent rod ,,cc passing through an ebonite cork#aja4 ,the rod ,c is connected to one terminal of a battery of small accumulators of #bjj to #cjj volts4 ,if these are absent1 the system can be charged by means of a rod of sealing-wax4 ,the charging rod ,,cc is then removed from contact with the gold-leaf system4 ,the rods ,p and ,c and the cylinder are then connected with earth4 ,the rate of movement of the gold-leaf is observed by a reading microscope through two holes in the cylinder1 covered with thin mica4 ,in cases where the natural ionization due to the enclosed air in the cylinder is to be measured accurately1 it is advisable to enclose the supporting and charging rod and sulphur bead inside a small metal cylinder ,m connected to earth1 so that only the charged gold-leaf system is exposed in the main volume of the air4 ,in an apparatus of this kind the small leakage over the sulphur bead can be eliminated almost completely by keeping the rod ,p charged to the average potential of the gold-leaf system during the observation4 ,this method has been used with great success by ,c4 ,t4 ,r4 ,wilson 7loc4 cit474 ,such refinements1 however1 are generally unnecessary1 except in investigations of the natural ionization of gases at low pressures1 when the conduction leak over the sulphur bead is comparable with the discharge due to the ionized gas4 #eg4 ,the electric capacity ,c of a gold-leaf system about #d cms4 long is usually about #a electrostatic unit4 ,if ,v is the decrease of potential of the gold-leaf system in t seconds1 the current i through the gas is given by ,,cv i ---- t4 ,with a well cleaned brass electroscope of volume #a litre1 the fall of potential due to the natural ionization of the air was found to be about #f volts per hour4 ,since the capacity of the gold-leaf system was about #a electrostatic unit #f i #a ---------- #ef #aj ,e4,s4 units #ai #aj amperes4 #cfjj #cjj ,with special precautions a rate of discharge of or even of this amount can be measured accurately4 ,the number of ions produced in the gas can be calculated if the charge on an ion is known4 ,j4 ,j4 ,thomson has shown that the charge e on an ion is equal to #cd #aj electrostatic units or #aac #aj coulombs4 ,let q number of ions produced per second per cubic centimetre throughout the volume of the electroscope1 ,s volume of electroscope in cubic centimetres4 ,if the ionization be uniform1 the saturation current i is given by i q,se4 ,now for an electroscope with a volume of #ajjj c4c41 i was equal to about #ai #aj amperes4 ,substituting the values given above q #ag ions per cubic centimetre per second4 ,with suitable precautions an electroscope can thus readily measure an ionization current corresponding to the production of #a ion per cubic centimetre per second4 ,the great advantage of an apparatus of this kind lies in the fact that the current measured is due to the ionization inside the vessel and is not influenced by the ionization of the external air or by electrostatic disturbances#ajb4 ,such an apparatus is very convenient for investigating the very penetrating radiations from the radio-elements1 since these rays pass readily through the walls of the electroscope4 ,when the electroscope is placed on a lead plate #c or #d mms4 thick1 the ionization in the electroscope1 due to a radio-active body placed under the lead1 is due entirely to the very penetrating rays1 since the other two types of rays are completely absorbed in the lead plate4 ,if a circular opening is cut in the base of the electroscope and covered with thin aluminium of sufficient thickness to absorb the rays1 measurements of the intensity of the rays from an active substance placed under it1 can be made with ease and certainty4 #eh4 ,a modified form of electroscope1 which promises to be of great utility for measuring currents even more minute than those to be observed with the type of instrument already described1 has recently been devised by ,c4 ,t4 ,r4 ,wilson#ajc4 ,the construction of the apparatus is shown in ,fig4 #ac4 ,the case consists of a rectangular brass box #d cms4 #d cms4 #c cms4 ,a narrow gold-leaf ,l is attached to a rod ,r passing through a clean sulphur cork4 ,opposite the gold-leaf is fixed an insulated brass plate ,p1 placed about #a mm4 from the wall of the box4 ,the movement of the gold-leaf is observed through two small windows by means of a microscope provided with a micrometer scale4 ,the plate ,p is maintained at a constant potential 7generally about #bjj volts74 ,the electrometer case is placed in an inclined position as shown in the figure1 the angle of inclination and the potential of the plate being adjusted to give the desired sensitiveness4 ,the gold-leaf is initially connected to the case1 and the microscope adjusted so that the gold-leaf is seen in the centre of the scale4 ,for a given potential of the plate1 the sensitiveness depends on the angle of tilt of the case4 ,there is a certain critical inclination below which the gold-leaf is unstable4 ,the most sensitive position lies just above the critical angle4 ,in a particular experiment ,wilson found that with an angle of tilt of #cj and with the plate at a constant potential of #bjg volts1 the gold-leaf1 when raised to a potential of one volt above the case1 moved over #bjj scale divisions of the eye-piece1 #ed divisions corresponding to one millimetre4 ,illustration3 ,fig4 #ac4 ,in use1 the rod ,r is connected with the external insulated system whose rise or fall of potential is to be measured4 ,on account of the small capacity of the system and the large movement of the gold-leaf for a small difference of potential1 the electroscope is able to measure extraordinarily minute currents4 ,the apparatus is portable4 ,if the plate ,p be connected to one pole of a dry pile the gold-leaf is stretched out towards the plate1 and in this position can be carried without risk of injury4 #ei4 ,electrometers4 ,although the electroscope can be used with advantage in special cases1 it is limited in its application4 ,the most generally convenient apparatus for measurement of ionization currents through gases is one of the numerous types of quadrant electrometer4 ,with the help of auxiliary capacities1 the electrometer can be used to measure currents with accuracy over a wide range1 and can be employed for practically every kind of measurement required in radio-activity4 ,the elementary theory of the symmetrical quadrant electrometer as given in the text-books is very imperfect4 ,it is deduced that the sensibility of the electrometer--measured by the deflection of the needle for #a volt ,p4,d4 between the quadrants--varies directly as the potential of the charged needle1 provided that this potential is high compared with the ,p4,d4 between the quadrants4 ,in most electrometers however1 the sensibility rises to a maximum1 and then decreases with increase of potential of the needle4 ,for electrometers in which the needle lies close to the quadrants1 this maximum sensibility is obtained for a comparatively low potential of the needle4 ,a theory of the quadrant electrometer1 accounting for this action1 has been recently given by ,g4 ,w4 ,walker#ajd4 ,the effect appears to be due to the presence of the air space that necessarily exists between adjoining quadrants4 ,illustration3 ,fig4 #ad4 ,suppose that it is required to measure with an electrometer the ionization current between two horizontal metal plates ,a and ,b 7,fig4 #ad7 on the lower of which some active material has been spread4 ,if the saturation current is required1 the insulated plate ,a is connected with one pole of a battery of sufficient ,e4,m4,f4 to produce saturation1 the other pole being connected to earth4 ,the insulated plate ,b is connected with one pair of quadrants of the electrometer1 the other pair being earthed4 ,by means of a suitable key ,k1 the plate ,b and the pair of quadrants connected with it may be either insulated or connected with earth4 ,when a measurement is to be taken1 the earth connection is broken4 ,if the positive pole of the battery is connected with ,a1 the plate ,b and the electrometer connections immediately begin to be charged positively1 and the potential1 if allowed1 will steadily rise until it is very nearly equal to the potential of ,a4 ,as soon as the potential of the electrometer system begins to rise1 the electrometer needle commences to move at a uniform rate4 ,observations of the angular movement of the needle are made either by the telescope and scale or by the movement of the spot of light on a scale in the usual way4 ,if the needle is damped so as to give a uniform motion over the scale1 the rate of movement of the needle1 i4e4 the number of divisions of the scale passed over per second1 may be taken as a measure of the current through the gas4 ,the rate of movement is most simply obtained by observing with a stop-watch the time taken for the spot of light1 after the motion has become steady1 to pass over #ajj divisions of the scale4 ,as soon as the observation is made1 the plate ,b is again connected with earth1 and the electrometer needle returns to its original position4 ,in most experiments on radio-activity1 only comparative measurements of saturation currents are required4 ,if these measurements are to extend over weeks or months1 as is sometimes the case1 it is necessary to adopt some method of standardizing the electrometer from day to day1 so as to correct for variation in its sensibility4 ,this is done most simply by comparing the current to be measured with that due to a standard sample of uranium oxide1 which is placed in a definite position in a small testing vessel1 always kept in connection with the electrometer4 ,uranium oxide is a very constant source of radiation1 and the saturation current due to it is the same from day to day4 ,by this method of comparison accurate observations may be made on the variation of activity of a substance over long intervals of time1 although the sensibility of the electrometer may vary widely between successive measurements4 #fj4 ,construction of electrometers4 ,as the quadrant electrometer has gained the reputation of being a difficult and uncertain instrument for accurate measurements of current1 it may be of value to give some particular details in regard to the best method of construction and insulation4 ,in most of the older types of quadrant electrometers the needle system was made unnecessarily heavy4 ,in consequence of this1 if a sensibility of the order of #ajj mms4 deflection for #a volt was required1 it was necessary to charge the ,leyden jar connected to the needle to a fairly high potential4 ,this at once introduced difficulties1 for at a high potential it is not easy to insulate the ,leyden jar satisfactorily1 or to charge it to the same potential from day to day4 ,this drawback is to a large extent avoided in the ,white pattern of the ,kelvin electrometer1 which is provided with a replenisher and attracted disc for keeping the potential of the needle at a definite value4 ,if sufficient trouble is taken in insulating and setting up this type of electrometer1 it proves a very useful instrument of moderate sensibility1 and will continue in good working order for a year or more without much attention4 ,simpler types of electrometer of greater sensibility can however be readily constructed to give accurate results4 ,the old type of quadrant electrometer1 to be found in every laboratory1 can readily be modified to prove a useful and trustworthy instrument4 ,a light needle can be made of thin aluminium1 of silvered paper or of a thin plate of mica1 covered with gold-leaf to make it conducting4 ,the aluminium wire and mirror attached should be made as light as possible4 ,the needle should be supported either by a fine quartz fibre or a long bifilar suspension of silk4 ,a very fine phosphor bronze wire of some length is also very satisfactory4 ,a magnetic control is not very suitable1 as it is disturbed by coils or dynamos working in the neighbourhood4 ,in addition1 the zero point of the needle is not as steady as with the quartz or bifilar suspension4 ,when an electrometer is used to measure a current by noting the rate of movement of the needle1 it is essential that the needle should be damped sufficiently to give a uniform motion of the spot of light over the scale4 ,the damping requires fairly accurate adjustment4 ,if it is too little1 the needle has an oscillatory movement superimposed on the steady motion2 if it is too great1 it moves too sluggishly from rest and takes some time to attain a state of uniform motion4 ,with a light needle1 very little1 if any1 extra damping is required4 ,a light platinum wire with a single loop dipping in sulphuric acid is generally sufficient for the purpose4 ,with light needle systems and delicate suspensions1 it is only necessary to charge the needle to a potential of a few hundred volts to give a sensibility of several thousand divisions for a volt4 ,with such low potentials1 the difficulty of insulation of the condenser1 with which the needle is in electrical connection1 is much reduced4 ,it is convenient to use a condenser such that the potential of the needle does not fall more than a few per cent4 per day4 ,the ordinary short glass jar partly filled with sulphuric acid is1 in most cases1 not easy to insulate to this extent4 ,it is better to replace it by an ebonite 7or sulphur7 condenser#aje such as is shown in ,fig4 #ae4 ,illustration3 ,fig4 #ae4 ,a circular plate of ebonite about #a cm4 thick is turned down until it is not more than mm4 thick in the centre4 ,into this circular recess a brass plate ,b fits loosely4 ,the ebonite plate rests on another brass plate ,c connected with earth4 ,the condenser thus formed has a considerable capacity and retains a charge for a long time4 ,in order to make connection with the needle1 a small glass vessel ,d1 partly filled with sulphuric acid1 is placed on the plate ,b and put in connection with the needle by means of a fine platinum wire4 ,the platinum wire from the needle dips into the acid1 and serves to damp the needle4 ,in a dry atmosphere1 a condenser of this kind will not lose more than #bj per cent4 of its charge in a week4 ,if the insulation deteriorates1 it can readily be made good by rubbing the edge of the ebonite ,a with sand-paper1 or removing its surface in a lathe4 ,if a sufficient and steady ,e4,m4,f4 is available1 it is much better to keep the battery constantly connected with the needle1 and to avoid the use of the condenser altogether4 ,if a battery of small accumulators is used1 their potential can be kept at a constant value1 and the electrometer always has a constant sensibility4 #fa4 ,a very useful electrometer of great sensibility has been devised by ,dolezalek#ajf4 ,it is of the ordinary quadrant type with a very light needle of silvered paper1 spindle shaped1 which lies fairly close to the quadrants4 ,a very fine quartz suspension is employed4 ,in consequence of the lightness of the needle and its nearness to the quadrants1 it acts as its own damper4 ,this is a great advantage1 for difficulties always arise when the wire dips into sulphuric acid1 on account of the thin film which collects after some time on the surface of the acid4 ,this film obstructs the motion of the platinum wire dipping into the acid1 and has to be removed at regular intervals4 ,these instruments can readily be made to give a sensibility of several thousand divisions for a volt when the needle is charged to about one hundred volts4 ,the sensibility of the electrometer passes through a maximum as the potential of the needle is increased4 ,it is always advisable to charge the needle to about the value of this critical potential4 ,the capacity of the electrometer is in general high 7about #ej electrostatic units7 but the increased sensibility more than compensates for this4 ,the needle may either be charged by lightly touching it with one terminal of a battery1 or it may be kept charged to a constant potential through the quartz suspension4 ,dolezalek states that the fibre can be made sufficiently conducting for the purpose by dipping it into a dilute solution of calcium chloride or phosphoric acid4 ,i have not found this method satisfactory in dry climates as in many cases the fibre practically loses its conductivity after a few days exposure to dry air4 ,in addition to its great sensibility1 the advantage of this instrument is in the steadiness of the zero and in the self-damping4 ,a sensibility of #aj1jjj millimetre divisions per volt can be readily obtained with this electrometer1 if a very fine fibre be used4 ,the use of such high sensibilities cannot1 however1 be recommended except for very special experiments4 ,the period of swing of the needle under these conditions is several minutes and the natural leak of the testing vessels employed1 as well as electrostatic and other disturbances1 make themselves only too manifest4 ,if measurements of minute currents are required1 an electroscope of the type described in ,section #ef is much to be preferred to a very sensitive electrometer4 ,the electroscope readings in such a case are more accurate than similar measurements made by an electrometer4 ,for most measurements in radio-activity1 an electrometer which has a sensibility of #ajj divisions per volt is very suitable1 and no advantage is gained by using an electrometer of greater sensibility4 ,if still smaller effects require to be measured1 the sensibility may be increased to several thousand divisions per volt4 #fb4 ,adjustment and screening4 ,in adjusting an electrometer1 it is important to arrange that the needle shall lie symmetrically with regard to the quadrants4 ,this is best tested by observing whether the needle is deflected on charging1 the quadrants all being earthed4 ,in most electrometers there is an adjustable quadrant1 the position of which may be altered until the needle is not displaced on charging4 ,when this condition is fulfilled1 the zero reading of the electrometer remains unaltered as the needle loses its charge1 and the deflection on both sides of the zero should be the same for equal and opposite quantities of electricity4 ,the supports of the quadrants require to be well insulated4 ,ebonite rods are as a rule more satisfactory for this purpose than glass4 ,in testing for the insulation of the quadrants and the connections attached1 the system is charged to give a deflection of about #bjj scale divisions4 ,if the needle does not move more than one or two divisions after standing for one minute1 the insulation may be considered quite satisfactory4 ,when a suitable desiccator is placed inside the tight-fitting electrometer case1 the insulation of the quadrants should remain good for months4 ,if the insulation of the ebonite deteriorates1 it can easily be made good by removing the surface of the ebonite in a lathe4 ,in working with a sensitive instrument like the ,dolezalek electrometer1 it is essential that the electrometer and the testing apparatus should be completely enclosed in a screen of wire-gauze connected with earth1 in order to avoid electrostatic disturbances4 ,if an apparatus is to be tested at some distance from the electrometer1 the wires leading to it should be insulated in metal cylinders connected with earth4 ,the size of the insulators used at various points should be made as small as possible1 in order to avoid disturbances due to their electrification4 ,in damp climates1 paraffin1 amber1 or sulphur insulates better than ebonite4 ,the objection to paraffin as an insulator for sensitive electrometers lies in the difficulty of getting entirely rid of any electrification on its surface4 ,when paraffin has been once charged1 the residual charge1 after diselectrifying it with a flame1 continues to leak out for a long interval4 ,all insulators should be diselectrified by means of a spirit-lamp or still better by leaving some uranium near them4 ,care should be taken not to touch the insulation when once diselectrified4 ,in accurate work it is advisable to avoid the use of gas jets or ,bunsen flames in the neighbourhood of the electrometer1 as the flame gases are strongly ionized and take some time to lose their conductivity4 ,if radio-active substances are present in the room1 it is necessary to enclose the wires leading to the electrometer in fairly narrow tubes1 connected with earth4 ,if this is not done1 it will be found that the needle does not move at a constant rate1 but rapidly approaches a steady deflection where the rate of loss of charge of the electrometer and connections1 due to the ionization of the air around them1 is balanced by the current to be measured4 ,this precaution must always be taken when observations are made on the very penetrating rays from active substances4 ,these rays readily pass through ordinary screens1 and ionize the air around the electrometer and connecting wires4 ,for this reason it is impossible to make accurate measurements of small currents in a room which is used for the preparation of radio-active material4 ,in course of time the walls of the room become radio-active owing to the dissemination of dust and the action of the radio-active emanations#ajg4 #fc4 ,electrometer key4 ,for work with electrometers of high sensibility1 a special key is necessary to make and break from a distance the connection of the quadrants with earth in order to avoid electrostatic disturbances at the moment the current is to be measured4 ,the simple key shown in ,fig4 #af has been found very satisfactory for this purpose4 ,a small brass rod ,,bm1 to which a string is attached1 can be moved vertically up and down in a brass tube ,a1 which is rigidly attached to a bent metal support connected with earth4 ,when the string is released1 this rod makes contact with the mercury ,m1 which is placed in a small metal vessel resting on a block of ebonite ,p4 ,the electrometer and testing vessel are connected with the mercury4 ,when the string is pulled1 the rod ,,bm is removed from the mercury and the earth connection of the electrometer system is broken4 ,on release of the string1 the rod ,,bm falls and the electrometer is again earthed4 ,by means of this key1 which may be operated at any distance from the electrometer1 the earth connection may be made and broken at definite intervals without any appreciable disturbance of the needle4 ,illustration3 ,fig4 #af4 #fd4 ,testing apparatus4 ,the arrangement shown in ,fig4 #ag is very convenient for many measurements in radio-activity4 ,two parallel insulated metal plates ,a and ,b are placed inside a metal vessel ,v1 provided with a side door4 ,the plate ,a is connected with one terminal of a battery of small storage cells1 the other pole of which is earthed2 the plate ,b with the electrometer1 and the vessel ,v with earth4 ,the shaded areas in the figure indicate the position of ebonite insulators4 ,the active material to be tested is spread uniformly in a shallow groove 7about #e cms4 square and #b mms4 deep7 in the brass plate ,a4 ,in order to avoid breaking the battery connection every time the plate ,a is removed1 the wire from the battery is permanently connected with the metal block ,n resting on the ebonite support4 ,in this arrangement there is no possibility of a conduction leak from the plate ,a to ,b1 since the earth-connected vessel ,v intervenes4 ,illustration3 ,fig4 #ag4 ,an apparatus of this kind is very convenient for testing the absorption of the radiations by solid screens1 as well as for making comparative studies of the activity of different bodies4 ,unless very active preparations of radium are employed1 a battery of #cjj volts is sufficient to ensure saturation when the plates are not more than #e centimetres apart4 ,if substances which give off a radio-active emanation are being tested1 the effect of the emanation can be eliminated by passing a steady current of air from a gas bag between the plates4 ,this removes the emanation as fast as it is produced4 ,if a clean plate is put in the place of ,a1 a small movement of the electrometer needle is always observed4 ,if there is no radio-active substance in the neighbourhood1 this effect is due to the small natural ionization of the air4 ,we can correct for this natural leak when necessary4 #fe4 ,we have often to measure the activity due to the emanations of thorium or radium1 or the excited activity produced by those emanations on rods or wires4 ,a convenient apparatus for this purpose is shown in ,fig4 #ah4 ,the cylinder ,b is connected with the battery in the usual way1 and the central conductor ,a with the electrometer4 ,this central rod is insulated from the external cylinder by an ebonite cork1 which is divided into two parts by a metal ring ,,cc connected to earth4 ,this ring acts the part of a guard-ring1 and prevents any conduction leak between ,b and ,a4 ,the ebonite is thus only required to insulate satisfactorily for the small rise of potential produced on ,a during the experiment4 ,in all accurate measurements of current in radio-activity the guard-ring principle should always be used to ensure good insulation4 ,this is easily secured when the ebonite is only required to insulate for a fraction of a volt1 instead of for several hundred volts1 as is the case when the guard-ring is absent4 ,illustration3 ,fig4 #ah4 #ff4 ,for measurements of radio-activity with an electrometer1 a steady source of ,e4,m4,f4 of at least #cjj volts is necessary4 ,this is best obtained by a battery of small cells simply made by immersing strips of lead in dilute sulphuric acid1 or by a battery of small accumulators of the usual construction4 ,small accumulators of capacity about one-half ampere-hour can now be obtained at a moderate price1 and are more constant and require less attention than simple lead cells4 ,in order to measure currents over a wide range1 a graduated series of capacities is required4 ,the capacity of an electrometer and testing apparatus is usually about #ej electrostatic units or #jjjjef microfarads4 ,subdivided condensers of mica are constructed in which capacities varying from #jja to #b microfarads are provided4 ,with such a condenser1 another extra capacity is required to bridge over the gap between the capacity of the electrometer and the lowest capacity of the condenser4 ,this capacity of value about #bjj electrostatic units can readily be made by using parallel plates or still better concentric cylinders4 ,with this series of capacities1 currents may be measured between #c #aj and #c #aj amperes--a range of over one million4 ,still larger currents can be measured if the sensibility of the electrometer is reduced1 or if larger capacities are available4 ,in a room devoted to electrometer measurements of radio-activity1 it is desirable to have no radio-active matter present except that to be tested4 ,the room should also be as free from dust as possible4 ,the presence of a large quantity of dust in the air 7see section #ca7 is a very disturbing factor in all radio-active measurements4 ,a larger ,e4,m4,f4 is required to produce saturation on account of the diffusion of the ions to the dust particles4 ,the presence of dust in the air also leads to uncertainty in the distribution of excited activity in an electric field 7see section #aha74 #fg4 ,measurement of ,current4 ,in order to determine the current in the electrometer circuit by measuring the rate of movement of the needle1 it is necessary to know both the capacity of the circuit and the sensibility of the electrometer4 ,let ,c capacity of electrometer and its connections in ,e4,s4 units1 d number of divisions of the scale passed over per second1 ,d sensibility of the electrometer measured in scale divisions for #a volt ,p4,d4 between the quadrants4 ,the current i is given by the product of the capacity of the system and the rate of rise of potential4 ,thus ,cd i ----- ,e4,s4 units1 #cjj;,d ,cd ----------- amperes4 #i #aj ,d ,suppose1 for example1 ,c #ej1 d #e1 ,d #ajjj2 then i #bh #aj amperes4 ,since the electrometer can readily measure a current corresponding to a movement of half a scale division per second1 we see that an electrometer can measure a current of #c #aj amperes1 which is considerably below the range of the most sensitive galvanometer4 ,the capacity of the electrometer itself must not be considered as equal to that of the pair of quadrants and the needle when in a position of rest4 ,the actual capacity is very much larger than this1 on account of the motion of the charged needle4 ,suppose1 for example1 that the needle is charged to a high negative potential1 and kept at the zero position by an external constraint4 ,if a quantity ,q of positive electricity is given to the electrometer and its connections1 the whole system is raised to a potential ,v1 such that ,q ,,cv1 where ,c is the capacity of the system4 ,when however the needle is allowed to move1 it is attracted into the charged pair of quadrants4 ,this corresponds to the introduction of a negatively charged body between the quadrants1 and in consequence the potential of the system is lowered to ,v4 ,the actual capacity ,c of the system when the needle moves is thus greater than ,c1 and is given by ,c,v ,,cv4 ,thus the capacity of the electrometer is not a constant1 but depends on the potential of the needle1 i4e4 on the sensibility of the electrometer4 ,an interesting result of practical importance follows from the variation of the capacity of the electrometer with the potential of the needle4 ,if the external capacity attached to the electrometer is small compared with that of the electrometer itself1 the rate of movement of the needle for a constant current is1 in some cases1 independent of the sensibility4 ,an electrometer may be used for several days or even weeks to give nearly equal deflections for a constant current1 without recharging the needle1 although its potential has been steadily falling during the interval4 ,in such a case the decrease in sensibility is nearly proportional to the decrease in capacity of the electrometer1 so that the deflection for a given current is only slightly altered4 ,the theory of this action has been given by ,j4 ,j4 ,thomson#ajh4 #fh4 ,the capacity of the electrometer and its connections cannot be measured by any of the commutator methods used for the determination of small capacities1 for in such cases the needle does not move1 and the capacity measured is not that of the electrometer system when in actual use4 ,the value of the capacity may1 however1 be determined by the method of mixtures4 ,let ,c capacity of electrometer and connections1 ,c capacity of a standard condenser4 ,the electrometer and its connections are charged to a potential ,v by a battery1 and the deflection d of the needle is noted4 ,by means of an insulated key1 the capacity of the standard condenser is added in parallel with the electrometer system4 ,let ,v be the potential of the system1 and d the new deflection4 ,then ,,cv 7,c ,c7 ,v1 ,c ,c ,v d -------- ---- ----- ,c ,v d d and ,c ,c ---------- d d ,illustration3 ,fig4 #ai4 ,a simple standard capacity for this purpose can be constructed of two concentric brass tubes the diameters of which can be accurately measured4 ,the external cylinder ,d 7,fig4 #ai7 is mounted on a wooden base1 which is covered with a sheet of metal or tinfoil connected to earth4 ,the tube ,c is supported centrally on ebonite rods at each end4 ,the capacity is given approximately by the formula ,c frac l #b loge frac b a 1 where b is the internal diameter of ,d1 a the external diameter of ,c1 and l the length of the tubes4 ,the following method can be used in some cases with advantage4 ,while a testing vessel is in connection with the electrometer1 a sample of uranium is placed on the lower plate ,a4 ,let d and d be the number of divisions passed over per second by the needle with and without the standard capacity in connection4 ,c ,c d ,then ---------- ----1 ,c d d and ,c ,c --------- d d ,this method has the advantage that the relative capacities are expressed in terms of the motion of the needle under the actual conditions of measurement4 #fi4 ,steady deflection method4 ,the methods of measurement previously described depend upon the rate of angular movement of a suspended gold-leaf or of an electrometer needle4 ,the galvanometer can only be employed for measurements with intensely active matter4 ,a need1 however1 has long been felt for a method in which ordinary ionization currents can be measured by means of a steady deflection of an electrometer needle4 ,this is especially the case1 where measurements have to be made with active substances whose activity alters rapidly in the course of a few minutes4 ,this can obviously be secured if the electrometer system 7one pair of quadrants being earthed7 is connected to earth through a suitable high resistance4 ,a steady deflection of the electrometer needle will be obtained when the rate of supply of electricity to the electrometer system is balanced by the loss due to conduction through the resistance4 ,if the high resistance obeys ,ohm's law1 the deflection should be proportional to the ionization current to be measured4 ,a simple calculation shows that the resistance required is very great4 ,suppose1 for example1 that a current is to be measured corresponding to a rate of movement of the needle of #e divisions per second1 with a sensibility of #ajjj divisions per volt1 and where the capacity of the electrometer system is #ej electrostatic units4 ,this current is equal to #bh #aj amperes4 ,if a steady deflection of #aj divisions is required1 which corresponds to a rise of potential of the system of of a volt1 the resistance should be #cf1jjj megohms4 ,for a deflection of #ajj divisions1 the resistance should be #aj times as large4 ,dr ,bronson#aji1 working in the laboratory of the writer1 has recently made some experiments in order to devise a practical method for measurements of this character4 ,it is difficult to obtain sufficiently high and constant resistances to answer the purpose4 ,tubes of xylol had too great a resistance1 while special carbon resistances were not sufficiently constant4 ,the difficulty was finally got over by the use of what may be called an 8air resistance40 ,the arrangement of the experiment is shown in ,fig4 #bj4 ,illustration3 ,fig4 #bj4 ,the electrometer system was connected with the upper of two insulated parallel plates ,,ab1 on the lower of which was spread a layer of a very active substance4 ,an active bismuth plate1 coated with radio-tellurium1 which had been obtained from ,sthamer of ,hamburg1 proved very convenient for this purpose4 ,the lower plate ,b was connected to earth4 ,the charge communicated to the upper plate of the testing vessel ,,cd and the electrometer system leaked away in consequence of the strong ionization between the plates ,,ab1 and a steady deflection was obtained when the rate of supply was equal to the rate of discharge4 ,this air resistance obeyed ,ohm's law over a considerable range1 i4e4 the steady deflection was proportional to the current4 ,it is advisable1 in such an arrangement1 to test whether the deflection is proportional to the ionization current over the range required for measurement4 ,this can readily be done by the use of a number of metal vessels filled with a constant radio-active substance like uranium oxide4 ,the effect of these1 when placed in the testing vessel1 can be tested separately and in groups1 and in this way the scale can be calibrated accurately4 ,the plates ,,ab were placed inside a closed vessel to avoid air currents4 ,the contact difference of potential between the plates ,,ab1 which shows itself by a steady deflection when no radio-active matter is present in ,,cd1 was for the most part eliminated by covering the surface of the plates ,a and ,b with very thin aluminium foil4 ,this method proved very accurate and convenient for measurement of rapid changes in activity1 and possesses many advantages over the ordinary rate-method of use of an electrometer4 ,a thin layer of radium of moderate activity would probably serve in place of the radio-tellurium1 but the emanation and the and rays emitted from it would be a possible source of disturbance to the measurements4 ,the deflection of the electrometer needle in this arrangement is independent of the capacity of the electrometer system1 and thus comparative measurements of current can be made without the necessity of determining the capacity in each case4 #gj4 ,quartz piezo-electrique4 ,in measurements of the strength of currents by electrometers1 it is always necessary to determine the sensibility of the instrument and the capacity of the electrometer and the apparatus attached thereto4 ,by means of the quartz piezo-electrique devised by the brothers ,,mm4 ,j4 and ,p4 ,curie#aaj1 measurements of the current can be made with rapidity and accuracy over a wide range4 ,these measurements are quite independent of the capacity of the electrometer and external circuit4 ,the essential part of this instrument consists of a plate of quartz which is cut in a special manner4 ,when this plate is placed under tension1 there is a liberation of electricity equal in amount but opposite in sign on the two sides of the plate4 ,the plate of quartz ,,ab 7,fig4 #ba7 is hung vertically and weights are added to the lower end4 ,the plate is cut so that the optic axis of the crystal is horizontal and at right angles to the plane of the paper4 ,illustration3 ,fig4 #ba4 ,the two faces ,a and ,b are normal to one of the binary axes 7or electrical axes7 of the crystal4 ,the tension must be applied in a direction normal to the optic and electric axes4 ,the two faces ,a and ,b are silvered1 but the main portion of the plate is electrically insulated by removing a narrow strip of the silvering near the upper and lower ends of the plate4 ,one side of the plate is connected with the electrometer and with the conductor1 the rate of leak of which is to be measured4 ,the quantity of electricity set free on one face of the plate is accurately given by ,l ,q #jfc ---- ,f b where ,l is the length of the insulated portion of the plate1 b the thickness ,,ab1 and ,f the weight attached in kilogrammes4 ,q is then given in electrostatic units4 ,suppose1 for example1 that it is required to measure the current between the plates ,,cd 7,fig4 #ba7 due to some radio-active material on the plate ,c1 for a given difference of potential between ,c and ,d4 ,at a given instant the connection of the quadrants of the electrometer with the earth is broken4 ,the weight is attached to the quartz plate1 and is held in the hand so as to apply the tension gradually4 ,this causes a release of electricity opposite in sign to that given to the plate ,d4 ,the electrometer needle is kept at the position of rest as nearly as possible by adjusting the tension by hand4 ,the tension being fully applied1 the moment the needle commences to move steadily from zero is noted4 ,the current between the plates ,,cd is then given by ,q_/t where t is the time of the observation4 ,the value of ,q is known from the weight attached4 ,in this method the electrometer is only used as a detector to show that the system is kept at zero potential4 ,no knowledge of the capacity of the insulated system is required4 ,with practice1 measurements of the current can be made in this way with rapidity and certainty4 ,footnote #ii3 ,soddy1 ,trans4 ,chem4 ,soc4 ,vol4 #ha1 p4 #hfj1 #aijb4 ,footnote #ajj3 ,wilson1 ,proc4 ,roy4 ,soc4 ,vol4 #fh1 p4 #aeb1 #aija4 ,footnote #aja3 ,if the apparatus is required to be air-tight1 the gold-leaf system can be charged by means of a piece of magnetized steel wire1 which is made to touch the rod ,r by the approach of a magnet4 ,footnote #ajb3 ,it is sometimes observed that the motion of the gold-leaf1 immediately after charging1 is irregular4 ,in many cases1 this can be traced to air currents set up in the electroscope in consequence of unsymmetrical heating by the source of light used for illumination4 ,footnote #ajc3 ,wilson1 ,proc4 ,camb4 ,phil4 ,soc4 ,vol4 #ab1 ,part ,,ii4 #aijc4 ,footnote #ajd3 ,walker1 ,phil4 ,mag4 ,aug4 #aijc4 ,footnote #aje3 ,strutt1 ,phil4 ,trans4 ,a1 p4 #ejg1 #aija4 ,footnote #ajf3 ,dolezalek1 ,instrumentenkunde1 p4 #cde1 ,dec4 #aija4 ,footnote #ajg3 ,it is very desirable that care should be taken not to release large quantities of the radium emanation inside a laboratory4 ,this emanation has a slow rate of decay and is carried by currents of air throughout the whole building and finally leaves behind an active deposit of very slow rate of change 7see chapter ,,xi474 ,eve 7,nature1 ,march #af1 #aije7 has drawn attention to the difficulty of making refined radio-active measurements under such conditions4 ,footnote #ajh3 ,j4 ,j4 ,thomson1 ,phil4 ,mag4 #df1 p4 #ecg1 #ahih4 ,footnote #aji3 ,bronson1 ,amer4 ,journ4 ,science1 ,feb4 #aije4 ,footnote #aaj3 ,j4 and ,p4 ,curie1 ,c4 ,r4 #ia1 pp4 #ch and #bid1 #ahhj4 ,see also ,friedel and ,j4 ,curie1 ,c4 ,r4 #if1 pp4 #abfb and #achi1 #ahhc1 and ,lord ,kelvin1 ,phil4 ,mag4 #cf1 pp4 #cca1 #cdb1 #chd1 #dad1 #dec1 #ahic4 ,,chapter ,,iv4 ,,nature ,,of ,,the ,,radiations4 ,,part ,i4 ,comparison of the ,radiations4 #ga4 ,the ,three ,types of ,radiation4 ,all the radio-active substances possess in common the power of acting on a photographic plate and of ionizing the gas in their immediate neighbourhood4 ,the intensity of the radiations may be compared by means of their photographic or electrical action2 and1 in the case of the strongly radio-active substances1 by the power they possess of lighting up a phosphorescent screen4 ,such comparisons1 however1 do not throw any light on the question whether the radiations are of the same or of different kinds1 for it is well known that such different types of radiations as the short waves of ultra-violet light1 ,rntgen and cathode rays1 all possess the property of producing ions throughout the volume of a gas1 lighting up a fluorescent screen1 and acting on a photographic plate4 ,neither can the ordinary optical methods be employed to examine the radiations under consideration1 as they show no trace of regular reflection1 refraction1 or polarization4 ,two general methods can be used to distinguish the types of the radiations given out by the same body1 and also to compare the radiations from the different active substances4 ,these methods are as follows3 7#a7 ,by observing whether the rays are appreciably deflected in a magnetic field4 7#b7 ,by comparing the relative absorption of the rays by solids and gases4 ,examined in these ways1 it has been found that there are three different types of radiation emitted from radio-active bodies1 which for brevity and convenience have been termed by the writer the 1 1 and rays4 7i7 ,the rays are very readily absorbed by thin metal foil and by a few centimetres of air4 ,they have been shown to consist of positively charged bodies projected with a velocity of about the velocity of light4 ,they are deflected by intense magnetic and electric fields1 but the amount of deviation is minute in comparison with the deviation1 under the same conditions1 of the cathode rays produced in a vacuum tube4 7ii7 ,the rays are far more penetrating in character than the rays1 and consist of negatively charged bodies projected with velocities of the same order as the velocity of light4 ,they are far more readily deflected than the rays1 and are in fact identical with the cathode rays produced in a vacuum tube4 7iii7 ,the rays are extremely penetrating1 and non-deviable by a magnetic field4 ,their true nature is not definitely settled1 but they are analogous in most respects to very penetrating ,rntgen rays4 ,the three best known radio-active substances1 uranium1 thorium1 and radium1 all give out these three types of rays1 each in an amount approximately proportional to its relative activity measured by the rays4 ,polonium stands alone in giving only the or easily absorbed rays#aaa4 #gb4 ,deflection of the rays4 ,the rays emitted from the active bodies thus present a very close analogy with the rays which are produced in a highly exhausted vacuum tube when an electric discharge passes through it4 ,the rays correspond to the canal rays1 discovered by ,goldstein1 which have been shown by ,wien to consist of positively charged bodies projected with great velocity 7see section #ea74 ,the rays are the same as the cathode rays1 while the rays resemble the ,rntgen rays4 ,in a vacuum tube1 a large amount of electric energy is expended in producing the rays1 but1 in the radio-active bodies1 the rays are emitted spontaneously1 and at a rate uninfluenced by any chemical or physical agency4 ,the and rays from the active bodies are projected with much greater velocity than the corresponding rays in a vacuum tube1 while the rays are of much greater penetrating power than ,rntgen rays4 ,the effect of a magnetic field on a pencil of rays from a radio-active substance giving out the three kinds of rays is very well illustrated in ,fig4 #bbaab4 ,illustration3 ,fig4 #bb4 ,some radium is placed in the bottom of a narrow cylindrical lead vessel ,r4 ,a narrow pencil of rays consisting of 1 1 and rays escapes from the opening4 ,if a strong uniform magnetic field is applied at right angles to the plane of the paper1 and directed towards the paper1 the three types of rays are separated from one another4 ,the rays continue in a straight line without any deviation4 ,the rays are deflected to the right1 describing circular orbits the radii of which vary within wide limits4 ,if the photographic plate ,,ac is placed under the radium vessel1 the rays produce a diffuse photographic impression on the right of the vessel ,r4 ,the rays are bent in the direction opposite to that of the rays1 and describe a portion of the arc of a circle of large radius1 but they are rapidly absorbed after traversing a distance of a few centimetres from the vessel ,r4 ,the amount of the deviation of the rays compared with that of the rays is much exaggerated in the figure4 #gc4 ,ionizing and penetrating power of the rays4 ,of the three kinds of rays1 the rays produce most of the ionization in the gas and the rays the least4 ,with a thin layer of unscreened active material spread on the lower of two parallel plates #e cms4 apart1 the amount of ionization due to the 1 1 and rays is of the relative order #aj1jjj1 #ajj1 and #a4 ,these numbers are only rough approximations1 and the differences become less marked as the thickness of the radio-active layer increases4 ,the average penetrating power of the rays is shown below4 ,in the first column is given the thickness of the aluminium1 which cuts each radiation down to half its value1 and in the second the relative power of penetration of the rays4 ,radiation ,thickness of ,relative ,aluminium in cms4 power of which cuts off half penetration the radiation rays #jjjje cms4 #a #jje cms4 #ajj #h cms4 #ajjjj ,the relative power of penetration is thus approximately inversely proportional to the relative ionization4 ,these numbers1 however1 only indicate the order of relative penetrating power4 ,this power varies considerably for the different active bodies4 ,the rays from uranium and polonium are the least penetrating1 and those from thorium the most4 ,the radiations from thorium and radium are very complex1 and consist of rays widely different in penetrating power4 ,some of the rays from these substances are much less and others much more penetrating than those from uranium1 which gives out fairly homogeneous rays4 #gd4 ,difficulties of comparative measurements4 ,it is difficult to make quantitative or even qualitative measurements of the relative intensity of the three types of rays from active substances4 ,the three general methods employed depend upon the action of the rays in ionizing the gas1 in acting on a photographic plate1 and in causing phosphorescent or fluorescent effects in certain substances4 ,in each of these methods the fraction of the rays which is absorbed and transformed into another form of energy is different for each type of ray4 ,even when one specific kind of ray is under observation1 comparative measurements are rendered difficult by the complexity of that type of rays4 ,for example1 the rays from radium consist of negatively charged particles projected with a wide range of velocity1 and1 in consequence1 they are absorbed in different amounts in passing through a definite thickness of matter4 ,in each case1 only a fraction of the energy absorbed is transformed into the particular type of energy1 whether ionic1 chemical1 or luminous1 which serves as a means of measurement4 ,the rays which are the most active electrically are the least active photographically4 ,under ordinary conditions1 most of the photographic action of uranium1 thorium1 and radium1 is due to the or cathodic rays4 ,the rays from uranium and thorium1 on account of their weak action1 have not yet been detected photographically4 ,with active substances like radium and polonium1 the rays readily produce a photographic impression4 ,so far the rays have been detected photographically from radium only4 ,that no photographic action of these rays has yet been established for uranium and thorium is probably due merely to the fact that the effect sought for is very small1 and during exposures for long intervals it is very difficult to avoid fogging of the plates owing to other causes4 ,considering the similarity of the radiations in other respects1 there can be little doubt that the rays do produce some photographic action1 though it is too small to observe with certainty4 ,these differences in the photographic and ionizing properties of the radiations must always be taken into account in comparing results obtained by the two methods4 ,the apparent contradiction of results obtained by different observers using these two methods is found to be due to their differences in relative photographic and ionizing action4 ,for example1 with the unscreened active material1 the ionization observed by the electrical method is due almost entirely to rays1 while the photographic action under the same condition is due almost entirely to the rays4 ,it is often convenient to know what thickness of matter is sufficient to absorb a specific type of radiation4 ,a thickness of aluminium or mica of #ja cms4 or a sheet of ordinary writing-paper is sufficient to absorb completely all the rays4 ,with such a screen over the active material1 the effects are due only to the and rays1 which pass through with a very slight absorption4 ,most of the rays are absorbed in #e mms4 of aluminium or #b mms4 of lead4 ,the radiation passing through such screens consists very largely of the rays4 ,as a rough working rule1 it may be taken that a thickness of matter required to absorb any type of rays is inversely proportional to the density of the substance1 i4e4 the absorption is proportional to the density4 ,this rule holds approximately for light substances1 but1 in heavy substances like mercury and lead1 the radiations are about twice as readily absorbed as the density rule would lead us to expect4 ,,part ,,ii4 ,the or ,cathodic ,rays4 #ge4 ,discovery of the rays4 ,a discovery which gave a great impetus to the study of the radiations from active bodies was made in #ahii1 almost simultaneously in ,germany1 ,france1 and ,austria4 ,it was observed that preparations of radium gave out some rays which were deviable by a magnetic field1 and very similar in character to the cathode rays produced in a vacuum tube4 ,the observation of ,elster and ,geitel that a magnetic field altered the conductivity produced in air by radium rays1 led ,giesel#aac to examine the effect of a magnetic field on the radiations4 ,in his experiments1 the radio-active preparation was placed in a small vessel between the poles of an electromagnet4 ,the vessel was arranged to give a pencil of rays which was approximately perpendicular to the field4 ,the rays caused a small fluorescent patch on the screen4 ,on exciting the electromagnet1 the fluorescent zone was observed to broaden out on one side4 ,on reversing the field1 the extension of the zone was in the opposite direction4 ,the deviation of the rays thus indicated was in the same direction and of the same order of magnitude as that for cathode rays4 ,s4 ,meyer and ,schweidler#aad also obtained similar results4 ,they showed1 in addition1 the deviation of the rays by the alteration of the conductivity of the air when a magnetic field was applied4 ,becquerel#aae1 a little later1 showed the magnetic deflection of the radium rays by using the photographic method4 ,p4 ,curie#aaf1 by the electrical method1 showed furthermore that the rays from radium consisted of two kinds1 one apparently non-deviable and easily absorbed 7now known as the rays71 and the other penetrating and deviable by a magnetic field 7now known as the rays74 ,the ionization effect due to the rays was only a small fraction of that due to the rays4 ,at a later date ,becquerel1 by the photographic method1 showed that uranium gave out some deflectable rays4 ,it had been shown previously#aag that the rays from uranium consisted of and rays4 ,the deflected rays in ,becquerel's experiment consisted entirely of rays1 as the rays from uranium produce no appreciable photographic action4 ,rutherford and ,grier#aah1 using the electric method1 showed that compounds of thorium1 like those of uranium1 gave out1 besides rays1 some penetrating rays1 deviable in a magnetic field4 ,as in the case of radium1 the ionization due to the rays of uranium and thorium is large compared with that due to the rays4 #gf4 ,examination of the magnetic deviation by the photographic method4 ,becquerel has made a very complete study1 by the photographic method1 of the rays from radium1 and has shown that they behave in all respects like cathode rays1 which are known to be negatively charged particles moving with a high velocity4 ,the motion of a charged ion acted on by a magnetic field has been discussed in section #di4 ,it has been shown that if a particle of mass m and charge e is projected with a velocity u1 at an angle with the direction of a uniform field of strength ,h1 it will describe a helix round the magnetic lines of force4 ,this helix is wound on a cylinder of radius ,r1 with the axis parallel to the field1 where ,r is given by mu ,r ---- sin 4 ,he ,when _/#b1 i4e4 when the rays are projected normally to the field1 the particles describe circles of radius mu ,r ---- ,he ,the planes of these circles are normal to the field4 ,thus1 for a particular velocity u1 the value of ,r varies inversely as the strength of the field4 ,in a uniform field the rays projected normally to the field describe circles1 and their directions of projection are the tangents at the origin4 ,this conclusion has been verified experimentally by ,becquerel for the rays of radium1 by an arrangement similar to that shown in ,fig4 #bc4 ,illustration3 ,fig4 #bc4 ,a photographic plate ,p1 with the film downwards1 is enveloped in black paper and placed horizontally in the uniform horizontal magnetic field of an electromagnet4 ,the magnetic field is supposed to be uniform1 and1 in the figure1 is at right angles to the plane of the paper4 ,the plate was covered with a sheet of lead1 and on the edge of the plate1 in the centre of the magnetic field1 is placed a small lead vessel ,r containing the radio-active matter4 ,on exciting the magnet1 so that the rays are bent to the left of the figure1 it is observed that a photographic impression is produced directly below the source of the rays1 which have been bent round by the magnetic field4 ,the active matter sends out rays equally in all directions4 ,the rays perpendicular to the field describe circles1 which strike the plate immediately under the source4 ,a few of these rays1 ,a1 ,a1 ,a1 are shown in the figure4 ,the rays1 normal to the plate1 strike the plate almost normally1 while the rays nearly parallel to the plate strike the plate at grazing incidence4 ,the rays1 inclined to the direction of the field1 describe spirals and produce effects on an axis parallel to the field passing through the source4 ,in consequence of this1 any opaque screen placed in the path of the rays has its shadow thrown near the edge of the photographic plate4 #gg4 ,complexity of the rays4 ,the deviable rays from radium are complex1 i4e4 they are composed of a flight of particles projected with a wide range of velocity4 ,in a magnetic field every ray describes a path1 of which the radius of curvature is directly proportional to the velocity of projection4 ,the complexity of the radiation has been shown very clearly by ,becquerel#aai in the following way4 ,an uncovered photographic plate1 with the film upwards1 was placed horizontally in the horizontal uniform magnetic field of an electromagnet4 ,a small1 open1 lead box1 containing the radio-active matter1 was placed in the centre of the field1 on the photographic plate4 ,the light1 due to the phosphorescence of the radio-active matter1 therefore1 could not reach the plate4 ,the whole apparatus was placed in a dark room4 ,the impression on the plate took the form of a large1 diffuse1 but continuous band1 elliptic in shape1 produced on one side of the plate4 ,such an impression is to be expected if the rays are sent out in all directions1 even if their velocities of projection are the same1 for it can readily be shown theoretically1 that the path of the rays is confined within an ellipse whose minor axis1 which is at right angles to the field1 is equal to #b,r1 and whose major axis is equal to ,r4 ,if1 however1 the active matter is placed in the bottom of a deep lead cylinder of small diameter1 the rays have practically all the same direction of projection1 and in that case each part of the plate is acted on by rays of a definite curvature4 ,in this case also1 a diffuse impression is observed on the plate1 giving1 so to speak1 a continuous spectrum of the rays and showing that the radiation is composed of rays of widely different curvatures4 ,fig4 #bd shows a photograph of this kind obtained by ,becquerel1 with strips of paper1 aluminium1 and platinum placed on the plate4 ,illustration3 ,fig4 #bd4 ,if screens of various thickness are placed on the plate1 it is observed that the plate is not appreciably affected within a certain distance from the active matter1 and that this distance increases with the thickness of the screen4 ,this distance is obviously equal to twice the radius of curvature of the path of the rays1 which are just able to produce an impression through the screen4 ,these experiments show very clearly that the most deviable rays are those most readily absorbed by matter4 ,by observations of this kind ,becquerel has determined approximately the inferior limit of the value of ,,hr for rays which are transmitted through different thicknesses of matter4 ,the results are given in the table below3 ,substance ,thickness ,inferior limit in mms4 of ,,hr for transmitted rays ,black paper #jjfe #fej ,aluminium #jjaj #cej #jajj #ajjj #jbjj #adhj ,mica #jjbe #ebj ,glass #jaee #aacj ,platinum #jjcj #acaj ,copper #jjhe #agdj ,lead #jacj #bfaj ,if e_/m is a constant for all the rays1 the value of ,,hr is proportional to the velocity of the rays1 and it follows from the table that the velocity of the rays which just produce an effect on the plate through #ac mms4 of lead is about #g times that of the rays which just produce an impression through #ja mm4 of aluminium4 ,it will be shown1 however1 in section #hb1 that e_/m is not a constant for all speeds1 but decreases with increase of velocity of the rays4 ,the difference in velocity between the rays is in consequence not as great as this calculation would indicate4 ,on examination of the rays from uranium1 ,becquerel found that the radiation is not as complex as that from radium1 but consists wholly of rays for which the value of ,,hr is about #bjjj4 #gh4 ,examination of the rays by the electric method4 ,the presence of easily deviable rays given off from an active substance can most readily be shown by the photographic method1 but it is necessary1 in addition1 to show that the penetrating rays which produce the ionization in the gas are the same as those which cause the photographic action4 ,this can be conveniently tested in an arrangement similar to that shown in ,fig4 #be4 ,illustration3 ,fig4 #be4 ,the radio-active matter ,a is placed on a lead block ,b between the two parallel lead plates ,,bb4 ,the rays pass between the parallel plates and ionize the gas between the plates ,,pp of the testing vessel4 ,the magnetic field is applied at right angles to the plane of the paper4 ,the dotted rectangle ,,eeee represents the position of the pole piece4 ,if a compound of radium or thorium is under investigation1 a stream of air is required to prevent the diffusion of the radio-active emanations into the testing vessel4 ,when a layer of uranium1 thorium or radium compound is placed at ,a1 the ionization in the testing vessel is due mainly to the action of the and rays4 ,the rays are cut off by adding a layer of aluminium #ja cm4 thick over the active material4 ,when the layer of active matter is not more than a few millimetres thick1 the ionization due to the rays is small compared with that produced by the rays1 and may be neglected4 ,on the application of a magnetic field at right angles to the mean direction of the rays1 the ionization in the testing vessel due to the rays steadily decreases as the strength of the field increases1 and in a strong field it is reduced to a very small fraction of its original value4 ,in this case the rays are bent so that none of them enter the testing vessel4 ,examined in this way1 it has been found that the rays of uranium1 thorium1 and radium consist entirely of rays readily deflected by a magnetic field4 ,the rays from polonium consist entirely of rays1 the deviation of which can be detected only in very intense magnetic fields4 ,when the screen covering the active material is removed1 in a strong magnetic field1 the ionization in the vessel is mainly due to the rays4 ,on account of the slight deviation of the rays under ordinary experimental conditions1 a still greater increase of the magnetic field does not appreciably alter the current due to them in the testing vessel4 ,the action of a magnetic field on a very active substance like radium is easily shown by the electrical method1 as the ionization current due to the deviable rays is large4 ,with substances of small activity like uranium and thorium1 the ionization current due to the deviable rays is very small1 and a sensitive electrometer or an electroscope is required to determine the variation1 in a magnetic field1 of the very small current involved4 ,this is especially the case for thorium oxide1 which gives out only about of the amount of deviable rays given out by the same weight of uranium oxide4 #gi4 ,experiments with a fluorescent screen4 ,the rays from a few milligrams of pure radium bromide produce intense fluorescence in barium platinocyanide and other substances which can be made luminous under the influence of the cathode rays4 ,using a centigram of radium bromide1 the luminosity on a screen1 placed upon it1 is bright enough to be observed in daylight4 ,with the aid of such a screen in a dark room many of the properties of the rays may be simply illustrated and their complex nature clearly shown4 ,a small quantity of radium is placed in the bottom of a short1 narrow1 lead tube open at one end4 ,this is placed between the pole pieces of an electromagnet1 and the screen placed below it4 ,with no magnetic field1 a faint luminosity of the screen is observed due to the very penetrating rays which readily pass through the lead4 ,when the magnetic field is put on1 the screen is brightly lighted up on one side over an area elliptical in shape 7section #gg74 ,the direction of deviation is reversed by reversal of the field4 ,the broad extent of the illumination shows the complex nature of the rays4 ,on placing a metallic object at various points above the screen1 the trajectory of the rays can readily be traced by noticing the position of the shadow cast upon the screen4 ,by observing the density of the shadow1 it can be seen that the rays most easily deviated are the least penetrating4 ,comparison of the rays with cathode rays4 #hj4 ,means of comparison4 ,in order to prove the identity of the rays from active bodies with the cathode rays produced in a vacuum tube1 it is necessary to show 7#a7 ,that the rays carry with them a negative charge2 7#b7 ,that they are deviated by an electric as well as by a magnetic field2 7#c7 ,that the ratio e_/m is the same as for the cathode rays4 ,electric charge carried by the rays4 ,the experiments of ,perrin and ,j4 ,j4 ,thomson have shown that the cathode rays carry with them a negative charge4 ,in addition1 ,lenard has shown that the rays still carry a charge after traversing thin layers of matter4 ,when the rays are absorbed1 they give up their charge to the body which absorbs them4 ,the total amount of charge carried by the rays from even a very active preparation of radium is1 in general1 small compared with that carried by the whole of the cathode rays in a vacuum tube1 and can be detected only by delicate methods4 99 99 99 99 99 ,suppose that a layer of very active radium is spread on a metal plate connected to earth1 and that the rays are absorbed by a parallel plate connected with an electrometer4 ,if the rays are negatively charged1 the top plate should receive a negative charge increasing with the time4 ,on account1 however1 of the great ionization produced by the rays between the plates1 any charge given to one of them is almost instantly dissipated4 ,in many cases1 the plate does become charged to a definite positive or negative potential depending on the metal1 but this is due to the contact difference of potential between the plates1 and would be produced whether the rays were charged or not4 ,the ionization of the gas is greatly diminished by placing over the active material a metal screen which absorbs the rays1 but allows the rays to pass through with little absorption4 ,the rapid loss of any charge communicated to the top plate can be very much reduced1 either by diminishing the pressure of the gas surrounding it or by enclosing the plate with suitable insulators4 ,in their experiments to determine the amount of charge carried by the radium rays1 ,m4 and ,mme ,curie#abj used the second method4 ,a metal disc ,,mm 7,fig4 #bf7 is connected with an electrometer by the wire ,t4 ,the disc and wire are completely surrounded by insulating matter ii4 ,the whole is surrounded by a metal envelope ,,eeee connected with earth4 ,on the lower side of the disc1 the insulator and the metallic covering are very thin4 ,this side is exposed to the rays of the radium ,r placed in a depression in a lead plate ,,aa4 ,illustration3 ,fig4 #bf4 ,the rays of the radium pass through the metal cover and insulator with little absorption1 but they are completely absorbed by the disc ,,mm4 ,it was observed that the disc received a negative charge which increased uniformly with the time1 showing that the rays carry with them a negative charge4 ,the current observed was very small4 ,with an active preparation of radium#aba1 forming a layer #be sq4 cms4 in area and #b mms4 thick1 a current of the order of #aj amperes was observed after the rays had traversed a layer of aluminium #ja mm4 thick and a layer of ebonite #c mm4 thick4 ,the current was the same with discs of lead1 copper1 and zinc1 and also when the ebonite was replaced by paraffin4 ,curie also observed in another experiment of a similar character that the radium itself acquired a positive charge4 ,this necessarily follows if the rays carry with them a negative charge4 ,if the rays alone carried with them a charge1 a pellet of radium1 if perfectly insulated1 and surrounded by a non-conducting medium1 would in the course of time be raised to a high positive potential4 ,since1 however1 the rays carry with them a charge opposite in sign to the rays1 the ratio of the charge carried off by the two types of rays must be determined1 before it can be settled whether the radium would acquire a positive or a negative charge4 ,if1 however1 the radium is placed in an insulated metal vessel of a thickness sufficient to absorb all the rays1 but not too thick to allow most of the rays to escape1 the vessel will acquire a positive charge in a vacuum4 ,an interesting experimental result bearing upon this point has been described by ,dorn#abb4 ,a small quantity of radium was placed in a sealed glass tube and left for several months4 ,on opening the tube with a file1 a bright electric spark was observed at the moment of fracture1 showing that there was a large difference of potential between the inside of the tube and the earth4 ,in this case the rays were absorbed in the walls of the tube1 but a large proportion of the rays escaped4 ,the inside of the tube thus became charged1 in the course of time1 to a high positive potential2 a steady state would be reached when the rate of escape of negative electricity was balanced by the leakage of positive electricity through the walls of the tube4 ,the external surface of the glass would be always practically at zero potential1 on account of the ionization of the air around it4 ,strutt#abc has recently described a simple and striking experiment to illustrate still more clearly that a radium preparation acquires a positive charge1 if it is enclosed in an envelope thick enough to absorb all the particles1 but thin enough to allow most of the particles to escape4 ,the experimental arrangement is clearly seen in ,fig4 #bg4 ,a sealed tube ,,aa containing the radium1 was attached at one end to a pair of thin gold leaves in metallic connection with the radium1 and was insulated inside a larger tube by means of a quartz rod ,b4 ,the inner surface of the tube was coated with tinfoil ,,ee connected to earth4 ,the glass surface of ,,aa was made conducting by a thin coating of phosphoric acid4 ,the air in the outer tube was exhausted as completely as possible by means of a mercury pump1 in order to reduce the ionization in the gas1 and consequently the loss of any charge gained by the gold leaves4 ,after an interval of #bj hours1 the gold leaves were observed to diverge to their full extent1 indicating that they had acquired a large positive charge4 ,in this experiment ,strutt used gram of radiferous barium of activity only #ajj times that of uranium4 ,illustration3 ,fig4 #bg4 ,if the tube is filled with #cj mgrs4 of pure radium bromide1 the leaves diverge to their full extent in the course of about a minute4 ,if it is arranged that the gold leaf1 at a certain angle of divergence1 comes in contact with a piece of metal connected with earth1 the apparatus can be made to work automatically4 ,the leaf diverges1 touches the metal1 and at once collapses1 and this periodic movement of the leaf will continue1 if not indefinitely1 at any rate as long as the radium lasts4 ,this 8radium clock0 should work at a sensibly uniform rate for many years1 but1 from evidence considered later 7,section #bfa71 there is reason to believe that the number of particles emitted would decrease exponentially with the time1 falling to half value in about #abjj years4 ,the period of movement of the leaf should thus gradually increase with the time1 and ultimately the effect would become too small to observe4 ,the action of this radium clock is the nearest approach to an apparent perpetual motion that has so far been observed4 ,a determination of the amount of the charge carried off by the rays of radium has been made by ,wien#abd4 ,a small quantity of radium1 placed in a sealed platinum vessel1 was hung by an insulating thread inside a glass cylinder1 which was exhausted to a low pressure4 ,a connection between the platinum vessel and an electrode sealed on to the external glass cylinder could be made1 when required1 by tilting the tube4 ,wien found that in a good vacuum the platinum vessel became charged to about #ajj volts4 ,the rate of escape of negative electricity from the platinum vessel containing #d milligrams of radium bromide corresponded to #bia #aj amperes4 ,if the charge on each particle is taken as #aa #aj electromagnetic units1 this corresponds to an escape of #bff #aj particles per second4 ,from #a gram of radium bromide the corresponding number would be #ff #aj per second4 ,since some of the rays are absorbed in their passage through the walls of the containing vessel and through the radium itself1 the actual number projected per second from #a gram of radium bromide must be greater than the above value4 ,this has been found by the writer to be the case4 ,the method employed reduced the absorption of the rays to a minimum1 and the total number emitted per second by #a gram of radium bromide in radio-active equilibrium was found to be #da #aj1 or about six times the number found by ,wien4 ,a detailed account of the method employed cannot be given with advantage at this stage1 but will be found later in ,section #bec4 #ha4 ,determination of e_/m4 ,we have seen 7,section #ej7 that1 in their passage between the plates of a condenser1 the cathode rays are deflected towards the positive plate4 ,shortly after the discovery of the magnetic deviation of the rays from radium1 ,dorn#abe and ,becquerel#abf showed that they also were deflected by an electric field4 ,by observing separately the amount of the electric and magnetic deviation1 ,becquerel was able to determine the ratio of e_/m and the velocity of the projected particles4 ,two rectangular copper plates1 #cde cms4 high and #a cm4 apart1 were placed in a vertical plane and insulated on paraffin blocks4 ,one plate was charged to a high potential by means of an influence machine1 and the other was connected with earth4 ,the active matter was placed in a narrow groove cut in a lead plate parallel to the copper plates and placed midway between them4 ,the photographic plate1 enveloped in black paper1 was placed horizontally above the plate containing the active substance4 ,the large and diffuse pencil of rays thus obtained was deflected by the electric field1 but the deviation amounted to only a few millimetres and was difficult to measure4 ,the method finally adopted was to place vertically above the active matter a thin screen of mica1 which cut the field into two equal parts4 ,thus1 in the absence of an electric field1 a narrow rectangular shadow was produced on the plate4 ,when the electric field was applied1 the rays were deflected and a part of the pencil of rays was stopped by the mica screen4 ,a shadow was thus cast on the plate which showed the direction of deviation and corresponded to the least deviable rays which gave an impression through the black paper4 ,if a particle of mass m1 charge e1 and velocity u1 is projected normally to an electric field of strength ,x1 the acceleration is in the direction of the field1 and is given by ,xe ----- 4 m ,since the particle moves with a constant acceleration parallel to the field1 the path of the particle is the same as that of a body projected horizontally from a height with a constant velocity and acted on by gravity4 ,the path of the particle is thus a parabola1 whose axis is parallel to the field and whose apex is at the point where the particle enters the electric field4 ,the linear deviation d of the ray parallel to the field after traversing a distance l is given by #a ,xe l d -- -- ---- ---- 4 #b m u ,on leaving the electric field1 the particle travels in the direction of the tangent to the path at that point4 ,if is the angular deviation of the path at that point e,xl tan ----- 4 mu ,the photographic plate was at a distance h above the extremity of the field4 ,thus the particles struck the plate at a distance d from the original path given by d#b h tan theta d#a frac ,xle mu#b 7frac l #b h7 ,in the experimental arrangement the values were d #d cms42 ,x #ajb #aj2 l #cde cms42 h #ab cms4 ,if the radius ,r of curvature of the path of the same rays is observed in a magnetic field of strength ,h perpendicular to the rays1 e ,v --- ---- m ,,hr ,combining these two equations we get u frac ,x 4 l 7frac l #b h7 ,h 4 ,r 4 d#b 4 ,a difficulty arose in identifying the part of the complex pencil of rays for which the electric and magnetic deviations were determined4 ,becquerel estimated that the value of ,,hr for the rays deflected by the electric field was about #afjj ,c4,g4,s4 units4 ,thus u #af #aj cms4 per second1 and e ---- #aj4 m ,thus these rays had a velocity more than half the velocity of light1 and an apparent mass about the same as the cathode ray particles1 i4e4 about of the mass of the hydrogen atom4 ,the ray is therefore analogous in all respects to the cathode ray1 except that it differs in velocity4 ,in a vacuum tube the cathode rays generally have a velocity of about #b #aj cms4 per sec4 ,in special tubes with strong fields the velocity may be increased to about #aj cms4 per sec4 ,these particles1 then1 behave like isolated units of negative electricity1 identical with the electrons set free by an electric discharge in a vacuum tube4 ,the electrons projected from radium have velocities varying from about #jb,v to at least #jif,v1 where ,v is the velocity of light1 and thus have an average speed considerably greater than that of the electrons produced in a vacuum tube4 ,these moving electrons are able to pass through much greater thicknesses of matter before they are absorbed than the slower electrons produced in a vacuum tube1 but the difference is one merely of degree and not of kind4 ,since electrons are continuously and spontaneously expelled from radium with enormous velocities1 they must acquire their energy of motion from the matter itself4 ,it is difficult to avoid the conclusion1 that this velocity has not been suddenly impressed on the electron4 ,such a sudden gain of velocity would mean an immense and sudden concentration of energy on a small particle1 and it is more probable that the electron before its expulsion has been in rapid orbital or oscillatory motion in the atom1 and1 by some means1 suddenly escapes from its orbit4 ,according to this view1 the energy of the electron is not suddenly created but is only made obvious by its escape from the system to which it belongs4 #hb4 ,variation of e_/m with the velocity of the electron4 ,the fact that radium throws off electrons with rates of speed varying from to the velocity of light has been utilised by ,kaufmann#abg to examine whether the ratio e_/m of the electrons varies with the speed4 ,we have seen 7,section #dh7 that1 according to the electromagnetic theory1 a charge of electricity in motion behaves as if it had apparent mass4 ,for small speeds1 this additional electrical mass is equal to #b e -- ----1 #c a where a is the radius of the body1 but it increases rapidly as the speed of light is approached4 ,it is very important to settle whether the mass of the electron is due partly to mechanical and partly to electrical mass1 or whether it can be explained by virtue of electricity in motion independently of the usual conception of mass4 ,slightly different formulae expressing the variation of mass with speed have been developed by ,j4 ,j4 ,thomson1 ,heaviside1 and ,searle4 ,to interpret his results ,kaufmann used a formula developed by ,m4 ,abraham#abh4 ,let m mass of electron for slow speeds2 m apparent mass of electron at any speed2 u velocity of electron2 ,v velocity of light4 ,let u_/,v2 then it can be shown that frac m m frac #c #d psi 7beta7 7#a71 where psi 7beta7 frac #a;beta#b 7frac #a beta#b #b;beta log frac #a beta #a beta #a7 7#b74 ,the experimental method employed to determine e_/m and u is similar to the method of crossed spectra4 ,some strongly active radium was placed at the bottom of a brass box4 ,the rays from this passed between two brass plates insulated and about #ab mm4 apart4 ,these rays fell on a platinum diaphragm1 containing a small tube about #jb mm4 in diameter1 which allowed a narrow bundle of rays to pass4 ,the rays then struck a photographic plate enveloped in a thin layer of aluminium4 ,in the experiments the diaphragm was about #b cms4 from the active material and at the same distance from the photographic plate4 ,when the whole apparatus was placed in a vacuum1 a ,p4,d4 of from #bjjj to #ejjj volts could be applied between the plates without a spark4 ,the rays were deflected in their passage through the electric field1 and produced what may be termed an electric spectrum on the plate4 ,illustration3 ,fig4 #bh4 ,if a magnetic field is superimposed parallel to the electric field by means of an electromagnet1 a magnetic spectrum is obtained perpendicular to the electric spectrum4 ,the combination of the two spectra gives rise to a curved line on the plate4 ,the double trace obtained on the photographic plate with reversal of the magnetic field is shown in ,fig4 #bh4 ,disregarding some small corrections1 it can readily be shown that if y and z are the electric and magnetic deviations respectively1 z ----- 7#c71 y and e z --- ---- 7#d74 m y ,from these two equations1 combined with 7#a71 we obtain frac y z#b psi 7kappa#a frac z y7 kappa#b 444 7#e71 where 1 1 are constants4 ,equation 7#e7 gives the curve that should be obtained on the plate according to the electromagnetic theory4 ,this is compared by trial with the actual curve obtained on the plate4 ,in this way ,kaufmann#abi found that the value of e_/m decreased with the speed1 showing that1 assuming the charge constant1 the mass of the electron increased with the speed4 ,the following numbers give some of the preliminary results obtained by this method4 ,velocity of electron e_/m #bcf #aj cms4 per #aca #aj sec4 #bdh #aag #aj #bei #jig #aj #bgb #jgg #aj #bhe #jfc #aj ,for the cathode rays ,s4 ,simon#acj obtained a value for e_/m of #ahf #aj for an average speed of about #g #aj cms4 per second4 ,in a later paper#aca with some very active radium1 more satisfactory photographs were obtained1 which allowed of accurate measurement4 ,the given equation of the curve was found to agree satisfactorily with experiment4 ,the table given below1 deduced from the results given by ,kaufmann1 shows the agreement between the theoretical and experimental values1 u being the velocity of the electron and ,v that of light4 ,the average percentage error between the observed and calculated value is thus not much more than one per cent4 ,it is remarkable how nearly the velocity of the electron has to approach the velocity of light before the value of m_/m becomes large4 ,this is shown in the following table which gives the calculated values of m_/m for different velocities of the electron4 ,value of ,observed value ,percentage u_/,v of m_/m difference from theoretical values ,small #a #gcb #acd #ae #geb #acg #ji #ggg #adb #jf #hja #adg #je #hcj #aede #je #hfj #afe #j #hhc #agc #bh #icc #bje #gh 8 #idi #bade #ab #ifc #bdb #jd ,value of small #a #e #i #ii #iii #iiii #iiiiii u_/,v ,calculated #ajj #ajae #aab #aha #cbh #dif #ffh #aja value m_/m ,thus for velocities varying from #j to the velocity of light1 the mass of the electron is practically constant4 ,the increase of mass becomes appreciable at about half the velocity of light1 and increases steadily as the velocity of light is approached4 ,theoretically the mass becomes infinite at the velocity of light1 but even when the velocity of the electron only differs from that of light by one part in a million1 its mass is only #aj times the value for slow speeds4 ,the above results are therefore in agreement with the view that the mass of the electron is altogether electrical in origin and can be explained purely by electricity in motion4 ,the value of e_/m1 for slow speeds1 deduced from the results was #ahd #aj1 which is in very close agreement with the value obtained by ,simon for the cathode rays1 viz4 #ahf #aj4 ,if the electricity carried by the electron is supposed to be distributed uniformly over a sphere of radius a1 for speeds slow compared with the velocity of light1 the apparent mass #b e m --- ---- #c a ,therefore #b e a --- ---- 4 e #c m ,taking the value of e as #aac #aj1 a is #ad #aj cms4 ,thus the diameter of an electron is minute compared with the diameter of an atom4 #hc4 ,distribution of velocity amongst the particles4 ,some interesting experiments have been recently made by ,paschen#acb to determine the relative number of particles which are expelled from radium at the different speeds4 ,the experimental arrangement is shown in ,fig4 #bi4 ,illustration3 ,fig4 #bi4 ,a small thin silvered glass tube b1 containing #ae mgrs4 of radium bromide1 was placed in the axis of a number of lead vanes arranged round a cylinder of diameter #b cms4 and length #bb cms4 ,when no magnetic field was acting1 the particles from the radium passed through the openings and were absorbed in an outer concentric cylinder aa of lead of inner diameter #cg cms4 and of thickness #ee mms4 ,this outer cylinder was rigidly connected to the inner cylinder cc by quartz rods ii1 which also served to insulate it4 ,the cylinder c and the radium were connected with earth4 ,a gold-leaf electroscope ,e was attached to a1 and the whole apparatus was enclosed in a glass vessel which was exhausted to a low vacuum by means of a mercury pump4 ,the glass vessel was placed in the uniform field of a large electromagnet1 so that the axis of the lead cylinder was parallel to the lines of force4 ,the outer cylinder gains a negative charge on account of the particles which are absorbed in it4 ,this negative charge1 which is indicated by the movement of the gold-leaf1 tends to be dissipated by the small ionization produced in the residual gas by the passage of the rays4 ,this action of the gas can be eliminated by observing the rate of movement of the gold leaf when charged alternately to an initial positive and negative potential4 ,the mean of the two rates is proportional to the number of particles which give up their charge to the lead cylinder4 ,this is evidently the case1 since1 when the charge is positive1 the ionization of the gas assists the rate of movement of the gold-leaf1 and1 when negative1 diminishes it to an equal extent4 ,when a magnetic field is applied1 each of the particles describes a curved path1 whose radius of curvature depends on the velocity of the particle4 ,for weak fields1 only the particles of smallest velocity will be deflected sufficiently not to strike the outer cylinder1 but1 as the field is raised1 the number will increase until finally all the particles fail to reach the outer cylinder4 ,the decrease of the charge communicated to the outer cylinder with the increase of the strength of the magnetic field is shown graphically in ,fig4 #cj1 ,curve ,i4 ,the ordinates represent in arbitrary units the charge communicated to the lead cylinder per second1 and thus serve as a measure of the number of particles which reach the cylinder4 ,knowing the dimensions of the apparatus1 and assuming the value e_/m found by ,kaufmann1 the velocity of the particles which just fail to reach the lead cylinder can be deduced from any strength of the magnetic field4 ,curve ,,ii1 ,fig4 #cj is the first differential of ,curve ,i1 and the ordinates represent the relative number of particles which are projected at each velocity4 ,illustration3 ,fig4 #cj4 ,from the data given by ,kaufmann 7see section #hb7 ,paschen deduced that the group of rays examined by the former1 which had velocities lying between #bab #aj and #bij #aj cms4 per second1 corresponded to the group of rays between the points ,a and ,b1 that is1 to the group of rays which were completely deflected from the lead cylinder between the magnetic fields of strengths of #ahge and #dica ,c4,g4,s4 units4 ,since radium gives off particles which require a field of strength over #gjjj units to deflect them1 ,paschen concluded that particles are expelled from radium with still greater velocities than the highest recorded by ,kaufmann4 ,paschen considered that the small charge observed in still higher fields was mainly due to the rays4 ,the effect is small and is probably not due to an actual charge carried by the rays but to a secondary effect produced by them4 ,this question will be discussed in more detail in section #aab4 ,there is a group of low velocity particles emitted by radium 7see ,fig4 #cj7 which have about the same speed as the electrons set free in a vacuum tube4 ,in consequence of their small velocity1 these probably produce a large proportion of the ionization due to the rays at short distances from the radium1 for it will be shown 7section #ajc7 that the ionization produced by an electron per unit length of path steadily decreases with increase of its velocity above a small limiting value4 ,this observation is confirmed by experiments on the absorption of the rays in passing through matter4 ,in ,paschen's experiments1 the glass tube containing the radium was #e mms4 thick1 so that a considerable proportion of the low velocity particles must have been stopped by it4 ,this is borne out by some later experiments of ,seitz which will be described in section #he4 #hd4 ,absorption of the rays by matter4 ,the particles produce ions in their passage through the gas and their energy of motion is consequently diminished4 ,a similar action takes place also when the rays pass through solid and liquid media1 and the mechanism of absorption is probably similar in all cases4 ,some of the particles in their passage through matter are completely stopped1 while others have their velocity reduced4 ,in addition1 there is a considerable scattering or diffuse reflection of the rays in traversing matter4 ,the amount of this scattering depends upon the density of the substance and also upon the angle of incidence of the rays4 ,this scattering of the rays will be discussed later in section #aaa4 ,there are two general methods of determining the absorption of the rays4 ,in the first method1 the variation of the ionization current is observed in a testing vessel when the active matter is covered by screens differing in material and thickness4 ,this ionization in the vessel depends upon two quantities1 viz4 the number of particles which pass through the matter and also upon the number of ions produced by them per unit path4 ,in the absence of any definite information in regard to the variation of ionization by the electron with its velocity1 no very definite conclusions can be drawn from such experiments4 ,the advent of pure radium-bromide has made it possible to determine the actual number of electrons which are absorbed in their passage through a definite thickness of matter1 by measuring the negative charge carried by the issuing rays4 ,experiments of this character have been made by ,seitz and will be considered later4 ,these two methods of determining the absorption of rays are quite distinct in principle1 and it is not to be expected that the values of the coefficients of absorption obtained in the two cases should be the same4 ,the whole question of the absorption of electrons by matter is very complicated1 and the difficulty is still further increased by the complexity of the rays emitted by the radio-active substances4 ,many of the results obtained by different methods1 while pointing to the same general conclusion1 are quantitatively in wide disagreement4 ,before any definite advance can be made to a better understanding of the mechanism of absorption1 it will be necessary to determine the variation of the ionization with the speed of the electron over a very wide range4 ,some work has already been done in this direction but not between sufficiently wide limits4 ,ionization method4 ,we shall first consider the results obtained on the absorption of rays by measuring the variation of the ionization current1 when screens of different thickness are placed over the active substance4 ,when the active matter is covered with aluminium foil of thickness #a mm41 the current in a testing vessel such as is shown in ,fig4 #ag1 is due almost entirely to the rays4 ,if a uranium compound is used1 it is found that the saturation current decreases with the thickness of matter traversed nearly according to an exponential law4 ,taking the saturation current as a measure of the intensity of the rays1 the intensity ,i after passing through a thickness d of matter is given by frac ,i ,i e- d 1 where is the constant of absorption of the rays and ,i is the initial intensity4 ,for uranium rays1 the current is reduced to half its value after passing through about #e mm4 of aluminium4 ,if a compound of thorium or radium is examined in the same way1 it is found that the current does not decrease regularly according to the above equation4 ,results of this kind for radium rays have been given by ,meyer and ,schweidler#acc4 ,the amount of absorption of the rays by a certain thickness of matter decreases with the thickness traversed4 ,this is exactly opposite to what is observed for the rays4 ,this variation in the absorption is due to the fact that the rays are made up of rays which vary greatly in penetrating power4 ,the rays from uranium are fairly homogeneous in character1 i4e4 they consist of rays projected with about the same velocity4 ,the rays from radium and thorium are complex1 i4e4 they consist of rays projected with a wide range of velocity and consequently with a wide range of penetrating power4 ,the electrical examination of the deviable rays thus leads to the same results as their examination by the photographic method4 ,results on the absorption of cathode rays have been given by ,lenard#acd1 who has shown that the absorption of cathode rays is nearly proportional to the density of the absorbing matter1 and is independent of its chemical state4 ,if the deviable rays from active bodies are similar to cathode rays1 a similar law of absorption is to be expected4 ,strutt#ace1 working with radium rays1 has determined the law of absorption1 and has found it roughly proportional to the density of matter over a range of densities varying from #jjda for sulphur dioxide to #bae for platinum4 ,in the case of mica and cardboard1 the values of divided by the density were #cid and #chd respectively1 while the value for platinum was #gcd4 ,in order to deduce the absorption coefficient1 he assumed that the radiation fell off according to an exponential law with the distance traversed4 ,as the rays from radium are complex1 we have seen that this is only approximately the case4 ,since the rays from uranium are fairly homogeneous1 and are at the same time penetrating in character1 they are more suitable for such a determination than the complex rays of radium4 ,i have in consequence made some experiments with uranium rays to determine the dependence of absorption on the density4 ,the results obtained are given in the following table1 where is the coefficient of absorption4 ,substance ,density _/,density ,glass #adj #bde #eg ,mica #adb #bgh #ea ,ebonite #fe #aad #eg ,wood #baf #dj #ed ,cardboard #cg #gj #ec ,iron #dd #gh #ef ,aluminium #adj #bfj #ed ,copper #fj #hf #gj ,silver #ge #aje #ga ,lead #abb #aae #ajh ,tin #if #gc #acb ,it will be observed that the value of the absorption constant divided by the density is very nearly the same for such different substances as glass1 mica1 ebonite1 wood1 iron and aluminium4 ,the divergences from the law are great1 however1 for the other metals examined1 viz4 copper1 silver1 lead and tin4 ,in tin the value of divided by the density is #be times its value for iron and aluminium4 ,these differences show that a law for the absorption of the rays depending only on the density does not hold for all substances4 ,with an exception in the case of tin1 the value of divided by the density for the metals increases in the same order as their atomic weights4 ,the absorption of the rays by matter decreases very rapidly with increase of speed4 ,for example1 the absorption of cathode rays in ,lenard's experiment 7loc4 cit47 is about #ejj times as great as for the uranium rays4 ,the velocity of the rays of uranium was found by ,becquerel to be about #af #aj cms4 per sec4 ,the velocity of the cathode rays used in ,lenard's experiment was certainly not less than of this1 so that1 for a decrease of speed of less than #aj times1 the absorption has increased over #ejj times4 #he4 ,number of electrons stopped by matter4 ,an account will now be given of the experiments made by ,seitz#acf1 to determine the relative number of electrons which are stopped in their passage through different thicknesses of matter4 ,the experimental arrangement is shown in ,fig4 #ca4 ,illustration3 ,fig4 #ca4 ,the radium was placed outside a glass vessel containing an insulated brass plate ,p1 the connection of which with a wire leading to the electrometer could be made or broken by a simple electromagnetic device4 ,the rays from the radium ,r1 after passing through openings in a brass plate ,a1 covered with thin aluminium foil1 were absorbed in the plate ,p4 ,the glass vessel was exhausted1 and the charge communicated to ,p by the rays was measured by an electrometer4 ,in a good vacuum1 the magnitude of the current observed is a measure of the number of particles absorbed by the upper plate#acg4 ,the following table shows the results obtained when different thicknesses of tin foil were placed over the radium4 ,the second table gives the ratio ,i_/,i where ,i is the rate of discharge observed before the absorbing screen is introduced4 ,the mean value of the absorption constant was deduced from the equation frac ,i ,i e- d where d is the thickness of matter traversed4 ,the values included in the brackets have not the same accuracy as the others4 ,there is thus a wide difference in penetrating power of the particles emitted from radium1 and some of them are very readily absorbed4 ,when a lead screen #c mms4 thick was placed over the radium--a thickness sufficient to absorb all the readily deflectable rays--a small negative charge was still given to the plate1 corresponding to #bi per cent4 of the maximum4 ,this is a very much smaller value than was observed by ,paschen 7see ,fig4 #cj74 ,thickness of ,i_/,i ,tin in mms4 #jjjhcd #hfi #age #jjaff #hjb #acbe #jjdba #fec #ajae #jjhah #dff #ice #jabd #cei #hbe #jaff #bhi #gdi #jbje #bcj #gae #jbgj #agj #fed #jeah #jfe #ec #jghi #jca #dd #aehe #jjei #cb #baf #jjdc #be ,this difference may1 in part1 be due to the fact that1 in ,paschen's experiments1 a large proportion of the slow velocity electrons were absorbed in the glass tube of #e mm4 thickness containing the radium4 ,seitz also determined the relative thickness1 compared with tin1 of different substances which reduced the negative charge communicated to ,p by a definite amount4 ,a few of the numbers are given below1 and expressed in terms of tin as unity4 ,substance ,thickness ,tin #a ,lead #gde ,gold #hc ,platinum #hd ,silver #a ,steel #abi ,aluminium #aef ,water #aff ,paraffin #afi ,the thickness required to stop a given proportion of the rays thus decreases with the density1 but not nearly so fast as the density increases4 ,these results are difficult to reconcile with the density-law of absorption found by ,lenard from the cathode rays1 or with the results of the ionization method already considered4 ,a further experimental examination of the whole question is very much to be desired4 #hf4 ,variation of the amount of radiation with the thickness of the layer of radiating material4 ,the radiations are sent out equally from all portions of the active mass1 but the ionization of the gas which is measured is due only to the radiations which escape into the air4 ,the depth from which the radiations can reach the surface depends on the absorption of the radiation by the active matter itself4 ,let be the absorption constant of the homogeneous radiation by the active material4 ,it can readily be shown that the intensity ,i of the rays issuing from a layer of active matter1 of thickness d1 is given by frac ,i ,i#j #a e- d 1 where ,i is the intensity at the surface due to a very thick layer4 ,this equation has been confirmed experimentally by observing the current due to the rays for different thicknesses of uranium oxide4 ,in this case ,i 77,i for a thickness of oxide corresponding to #aa gr4 per sq4 cm4 ,this gives a value of divided by density of #fc4 ,this is a value slightly greater than that observed for the absorption of the same rays in aluminium4 ,such a result shows clearly that the substance which gives rise to the rays does not absorb them to a much greater extent than does ordinary matter of the same density4 ,the value of will vary1 not only for the different active substances1 but also for the different compounds of the same substance4 ,,part ,,iii4 ,the ,rays4 #hg4 ,the rays4 ,the magnetic deviation of the rays was discovered towards the end of #ahii1 at a comparatively early stage in the history of radio-activity1 but three years elapsed before the true character of the rays was disclosed4 ,it was natural that great prominence should have been given in the early stages of the subject to the rays1 on account of their great penetrating power and marked action in causing phosphorescence in many substances4 ,the rays were1 in comparison1 very little studied1 and their importance was not generally recognized4 ,it will1 however1 be shown that the rays play a far more important part in radio-active processes than the rays1 and that the greater portion of the energy emitted in the form of ionizing radiations is due to them4 #hh4 ,the nature of the rays4 ,the nature of the rays was difficult to determine1 for a magnetic field sufficient to cause considerable deviation of the rays produced no appreciable effect on the rays4 ,it was suggested by several observers that they were1 in reality1 secondary rays set up by the or cathode rays in the active matter from which they were produced4 ,such a view1 however1 failed to explain the radio-activity of polonium1 which gave out rays only4 ,later work also showed that the matter1 which gave rise to the rays from uranium1 could be chemically separated from the uranium1 while the intensity of the rays was unaffected4 ,these and other results show that the and rays are produced quite independently of one another4 ,the view that they are an easily absorbed type of ,rntgen rays fails to explain a characteristic property of the rays1 viz4 that the absorption of the rays in a given thickness of matter1 determined by the electrical method1 increases with the thickness of matter previously traversed4 ,it does not seem probable that such an effect could be produced by a radiation like ,x rays1 but the result is to be expected if the rays consist of projected bodies1 which fail to ionize the gas when their velocity is reduced below a certain value4 ,from observations of the relative ionization produced in gases by the and rays1 ,strutt#ach suggested in #aija that the rays might consist of positively charged bodies projected with great velocity4 ,sir ,william ,crookes#aci1 in #aijb1 advanced the same hypothesis4 ,from a study of the rays of polonium ,mme4 ,curie#adj in #aijj suggested the probability that these rays consisted of bodies1 projected with great velocity1 which lost their energy by passing through matter4 ,the writer was led independently to the same view by a mass of indirect evidence which received an explanation only on the hypothesis that the rays consisted of matter projected with great velocity4 ,preliminary experiments with radium of activity #ajjj showed that it was very difficult to determine the magnetic deviation of the rays4 ,when the rays were passed through slits sufficiently narrow to enable a minute deviation of the rays to be detected1 the ionizing effect of the issuing rays was too small to be measured with certainty4 ,it was not until radium of activity #ai1jjj was obtained that it was possible to detect the deviation of these rays in an intense magnetic field4 ,how small the magnetic deviation is may be judged from the fact that the rays1 projected at right angles to a magnetic field of #aj1jjj ,c4,g4,s4 units1 describe the arc of a circle of about #ci cms4 radius1 while under the same conditions the cathode rays produced in a vacuum tube would describe a circle of about #ja cm4 radius4 ,it is therefore not surprising that the rays were for some time thought to be non-deviable in a magnetic field4 #hi4 ,magnetic deviation of the rays4 ,the general method employed#ada to detect the magnetic deviation of the rays was to allow the rays to pass through narrow slits and to observe whether the rate of discharge of an electroscope1 due to the issuing rays1 was altered by the application of a strong magnetic field4 ,fig4 #cb shows the general arrangement of the experiment4 ,the rays from a thin layer of radium of activity #ai1jjj passed upwards through a number of narrow slits ,g1 in parallel1 and then through a thin layer of aluminium foil1 #jjjcd cm4 thick1 into the testing vessel ,v4 ,the ionization produced by the rays in the testing vessel was measured by the rate of movement of the leaves of a gold-leaf electroscope ,b4 ,the gold-leaf system was insulated inside the vessel by a sulphur bead ,c1 and could be charged by means of a movable wire ,d1 which was afterwards earthed4 ,the rate of movement of the gold-leaf was observed through small mica windows in the testing vessel by means of a microscope provided with a micrometer eye-piece4 ,illustration3 ,fig4 #cb4 ,in order to increase the ionization in the testing vessel1 the rays passed through #bj to #be slits of equal width1 placed side by side4 ,this was arranged by cutting grooves at regular intervals in side-plates into which brass plates were slipped4 ,the width of the slit varied in different experiments between #jdb cm4 and #a cm4 ,the magnetic field was applied perpendicular to the plane of the paper1 and parallel to the plane of the slits4 ,the rays are thus deflected in a direction perpendicular to the plane of the slits and a very small amount of deviation is sufficient to cause the rays to impinge on the sides of the plate where they are absorbed4 ,the testing vessel and system of plates were waxed to a lead plate ,p so that the rays entered the vessel ,v only through the aluminium foil4 ,it is necessary in these experiments to have a steady stream of gas passing downwards between the plates in order to prevent the diffusion of the emanation from the radium upwards into the testing vessel4 ,the presence in the testing vessel of a small amount of this emanation1 which is always given out by radium1 would produce great ionization and completely mask the effect to be observed4 ,for this purpose1 a steady current of dry electrolytic hydrogen of about #b c4c4 per second was passed into the testing vessel2 it then streamed through the porous aluminium foil1 and passed between the plates carrying the emanation with it away from the apparatus4 ,the use of a stream of hydrogen instead of air greatly simplifies the experiment1 for it increases the ionization current due to the rays in the testing vessel1 and at the same time greatly diminishes that due to the and rays4 ,this is caused by the fact that the rays are much more readily absorbed in air than in hydrogen1 while the rate of production of ions due to the and rays is much less in hydrogen than in air4 ,the intensity of the rays after passing between the plates is consequently greater when hydrogen is used2 and since the rays pass through a sufficient distance of hydrogen in the testing vessel to be largely absorbed1 the total amount of ionization produced by them is greater with hydrogen than with air4 ,the following is an example of an observation on the magnetic deviation3-- ,pole-pieces #aij #bej cms4 ,strength of field between pole-pieces #hcgj units4 ,apparatus of #be parallel plates of length #cgj cms41 width #gj cm41 with an average air-space between plates of #jdb cm4 ,distance of radium below plates #ad cm4 ,rate of discharge of electroscope in volts per minute 7#a7 ,without magnetic field #hcc 7#b7 ,with magnetic field #agb 7#c7 ,radium covered with thin layer of #jic mica to absorb all rays 7#d7 ,radium covered with mica and #jib magnetic field applied ,the mica plate1 #ja cm4 thick1 was of sufficient thickness to absorb completely all the rays1 while it allowed the rays and rays to pass through without appreciable absorption4 ,the difference between 7#a7 and 7#c71 #gdj volts per minute1 gives the rate of discharge due to the rays alone2 the difference between 7#b7 and 7#c71 #jgi volts per minute1 that due to the rays not deviated by the magnetic field employed4 ,the amount of rays not deviated by the field is thus about #aa of the total4 ,the small difference between 7#c7 and 7#d7 measures the small ionization due to the rays1 for they would be completely deviated by the magnetic field2 7#d7 comprises the effect of the rays together with the natural leak of the electroscope in hydrogen4 ,in this experiment there was a good deal of stray magnetic field acting on the rays before they reached the pole-pieces4 ,the diminution of the rate of discharge due to the rays was found to be proportional to the strength of field between the pole-pieces4 ,with a more powerful magnetic field1 the whole of the rays were deviated1 showing that they consisted entirely of projected charged particles4 ,in order to determine the direction of deviation of the rays1 the rays were passed through slits one mm4 in width1 each of which was half covered with a brass strip4 ,the diminution of the rate of discharge in the testing vessel for a given magnetic field in such a case depends upon the direction of the field4 ,in this way it was found that the rays were deviated in the opposite sense to the cathode rays4 ,since the latter consist of negatively charged particles1 the rays must consist of positively charged particles4 ,these results were soon after confirmed by ,becquerel#adb1 by the photographic method1 which is very well adapted to determine the character of the path of the rays acted on by a magnetic field4 ,the radium was placed in a linear groove cut in a small block of lead4 ,above this source1 at a distance of about #a centimetre1 was placed a metallic screen1 formed of two plates1 leaving between them a narrow opening parallel to the groove4 ,above this was placed the photographic plate4 ,the whole apparatus was placed in a strong magnetic field parallel to the groove4 ,the strength of the magnetic field was sufficient to deflect the rays completely away from the plate4 ,when the plate was parallel to the opening1 there was produced on it an impression1 due to the rays alone1 which became more and more diffuse as the distance from the opening increased4 ,this distance should not exceed #a or #b centimetres on account of the absorption of the rays in air4 ,if1 during the exposure1 the magnetic field is reversed for equal lengths of time1 on developing the plate two images of the rays are observed which are deflected in opposite directions4 ,this deviation1 even in a strong field1 is small though quite appreciable and is opposite in sense to the deviation observed for the or cathodic rays from the same material4 ,m4 ,becquerel#adc1 by the same method1 found that the rays from polonium were deviated in the same direction as the rays from radium2 and thus that they also consist of projected positive bodies4 ,in both cases1 the photographic impressions were sharply marked and did not show the same diffusion which always appears in photographs of the rays4 #ij4 ,electrostatic deviation of the rays4 ,if the rays are charged bodies1 they should be deflected in passing through a strong electric field4 ,this was found by the writer to be the case1 but the electric deviation is still more difficult to detect than the magnetic deviation1 as the intensity of the electric field must of necessity be less than that required to produce a spark in the presence of radium4 ,the apparatus was similar to that employed for the magnetic deviation 7,fig4 #cb7 with this exception1 that the brass sides which held the plates in position1 were replaced by ebonite4 ,alternate plates were connected together and charged to a high potential by means of a battery of small accumulators4 ,the discharge in the electroscope1 due to the rays1 was found to be diminished by application of the electric field4 ,with plates #jee cm4 apart and #de cms4 high1 the diminution was only #g with a ,p4,d4 of #fjj volts between the slits4 ,with a special arrangement of plates1 with slits only #ja cm4 apart1 the discharge was diminished about #de with an electric field corresponding to #aj1jjj volts per cm4 #ia4 ,determination of the constants of the rays4 ,if the deviation of the rays in both an electric and magnetic field is known1 the values of the velocity of the rays1 and the ratio e_/m of the charge of the particle to its mass can be determined by the method1 first used by ,j4 ,j4 ,thomson for the cathode rays1 which is described in section #ej4 ,from the equations of a moving charged body1 the radius of curvature of the path of the rays in a magnetic field of strength ,h perpendicular to the path of the rays is given by m ,h ---- ,v 4 e ,if the particle1 after passing through a uniform magnetic field for a distance l1 is deviated through a small distance d from its original direction1 #b;d l or l e ,h d ----- ---- ---- 7#a74 #b m ,v ,if the rays pass through a uniform electric field of strength ,x and length l with a deviation d1 #a ,xel d --- ------ 7#b71 #b m,v since ,xe_/m is the acceleration of the particle1 at right angles to its direction1 and l_/,v is the time required to travel through the electric field4 ,from equations 7#a7 and 7#b7 d l ,x ,v ---- ----- -----1 d l ,h and e #b;d ,v ---- ---- --- 4 m l ,h ,the values of ,v and e_/m are thus completely determined from the combined results of the electric and magnetic deviation4 ,it was found that ,v #be #aj cms4 per sec4 e_/m #f #aj4 ,on account of the difficulty of obtaining a large electrostatic deviation1 these values are only approximate in character4 ,the results on the magnetic and electric deviation of the rays of radium have been confirmed by ,des ,coudres#add1 by the photographic method4 ,some pure radium bromide was used as a source of radiation4 ,the whole apparatus was enclosed in a vessel which was exhausted to a low vacuum4 ,in this way1 not only was he able to determine the photographic action of the rays at a much greater distance from the source1 but he was also able to apply a stronger electric field without the passage of a spark4 ,he found values of the constants given by ,v #afe #aj cms4 per sec4 e_/m #fd #aj4 ,these values are in very good agreement with the numbers found by the electric method4 ,the rays from radium are complex1 and probably consist of a stream of positively charged bodies projected at velocities lying between certain limits4 ,the amount of deviation of the particles in a magnetic field will thus differ according to the velocity of the particle4 ,the photographic results of ,becquerel seem to indicate that the velocity of the rays of radium can vary only within fairly narrow limits1 since the trajectory of the rays in a magnetic field is sharply marked and not nearly as diffuse as in similar experiments with the rays4 ,the evidence1 however1 discussed in the following section1 shows that the velocities of the particles from a thick layer of radium vary over a considerable range4 #ib4 ,becquerel#ade has examined the amount of magnetic deviation of the rays at different distances from the source of the rays in a very simple way4 ,a narrow vertical pencil of the rays1 after its passage through a narrow slit1 fell on a photographic plate1 which was inclined at a small angle to the vertical and had its lower edge perpendicular to the slit4 ,the trajectory of the rays is shown by a fine line traced on the plate4 ,if a strong magnetic field is applied parallel to the slit1 the trajectory of the rays is displaced to the right or left according to the direction of the field4 ,if equal times of exposure are given for the magnetic field in the two directions1 on developing the plate two fine diverging lines are found traced on the plate4 ,the distance between these lines at any point is a measure of twice the average deviation at that point1 corresponding to the value of the magnetic field4 ,by measuring the distance between the trajectories at various points1 ,becquerel found that the radius of curvature of the path of the rays increased with the distance from the slit4 ,the product ,h of the strength of the field and the radius of curvature of the path of the rays is shown in the following table4 ,distance in mms4 ,h from the slit #a #bia #aj #c #bii #e #cjf #g #cae #h #cbg #i #cda ,the writer 7loc4 cit47 showed that the maximum value of ,h for complete deviation of the rays was #cij1jjj4 ,the results are thus in good agreement4 ,since m ,h ----- ,v e these results show that the values either of ,v or of e_/m for the projected particles vary at different distances from the source4 ,becquerel considered that the rays were homogeneous1 and1 in order to explain the results1 has suggested that the charge on the projected particles may gradually decrease with the distance traversed1 so that the radius of curvature of the path steadily increases with the distance from the source4 ,it1 however1 seems more probable that the rays consist of particles projected with different velocities1 and that the slower particles are more quickly absorbed in the gas4 ,in consequence of this1 only the swifter particles are present some distance from the source4 ,this conclusion is borne out by some recent experiments of ,bragg and ,kleeman#adf on the nature of the absorption of particles by matter1 which are discussed in more detail in sections #ajc and #ajd4 ,they found that the particles from a thick layer of radium are complex1 and have a wide range of penetrating power and presumably of velocity4 ,this is due to the fact that the particles emitted from the radium come from different depths4 ,since their velocity is reduced in their transit through matter1 a pencil of rays will consist of particles which differ considerably in speed4 ,those which are just able to emerge from the radium will be absorbed in a very short depth of air1 while those that come from the surface will be able to pass through several centimetres of air before they lose their power of ionizing the gas4 ,since the particles have different velocities1 they will be unequally deflected by the magnetic field1 the slower moving particles describing a more curved path than the swifter ones4 ,consequently1 the outer edge of the trace of the pencil of rays on the photographic plate1 as obtained by ,becquerel1 will be the locus of the points where the photographic action of the particles end4 ,it was found that the particles are most efficient as ionizers of the gas just before their power of ionizing ends4 ,the loss of ionizing power of the particles seems to be fairly abrupt1 and1 for particles of the same velocity1 to occur always after traversing a definite distance in air4 ,on the assumption that the photographic as well as the ionizing action is most intense just before the particles are stopped1 and ceases fairly abruptly1 ,bragg has been able to account numerically for the measurements 7see above table7 recorded by ,becquerel4 ,quite apart from the special assumptions required for such a quantitative comparison of theory with experiment1 there can be little doubt that the increase of value of ,h with distance can be satisfactorily explained as a consequence of the complex character of the pencil of rays#adg4 ,becquerel states that the amount of deviation1 in a given magnetic field1 was the same for the rays of polonium and of radium4 ,this shows that the value of m ---- ,v e is the same for the rays from the two substances4 ,since the rays from polonium are far more readily absorbed than the rays from radium1 this result would indicate that the value of m_/e is greater for the particles of polonium than of radium4 ,further experimental evidence is required on this important point4 #ic4 ,charge carried by the rays4 ,we have seen that the negative charge carried by the particles has been readily measured4 ,since there is reason to believe 7section #bbi7 that four particles are expelled from radium for each particle1 it is to be expected that the positive charge carried by the particles should be determined still more readily4 ,all the initial experiments1 however1 made to detect this charge1 gave negative results2 and1 before successful results were obtained1 it was found necessary to eliminate some secondary actions1 which at first completely masked the effects to be looked for4 ,in consequence of the importance of this question1 a brief account will be given of the methods of measurement adopted and the special experimental difficulties which have arisen4 ,in the first place1 it must be remembered that only a small fraction of the rays1 emitted from a layer of powdered radium bromide1 escape into the surrounding gas4 ,on account of the ease with which the rays are stopped in their passage through matter1 only those escape which are expelled from a superficial layer1 and the rest are absorbed by the radium itself4 ,on the other hand1 a much larger proportion of the rays escape1 on account of their greater power of penetration4 ,in the second place1 the particle is a far more efficient ionizer of the gas than the particle1 and1 in consequence1 if the charge carried by the rays is to be determined by methods similar to those employed for the rays 7see section #hj71 the pressure of the gas surrounding the conductor to be charged must be very small in order to eliminate1 as far as possible1 the loss of charge resulting from the ionization of the residual gas by the rays#adh4 ,the experimental arrangement used by the writer is shown in ,fig4 #cc4 ,a thin film of radium was obtained on a plate ,a by evaporation of a radium solution containing a known weight of radium bromide4 ,some hours after evaporation1 the activity of the radium1 measured by the rays1 is about #be per cent4 of its maximum value1 and the rays are almost completely absent4 ,the activity measured by the and rays is then slowly regained1 and recovers its original value after about a month's interval 7see chapter ,,xi474 ,the experiments were made on the active plate when its activity was a minimum1 in order to avoid complications due to the presence of rays4 ,the film of radium was so thin that only a very small fraction of the rays was absorbed4 ,illustration3 ,fig4 #cc4 ,the active plate ,a was insulated in a metal vessel ,d1 and was connected to one pole of the battery1 the other pole being earthed4 ,the upper electrode1 which was insulated and connected with a ,dolezalek electrometer1 consisted of a rectangular copper vessel ,,bc1 the lower part of which was covered with a thin sheet of aluminium foil4 ,the rays passed through the foil1 but were stopped by the copper sides of the vessel4 ,this arrangement was found to reduce the secondary ionization produced at the surface of the upper plate4 ,the outside vessel ,d could be connected with either ,a or ,b or with earth4 ,by means of a mercury pump1 the vessel was exhausted to a very low pressure4 ,if the rays carry a positive charge1 the current between the two plates measured by the electrometer should be greater when ,a is charged positively4 ,no certain difference1 however1 between the currents in the two directions was observed1 even when a very good vacuum was obtained4 ,in some arrangements1 it was found that the current was even greater when the lower plate was negative than when it was positive4 ,an unexpected experimental result was also noticed4 ,the current between the parallel plates at first diminished with the pressure1 but soon reached a limiting value which was not altered however good a vacuum was produced4 ,for example1 in one experiment1 the current between the two parallel plates1 placed about #c mms4 apart1 was initially #fe #aj amperes and fell off directly as the pressure4 ,the current reached a limiting value of about #f #aj amperes1 or about of the value at atmospheric pressure4 ,the magnitude of this limiting current was not much altered if the air was replaced by hydrogen4 ,experiments of a similar character have been made by ,strutt#adi and ,j4 ,j4 ,thomson#aej2 using an active bismuth plate coated with radio-tellurium 7polonium7 after ,marckwald's method4 ,this substance emits only rays1 and is thus especially suitable for experiments of this kind4 ,strutt employed the method used by him to show the charge carried by the rays 7,fig4 #bg74 ,he found1 however1 that1 even in the lowest possible vacuum1 the electroscope rapidly lost its charge and at the same rate whether it was charged positively or negatively4 ,this is in agreement with the results found by the writer with radium4 ,in the experiments of ,j4 ,j4 ,thomson1 the electroscope was attached to a metal disc placed #c cms4 from the plate of radio-tellurium4 ,a very low vacuum was produced by ,dewar's method by absorbing the residual gas in cocoanut charcoal immersed in liquid air4 ,when the electroscope was charged negatively1 an extremely slow rate of leak was observed1 but when charged positively the leak was about #ajj times greater4 ,this showed that the polonium gave out large quantities of negative electricity1 but not enough positive to be detected4 ,by placing the apparatus in a strong magnetic field1 the negative particles were prevented from reaching the electroscope and the positive leak was stopped4 ,these results indicate that these negative particles are not projected with sufficient velocity to move against the repulsion exerted by the electrified body1 and are bent by a magnetic field4 ,there thus seems little doubt that a stream of negative particles 7electrons7 is projected from the active surface at a very slow speed4 ,such low velocity electrons are also projected from uranium and radium4 ,it is probable that these electrons are a type of secondary radiation1 set up at the surfaces on which the rays fall4 ,the particles would be extremely readily absorbed in the gas1 and their presence would be difficult to detect except in low vacua4 ,j4 ,j4 ,thomson at first obtained no evidence that the particles of polonium were charged2 but in later experiments1 where the plates were closer together1 the electroscope indicated that the rays did carry a positive charge4 ,in order to see whether the positive charge due to the rays from radium could be detected when the slow moving ions were prevented from escaping by a magnetic field1 ,i placed the apparatus of ,fig4 #cc between the pole-pieces of a large electromagnet1 so that the magnetic field was parallel to the plane of the plates#aea4 ,a very marked alteration was observed both on the magnitude of the positive and negative currents4 ,in a good vacuum1 the upper plate received a positive charge1 independently of whether the lower plate was charged positively or negatively or was connected with earth4 ,after the magnetic field had reached a certain value1 a great increase in its strength had no appreciable effect on the magnitude of the current4 ,the following table illustrates the results obtained when the two plates were #c mms4 apart1 and were both coated with thin aluminium foil4 ,potential ,current in arbitrary units of lower plate ,without magnetic ,with magnetic field field #j -- #cf #b volts #bj #df #ci #b #be #cc #d #bh #dg #da #d #ce #ce #h #ca #ef #dc #h #dj #ca #hd #ce #gg #ej #hd #eb #bd ,let n be the number of particles1 carrying a charge e1 which are absorbed in the upper plate4 ,let be the current due to the slight ionization of the residual gas4 ,if only a small potential is applied to the lower plate1 this current should be equal in magnitude but opposite in sign when the potential is reversed4 ,let be the charge per sec4 communicated to the upper electrode when the lower plate is charged positively and the value when charged negatively4 ,then ne1 ne2 adding we get ne ------ 4 #b ,now in the third column of the above table it is seen that 7 7_/#b has the values #ci1 #da1 #dc for #b1 #d1 and #h volts respectively4 ,the numbers are thus in fairly good agreement4 ,similar results were obtained when a brass plate was substituted for the upper electrode shown in the figure4 ,taking into consideration that the magnitude of ne is independent of the strength of the magnetic field above a certain small value1 and the good agreement of the numbers obtained with variation of voltage1 ,i think that there can be no doubt that the positive charge communicated to the upper electrode was carried by the particles4 ,this positive charge was not small1 for using a weight of #dh mgrs4 radium bromide spread in a thin foil over an area of about #bj sq4 cms41 the charge communicated by the particles corresponded to a current #hh #aj amperes1 and1 with the ,dolezalek electrometer employed1 it was necessary to add a capacity of #jjbd microfarads to the electrometer system4 ,in these experiments1 the film of radium bromide was so thin1 that only a very small percentage of the particles was stopped by the radium itself4 ,assuming that each particle carries the same charge as an ion1 viz4 #aa #aj coulombs1 and remembering that half of the particles are absorbed in the lower plate1 the total number ,n of particles expelled per second from one gram of radium bromide 7at its minimum activity7 can be deduced4 ,in two separate experiments where the amount of radium used was #aid and #dhd mgrs4 respectively1 the values of ,n were in close agreement and equal to #cf #aj4 ,now it will be shown later that in radium there are three other products in radio-active equilibrium1 each of which probably gives out the same number of particles as radium itself4 ,if this is the case1 the total number of particles expelled per second from #a gram of radium bromide in radio-active equilibrium is #d,n or #add #aj4 ,assuming the composition of radium bromide as ,ra,br1 the number per second per gram of radium is #be #aj4 ,this number will be found to be in very good agreement with that deduced from indirect data 7chapter ,,xiii474 ,the value of ,n is of great importance in determining the magnitude of various quantities in radio-active calculations4 #id4 ,mass and energy of the particle4 ,it has been pointed out that the rays from radium and polonium are analogous to the ,canal rays of ,goldstein1 for both carry a positive charge and are difficult to deflect by a magnetic field4 ,the experiments of ,wien have shown that the velocity of projection of the canal rays varies with the gas in the tube and the intensity of the electric field applied1 but it is generally about of the velocity of the particle from radium4 ,the value of e_/m is also variable1 depending upon the gas in the tube4 ,it has been shown that for the rays of radium e ,v #be #aj and ---- #f #aj4 m ,now the value of e_/m for the hydrogen atom1 liberated in the electrolysis of water1 is #aj4 ,assuming the charge carried by the particle to be the same as that carried by the hydrogen atom1 the mass of the particle is about twice that of the hydrogen atom4 ,taking into consideration the uncertainty attaching to the experimental value of e_/m for the particle1 if the particle consists of any known kind of matter1 this result indicates that it consists either of projected helium or hydrogen4 ,further evidence on this important question is given in section #bfj4 ,the rays from all the radio-active substances and their products1 such as the radio-active emanations and the matter causing excited activity1 possess the same general properties and do not vary very much in penetrating power4 ,it is thus probable that in all cases the rays from the different radio-active substances consist of positively charged bodies projected with great velocity4 ,since the rays from radium are made up in part of rays from the emanation stored in the radium1 and from the excited activity which it produces1 the rays from each of these products must consist of positively charged bodies2 for it has been shown that all the rays from radium are deviated in a strong magnetic field4 ,the kinetic energy of each projected particle is enormous1 compared with its mass4 ,the kinetic energy of each particle is #a #a m --- m,v -- --- ,ve #ei #aj ergs4 #b #b e ,taking the velocity of a rifle bullet as #aj cms4 per second1 it is seen that1 mass for mass1 the energy of motion of the rays is #f #aj times as great as that of the rifle bullet4 ,in this projection of bodies atomic in size with great velocity probably lies the principal cause of the heating effects produced by radium 7chapter ,,xii74 #ie4 ,atomic disintegration4 ,the radio-activity of the radio-elements is an atomic and not a molecular property4 ,the rate of emission of the radiations depends only on the amount of the element present and is independent of its combination with inactive substances4 ,in addition1 it will be shown later that the rate of emission is not affected by wide variations of temperature1 or by the application of any known chemical or physical forces4 ,since the power of radiating is a property of the radio-atoms1 and the radiations consist for the most part of positively and negatively charged masses projected with great velocity1 it is necessary to suppose that the atoms of the radio-elements are undergoing disintegration1 in the course of which parts of the atom escape from the atomic system4 ,it seems very improbable that the and particles can suddenly acquire their enormous velocity of projection by the action of forces existing inside or outside the atom4 ,for example1 the particle would have to travel from rest between two points differing in potential by #eb million volts in order to acquire the kinetic energy with which it escapes4 ,thus it seems probable that these particles are not set suddenly in motion1 but that they escape from an atomic system in which they were already in rapid oscillatory or orbital motion4 ,on this view1 the energy is not communicated to the projected particles1 but exists beforehand in the atoms from which they escape4 ,the idea that the atom is a complicated structure consisting of charged parts in rapid oscillatory or orbital motion has been developed by ,j4 ,j4 ,thomson1 ,larmor and ,lorentz4 ,since the particle is atomic in size1 it is natural to suppose that the atoms of the radio-active elements consist not only of the electrons in motion1 but also of positively charged particles whose mass is about the same as that of the hydrogen or helium atom4 ,it will be shown later that only a minute fraction of the atoms of the radio-element need break up per second in order to account for the radiations even of an enormously active element like radium4 ,the question of the possible causes which lead to this atomic disintegration and the consequences which follow from it will be discussed later in chapter ,,xiii4 #if4 ,experiments with a zinc sulphide screen4 ,a screen of ,sidot's hexagonal blend 7phosphorescent crystalline zinc sulphide7 lights up brightly under the action of the rays of radium and polonium4 ,if the surface of the screen is examined with a magnifying glass1 the light from the screen is found not to be uniformly distributed but to consist of a number of scintillating points of light4 ,no two flashes succeed one another at the same point1 but they are scattered over the surface1 coming and going rapidly without any movement of translation4 ,this remarkable action of the radium and polonium rays on a zinc sulphide screen was discovered by ,sir ,william ,crookes#aeb1 and independently by ,elster and ,geitel#aec1 who observed it with the rays given out from a wire which has been charged negatively either in the open air or in a vessel containing the emanation of thorium4 ,in order to show the scintillations of radium on the screen1 ,sir ,william ,crookes has devised a simple apparatus which he has called the 8,spinthariscope40 ,a small piece of metal1 which has been dipped in a radium solution1 is placed several millimetres away from a small zinc sulphide screen4 ,this screen is fixed at one end of a short brass tube and is looked at through a lens fixed at the other end of the tube4 ,viewed in this way1 the surface of the screen is seen as a dark background1 dotted with brilliant points of light which come and go with great rapidity4 ,the number of points of light per unit area to be seen at one time falls off rapidly as the distance from the radium increases1 and1 at several centimetres distance1 only an occasional one is seen4 ,the experiment is extremely beautiful1 and brings vividly before the observer the idea that the radium is shooting out a stream of projectiles1 the impact of each of which on the screen is marked by a flash of light4 ,the scintillating points of light on the screen are the result of the impact of the particles on its surface4 ,if the radium is covered with a layer of foil of sufficient thickness to absorb all the rays the scintillations cease4 ,there is still a phosphorescence to be observed on the screen due to the and rays1 but this luminosity is not marked by scintillations to any appreciable extent4 ,sir ,william ,crookes showed that the number of scintillations was about the same in vacuo as in air at atmospheric pressure4 ,if the screen was kept at a constant temperature1 but the radium cooled down to the temperature of liquid air1 no appreciable difference in the number of scintillations was observed4 ,if1 however1 the screen was gradually cooled to the temperature of liquid air1 the scintillations diminished in number and finally ceased altogether4 ,this is due to the fact that the screen loses to a large extent its power of phosphorescence at such a low temperature4 ,not only are scintillations produced by radium1 actinium1 and polonium1 but also by the emanations and other radio-active products which emit rays4 ,in addition1 ,f4 ,h4 ,glew#aed has found that they can be observed from the metal uranium1 thorium compounds and various varieties of pitchblende4 ,in order to show the scintillations produced by pitchblende1 a flat surface was ground1 and a transparent screen1 whose lower surface was coated with zinc sulphide1 placed upon it4 ,glew has designed a modified and very simple form of spinthariscope4 ,a transparent screen1 coated on one side with a thin layer of zinc sulphide1 is placed in contact with the active material1 and the scintillations observed by a lens in the usual way4 ,since there is no absorption in the air1 the luminosity is a maximum4 ,the relative transparency of different substances placed between the active material and the screen may1 in this way1 be directly studied4 ,the production of scintillations appears to be a general property of the rays from all radio-active substances4 ,the scintillations are best shown with a zinc sulphide screen2 but are also observed with willemite 7zinc silicate71 powdered diamond1 and potassium platinocyanide 7,glew1 loc4 cit474 ,if a screen of barium platinocyanide is exposed to the rays from radium1 the scintillations are difficult to observe1 and the luminosity is far more persistent than for a zinc sulphide screen exposed under the same conditions4 ,the duration of the phosphorescence in this case probably accounts for the absence of visible scintillations4 ,there can be no doubt that the scintillations result from the continuous bombardment of the sensitive screen by the particles4 ,each of these particles moves with enormous velocity1 and has a considerable energy of motion4 ,on account of the ease with which these particles are stopped1 most of this energy is given up at the surface of the screen1 and a portion of the energy is in some way transformed into light4 ,zinc sulphide is very sensitive to mechanical shocks4 ,luminosity is observed if a penknife is drawn across the screen1 or if a current of air is directed on to the screen4 ,the disturbance effected by the impact of the particle extends over a distance very large compared with the size of the impinging particle1 so that the spots of light produced have an appreciable area4 ,recently ,becquerel#aee has made an examination of the scintillations produced by different substances1 and has concluded that the scintillations are due to irregular cleavages in the crystals composing the screen1 produced by the action of the rays4 ,scintillations can be mechanically produced by crushing a crystal4 ,tommasina#aef found that a zinc sulphide screen removed from the action of the radium rays for several days1 showed the scintillations again when an electrified rod was brought near it4 ,the number of scintillations produced in zinc sulphide depends upon the presence of a slight amount of impurity and on its crystalline state4 ,it can be shown that even with the most sensitive zinc sulphide screens1 the number of scintillations is probably only a small fraction of the total number of particles which fall upon it4 ,it would appear that the crystals are in some way altered by the bombardment of the particles1 and that some of the crystals occasionally break up with emission of light#aeg4 ,although the scintillations from a particle of pure radium bromide are very numerous1 they are not too numerous to be counted4 ,close to the radium1 the luminosity is very bright1 but by using a high power microscope the luminosity can still be shown to consist of scintillations4 ,since the number of scintillations probably bears no close relation to the number of particles emitted1 a determination of the number of scintillations would have no special physical significance4 ,the relation between the number of particles and the number of scintillations would probably be variable1 depending greatly on the exact chemical composition of the sensitive substance and also upon its crystalline state4 #ig4 ,absorption of the rays by matter4 ,the rays from the different radio-active substances can be distinguished from one another by the relative amounts of their absorption by gases or by thin screens of solid substances4 ,when examined under the same conditions1 the rays from the active substances can be arranged in a definite order with reference to the amount of absorption in a given thickness of matter4 ,in order to test the amount of absorption of the rays for different thicknesses of matter1 an apparatus similar to that shown in ,fig4 #ag1 p4 #ih1 was employed#aeh4 ,a thin layer of the active material was spread uniformly over an area of about #cj sq4 cms41 and the saturation current observed between two plates #ce cms4 apart4 ,with a thin layer#aei of active material1 the ionization between the plates is due almost entirely to the rays4 ,the ionization due to the and rays is generally less than #a of the total4 ,the following table shows the variation of the saturation current between the plates due to the rays from radium and polonium1 with successive layers of aluminium foil interposed1 each #jjjcd cm4 in thickness4 ,in order to get rid of the ionization due to the rays from radium1 the radium chloride employed was dissolved in water and evaporated4 ,this renders the active compound1 for the time1 nearly free from rays4 ,the initial current with #a layer of aluminium over the active material is taken as #ajj4 ,it will be observed that the current due ,polonium4 ,radium4 ,layers of ,current ,ratio of ,layers of ,current ,ratio of aluminium decrease aluminium decrease for each for each layer layer #j #ajj #j #ajj #da #dh #a #da #a #dh #ca #dh #b #abf #b #bc #ag #fj #c #ba #c #acf #jfg #dg #d #ad #d #fd #ci #e #j #e #be #cf #f #i #g #j to the radium rays decreases very nearly by half its value for each additional thickness until the current is reduced to about #f of the maximum4 ,it then decays more rapidly to zero4 ,thus1 for radium1 over a wide range1 the current decreases approximately according to an exponential law with the thickness of the screen1 or frac i i e d 1 where i is the current for a thickness d1 and i the initial current4 ,in the case of polonium1 the decrease is far more rapid than would be indicated by the exponential law4 ,by the first layer1 the current is reduced to the ratio #da4 ,the addition of the third layer cuts the current down to a ratio of #ag4 ,for most of the active bodies1 the current diminishes slightly faster than the exponential law would lead one to expect1 especially when the radiation is nearly all absorbed4 #ih4 ,the increase of absorption of the rays of polonium with the thickness of matter traversed has been very clearly shown in some experiments made by ,mme ,curie4 ,the apparatus employed is shown in ,fig4 #cd4 ,illustration3 ,fig4 #cd4 ,the saturation current was measured between two parallel plates ,,pp #c cms4 apart4 ,the polonium ,a was placed in the metal box ,,cc1 and the rays from it1 after passing through an opening in the lower plate ,p1 covered with a layer of thin foil ,t1 ionized the gas between the plates4 ,for a certain distance ,,at1 of #d cms4 or more1 no appreciable current was observed between ,p and ,p4 ,as the distance ,,at was diminished1 the current increased in a very sudden manner1 so that for a small variation of the distance ,,at there was a large increase of current4 ,with still further decrease of distance the current increases in a more regular manner4 ,the results are shown in the following table1 where the screen ,t consisted of one and two layers of aluminium foil respectively4 ,the current due to the rays1 without the aluminium screen1 is in each case taken as #ajj4 ,distance ,,at in cms4 #ce #be #ai #ade #je ,for #ajj rays #j #j #e #aj #be transmitted by one layer ,for #ajj rays #j #j #j #j #jg transmitted by two layers ,the metallic screen thus cuts off a greater proportion of the rays the greater the distance of air which the radiations traverse4 ,the effects are still more marked if the plates ,,pp are close together4 ,results similar but not so marked are found if radium is substituted for the polonium4 ,it follows from these experiments that the ionization per unit volume1 due to a large plate uniformly covered with the radio-active matter1 falls off rapidly with the distance from the plate4 ,at a distance of #aj cms4 the rays from uranium1 thorium1 or radium have been completely absorbed in the gas1 and the small ionization then observed in the gas is due to the more penetrating and rays4 ,the relative amount of the ionization observed at a distance from the source will increase with the thickness of the layer of active matter1 but will reach a maximum for a layer of a certain thickness4 ,the greater proportion of the ionization1 due to unscreened active matter1 is thus entirely confined to a shell of air surrounding it not more than #aj cms4 in depth4 ,illustration3 ,fig4 #ce4 #ii4 ,the rays from different compounds of the same active element1 although differing in amount1 have about the same average penetrating power4 ,experiments on this point have been made by the writer#afj and by ,owens#afa4 ,thus in comparing the relative power of penetration of the rays from the different radio-elements1 it is only necessary to determine the penetrating power for one compound of each of the radio-elements4 ,rutherford and ,miss ,brooks#afb have determined the amount of absorption of the rays from the different active substances in their passage through successive layers of aluminium foil #jjjcd cm4 thick4 ,the curves of absorption are given in ,fig4 #ce4 ,for the purpose of comparison in each case1 the initial current with the bare active compound was taken as #ajj4 ,a very thin layer of the active substance was used1 and1 in the case of thorium and radium1 the emanations given off were removed by a slow current of air through the testing vessel4 ,a potential difference of #cjj volts was applied between the plates1 which was sufficient to give the maximum current in each case4 ,curves for the minerals organite and thorite were very nearly the same as for thoria4 ,for comparison1 the absorption curves of the excited radiations of thorium and radium are given1 as well as the curve for the radio-elements uranium1 thorium1 radium1 and polonium4 ,the radiations may be arranged in the following order1 as regards their power of penetration1 beginning with the most penetrating4 ,thorium ,radium excited radiation4 ,thorium4 ,radium4 ,polonium4 ,uranium4 ,the same order is observed for all the absorbing substances examined1 viz41 aluminium1 ,dutch metal1 tinfoil1 paper1 and air and other gases4 ,the differences in the absorption of the rays from the active bodies are thus considerable1 and must be ascribed either to a difference of mass or of velocity of the particles or to a variation in both these quantities4 ,since the rays differ either in mass or velocity1 it follows that they cannot be ascribed to any single radio-active impurity common to all radio-active bodies4 #ajj4 ,absorption of the rays by gases4 ,the rays from the different radio-active substances are quickly absorbed in their passage through a few centimetres of air at atmospheric pressure and temperature4 ,in consequence of this1 the ionization of the air1 due to the rays1 is greatest near the surface of the radiating body and falls off very rapidly with the distance 7see section #ih74 ,illustration3 ,fig4 #cf4 ,a simple method of determining the absorption in gases is shown in ,fig4 #cf4 ,the maximum current is measured between two parallel plates ,a and ,b kept at a fixed distance of #b cms4 apart1 and then moved by means of a screw to different distances from the radio-active surface4 ,the radiation from this active surface passed through a circular opening in the plate ,a1 covered with thin aluminium foil1 and was stopped by the upper plate4 ,for observations on other gases besides air1 and for examining the effect at different pressures1 the apparatus is enclosed in an air-tight cylinder4 ,if the radius of the active surface is large compared with the distance of the plate ,a from it1 the intensity of the radiation is approximately uniform over the opening in the plate ,a1 and falls off with the distance x traversed according to an exponential law4 ,thus frac ,i ,i e- x 1 where is the 8absorption constant0 of the radiation for the gas under consideration#afc4 ,let x distance of lower plate from active material1 l distance between the two fixed plates4 ,the energy of the radiation at the lower plate is then ,i e- x 1 and at the upper plate ,i e- 7l x7 4 ,the total number of ions produced between the parallel plates ,a and ,b is therefore proportional to e- x e- 7l x7 e- x 7#a - e- l7 4 ,since the factor #a e- l is a constant1 the saturation current between ,a and ,b varies as e- x 1 i4e4 it decreases according to an exponential law with the distance traversed4 ,illustration3 ,fig4 #cg4 ,the variation of the current between ,a and ,b with the distance from a thin layer of uranium oxide is shown in ,fig4 #cg for different gases4 ,the initial measurements were taken at a distance of about #ce mms4 from the active surface4 ,the actual values of this initial current were different for the different gases1 but1 for the purposes of comparison1 the value is in each case taken as unity4 ,it will be seen that the current falls off with the distance approximately in a geometrical progression1 a result which is in agreement with the simple theory given above4 ,the distance through which the rays pass before they are absorbed is given below for different gases4 ,gas ,distance in mms4 to absorb half of radiation ,carbonic acid #c ,air #dc ,coal-gas #ge ,hydrogen #af ,the results for hydrogen are only approximate1 as the absorption is small over the distance examined4 ,the absorption is least in hydrogen and greatest in carbonic acid1 and follows the same order as the densities of the gases4 ,in the case of air and carbonic acid1 the absorption is proportional to the density1 but this rule is widely departed from in the case of hydrogen4 ,results for the relative absorption by air of the rays from the different active bodies are shown in ,fig4 #ch4 ,illustration3 ,fig4 #ch4 ,the initial observation was made about #b mms4 from the active surface1 and the initial current is in each case taken as #ajj4 ,the current1 as in the case of uranium1 falls off at first approximately in geometrical progression with the distance4 ,the thickness of air1 through which the radiation passes before the intensity is reduced to half value1 is given below4 ,distance in mms4 ,uranium #dc ,radium #ge ,thorium #aj ,excited radiation #afe from ,thorium and ,radium ,the order of absorption by air of the radiations from the active substances is the same as the order of absorption by the metals and solid substances examined4 #aja4 ,connection between absorption and density4 ,since in all cases the radiations first diminish approximately according to an exponential law with the distance traversed1 the intensity ,i after passing through a thickness x is given by ,i ,i e- x where is the absorption constant and ,i the initial intensity4 ,the following table shows the value of with different radiations for air and aluminium4 ,radiation for for air aluminium ,excited radiation #hcj #db ,thorium #abej #fi ,radium #afjj #ij ,uranium #bgej #af ,taking the density of air at #bj ,c4 and #gfj mms4 as #jjjabj compared with water as unity1 the following table shows the value of divided by density for the different radiations4 ,radiation ,aluminium ,air ,excited radiation #cbj #cej ,thorium #dhj #eej ,radium #fbj #gdj ,uranium #ajfj #acjj ,comparing aluminium and air1 the absorption is thus roughly proportional to the density for all the radiations4 ,the divergence1 however1 between the absorption-density numbers is large when two metals like tin and aluminium are compared4 ,the value of for tin is not much greater than for aluminium1 although the density is nearly three times as great4 ,if the absorption is proportional to the density1 the absorption in a gas should vary directly as the pressure1 and this is found to be the case4 ,some results on this subject have been given by the writer 7loc4 cit47 for uranium rays between pressures of and #a atmosphere4 ,owens 7loc4 cit47 examined the absorption of the radiation in air from thoria between the pressures of #je to #c atmospheres and found that the absorption varied directly as the pressure4 ,the variation of absorption with density for the projected positive particles is thus very similar to the law for the projected negative particles and for cathode rays4 ,the absorption1 in both cases1 depends mainly on the density1 but is not in all cases directly proportional to it4 ,since the absorption of the rays in gases is probably mainly due to the exhaustion of the energy of the rays by the production of ions in the gas1 it seems probable that the absorption in metals is due to a similar cause4 #ajb4 ,relation between ionization and absorption in gases4 ,it has been shown 7section #de7 that if the rays are completely absorbed in a gas1 the total ionization produced is about the same for all the gases examined4 ,since the rays are unequally absorbed in different gases1 there should be a direct connection between the relative ionization and the relative absorption4 ,this is seen to be the case if the results of ,strutt 7section #de7 are compared with the relative absorption constants 7section #ajj74 ,gas ,relative ,relative absorption ionization ,air #a #a ,hydrogen #bg #bbf ,carbon dioxide #adc #aec ,considering the difficulty of obtaining accurate determinations of the absorption1 the relative ionization in a gas is seen to be directly proportional to the relative absorption within the limits of experimental error4 ,this result shows that the energy absorbed in producing an ion is about the same in air1 hydrogen1 and carbon dioxide4 #ajc4 ,mechanism of the absorption of rays by matter4 ,the experiments1 already described1 show that the ionization of the gas1 due to the rays from a large plane surface of radio-active matter1 falls off in most cases approximately according to an exponential law1 until most of the rays are absorbed1 whereupon the ionization decreases at a much faster rate4 ,in the case of polonium1 the ionization falls off more rapidly than is to be expected on the simple exponential law4 ,the ionization produced in the gas is due to the collision of the rapidly moving particles with the molecules of the gas in their path4 ,on account of its large mass1 the particle is a far more efficient ionizer than the particle moving at the same speed4 ,it can be deduced from the results of experiment that each projected particle is able to produce about #ajj1jjj ions in passing through a few centimetres of the gas before its velocity is reduced to the limiting value1 below which it no longer ionizes the gas in its path4 ,energy is required to ionize the gas1 and this energy can only be obtained at the expense of the kinetic energy of the projected particle4 ,thus it is to be expected that the particle should gradually lose its velocity and energy of motion in its passage through the gas4 ,since the rate of absorption of the rays in gases is deduced from measurements of the ionization of the gas at different distances from the source of radiation1 a knowledge of the law of variation of the ionizing power of the projected particle with its speed is required in order to interpret the results4 ,the experimental data on this question are1 however1 too incomplete to be applied directly to a solution of this question4 ,townsend#afd has shown that a moving electron produces ions in the gas after a certain limiting velocity is reached4 ,the number of ions produced per centimetre of its path through the gas then rises to a maximum1 and for still higher speeds continuously decreases4 ,for example1 ,townsend found that the number of ions produced by an electron moving in an electric field was small at first for weak fields1 but increased with the strength of the electric field to a maximum corresponding to the production of #bj ions per cm4 of path in air at a pressure of #a mm4 of mercury4 ,durack#afe found that the electrons1 generated in a vacuum tube1 moving with a velocity of about #e #aj cms4 per second produced a pair of ions every #e cms4 of path at #a mm4 pressure4 ,in a later paper1 ,durack showed that for the electrons from radium1 which are projected with a velocity greater than half the velocity of light1 a pair of ions was produced every #aj cms4 of path4 ,the high speed electron from radium is thus a very inefficient ionizer and produces only about of the ionization per unit path observed by ,townsend for the slow moving electron4 #ajd4 ,in the case of the particle1 no direct measurements have been made upon the variation of the ionization with the velocity of the particle1 so that the law of absorption of the rays cannot be deduced directly4 ,an indirect attack upon the question has1 however1 been made recently by ,bragg and ,kleeman#aff who have formulated a simple theory to account for the experimental results which they have obtained upon the absorption of the rays4 ,the particles from each simple type of radio-active matter are supposed to be projected with the same velocity1 and to pass through a definite distance a in air at atmospheric pressure and temperature before they are all absorbed4 ,as a first approximation the ionization per unit path is supposed to be the same over the whole length traversed before absorption1 and to cease fairly suddenly at a definite distance from the source of radiation4 ,this is in agreement with the observed fact that the ionization between parallel plates increases very rapidly when it approaches nearer than a certain distance to the radiant source4 ,the range a depends upon the initial energy of motion of the particle and will thus be different for different kinds of radio-active matter4 ,if a thick layer of radio-active matter is employed1 only the particles from the surface have a range a4 ,those which reach the surface from a depth d have their range diminished by an amount d1 where is the density of the radio-active matter compared with air4 ,this is merely an expression of the fact that the absorption of the rays is proportional to the thickness and density of matter traversed4 ,the rays from a thick layer of active matter will thus be complex1 and will consist of particles of different velocity whose ranges have all values between #j and a4 ,suppose that a narrow pencil of rays is emitted from a thick layer of radio-active material1 and confined by metal stops as in ,fig4 #ci4 ,illustration3 ,fig4 #ci4 ,the pencil of rays passes into an ionization vessel ,,ab through a fine wire gauze ,a4 ,the amount of ionization is to be determined between ,a and ,b for different distances h from the source of the rays ,r to the plate ,a4 ,all the particles coming from a depth x of the material given by h a x will enter the ionization vessel4 ,the number of ions produced in a depth dh of the ionization vessel is equal to nxdh1 i4e4 to a h n --------- dh1 where n is a constant4 ,if the depth of the ionization vessel be b1 the total number of ions produced in the vessel is inthhb n frac a-h rho dh frac nb rho 7a h - frac b #b7 4 ,this supposes that the stream of particles passes completely across the vessel4 ,if not1 the expression becomes intha n frac a h rho dh frac n 7a h7#b #brho 4 ,if the ionization in the vessel ,,ab is measured1 and a curve plotted showing its relation to h1 the curve in the former case should be a straight line whose slope is nb_/ and in the latter a parabola4 ,thus if a thin layer of radio-active material is employed and a shallow ionization vessel1 the ionization would be represented by a curve such as ,,apm 7,fig4 #dj71 where the ordinates represent distances from the source of radiation1 and the abscissae the ionization current between the plates ,,ab4 ,illustration3 ,fig4 #dj4 ,in this case1 ,,pm is the range of the particles from the lowest layer of the radio-active matter4 ,the current should be constant for all distances less than ,,pm4 ,for a thick layer of radio-active matter1 the curve should be a straight line such as ,,apb4 ,curves of the above character should only be obtained when definite cones of rays are employed1 and where the ionization vessel is shallow and includes the whole cone of rays4 ,in such a case the inverse square law need not be taken into account4 ,in the experiments previously recorded 7sections #ii and #ajj71 the ionization was measured between parallel plates several centimetres apart for a large area of radio-active material4 ,such an arrangement was necessary at the time at which the experiments were made1 as only weak radio-active material was available4 ,measurable electrical effects could not then be obtained with narrow cones of rays and shallow ionization vessels1 but this disadvantage is removed by the advent of pure radium bromide as a source of radiation4 ,the interesting experiments described by ,bragg and ,kleeman show that the theoretical curves are approximately realized in practice4 ,the chief difficulty experienced in the analysis of the experimental results was due to the fact that radium is a complex radio-active substance and contains four radio-active products each of which gives rise to rays which have different ranges4 ,the general character of the results obtained from radium are shown graphically in ,fig4 #da1 curves ,a1 ,b1 ,c1 ,d4 ,illustration3 ,fig4 #da4 ,the ordinates represent the distance between the radium and the gauze of the testing vessel2 the abscissae the current in the ionization vessel in arbitrary units4 ,five milligrams of radium bromide were used1 and the depth of the ionization vessel was about #e mms4 ,curve ,a is for a cone of rays of angle #bj4 ,the initial current at a distance of #g cms4 is due to the and rays and natural leak4 ,this curve is initially parabolic1 and then is made up of two straight lines4 ,curve ,b is for a smaller cone1 and shows the straight line character of the curve to within a short distance of the radium4 ,curve ,c was obtained under the same condition as curve ,a1 but with a layer of gold beater's skin placed over the radium4 ,the effect of this is to reduce all the ordinates of curve ,a by the same quantity4 ,this is to be expected on the simple theory already considered4 ,curve ,d was obtained when the radium was heated so as to get rid of the emanation and its products4 ,the particles of greatest range are quite absent and the curve is simpler in character4 ,illustration3 ,fig4 #db4 ,the complex character of the radium curves are more clearly brought out by a careful examination of a portion of the curve at distances between #b and #e cms4 from the radium1 using an ionization vessel of depth only #b mms4 ,the results are shown in ,fig4 #db1 where the curve is seen to consist approximately of four straight lines of different slopes represented by ,,pq1 ,,qr1 ,,rs1 ,,st4 ,such a result is to be expected1 for it will be shown later that four distinct ray products exist in radium when in radio-active equilibrium4 ,each of these products of radium emits an equal number of particles per second1 but the range of each is different4 ,if a is the range of one stream1 a of another1 the ionization in the vessel ,,ab1 when two streams enter the vessel1 should be nb nb ---- 7a-h-b_/#b7 ----- 7a h b_/#b71 i4e4 nb ---- 7a a #b;h b7 4 ,thus the slope of the curve should in this case be #bnb_/1 while if only one stream enters1 it should be nb_/4 ,when three reach it1 the slope should be #cnb_/ and for four #dnb_/4 ,these results are realized fairly closely in practice4 ,the curve 7,fig4 #db7 consists of four parts1 whose slopes are in the proportion #af1 #cd1 #de1 #fe1 i4e4 very nearly in the ratio #a1 #b1 #c1 #d4 ,experiments were also made with very thin layers of radium bromide1 when1 as we have seen 7,fig4 #dj7 a very different shape of curve is to be expected4 ,an example of the results is shown in ,fig4 #dc1 curves ,i41 ,,ii4 and ,,iii4 ,curve ,i4 is obtained from radium bromide which has been heated to drive off the emanation1 and curves ,,ii4 and ,,iii4 from the same substance several days later1 when the emanation was again accumulating4 ,the portion ,,pq1 which is absent in the first curve1 is probably due to the 8excited0 activity produced by the emanation4 ,by careful examination of the successive changes in the curves after the radium has been heated to drive off the emanation1 it is possible to tell the range of the rays from each of the different products1 and this has been done to some extent by ,bragg and ,kleeman4 ,it will be seen later that the results here obtained support in a novel way the theory of radio-active changes which has been advanced from data of quite a different character4 ,the inward slope of the curve in ,fig4 #dc due to the radium indicates that the particles become more efficient ionizers as their velocity decreases4 ,this is in agreement with observations on the rays4 ,in some cases ,bragg also observed that the particles are the most efficient ionizers just before they lose their power of ionizing the gas4 ,illustration3 ,fig4 #dc4 ,thus we may conclude from these experiments that the particles from a simple radio-active substance traverse a definite distance in air1 at a definite pressure and temperature1 and that the ionization ends fairly abruptly4 ,if the rays traverse a sheet of metal1 the effective range of ionization is diminished by a distance corresponding to d1 where is the density of the material compared with air and d its thickness4 ,the rays from a thick layer of a simple radio-active substance consist of particles of different velocities1 which have ranges in air lying between #j and the maximum range4 ,the ionization of the particles per unit path is greatest near the end of its range1 and decreases somewhat as we approach the radiant source4 ,a complex source of rays like radium gives out four types of rays1 each of which has a different but distinct range4 ,from this theory it is possible to calculate approximately the decrease of current to be observed when sheets of metal foil are placed over a large area of radio-active substance4 ,this is the method that has been employed to obtain the curves of ,figs4 #ce and #ch4 ,suppose a very thin layer of simple radio-active matter is employed 7for example a bismuth plate covered with radio-tellurium or a metal plate made active by exposure to the presence of the thorium or radium emanations7 and that the ionization vessel is of sufficient depth to absorb the rays completely4 ,let d be the thickness of the metal plate1 its density compared with air4 ,consider a point ,p close to the upper side of the plate4 ,the range of the particles moving from a point1 when the path makes an angle with the normal at ,p1 is a d sec 1 where a is the range in air4 ,the rays coming from points such that the paths make an angle with the normal greater than cos#a frac rho d a will thus be absorbed in the plate4 ,by integrating over the circular area under the point ,p1 it is easy to show that the total ionization in the vessel is proportional to intcos#a frac rho d a #b pi sin theta cos theta 7a rho d sec theta7 dtheta frac pi 7a rho d7#b a 4 ,the curves showing the relation between current and distance of metal traversed should thus be parabolic with respect to d4 ,this is approximately the case for a simple substance like radio-tellurium4 ,the curve for a thick layer of radium would be difficult to calculate on account of the complexity of the rays1 but we know from experiment that it is approximately exponential4 ,an account of some recent investigations made to determine the range of velocity over which the particle is able to ionize the gas is given in ,appendix ,a4 ,the results there given strongly support the theory of absorption of the rays discussed above4 ,,part ,,iv4 ,the or very penetrating ,rays4 #aje4 ,in addition to the and rays1 the three active substances1 uranium1 thorium1 and radium1 all give out a radiation of an extraordinarily penetrating character4 ,these rays are considerably more penetrating than the ,x rays produced in a 8hard0 vacuum tube4 ,their presence can readily be observed for an active substance like radium1 but is difficult to detect for uranium and thorium unless a large quantity of active material is used4 ,villard#afg1 using the photographic method1 first drew attention to the fact that radium gave out these very penetrating rays1 and found that they were non-deviable by a magnetic field4 ,this result was confirmed by ,becquerel#afh4 ,using a few milligrams of radium bromide1 the rays can be detected in a dark room by the luminosity they excite in the mineral willemite or a screen of platinocyanide of barium4 ,the and rays are completely absorbed by placing a thickness of #a centimetre of lead over the radium1 and the rays which then pass through the lead consist entirely of rays4 ,the very great penetrating power of these rays is easily observed by noting the slight diminution of the luminosity of the screen when plates of metal several centimetres thick are placed between the radium and the screen4 ,these rays also produce ionization in gases and are best investigated by the electrical method4 ,the presence of the rays from #cj mgrs4 of radium bromide can be observed in an electroscope after passing through #cj cms4 of iron4 #ajf4 ,absorption of the rays4 ,in an examination of the active substances by the electrical method1 the writer#afi found that both uranium and thorium gave out rays in amount roughly proportional to their activity4 ,an electroscope of the type shown in ,fig4 #ab was employed4 ,this was placed on a large lead plate #fe cm4 thick1 the active substance being placed in a closed vessel beneath4 ,the discharge due to the natural ionization of the air in the electroscope was first observed4 ,the additional ionization due to the active substance must be that produced by rays which have passed through the lead plate and the walls of the electroscope4 ,the following table shows that the discharge due to these rays decreases approximately according to an exponential law with the thickness of lead traversed4 ,thickness of lead ,rate of discharge #fb cms4 #ajj #fd cms4 #fg #bhf #bc #ejh #h ,using #ajj grs4 of uranium and thorium1 the discharge due to the rays through #a cm4 of lead was quite appreciable1 and readily measured4 ,the results showed that the amount of rays was about the same for equal weights of thorium and uranium oxides4 ,the penetrating power was also about the same as for the radium rays4 ,illustration3 ,fig4 #dd4 ,the writer showed that the absorption of the rays from radium was approximately proportional to the density of the substance traversed4 ,a more detailed examination of the absorption of these rays in various substances has been recently made by ,mc,clelland#agj4 ,the curve 7,fig4 #dd7 shows the decrease of the ionization current in a testing vessel due to the and rays with successive layers of lead4 ,it is seen that the rays are almost completely stopped by #d mms4 of lead2 the ionization is then due entirely to the rays4 ,in order to leave no doubt that all the rays were absorbed1 the radium was covered with a thickness of #h mms4 of lead1 and measurements of the coefficient of absorption were made for additional thicknesses4 ,the average value of was calculated from the usual equation frac ,i ,i e- d 1 where d is the thickness of matter traversed4 ,the following table shows the value of 1 7,i7 for the first #be mms4 of matter traversed 7after initially passing through #h mms4 of lead71 7,,ii7 for the thickness #be to #e mms41 7,,iii7 for #e to #aj mms41 7,,iv7 #aj to #ae mms4 ,,table ,a4 ,substance ,i ,,ii ,,iii ,,iv ,platinum #aafg ,mercury #gbf #ffa #ech #dic ,lead #fda #efc #dhj #ddj ,zinc #bhb #bff #bdh #bff ,aluminium #ajd #ajd #ajd #ajd ,glass #jhg #jhg #jhg #jhg ,water #jcd #jcd #jcd #jcd ,in the above table1 the absorption in aluminium1 glass and water was too small to determine with accuracy the variation of with distance traversed4 ,it will be observed that1 for the denser substances1 the coefficient of absorption decreases with the distance through which the rays have passed4 ,this indicates that the rays are heterogeneous4 ,the variation of is more marked in heavy substances4 ,table ,b gives the values of divided by density for the above numbers4 ,if the absorption were directly proportional to the density1 the quotient would be the same in all cases4 ,,table ,b4 divided by density4 ,substance ,i ,,ii ,,iii ,,iv ,platinum #jed ,mercury #jec #jdh #jci #jcf ,lead #jef #jdi #jdb #jcg ,zinc #jci #jcg #jcd #jcc ,aluminium #jch #jch #jch #jch ,glass #jcd #jcd #jcd #jcd ,water #jcd #jcd #jcd #jcd ,the numbers in column ,i vary considerably1 but the agreement becomes closer in the succeeding columns1 until in column ,,iv the absorption is very nearly proportional to the density4 ,it is seen that the absorption of all three types of rays from radio-active substances is approximately proportional to the density of the substance traversed--a relation first observed by ,lenard for the cathode rays4 ,this law of absorption thus holds for both positively and negatively electrified particles projected from the radio-active substances1 and also for the electromagnetic pulses which are believed to constitute the rays2 although the absorption of the rays1 for example1 is #aj1jjj times greater than for the rays4 ,we have seen in section #hd that the value of the absorption constant for lead is #abb for the rays from uranium4 ,the value for the rays from radium varies between #fd and #dd1 showing that the rays are more than #bjj times as penetrating as the rays4 #ajg4 ,nature of the rays4 ,in addition to their great penetrating power1 the rays differ from the and rays in not being deflected to an appreciable degree by a magnetic or electric field4 ,in a strong magnetic field1 it can be shown1 using the photographic method1 that there is an abrupt discontinuity between the and rays1 for the former are bent completely away from the latter4 ,this indicates that1 as regards the action of a magnetic field1 there is no gradual transition of magnetic properties between the and rays4 ,paschen#aga has examined the rays in a very intense magnetic field1 and1 from the absence of deflection of these rays1 has calculated that1 if they consist of electrified particles carrying an ionic charge1 and projected with a velocity approaching that of light1 their apparent mass must be at least #de times greater than that of the hydrogen atom4 ,it now remains for us to consider whether the rays are corpuscular in character1 or whether they are a type of electromagnetic pulse in the ether similar to ,rntgen rays4 ,they resemble ,rntgen rays in their great penetrating power and in their absence of deflection in a magnetic field4 ,earlier experiments seemed to indicate an important difference between the action of and ,x rays4 ,it is well known that ordinary ,x rays produce much greater ionization in gases such as sulphuretted hydrogen and hydrochloric acid gas1 than in air1 although the differences in density are not large4 ,for example1 exposed to ,x rays1 sulphuretted hydrogen has six times the conductivity of air1 while with rays the conductivity only slightly exceeds that of air4 ,the results obtained by ,strutt1 in this connection1 have already been given in section #de4 ,it is there shown that the relative conductivity of gases exposed to rays 7and also to and rays7 is1 in most cases1 nearly proportional to their relative densities2 but1 under ,x rays1 the relative conductivity for some gases and vapours is very much greater than for the rays4 ,it must be remembered1 however1 that the results obtained by ,strutt were for 8soft ,x rays10 whose penetrating power was very much less than that of the rays4 ,in order to see if the relative conductivity of gases produced by ,x rays depended upon their penetrating power1 ,a4 ,s4 ,eve#agb made some experiments with a very 8hard0 ,x ray bulb1 which gave an unusually penetrating type of rays4 ,the results of the measurements are shown in the table below1 where the conductivity for each type of rays is expressed relative to air as unity4 ,the results obtained for 8soft0 ,x rays by ,strutt and by ,eve for rays are added for comparison4 ,it is seen that the hard rays show a much closer agreement than the soft rays with the density law found for the rays4 ,the high values previously obtained for the vapours of chloroform and carbon tetrachloride are greatly reduced1 and are very nearly the same as for the rays4 ,on the other hand1 the vapour of methyl iodide is an exception1 and still shows a high conductivity4 ,the rays were1 however1 forty times as penetrating as the hard ,x rays1 and it is probable that the value of methyl iodide would be reduced with still more penetrating ,x rays4 ,relative conductivities of gases4 ,gas ,relative 8,soft0 8,hard0 rays ,density ,x rays ,x rays ,hydrogen #jg #aa #db #ai ,air #aj #aj #aj #aj ,sulphuretted #ab #f #i #abc ,hydrogen ,chloroform #dc #cb #df #dh ,methyl ,iodide #ej #gb #ace #ef ,carbon #ec #de #di #eb ,tetrachloride ,the hard ,x rays were found to give far more secondary radiation than the rays1 but this effect is probably also a function of the penetrating power of the primary rays4 ,it will be seen later 7section #aab7 that rays give rise to a secondary radiation of the ray type4 ,this has also been observed for the ,x rays4 ,considering the experimental evidence as a whole1 there is undoubtedly a very marked similarity between the properties of and ,x rays4 ,the view that the rays are a type of very penetrating ,x rays1 also receives support from theoretical considerations4 ,we have seen 7section #eb7 that the ,x rays are believed to be electromagnetic pulses1 akin in some respects to short light waves1 which are set up by the sudden stoppage of the cathode ray particles4 ,conversely1 it is also to be expected that ,x rays will be produced at the sudden starting1 as well as at the sudden stopping1 of electrons4 ,since most of the particles from radium are ejected from the radium atom with velocities much greater than the cathode particles in a vacuum tube1 ,x rays of a very penetrating character will arise4 ,but the strongest argument in support of this view is derived from an examination of the origin and connection of the and rays from radio-active substances4 ,it will be shown later that the ray activity observed in radium arises from several disintegration products1 stored up in the radium1 while the and rays arise only from one of these products named radium ,c4 ,it is found1 too1 that the activity measured by the rays is always proportional to the activity measured by the rays1 although by separation of the products the activity of the latter may be made to undergo great variations in value4 ,thus the intensity of the rays is always proportional to the rate of expulsion of particles1 and this result indicates that there is a close connection between the and rays4 ,such a result is to be expected if the particle is the parent of the ray1 for the expulsion of each electron from radium will give rise to a narrow spherical pulse travelling from the point of disturbance with the velocity of light4 #ajh4 ,there is another possible hypothesis in regard to the nature of these rays4 ,it has been shown 7sections #dh and #hb7 that the apparent mass of an electron increases as the speed of light is approached2 theoretically it should be very great when the velocity of the electron is exceedingly close to the velocity of light4 ,in such a case1 a moving electron would be difficult to deflect by a magnetic or electric field4 ,the view that the rays are electrons carrying a negative charge and moving with a velocity nearly equal to that of light has recently been advocated by ,paschen#agc4 ,he concluded from experiment that the rays like the rays carried a negative charge4 ,we have seen 7section #he7 that ,seitz also observed that a small negative charge was communicated to bodies on which the rays impinged1 but the magnitude of this charge was much smaller than that observed by ,paschen4 ,i do not think that much weight can be attached to observations that a small positive or negative charge is communicated to bodies on which the rays fall1 for it will be shown later that a strong secondary radiation1 consisting in part of electrons1 is set up during the passage of the rays through matter4 ,it is not improbable that the small charge observed is not a direct result of the charge carried by the rays1 but is an indirect effect due to the secondary radiations emitted from the surface of bodies4 ,there is no doubt that a thick lead vessel1 completely enclosing a quantity of radium1 acquires a small positive charge1 but this result would follow whether the rays carry a charge or not1 since the secondary radiations from the lead surface consist of projected particles which carry with them a negative charge4 ,on this corpuscular theory of the nature of the rays1 each electron must have a large apparent mass1 or otherwise it would be appreciably deflected by an intense magnetic field4 ,the energy of motion of the electron must1 in consequence1 be very great1 and1 if the number of the electrons constituting the rays is of the same order of magnitude as the number of the particles1 a large heating effect is to be expected when the rays are stopped in matter4 ,paschen#agd made some experiments on the heat emission of radium due to the rays2 he concluded that the rays were responsible for more than half of the total heat emission of radium and carried away energy at the rate of over #ajj gram calories per hour per gram of radium4 ,this result was not confirmed by later experiments of ,rutherford and ,barnes#age1 who found that the heating effect of the rays could not be more than a few per cent4 of the total heat emission of radium4 ,these results will be considered later in chapter ,,xii4 ,the weight of evidence1 both experimental and theoretical1 at present supports the view that the rays are of the same nature as the ,x rays but of a more penetrating type4 ,the theory that the ,x rays consist of non-periodic pulses in the ether1 set up when the motion of electrons is arrested1 has found most favour1 although it is difficult to provide experimental tests to decide definitely the question4 ,the strongest evidence in support of the wave nature of the ,x rays is derived from the experiments of ,barkla#agf1 who found that the amount of secondary radiation set up by the ,x rays on striking a metallic surface depended on the orientation of the ,x ray bulb4 ,the rays thus showed evidence of a one-sidedness or polarization which is only to be expected if the rays consist of a wave motion in the ether4 ,,part ,v4 ,secondary ,rays4 #aji4 ,production of secondary rays4 ,it has long been known that ,rntgen rays1 when they impinge on solid obstacles1 produce secondary rays of much less penetrating power than the incident rays4 ,this was first shown by ,perrin and has been investigated in detail by ,sagnac1 ,langevin1 ,townsend and others4 ,thus it is not surprising that similar phenomena should be observed for the radiation from radio-active substances4 ,by means of the photographic method1 ,becquerel#agg has made a close study of the secondary radiations produced by radio-active substances4 ,in his earliest observations1 he noticed that radiographs of metallic objects were always surrounded by a diffuse border4 ,this effect is due to the secondary rays set up by the incident rays at the surface of the screen4 ,the secondary rays produced by the rays are very feeble4 ,they are best shown by polonium1 which gives out only rays1 so that the results are not complicated by the action of the rays4 ,strong secondary rays are set up at the point of impact of the or cathodic rays4 ,becquerel found that the magnitude of this action depended greatly on the velocity of the rays4 ,the rays of lowest velocity gave the most intense secondary action1 while the penetrating rays gave1 in comparison1 scarcely any secondary effect4 ,in consequence of the presence of this secondary radiation1 the photographic impression of a screen pierced with holes is not clear and distinct4 ,in each case there is a double photographic impression1 due to the primary rays and the secondary rays set up by them4 ,these secondary rays are deviable by a magnetic field1 and in turn produce tertiary rays and so on4 ,the secondary rays are in all cases more readily deviated and absorbed than the primary rays1 from which they arise4 ,the very penetrating rays give rise to secondary rays1 which cause intense action on the photographic plate4 ,when some radium was placed in a cavity inside a deep lead block1 rectangular in shape1 besides the impression due to the direct rays through the lead1 ,becquerel observed that there was also a strong impression due to the secondary rays emitted from the surface of the lead4 ,the action of these secondary rays on the plate is so strong that the effect on the plate is1 in many cases1 increased by adding a metal screen between the active material and the plate4 ,the comparative photographic action of the primary and secondary rays cannot be taken as a relative measure of the intensity of their radiations4 ,for example1 only a small portion of the energy of the rays is in general absorbed in the sensitive film4 ,since the secondary rays are far more easily absorbed than the primary rays1 a far greater proportion of their energy is expended in producing photographic action than in the case of the primary rays4 ,it is thus not surprising that the secondary rays set up by the and rays may in some cases produce a photographic impression comparable with1 if not greater than1 the effect of the incident rays4 ,on account of these secondary rays1 radiographs produced by the rays of radium in general show a diffuse border round the shadow of the object4 ,for this reason radiographs of this kind lack the sharpness of outline of ,x ray photographs4 #aaj4 ,secondary radiation produced by rays4 ,mme ,curie#agh has shown by the electric method that the rays of polonium produce secondary rays4 ,the method adopted was to compare the ionization current between two parallel plates1 when two screens of different material1 placed over the polonium1 were interchanged4 ,these results show that the rays of polonium are modified in passing through matter1 and that the amount of secondary rays set up varies with screens of different material4 ,mme ,curie1 using the same method1 was unable to observe any such effect for the rays of radium4 ,the production of secondary rays by the rays of radium is1 however1 readily shown by the photographic method4 ,we have already seen 7section #ic7 that very low velocity electrons accompany the rays from radium or radio-tellurium spread on a metal plate4 ,these electrons are probably liberated when the rays escape from or impinge upon matter1 and the number emitted depends upon the kind of matter used as a screen4 ,the differences shown in the above table when the screens were interchanged are explained simply in this way4 ,screens employed ,thickness ,current in mms4 observed ,aluminium #jja ,cardboard #jjje #agi ,cardboard #jjje ,aluminium #jja #fg ,aluminium #jja ,tin #jjje #aej ,tin #jjje ,aluminium #jja #abf ,tin #jjje ,cardboard #jjje #aci ,cardboard #jjje ,tin #jjje #dd ,illustration3 ,fig4 #de4 #aaa4 ,secondary rays produced by and rays4 ,an examination of the amount and character of the secondary radiation emitted by various substances1 when exposed to the and rays of radium1 has recently been made by ,a4 ,s4 ,eve#agi4 ,the general experimental method employed is shown in ,fig4 #de4 ,the electroscope 7,fig4 #de7 was placed behind a lead screen #de cms4 thick1 which stopped all the rays and absorbed the greater proportion of the rays from the radium tube placed at ,r4 ,on bringing near a plate of matter ,m1 the primary rays fell upon it and some of the secondary rays1 emitted in all directions1 passed into the side of the electroscope1 which was covered with aluminium foil of thickness #je mm4 ,before the plate ,m was placed in position the rate of discharge of the electroscope was due to the natural leak and the rays from ,r1 and the secondary radiation from the air4 ,on bringing the radiator ,m into position1 the rate of discharge was much increased1 and the difference between the rate of movement of the gold-leaf in the two cases was taken as a measure of the amount of secondary rays from ,m4 ,the absorption of the secondary rays was tested by placing an aluminium plate #he mm4 thick before the face of the electroscope4 ,the secondary rays were found to be fairly homogeneous1 for the intensity fell off according to an exponential law with the distance traversed4 ,the value of the absorption constant was determined from the usual equation frac ,i ,i e- d 1 where d is the thickness of the screen4 ,the table given below shows the results obtained when thick plates of different substances of the same dimensions were placed in a definite position at ,m4 ,the secondary radiation from fluids was obtained by a slight alteration of the experimental arrangements4 ,thirty milligrammes of radium bromide were used1 and the results are expressed in terms of the number of scale divisions passed over per second by the gold-leaf4 ,it will be noticed that the amount of secondary radiation follows in most cases the same order as the densities1 and is greatest for mercury4 ,the value of 7secondary radiation7_/density is not a constant1 but varies considerably1 being greatest for light substances4 ,the absorption constant of the secondary rays from different radiators is not very different1 with the exception of substances such as granite1 brick1 and cement1 which give out secondary rays of nearly twice the penetrating power of other substances4 and rays4 ,radiator ,density ,secondary ,sec4 ,aluminium ,radiation ,rad4 _/ #jhe cm4 ,density ,mercury #acf #adg #ajh ,lead #aad #ada #abd #ahe ,copper #hh #gi #ij #bj ,brass #hd #ha #if #ba ,iron #gh #ge #if #bj 7wrought7 ,tin #gd #gc #ii #bjc ,zinc #gj #gi #aac ,granite #bg #ed #bjj #abd ,slate #bf #ec #bjd #aba ,aluminium #bf #db #afa #bd ,glass #be #dd #agf #bd ,cement #bd #dg #aif #ace ,brick #bb #di #bbc #acj ,ebonite #aa #cb #bia #bf ,water #aj #bd #bdj #ba ,ice #ib #bf #bhb ,paraffin #i #ag #ahh #ba solid liquid #he #af #ahh ,mahogany #ef #bad #chb #bc ,paper #d8 #baj #eb #bb ,millboard #d8 #aid #dh #bje ,papier-mch 444 #bai ,basswood #cf #bjg #eg #bb ,pine #ce #bah #fb #ba ,x ray screen #geb #bcf ,the secondary radiation not only comes from the surface of the radiator but from a considerable depth4 ,the amount of secondary rays increases with the thickness of the radiator1 and1 in the case of glass and aluminium1 reaches a practical maximum for a plate about #c mms4 thick4 ,in the above table1 the secondary radiation arises from both the rays and rays together4 ,when the rays were cut off by a layer of lead #fc mms4 thick1 placed between the radium and the radiator1 the effect on the electroscope was reduced to less than #bj per cent4 of its former value1 showing that the rays supplied more than #hj per cent4 of the secondary radiation4 ,the following table shows the relative amount of secondary rays from different substances when exposed to and rays together and to rays alone4 ,the amount from lead in each case is taken as a standard and equal to #ajj4 ,the amount of secondary radiation found by ,townsend from soft ,x rays is added for comparison4 ,secondary ,radiations4 and ,rntgen ,radiator rays rays ,lead #ajj #ajj #ajj ,copper #eg #fa #bia ,brass #eh #ei #bfc ,zinc #eg 444 #bhb ,aluminium #cj #cj #be ,glass #ca #ce #ca ,paraffin #ab #bj #abe ,it will be observed that the relative amounts are about the same for the rays alone as for the and rays together4 ,on the other hand1 the amount of secondary radiation set up by ,x rays is very different1 lead for example giving much less than brass or copper4 ,the secondary rays from the rays alone are slightly less penetrating than for the and rays together1 but are far more penetrating than the secondary radiation from the ,x rays examined by ,townsend4 ,the amount of secondary radiation set up by the and rays is mainly independent of the state of the surface of the radiator4 ,about the same amount is obtained from iron as from iron filings2 from liquid as from solid paraffin2 and from ice as from water#ahj4 ,becquerel has shown that the secondary rays set up by the rays are deflected by a magnet and consist of negatively charged particles 7electrons74 ,it has been pointed out in section #eb that the cathode rays are diffusely reflected from the metal on which they fall4 ,these secondary rays consist in part of electrons moving with about the same velocity as the primary1 and in part of some electrons with a much slower speed4 ,the secondary rays set up by the rays of radium have on an average less penetrating power than the primary rays1 and consequently less velocity than the primary rays4 ,it must be remembered that the rays from radium are very complex1 and consist of electrons projected with a considerable range of velocities4 ,the secondary rays are1 on an average1 certainly more penetrating than the most easily absorbed rays emitted from radium1 and probably move with a velocity of about half that of light4 ,it is still uncertain whether the secondary rays are produced by the action of the primary rays on matter1 or whether they consist of a portion of the primary rays whose direction of motion has been deflected in their passage through matter1 so that they emerge again with diminished velocity from the surface4 #aab4 ,magnetic deflection of secondary rays from rays4 ,it has been seen that the secondary rays set up by the rays alone are very similar in character to those caused by the rays4 ,this result was still further confirmed by ,eve1 who showed that the secondary rays produced by the rays are readily deflected by a magnetic field4 ,the experimental arrangement is shown in ,fig4 #df4 ,illustration3 ,fig4 #df4 ,a small electroscope was mounted on one side of a lead platform #ab cms4 thick1 which rested on a lead cylinder #aj cms4 high and #aj cms4 in diameter4 ,the radium was placed at the bottom of a hole reaching to the centre of the cylinder4 ,on applying a strong magnetic field1 at right angles to the plane of the paper1 so as to bend the secondary rays from the platform towards the electroscope1 the rate of discharge was much increased4 ,on reversing the field1 the effect was much diminished4 ,since the rays are not themselves deflected by a magnetic field1 this result shows that the secondary radiation is quite different in character from the primary rays1 and consists of electrons projected with a velocity 7deduced from the penetrating power7 of about half the velocity of light4 ,we have already pointed out that the emission of electrons from a substance traversed by the rays will account sufficiently well for the charge observed by ,paschen1 without the necessity of assuming that the rays carry a negative charge of electricity4 ,the secondary radiation set up by ,rntgen rays1 like that due to the and rays1 consists in part of electrons projected with considerable velocity4 ,these three types of rays seem about equally efficient in causing the expulsion of electrons from the substance through which they pass4 ,we have seen that the ,x and rays are1 in all probability1 electromagnetic pulses set up by the sudden starting or stopping of electrons1 and1 since these rays in turn cause the removal of electrons1 the process appears to be reversible4 ,since the rays pass through some thickness of matter before their energy of motion is arrested1 theory would lead us to expect that a type of soft ,x rays should be generated in the absorbing matter4 ,,part ,,vi4 #aac4 ,comparison of the ionization produced by the and rays4 ,with unscreened active material the ionization produced between two parallel plates1 placed as in ,fig4 #ag1 is mainly due to the rays4 ,on account of the slight penetrating power of the rays1 the current due to them practically reaches a maximum with a small thickness of radio-active material4 ,the following saturation currents were observed#aha for different thicknesses of uranium oxide between parallel plates sufficiently far apart for all the rays to be absorbed in the gas between them4 ,surface of uranium oxide #ch sq4 cms4 ,weight of uranium ,saturation current oxide in grammes per in amperes per sq4 sq4 cm4 of surface cm4 of surface #jjcf #ag #aj #jjif #cb #aj #jahi #dj #aj #jcej #dd #aj #jiee #dg #aj ,the current reached about half its maximum value for a weight of oxide #jjee gr4 per sq4 cm4 ,if the rays are cut off by a metallic screen1 the ionization is then mainly due to the rays1 since the ionization produced by the rays is small in comparison4 ,for the rays from uranium oxide it has been shown 7section #hf7 that the current reaches half its maximum value for a thickness of #jaa gr4 per sq4 cm4 ,meyer and ,schweidler#ahb have found that the radiation from a water solution of uranium nitrate is very nearly proportional to the amount of uranium present in the solution4 ,on account of the difference in the penetrating power of the and rays1 the ratio of the ionization currents produced by them depends on the thickness of the radio-active layer under examination4 ,the following comparative values of the current due to the and rays were obtained for very thin layers of active matter#ahc4 ,a weight of gramme of fine powder1 consisting of uranium oxide1 thorium oxide1 or radium chloride of activity #bjjj1 was spread as uniformly as possible over an area of #hj sq4 cms4 ,the saturation current was observed between parallel plates #eg cms4 apart4 ,this distance was sufficient to absorb most of the rays from the active substances4 ,a layer of aluminium #jji cm4 thick absorbed all the rays4 ,current ,current ,ratio of due to due to currents rays rays _/ ,uranium #a #a #jjgd ,thorium #a #bg #jjbj ,radium #bjjj #acej #jjcc ,in the above table the saturation current due to the and rays of uranium is1 in each case1 taken as unity4 ,the third column gives the ratio of the currents observed for equal weights of substance4 ,the results are only approximate in character1 for the ionization due to a given weight of substance depends on its fineness of division4 ,in all cases1 the current due to the rays is small compared with that due to the rays1 being greatest for uranium and least for thorium4 ,as the thickness of layer increases1 the ratio of currents _/ steadily increases to a constant value4 #aad4 ,comparison of the energy radiated by the and rays4 ,it has not yet been found possible to measure directly the energy of the and rays4 ,a comparison of the energy radiated in the two forms of rays can1 however1 be made indirectly by two distinct methods4 ,if it be assumed that the same amount of energy is required to produce an ion by either the or the ray1 and that the same proportion of the total energy is used up in producing ions1 an approximate estimate can be made of the ratio of the energy radiated by the and rays by measuring the ratio of the total number of ions produced by them4 ,if is the coefficient of absorption of the rays in air1 the rate of production of ions per unit volume at a distance x from the source is q e- x where q is the rate of ionization at the source4 ,the total number of ions produced by complete absorption of the rays is intinfty q e- x dx frac q 1 ,now is difficult to measure experimentally for air1 but an approximate estimate can be made of its value from the known fact that the absorption of rays is approximately proportional to the density of any given substance4 ,for rays from uranium the value of for aluminium is about #ad1 and divided by the density is #ed4 ,taking the density of air as #jjab1 we find that for air #jjfe4 ,the total number of ions produced in air is thus #aedq when the rays are completely absorbed4 ,now from the above table the ionization due to the rays is #jjgd of that produced by rays1 when the rays passed through a distance of #eg cms4 of air4 ,thus we have approximately ,total number of ions produced by rays #jjgd ---------------------------------- ----- #aed #jbj4 ,total number of ions produced by rays #eg ,therefore about of the total energy radiated into air by a thin layer of uranium is carried by the rays or electrons4 ,the ratio for thorium is about and for radium about 1 assuming the rays to have about the same average value of 4 ,this calculation takes into account only the energy which is radiated out into the surrounding gas2 but on account of the ease with which the rays are absorbed1 even with a thin layer1 the greater proportion of the radiation is absorbed by the radio-active substance itself4 ,this is seen to be the case when it is recalled that the radiation of thorium or radium is reduced to half value after passing through a thickness of about #jjjje cm4 of aluminium4 ,taking into consideration the great density of the radio-active substances1 it is probable that most of the radiation which escapes into the air is due to a thin skin of the powder not much more than #jjja cm4 in thickness4 99 99 99 99 99 ,an estimate1 however1 of the relative rate of emission of energy by the and rays from a thick layer of material can be made in the following way3--,for simplicity suppose a thick layer of radio-active substance spread uniformly over a large plane area4 ,there seems to be no doubt that the radiations are emitted uniformly from each portion of the mass2 consequently1 the radiation1 which produces the ionizing action in the gas above the radio-active layer1 is the sum total of all the radiation which reaches the surface of the layer4 99 99 99 99 99 ,let be the average coefficient of absorption of the rays in the radio-active substance itself and the specific gravity of the substance4 ,let ,e be the total energy radiated per sec4 per unit mass of the substance when the absorption of the rays in the substance itself is disregarded4 ,the energy per sec4 radiated to the upper surface by a thickness dx of a layer of unit area at a distance x from the surface is given by frac #a #b ,e#a sigma e-#a x dx 4 ,the total energy ,w per unit area radiated to the surface per sec4 by a thickness d is given by ,w#a frac #ab intd ,e#a sigma e-#a x dx frac ,e#a sigma #b #a 7#a e-#a d7 frac ,e#a sigma #b #a if d is large4 99 99 99 99 99 ,in a similar way it may be shown that the energy ,w of the rays reaching the surface is given by ,w#b frac ,e#b sigma #b #b where ,e and are the values for the rays corresponding to ,e and for the rays4 ,thus it follows that ,e ,w ---- ------ ,e ,w and are difficult to determine directly for the radio-active substance itself1 but it is probable that the ratio _/ is not very different from the ratio for the absorption coefficients for another substance like aluminium4 ,this follows from the general result that the absorption of both and rays is proportional to the density of the substance2 for it has already been shown in the case of the rays from uranium that the absorption of the rays in the radio-active material is about the same as for non-radio-active matter of the same density4 ,with a thick layer of uranium oxide spread over an area of #bb sq4 cms41 it was found that the saturation current between parallel plates #fa cms4 apart1 due to the rays1 was #abg times as great as the current due to the rays4 ,since the rays were entirely absorbed between the plates and the total ionization produced by the rays is #aed times the value at the surface of the plates1 ,w total number of ions due to rays ----- ---------------------------------- ,w total number of ions due to rays #abg #fa ------ #je approximately4 #aed ,now the value of for aluminium is #bgdj and of for the same metal #ad1 thus ,e ,w ----- ---------- #ajj approximately ,e ,w ,this shows that the energy radiated from a thick layer of material by the rays is only about #a per cent4 of the energy radiated in the form of rays4 ,this estimate is confirmed by calculations based on independent data4 ,let m1 m be the masses of the and particles respectively and v1 v their velocities4 frac ,energy of one alpha particle ,energy of one beta particle frac m#a v#ab m#b v#bb frac frac m#a e v#ab frac m#b e v#bb 4 ,now it has been shown that for the rays of radium v #be #aj1 e ---- #f #aj4 m ,the velocity of the rays of radium varies between wide limits4 ,taking for an average value v #ae #aj1 e ---- #ah #aj1 m it follows that the energy of the particle from radium is almost #hc times the energy of the particle4 ,if equal numbers of and particles are projected per second1 the total energy radiated in the form of rays is about #hc times the amount in the form of rays4 ,evidence will be given later 7section #bec7 to show that the number of particles projected is probably four times the number of particles2 so that a still greater proportion of the energy is emitted in the form of rays4 ,these results thus lead to the conclusion that1 from the point of view of the energy emitted1 the rays are far more important than the rays4 ,this conclusion is supported by other evidence which is discussed in chapters ,,xii and ,,xiii1 where it will be shown that the rays play by far the most important part in the changes occurring in radio-active bodies1 and that the rays only appear in the latter stages of the radio-active processes4 ,from data based on the relative absorption and ionization of the and rays in air1 it can be shown that the rays carry off about the same amount of energy as the rays4 ,these conclusions are confirmed by direct measurement of the heating effect of radium1 which is discussed in detail in chapter ,,xii4 ,footnote #aaa3 ,in an examination of uranium the writer 7,phil4 ,mag4 p4 #aaf1 ,jan4 #ahii7 found that the rays from uranium consist of two kinds1 differing greatly in penetrating power1 which were called the and rays4 ,later1 it was found that similar types of rays were emitted by thorium and radium4 ,on the discovery that very penetrating rays were given out by uranium and thorium as well as by radium1 the term was applied to them by the writer4 ,the word 8ray0 has been retained in this work1 although it is now settled that the and rays consist of particles projected with great velocity4 ,the term is thus used in the same sense as by ,newton1 who applied it in the ,principia to the stream of corpuscles which he believed to be responsible for the phenomenon of light4 ,in some recent papers1 the and rays have been called the and 8emanations40 ,this nomenclature cannot fail to lead to confusion1 since the term 8radio-active emanation0 has already been generally adopted in radio-activity as applying to the material substance which gradually diffuses from thorium and radium compounds1 and itself emits rays4 ,footnote #aab3 ,this method of illustration is due to ,mme ,curie1 ,thse prsente la ,facult des ,sciences de ,paris1 #aijc4 ,footnote #aac3 ,giesel1 ,annal4 d4 ,phys4 #fi1 p4 #hcd1 #ahii4 ,footnote #aad3 ,meyer and ,schweidler1 ,phys4 ,zeit4 #a1 pp4 #ij1 #aac1 #ahii4 ,footnote #aae3 ,becquerel1 ,c4 ,r4 #abi1 pp4 #iig1 #abje1 #ahii4 ,footnote #aaf3 ,curie1 ,c4 ,r4 #acj1 p4 #gc1 #aijj4 ,footnote #aag3 ,rutherford1 ,phil4 ,mag4 ,january1 #ahii4 ,footnote #aah3 ,rutherford and ,grier1 ,phil4 ,mag4 ,september1 #aijb4 ,footnote #aai3 ,becquerel1 ,c4 ,r4 #acj1 pp4 #bjf1 #cgb1 #haj1 #igi4 #aijj4 ,footnote #abj3 ,m4 and ,mme ,curie1 ,c4 ,r4 #acj1 p4 #fdg1 #aijj4 ,footnote #aba3 ,the activity of the radium preparation was not stated in the paper4 ,footnote #abb3 ,dorn1 ,phys4 ,zeit4 #d1 ,no4 #ah1 p4 #ejg1 #aijc4 ,footnote #abc3 ,strutt1 ,phil4 ,mag4 ,nov4 #aijc4 ,footnote #abd3 ,wien1 ,phys4 ,zeit4 #d1 ,no4 #bc1 p4 #fbd1 #aijc4 ,footnote #abe3 ,dorn1 ,c4 ,r4 #acj1 p4 #aabi1 #aijj4 ,footnote #abf3 ,becquerel1 ,c4 ,r4 #acj1 p4 #hji1 #aijj4 ,footnote #abg3 ,kaufmann1 ,phys4 ,zeit4 #d1 ,no4 #a b1 p4 #ed1 #aijb4 ,footnote #abh3 ,abraham1 ,phys4 ,zeit4 #d1 ,no4 #a b1 p4 #eg1 #aijb4 ,footnote #abi3 ,kaufmann1 ,nachrichten d4 ,ges4 d4 ,wiss4 zu ,gtt41 ,nov4 #h1 #aija4 ,footnote #acj3 ,simon1 ,annal4 d4 ,phys4 p4 #ehi1 #ahii4 ,footnote #aca3 ,kaufmann1 ,phys4 ,zeit4 #d1 ,no4 #a b1 p4 #ed1 #aijb4 ,footnote #acb3 ,paschen1 ,annal4 d4 ,phys4 #ad1 p4 #chi1 #aijd4 ,footnote #acc3 ,meyer and ,schweidler1 ,phys4 ,zeit4 pp4 #ij1 #aac1 #bji1 #aijj4 ,footnote #acd3 ,lenard1 ,annal4 d4 ,phys4 #ef1 p4 #bge1 #ahie4 ,footnote #ace3 ,strutt1 ,nature1 p4 #eci1 #aijj4 ,footnote #acf3 ,seitz1 ,phys4 ,zeit4 #e1 ,no4 #ad1 p4 #cie1 #aijd4 ,footnote #acg3 ,it is presumed that the results were corrected1 if necessary1 for the discharging action due to the ionized gas1 although no direct mention of this is made in the paper by ,seitz4 ,footnote #ach3 ,strutt1 ,phil4 ,trans4 ,a1 p4 #ejg1 #aija4 ,footnote #aci3 ,crookes1 ,proc4 ,roy4 ,soc4 #aijb4 ,chem4 ,news1 #he1 p4 #aji1 #aijb4 ,footnote #adj3 ,mme ,curie1 ,c4 ,r4 #acj1 p4 #gf1 #aijj4 ,footnote #ada3 ,rutherford1 ,phil4 ,mag4 ,feb4 #aijc4 ,phys4 ,zeit4 #d1 p4 #bce1 #aijb4 ,footnote #adb3 ,becquerel1 ,c4 ,r4 #acf1 p4 #aii1 #aijc4 ,footnote #adc3 ,becquerel1 ,c4 ,r4 #acf1 p4 #dca1 #aijc4 ,footnote #add3 ,des ,coudres1 ,phys4 ,zeit4 #d1 ,no4 #ag1 p4 #dhc1 #aijc4 ,footnote #ade3 ,becquerel1 ,c4 ,r4 #acf1 p4 #aeag1 #aijc4 ,footnote #adf3 ,bragg1 ,phil4 ,mag4 ,dec4 #aijd2 ,bragg and ,kleeman1 ,phil4 ,mag4 ,dec4 #aijd4 ,footnote #adg3 ,further experimental results bearing on this important question are given in an ,appendix to this book4 ,footnote #adh3 ,bakerian ,lecture1 ,phil4 ,trans4 ,a1 p4 #afi1 #aijd4 ,footnote #adi3 ,strutt1 ,phil4 ,mag4 ,aug4 #aijd4 ,footnote #aej3 ,j4 ,j4 ,thomson1 ,proc4 ,camb4 ,phil4 ,soc4 #ac1 ,pt4 ,i4 p4 #ci1 #aije4 ,nature1 ,dec4 #ae1 #aijd4 ,footnote #aea3 ,rutherford1 ,nature1 ,march #b1 #aije4 ,j4 ,j4 ,thomson1 ,nature1 ,march #i1 #aije4 ,footnote #aeb3 ,crookes1 ,proc4 ,roy4 ,soc4 #ha1 p4 #dje1 #aijc4 ,footnote #aec3 ,elster and ,geitel1 ,phys4 ,zeit4 ,no4 #ae1 p4 #dcg1 #aijc4 ,footnote #aed3 ,glew1 ,arch4 ,rntgen ,ray1 ,june #aijd4 ,footnote #aee3 ,becquerel1 ,c4 ,r4 #acg1 ,oct4 #bg1 #aijc4 ,footnote #aef3 ,tommasina1 ,c4 ,r4 #acg1 ,nov4 #i1 #aijc4 ,footnote #aeg3 ,an interesting side-light is thrown on this question by the experiments described in ,appendix ,a of this book4 ,footnote #aeh3 ,rutherford and ,miss ,brooks1 ,phil4 ,mag4 ,july #aijb4 ,footnote #aei3 ,in order to obtain a thin layer1 the compound to be tested is ground to a fine powder and then sifted through a fine gauge uniformly over the area1 so that the plate is only partially covered4 ,footnote #afj3 ,rutherford1 ,phil4 ,mag4 ,jan4 #ahii4 ,footnote #afa3 ,owens1 ,phil4 ,mag4 ,oct4 #ahii4 ,footnote #afb3 ,rutherford and ,miss ,brooks1 ,phil4 ,mag4 ,july1 #aijj4 ,footnote #afc3 ,since the ionization at any point above the plate is the resultant effect of the particles coming from all points of the large radio-active layer1 is not the same as the coefficient of absorption of the rays from a point source4 ,it will however be proportional to it4 ,for this reason is called the 8absorption constant40 ,footnote #afd3 ,townsend1 ,phil4 ,mag4 ,feb4 #aija4 ,footnote #afe3 ,durack1 ,phil4 ,mag4 ,july #aijb1 ,may #aijc4 ,footnote #aff3 ,bragg and ,bragg and ,kleeman1 ,phil4 ,mag4 ,dec4 #aijd4 ,footnote #afg3 ,villard1 ,c4 ,r4 #acj1 pp4 #ajaj1 #aagh1 #aijj4 ,footnote #afh3 ,becquerel1 ,c4 ,r4 #acj1 p4 #aaed1 #aijj4 ,footnote #afi3 ,rutherford1 ,phys4 ,zeit4 #c1 p4 #eag1 #aijb4 ,footnote #agj3 ,mc,clelland1 ,phil4 ,mag4 ,july #aijd4 ,footnote #aga3 ,paschen1 ,phys4 ,zeit4 #e1 ,no4 #ah1 p4 #efc1 #aijd4 ,footnote #agb3 ,a4 ,s4 ,eve1 ,phil4 ,mag4 ,nov4 #aijd4 ,footnote #agc3 ,paschen1 ,annal4 d4 ,physik1 #ad1 p4 #aad1 #aijd2 #ad1 #b1 p4 #chi1 #aijd4 ,phys4 ,zeit4 #e1 ,no4 #ah1 p4 #efc1 #aijd4 ,footnote #agd3 ,paschen1 ,phys4 ,zeit4 #e1 ,no4 #ah1 p4 #efc1 #aijd4 ,footnote #age3 ,rutherford and ,barnes1 ,phil4 ,mag4 ,may #aije4 ,nature1 p4 #aea1 ,dec4 #ae1 #aijd4 ,footnote #agf3 ,barkla1 ,nature1 ,march #ag1 #aijd4 ,footnote #agg3 ,becquerel1 ,c4,r4 #acb1 pp4 #cga1 #gcd1 #abhf4 #aija4 ,footnote #agh3 ,mme ,curie1 ,thse prsente la ,facult des ,sciences1 ,paris #aijc1 p4 #he4 ,footnote #agi3 ,a4 ,s4 ,eve1 ,phil4 ,mag4 ,dec4 #aijd4 ,footnote #ahj3 ,in a recent paper 7,phil4 ,mag4 ,feb4 #aije71 ,mc,clelland has1 in the main1 confirmed the experimental results obtained by ,eve4 ,an electrometer was used instead of an electroscope4 ,he finds1 in addition1 that the amount of secondary radiation depends on the angle of incidence of the primary rays1 and is greatest for an angle of #de4 ,in a letter to ,nature 7,feb4 #bc1 p4 #cij1 #aije71 he states that more recent experiments have shown that the amount of secondary radiation from different substances is a function of their atomic weights rather than of their densities4 ,in every case examined1 the amount of secondary radiation increases with the atomic weight1 but is not proportional to it4 ,footnote #aha3 ,rutherford and ,mc,clung1 ,phil4 ,trans4 ,a4 p4 #be1 #aija4 ,footnote #ahb3 ,meyer and ,schweidler1 ,wien ,ber4 #aac1 ,july1 #aijd4 ,footnote #ahc3 ,rutherford and ,grier1 ,phil4 ,mag4 ,sept4 #aijb4 ,,chapter ,v4 ,,properties ,,of ,,the ,,radiations4 #aae4 ,besides their power of acting on a photographic plate1 and of ionizing gases1 the radiations from active bodies are able to produce marked chemical and physical actions in various substances4 ,most of these effects are due either to the or rays4 ,the rays produce little effect in comparison4 ,since the rays are similar in all respects to high velocity cathode rays1 it is to be expected that they will produce effects similar in character to those produced by the cathode rays in a vacuum tube4 ,phosphorescent action4 ,becquerel#ahd has studied the action of radium rays in producing phosphorescence in various bodies4 ,the substance to be tested was placed above the radium in the form of powder on a very thin mica plate4 ,examination was made of the sulphides of calcium and strontium1 ruby1 diamond1 varieties of spar1 phosphorus and hexagonal blende4 ,substances like the ruby and spar1 which phosphoresce under luminous rays1 did not phosphoresce under the radium rays4 ,on the other hand1 those which were made luminous by ultra-violet light were also luminous under the action of radium rays4 ,the radium rays show distinct differences from ,x rays4 ,for example1 a diamond which was very luminous with radium rays was unaffected by ,x rays4 ,the double sulphate of uranium and potassium is more luminous than hexagonal blende under ,x rays1 but the reverse is true for radium rays2 under the influence of these rays1 sulphide of calcium gave a blue luminosity but was hardly affected by ,x rays4 ,the following table shows the relative phosphorescence excited in various bodies4 ,substance ,without screen4 ,across screen ,intensity of black paper ,hexagonal blende #accf #jd ,platino-cyanide of #aii #je barium ,diamond #aad #ja ,double sulphate of #ajj #ca ,uranium and ,potassium ,calcium ,fluoride #cj #jb ,in the last column the intensity without the screen is in each case taken as unity4 ,the great diminution of intensity after the rays have passed through black paper shows that most of the phosphorescence developed without the screen is1 in the majority of cases1 due to the rays4 ,bary#ahe has made a very complete examination of the class of substances which become luminous under radium rays4 ,he found that the great majority of substances belong to the alkali metals and alkaline earths4 ,all these substances were also phosphorescent under the action of ,x rays4 ,crystalline zinc sulphide 7,sidot's blende7 phosphoresces very brightly under the influence of the rays from radium and other very active substances4 ,this was observed by ,curie and ,debierne in their study of the radium emanation and the excited activity produced by it4 ,it has also been largely used by ,giesel as an optical means of detecting the presence of emanations from very active substances4 ,it is an especially sensitive means of detecting the presence of rays1 when it exhibits the 8scintillating0 property already discussed in section #if4 ,in order to show the luminosity due to the rays1 the screen should be held close to the active substance1 as the rays are absorbed in their passage through a few centimetres of air4 ,zinc sulphide is also luminous under the action of the rays1 but the phosphorescence is far more persistent than when produced by the rays4 ,very beautiful luminous effects are produced by large crystals of the platinocyanides exposed to the radium rays4 ,those containing lithium give a brilliant pink colour4 ,the calcium and barium salts fluoresce with a deep green light1 and the sodium compound with a lemon yellow4 ,the mineral willemite 7zinc silicate7 was recently found by ,kunz to be an even more sensitive means of detecting the presence of the radiations than platinocyanide of barium4 ,it fluoresces showing a beautiful greenish colour1 and a piece of mineral exposed to the action of the rays appears quite translucent4 ,the crystals of the platinocyanides of barium and lithium are especially suited for showing the action of the rays1 and1 in this respect1 are superior to willemite4 ,a very striking effect is shown by the mineral kunzite--a new variety of spodumene discovered by ,kunz#ahf4 ,this is a transparent gem like crystal1 often of very large size1 which glows with a beautiful reddish colour under the action of the or rays1 but does not appear to be sensitive to the rays4 ,the luminosity extends throughout the crystal1 but is not so marked as in the platinocyanides or willemite4 ,the mineral sparteite#ahg1 a form of calcite containing a few per cent4 of manganese1 has been found by ,ambrecht to fluoresce with a very deep orange light under the and rays4 ,the colour appears to depend on the intensity of the rays1 and is deeper close to the radium than at some distance away4 ,if kunzite and sparteite are exposed to the action of the cathode rays in a vacuum tube1 the colour is different from that produced by the radium rays4 ,the former appears a deep yellow1 instead of the deep red observed with the radium rays4 ,the different actions of the radium rays on these fluorescent substances can be illustrated very simply and beautifully by the following experiment4 ,a small ,u tube is filled with fragments of the fluorescent substance arranged in layers4 ,the ,u tube is immersed in liquid air and the emanation from about #cj mgrs4 of radium bromide is condensed in the tube4 ,on closing the tube and removing it from the liquid air1 the emanation distributes itself uniformly in the tube4 ,the shades of colour produced in the different substances are clearly seen4 ,it is observed that all the crystals increase in luminosity for several hours1 on account of the excited activity produced by the emanation4 ,this effect is especially observed in kunzite1 which at first hardly responds to the rays1 since the and rays1 which causes it to fluoresce1 are not given out by the emanation itself but by one of its later products4 ,the intensity of the and rays is1 in consequence1 small at first but rises to a maximum after several hours2 the luminosity observed varies in a corresponding manner4 ,sir ,william ,crookes#ahh has made an examination of the effect of continued exposure of a diamond to the radium rays4 ,an 8off-colour0 diamond1 of a pale yellow colour1 was placed inside a tube with radium bromide4 ,after #gh days' exposure1 the diamond had darkened and become bluish green in tint2 when heated at #ej in a mixture of potassium chlorate for ten days1 the diamond lost its dull surface colour and was bright and transparent1 and its tint had changed to a pale bluish green4 ,the rays have thus a double action on the diamond2 the less penetrating rays produce a superficial darkening due to the change of the surface into graphite1 while the more penetrating rays and the rays produce a change of colour throughout its mass4 ,the diamond phosphoresced brightly during the whole course of its exposure to the rays4 ,crookes also observed that the diamond still retained enough activity to affect a photographic plate #ce days after removal1 although1 during the period of #aj days1 it was heated in a mixture sufficiently powerful to remove the outer skin of graphite4 ,this residual activity may possibly be due to a slow transformation product of the emanation which is deposited on the surface of bodies 7see chapter ,,xi74 ,marckwald observed that the rays from radio-tellurium produced marked phosphorescence on some kinds of diamonds4 ,an account of the various luminous effects produced on different gems by exposure to the radium and actinium rays has been given by ,kunz and ,baskerville#ahi4 ,both zinc sulphide and platinocyanide of barium diminish in luminosity after exposure for some time to the action of the rays4 ,to regenerate a screen of the latter1 exposure to solar light is necessary4 ,a similar phenomenon has been observed by ,villard for a screen exposed to ,rntgen rays4 ,giesel made a screen of platinocyanide of radio-active barium4 ,the screen1 very luminous at first1 gradually turned brown in colour1 and at the same time the crystals became dichroic4 ,in this condition the luminosity was much less1 although the active substance had increased in activity after preparation4 ,many of the substances which are luminous under the rays from active substances lose this property to a large extent at low temperatures#aij4 #aaf4 ,luminosity of radium compounds4 ,all radium compounds are spontaneously luminous4 ,this luminosity is especially brilliant in the dry haloid salts1 and persists for long intervals of time4 ,in damp air the salts lose a large amount of their luminosity1 but they recover it on drying4 ,with very active radium chloride1 the ,curies have observed that the light changes in colour and intensity with time4 ,the original luminosity is recovered if the salt is dissolved and dried4 ,many inactive preparations of radiferous barium are strongly luminous4 ,the writer has seen a preparation of impure radium bromide which gave out a light sufficient to read by in a dark room4 ,the luminosity of radium persists over a wide range of temperature and is as bright at the temperature of liquid air as at ordinary temperatures4 ,a slight luminosity is observed in a solution of radium1 and if crystals are being formed in the solution1 they can be clearly distinguished in the liquid by their greater luminosity4 #aag4 ,spectrum of the phosphorescent light of radium and actinium4 ,compounds of radium1 with a large admixture of barium1 are usually strongly self-luminous4 ,this luminosity decreases with increasing purity1 and pure radium bromide is only very feebly self-luminous4 ,a spectroscopic examination of the slight phosphorescent light of pure radium bromide has been made by ,sir ,william and ,lady ,huggins#aia4 ,on viewing the light with a direct vision spectroscope1 there were faint indications of a variation of luminosity at different points along the spectrum4 ,in order to get a photograph of the spectrum within a reasonable time1 they made use of a quartz spectroscope of special design which had been previously employed in a spectroscopic examination of faint celestial objects4 ,after three days' exposure with a slit of of an inch in width1 a negative was obtained which showed a number of bright lines4 ,the magnified spectrum is shown in ,fig4 #df ,a4 ,the lines of this spectrum were found to agree not only in position but also in relative intensity with the band spectrum of nitrogen4 ,the band spectrum of nitrogen and also the spark spectrum#aib of radium are shown in the same figure4 ,some time afterwards ,sir ,william ,crookes and ,prof4 ,dewar showed that this spectrum of nitrogen was not obtained if the radium was contained in a highly exhausted tube4 ,thus it appears that the spectrum is due to the action of the radium rays either on occluded nitrogen or the nitrogen in the atmosphere surrounding the radium4 ,it is very remarkable that a phosphorescent light1 like that of radium bromide1 should show a bright line spectrum of nitrogen4 ,it shows that radium at ordinary temperatures is able to set up radiations which are produced only by the electric discharge under special conditions4 ,sir ,william and ,lady ,huggins were led to examine the spectrum of the natural phosphorescent light of radium with the hope that some indications might be obtained thereby of the processes occurring in the radium atom4 ,since the main radiation from radium consists of positively charged atoms projected with great velocity1 radiations must be set up both in the expelled body and in the system from which it escapes4 ,illustration3 ,fig4 #df;a4 ,giesel#aic observed that the spectrum of the phosphorescent light of actinium consists of three bright lines4 ,measurements of the wave length were made by ,hartmann#aid4 ,the luminosity was very slight and a long exposure was required4 ,the lines observed were in the red1 blue and green4 ,the wave length and velocity are shown below4 ,line ,intensity #a #aj #dhhed #ja ngstrm units #b #f #ecjj #f #c #a #eiji #aj ,the line #dhhe was very broad2 the other two lines were so feeble that it was difficult to determine their wave length with accuracy4 ,hartmann suggests that these lines may be found in the spectrum of the new stars4 ,the lines observed have no connection with radium or its emanation#aie4 #aah4 ,thermo-luminescence4 ,e4 ,wiedemann and ,schmidt#aif have shown that certain bodies after exposure to the cathode rays or the electric spark become luminous when they are heated to a temperature much below that required to cause incandescence4 ,this property of thermo-luminescence is most strikingly exhibited in certain cases where two salts1 one of which is much in excess of the other1 are precipitated together4 ,it is to be expected that such bodies would also acquire the property when exposed to the or cathodic rays of radium4 ,this has been found to be the case by ,wiedemann#aig4 ,becquerel showed that fluor-spar1 exposed to the radium rays1 was luminous when heated4 ,the glass tubes in which radium is kept are rapidly blackened4 ,on heating the tube1 a strong luminosity is observed1 and the coloration to a large extent disappears4 ,the peculiarity of many of these bodies lies in the fact that the property of becoming luminous when heated is retained for a long interval of time after the body is removed from the influence of the exciting cause4 ,it appears probable that the rays cause chemical changes in these bodies1 which are permanent until heat is applied4 ,a portion of the chemical energy is then released in the form of visible light4 ,physical actions4 #aai4 ,some electric effects4 ,radium rays have the same effect as ultra-violet light and ,rntgen rays in increasing the facility with which a spark passes between electrodes4 ,elster and ,geitel#aih showed that if two electrodes were separated by a distance such that the spark just refused to pass1 on bringing near a specimen of radium the spark at once passes4 ,this effect is best shown with short sparks from a small induction coil4 ,the ,curies have observed that radium completely enveloped by a lead screen #a cm4 thick produces a similar action4 ,the effect in that case is due to the rays alone4 ,this action of the rays can be very simply illustrated by connecting two spark-gaps with the induction coil in parallel4 ,the spark-gap of one circuit is adjusted so that the discharge just refuses to pass across it1 but passes by the other4 ,when some radium is brought near the silent spark-gap1 the spark at once passes and ceases in the other#aii4 ,hemptinne#bjj found that the electrodeless discharge in a vacuum tube began at a higher pressure when a strong preparation of radium was brought near the tube4 ,in one experiment the discharge without the rays began at #ea mms4 but with the radium rays at #fh mms4 ,the colour of the discharge was also altered4 ,himstedt#bja found that the resistance of selenium was diminished by the action of radium rays in the same way as by ordinary light4 ,f4 ,henning#bjb examined the electrical resistance of a barium chloride solution containing radium of activity #ajjj1 but could observe no appreciable difference between it and a similar pure solution of barium chloride4 ,this experiment shows that the action of the rays from the radium does not produce any appreciable change in the conductivity of the barium solution4 ,kohlrausch and ,henning#bjc have recently made a detailed examination of the conductivity of pure radium bromide solutions1 and have obtained results very similar to those for the corresponding barium solutions4 ,kohlrausch#bjd found that the conductivity of water exposed to the radiations from radium increased more rapidly than water which had not been exposed4 ,this increase of conductivity may have been due to an increase of the conductivity of the water itself1 or to an increased rate of solution of the glass of the containing vessel4 ,specimens of strongly active material have been employed to obtain the potential at any point of the atmosphere4 ,the ionization due to the active substance is so intense that the body to which it is attached rapidly takes up the potential of the air surrounding the active substance4 ,in this respect it is more convenient and rapid in its action than the ordinary taper or water dropper1 but on account of the disturbance of the electric field by the strong ionization produced1 it is probably not so accurate a method as that of the water dropper4 #abj4 ,effect on liquid and solid dielectrics4 ,p4 ,curie#bje made the very important observation that liquid dielectrics became partial conductors under the influence of radium rays4 ,in these experiments the radium1 contained in a glass tube1 was placed in an inner thin cylinder of copper4 ,this was surrounded by a concentric copper cylinder1 and the liquid to be examined filled the space between4 ,a strong electric field was applied1 and the current through the liquid measured by means of an electrometer4 ,the following numbers illustrate the results obtained3 ,substance ,conductivity in megohms per #a cm4 ,carbon bisulphide #bj #aj ,petroleum ether #ae ,amyline #ad ,carbon chloride #h ,benzene #d ,liquid air #ac ,vaseline oil #af ,liquid air1 vaseline oil1 petroleum ether1 amyline1 are normally nearly perfect insulators4 ,the conductivity of amyline and petroleum ether due to the rays at #ag ,c4 was only of its value at #j ,c4 ,there is thus a marked action of temperature on the conductivity4 ,for very active material the current was proportional to the voltage4 ,with material of only of the activity1 it was found that ,ohm's law was not obeyed4 ,the following numbers were obtained3 ,volts ,current #ej #aji #ajj #ahe #bjj #bee #djj #cce ,for an increase of voltage of #h times1 the current only increases about #c times4 ,the current in the liquid thus tends to become 8saturated0 as does the ordinary ionization current through a gas4 ,these results have an important bearing on the ionization theory1 and show that the radiation probably produces ions in the liquid as well as in the gas4 ,it was also found that ,x rays increased the conductivity to about the same extent as the radium rays4 ,becquerel#bjf has recently shown that solid paraffin exposed to the and rays of radium acquires the property of conducting electricity to a slight extent4 ,after removal of the radium the conductivity diminishes with time according to the same law as for an ionized gas4 ,these results show that a solid as well as a liquid and gaseous dielectric is ionized under the influence of radium rays4 #aba4 ,effect of temperature on the radiations4 ,becquerel#bjg1 by the electric method1 determined the activity of uranium at the temperature of liquid air1 and found that it did not differ more than #a per cent4 from the activity at ordinary temperatures4 ,in his experiments1 the rays from the uranium were absorbed before reaching the testing vessel1 and the electric current measured was due to the rays alone4 ,p4 ,curie#bjh found that the luminosity of radium and its power of exciting fluorescence in bodies were retained at the temperature of liquid air4 ,observations by the electric method showed that the activity of radium was unaltered at the temperature of liquid air4 ,if a radium compound is heated in an open vessel1 it is found that the activity1 measured by the rays1 falls to about #be per cent4 of its original value4 ,this is however not due to a change in the radio-activity1 but to the release of the radio-active emanation1 which is stored in the radium4 ,no alteration is observed if the radium is heated in a closed vessel from which none of the radio-active products are able to escape4 #abb4 ,motion of radium in an electric field4 ,joly#bji found that a disc1 one side of which is coated with a few milligrams of radium bromide1 exhibits1 when an electrified body is brought near it1 motions very different to those observed in the case of an inactive substance4 ,the electrified body1 whether positive or negative1 repels the suspended body if brought up to it on the side coated with radium1 but attracts it if presented to the naked side4 ,this effect is very simply shown by constructing a small apparatus like a radiometer4 ,two covered glasses are attached to the end of a glass fibre about #f cms4 long1 the surfaces lying in the same plane4 ,the apparatus is free to rotate on a pivot4 ,the two vanes are coated on alternate faces with radium bromide1 and the whole apparatus contained within a glass receiver4 ,if an electrified rod of ebonite or sealing wax is brought up close to the receiver1 a rotation is communicated to the vane which increases as the pressure of the air is lowered to #e or #f cms4 of mercury4 ,by placing the apparatus between parallel plates connected with the terminals of a ,wimshurst machine1 a steady rotation is communicated to the vanes4 ,the rotation is always in such a direction that the radium coated surface is repelled from the electrified body4 ,this action was examined still further by attaching the vanes to the glass beam of a ,coulomb's balance4 ,a metal sphere1 which could be charged from without1 was fixed facing the side coated with radium4 ,a repulsion was always observed except when the charge was very strong and the vane near the sphere4 ,if1 however1 the two vanes were connected by a light wire and a similar sphere placed exactly opposite the other1 an attraction was observed if one sphere was charged1 but a repulsion if both were charged4 ,these effects were observed whether the vanes were of aluminium or glass4 ,joly found that the effect could not be explained by any direct action due to the movement of the ions in an electric field4 ,the recoil1 due to the expulsion of particles from one side of the vane1 is far too small to account for the movement observed4 ,this effect can1 ,i think1 be simply accounted for by taking into consideration the difference in conductivity of the gas on the two sides of the radium coated vane4 ,if a small vane1 coated uniformly with radium on both sides1 and mounted on an insulating support1 be brought near a charged body kept at a constant potential1 it acts like a water dropper and rapidly acquires very nearly the average potential which existed at that point before the vane was brought up4 ,the mechanical force acting on the vane will1 in consequence1 be small4 ,if1 however1 the vane is only coated with radium on the side near the charged body1 the ionization and consequently the conductivity of the gas is much greater between the vane and the charged body than on the opposite side4 ,suppose1 for simplicity1 the body is charged to a positive potential4 ,on account of the greater conductivity of the gas on the side facing the charged body1 it will rapidly acquire a positive charge1 and the potential of the vane will reach a higher value than existed at that place before the vane was introduced4 ,this will result in a repulsion of the vane4 ,this also accounts for the attraction observed in the experiment with the ,coulomb's balance already referred to4 ,suppose that one sphere is positively charged and the other earthed1 and the two vanes metallically connected together4 ,the vane next to the charged body will become charged positively1 but this charge will be dissipated rapidly on account of the ionization of the gas close to the opposite vane1 and1 in most conditions1 this loss of charge will be so rapid that the potential of the vane is unable to reach the value which would exist at that place in the field1 if the vane were removed4 ,there will1 in consequence1 be an attracting force acting on the vane towards the sphere4 ,the repulsion observed by ,joly is thus only an indirect result of the ionization in the gas produced by the radium1 and should be shown under conditions where similar unequal distribution of ionization is produced by any other sources4 ,since radium gives out heat at a fairly rapid rate1 a radiometer in which the vanes were coated on one side with radium instead of lampblack1 should rotate at low pressure of the gas1 even if no source of light is brought near it4 ,this should evidently be the case1 since the face coated with radium should reach a slightly higher temperature than the other4 ,this experiment has been tried1 but the effect seems too small to produce rotation of the vanes4 ,chemical actions4 #abc4 ,rays from active radium preparations change oxygen into ozone#baj4 ,its presence can be detected by the smell or by the action on iodide of potassium paper4 ,this effect is due to the and rays from the radium1 and not to the luminous rays from it4 ,since energy is required to produce ozone from oxygen1 this must be derived from the energy of the radiations4 ,the ,curies found that radium compounds rapidly produced coloration in glass4 ,for moderately active material the colour is violet1 for more active material it is yellow4 ,long continued action blackens the glass1 although the glass may have no lead in its composition4 ,this coloration gradually extends through the glass1 and is dependent to some extent on the kind of glass used4 ,giesel#baa found that he could obtain as much coloration in rock-salt and fluor-spar by radium rays1 as by exposure to the action of cathode rays in a vacuum tube4 ,the coloration1 however1 extended much deeper than that produced by the cathode rays4 ,this is to be expected1 since the radium rays have a higher velocity1 and consequently greater penetrating power1 than the cathode rays produced in an ordinary vacuum tube4 ,goldstein observed that the coloration is far more intense and rapid when the salts are melted or heated to a red heat4 ,melted potassium sulphate1 under the action of a very active preparation of radium1 was rapidly coloured a strong greenish blue which gradually changed into a dark green4 ,salomonsen and ,dreyer#bab found that plates of quartz were coloured by exposure to radium rays4 ,when examined minutely1 plates cut perpendicular to the optic axis showed the presence of lines and striae1 parallel to the binary axes4 ,adjacent portions of the striated system differed considerably in intensity of coloration and clearly revealed the heterogeneity of structures of the crystal4 ,the cause of these colorations by cathode and radium rays has been the subject of much discussion4 ,elster and ,geitel#bac observed that a specimen of potassium sulphate1 coloured green by radium rays1 showed a strong photo-electric action1 i4e4 it rapidly lost a negative charge of electricity when exposed to the action of ultra-violet light4 ,all substances coloured by cathode rays show a strong photo-electric action1 and1 since the metals sodium and potassium themselves show photo-electric action to a very remarkable degree1 ,elster and ,geitel have suggested that the colorations are caused by a solid solution of the metal in the salt4 ,although the coloration due to radium rays extends deeper than that due to the cathode rays1 when exposed to light the colour fades away at about the same rate in the two cases4 ,becquerel#bad found that white phosphorus is changed into the red variety by the action of radium rays4 ,this action was shown to be due mainly to the rays4 ,the secondary radiation set up by the primary rays also produced a marked effect4 ,radium rays1 like ordinary light rays1 also caused a precipitate of calomel in the presence of oxalic acid4 ,hardy and ,miss ,willcock#bae found that a solution of iodoform in chloroform turned purple after exposure for #e minutes to the rays from #e milligrams of radium bromide4 ,this action is due to the liberation of iodine4 ,by testing the effect of screens of different thicknesses1 over the radium1 this action was found to be mainly due to the rays from the radium4 ,rntgen rays produce a similar coloration4 ,hardy#baf also observed an action of the radium rays on the coagulation of globulin4 ,two solutions of globulin from ox serum were used1 one made electro-positive by adding acetic acid1 and the other electro-negative by adding ammonia4 ,when the globulin was exposed close to the radium in naked drops1 the opalescence of the electro-positive solution rapidly diminished1 showing that the solution became more complete4 ,the electro-negative solution was rapidly turned to a jelly and became opaque4 ,these actions were found to be due to the rays of radium alone4 ,this is further evidence in favour of the view that the rays consist of projected positively charged bodies of atomic dimensions1 for a similar coagulation effect is produced by the metallic ions of liquid electrolytes1 and has been shown by ,w4 ,c4 ,d4 ,whetham#bag to be due to the electric charges carried by the ions4 #abd4 ,gases evolved from radium4 ,curie and ,debierne#bah observed that radium preparations placed in a vacuum tube continually lowered the vacuum4 ,the gas evolved was always accompanied by the emanation1 but no new lines were observed in its spectrum4 ,giesel#bai has observed a similar evolution of gas from solutions of radium bromide4 ,giesel forwarded some active material to ,runge and ,bdlander1 in order that they might test the gas spectroscopically4 ,from #a gram of a #e per cent4 radium preparation they obtained #ce c4c4 of gas in #af days4 ,this gas was found1 however1 to be mainly hydrogen1 with #ab per cent4 of oxygen4 ,in later experiments ,ramsay and ,soddy#bbj found that #ej milligrams of radium bromide evolved gases at the rate of about #je c4c4 per day4 ,this is a rate of evolution about twice that observed by ,runge and ,bdlander4 ,on analysing the gases about #bhi per cent4 consisted of oxygen1 and the rest hydrogen4 ,the slight excess of hydrogen over that attained in the decomposition of water1 they consider to be due to the action of oxygen on the grease of the stop-cocks4 ,the radio-active emanation from radium has a strong oxidizing action and rapidly produces carbon dioxide1 if carbonaceous matter is present4 ,the production of gas is probably due to the action of the radiations in decomposing water4 ,the amount of energy required to produce the rate of decomposition observed by ,ramsay and ,soddy--about #aj c4c4 per day for #a gram of radium bromide--corresponds to about #cj gram-calories per day4 ,this amount of energy is about two per cent4 of the total energy emitted in the form of heat4 ,ramsay and ,soddy 7loc4 cit47 have also observed the presence of helium in the gases evolved by solution of radium bromide4 ,this important result is considered in detail in section #bfg4 ,physiological actions4 #abe4 ,walkhoff first observed that radium rays produce burns of much the same character as those caused by ,rntgen rays4 ,experiments in this direction have been made by ,giesel1 ,curie and ,becquerel1 and others1 with very similar results4 ,there is at first a painful irritation1 then inflammation sets in1 which lasts from #aj to #bj days4 ,this effect is produced by all preparations of radium1 and appears to be due mainly to the and rays4 ,care has to be taken in handling radium on account of the painful inflammation set up by the rays4 ,if a finger is held for some minutes at the base of a capsule containing a radium preparation1 the skin becomes inflamed for about #ae days and then peels off4 ,the painful feeling does not disappear for two months4 ,danysz#bba found that this action is mainly confined to the skin1 and does not extend to the underlying tissue4 ,caterpillars subjected to the action of the rays lost their power of motion in several days and finally died4 ,radium rays have been found beneficial in certain cases of cancer4 ,the effect is apparently similar to that produced by ,rntgen rays1 but the use of radium possesses the great advantage that the radiating source can be enclosed in a fine tube and introduced at the particular point at which the action of the rays is required4 ,the rays have also been found to hinder or stop the development of microbes#bbb4 ,it would be out of place here to give an account of the numerous experiments that have been made by physicists and physiologists on the action of the rays of radium and of other radio-active substances on different organisms1 such as caterpillars1 mice and guinea-pigs4 ,in some cases1 the experiments have been carried out by placing the organisms in an atmosphere impregnated with the radium emanation4 ,the effect of an exposure under such conditions for several days or weeks has been found generally harmful and in many cases fatal4 ,the literature in this new department of study is already large and is increasing rapidly4 ,another interesting action of the radium rays has been observed by ,giesel4 ,on bringing up a radium preparation to the closed eye1 in a dark room1 a sensation of diffuse light is observed4 ,this effect has been examined by ,himstedt and ,nagel#bbc who have shown that it is due to a fluorescence produced by the rays in the eye itself4 ,the blind are able to perceive this luminosity if the retina is intact1 but not if the retina is diseased4 ,hardy and ,anderson#bbd have examined this effect in some detail4 ,the sensation of light is produced both by the and rays4 ,the eyelid practically absorbs all the rays1 so that the luminosity observed with a closed eye is due to the rays alone4 ,the lens and retina of the eye are strongly phosphorescent under the action of the and rays4 ,hardy and ,anderson consider that the luminosity observed in a dark room with the open eye 7the phosphorescent light of the radium itself being stopped by black paper7 is to a large extent due to the phosphorescence set up in the eyeball4 ,the rays1 for the most part1 produce the sensation of light when they strike the retina4 ,tommasina stated that the air exhaled by man contained a larger proportion of ions than ordinary air1 and1 in consequence1 caused an increased rate of discharge of an electroscope4 ,the experiment was repeated by ,elster and ,geitel but with negative results4 ,on the other hand1 they found that the breath of ,dr ,giesel1 of ,braunschweig1 who had been engaged continuously in the chemical separation of the radio-active bodies1 caused a rapid loss of charge of an electroscope4 ,this increased rate of discharge was probably mainly due to the radium emanation1 with which his system had become impregnated by inhaling the emanation-laden air of the laboratory4 ,footnote #ahd3 ,becquerel1 ,c4 ,r4 #abi1 p4 #iab1 #ahii4 ,footnote #ahe3 ,bary1 ,c4 ,r4 #acj1 p4 #ggf1 #aijj4 ,footnote #ahf3 ,kunz and ,baskerville1 ,amer4 ,journ4 ,science ,,xvi4 p4 #cce1 #aijc4 ,footnote #ahg3 ,see ,nature1 p4 #ebc1 ,march #ca1 #aijd4 ,footnote #ahh3 ,crookes1 ,proc4 ,roy4 ,soc4 #gd1 p4 #dg1 #aijd4 ,footnote #ahi3 ,kunz and ,baskerville1 ,science ,,xviii1 p4 #gfi1 ,dec4 #ah1 #aijc4 ,footnote #aij3 ,beilby in a recent communication to the ,royal ,society 7,feb4 #i and #bc1 #aije7 has examined in some detail the production of phosphorescence by the and rays of radium and has put forward a theory to account for the different actions observed4 ,footnote #aia3 ,huggins1 ,proc4 ,roy4 ,soc4 #gb1 pp4 #aif and #dji1 #aijc4 ,footnote #aib3 ,the spark spectrum of the radium bromide showed the ,h and ,k lines of calcium and also faintly some of the strong lines of barium4 ,the characteristic lines of radium of wave-lengths #chadei1 #cfdig1 #dcdjf and #bgjhf1 as shown by ,demaray and others are clearly shown in the figure4 ,the strong line of wave-length about #bhad is due to radium4 ,footnote #aic3 ,giesel1 ,ber4 d4 ,d4 ,chem4 ,ges4 #cg1 p4 #afif1 #aijd4 ,footnote #aid3 ,hartmann1 ,phys4 ,zeit4 #e1 ,no4 #ah1 p4 #egj1 #aijd4 ,footnote #aie3 ,in a recent paper1 ,giesel 7,ber4 d4 ,d4 ,chem4 ,ges4 ,no4 #c1 p4 #gge1 #aije7 has shown that the bright lines are due to didymium1 which is present as an impurity4 ,exposure of didymium to the radium rays also causes the appearance of the lines4 ,footnote #aif3 ,wiedemann and ,schmidt1 ,wied4 ,annal4 #ei1 p4 #fjd1 #ahie4 ,footnote #aig3 ,wiedemann1 ,phys4 ,zeit4 #b1 p4 #bfi1 #aija4 ,footnote #aih3 ,elster and ,geitel1 ,annal4 d4 ,phys4 #fi1 p4 #fgc1 #ahii4 ,footnote #aii3 ,willows and ,peck 7,phil4 ,mag4 ,march1 #aije7 found that under some conditions1 especially for long sparks1 the rays of radium hindered the passage of the spark4 ,footnote #bjj3 ,hemptinne1 ,c4 ,r4 #acc1 p4 #icd1 #aija4 ,footnote #bja3 ,himstedt1 ,phys4 ,zeit4 p4 #dgf1 #aijj4 ,footnote #bjb3 ,henning1 ,annal4 d4 ,phys4 p4 #efb1 #aijb4 ,footnote #bjc3 ,kohlrausch and ,henning1 ,verh4 ,deutsch4 ,phys4 ,ges4 #f1 p4 #add1 #aijd4 ,footnote #bjd3 ,kohlrausch1 ,verh4 ,deutsch4 ,phys4 ,ges4 #e1 p4 #bfa1 #aijd4 ,footnote #bje3 ,p4 ,curie1 ,c4 ,r4 #acd1 p4 #dbj1 #aijb4 ,footnote #bjf3 ,becquerel1 ,c4 ,r4 #acf1 p4 #aagc1 #aijc4 ,footnote #bjg3 ,becquerel1 ,c4 ,r4 #acc1 p4 #aii1 #aija4 ,footnote #bjh3 ,p4 ,curie1 ,socit de ,physique1 ,march #b1 #aijj4 ,footnote #bji3 ,joly1 ,phil4 ,mag4 ,march1 #aijd4 ,footnote #baj3 ,s4 and ,p4 ,curie1 ,c4 ,r4 #abi1 p4 #hbc1 #ahii4 ,footnote #baa3 ,giesel1 ,verhandlg4 d4 ,d4 ,phys4 ,ges4 ,jan4 #e1 #aijj4 ,footnote #bab3 ,salomonsen and ,dreyer1 ,c4 ,r4 #aci1 p4 #ecc1 #aijd4 ,footnote #bac3 ,elster and ,geitel1 ,phys4 ,zeit4 p4 #aac1 ,no4 #c1 #aijb4 ,footnote #bad3 ,becquerel1 ,c4 ,r4 #acc1 p4 #gji1 #aija4 ,footnote #bae3 ,hardy and ,miss ,willcock1 ,proc4 ,roy4 ,soc4 #gb1 p4 #bjj1 #aijc4 ,footnote #baf3 ,hardy1 ,proc4 ,physiolog4 ,soc4 ,may #af1 #aijc4 ,footnote #bag3 ,whetham1 ,phil4 ,mag4 ,nov4 #ahii2 ,theory of ,solution1 ,camb4 #aijb1 p4 #cif4 ,footnote #bah3 ,curie and ,debierne1 ,c4 ,r4 #acb1 p4 #gfh1 #aija4 ,footnote #bai3 ,giesel1 ,ber4 ,d4 d4 ,chem4 ,ges4 #ce1 p4 #cfje1 #aijb4 ,footnote #bbj3 ,ramsay and ,soddy1 ,proc4 ,roy4 ,soc4 #gb1 p4 #bjd1 #aijc4 ,footnote #bba3 ,danysz1 ,c4 ,r4 #acf1 p4 #dfa1 #aijc4 ,footnote #bbb3 ,aschkinass and ,caspari1 ,arch4 d4 ,ges4 ,physiologie1 #hf1 p4 #fjc1 #aija4 ,footnote #bbc3 ,himstedt and ,nagel1 ,drude's ,annal4 #d1 p4 #ecg1 #aija4 ,footnote #bbd3 ,hardy and ,anderson1 ,proc4 ,roy4 ,soc4 #gb1 p4 #cic1 #aijc4 ,,chapter ,,vi4 ,,continuous ,,production ,,of ,,radio-,,active ,,matter4 #abf4 ,an account will now be given of some experiments which have thrown much light1 not only on the nature of the processes which serve to maintain the radio-activity of the radio-active bodies1 but also on the source of the energy continuously emitted by those bodies4 ,in this chapter1 for simplicity1 the radio-activity of uranium and thorium will alone be considered1 for it will be seen later that the changes taking place in these two substances are typical of those which occur in all radio-active substances4 ,we have seen 7section #bc7 that there is some doubt whether the radio-activity of thorium is due to that element itself1 or to an unknown radio-active constituent associated with it4 ,this uncertainty1 however1 will present no serious difficulty when we are discussing the radio-activity of thorium1 for the general conclusions are1 for the most part1 independent of whether thorium is the primary radio-active constituent or not4 ,for simplicity1 however1 it will be assumed for the present that the radio-activity is due to thorium itself4 ,if future research should definitely show that the radio-activity1 ordinarily observed in thorium1 is due to a new radio-active element mixed with it1 the radio-active processes considered will refer to this new element4 #abg4 ,uranium ,x4 ,the experiments of ,mme ,curie show that the radio-activity of uranium and radium is an atomic phenomenon4 ,the activity of any uranium compound depends only on the amount of that element present1 and is unaffected by its chemical combination with other substances1 and is not appreciably affected by wide variations of temperature4 ,it would thus seem probable1 since the activity of uranium is a specific property of the element1 that the activity could not be separated from it by chemical agencies4 ,in #aijj1 however1 ,sir ,william ,crookes#bbe showed that1 by a single chemical operation1 uranium could be obtained photographically inactive while the whole of the activity could be concentrated in a small residue free from uranium4 ,this residue1 to which he gave the name of ,ur ,x1 was many hundred times more active photographically1 weight for weight1 than the uranium from which it had been separated4 ,the method employed for this separation was to precipitate a solution of the uranium with ammonium carbonate4 ,on dissolving the precipitate in an excess of the reagent1 a light precipitate remained behind4 ,this was filtered1 and constituted the ,ur ,x4 ,the active substance ,ur ,x was probably present in very small quantity1 mixed with impurities derived from the uranium4 ,no new lines were observed in its spectrum4 ,a partial separation of the activity of uranium was also effected by another method4 ,crystallized uranium nitrate was dissolved in ether1 when it was found that the uranium divided itself between the ether and water present in two unequal fractions4 ,the small part dissolved in the water layer was found to contain practically all the activity when examined by the photographic method1 while the other fraction was almost inactive4 ,these results1 taken by themselves1 pointed very strongly to the conclusion that the activity of uranium was not due to the element itself1 but to some other substance1 associated with it1 which had distinct chemical properties4 ,results of a similar character were observed by ,becquerel#bbf4 ,it was found that barium could be made photographically very active by adding barium chloride to the uranium solution and precipitating the barium as sulphate4 ,by a succession of precipitations the uranium was rendered photographically almost inactive1 while the barium was strongly active4 ,the inactive uranium and the active barium were laid aside2 but1 on examining them a year later1 it was found that the uranium had completely regained its activity1 while that of the barium had completely disappeared4 ,the loss of activity of uranium was thus only temporary in character4 ,in the above experiments1 the activity of uranium was examined by the photographic method4 ,the photographic action produced by uranium is due almost entirely to the rays4 ,the rays1 in comparison1 have little if any effect4 ,now the radiation from ,ur ,x consists entirely of rays1 and is consequently photographically very active4 ,if the activity of uranium had been measured electrically without any screen over it1 the current observed would have been due very largely to the rays1 and little change would have been observed after the removal of ,ur ,x1 since only the constituent responsible for the rays was removed4 ,this important point is discussed in more detail in section #bje4 #abh4 ,thorium ,x4 ,rutherford and ,soddy#bbg1 working with thorium compounds1 found that an intensely active constituent could be separated from thorium by a single chemical operation4 ,if ammonia is added to a thorium solution1 the thorium is precipitated1 but a large amount of the activity is left behind in the filtrate1 which is chemically free from thorium4 ,this filtrate was evaporated to dryness1 and the ammonium salts driven off by ignition4 ,a small residue was obtained which1 weight for weight1 was in some cases several thousand times more active than the thorium from which it was obtained1 while the activity of the precipitated thorium was reduced to less than one half of its original value4 ,this active constituent was named ,th ,x from analogy to ,crookes' ,ur ,x4 ,the active residue was found to consist mainly of impurities from the thorium2 the ,th ,x could not be examined chemically1 and probably was present only in minute quantity4 ,it was also found that an active constituent could be partly separated from thorium oxide by shaking it with water for some time4 ,on filtering the water1 and evaporating down1 a very active residue was obtained which was analogous in all respects to ,th ,x4 ,on examining the products a month later1 it was found that the ,th ,x was no longer active1 while the thorium had completely regained its activity4 ,a long series of measurements was then undertaken to examine the time-rate of these processes of decay and recovery of activity4 ,illustration3 ,fig4 #dg4 ,the results are shown graphically in ,fig4 #dg1 where the final activity of the thorium and the initial activity of the ,th ,x are in each case taken as #ajj4 ,the ordinates represent the activities determined by means of the ionization current1 and the abscissae represent the time in days4 ,it will be observed that both curves are irregular for the first two days4 ,the activity of the ,th ,x increased at first1 while the activity of the thorium diminished4 ,disregarding these initial irregularities of the curves1 which will be explained in detail in section #bjh1 it will be seen that1 after the first two days1 the time taken for the thorium to recover half its lost activity is about equal to the time taken by the ,th ,x to lose half its activity4 ,this time in each case is about four days4 ,the percentage proportion of the activity regained by the thorium1 over any given interval1 is approximately equal to the percentage proportion of the activity lost by the ,th ,x during the same interval4 ,illustration3 ,fig4 #dh4 ,if the recovery curve is produced backwards to meet the vertical axis1 it does so at a minimum of #be per cent41 and the above conclusions hold more accurately1 if the recovery is assumed to start from this minimum4 ,this is clearly shown by ,fig4 #dh1 where the percentages of activity recovered1 reckoned from the #be per cent4 minimum1 are plotted as ordinates4 ,in the same figure the decay curve1 after the second day1 is shown on the same scale4 ,the activity of the ,th ,x decays with the time according to an exponential law1 falling to half value in about four days4 ,if ,i is the initial activity and ,it is the activity after a time t1 then frac ,i ,it e-t 1 where is a constant and e the natural base of logarithms4 ,the experimental curve of the rise of activity from a minimum to a maximum value is therefore expressed by the equation frac ,it ,i #a e-t 1 where ,i is the amount of activity recovered when the state of constant activity is reached1 ,it the activity recovered after a time t1 and is the same constant as before4 #abi4 ,uranium ,x4 ,similar results were obtained when uranium was examined4 ,the ,ur ,x was separated by ,becquerel's method of successive precipitations with barium4 ,the decay of the separated activity and the recovery of the lost activity are shown graphically in ,fig4 #di4 ,a more detailed discussion of this experiment is given in section #bje4 ,illustration3 ,fig4 #di4 ,the curves of decay and recovery exhibit the same peculiarities and can be expressed by the same equations as in the case of thorium4 ,the time-rate of decay and recovery is1 however1 much slower than for thorium1 the activity of the ,ur ,x falling to half its value in about #bb days4 ,a large number of results of a similar character have been obtained from other radio-active products1 separated from the radio-elements1 but the cases of thorium and uranium will suffice for the present to form a basis for the discussion of the processes that are taking place in radio-active bodies4 #acj4 ,theory of the phenomena4 ,these processes of decay and recovery go on at exactly the same rate if the substances are removed from the neighbourhood of one another1 or enclosed in lead1 or placed in a vacuum tube4 ,it is at first sight a remarkable phenomenon that the processes of decay and recovery should be so intimately connected1 although there is no possibility of mutual interaction between them4 ,these results1 however1 receive a complete explanation on the following hypotheses3 7#a7 ,that there is a constant rate of production of fresh radio-active matter by the radio-active body2 7#b7 ,that the activity of the matter so formed decreases according to an exponential law with the time from the moment of its formation4 ,suppose that q particles of new matter are produced per second from a given mass of matter4 ,the rate of emission of energy due to the particles produced in the time dt1 is1 at the moment of their formation1 equal to ,kqdt1 where ,k is a constant4 ,it is required to find the activity due to the whole matter produced after the process has continued for a time ,t4 ,the activity d,i1 due to the matter produced during the time dt at the time t1 decays according to an exponential law during the time ,t t that elapses before its activity is estimated1 and in consequence is given by d,i ,kqe- 7,t-t7 dt 1 where is the constant of decay of activity of the active matter4 ,the activity ,i,t due to the whole matter produced in the time ,t is thus given by ,it int,t ,kqe- 7,t-t7 dt frac ,kq 7#a e- ,t7 4 ,the activity reaches a maximum value ,i when ,t is very great1 and is then given by ,kq ,i ----- thus frac ,i,t ,i #a e- ,t 4 ,this equation agrees with the experimental results for the recovery of lost activity4 ,another method for obtaining this equation is given later in section #acc4 ,a state of equilibrium is reached when the rate of loss of activity of the matter already produced is balanced by the activity supplied by the production of new active matter4 ,according to this view1 the radio-active bodies are undergoing change1 but the activity remains constant owing to the action of two opposing processes4 ,now1 if this active matter can at any time be separated from the substance in which it is produced1 the decay of its activity1 as a whole1 should follow an exponential law with the time1 since each portion of the matter decreases in activity according to an exponential law with the time1 whatever its age may be4 ,if ,i is the initial activity of the separated product1 the activity ,it after an interval t is given by frac ,i,t ,i e-t 4 ,thus1 the two assumptions--of uniform production of active matter and of the decay of its activity in an exponential law from the moment of its formation--satisfactorily explain the relation between the curves of decay and recovery of activity4 #aca4 ,experimental evidence4 ,it now remains to consider further experimental evidence in support of these hypotheses4 ,the primary conception is that the radio-active bodies are able to produce from themselves matter of chemical properties different from those of the parent substance1 and that this process goes on at a constant rate4 ,this new matter initially possesses the property of activity1 and loses it according to a definite law4 ,the fact that a proportion of the activity of radium and thorium can be concentrated in small amounts of active matter like ,th ,x or ,ur ,x does not1 of itself1 prove directly that a material constituent responsible for the activity has been chemically separated4 ,for example1 in the case of the separation of ,th ,x from thorium1 it might be supposed that the non-thorium part of the solution is rendered temporarily active by its association with thorium1 and that this property is retained through the processes of precipitation1 evaporation1 and ignition1 and finally manifests itself in the residue remaining4 ,according to this view it is to be expected that any precipitate capable of removing the thorium completely from its solution should yield active residues similar to those obtained from ammonia4 ,no such case has1 however1 been observed4 ,for example1 when thorium nitrate is precipitated by sodium or ammonium carbonate1 the residue from the filtrate after evaporation and ignition is free from activity and the thorium carbonate obtained has the normal amount of activity4 ,in fact1 ammonia is the only reagent yet found capable of completely separating ,th ,x from thorium4 ,a partial separation of the ,th ,x can be made by shaking thorium oxide with water owing to the greater solubility of ,th ,x in water4 ,thorium and uranium behave quite differently with regard to the action of ammonia and ammonium carbonate4 ,ur ,x is completely precipitated with the uranium in an ammonia solution and the filtrate is inactive4 ,ur ,x is separated by ammonium carbonate1 while ,th ,x under the same conditions is completely precipitated with the thorium4 ,the ,ur ,x and the ,th ,x thus behave like distinct types of matter with well-marked chemical properties quite distinct from those of the substances in which they are produced4 ,the removal of ,ur ,x by the precipitation of barium is probably not directly connected with the chemical properties of ,ur ,x4 ,the separation is probably due to the dragging down of the ,ur ,x with the dense barium precipitate4 ,sir ,william ,crookes found that the ,ur ,x was dragged down by precipitates when no question of insolubility was involved1 and such a result is to be expected if the ,ur ,x exists in extremely minute quantity4 ,it must be borne in mind that the actual amount of the active constituents ,th ,x and ,ur ,x1 separated from thorium and uranium1 is probably infinitesimal1 and that the greater proportion of the residues is due to impurities present in the salt and the reagents1 a very small amount of active matter being mixed with them4 #acb4 ,rate of production of ,th ,x4 ,if the recovery of the activity of uranium or thorium is due to the continuous production of new active matter1 it should be possible to obtain experimental evidence of the process4 ,as the case of thorium has been most fully investigated1 a brief account will be given of some experiments made by ,rutherford and ,soddy#bbh to show that ,th ,x is produced continuously at a constant rate4 ,preliminary experiments showed that three successive precipitations were sufficient to remove the ,th ,x almost completely from the thorium4 ,the general method employed was to precipitate a solution of #e grams of thorium-nitrate with ammonia4 ,the precipitate was then redissolved in nitric acid and the thorium again precipitated as before1 as rapidly as possible1 so that the ,th ,x produced in the time between successive precipitations should not appreciably affect the results4 ,the removal of the ,th ,x was followed by measurements of the activity of the residues obtained from successive filtrates4 ,in three successive precipitations the activities of the residues were proportional to #ajj1 #h1 #af respectively4 ,thus two precipitations are nearly sufficient to free the thorium from ,th ,x4 ,the thorium freed from ,th ,x was then allowed to stand for a definite time1 and the amount of ,th ,x formed during that time found by precipitating it1 and measuring its radio-activity4 ,according to the theory1 the activity ,it of the thorium formed in the time t is given by frac ,it ,i #a e-t 1 where ,i is the total activity of ,th ,x1 when there is radio-active equilibrium4 ,if t is small1 ,it ------ t4 ,i ,since the activity of ,th ,x falls to half value in #d days1 the value of expressed in hours #jjgb4 ,after standing a period of #a hour about 1 after #a day 1 after #d days of the maximum should be obtained4 ,the experimental results obtained showed an agreement1 as good as could be expected1 with the equation expressing the result that the ,th ,x was being produced at a constant rate4 ,the thorium-nitrate which had been freed from ,th ,x was allowed to stand for one month1 and then it was again subjected to the same process4 ,the activity of the ,th ,x was found to be the same as that obtained from an equal amount of the original thorium-nitrate4 ,in one month1 therefore1 the ,th ,x had been regenerated1 and had reached a maximum value4 ,by leaving the thorium time to recover fully its activity1 this process can be repeated indefinitely1 and equal amounts of ,th ,x are obtained at each precipitation4 ,ordinary commercial thorium-nitrate and the purest nitrate obtainable showed exactly the same action1 and equal amounts of ,th ,x could be obtained from equal weights4 ,these processes thus appear to be independent of the chemical purity of the substance#bbi4 ,the process of the production of ,th ,x is continuous1 and no alteration has been observed in the amount produced in the given time after repeated separations4 ,after #bc precipitations extending over #i days1 the amount produced in a given interval was about the same as at the beginning of the process4 ,these results are all in agreement with the view that the ,th ,x is being continuously produced from the thorium compound at a constant rate4 ,the amount of active matter produced from #a gram of thorium is probably extremely minute1 but the electrical effects due to its activity are so large that the process of production can be followed after extremely short intervals4 ,with a sensitive electrometer the amount of ,th ,x produced per minute in #aj grams of thorium-nitrate gives a rapid movement to the electrometer needle4 ,for larger intervals it is necessary to add additional capacity to the system to bring the effects within range of the instrument4 #acc4 ,rate of decay of activity4 ,it has been shown that the activity of ,ur ,x and ,th ,x decays according to an exponential law with the time4 ,this1 we shall see later1 is the general law of decay of activity in any type of active matter1 obtained by itself1 and freed from any secondary active products which it may1 itself1 produce4 ,in any case1 when this law is not fulfilled1 it can be shown that the activity is due to the superposition of two or more effects1 each of which decays in an exponential law with the time4 ,the physical interpretation of this law still remains to be discussed4 ,it has been shown that in uranium and thorium compounds there is a continuous production of active matter which keeps the compound in radio-active equilibrium4 ,the changes by which the active matter is produced must be chemical in nature1 since the products of the action are different in chemical properties from the matter in which the changes take place4 ,the activity of the products has afforded the means of following the changes occurring in them4 ,it now remains to consider the connection between the activity at any time1 and the amount of chemical change taking place at that time4 ,in the first place1 it is found experimentally that the saturation ionization current it1 after the active product has been allowed to decay for a time t1 is given by frac it i e-t 1 where i is the initial saturation current and the constant of decay4 ,now the saturation current is a measure of the total number of ions produced per second in the testing vessel4 ,it has already been shown that the rays1 which produce the greater proportion of ionization in the gas1 consist of positively charged particles projected with great velocity4 ,suppose for simplicity that each atom of active matter1 in the course of its change1 gives rise to one projected particle4 ,each particle will produce a certain average number of ions in its path before it strikes the boundaries or is absorbed in the gas4 ,since the number of projected particles per second is equal to the number of atoms changing per second1 the number of atoms nt which change per second at the time t is given by frac nt n e-t 1 where n is the initial number which change per second4 ,on this view1 then1 the law of decay expresses the result that the number of atoms changing in unit time1 diminishes according to an exponential law with the time4 ,the number of atoms ,nt which remain unchanged after an interval t is given by ,nt inttinfty nt dt frac n e-t 4 ,if ,n is the number of atoms at the beginning1 ,n frac n 1 ,thus frac ,nt ,n e-t 7#a74 or the law of decay expresses the fact that the activity of a product at any time is proportional to the number of atoms which remain unchanged at that time4 ,this is the same as the law of monomolecular change in chemistry1 and expresses the fact that there is only one changing system4 ,if the change depended on the mutual action of two systems1 the law of decay would be different1 since the rate of decay in that case would depend on the relative concentration of the two reacting substances4 ,this is not so1 for not a single case has yet been observed in which the law of decay was affected by the amount of active matter present4 ,from the above equation 7#a7 d,nt ------- -,nt1 dt or the number of systems changing in unit time is proportional to the number unchanged at that time4 ,in the case of recovery of activity1 after an active product has been removed1 the number of systems changing in unit time1 when radio-active equilibrium is produced1 is equal to ,n4 ,this must be equal to the number q of new systems applied in unit time1 or q ,n1 q and ------2 ,n has thus a distinct physical meaning1 and may be defined as the proportion of the total number of systems present which change per second4 ,it has different values for different types of active matter1 but is invariable for any particular type of matter4 ,for this reason1 will be termed the 8radio-active constant0 of the product4 ,we are now in a position to discuss with more physical definiteness the gradual growth of ,th ,x in thorium1 after the ,th ,x has been completely removed from it4 ,let q particles of ,th ,x be produced per second by the thorium1 and let ,n be the number of particles of ,th ,x present at any time t after the original ,th ,x was removed4 ,the number of particles of ,th ,x which change every second is ,n1 where is the radio-active constant of ,th ,x4 ,now1 at any time during the process of recovery1 the rate of increase of the number of particles of ,th ,x the rate of production the rate of change2 that is d,n ------ q ,n4 dt ,the solution of this equation is of the form ,n ae-t b 1 where a and b are constants4 ,now when t is very great1 the number of particles of ,th ,x present reach a maximum value ,n4 ,thus1 since ,n ,n when t infinity1 b ,n2 since ,n #j when t #j1 a b #j2 hence b -a ,n1 and the equation becomes frac ,n ,n #a e-t 4 ,this is equivalent to the equation already obtained in section #acj1 since the intensity of the radiation is always proportional to the number of particles present4 #acd4 ,influence of conditions on the rate of decay4 ,since the activity of any product1 at any time1 may be taken as a measure of the rate at which chemical change takes place1 it may be used as a means of determining the effect of conditions on the changes occurring in radio-active matter4 ,if the rate of change should be accelerated or retarded1 it is to be expected that the value of the radio-active constant will be increased or decreased1 i4e4 that the decay curve will be different under different conditions4 ,no such effect1 however1 has yet been observed in any case of radio-active change1 where none of the active products produced are allowed to escape from the system4 ,the rate of decay is unaltered by any chemical or physical agency1 and in this respect the changes in radio-active matter are sharply distinguished from ordinary chemical changes4 ,for example1 the rate of decay of activity from any product takes place at the same rate when the substance is exposed to light as when it is kept in the dark1 and at the same rate in a vacuum as in air or any other gas at atmospheric pressure4 ,its rate of decay is unaltered by surrounding the active matter by a thick layer of lead under conditions where no ordinary radiation from outside can affect it4 ,the activity of the matter is unaffected by ignition or chemical treatment4 ,the material giving rise to the activity can be dissolved in acid and re-obtained by evaporation of the solution without altering the activity4 ,the rate of decay is the same whether the active matter is retained in the solid state or kept in solution4 ,when a product has lost its activity1 resolution or heat does not regenerate it1 and as we shall see later1 the rate of decay of the active products1 so far examined1 is the same at a red heat as at the temperature of liquid air4 ,in fact1 no variation of physical or chemical conditions has led to any observable difference in the decay of activity of any of the numerous types of active matter which have been examined4 #ace4 ,effect of conditions on the rate of recovery of activity4 ,the recovery of the activity of a radio-element with time1 when an active product is separated from it1 is governed by the rate of production of fresh active matter and by the decay of activity of that already produced4 ,since the rate of decay of the activity of the separated product is independent of conditions1 the rate of recovery of activity can be modified only by a change of the rate of production of fresh active matter4 ,as far as experiments have gone1 the rate of production1 like the rate of decay1 is independent of chemical or physical conditions4 ,there are indeed certain cases which are apparent exceptions to this rule4 ,for example1 the escape of the radio-active emanations from thorium and radium is readily affected by heat1 moisture and solution4 ,a more thorough investigation1 however1 shows that the exception is only apparent and not real4 ,these cases will be discussed more in detail in chapter ,,vii1 but it may be stated here that the differences observed are due to differences in the rate of escape of the emanations into the surrounding gas1 and not to differences in the rate of production4 ,for this reason it is difficult to test the question at issue in the case of the thorium compounds1 which in most cases readily allow the emanation produced by them to escape into the air4 ,in order to show that the rate of production is independent of molecular state1 temperature1 etc41 it is necessary in such a case to undertake a long series of measurements extending over the whole time of recovery4 ,it is impossible to make accurate relative comparisons to see if the activity is altered by the conversion of one compound into another4 ,the relative activity in such a case1 when measured by spreading a definite weight of material uniformly on a metal plate1 varies greatly with the physical conditions of the precipitate1 although the total activity of two compounds may be the same4 ,the following method#bcj offers an accurate and simple means of studying whether the rate of production of active matter is influenced by molecular state4 ,the substance is chemically converted into any compound required1 care being taken that active products are recovered during the process4 ,the new compound is then spread on a metal plate and compared with a standard sample of uranium for several days or weeks as required4 ,if the rate of production of active matter is altered by the conversion1 there should be an increase or decrease of activity to a new steady value1 where the production of active matter is again balanced by the rate of decay4 ,this method has the great advantage of being independent of the physical condition of the precipitate4 ,it can be applied satisfactorily to a compound of thorium like the nitrate and the oxide which has been heated to a white heat1 after which treatment only a slight amount of emanation escapes4 ,the nitrate was converted into the oxide in a platinum crucible by treatment with sulphuric acid and ignition to a white heat4 ,the oxide so obtained was spread on a plate1 but no change of its activity was observed with time1 showing that in this case the rate of production was independent of molecular state4 ,this method1 which is limited in the case of thorium1 may be applied generally to the uranium compounds where the results are not complicated by the presence of an emanation4 ,no differences have yet been observed in the recovery curves of different thorium compounds after the removal of ,th ,x4 ,for example1 the rate of recovery is the same whether the precipitated hydroxide is converted into the oxide or into the sulphate4 #acf4 ,disintegration hypothesis4 ,in the discussion of the changes in radio-active bodies1 only the active products ,ur ,x and ,th ,x have been considered4 ,it will1 however1 be shown later that these two products are only examples of many other types of active matter which are produced by the radio-elements1 and that each of these types of active matter has definite chemical as well as radio-active properties1 which distinguish it1 not only from the other active products1 but also from the substance from which it is produced4 ,the full investigation of these changes will be shown to verify in every particular the hypothesis that radio-activity is the accompaniment of chemical changes of a special kind occurring in matter1 and that the constant activity of the radio-elements is due to an equilibrium process1 in which the rate of production of fresh active matter balances the rate of change of that already formed4 ,the nature of the process taking place in the radio-elements1 in order to give rise to the production at a constant rate of new kinds of active matter1 will now be considered4 ,since in thorium or uranium compounds there is a continuous production of radio-active matter1 which differs in chemical properties from the parent substance1 some kind of change must be taking place in the radio-element4 ,this change1 by which new matter is produced1 is very different in character from the molecular changes dealt with in chemistry1 for no chemical change is known which proceeds at the same rate at the temperatures corresponding to a red heat and to liquid air1 and is independent of all physical and chemical actions4 ,if1 however1 the production of active matter is supposed to be the result of changes1 not in the molecule1 but in the atom itself1 it is not to be expected that the temperature would exert much influence4 ,the general experience of chemistry in failing to transform the elements by the action of temperature is itself strong evidence that wide ranges of temperature have not much effect in altering the stability of the chemical atom4 ,the view that the atoms of the radio-elements are undergoing spontaneous disintegration was put forward by ,rutherford and ,soddy as a result of evidence of this character4 ,the discovery of the material nature of the rays added strong confirmation to the hypothesis2 for it has been pointed out 7section #ie7 that the expulsion of particles must be the result of a disintegration of the atoms of the radio-element4 ,taking the case of thorium as an example1 the processes occurring in the atom may be pictured in the following way4 ,it must be supposed that the thorium atoms are not permanently stable systems1 but1 on an average1 a constant small proportion of them--about one atom in every #aj will suffice--break up per second4 ,the disintegration consists in the expulsion from the atom of one or more particles with great velocity4 ,for simplicity1 it will be supposed that each atom expels one particle4 ,it has been shown that the particle of radium has a mass about twice that of the hydrogen atom4 ,from the similarity of the rays from thorium and radium1 it is probable that the particle of thorium does not differ much in mass from that of radium1 and may be equal to it4 ,the particles expelled from the thorium atoms as they break up constitute what is known as the 8non-separable activity0 of thorium4 ,this activity1 measured by the rays1 is about #be per cent4 of the maximum4 ,after the escape of an particle1 the part of the atom left behind1 which has a mass slightly less than that of the thorium atom1 tends to rearrange its components to form a temporarily stable system4 ,it is to be expected that it will differ in chemical properties from the thorium atom from which it was derived4 ,the atom of the substance ,th ,x is1 on this view1 the thorium atom minus one particle4 ,the atoms of ,th ,x are far more unstable than the atoms of thorium1 and one after the other they break up1 each atom expelling one particle as before4 ,these projected particles give rise to the radiation from the ,th ,x4 ,since the activity of ,th ,x falls to half its original value in about four days1 on an average half of the atoms of ,th ,x break up in four days1 the number breaking up per second being always proportional to the number present4 ,after an atom of ,th ,x has expelled an particle1 the mass of the system is again reduced1 and its chemical properties are changed4 ,it will be shown 7section #aed7 that the ,th ,x produces the thorium emanation1 which exists as a radio-active gas1 and that this in turn is transformed into matter which is deposited on solid bodies and gives rise to the phenomena of excited activity4 ,the first few successive changes occurring in thorium are shown diagrammatically below 7,fig4 #ej74 ,illustration3 ,fig4 #ej4 ,thus as a result of the disintegration of the thorium atom1 a series of chemical substances is produced1 each of which has distinctive chemical properties4 ,each of these products is radio-active1 and loses its activity according to a definite law4 ,since thorium has an atomic weight of #bcg1 and the weight of the particle is about #b1 it is evident that1 if only one particle is expelled at each change1 the process of disintegration could pass through a number of successive stages and yet leave behind1 at the end of the process1 a mass comparable with that of the parent atom4 ,it will be shown later that a process of disintegration1 very similar to that already described for thorium1 must be supposed to take place also in uranium1 actinium and radium4 ,the full discussion of this subject cannot be given with advantage until two of the most important products of the three substances thorium1 radium and actinium1 viz4 the radio-active emanations and the matter which causes excited activity1 have been considered in detail4 #acg4 ,magnitude of the changes4 ,it can be calculated by several independent methods 7see section #bdf7 that1 in order to account for the radio-activity observed in thorium1 about #c #aj atoms in each gram of thorium suffer disintegration per second4 ,it is well known 7section #ci7 that #a cubic centimetre of hydrogen at atmospheric pressure and temperature contains about #cf #aj molecules4 ,from this it follows that one gram of thorium contains #cf #aj atoms4 ,the fraction which breaks up per second is thus about #aj4 ,this is an extremely small ratio1 and it is evident that the process could continue for long intervals of time1 before the amount of matter changed would be capable of detection by the spectroscope or by the balance4 ,with the electroscope it is possible to detect the radiation from #aj gram of thorium1 i4e4 the electroscope is capable of detecting the ionization which accompanies the disintegration of a single thorium atom per second4 ,the electroscope is thus an extraordinarily delicate means for detection of minute changes in matter1 which are accompanied1 as in the case of the radio-elements1 by the expulsion of charged particles with great velocity4 ,it is possible to detect by its radiation the amount of ,th ,x produced in a second from #a gram of thorium1 although the process would probably need to continue thousands of years before it could be detected by the balance or the spectroscope4 ,it is thus evident that the changes occurring in thorium are of an order of magnitude quite different from that of ordinary chemical changes1 and it is not surprising that they have never been observed by direct chemical methods4 ,footnote #bbe3 ,crookes1 ,proc4 ,roy4 ,soc4 #ff1 p4 #dji1 #aijj4 ,footnote #bbf3 ,becquerel1 ,c4 ,r4 #aca1 p4 #acg1 #aijj2 #acc1 p4 #igg1 #aija4 ,footnote #bbg3 ,rutherford and ,soddy1 ,phil4 ,mag4 ,sept4 and ,nov4 #aijb4 ,trans4 ,chem4 ,soc4 #ha1 pp4 #cba and #hcg1 #aijb4 ,footnote #bbh3 ,rutherford and ,soddy1 ,phil4 ,mag4 ,sept4 #aijb4 ,footnote #bbi3 ,the general method of regarding the subject would be unchanged1 even if it were proved that the radio-activity of thorium is not due to thorium at all but to a small constant amount of a radio-active impurity mixed with it4 ,footnote #bcj3 ,rutherford and ,soddy1 ,phil4 ,mag4 ,sept4 #aijb4 ,,chapter ,,vii4 ,,radio-,,active ,,emanations4 #ach4 ,introduction4 ,a most important and striking property possessed by radium1 thorium1 and actinium1 but not by uranium or polonium1 is the power of continuously emitting into the surrounding space a material emanation1 which has all the properties of a radio-active gas4 ,this emanation is able to diffuse rapidly through gases and through porous substances1 and may be separated from the gas with which it is mixed by condensation by the action of extreme cold4 ,this emanation forms a connecting link between the activity of the radio-elements themselves and their power of exciting activity on surrounding objects1 and has been studied more closely than the other active products on account of its existence in the gaseous state4 ,the emanations from the three active bodies all possess similar radio-active properties1 but the effects are more marked in the case of the emanation from radium1 on account of the very great activity of that element4 ,thorium ,emanation4 #aci4 ,discovery of the emanation4 ,in the course of examination of the radiations of thorium1 several observers had noted that some of the thorium compounds1 and especially the oxide1 were very inconstant sources of radiation1 when examined in open vessels by the electrical method4 ,owens#bca found that this inconstancy was due to the presence of air currents4 ,when a closed vessel was used1 the current1 immediately after the introduction of the active matter1 increased with the time1 and finally reached a constant value4 ,by drawing a steady stream of air through the vessel the value of the current was much reduced4 ,it was also observed that the radiations could apparently pass through large thicknesses of paper1 which completely absorbed the ordinary radiation4 ,in an investigation of these peculiar properties of thorium compounds1 the writer#bcb found that the effects were due to an emission of radio-active particles of some kind from the thorium compounds4 ,this 8emanation10 as it was termed for convenience1 possesses the properties of ionizing the gas and acting on a photographic plate1 and is able to diffuse rapidly through porous substances like paper and thin metal foil4 ,the emanation1 like a gas1 is completely prevented from escaping by covering the active matter with a thin plate of mica4 ,the emanation can be carried away by a current of air2 it passes through a plug of cotton-wool and can be bubbled through solutions without any loss of activity4 ,in these respects1 it behaves very differently from the ions produced in the gas by the rays from active substances1 for these give up their charges completely under the same conditions4 ,since the emanation passes readily through large thicknesses of cardboard1 and through filters of tightly packed cotton-wool1 it does not seem likely that the emanation consists of particles of dust given off by the active matter4 ,this point was tested still further by the method used by ,aitken and ,wilson1 for detecting the presence of dust particles in the air4 ,the oxide1 enclosed in a paper cylinder1 was placed in a glass vessel1 and the dust was removed by repeated small expansions of the air over a water surface4 ,the dust particles act as nuclei for the formation of small drops and are then removed from the air by the action of gravity4 ,after repeated expansions1 no cloud was formed1 and the dust was considered to be removed4 ,after waiting for some time to allow the thorium emanation to collect1 further expansions were made but no cloud resulted1 showing that for the small expansions used1 the particles were too small to become centres of condensation4 ,the emanation then could not be regarded as dust emitted from thorium4 ,since the power of diffusing rapidly through porous substances1 and acting on a photographic plate1 is also possessed by a chemical substance like hydrogen peroxide1 some experiments were made to see if the emanation could be an agent of that character4 ,it was found1 however1 that hydrogen peroxide is not radio-active1 and that its action on the plate is a purely chemical one1 while it is the radiation from the emanation and not the emanation itself that produces ionizing and photographic effects4 #adj4 ,experimental arrangements4 ,the emanation from thorium is given off in minute quantity4 ,no appreciable lowering of the vacuum is observed when an emanating compound is placed in a vacuum tube and no new spectrum lines are observed4 ,for an examination of the emanation1 an apparatus similar in principle to that shown in ,fig4 #ea is convenient4 ,illustration3 ,fig4 #ea4 ,the thorium compound1 either bare or enclosed in a paper envelope1 was placed in a glass tube ,c4 ,a current of air from a gasometer1 after passing through a tube containing cotton-wool to remove dust particles1 bubbled through sulphuric acid in the vessel ,a4 ,it then passed through a bulb containing tightly packed cotton-wool to prevent any spray being carried over4 ,the emanation1 mixed with air1 was carried from the vessel ,c through a plug of cotton-wool ,d1 which removed completely all the ions carried with the emanation4 ,the latter then passed into a long brass cylinder1 #ge cm4 in length and #f cm4 in diameter4 ,the insulated cylinder was connected with a battery in the usual way4 ,three insulated electrodes1 ,e1 ,f1 ,h1 of equal lengths1 were placed along the axis of the cylinder1 supported by brass rods passing through ebonite corks in the side of the cylinder4 ,the current through the gas1 due to the presence of the emanation1 was measured by means of an electrometer4 ,an insulating key was arranged so that any one of the electrodes ,e1 ,f1 ,h could be rapidly connected with one pair of quadrants of the electrometer1 the other two being always connected with earth4 ,the current observed in the testing cylinder vessel was due entirely to the ions produced by the emanation carried into the vessel by the current of air4 ,on substituting a uranium compound for the thorium1 not the slightest current was observed4 ,after a constant flow has passed for about #aj minutes1 the current due to the emanation reaches a constant value4 ,the variation of the ionization current with the voltage is similar to that observed for the gas ionized by the radiations from the active bodies4 ,the current at first increases with the voltage1 but finally reaches a saturation value4 #ada4 ,duration of the activity of the emanation4 ,the emanation rapidly loses its activity with time4 ,this is very readily shown with the apparatus of ,fig4 #ea4 ,the current is found to diminish progressively along the cylinder1 and the variation from electrode to electrode depends on the velocity of the flow of air4 ,if the velocity of the air current is known1 the decay of activity of the emanation with time can be deduced4 ,if the flow of air is stopped1 and the openings of the cylinder closed1 the current steadily diminishes with time4 ,the following numbers illustrate the variation with time of the saturation current1 due to the emanation in a closed vessel4 ,the observations were taken successively1 and as rapidly as possible after the current of air was stopped4 ,time in seconds ,current #j #ajj #bh #fi #fb #ea #aah #be #aee #ad #baj #fg #bgb #da #cfj #ah ,curve ,a1 ,fig4 #eb1 shows the relation existing between the current through the gas and the time4 ,the current just before the flow of air was stopped is taken as unity4 ,the current through the gas1 which is a measure of the activity of the emanation1 diminishes according to an exponential law with the time like the activity of the products ,ur ,x and ,th ,x4 ,the rate of decay is1 however1 much more rapid1 the activity of the emanation decreasing to half value in about one minute4 ,according to the view developed in section #acf1 this implies that half of the emanation particles have undergone change in one minute4 ,after an interval of #aj minutes the current due to the emanation is very small1 showing that practically all the emanation particles present have undergone change4 ,illustration3 ,fig4 #eb4 ,the rate of decay has been more accurately determined by ,rossignol and ,gimingham#bcc who found that the activity fell to half value in about #ea seconds4 ,bronson#bcd1 using the steady deflection method described in section #fi1 found the corresponding time #ed seconds4 ,the decrease of the current with the time is an actual measure of the decrease of the activity of the emanation1 and is not in any way influenced by the time that the ions produced take to reach the electrodes4 ,if the ions had been produced from a uranium compound the duration of the conductivity for a saturation voltage would only have been a fraction of a second4 ,the rate of decay of the activity of the emanation is independent of the electromotive force acting on the gas4 ,this shows that the radio-active particles are not destroyed by the electric field4 ,the current through the gas at any particular instant1 after stoppage of the flow of air1 was found to be the same whether the electromotive force had been acting the whole time or had been just applied for the time of the test4 ,the emanation itself is unaffected by a strong electric field and so cannot be charged4 ,by testing its activity after passing it through long concentric cylinders1 charged to a high potential1 it was found that the emanation certainly did not move with a velocity greater than #jjjja cm4 per second1 for a gradient of #a volt per cm41 and there was no evidence to show that it moved at all4 ,this conclusion has been confirmed by the experiments of ,mc,clelland#bce4 ,the rate at which the emanation is produced is independent of the gas surrounding the active matter4 ,if in the apparatus of ,fig4 #ea air is replaced by hydrogen1 oxygen1 or carbonic acid1 similar results are obtained1 though the current observed in the testing vessel varies for the different gases on account of the unequal absorption by them of the radiation from the emanation4 ,if a thorium compound1 enclosed in paper to absorb the radiation1 is placed in a closed vessel1 the saturation current due to the emanation is found to vary directly as the pressure4 ,since the rate of ionization is proportional to the pressure for a constant source of radiation1 this experiment shows that the rate of emission of the emanation is independent of the pressure of the gas4 ,the effect of pressure on the rate of production of the emanation is discussed in more detail later in section #aeg4 #adb4 ,effect of thickness of layer4 ,the amount of emanation emitted by a given area of thorium compound depends on the thickness of the layer4 ,with a very thin layer1 the current between two parallel plates1 placed in a closed vessel as in ,fig4 #ag1 is due very largely to the rays4 ,since the radiation is very readily absorbed1 the current due to it practically reaches a maximum when the surface of the plate is completely covered by a thin layer of the active material4 ,on the other hand the current produced by the emanation increases until the layer is several millimetres in thickness1 and then is not much altered by adding fresh active matter4 ,this falling off of the current after a certain thickness has been reached is to be expected1 since the emanation1 which takes several minutes to diffuse through the layer above it1 has already lost a large proportion of its activity4 ,with a thick layer of thorium oxide in a closed vessel1 the current between the plates is largely due to the radiation from the emanation lying between the plates4 ,the following tables illustrate the way in which the current varies with the thickness of paper for both a thin and a thick layer4 ,,table ,i4 ,thin ,layer4 ,thickness of sheets of paper #jjbg4 ,no4 of ,current layers of paper #j #a #a #cg #b #af #c #jh ,,table ,,ii4 ,thick ,layer4 ,thickness of paper #jjh cm4 ,no4 of ,current layers of paper #j #a #a #gd #b #gd #e #gb #aj #fg #bj #ee ,the initial current with the unscreened compound is taken as unity4 ,in ,table ,i1 for a thin layer of thorium oxide1 the current diminished rapidly with additional layers of thin paper4 ,in this case the current is due almost entirely to the rays4 ,in ,table ,,ii the current falls to #gd for the first layer4 ,in this case about #bf of the current is due to the rays1 which are practically absorbed by the layer #jjh cm4 in thickness4 ,the slow decrease with additional layers shows that the emanation diffuses so rapidly through a few layers of paper that there is little loss of activity during the passage4 ,the time taken to diffuse through #bj layers is however appreciable1 and the current consequently has decreased4 ,after passing through a layer of cardboard #af mms4 in thickness the current is reduced to about one-fifth of its original value4 ,in closed vessels the proportion of the total current1 due to the emanation1 varies with the distance between the plates as well as with the thickness of the layer of active material4 ,it also varies greatly with the compound examined4 ,in the nitrate1 which gives off only a small amount of emanation1 the proportion is very much smaller than in the hydroxide1 which gives off a large amount of emanation4 #adc4 ,increase of current with time4 ,the current due to the emanation does not reach its final value for some time after the active matter has been introduced into the closed vessel4 ,the variation with time is shown in the following table4 ,the saturation current due to thorium oxide1 covered with paper1 was observed between concentric cylinders of #ee cms4 and #h cm4 diameter4 ,immediately before observations on the current were made1 a rapid stream of air was blown through the apparatus4 ,this removed most of the emanation4 ,however1 the current due to the ionization of the gas by the emanation1 as it was carried along by the current of air1 was still appreciable4 ,the current consequently does not start from zero4 ,time in ,current seconds #j #i #bc #be #ec #di #if #fg #abe #gf #aid #hh #bdd #ih #cjd #ii #dhd #ajj ,the results are shown graphically in ,fig4 #eb1 curve ,b4 ,the decay of the activity of the emanation with time1 and the rate of increase of the activity due to the emanation in a closed space1 are connected in the same way as the decay and recovery curves of ,th ,x and ,ur ,x4 ,with the previous notation1 the decay curve is given by frac ,i,t ,i e-t 4 and the recovery curve by frac ,it ,i #a e-t 1 where is the radio-active constant of the emanation4 ,this relation is to be expected1 since the decay and recovery curves of the emanation are determined by exactly the same conditions as the decay and recovery curves of ,ur ,x and ,th ,x4 ,in both cases there is3 7#a7 ,a supply of fresh radio-active particles produced at a constant rate4 7#b7 ,a loss of activity of the particles following an exponential law with the time4 ,in the case of ,ur ,x and ,th ,x1 the active matter produced manifests its activity in the position in which it is formed2 in this new phenomenon1 a proportion of the active matter in the form of the emanation escapes into the surrounding gas4 ,the activity of the emanation1 due to a thorium compound kept in a closed vessel1 thus reaches a maximum when the rate of supply of fresh emanation particles from the compound is balanced by the rate of change of those already present4 ,the time for recovery of half the final activity is about #a minute1 the same as the time taken for the emanation1 when left to itself1 to lose half its activity4 ,if q is the number of emanation particles escaping into the gas per second1 and ,n the final number when radio-active equilibrium is reached1 then 7section #acc71 q ,n4 ,since the activity of the emanation falls to half value in #a minute 1 and ,n #hgq1 or the number of emanation particles present when a steady state is reached is #hg times the number produced per second4 ,radium ,emanation4 #add4 ,discovery of the emanation4 ,shortly after the discovery of the thorium emanation1 ,dorn#bcf repeated the results1 and1 in addition1 showed that radium compounds also gave off radio-active emanations1 and that the amount given off was much increased by heating the compound4 ,the radium emanation differs from the thorium emanation in the rate at which it loses its activity4 ,it decays far more slowly1 but in other respects the emanations of thorium and radium have much the same properties4 ,both emanations ionize the gas with which they are mixed1 and affect a photographic plate4 ,both diffuse readily through porous substances but are unable to pass through a thin plate of mica2 both behave like a temporarily radio-active gas1 mixed in minute quantity with the air or other gas in which they are conveyed4 #ade4 ,decay of activity of the emanation4 ,very little emanation escapes from radium chloride in the solid state1 but the amount is largely increased by heating1 or by dissolving the compound in water4 ,by bubbling air through a radium chloride solution1 or passing air over a heated radium compound1 a large amount of emanation may be obtained which can be collected1 mixed with air1 in a suitable vessel4 ,experiments to determine accurately the rate of decay of activity of the emanation have been made by ,p4 ,curie#bcg1 and ,rutherford and ,soddy#bch4 ,in the experiments of the latter1 the emanation mixed with air was stored over mercury in an ordinary gas-holder4 ,from time to time1 equal quantities of air mixed with the emanation were measured off by a gas pipette and delivered into a testing vessel4 ,the latter consisted of an air-tight brass cylinder carrying a central insulated electrode4 ,a saturation voltage was applied to the cylinder1 and the inner electrode was connected to the electrometer with a suitable capacity in parallel4 ,the saturation current was observed immediately after the introduction of the active gas into the testing vessel1 and was taken as a measure of the activity of the emanation present4 ,the current increased rapidly with the time owing to the production of excited activity on the walls of the containing vessel4 ,this effect is described in detail in chapter ,,viii4 ,the measurements were made at suitable intervals over a period of #cc days4 ,the following table expresses the results1 the initial activity being taken as #ajj4 ,time in ,relative hours ,activity #j #ajj #bjh #heg #ahgf #bdj #cedi #fi #ebai #ae #ghfi #jai ,the activity falls off according to an exponential law with the time1 and decays to half value in #cga days4 ,with the usual notation frac ,i,t ,i e-t 4 the mean value of deduced from the results is given by #baf #aj 4 ,p4 ,curie determined the rate of decay of activity of the emanation by another method4 ,the active matter was placed at one end of a sealed tube4 ,after sufficient time had elapsed the portion of the tube containing the radium compound was removed4 ,the loss of activity of the emanation1 stored in the other part1 was tested at regular intervals by observing the ionization current due to the rays which passed through the walls of the glass vessel4 ,the testing apparatus and the connections are shown clearly in ,fig4 #ec4 ,the ionization current is observed between the vessels ,,bb and ,,cc4 ,the glass tube ,a contains the emanation4 ,illustration3 ,fig4 #ec4 ,now it will be shown later that the emanation itself gives off only rays1 and these rays are completely absorbed by the glass envelope1 unless it is made extremely thin4 ,the rays producing ionization in the testing vessel were thus not due to the rays from the emanation at all1 but to the and rays due to the excited activity produced on the walls of the glass tube by the emanation inside it4 ,what was actually measured was thus the decay of the excited activity derived from the emanation1 and not the decay of activity of the emanation itself4 ,since1 however1 when a steady state is reached1 the amount of excited activity is nearly proportional at any time to the activity of the emanation1 the rate of decay of the excited activity on the walls of the vessel indirectly furnishes a measure of the rate of decay of the emanation itself4 ,this is only true if the emanation is placed for four or five hours in the tube before observations begin1 in order to allow the excited activity time to reach a maximum value4 ,using this method ,p4 ,curie obtained results similar to those obtained by ,rutherford and ,soddy by the direct method4 ,the activity decayed according to an exponential law with the time1 falling to half value in #cii days4 ,the experiments were performed under the most varied conditions but the rate of decay was found to remain unaltered4 ,the rate of decay did not depend on the material of the vessel containing the emanation or on the nature or pressure of the gas with which the emanation was mixed4 ,it was unaffected by the amount of emanation present1 or by the time of exposure to the radium1 provided sufficient time had elapsed to allow the excited activity to reach a maximum value before the observations were begun4 ,p4 ,curie#bci found that the rate of decay of activity was not altered by exposing the vessel containing the emanation to different temperatures1 ranging from #dej to #ahj ,c4 ,in this respect the emanations of thorium and radium are quite analogous4 ,the rate of decay seems to be unaffected by any physical or chemical agency1 and the emanations behave in exactly the same way as the radio-active products ,th ,x and ,ur ,x1 already referred to4 ,the radio-active constant is thus a fixed and unalterable quantity for both emanations1 although in one case its value is about #ejjj times greater than in the other4 ,emanations from ,actinium4 #adf4 ,debierne#bdj found that actinium gives out an emanation similar to the emanation of thorium and radium4 ,the loss of activity of the emanation is even more rapid than for the thorium emanation1 for its activity falls to half value in #ci seconds4 ,in consequence of the rapid decay of activity1 the emanation is able to diffuse through the air only a short distance from the active matter before it loses the greater proportion of its activity4 ,giesel early observed that the radio-active substance separated by him1 which we have seen 7section #ah7 is identical in radio-active properties with actinium1 gave off a large amount of emanation4 ,it was in consequence of this property1 that he gave it the name of the 8emanating substance0 and later 8emanium40 ,the impure preparations of this substance emit the emanation very freely and in this respect differ from most of the thorium compounds4 ,the emanation from actinium like those from thorium and radium possesses the property of exciting activity on inactive bodies1 but it has not yet been studied so completely as the better known emanations of thorium and radium4 ,experiments with large amounts of ,radium ,emanation4 #adg4 ,with very active specimens of radium a large amount of emanation can be obtained1 and the electrical1 photographic1 and fluorescent effects are correspondingly intense4 ,on account of the small activity of thorium and the rapid decay of its emanation the effects due to it are weak1 and can be studied only for a few minutes after its production4 ,the emanation from radium1 on the other hand1 in consequence of the slow decay of its activity1 may be stored mixed with air in an ordinary gas-holder1 and its photographic and electrical actions may be examined several days or even weeks after1 quite apart from those of the radium from which it was obtained4 ,it is1 in general1 difficult to study the radiation due to the emanation alone1 on account of the fact that the emanation is continually producing a secondary type of activity on the surface of the vessel in which the emanation is enclosed4 ,this excited activity reaches a maximum value several hours after the introduction of the emanation1 and1 as long as it is kept in the vessel1 this excited activity on the walls decays at the same rate as the emanation itself1 i4e4 it falls to half its initial value in about #d days4 ,if1 however1 the emanation is blown out1 the excited activity remains behind on the surface1 but rapidly loses its activity in the course of a few hours4 ,after several hours the intensity of the residual radiation is very small4 ,these effects and their connection with the emanation are discussed more fully in chapter ,,viii4 ,giesel#bda has recorded some interesting observations of the effect of the radium emanation on a screen of phosphorescent zinc sulphide4 ,when a few centigrams of moist radium bromide were placed on a screen any slight motion of the air caused the luminosity to move to and fro on the screen4 ,the direction of phosphorescence could be altered at will by a slow current of air4 ,the effect was still further increased by placing the active material in a tube and blowing the air through it towards the screen4 ,a screen of barium platinocyanide or of ,balmain's paint failed to give any visible light under the same conditions4 ,the luminosity was not altered by a magnetic field1 but it was affected by an electric field4 ,if the screen were charged the luminosity was more marked when it was negative than when it was positive4 ,giesel states that the luminosity was not equally distributed1 but was concentrated in a peculiar ring-shaped manner over the surface of the screen4 ,the concentration of luminosity on the negative1 rather than on the positive1 electrode is probably due to the excited activity1 caused by the emanation1 and not to the emanation itself1 for this excited activity is concentrated chiefly on the negative electrode in an electric field 7see chapter ,,viii74 ,an experiment to illustrate the phosphorescence produced in some substances by the rays from a large amount of emanation is described in section #afe4 #adh4 ,curie and ,debierne#bdb have investigated the emanation from radium1 and the excited activity produced by it4 ,some experiments were made on the amount of emanation given off from radium under very low pressures4 ,the tube containing the emanation was exhausted to a good vacuum by a mercury pump4 ,it was observed that a gas was given off from the radium which produced excited activity on the glass walls4 ,this gas was extremely active1 and rapidly affected a photographic plate through the glass4 ,it caused fluorescence on the surface of the glass and rapidly blackened it1 and was still active after standing ten days4 ,when spectroscopically examined1 this gas did not show any new lines1 but generally those of the spectra of carbonic acid1 hydrogen1 and mercury4 ,in the light of the results described in section #abd the gas1 given off by the radium1 was probably the non-active gases hydrogen and oxygen1 in which the active emanation was mixed in minute quantity4 ,it will be shown later 7section #bdb7 that the energy radiated from the emanation is enormous compared with the amount of matter involved1 and that the effects observed1 in most cases1 are produced by an almost infinitesimal amount of the emanation4 ,in further experiments1 ,curie and ,debierne#bdc found that many substances were phosphorescent under the action of the emanation and the excited activity produced by it4 ,in their experiments1 two glass bulbs ,a and ,b 7,fig4 #ed7 were connected with a glass tube4 ,the active material was placed in the bulb ,a and the substance to be examined in the other4 ,illustration3 ,fig4 #ed4 ,they found that1 in general1 substances that were phosphorescent in ordinary light became luminous4 ,the sulphide of zinc was especially brilliant and became as luminous as if exposed to a strong light4 ,after sufficient time had elapsed the luminosity reached a constant value4 ,the phosphorescence is partly due to the excited activity produced by the emanation on its surface1 and partly to the direct radiation from the emanation4 ,phosphorescence was also produced in glass4 ,thuringian glass showed the most marked effects4 ,the luminosity of the glass was found to be about the same in the two bulbs1 but was more marked in the connecting tube4 ,the effect in the two bulbs was the same even if connected by a very narrow tube4 ,some experiments were also made with a series of phosphorescent plates placed in the vessel at varying distances apart4 ,with the plates #a mm4 apart the effect was very feeble1 but increased directly as the distance and was large for a distance of #c cms4 ,these effects receive a general explanation on the views already put forward4 ,when the radium is placed in the closed vessel1 the emanation is given off at a constant rate and gradually diffuses throughout the enclosure4 ,since the time taken for diffusion of the emanation through tubes of ordinary size is small compared with the time required for the activity to be appreciably reduced1 the emanation1 and also the excited activity due to it1 will be nearly equally distributed throughout the vessel4 ,the luminosity due to it should thus be equal at each end of the tube4 ,even with a capillary tube connecting the two bulbs1 the gas continuously given off by the radium will always carry the emanation with it and cause a practically uniform distribution4 ,the gradual increase of the amount of emanation throughout the tube will be given by the equation frac ,nt ,n #a e-t 4 where ,nt is the number of emanation particles present at the time t1 ,n the number present when radio-active equilibrium is reached1 and is the radio-active constant of the emanation4 ,the phosphorescent action1 which is due partly to the radiations from the emanation and partly to the excited activity on the walls1 should thus reach half the maximum value in four days and should practically reach its limit after three weeks' interval4 ,the variation of luminosity with different distances between the screens is to be expected4 ,the amount of excited activity deposited on the boundaries is proportional to the amount of emanation present4 ,since the emanation is equally distributed1 the amount of excited activity deposited on the screens1 due to the emanation between them1 varies directly as the distance1 provided the distance between the screens is small compared with their dimensions4 ,such a result would also follow if the phosphorescence were due to the radiation from the emanation itself1 provided that the pressure of the gas was low enough to prevent absorption of the radiation from the emanation in the gas itself between the screens4 ,measurements of ,emanating ,power4 #adi4 ,emanating power4 ,the compounds of thorium in the solid state vary very widely in the amount of emanation they emit under ordinary conditions4 ,it is convenient to use the term emanating power to express the amount of emanation given off per second by one gram of the compound4 ,since1 however1 we have no means of determining absolutely the amount of emanation present1 all measurements of emanating power are of necessity comparative4 ,in most cases1 it is convenient to take a given weight of a thorium compound1 kept under conditions as nearly as possible constant1 and to compare the amount of emanation of the compound to be examined with this standard4 ,in this way comparisons of the emanating power of thorium compounds have been made by ,rutherford and ,soddy#bdd1 using an apparatus similar to that shown in ,fig4 #ea on page #bdj4 ,a known weight of the substance to be tested was spread on a shallow dish1 placed in the glass tube ,c4 ,a stream of dry dust-free air1 kept constant during all the experiments1 was passed over the compound and carried the emanation into the testing vessel4 ,after ten minutes interval1 the current due to the emanation in the testing vessel reached a constant value4 ,the compound was then removed1 and the standard comparison sample of equal weight substituted2 the saturation current was observed when a steady state was again reached4 ,the ratio of these two currents gives the ratio of the emanating power of the two samples4 ,it was found experimentally that1 for the velocities of air current employed1 the saturation current in the testing vessel was directly proportional to the weight of thorium1 for weights up to #bj grams4 ,this is explained by the supposition that the emanation is removed by the current of air from the mass of the compound1 as fast as it is formed4 ,let i saturation current due to a weight of the standard1 i of the sample to be tested4 7emanating power of specimen7 i ,then ---------------------------- ---- --- 7emanating power of standard7 i ,by means of this relation the emanating power of compounds which are not of equal weight can be compared4 ,it was found that thorium compounds varied enormously in emanating power1 although the percentage proportion of thorium present in the compound was not very different4 ,for example1 the emanating power of thorium hydroxide was generally #c to #d times greater than that of ordinary thoria1 obtained from the manufacturer4 ,thorium nitrate1 in the solid state1 had only of the emanating power of ordinary thoria1 while preparations of the carbonate were found to vary widely among themselves in emanating power1 which depended upon slight variations in the method of preparation4 #aej4 ,effect of conditions on emanating power4 ,the emanating power of different compounds of thorium and radium is much affected by the alteration of chemical and physical conditions4 ,in this respect the emanating power1 which is a measure of the rate of escape of the emanation into the surrounding gas1 must not be confused with the rate of decay of the activity of the emanations themselves1 which has already been shown to be unaffected by external conditions4 ,dorn 7loc4 cit47 first observed that the emanating power of thorium and radium compounds was much affected by moisture4 ,in a fuller investigation of this point by ,rutherford and ,soddy1 it was found that the emanating power of thoria is from two to three times greater in a moist than in a dry gas4 ,continued desiccation of the thoria in a glass tube1 containing phosphorus pentoxide1 did not reduce the emanating power much below that observed in ordinary dry air4 ,in the same way radium chloride in the solid state gives off very little emanation when in a dry gas1 but the amount is much increased in a moist gas4 ,the rate of escape of emanation is much increased by solution of the compound4 ,for example1 thorium nitrate1 which has an emanating power of only that of thoria in the solid state1 has in solution an emanating power of #c to #d times that of thoria4 ,p4 ,curie and ,debierne observed that the emanating power of radium was also much increased by solution4 ,temperature has a very marked effect on the emanating power4 ,the writer#bde showed that the emanating power of ordinary thoria was increased three to four times by heating the substance to a dull red heat in a platinum tube4 ,if the temperature was kept constant the emanation continued to escape at the increased rate1 but returned to its original value on cooling4 ,if1 however1 the compound was heated to a white heat1 the emanating power was greatly reduced1 and it returned on cooling to about #aj of the original value4 ,such a compound is said to be de-emanated4 ,the emanating power of radium compounds varies in a still more striking manner with rise of temperature4 ,the rate of escape of the emanation is momentarily increased even #aj1jjj times by heating to a dull red heat4 ,this effect does not continue1 for the large escape of the emanation by heating is in reality due to the release of the emanation stored up in the radium compound4 ,like thoria1 when the compound has once been heated to a very high temperature1 it loses its emanating power and does not regain it4 ,it regains its power of emanating1 however1 after solution and re-separation4 ,a further examination of the effect of temperature was made by ,rutherford and ,soddy#bdf4 ,the emanating power of thoria decreases very rapidly with lowering of temperature1 and at the temperature of solid carbonic acid it is only about #aj of its ordinary value4 ,it rapidly returns to its original value when the cooling agent is removed4 ,increase of temperature from #hj ,c4 to a dull red heat of platinum thus increases the emanating power about #dj times1 and the effects can be repeated again and again1 with the same compound1 provided the temperature is not raised to the temperature at which de-emanation begins4 ,de-emanation sets in above a red heat1 and the emanating power is then permanently diminished1 but even long-continued heating at a white heat never entirely destroys the emanating power4 #aea4 ,regeneration of emanating power4 ,an interesting question arises whether the de-emanation of thorium and radium is due to a removal or alteration of the substance which produces the emanation1 or whether intense ignition merely changes the rate of escape of the emanation from the solid into the surrounding atmosphere4 ,it is evident that the physical properties of the thoria are much altered by intense ignition4 ,the compound changes in colour from white to pink2 it becomes denser and also far less readily soluble in acids4 ,in order to test if the emanating power could be regenerated by a cyclic chemical process1 the de-emanated thoria was dissolved1 precipitated as hydroxide and again converted into oxide4 ,at the same time a specimen of the ordinary oxide was subjected to an exactly parallel process4 ,the emanating power of both these compounds was the same1 and was from two to three times greater than that of ordinary thoria4 ,thus de-emanation does not permanently destroy the power of thorium of giving out an emanation1 but merely produces an alteration of the amount of the emanation which escapes from the compound4 #aeb4 ,rate of production of the emanation4 ,the emanating power of thorium compounds1 then1 is a very variable quantity1 much affected by moisture1 heat1 and solution4 ,speaking generally1 increased temperatures and solution greatly increase the emanating power of both thorium and radium4 ,the wide differences between the emanating powers of these substances in the solid state and in solution pointed to the conclusion that the differences were probably due to the rate of escape of the emanation into the surrounding gas1 and not to a variation of the rate of reaction which gave rise to the emanation4 ,it is obvious that a very slight retardation in the rate of escape of the thorium emanation from the compound into the gas1 will1 on account of the rapid decay of activity of the emanation1 produce great changes in emanating power4 ,the regeneration of the emanating power of de-emanated thoria and radium by solution and chemical treatment made it evident that the original power of thorium and radium of producing the emanation still persisted in an unaltered degree4 ,the question whether the emanation was produced at the same rate in emanating as in non-emanating compounds can be put to a sharp quantitative test4 ,if the rate of production of emanation goes on at the same rate in the solid compound where very little escapes1 as in the solution where probably all escapes1 the emanation must be occluded in the compound1 and consequently there must be a sudden release of this emanation on solution of the compound4 ,on account of the very slow decay of the activity of the emanation of radium1 the effects should be far more marked in that compound than in thorium4 ,from the point of view developed in section #acc1 the exponential law of decay of the emanation expresses the result that ,nt the number of particles remaining unchanged at the time t is given by frac ,nt ,n e-t 7#a74 where ,n is the initial number of particles present4 ,when a steady state is reached1 the rate of production q of fresh emanation particles is exactly balanced by the rate of change of the particles ,n already present1 i4e4 q ,n1 ,n in this case represents the amount of emanation 8occluded0 in the compound4 ,substituting the value of found for the radium emanation in section #ade1 ,n ----- #a_/ #dfc1jjj4 q ,the amount of emanation stored in a non-emanating radium compound should therefore be nearly #ejj1jjj times the amount produced per second by the compound4 ,this result was tested in the following way#bdg4 ,a weight of #jc gr4 of radium chloride of activity #ajjj times that of uranium was placed in a ,drechsel bottle and a sufficient amount of water drawn in to dissolve it4 ,the released emanation was swept out by a current of air into a small gas holder and then into a testing cylinder4 ,the initial saturation current was proportional to ,n4 ,a rapid current of air was then passed through the radium solution for some time in order to remove any slight amount of emanation which had not been removed initially4 ,the ,drechsel bottle was closed air-tight1 and allowed to stand undisturbed for a definite time t4 ,the accumulated emanation was then swept out as before into the testing vessel4 ,the new ionization current represents the value of ,nt the amount of emanation formed in the compound during the interval t4 ,in the experiment t #aje minutes1 and the observed value ,nt ----- #jaca4 ,n ,assuming that there is no decay during the interval1 ,nt #aje #fj q4 ,n ,thus --- #dhj1jjj4 q ,making the small correction for the decay of activity during the interval1 ,n ----- #dgg1jjj4 q ,we have previously shown that from the theory ,n #a ----- ---- #dfc1jjj4 q ,the agreement between theory and experiment is thus as close as could be expected from the nature of the experiments4 ,this experiment proves conclusively that the rate of production of emanation in the solid compound is the same as in the solution4 ,in the former case it is occluded1 in the latter it escapes as fast as it is produced4 ,it is remarkable how little emanation1 compared with the amount stored up in the compound1 escapes from solid radium chloride in a dry atmosphere4 ,one experiment showed that the emanating power in the dry solid state was less than of the emanating power of the solution4 ,since nearly #ejj1jjj times as much emanation is stored up in the solid compound as is produced per second1 this result showed that the amount of emanation which escaped per second was less than #aj of that occluded in the compound4 ,if a solid radium chloride compound is kept in a moist atmosphere1 the emanating power becomes comparable with the amount produced per second in the solution4 ,in such a case1 since the rate of escape is continuous1 the amount occluded will be much less than the amount for the non-emanating material4 ,the phenomenon of occlusion of the radium emanation is probably not connected in any way with its radio-activity1 although this property has here served to measure it4 ,the occlusion of helium by minerals presents almost a complete analogy to the occlusion of the radium emanation4 ,part of the helium is given off by fergusonite1 for example1 when it is heated and all of it when the mineral is dissolved4 #aec4 ,similar results hold for thorium1 but1 on account of the rapid loss of activity of the emanation1 the amount of emanation occluded in a non-emanating compound is very small compared with that observed for radium4 ,if the production of the thorium emanation proceeds at the same rate under all conditions1 the solution of a solid non-emanating compound should be accompanied by a rush of emanation greater than that subsequently produced4 ,with the same notation as before we have for the thorium emanation1 ,n #a ----- ----- #hg4 q ,this result was tested as follows3 a quantity of finely powdered thorium nitrate1 of emanating power of ordinary thoria1 was dropped into a ,drechsel bottle containing hot water and the emanation rapidly swept out into the testing vessel by a current of air4 ,the ionization current rose quickly to a maximum1 but soon fell again to a steady value2 showing that the amount of emanation released when the nitrate dissolves1 is greater than the subsequent amount produced from the solution4 ,the rapid loss of the activity of the thorium emanation makes a quantitative comparison like that for radium very difficult4 ,by slightly altering the conditions of the experiment1 however1 a definite proof was obtained that the rate of production of emanation is the same in the solid compound as in the solution4 ,after dropping in the nitrate1 a rapid air stream was blown through the solution for #be seconds into the testing vessel4 ,the air stream was stopped and the ionization current immediately measured4 ,the solution was then allowed to stand undisturbed for #aj minutes4 ,in that time the accumulation of the emanation again attained a practical maximum and again represented a steady state4 ,the stream of air was blown through1 as before1 for #be seconds1 stopped and the current again measured4 ,in both cases1 the electrometer recorded a movement of #adf divisions per second4 ,by blowing the same stream of air continuously through the solution the final current corresponded to #gi divisions per second or about one-half of that observed after the first rush4 ,thus the rate of production of emanation is the same in the solid nitrate as in the solution1 although the emanating power1 i4e4 the rate of escape of the emanation1 is over #fjj times greater in the solution than in the solid4 ,it seems probable that the rate of production of emanation by thorium1 like the rate of production of ,ur ,x and ,th ,x1 is independent of conditions4 ,the changes of emanating power of the various compounds by moisture1 heat1 and solution must therefore be ascribed solely to an alteration in the rate of escape of the emanation into the surrounding gas and not to an alteration in the rate of its production in the compound4 ,on this view1 it is easy to see that slight changes in the mode of preparation of a thorium compound may produce large changes in emanating power4 ,such effects have been often observed1 and must be ascribed to slight physical changes in the precipitate4 ,the fact that the rate of production of the emanation is independent of the physical or chemical conditions of the thorium1 in which it is produced1 is thus in harmony with what had previously been observed for the radio-active products ,ur ,x and ,th ,x4 ,source of the ,thorium ,emanation4 #aed4 ,some experiments of ,rutherford and ,soddy#bdh will now be considered1 which show that the thorium emanation is produced1 not directly by the thorium itself1 but by the active product ,th ,x4 ,when the ,th ,x1 by precipitation with ammonia1 is removed from a quantity of thorium nitrate1 the precipitated thorium hydroxide does not at first possess appreciable emanating power4 ,this loss of emanating power is not due1 as in the case of the de-emanated oxide1 to a retardation in the rate of escape of the emanation produced2 for the hydroxide1 when dissolved in acid1 still gives off no emanation4 ,on the other hand1 the solution1 containing the ,th ,x1 possesses emanating power to a marked degree4 ,when the precipitated hydroxide and the ,th ,x is left for some time1 it is found that the ,th ,x decreases in emanating power1 while the hydroxide gradually regains its emanating power4 ,after about a month's interval1 the emanating power of the hydroxide has nearly reached a maximum1 while the emanating power of the ,th ,x has almost disappeared4 ,the curves of decay and recovery of emanating power with time are found to be exactly the same as the curves of decay and recovery of activity of ,th ,x and the precipitated hydroxide respectively1 shown in ,fig4 #dg4 ,the emanating power of ,th ,x1 as well as its activity1 falls to half value in four days1 while the hydroxide regains half its final emanating power as well as half its lost activity in the same interval4 ,it follows from these results that the emanating power of ,th ,x is directly proportional to its activity1 i4e4 that the rate of production of emanating particles is always proportional to the number of particles1 projected from the ,th ,x per second4 ,the radiation from ,th ,x thus accompanies the change of the ,th ,x into the emanation4 ,since the emanation has chemical properties distinct from those of the ,th ,x1 and also a distinctive rate of decay1 it cannot be regarded as a vapour of ,th ,x1 but it is a distinct chemical substance1 produced by the changes occurring in ,th ,x4 ,on the view advanced in section #acf1 the atom of the emanation consists of the part of the atom of ,th ,x left behind after the expulsion of one or more particles4 ,the atoms of the emanation are unstable1 and in turn expel particles4 ,this projection of particles constitutes the radiation from the emanation1 which serves as a measure of the amount of emanation present4 ,since the activity of the emanation falls to half value in one minute while that of ,th ,x falls to half value in four days1 the emanation consists of atoms which disintegrate at intervals nearly #fjjj times shorter than those of the atoms of ,th ,x4 ,source of the ,radium and ,actinium ,emanation4 #aee4 ,no intermediate stage--,radium ,x--between radium and its emanation1 corresponding to the ,th ,x for thorium1 has so far been observed4 ,the emanation from radium is probably produced directly from that element4 ,in this respect1 the radium emanation holds the same position in regard to radium as ,th ,x does to thorium1 and its production from radium can be explained on exactly similar lines4 ,it will be shown later in chapter ,x1 that the emanation of actinium1 like that of thorium1 does not arise directly from the parent element but from an intermediate product actinium ,x1 which is very analogous in physical and chemical properties to ,th ,x4 ,radiations from the ,emanations4 #aef4 ,special methods are necessary to examine the nature of the radiation from the emanations1 for the radiations arise from the volume of the gas in which the emanations are distributed4 ,some experiments to examine the radiations from the thorium emanation were made by the writer in the following way4 ,illustration3 ,fig4 #ee4 ,a highly emanating thorium compound wrapped in paper was placed inside a lead box ,b about #a cm4 deep1 shown in ,fig4 #ee4 ,an opening was cut in the top of the box1 over which a very thin sheet of mica was waxed4 ,the emanation rapidly diffused through the paper into the vessel1 and after ten minutes reached a state of radio-active equilibrium4 ,the penetrating power of the radiation from the emanation which passed through the thin mica window was examined by the electrical method in the usual way by adding screens of thin aluminium foil4 ,the results are expressed in the following table3 ,thickness of mica window #jjae cm4 ,thickness of aluminium foil #jjjcd cm4 ,layers of ,current foil #j #ajj #a #ei #b #cj #c #aj #d #cb ,the greater proportion of the conductivity is thus due to rays1 as in the case of the radio-active elements4 ,the amount of absorption of these rays by aluminium foil is about the same as that of the rays from the active bodies4 ,no direct comparison can be made1 for the rays from the emanation show the characteristic property of increased rate of absorption with thickness of matter traversed4 ,before testing1 the rays have been largely absorbed by the mica window1 and the penetrating power has consequently decreased4 ,no alteration in the radiation from the emanation was observed on placing an insulated wire inside the emanation vessel1 and charging it to a high positive or negative potential4 ,when a stream of air through the vessel carried away the emanation as fast as it was produced1 the intensity of the radiation fell to a small fraction of its former value4 ,no evidence of any rays in the radiations was found in these experiments1 although a very small effect would have been detected4 ,after standing some hours1 however1 rays began to appear4 ,these were due to the excited activity deposited on the walls of the vessel from the emanation1 and not directly to the emanation itself4 ,the radium emanation1 like that of thorium1 only gives rise to rays4 ,this was tested in the following way#bdi3 ,a large amount of emanation was introduced into a cylinder made of sheet copper #jje cm4 thick1 which absorbed all the rays but allowed the and rays1 if present1 to pass through with but little loss4 ,the external radiation from the cylinder was determined at intervals1 commencing about two minutes after the introduction of the emanation4 ,the amount observed at first was extremely small1 but increased rapidly and practically reached a maximum in three or four hours4 ,thus the radium emanation only gives out rays1 the rays appearing as the excited activity is produced on the walls of the vessel4 ,on sweeping out the emanation by a current of air1 there was no immediately appreciable decrease of the radiation4 ,this is another proof that the emanation does not emit any rays4 ,in a similar way it can be shown that the emanation does not give out rays2 these rays always make their appearance at the same time as the rays4 ,the method of examination of the radiations from the emanations has been given in some detail1 as the results are of considerable importance in the discussion1 which will be given later in chapters ,x and ,,xi1 of the connection between the changes occurring in radio-active products and the radiations they emit4 ,there is no doubt that the emanations1 apart from the excited activity to which they give rise1 only give out rays1 consisting most probably of positively charged bodies projected with great velocity4 ,effect of pressure on the rate of production of the ,emanation4 #aeg4 ,it has already been mentioned that the conductivity due to the thorium emanation is proportional to the pressure of the gas1 pointing to the conclusion that the rate of production of the emanation is independent of the pressure1 as well as of the nature of the surrounding gas4 ,this result was directly confirmed with the apparatus of ,fig4 #ee4 ,when the pressure of the gas under the vessel was slowly reduced1 the radiation1 tested outside the window1 increased to a limit1 and then remained constant over a wide range of pressure4 ,this increase1 which was far more marked in air than in hydrogen1 is due to the fact that the rays from the emanation were partially absorbed in the gas inside the vessel when at atmospheric pressure4 ,at pressures of the order of #a millimetre of mercury the external radiation decreased1 but experiment showed that this must be ascribed to a removal of the emanation by the pump1 and not to a change in the rate of production4 ,the thorium compounds very readily absorb water-vapour1 which is slowly given off at low pressures1 and in consequence some of the emanation is carried out of the vessel with the water-vapour4 ,curie and ,debierne#bej found that both the amount of excited activity produced in a closed vessel containing active samples of radium1 and also the time taken to reach a maximum value1 were independent of the pressure and nature of the gas4 ,this was true in the case of a solution down to the pressure of the saturated vapour1 and in the case of solid salts to very low pressures4 ,when the pump was kept going at pressures of the order of #jja mm4 of mercury1 the amount of excited activity was much diminished4 ,this was probably not due to any alteration of the rate of escape of the emanation1 but to the removal of the emanation by the action of the pump as fast as it was formed4 ,since the amount of excited activity1 when in a state of radio-active equilibrium1 is a measure of the amount of emanation producing it1 these results show that the amount of emanation present when the rate of production balances the rate of decay is independent of the pressure and nature of the gas4 ,it was also found that the time taken to reach the point of radio-active equilibrium was independent of the size of the vessel or the amount of active matter present4 ,this proves that the state of equilibrium cannot in any way be ascribed to the possession by the emanation of any appreciable vapour pressure2 for if such were the case1 the time taken to reach the equilibrium value should depend on the size of the vessel and the amount of active matter present4 ,the results are1 however1 in agreement with the view that the emanation is present in minute quantity in the tube1 and that the equilibrium is governed purely by the radio-active constant 1 the constant of decay of activity of the emanation4 ,this has been seen to be the same under all conditions of concentration1 pressure and temperature1 and1 provided the rate of supply of the emanation from the active compound is not changed1 the time-rate of increase of activity to the equilibrium value will always be the same1 whatever the size of the vessel or the nature and pressure of the surrounding gas4 ,chemical ,nature of the ,emanations4 #aeh4 ,we shall now consider some experiments on the physical and chemical properties of the emanations themselves1 without reference to the material producing them1 in order to see if they possess any properties which connect them with any known kind of matter4 ,it was soon observed that the thorium emanation passed unchanged through acid solutions1 and later the same result was shown to hold true in the case of both emanations for every reagent that was tried4 ,preliminary observations#bea showed that the thorium emanation1 obtained in the usual way by passing air over thoria1 passed unchanged in amount through a platinum tube heated electrically to the highest temperature obtainable4 ,the tube was then filled with platinum-black1 and the emanation passed through it in the cold1 and with gradually increasing temperatures1 until the limit was reached4 ,in another experiment1 the emanation was passed through a layer of red-hot lead-chromate in a glass tube4 ,the current of air was replaced by a current of hydrogen1 and the emanation was sent through red-hot magnesium-powder and red-hot palladium-black1 and1 by using a current of carbon dioxide1 through red-hot zinc-dust4 ,in every case the emanation passed through without sensible change in the amount4 ,if anything1 a slight increase occurred1 owing to the time taken for the gas-current to pass through the tubes when hot being slightly less than when cold1 the decay en route being consequently less4 ,the only known gases capable of passing in unchanged amount through all the reagents employed are the recently discovered members of the argon family4 ,but another possible interpretation might be put upon the results4 ,if the emanation were the manifestation of a type of excited radio-activity on the surrounding atmosphere1 then1 since from the nature of the experiments it was necessary to employ in each case as the atmosphere1 a gas not acted on by the reagent employed1 the result obtained might be expected4 ,red-hot magnesium would not retain an emanation consisting of radio-active hydrogen1 nor red-hot zinc-dust an emanation consisting of radio-active carbon dioxide4 ,the incorrectness of this explanation was shown in the following way4 ,carbon dioxide was passed over thoria1 then through a ,t-tube1 where a current of air met and mixed with it1 both passing on to the testing-cylinder4 ,but between this and the ,t-tube a large soda-lime tube was introduced1 and the current of gas was thus freed from its admixed carbon dioxide1 before being tested in the cylinder for the emanation4 ,the amount of emanation found was quite unchanged1 whether carbon dioxide was sent over thoria in the manner described1 or whether1 keeping the other arrangements as before1 an equally rapid current of air was substituted for it4 ,the theory that the emanation is an effect of the excited activity on the surrounding medium is thus excluded4 ,experiments of a similar kind on the radium emanation were made later4 ,a steady stream of gas was passed through a radium chloride solution and then through the reagent to be employed1 into a testing-vessel of small volume1 so that any change in the amount of emanation passing through could readily be detected4 ,the radium emanation1 like that of thorium1 passed unchanged in amount through every reagent used4 ,in later experiments by ,sir ,william ,ramsay and ,mr ,soddy#beb1 the emanation from radium was exposed to still more drastic treatment4 ,the emanation in a glass tube was sparked for several hours with oxygen over alkali4 ,the oxygen was then removed by ignited phosphorus and no visible residue was left4 ,when1 however1 another gas was introduced1 mixed with the minute amount of emanation in the tube and withdrawn1 the activity of emanation was found to be unaltered4 ,in another experiment1 the emanation was introduced into a magnesium lime tube1 which was heated for three hours at a red heat4 ,the emanation was then removed and tested1 but no diminution in its discharging power was observed4 ,the emanations of thorium and radium thus withstand chemical treatment in a manner hitherto unobserved except in gases of the argon family4 #aei4 ,ramsay and ,soddy 7loc4 cit47 record an interesting experiment to illustrate the gaseous nature of the emanation4 ,a large amount of the radium emanation was collected in a small glass tube4 ,this tube phosphoresced brightly under the influence of the rays from the emanation4 ,the passage of the emanation from point to point was observed in a darkened room by the luminosity excited in the glass4 ,on opening the stop-cock connecting with the ,tpler pump1 the slow flow through the capillary tube was noticed1 the rapid passage along the wider tubes1 the delay in passing through a plug of phosphorous pentoxide1 and the rapid expansion into the reservoir of the pump4 ,when compressed1 the luminosity of the emanation increased1 and became very bright as the small bubble containing the emanation was expelled through the fine capillary tube4 ,diffusion of the ,emanations4 #afj4 ,it has been shown that the emanations of thorium and radium behave like radio-active gases1 distributed in minute amount in the air or other gas in which they are tested4 ,with the small quantities of active material so far investigated1 the emanations have not yet been collected in sufficient amount to determine their density4 ,although the molecular weight of the emanations cannot yet be obtained by direct chemical methods1 an indirect estimate of it can be made by determining the rate of their inter-diffusion into air or other gases4 ,the coefficients of inter-diffusion of various gases have long been known1 and the results show that the coefficient of diffusion of one gas into another is1 for the simpler gases1 approximately inversely proportional to the square root of the product of their molecular weights4 ,if1 therefore1 the coefficient of diffusion of the emanation into air is found to have a value1 lying between that of two known gases ,a and ,b1 it is probable that the molecular weight of the emanation lies between that of ,a and ,b4 ,although the volume of the emanation given off from radium is very small1 the electrical conductivity produced by the emanation in the gas1 with which it is mixed1 is often very large1 and offers a ready means of measuring the emanation present4 ,some experiments have been made by ,miss ,brooks and the writer#bec to determine the rate of the diffusion of the radium emanation into air1 by a method similar to that employed by ,loschmidt#bed in #ahga1 in his investigations of the coefficient of inter-diffusion of gases4 ,illustration3 ,fig4 #ef4 ,fig4 #ef shows the general arrangement4 ,a long brass cylinder ,,ab1 of length #gc cms41 and diameter #f cms41 was divided into two equal parts by a movable metal slide ,s4 ,the ends of the cylinder were closed with ebonite stoppers4 ,two insulated brass rods1 a and b1 each half the length of the tube1 passed through the ebonite stoppers and were supported centrally in the tube4 ,the cylinder was insulated and connected with one pole of a battery of #cjj volts1 the other pole of which was earthed4 ,the central rods could be connected with a sensitive quadrant electrometer4 ,the cylinder was covered with a thick layer of felt1 and placed inside a metal box filled with cotton wool in order to keep temperature conditions as steady as possible4 ,in order to convey a sufficient quantity of emanation into the half-cylinder ,a1 it was necessary to heat the radium slightly4 ,the slide ,s was closed and the side tubes opened4 ,a slow current of dry air from a gasometer was passed through a platinum tube1 in which a small quantity of radium compound was placed4 ,the emanation was carried with the air into the cylinder ,a4 ,when a sufficient quantity had been introduced1 the stream of air was stopped4 ,the side tubes were closed by fine capillary tubes4 ,these prevented any appreciable loss of gas due to the diffusion1 but served to keep the pressure of the gas inside ,a at the pressure of the outside air4 ,the three entrance tubes into the cylinder1 shown in the figure1 were for the purpose of initially mixing the emanation and gas as uniformly as possible4 ,after standing several hours to make temperature conditions steady1 the slide was opened1 and the emanation began to diffuse into the tube ,b4 ,the current through the tubes ,a and ,b was measured at regular intervals by an electrometer1 with a suitable capacity in parallel4 ,initially there is no current in ,b1 but after the opening of the slide1 the amount in ,a decreased and the amount in ,b steadily increased4 ,after several hours the amount in each half is nearly the same1 showing that the emanation is nearly uniformly diffused throughout the cylinder4 ,it can readily be shown#bee that if ,k coefficient of diffusion of the emanation into air1 t duration of diffusion experiments in secs41 a total length of cylinder1 ,s partial pressure of emanation in tube ,a at end of diffusion1 ,s partial pressure of emanation in tube ,b at end of diffusion1 then frac ,s#a ,s#b ,s#a ,s#b frac #h pi#b 7efrac pi#b ,kt a#b frac #ai;e-frac #i pi#b ,kt a#b 4447 4 ,now the values of ,s and ,s are proportional to the saturation ionization currents due to the emanations in the two halves of the cylinder4 ,from this equation ,k can be determined1 if the relative values of ,s and ,s are observed after diffusion has been in progress for a definite interval t4 ,the determination of ,s and ,s is complicated by the excited activity produced on the walls of the vessel4 ,the ionization due to this must be subtracted from the total ionization observed in each half of the cylinder1 for the excited activity is produced from the material composing the emanation1 and is removed to the electrodes in an electric field4 ,the ratio of the current due to excited activity to the current due to the emanation depends on the time of exposure to the emanation1 and is only proportional to it for exposures of several hours4 ,the method generally adopted in the experiments was to open the slide for a definite interval1 ranging in the experiments from #ae to #abj minutes4 ,the slide was then closed and the currents in each half determined at once4 ,the central rods1 which had been kept negatively charged during the experiments1 had most of the excited activity concentrated on their surfaces4 ,these were removed1 new rods substituted and the current immediately determined4 ,the ratio of the currents in the half cylinders under these conditions was proportional to ,s and ,s1 the amounts of emanation present in the two halves of the cylinder4 ,the values of ,k1 deduced from different values of t1 were found to be in good agreement4 ,in the earlier experiments the values of ,k were found to vary between #jh and #ab4 ,in some later experiments1 where great care was taken to ensure that temperature conditions were very constant1 the values of ,k were found to vary between #jg and #ji4 ,the lower value #jg is most likely nearer the true value1 as temperature disturbances tend to give too large a value of ,k4 ,no certain differences were observed in the value of ,k whether the air was dry or damp1 or whether an electric field was acting or not4 #afa4 ,some experiments on the rate of diffusion of the radium emanation into air were made at a later date by ,p4 ,curie and ,danne#bef4 ,if the emanation is contained in a closed reservoir1 it has been shown that its activity1 which is a measure of the amount of emanation present1 decreases according to an exponential law with the time4 ,if the reservoir is put in communication with the outside air through a capillary tube1 the emanation slowly diffuses out1 and the amount of emanation in the reservoir is found to decrease according to the same law as before1 but at a faster rate4 ,using tubes of different lengths and diameters1 the rate of diffusion was found to obey the same laws as a gas4 ,the value of ,k was found to be #jajj4 ,this is a slightly greater value of ,k than the lowest value #jjg found by ,rutherford and ,miss ,brooks4 ,no mention is made by ,curie and ,danne of having taken any special precautions against temperature disturbances1 and this may account for the higher value of ,k obtained by them4 ,they also found that the emanation1 like a gas1 always divided itself between two reservoirs1 put in connection with one another1 in the proportion of their volumes4 ,in one experiment one reservoir was kept at a temperature of #aj ,c4 and the other at #cej ,c4 ,the emanation divided itself between the two reservoirs in the same proportion as would a gas under the same conditions4 #afb4 ,for the purpose of comparison1 a few of the coefficients of inter-diffusion of gases1 compiled from ,landolt and ,bernstein's tables1 are given below4 ,gas or vapour ,coefficient of ,molecular diffusion into weight air ,water vapour #jaih #ah ,carbonic acid #jadb #dd gas ,alcohol vapour #jaja #df ,ether vapour #jjgg #gd ,radium #jjg 8 emanation ,the tables1 although not very satisfactory for the purpose of comparison1 show that the coefficient of inter-diffusion follows the inverse order of the molecular weights4 ,the value of ,k for the radium emanation is slightly less than for ether vapour1 of which the molecular weight is #gd4 ,we may thus conclude that the emanation is of greater molecular weight than #gd4 ,it seems likely that the emanation has a molecular weight somewhere in the neighbourhood of #ajj1 and is probably greater than this1 for the vapours of ether and alcohol have higher diffusion coefficients compared with carbonic acid than the theory would lead us to anticipate4 ,comparing the diffusion coefficients of the emanation and carbonic acid into air1 the value of the molecular weight of the emanation should be about #agf if the result observed for the simple gases1 viz4 that the coefficient of diffusion is inversely proportional to the square root of the molecular weights1 holds true in the present case4 ,bumstead and ,wheeler#beg compared the rates of diffusion of the radium emanation and of carbon dioxide through a porous plate1 and concluded that the molecular weight of the emanation was about #ahj4 ,on the disintegration theory1 the atom of the emanation is derived from the radium atom by the expulsion of one particle4 ,thus1 it is to be expected that its molecular weight would be over #bjj4 ,it is of interest to compare the value of ,k #jg with the value of ,k determined by ,townsend 7section #cg7 for the gaseous ions produced in air at ordinary pressure and temperature1 by ,rntgen rays or by the radiations from active substances4 ,townsend found that the value of ,k in dry air was #jbh for the positive ions and #jdc for the negative ions4 ,the radium emanation thus diffuses more rapidly than the ions produced by its radiation in the gas1 and behaves as if its mass were smaller than that of the ions produced in air1 but considerably greater than that of the air molecules with which it is mixed4 ,it is not possible to regard the emanation as a temporarily modified condition of the gas originally in contact with the active body4 ,under such conditions a much larger value of ,k would be expected4 ,the evidence derived from the experiments on diffusion strongly supports the view that the emanation is a gas of heavy molecular weight4 ,makower#beh has recently attacked the question of the molecular weight of the radium emanation by another method4 ,the rate of diffusion of the emanation through a porous plug of plaster-of-,paris was compared with that of the gases oxygen1 carbon dioxide1 and sulphur dioxide4 ,it was found that ,graham's law1 viz4 that the coefficient of diffusion ,k is inversely proportional to the square root of its molecular weight ,m1 was not strictly applicable4 ,the value of ,k ,m was not found to be constant for these gases1 but decreased with increase of molecular weight of the gas4 ,if1 however1 a curve was plotted with ,k ,m as ordinate and ,k as abscissa1 the points corresponding to the values of ,o1 ,,co and ,,so were found to lie on a straight line4 ,by linear extrapolation1 the molecular weight of the emanation was estimated4 ,the value obtained from experiments on three different porous plugs was #hee1 #ig1 and #ii respectively4 ,this method indicates that the molecular weight of the radium emanation is about #ajj2 but in all the experiments on diffusion1 it must be remembered that the emanation1 whose rate of inter-diffusion is being examined1 exists in minute quantity mixed with the gas1 and is compared with the rate of inter-diffusion of gases which are present in large quantity4 ,for this reason1 deductions of the molecular weight of the emanation may be subject to comparatively large errors1 for which it is difficult to make correction4 ,diffusion of the ,thorium ,emanation4 #afc4 ,on account of the rapid decay of the activity of the thorium emanation1 it is not possible to determine the value of ,k its coefficient of diffusion into air by the methods employed for the radium emanation4 ,the value of ,k has been determined by the writer in the following way4 ,a plate ,c1 ,fig4 #eg1 covered with thorium hydroxide1 was placed horizontally near the base of a long vertical brass cylinder ,p4 ,the emanation released from the thorium compound diffuses upwards in the cylinder4 ,illustration3 ,fig4 #eg4 ,let p be the partial pressure of the emanation at a distance x from the source ,c4 ,this will be approximately uniform over the cross section of the cylinder4 ,from the general principles of diffusion we get the equation dp dp ,k ----- ---- 4 dx dt ,the emanation is continuously breaking up and expelling particles4 ,the emanation-residue gains a positive charge1 and1 in an electric field1 is removed at once from the gas to the negative electrode4 ,since the activity of the emanation at any time is always proportional to the number of particles which have not broken up1 and since the activity decays with the time according to an exponential law1 p p#a e-t 1 where p is the value of p when t #j and is the radio-active constant of the emanation4 ,then dp ---- -p1 dt and dp ,k ----- p4 dx ,thus p ,ae-sqrt frac ,k x ,besqrt frac ,k x 4 ,since p #j when x infinity1 ,b #j4 ,if p p when x #j1 ,a p4 ,thus p p e-sqrt frac ,k x 4 ,it was not found convenient in the experiments to determine the activity of the emanation along the cylinder1 but an equivalent method was used which depends upon measuring the distribution of 8excited activity10 produced along a central rod ,,ab1 which is charged negatively4 ,it will be shown later 7section #agg7 that the amount of excited activity at any point is always proportional to the amount of emanation at that point4 ,the distribution of 8excited activity0 along the central rod from the plate ,c upwards thus gives the variation of p for the emanation along the tube4 ,in the experiments1 the cylinder was filled with dry air at atmospheric pressure and was kept at a constant temperature4 ,the central rod was charged negatively and exposed from one to two days in the presence of the emanation4 ,the rod was then removed1 and the distribution of the excited activity along it determined by the electric method4 ,it was found that the amount of excited activity fell off with the distance x according to an exponential law1 falling to half value in about #ai cms4 ,this is in agreement with the above theory4 ,since the activity of the emanation falls to half value in #a minute1 #jaae4 ,the value ,k #ji was deduced from the average of a number of experiments4 ,this is a slightly greater value than ,k #jg1 obtained for the radium emanation1 but the results show that the two emanations do not differ much from one another in molecular weight4 ,makower 7loc4 cit47 compared the rates of diffusion of the thorium and radium emanation through a porous plate1 and concluded that the two emanations were of about the same molecular weight1 thus confirming the results obtained by the above method4 ,diffusion of the ,emanation into ,liquids4 #afd4 ,experiments have been made by ,wallstabe#bei on the coefficient of diffusion of the radium emanation into various liquids4 ,the radium emanation was allowed to diffuse into a closed reservoir1 containing a cylinder of the liquid under observation4 ,the cylinder was provided with a tube and a stop-cock extending beyond the closed vessel1 so that different layers of the liquid could be removed4 ,the liquid was then placed in a closed testing vessel1 where the ionization current due to the escape of the emanation from the liquid was observed to rise to a maximum after several hours1 and then to decay4 ,this maximum value of the current was taken as a measure of the amount of emanation absorbed in the liquid4 ,the coefficient of diffusion ,k of the emanation into the liquid can be obtained from the same equation used to determine the diffusion of the thorium emanation into air1 p p e-sqrt frac ,k x 4 where is the constant of decay of activity of the radium emanation and x the depth of the layer of water from the surface4 ,putting a sqrt frac ,k 1 it was found that for water #af1 for toluol #ge4 ,the value of expressed in terms of a day as the unit of time is about #ag4 ,thus the value of ,k for the diffusion of the radium emanation into water #jff cm4 _/ day4 ,the value of ,k found by ,stefan#bfj for the diffusion of carbon dioxide into water was #acf cm4_/day4 ,these results are thus in harmony with the conclusion drawn from the diffusion of the radium emanation into air1 and show that the radium emanation behaves as a gas of high molecular weight4 ,condensation of the ,emanations4 #afe4 ,condensation of the emanations4 ,during an investigation of the effect of physical and chemical agencies on the thorium emanation1 ,rutherford and ,soddy#bfa found that the emanation passed unchanged in amount through a white-hot platinum tube and through a tube cooled to the temperature of solid carbon dioxide4 ,in later experiments the effects of still lower temperatures were examined1 and it was then found that at the temperature of liquid air both emanations were condensed#bfb4 ,if either emanation is conveyed by a slow stream of hydrogen1 oxygen1 or air through a metal spiral immersed in liquid air1 and placed in connection with a testing vessel as in ,fig4 #ea1 no trace of emanation escapes in the issuing gas4 ,when the liquid air is removed and the spiral plunged into cotton-wool1 several minutes elapse before any deflection of the electrometer needle is observed1 and then the condensed emanation volatilizes rapidly1 and the movement of the electrometer needle is very sudden1 especially in the case of radium4 ,with a fairly large amount of radium emanation1 under the conditions mentioned1 a very few seconds elapse after the first sign of movement before the electrometer needle indicates a deflection of several hundred divisions per second4 ,it is not necessary in either case that the emanating compound should be retained in the gas stream4 ,after the emanation is condensed in the spiral1 the thorium or radium compound may be removed and the gas stream sent directly into the spiral4 ,but in the case of thorium1 under these conditions1 the effects observed are naturally small owing to the rapid loss of the activity of the emanation with time1 which proceeds at the same rate at the temperature of liquid air as at ordinary temperatures4 ,if a large amount of radium emanation is condensed in a glass ,u tube1 the progress of the condensation can be followed by the eye1 by means of the phosphorescence which the radiations excite in the glass4 ,if the ends of the tube are sealed and the temperature allowed to rise1 the glow diffuses uniformly throughout the tube1 and can be concentrated at any point to some extent by local cooling of the tube with liquid air4 #aff4 ,experimental arrangements4 ,a simple experimental arrangement to illustrate the condensation and volatilization of the emanation and some of its characteristic properties is shown in ,fig4 #eh4 ,the emanation obtained from a few milligrams of radium bromide by solution or heating is condensed in the glass ,u tube ,t immersed in liquid air4 ,this ,u tube is then put into connection with a larger glass tube ,v1 in the upper part of which is placed a piece of zinc sulphide screen ,z1 and in the lower part of the tube a piece of the mineral willemite4 ,the stop-cock ,a is closed and the ,u tube and the vessel ,v are partially exhausted by a pump through the stop-cock ,b4 ,this lowering of the pressure causes a more rapid diffusion of the emanation when released4 ,the emanation does not escape if the tube ,t is kept immersed in liquid air4 ,the stop-cock ,b is then closed1 and the liquid air removed4 ,no luminosity of the screen or the willemite in the tube ,v is observed for several minutes1 until the temperature of ,t rises above the point of volatilization of the emanation4 ,the emanation is then rapidly carried into the vessel ,v1 partly by expansion of the gas in the tube ,t with rising temperature1 and partly by the process of diffusion4 ,the screen ,z and the willemite ,w are caused to phosphoresce brilliantly under the influence of the rays from the emanation surrounding them4 ,illustration3 ,fig4 #eh4 ,if the end of the vessel ,v is then plunged into liquid air1 the emanation is again condensed in the lower end of the tube1 and the willemite phosphoresces much more brightly than before4 ,this is not due to an increase of the phosphorescence of willemite at the temperature of the liquid air1 but to the effect of the rays from the emanation condensed around it4 ,at the same time the luminosity of the zinc sulphide gradually diminishes1 and practically disappears after several hours if the end of the tube is kept in the liquid air4 ,if the tube is removed from the liquid air1 the emanation again volatilizes and lights up the screen ,z4 ,the luminosity of the willemite returns to its original value after the lapse of several hours4 ,this slow change of the luminosity of the zinc sulphide screen and of the willemite is due to the gradual decay of the 8excited activity0 produced by the emanation on the surface of all bodies exposed to its action 7chapter ,,viii74 ,the luminosity of the screen is thus due partly to the radiation from the emanation and partly to the excited radiation caused by it4 ,as soon as the emanation is removed from the upper to the lower part of the tube1 the 8excited0 radiation gradually diminishes in the upper and increases in the lower part of the tube4 ,the luminosity of the screen gradually diminishes with the time as the enclosed emanation loses its activity1 but is still appreciable after an interval of several weeks4 ,an apparatus of a similar character to illustrate the condensation of the radium emanation has been described by ,p4 ,curie#bfc4 ,illustration3 ,fig4 #ei4 #afg4 ,determination of the temperature of condensation4 ,a detailed investigation was made by ,rutherford and ,soddy 7loc4 cit47 of the temperatures at which condensation and volatilization commenced for the two emanations4 ,the experimental arrangement of the first method is shown clearly in ,fig4 #ei4 ,a slow constant stream of gas1 entering at ,a1 was passed through a copper spiral ,s1 over #c metres in length1 immersed in a bath of liquid ethylene4 ,the copper spiral was made to act as its own thermometer by determining its electrical resistance4 ,the resistance temperature curve was obtained by observation of the resistances at #j1 the boiling point of liquid ethylene #ajce1 the solidification point of ethylene #afi and in liquid air4 ,the temperature of the liquid air was deduced from the tables given by ,baly for the boiling point of liquid air for different percentages of oxygen4 ,the resistance-temperature curve1 for the particular spiral employed1 was found to be nearly a straight line between #j and #aib;,c41 cutting the temperature axis if produced nearly at the absolute zero4 ,the resistance of the spiral1 deduced from readings on an accurately calibrated ,weston millivoltmeter1 with a constant current through the spiral1 was thus very approximately proportional to the absolute temperature4 ,the liquid ethylene was kept vigorously stirred by an electric motor1 and was cooled to any desired temperature by surrounding the vessel with liquid air4 ,the general method employed for the radium emanation was to pass a suitable amount of emanation1 mixed with the gas to be used1 from the gas holder ,b into the spiral1 cooled below the temperature of condensation4 ,after the emanation was condensed in the spiral1 a current of electrolytic hydrogen or oxygen was passed through the spiral4 ,the temperature was allowed to rise gradually1 and was noted at the instant when a deflection of the electrometer1 due to the presence of emanation in the testing vessel ,t1 was observed4 ,the resistance1 subject to a slight correction due to the time taken for the emanation to be carried into the testing vessel1 gave the temperature at which some of the emanation commenced to volatilize4 ,the ionization current in the testing vessel rose rapidly to a maximum value1 showing that1 for a small increase of temperature1 the whole of the radium emanation was volatilized4 ,the following table gives an illustration of the results obtained for a current of hydrogen of #ach cubic centimetres per second4 ,temperature ,divisions per second of the electrometer #afj #j #aef #j #aedc #a #aech #ba #aebe #bd ,the following table shows the results obtained for different currents of hydrogen and oxygen4 ,current of ,gas ,t ,t ,hydrogen #be c4c4 per sec4 #aeac #aej #cb #aecg #aea #ib #aeb #aea #ach #aed #aec #bc #afbe #afb ,oxygen #cd #aebe #aeae #eh #aee #aec ,the temperature ,t in the above table gives the temperature of initial volatilization1 ,t the temperature for which half of the condensed emanation had been released4 ,for slow currents of hydrogen and oxygen1 the values of ,t and ,t are in good agreement4 ,for a stream of gas as rapid as #bc cubic centimetres per second the value of ,t is much lower4 ,such a result is to be expected2 for1 in too rapid a stream1 the gas is not cooled to the temperature of the spiral1 and1 in consequence1 the inside surface of the spiral is above the mean temperature1 and some of the emanation escapes at a temperature apparently much lower4 ,in the case of oxygen1 this effect appears for a gas stream of #jeh cubic centimetres per second4 ,in the experiments on the thorium emanation1 on account of the rapid loss of activity1 a slightly different method was necessary4 ,the steady stream of gas was passed over the thorium compound1 and the temperature was observed at the instant when an appreciable movement of the electrometer appeared4 ,this gave the temperature at which a small fraction of the thorium emanation escaped condensation1 and not the value ,t observed for the radium emanation1 which gave the temperature for which a small fraction of the previously condensed emanation was volatilized4 ,the following table illustrates the results obtained4 ,current of ,gas ,temperature ,hydrogen #ga c4c4 per #aee ,c4 sec4 #ach #aei ,c4 ,oxygen #eh #aee ,c4 ,on comparing these results with the values obtained for the radium emanation1 it will be observed that with equal gas streams the temperatures are nearly the same4 ,a closer examination of the thorium emanation showed1 however1 that this apparent agreement was only accidental1 and that there was1 in reality1 a very marked difference in the effect of temperature on the two emanations4 ,it was found experimentally that the radium emanation was condensed very near the temperature at which volatilization commenced1 and that the points of condensation and volatilization were defined fairly sharply4 ,illustration3 ,fig4 #fj4 ,on the other hand1 the thorium emanation required a range of over #cj ,c4 after condensation had started in order to ensure complete condensation4 ,fig4 #fj is an example of the results obtained with a steady gas stream of #ach c4c4 per sec4 of oxygen4 ,the ordinates represent the percentage proportion of the emanation uncondensed at different temperatures4 ,it will be observed that condensation commences about #abj1 and that very little of the emanation escapes condensation at #aee ,c4 ,to investigate this difference of behaviour in the two emanations1 a static method was employed1 which allowed an examination of the two emanations to be made under comparable conditions4 ,the emanation1 mixed with a small amount of the gas to be used1 was introduced into the cool spiral1 which had been exhausted previously by means of a mercury pump4 ,the amount of emanation remaining uncondensed after definite intervals was rapidly removed by means of the pump1 and was carried with a constant auxiliary stream of gas into the testing vessel4 ,tested in this way1 it was found that the volatilization point of the radium emanation was very nearly the same as that obtained by the blowing method1 viz4 #aej ,c4 ,with thorium1 on the other hand1 the condensation started at about #abj ,c41 and1 as in the blowing method1 continued over a range of about #cj ,c4 ,the proportion of the emanation condensed at any temperature was found to depend on a variety of conditions1 although the point at which condensation commenced1 viz4 #abj ,c41 was about the same in each case4 ,it depended on the pressure and nature of the gas1 on the concentration of the emanation1 and on the time for which it was left in the spiral4 ,for a given temperature a greater proportion of the emanation was condensed1 the lower the pressure and the longer the time it was left in the spiral4 ,under the same conditions1 the emanation was condensed more rapidly in hydrogen than in oxygen4 #afh4 ,thus there is no doubt that the thorium emanation begins to condense at a temperature higher than that at which the radium emanation condenses4 ,the explanation of the peculiar behaviour of the thorium emanation is clear when the small number of emanation particles present in the gas are taken into consideration4 ,it has been shown that both emanations give out only rays4 ,it is probable that the particles from the two emanations are similar in character and produce about the same number of ions in their passage through the gas4 ,the number of ions produced by each particle before its energy is dissipated is probably about #gj1jjj4 7,see section #beb47 ,now1 in the experiment1 the electrometer readily measured a current of #aj electrostatic units4 ,taking the charge on an ion as #cd #aj electrostatic units1 this corresponds to a production in the testing vessel of about #c #aj ions per sec41 which would be produced by about #dj expelled particles per second4 ,each radiating particle cannot expel less than one particle and may expel more1 but it is likely that the number expelled by an atom of the thorium emanation is not greatly different from that expelled by an atom of the radium emanation4 ,in section #acc it has been shown that1 according to the law of decay1 ,n particles change per second when ,n are present4 ,thus1 to produce #dj particles1 ,n cannot be greater than #dj4 ,since for the thorium emanation is 1 it follows that ,n cannot be greater than #cejj4 ,the electrometer thus detected the presence of #cejj particles of the thorium emanation1 and since in the static method the volume of the condensing spiral was about #ae c4c41 this corresponded to a concentration of about #bcj particles per c4c4 ,an ordinary gas at atmospheric pressure and temperature probably contains about #cf #aj molecules per c4c4 ,thus the emanation would have been detected on the spiral if it had possessed a partial pressure of less than #aj of an atmosphere4 ,it is not surprising then that the condensation point of the thorium emanation is not sharply defined4 ,it is rather a matter of remark that condensation should occur so readily with so sparse a distribution of emanation particles in the gas2 for1 in order that condensation may take place1 it is probable that the particles must approach within one another's sphere of influence4 ,now in the case of the radium emanation1 the rate of decay is about #ejjj times slower than that of the thorium emanation1 and consequently the actual number of particles that must be present to produce the same ionization per second in the two cases must be about #ejjj times greater in the case of radium than in the case of thorium4 ,this conclusion involves only the assumption that the same number of rays is produced by a particle of emanation in each case1 and that the expelled particles produce in their passage through the gas the same number of ions4 ,the number of particles present1 in order to be detected by the electrometer1 in this experiment1 must therefore have been about #ejjj #cejj1 i4e4 about #b #aj4 ,the difference of behaviour in the two cases is well explained by the view that1 for equal electrical effects1 the number of radium emanation particles must be far larger than the number of thorium emanation particles4 ,the probability of the particles coming into each other's sphere of influence will increase very rapidly as the concentration of the particles increases1 and1 in the case of the radium emanation1 once the temperature of condensation is attained1 all but a small proportion of the total number of particles present will condense in a very short time4 ,in the case of the thorium emanation1 however1 the temperature might be far below that of condensation1 and yet a considerable portion remain uncondensed for comparatively long intervals4 ,on this view the experimental results obtained might reasonably be expected4 ,a greater proportion of emanation condenses the longer the time allowed for condensation under the same conditions4 ,the condensation occurs more rapidly in hydrogen than in oxygen1 as the diffusion is greater in the former gas4 ,for the same reason the condensation occurs faster the lower the pressure of the gas present4 ,finally1 when the emanation is carried by a steady stream of gas1 a smaller proportion condenses than in the other cases1 because the concentration of emanation particles per unit volume of gas is less under these conditions4 ,it is possible that the condensation of the emanations may not occur in the gas itself but at the surface of the containing vessel4 ,accurate observations of the temperature of condensation have so far only been made in a copper spiral1 but condensation certainly occurs in tubes of lead or glass at about the same temperature as in tubes of copper4 #afi4 ,in experiments that were made by the static method with a very large quantity of radium emanation1 a slight amount of escape of the condensed emanation was observed several degrees below the temperature at which most of the emanation was released4 ,this is to be expected1 since1 under such conditions1 the electrometer is able to detect a very minute proportion of the whole quantity of the emanation condensed4 ,special experiments1 with a large quantity of emanation1 that were made with the spiral immersed in a bath of rapidly boiling nitric oxide1 showed this effect very clearly4 ,for example1 the condensed emanation began to volatilize at #aee ,c4 ,in #d minutes the temperature had risen to #aece1 and the amount volatilized was four times as great as at #aee4 ,in the next #e minutes the temperature had increased to #aebc and practically the whole quantity1 which was at least fifty times the amount at the temperature of #aece1 had volatilized4 ,it thus seems probable that1 if the temperature were kept steady at the point at which volatilization was first observed1 and the released emanation removed at intervals1 the whole of the emanation would in course of time be liberated at that temperature4 ,curie and ,dewar and ,ramsay have observed that the emanation condensed in a ,u tube1 immersed in liquid air1 slowly escapes if the pump is kept steadily working4 ,these results point to the probability that the condensed emanation possesses a true vapour pressure1 but great refinements in experimental methods would be necessary before such a conclusion could be definitely established4 ,the true temperature of condensation of the thorium emanation is probably about #abj ,c41 and that of radium about #aej ,c4 ,thus there is no doubt that the two emanations are quite distinct from each other in this respect1 and also with regard to their radio-activity1 although they both possess the property of chemical inertness4 ,these results on the temperatures of condensation do not allow us to make any comparison of the condensation points of the emanations with those of known gases1 since the lowering of the condensation points of gases with diminution of pressure has not been studied at such extremely minute pressures4 #agj4 ,it has been found#bfd that the activity of the thorium emanation1 when condensed in the spiral at the temperature of liquid air1 decayed at the same rate as at ordinary temperatures4 ,this is in accord with results of a similar kind obtained by ,p4 ,curie for the radium emanation 7section #ade71 and shows that the value of the radio-active constant is unaffected by wide variations of temperature4 ,amount of ,emanation from ,radium and ,thorium4 #aga4 ,it has been shown in section #ic from experimental data that #a gram of radium bromide at its minimum activity emits about #cf #aj particles per second4 ,since the activity due to the emanation stored up in radium1 when in a state of radio-active equilibrium1 is about one quarter of the whole and about equal to the minimum activity1 the number of particles projected per second by the emanation from #a gram of radium bromide is about #cf #aj4 ,it has been shown in section #aeb that #dfc1jjj times the amount of emanation produced per second is stored up in the radium4 ,but1 in a state of radio-active equilibrium1 the number of emanation particles breaking up per second is equal to the number produced per second4 ,assuming that each emanation particle in breaking up expels one particle1 it follows that the number of emanation particles present in #a gram of radium bromide in radio-active equilibrium is #dfc1jjj #cf #aj1 i4e4 #ag #aj4 ,taking the number of hydrogen molecules in #a c4c4 of gas at atmospheric pressure and temperature as #cf #aj 7section #ci71 the volume of the emanation from #a gram of radium bromide is #df #aj cubic centimetres at atmospheric pressure and temperature4 ,assuming the composition of radium bromide as ,ra,br1 the amount from #a gram of radium in radio-active equilibrium is #jhb cubic millimetres4 ,quite independently of any method of calculation it was early evident that the volume of the emanation was very small1 for all the earlier attempts made to detect its presence by its volume were unsuccessful4 ,it will be seen1 however1 that1 when larger quantities of radium were available for experiment1 the emanation has been collected in volume sufficiently large to measure4 ,in the case of thorium1 the maximum quantity of emanation to be obtained from #a gram of the solid is very minute1 both on account of the small activity of thorium and of the rapid break up of the emanation after its production4 ,since the amount of emanation1 stored in a non-emanating thorium compound1 is only #hg times the rate of production1 while in radium it is #dfc1jjj times1 and the rate of production of the emanation by radium is about #a million times faster than by thorium1 it follows that the amount of emanation to be obtained from #a gram of thorium is not greater than #aj of the amount from an equal weight of radium1 i4e4 its volume is not greater than #aj c4c4 at the ordinary pressure and temperature4 ,even with large quantities of thorium1 the amount of emanation is too small ever to be detected by its volume4 #agb4 ,volume of the emanation from radium4 ,the evidence already considered points very strongly to the conclusion that the emanation possesses all the properties of a chemically inert gas of high molecular weight4 ,since the emanation continuously breaks up1 and is transformed into a solid type of matter1 which is deposited on the surface of bodies1 the volume of the emanation1 when separated from radium1 should contract at the same rate as it loses its activity1 i4e4 it should decrease to half value in about four days4 ,the amount of emanation to be obtained from a given quantity of radium is a maximum when the rate of production of new emanation balances its rate of change4 ,this condition is practically attained when the emanation has been allowed to collect for an interval of one month4 ,the probable volume of the emanation to be obtained from #a gram of radium was early calculated on certain assumptions1 and from data then available the writer#bfe deduced that the volume of the emanation from #a gram of radium lay between #jf and #f cubic millimetre at atmospheric pressure and temperature1 and was probably nearer the latter value4 ,the volume to be expected on the latest data has been discussed in the preceding section and shown to be about #hb cubic mm4 ,the volume of the emanation is thus very small1 but not too small to be detected if several centigrams of radium are available4 ,this has been proved to be the case by ,ramsay and ,soddy#bff who1 by very careful experiment1 finally succeeded in isolating a small quantity of the emanation and in determining its volume4 ,the experimental method employed by them will now be briefly described4 ,illustration3 ,fig4 #fa4 ,the emanation from #fj milligrams of radium bromide in solution was allowed to collect for #h days and then drawn off through the inverted siphon ,e 7,fig4 #fa7 into the explosion burette ,f4 ,this gas consisted for the most part of hydrogen and oxygen1 produced by the action of the radiations on the water of the solution4 ,after explosion1 the excess of hydrogen mixed with emanation was left some time in contact with caustic soda1 placed in the upper part of the burette1 in order to remove all trace of carbon dioxide4 ,in the meantime the upper part of the apparatus had been completely evacuated4 ,the connection ,c to the pump was closed1 and the hydrogen and emanation were allowed to enter the apparatus1 passing over a phosphorous pentoxide tube ,d4 ,the emanation was condensed in the lower part of the capillary tube ,a1 by surrounding it with the tube ,b filled with liquid air4 ,the process of condensation was rendered manifest by the brilliant luminosity of the lower part of the tube4 ,the mercury from the burette was then allowed to run to ,g1 and the apparatus again completely evacuated4 ,the connection of the pump was again closed1 the liquid air was removed and the volatilized emanation forced into the fine capillary tube ,a4 ,observations were then made1 from day to day1 of the volume of the emanation4 ,the results are given in the table below4 ,time ,volume ,time ,volume ,start #jabd cub4 #g days #jjjej cub4 mm4 mm4 #a day #jjbg #i #jjjda #c #jjaa #aa #jjjbj #d #jjjie #ab #jjjaa #f #jjjfc #bh #jjjjd ,the volume contracted with the time1 and was very small after a month's interval1 but the minute bubble of the emanation still retained its luminosity to the last4 ,the tube became deep purple in colour1 which rendered readings difficult except with a strong light4 ,there was a sudden decrease in the first day1 which may have been due to the mercury sticking in the capillary tube4 ,the experiments were repeated with another capillary tube and the volume of gas observed at normal pressure was #jjbed c4 mm4 ,the gas obtained was found to obey ,boyle's law within the limit of experimental error over a considerable range of pressure4 ,but1 unlike in the first experiment1 the gas did not contract but expanded rapidly during the first few hours1 and then more slowly1 finally reaching a volume after #bc days of #jbfb c4 mm4 or about #aj times the initial volume4 ,the measurements were complicated by the appearance of bubbles of gas in the top of the mercury column4 ,the differences observed in these two experiments are difficult to account for4 ,we shall see1 later1 that the emanation always produces helium1 and1 in the first experiment1 the decrease of the volume to zero indicates that the helium was buried or absorbed in the walls of the tube4 ,in the second case1 probably owing to some difference in the glass of the capillary tube1 the helium may have been released4 ,this suggestion is confirmed by the observation that the volume of gas1 after the experiment ended1 gave a brilliant spectrum of helium4 ,we shall see later that there is considerable evidence that the particles expelled from radio-active substances consist of helium atoms4 ,since the particles are projected with great velocity1 they will first be buried in the walls of the tube1 and then may gradually diffuse out into the gas again under conditions probably depending on the kind of glass employed4 ,since particles are projected from the emanation and also from two of the rapidly changing products which arise from it1 the volume of helium should1 on this view1 be three times the initial volume of the emanation4 ,if the helium produced escaped from the walls of the tube into the gas1 the apparent volume of the gas in the capillary should increase to three times the initial volume in a month's interval1 for during that time the emanation itself has been transformed into a solid type of matter deposited on the walls of the tube4 ,ramsay and ,soddy concluded from their experiments that the maximum volume of emanation to be obtained from #a gram of radium was about #a cubic millimetre at standard pressure and temperature1 and that the emanation was produced from #a gram of radium at the rate of #c #aj c4 mm4 per second4 ,this amount is in very good agreement with the calculated value1 and is a strong indication of the general correctness of the theory on which the calculations are based4 #agc4 ,spectrum of the emanation4 ,after the separation of the emanation and the determination of its volume1 ,ramsay and ,soddy made numerous attempts to obtain its spectrum4 ,in some of the earlier experiments several bright lines were seen for a short time1 but these lines were soon masked by the appearance of the hydrogen lines4 ,in later experiments ,ramsay and ,collie#bfg succeeded in obtaining a spectrum of the emanation1 which persisted for a short time1 during which a rapid determination of the wave-lengths was made4 ,they state that the spectrum was very brilliant1 consisting of very bright lines1 the spaces between being perfectly dark4 ,the spectrum bore a striking resemblance in general character to the spectrum of the gases of the argon family4 ,the spectrum soon faded1 and the spectrum of hydrogen began to appear4 ,the following table shows the wave-length of the lines observed in the spectrum4 ,the degree of coincidence of the lines of known wave-lengths shows that the error is probably less than five ngstrm units4 ,wave-length ,remarks #fefg ,hydrogen ,c2 true wave-length1 #fefc2 observed each time4 #fcjg ,observed only at first2 evanescent4 #eige #eiee #ehje ,observed each time2 persistent4 #egij ,mercury2 true wave-length1 #egij4 #egfh #egfi4 #egbe ,observed only at first2 evanescent4 #eeie ,observed each time2 persistent and strong4 #edfe ,mercury2 true wave-length1 #edfa4 #eaje ,not observed at first2 appeared after some seconds2 persisted and was visible during the second examination4 #dihe ,observed each time2 persistent and strong4 #dhfe ,hydrogen ,f2 true wave-length1 #dhfa4 #dfij ,observed only at first4 #dfej ,not observed when the emanation was examined again4 #dfcj #dcfj ,mercury3 true wave-length1 #dcei4 ,the experiments were repeated with a new supply of emanation1 and some of the stronger lines were observed again1 while some new lines made their appearance4 ,ramsay and ,collie suggest that the strong line #eeie may be identical with a line which was observed by ,pickering#bfh in the spectrum of lightning1 and was not identified with the spectrum of any known gas4 ,until large quantities of radium are available for the experimenter it would appear difficult to make sure how many of these lines must be ascribed to the spectrum of the emanation or to measure the wave-lengths with accuracy4 ,the results are of great interest1 as showing that the emanation has a definite and new spectrum of the same general character as the argon group of gases to which1 as we have seen1 it is chemically allied4 ,summary of ,results4 #agd4 ,the investigations into the nature of the radio-active emanations have thus led to the following conclusions3--,the radio-elements thorium1 radium and actinium continuously produce from themselves radio-active emanations at a rate which is constant under all conditions4 ,in some cases1 the emanations continuously diffuse from the radio-active compounds into the surrounding gas2 in other cases1 the emanations are unable to escape from the material in which they are produced1 but are occluded1 and can only be released by solution or by the action of heat4 ,the emanations possess all the properties of radio-active gases4 ,they diffuse through gases1 liquids1 and porous substances1 and can be occluded in some solids4 ,under varying conditions of pressure1 volume1 and temperature1 the emanations distribute themselves in the same way and according to the same laws as does a gas4 ,the emanations possess the important property of condensation under the influence of extreme cold1 and by that means can be separated from the gases with which they are mixed4 ,the radiation from the emanation is material in nature1 and consists of a stream of positively charged particles projected with great velocity4 ,the emanations possess the property of chemical inertness1 and in this respect resemble the gases of the argon family4 ,the emanations are produced in minute amount2 but a sufficient quantity of the radium emanation has been obtained to determine its volume and its spectrum4 ,with regard to their rates of diffusion1 the emanations of both thorium and radium behave like gases of high molecular weight4 ,these emanations have been detected and their properties investigated by the property they possess of emitting radiations of a special character4 ,these radiations consist entirely of rays1 i4e4 particles1 projected with great velocity1 which carry a positive charge and have a mass about twice that of the hydrogen atom4 ,the emanations do not possess the property of permanently radiating1 but the intensity of the radiations diminishes according to an exponential law with the time1 falling to half value1 from actinium in #d seconds1 from thorium in one minute1 and from radium in about four days4 ,the law of decay of activity does not seem to be influenced by any physical or chemical agency4 ,the emanation particles gradually break up1 each particle as it breaks up expelling a charged body4 ,the emanation after it has radiated ceases to exist as such1 but is transformed into a new kind of matter1 which is deposited on the surface of bodies and gives rise to the phenomena of excited activity4 ,this last property1 and the connection of the emanation with it1 are discussed in detail in the next chapter4 ,footnote #bca3 ,owens1 ,phil4 ,mag4 p4 #cfj1 ,oct4 #ahii4 ,footnote #bcb3 ,rutherford1 ,phil4 ,mag4 p4 #a1 ,jan4 #aijj4 ,footnote #bcc3 ,rossignol and ,gimingham1 ,phil4 ,mag4 ,july1 #aijd4 ,footnote #bcd3 ,bronson1 ,amer4 ,journ4 ,science1 ,feb4 #aije4 ,footnote #bce3 ,phil4 ,mag4 ,april1 #aijd4 ,footnote #bcf3 ,dorn1 ,abh4 der4 ,naturforsch4 ,ges4 fr ,halle-a-,s41 #aijj4 ,footnote #bcg3 ,p4 ,curie1 ,c4 ,r4 #ace1 p4 #heg1 #aijb4 ,footnote #bch3 ,rutherford and ,soddy1 ,phil4 ,mag4 ,april1 #aijc4 ,footnote #bci3 ,p4 ,curie1 ,c4 ,r4 #acf1 p4 #bbc1 #aijc4 ,footnote #bdj3 ,debierne1 ,c4 ,r4 #acf1 p4 #adf1 #aijc4 ,footnote #bda3 ,giesel1 ,ber4 ,d4 deutsch4 ,chem4 ,ges4 p4 #cfjh1 #aijb4 ,footnote #bdb3 ,curie and ,debierne1 ,c4 ,r4 #acb1 pp4 #edh and #gfh1 #aija4 ,footnote #bdc3 ,curie and ,debierne1 ,c4 ,r4 #acc1 p4 #ica1 #aija4 ,footnote #bdd3 ,rutherford and ,soddy1 ,trans4 ,chem4 ,soc4 p4 #cba1 #aijb4 ,phil4 ,mag4 ,sept4 #aijb4 ,footnote #bde3 ,rutherford1 ,phys4 ,zeit4 #b1 p4 #dbi1 #aija4 ,footnote #bdf3 ,rutherford and ,soddy1 ,phil4 ,mag4 ,nov4 #aijb4 ,footnote #bdg3 ,rutherford and ,soddy1 ,phil4 ,mag4 ,april1 #aijc4 ,footnote #bdh3 ,rutherford and ,soddy1 ,phil4 ,mag4 ,nov4 #aijb4 ,footnote #bdi3 ,rutherford and ,soddy1 ,phil4 ,mag4 ,april1 #aijc4 ,footnote #bej3 ,curie and ,debierne1 ,c4 ,r4 #acc1 p4 #ica1 #aija4 ,footnote #bea3 ,rutherford and ,soddy1 ,phil4 ,mag4 ,nov4 #aijb4 ,footnote #beb3 ,ramsay and ,soddy1 ,proc4 ,roy4 ,soc4 #gb1 p4 #bjd1 #aijc4 ,footnote #bec3 ,rutherford and ,miss ,brooks1 ,trans4 ,roy4 ,soc4 ,canada #aija1 ,chem4 ,news #aijb4 ,footnote #bed3 ,loschmidt1 ,sitzungsber4 d4 ,wien4 ,akad4 #fa1 ,,ii4 p4 #cfg1 #ahga4 ,footnote #bee3 ,see ,stefan1 ,sitzungsber4 d4 ,wien4 ,akad4 #fc1 ,,ii4 p4 #hb1 #ahga4 ,footnote #bef3 ,p4 ,curie and ,danne1 ,c4 ,r4 #acf1 p4 #acad1 #aijc4 ,footnote #beg3 ,bumstead and ,wheeler1 ,amer4 ,jour4 ,science1 ,feb4 #aijd4 ,footnote #beh3 ,makower1 ,phil4 ,mag4 ,jan4 #aije4 ,footnote #bei3 ,wallstabe1 ,phys4 ,zeit4 #d1 p4 #gba1 #aijc4 ,footnote #bfj3 ,stefan1 ,wien4 ,ber4 #b1 p4 #cga1 #ahgh4 ,footnote #bfa3 ,rutherford and ,soddy1 ,phil4 ,mag4 ,nov4 #aijb4 ,footnote #bfb3 ,phil4 ,mag4 ,may1 #aijc4 ,footnote #bfc3 ,p4 ,curie1 ,socit de ,physique1 #aijc4 ,footnote #bfd3 ,rutherford and ,soddy1 ,phil4 ,mag4 ,may1 #aijc4 ,footnote #bfe3 ,nature1 ,aug4 #bj1 #aijc4 ,footnote #bff3 ,proc4 ,roy4 ,soc4 #gc1 ,no4 #did1 p4 #cdf1 #aijd4 ,footnote #bfg3 ,proc4 ,roy4 ,soc4 #gc1 ,no4 #die1 p4 #dgj1 #aijd4 ,footnote #bfh3 ,pickering1 ,astrophys4 ,journ4 ,vol4 #ad1 p4 #cfh1 #aija4 ,,chapter ,,viii4 ,,excited ,,radio-,,activity4 #age4 ,excited radio-activity4 ,one of the most interesting and remarkable properties of thorium1 radium1 and actinium1 is their power of 8exciting0 or 8inducing0 temporary activity on all bodies in their neighbourhood4 ,a substance which has been exposed for some time in the presence of radium or thorium behaves as if its surface were covered with an invisible deposit of intensely radio-active material4 ,the 8excited0 body emits radiations capable of affecting a photographic plate and of ionizing a gas4 ,unlike the radio-elements themselves1 however1 the activity of the body does not remain constant after it has been removed from the influence of the exciting active material1 but decays with the time4 ,the activity lasts for several hours when due to radium and several days when due to thorium4 ,this property was first observed by ,m4 and ,mme4 ,curie#bfi for radium1 and independently by the writer#bgj for thorium#bga4 ,if any solid body is placed inside a closed vessel containing an emanating compound of thorium or radium1 its surface becomes radio-active4 ,for thorium compounds the amount of excited activity on a body is in general greater the nearer it is to the active material4 ,in the case of radium1 however1 provided the body has been exposed for several hours1 the amount of excited activity is to a large extent independent of the position of the body in the vessel containing the active material4 ,bodies are made active whether exposed directly to the action of the radio-active substance or screened from the action of the direct rays4 ,this has been clearly shown in some experiments of ,p4 ,curie4 ,a small open vessel a 7,fig4 #fb7 containing a solution of radium is placed inside a larger closed vessel ,v4 ,illustration3 ,fig4 #fb4 ,plates ,a1 ,b1 ,c1 ,d1 ,e are placed in various positions in the enclosure4 ,after exposure for a day1 the plates after removal are found to be radio-active even in positions completely shielded from the action of the direct rays4 ,for example1 the plate ,d shielded from the direct radiation by the lead plate ,p is as active as the plate ,e1 exposed to the direct radiation4 ,the amount of activity produced in a given time on a plate of given area in a definite position is independent of the material of the plate4 ,plates of mica1 copper1 cardboard1 ebonite1 all show equal amounts of activity4 ,the amount of activity depends on the area of the plate and on the amount of free space in its neighbourhood4 ,excited radio-activity is also produced in water if exposed to the action of an emanating compound4 #agf4 ,concentration of excited radio-activity on the negative electrode4 ,when thorium or radium is placed in a closed vessel1 the whole interior surface becomes strongly active4 ,in a strong electric field1 on the other hand1 the writer found that the activity was confined entirely to the negative electrode4 ,by suitable arrangements1 the whole of the excited activity1 which was previously distributed over the surface of the vessel1 can be concentrated on a small negative electrode placed inside the vessel4 ,an experimental arrangement for this purpose is shown in ,fig4 #fc4 ,illustration3 ,fig4 #fc4 ,the metal vessel ,v containing a large amount of thoria is connected with the positive pole of a battery of about #cjj volts4 ,the wire ,,ab to be made active is fastened to a stouter rod ,,bc1 passing through an ebonite cork inside a short cylinder ,d1 fixed in the side of the vessel4 ,this rod is connected with the negative pole of the battery4 ,in this way the wire ,,ab is the only conductor exposed in the field with a negative charge1 and it is found that the whole of the excited activity is concentrated upon it4 ,in this way it is possible to make a short thin metal wire over #aj1jjj times as active per unit surface as the thoria from which the excited activity is derived4 ,in the same way1 the excited activity due to radium can be concentrated mainly on the negative electrode4 ,in the case of thorium1 if the central wire be charged positively1 it shows no appreciable activity4 ,with radium1 however1 a positively charged body becomes slightly active4 ,in most cases1 the amount of activity produced on the positive electrode is not more than #e of the corresponding amount when the body is negatively charged4 ,for both thorium and radium1 the amount of excited activity on electrodes of the same size is independent of their material4 ,all metals are made active to equal extents for equal times of exposure4 ,when no electric field is acting1 the same amount of activity is produced on insulators like mica and glass as on conductors of equal dimensions4 #agg4 ,connection between the emanations and excited activity4 ,an examination of the conditions under which excited activity is produced shows that there is a very close connection between the emanation and the excited activity4 ,if a thorium compound is covered with several sheets of paper1 which cut off the rays but allow the emanation to pass through1 excited activity is still produced in the space above it4 ,if a thin sheet of mica is waxed down over the active material1 thus preventing the escape of the emanation1 no excited activity is produced outside it4 ,uranium and polonium which do not give off an emanation are not able to produce excited activity on bodies4 ,not only is the presence of the emanation necessary to cause excited activity1 but the amount of excited activity is always proportional to the amount of emanation present4 ,for example1 de-emanated thoria produces very little excited activity compared with ordinary thoria4 ,in all cases the amount of excited activity produced is proportional to the emanating power4 ,when passing through an electric field the emanation loses its property of exciting activity at the same rate as the radiating power diminishes4 ,this was shown by the following experiment4 ,a slow constant current of air from a gasometer1 freed from dust by its passage through cotton-wool1 passed through a rectangular wooden tube #gj cms4 long4 ,four equal insulated metal plates ,a1 ,b1 ,c1 ,d1 were placed at regular intervals along the tube4 ,the positive pole of a battery of #cjj volts was connected with a metal plate placed in the bottom of the tube1 while the negative pole was connected with the four plates4 ,a mass of thoria was placed in the bottom of the tube under the plate ,a1 and the current due to the emanation determined at each of the four plates4 ,after passing a current of air of #jb cm4 per second for #g hours along the tube1 the plates were removed and the amount of excited activity produced on them was tested by the electric method4 ,the following results were obtained4 ,relative ,relative current due excited to emanation activity ,plate ,a #a #a ,b #ee #dc ,c #ah #af ,d #jgb #jfa ,within the errors of measurement1 the amount of excited activity is thus proportional to the radiation from the emanation1 i4e4 to the amount of emanation present4 ,the same considerations hold for the radium emanation4 ,the emanation in this case1 on account of the slow loss of its activity1 can be stored mixed with air for long periods in a gasometer1 and its effects tested quite independently of the active matter from which it is produced4 ,the ionization current due to the excited activity produced by the emanation is always proportional to the current due to the emanation for the period of one month or more that its activity is large enough to be measured conveniently by an electrometer4 ,if1 at any time during the interval1 some of the emanation is removed and introduced into a new testing vessel1 the ionization current will immediately commence to increase1 rising in the course of four or five hours to about twice its original value4 ,this increase of the current is due to the excited activity produced on the walls of the containing vessel4 ,on blowing out the emanation1 the excited activity is left behind1 and at once begins to decay4 ,whatever its age1 the emanation still possesses the property of causing excited activity1 and in amount always proportional to its activity1 i4e4 to the amount of emanation present4 ,these results show that the power of exciting activity on inactive substances is a property of the radio-active emanations1 and is proportional to the amount of emanation present4 ,the phenomenon of excited activity cannot be ascribed to a type of phosphorescence produced by the rays from the emanation on bodies2 for it has been shown that the activity can be concentrated on the negative electrode in a strong electric field1 even if the electrode is shielded from the direct radiation from the active substance which gives off the emanation4 ,the amount of excited activity does not seem in any way connected with the ionization produced by the emanation in the gas with which it is mixed4 ,for example1 if a closed vessel is constructed with two large parallel insulated metal plates on the lower of which a layer of thoria is spread1 the amount of the excited activity on the upper plate when charged negatively1 is independent of the distance between the plates when that distance is varied from #a millimetre to #b centimetres4 ,this experiment shows that the amount of excited activity depends only on the amount of emanation emitted from the thoria2 for the ionization produced with a distance of #b centimetres between the plates is about ten times as great as with a distance of #a millimetre4 #agh4 ,if a platinum wire be made active by exposure to the emanation of thoria1 its activity can be removed by treating the wire with certain acids#bgb4 ,for example1 the activity is not much altered by immersing the wire in hot or cold water or nitric acid1 but more than #hj of it is removed by dilute or concentrated solutions of sulphuric or hydrochloric acid4 ,the activity has not been destroyed by this treatment but is manifested in the solution4 ,if the solution be evaporated1 the activity remains behind on the dish4 ,these results show that the excited activity is due to a deposit on the surface of bodies of radio-active matter which has definite properties as regards solution in acids4 ,this active matter is dissolved in some acids1 but1 when the solvent is evaporated1 the active matter is left behind4 ,this active matter is deposited on the surface of bodies1 for it can be partly removed by rubbing the body with a cloth1 and almost completely by scouring the plate with sand or emery paper4 ,if a negatively charged wire is placed in the presence of a large quantity of radium emanation1 it becomes intensely active4 ,if the wire1 after removal1 is drawn across a screen of zinc sulphide1 or willemite1 a portion of the active matter is rubbed off1 and a luminous trail is left behind on the screen4 ,the amount of active matter deposited is extremely small1 for no difference of weight has been detected in a platinum wire when made extremely active4 ,on examining the wire under a microscope1 no trace of foreign matter is observed4 ,it follows from these results that the matter which causes excited activity is many thousand times more active1 weight for weight1 than radium itself4 ,it is convenient to have a definite name for this radio-active matter1 for the term 8excited activity0 only refers to the radiation from the active matter and not to the matter itself4 ,the term 8active deposit0 will be generally applied to this matter4 ,the active deposit from the three substances thorium1 radium1 and actinium is1 in each case1 derived from its respective emanation1 and possesses the same general property of concentration on the negative electrode in an electric field and of acting as a non-volatile type of matter which is deposited from the gas on to the surface of bodies4 ,these active deposits1 while all soluble in strong acids1 are chemically distinct from each other4 ,the term 8active deposit0 can1 however1 only be used when the matter is spoken of as a whole2 for it will be shown later that the matter1 under ordinary conditions1 is complex and contains several constituents which have distinctive physical and chemical properties and also a distinctive rate of change4 ,according to the theory advanced in section #acf1 we may suppose that the emanation of thorium1 radium1 and actinium is unstable and breaks up with the expulsion of an particle4 ,the residue of the atom of the emanation diffuses to the sides of the vessel or is removed to the negative electrode in an electric field4 ,this active deposit is in turn unstable and breaks up in several successive stages4 ,the 8excited activity0 proper is the radiation set up by the active deposit in consequence of the changes occurring in it4 ,on this view1 the emanation is the parent of the active deposit in the same way that ,th ,x is the parent of the emanation4 ,the proportionality which always exists between the activity of the emanation and the excited activity to which it gives rise1 is at once explained1 if one substance be the parent of the other4 #agi4 ,decay of the excited activity produced by thorium4 ,the excited activity produced in a body after a long exposure to the emanations of thorium1 decays in an exponential law with the time1 falling to half value in about #aa hours4 ,the following table shows the rate of decay of the excited activity produced on a brass rod4 ,time in ,current hours #j #ajj #gi #fd #aah #dgd #bcd #aif #bib #ach #cbf #ajc #dib #cg #fba #ahf #gad #jhf ,the results are shown graphically in ,fig4 #fd1 ,curve ,a4 ,illustration3 ,fig4 #fd4 ,the intensity of the radiation ,i after any time t is given by frac ,i ,i e-t 1 where is the radio-active constant4 ,the rate of decay of excited activity1 like that of the activity of other radio-active products1 is not appreciably affected by change of conditions4 ,the rate of decay is independent of the concentration of the excited activity1 and of the material of the body on which it is produced4 ,it is independent also of the nature and pressure of the gas in which it decays4 ,the rate of decay is unchanged whether the excited activity is produced on the body with or without an electric field4 ,the amount of excited activity produced on a body increases at first with the time1 but reaches a maximum after an exposure of several days4 ,an example of the results is given in the following table4 ,in this experiment a rod was made the cathode in a closed vessel containing thoria4 ,it was removed at intervals for the short time necessary to test its activity and then replaced4 ,time in ,current hours #aeh #fc #cbe #aje #ehc #bi #ihc #dj #adjj #ei #bcda #gg #bihc #hc #dgjj #ij #gbej #ie #ifjj #ajj ,these results are shown graphically in ,curve ,b1 ,fig4 #fd4 ,it is seen that the decay and recovery curves may be represented approximately by the following equations4 ,for the decay curve ,a1 frac ,i ,i e-t 4 ,for the recovery curve ,b1 frac ,i ,i #a e-t 4 ,the two curves are thus complementary to one another2 they are connected in the same way as the decay and recovery curves of ,ur ,x1 and are susceptible of a similar explanation4 ,the amount of excited radio-activity reaches a maximum value when the rate of supply of fresh radio-active particles balances the rate of change of those already deposited4 #ahj4 ,excited radio-activity produced by a short exposure4 ,the initial portion of the recovery curve ,b1 ,fig4 #fd1 is not accurately represented by the above equation4 ,the activity for the first few hours increases more slowly than would be expected from the equation4 ,this result1 however1 is completely explained in the light of later results4 ,the writer#bgc found that1 for a short exposure of a body to the thorium emanation1 the excited activity upon it after removal1 instead of at once decaying at the normal rate1 increased for several hours4 ,in some cases the activity of the body increased to three or four times its original value in the course of a few hours and then decayed with the time at the normal rate4 ,for an exposure of #da minutes to the emanation the excited activity after removal rose to three times its initial value in about #c hours and then fell again at about the normal rate to half value in #aa hours4 ,with a longer time of exposure to the emanation1 the ratio of the increase after removal is much less marked4 ,for a day's exposure1 the activity after removal begins at once to diminish4 ,in this case1 the increase of activity of the matter deposited in the last few hours does not compensate for the decrease of activity of the active matter as a whole1 and consequently the activity at once commences to decay4 ,this increase of activity with time explains the initial irregularity in the recovery curve1 for the active matter deposited during the first few hours takes some time to reach its maximum activity1 and the initial activity is1 in consequence1 smaller than would be expected from the equation4 ,the increase of activity on a rod exposed for a short interval in the presence of the thorium emanation has been further investigated by ,miss ,brooks4 ,the curve ,c in ,fig4 #fe shows the variation with time of the activity of a brass rod exposed for #aj minutes in the emanation vessel filled with dust-free air4 ,the excited activity after removal increased in the course of #cg hours to five times its initial value1 and afterwards decayed at the normal rate4 ,the dotted line curve ,d represents the variation of activity to be expected if the activity decayed exponentially with the time4 ,the explanation of this remarkable action is considered in detail in section #bjg4 ,illustration3 ,fig4 #fe4 #aha4 ,effect of dust on the distribution of excited activity4 ,miss ,brooks#bgd1 working in the ,cavendish ,laboratory1 observed that the excited activity due to the thorium emanation appeared in some cases on the anode in an electric field1 and that the distribution of excited activity varied in an apparently capricious manner4 ,this effect was finally traced to the presence of dust in the air of the emanation vessel4 ,for example1 with an exposure of #e minutes the amount of excited activity to be observed on a rod depended on the time that the air had been allowed to remain undisturbed in the emanation vessel beforehand4 ,the effect increased with the time of standing1 and was a maximum after about #ah hours4 ,the amount of excited activity obtained on the rod was then about #bj times as great as the amount observed for air freshly introduced4 ,the activity of this rod did not increase after removal1 but with fresh air1 the excited activity1 for an exposure of #e minutes1 increased to five or six times its initial value4 ,this anomalous behaviour was found to be due to the presence of dust particles in the air of the vessel1 in which the bodies were made radio-active4 ,these particles of dust1 when shut up in the presence of the emanation1 become radio-active4 ,when a negatively charged rod is introduced into the vessel1 a part of the radio-active dust is concentrated on the rod and its activity is added to the normal activity produced on the wire4 ,after the air in the vessel has been left undisturbed for an interval sufficiently long to allow each of the particles of dust to reach a state of radio-active equilibrium1 on the application of an electric field1 all the positively charged dust particles will at once be carried to the negative electrode4 ,the activity of the electrode at once commences to decay1 since the decay of the activity of the dust particles on the wire quite masks the initial rise of the normal activity produced on the wire4 ,part of the radio-active dust is also carried to the anode1 and the proportion increases with the length of time during which the air has been undisturbed4 ,the greatest amount obtained on the anode was about #fj of that on the cathode4 ,these anomalous effects were found to disappear if the air was made dust-free by passing through a plug of glass wool1 or by application for some time of a strong electric field4 #ahb4 ,decay of excited activity from radium4 ,the excited activity produced on bodies by exposure to the radium emanation decays much more rapidly than the thorium excited activity4 ,for short times of exposure#bge to the emanation the decay curve is very irregular4 ,this is shown in ,fig4 #ff4 ,it was found that the intensity of the radiation measured by the rays decreased rapidly for the first #aj minutes after removal1 but about #ae minutes after removal reached a value which remained nearly constant for an interval of about #bj minutes4 ,it then decayed to zero1 finally following an exponential law1 the intensity falling to half value in about #bh minutes4 ,with longer times of exposure1 the irregularities in the curve are not so marked4 ,illustration3 ,fig4 #ff4 ,miss ,brooks has recently determined the decay curves of the excited activity of radium for different times of exposure1 measured by the rays4 ,the results are shown in ,fig4 #fg1 where the initial ordinates represent the activity communicated to the body from different times of exposure to a constant supply of emanation4 ,it will be observed that in all cases there is a sudden initial drop of activity1 which becomes less marked with increasing time of exposure4 ,the activity1 several hours after removal1 decreases exponentially in all cases1 falling to half value in about #bh minutes4 ,not only do the curves of variation of the excited activity after removal depend upon the time of exposure to the emanation1 but they also depend upon whether the or and rays are used as a means of measurement4 ,the curves obtained for the rays are identical with those from the rays1 showing that these two types of rays always occur together and in the same proportion4 ,the curves measured by the rays are very different1 especially for the case of a short exposure to the emanation4 ,this is clearly shown in ,fig4 #fh1 which gives the and ray curves for exposures of #aj minutes1 #dj minutes1 and #a hour1 and also the limiting case of an exposure of #bd hours4 ,illustration3 ,fig4 #fg4 ,illustration3 ,fig4 #fh4 ,about #be minutes after removal1 the activity decays approximately at the same rate in each case4 ,for convenience of representation1 the ordinates of the curves were adjusted so that they all passed through a common point4 ,we shall see later 7chapter ,,xi7 that the rates of decay are not identically the same until several hours after removal2 but1 in the above figure1 it is difficult to represent the slight variations4 ,it will be observed that for the short exposure of #aj minutes the activity measured by the rays is small at first but rises to a maximum in about #bb minutes1 and then dies away with the time4 ,the curve of decay of activity1 measured by the rays for a long exposure1 does not show the rapid initial drop which occurs in all the ray curves4 ,curie and ,danne#bgf made an investigation of the curves of decay of excited activity for different times of exposure to the radium emanation1 but apparently did not take into account the fact that measurements made by the and rays give quite different curves of decay4 ,some of the family of curves1 given in their paper1 refer to the rays and others to the rays4 ,they showed1 however1 the important fact that the curve of decay obtained by them for a long exposure 7which is identical with the ray curve7 could be empirically expressed by an equation of the form frac ,it ,i ae-#a t 7a #a7 e-#b t 1 where ,i is the initial intensity and ,it the intensity after any time t2 1 4 ,the numerical constant a #dbj4 ,after an interval of #be hours1 the logarithmic decay curve is nearly a straight line1 that is1 the activity falls off according to an exponential law with the time1 decreasing to half value in about #bh minutes4 ,the full explanation of this equation1 and of the peculiarities of the various decay curves of the excited activity of radium1 will be discussed in detail in chapter ,,xi4 ,as in the case of the excited activity from thorium1 the rate of decay of the excited activity from radium is for the most part independent of the nature of the body made active4 ,curie and ,danne 7loc4 cit47 observed that the active bodies gave off an emanation itself capable of exciting activity in neighbouring bodies4 ,this property rapidly disappeared1 and was inappreciable #b hours after removal4 ,in certain substances like celluloid and caoutchouc1 the decay of activity is very much slower than for the metals4 ,this effect becomes more marked with increase of time of exposure to the emanation4 ,a similar effect is exhibited by lead1 but to a less marked degree4 ,during the time the activity lasts1 these substances continue to give off an emanation4 ,it is probable that these divergencies from the general law are not due to an actual change in the rate of decay of the true excited activity but to an occlusion of the emanation by these substances during the interval of exposure4 ,after exposure the emanation gradually diffuses out1 and thus the activity due to this occluded emanation and the excited activity produced by it decays very slowly with the time4 #ahc4 ,active deposit of very slow decay4 ,m4 and ,mme ,curie#bgg have observed that bodies which have been exposed for a long interval in the presence of the radium emanation do not lose all their activity4 ,the excited activity at first decays rapidly at the normal rate1 falling to half value in about #bh minutes1 but a residual activity1 which they state is of the order of #j1jjj of the initial activity1 always remains4 ,a similar effect was observed by ,giesel4 ,the writer has examined the variation of this residual activity1 and has found that it increases for several years4 ,the results are discussed in detail in chapter ,,xi4 ,it will there be shown that this active deposit of slow transformation contains the radio-active constituents present in polonium1 radio-tellurium and radio-lead4 ,illustration3 ,fig4 #fi4 #ahd4 ,the excited activity from actinium4 ,the emanation of actinium1 like that of thorium and radium1 produces excited activity on bodies1 which is concentrated on the negative electrode in an electric field4 ,debierne#bgh found that the excited activity decays approximately according to an exponential law1 falling to half value in #da minutes4 ,giesel#bgi examined the rate of decay of the excited activity of 8emanium0--which1 we have seen1 probably contains the same radio-active constituents as actinium--and found that it decayed to half value in #cd minutes4 ,miss ,brooks#bhj found that the curves of decay of the excited activity from ,giesel's emanium varied with the time of exposure to the emanation4 ,the results are shown graphically in ,fig4 #fi1 for time exposures of #a1 #b1 #e1 #aj and #cj minutes1 and also for a long exposure of #ba hours4 ,after #aj minutes the curves have approximately the same rate of decay4 ,for convenience1 the ordinates of the curves are adjusted to pass through a common point4 ,for a very short exposure1 the activity is small at first1 but reaches a maximum about #i minutes later and finally decays exponentially to zero4 ,the curve of variation of activity for a very short exposure has been determined accurately by ,bronson2 it is shown later in ,fig4 #hc4 ,he found that the decay of activity is finally exponential1 falling to half value in #cf minutes4 ,the explanation of these curves is discussed in detail in chapter ,x1 section #bab4 #ahe4 ,physical and chemical properties of the active deposit4 ,on account of the slow decay of the activity of the active deposit from the thorium emanation1 its physical and chemical properties have been more closely examined than the corresponding deposit from radium4 ,it has already been mentioned that the active deposit of thorium is soluble in some acids4 ,the writer#bha found that the active matter was dissolved off the wire by strong or dilute solutions of sulphuric1 hydrochloric and hydrofluoric acids1 but was only slightly soluble in water or nitric acid4 ,the active matter was left behind when the solvent was evaporated4 ,the rate of decay of activity was unaltered by dissolving the active matter in sulphuric acid1 and allowing it to decay in the solution4 ,in the experiment1 the active matter was dissolved off an active platinum wire2 then equal portions of the solutions were taken at definite intervals1 evaporated down in a platinum dish1 and the activity of the residue tested by the electric method4 ,the rate of decay was found to be exactly the same as if the active matter had been left on the wire4 ,in another experiment1 an active platinum wire was made the cathode in a copper sulphate solution1 and a thin film of copper deposited on it4 ,the rate of decay of the activity was unchanged by the process4 ,a detailed examination of the physical and chemical properties of the active deposit of thorium has been made by ,f4 von ,lerch#bhb and some important and interesting results have been obtained4 ,a solution of the active deposit was prepared by dissolving the metal which had been exposed for some time in the presence of the thorium emanation4 ,in most cases the active matter was precipitated with the metal4 ,for example1 an active copper wire was dissolved in nitric acid and then precipitated by caustic potash4 ,the precipitate was strongly active4 ,an active magnesium wire1 dissolved in hydrochloric acid and then precipitated as phosphate1 also gave an active precipitate4 ,the activity of the precipitates decayed at the normal rate1 i4e4 the activity fell to half value in about #aa hours4 ,experiments were also made on the solubility of the active deposit in different substances4 ,a platinum plate was made active and then placed in different solutions1 and the decrease of the activity observed4 ,in addition to the acids already mentioned1 a large number of substances were found to dissolve the active deposit to some extent4 ,the active matter was however not dissolved to an appreciable extent in ether or alcohol4 ,many substances became active if added to the active solution and then precipitated4 ,for example1 an active solution of hydrochloric acid was obtained by dissolving the deposit on an active platinum wire4 ,barium chloride was then added and precipitated as sulphate4 ,the precipitate was strongly active1 thus suggesting that the active matter was carried down by the barium4 #ahf4 ,electrolysis of solutions4 ,dorn showed that1 if solutions of radiferous barium chloride were electrolysed1 both electrodes became temporarily active1 but the anode to a greater degree than the cathode4 ,f4 von ,lerch has made a detailed examination of the action of electrolysis on a solution of the active deposit of thorium4 ,the matter was dissolved off an active platinum plate by hydrochloric acid1 and then electrolysed between platinum electrodes4 ,the cathode was very active1 but there was no trace of activity on the anode4 ,the cathode lost its activity at a rate much faster than the normal4 ,with an amalgamated zinc cathode on the other hand1 the rate of decay was normal4 ,when an active solution of hydrochloric acid was electrolysed with an electromotive force smaller than that required to decompose water1 the platinum became active4 ,the activity decayed to half value in #dge hours while the normal fall is to half value in #aa hours4 ,these results point to the conclusion that the active matter is complex and consists of two parts which have different rates of decay of activity1 and can be separated by electrolysis4 ,under special conditions it was found possible to make the anode active4 ,this was the case if the anion attached itself to the anode4 ,for example1 if an active hydrochloric solution was electrolysed with a silver anode1 the chloride of silver formed was strongly active and its activity decayed at a normal rate4 ,the amount of activity obtained by placing different metals in active solutions for equal times varied greatly with the metal4 ,for example1 it was found that if a zinc plate and an amalgamated zinc plate1 which show equal potential differences with regard to hydrochloric acid1 were dipped for equal times in two solutions of equal activity1 the zinc plate was seven times as active as the other4 ,the activity was almost removed from the solution in a few minutes by dipping a zinc plate into it4 ,some metals became active when dipped into an active solution while others did not4 ,platinum1 palladium1 and silver remained inactive1 while copper1 tin1 lead1 nickel1 iron1 zinc1 cadmium1 magnesium1 and aluminium became active4 ,these results strongly confirm the view that excited activity is due to a deposit of active matter which has distinctive chemical behaviour4 ,g4 ,b4 ,pegram#bhc has made a detailed study of the active deposits obtained by electrolysis of pure and commercial thorium salts4 ,the commercial thorium nitrate obtained from ,p4 de ,haen gave1 when electrolysed1 a deposit of lead peroxide on the anode4 ,this deposit was radio-active1 and its activity decayed at the normal rate of the excited activity due to thorium4 ,from solutions of pure thorium nitrate1 no visible deposit was obtained on the anode1 but it was1 however1 found to be radio-active4 ,the activity decayed rapidly1 falling to half value in about one hour4 ,some experiments were also made on the effect of adding metallic salts to thorium solutions and then electrolysing them4 ,anode and cathode deposits of the oxides or metals obtained in this way were found to be radio-active1 but the activity fell to half value in a few minutes4 ,the gases produced by electrolysis were radio-active1 but this was due to the presence of the thorium emanation4 ,the explanation of the results obtained by ,pegram and von ,lerch will be considered later in section #bjg4 ,it will be shown that the active deposit of thorium contains two distinct substances which have different rates of transformation4 #ahg4 ,effect of temperature4 ,the activity of a platinum wire which has been exposed in the presence of the thorium emanation is almost completely lost by heating the wire to a white heat4 ,miss ,f4 ,gates#bhd found that the activity was not destroyed by the intense heat1 but manifested itself on neighbouring bodies4 ,when the active wire was heated electrically in a closed cylinder1 the activity was transferred from the wire to the interior surface of the cylinder in unaltered amount4 ,the rate of decay of the activity was not altered by the process4 ,by blowing a current of air through the cylinder during the heating1 a part of the active matter was removed from the cylinder4 ,similar results were found for the excited activity due to radium4 ,f4 von ,lerch 7loc4 cit47 determined the amount of activity removed at different temperatures4 ,the results are shown in the following table for a platinum wire excited by the thorium emanation#bhe4 ,temperature ,percentage of activity removed ,heated #b minutes #hjj ,c4 #j then minute #ajbj ,c4 #af more #abfj ,c4 #eb #adfj ,c4 #ii ,the effect of heat on the volatilization of the active deposit of radium has been examined in detail by ,curie and ,danne4 ,the interesting and important results obtained by them will be discussed in chapter ,,xi1 section #bbf4 #ahh4 ,effect of variation of ,e4,m4,f4 on amount of excited activity from thorium4 ,it has been shown that the excited activity is confined to the cathode in a strong electric field4 ,in weaker fields the activity is divided between the cathode and the walls of the vessel4 ,this was tested in an apparatus#bhf shown in ,fig4 #gj4 ,illustration3 ,fig4 #gj4 ,a is a cylindrical vessel of #ee cms4 diameter1 ,b the negative electrode passing through insulating ends ,c1 ,d4 ,for a potential difference of #ej volts1 most of the excited activity was deposited on the electrode ,b4 ,for about #c volts1 half of the total excited activity was produced on the rod ,b1 and half on the walls of the vessel4 ,whatever the voltage applied1 the sum of the activities on the central rod and the walls of the cylinder was found to be a constant when a steady state was reached4 ,when no voltage was applied1 diffusion alone was operative1 and in that case about #ac per cent4 of the total activity was on the rod ,b4 ,the application of an electric field has thus no influence on the sum total of excited activity1 but merely controls the proportion concentrated on the negative electrode4 ,a more detailed examination of the variation with strength of field of the amount on the negative electrode was made in a similar manner by ,f4 ,henning#bhg4 ,he found that in a strong electric field the amount of excited activity was practically independent of the diameter of the rod ,b1 although the diameter varied between #ei mm4 and #fj mms4 ,with a small voltage1 the amount on the negative electrode varied with its diameter4 ,the curves showing the relation between the amount of excited activity and voltage are very similar in character to those obtained for the variation of the current through an ionized gas with the voltage applied4 ,the amount of excited activity reaches a maximum when all the active matter is removed from the gas as rapidly as it is formed4 ,with weaker fields1 a portion diffuses to the sides of the vessel1 and produces excited activity on the positive electrode4 #ahi4 ,effect of pressure on distribution of excited activity4 ,in a strong electric field1 the amount of excited activity produced on the cathode is independent of the pressure down to a pressure of about #aj mms4 of mercury4 ,in some experiments made by the writer#bhh1 the emanating thorium compound was placed inside a closed cylinder about #d cms4 in diameter1 through which passed an insulated central rod4 ,the central rod was connected to the negative pole of a battery of #ej volts4 ,when the pressure was reduced below #aj mms4 of mercury1 the amount of excited activity produced on the negative electrode diminished1 and was a very small fraction of its original value at a pressure of mm4 ,some excited activity was in this case found to be distributed over the interior surface of the cylinder4 ,it may thus be concluded that at low pressures the excited activity appears on both anode and cathode1 even in a strong electric field4 ,the probable explanation of this effect is given in the next section4 ,curie and ,debierne#bhi observed that when a vessel containing an emanating radium compound was kept pumped down to a low pressure1 the amount of excited activity produced on the vessel was much reduced4 ,in this case the emanation given off by the radium was removed by the pump with the other gases continuously evolved from the radium compound4 ,on account of the very slow decay of activity of the emanation1 the amount of excited activity produced on the walls of the vessel1 in the passage of the emanation through it1 was only a minute fraction of the amount produced when none of the emanation given off was allowed to escape4 #aij4 ,transmission of excited activity4 ,the characteristic property of excited radio-activity is that it can be confined to the cathode in a strong electric field4 ,since the activity is due to a deposit of radio-active matter on the electrified surface1 the matter must be transported by positively charged carriers4 ,the experiments of ,fehrle#bij showed that the carriers of excited activity travel along the lines of force in an electric field4 ,for example1 when a small negatively charged metal plate was placed in the centre of a metal vessel containing an emanating thorium compound1 more excited activity was produced on the sides and corners of the plate than at the central part4 ,a difficulty however arises in connection with the positive charge of the carrier4 ,according to the view developed in section #acf and later in chapters ,x and ,,xi1 the active matter which is deposited on bodies and gives rise to excited activity1 is itself derived from the emanation4 ,the emanations of thorium and radium emit only rays1 i4e4 positively charged particles4 ,after the expulsion of an particle1 the residue1 which is supposed to constitute the primary matter of the active deposit1 should retain a negative charge1 and be carried to the anode in an electric field4 ,the exact opposite however is observed to be the case4 ,the experimental evidence does not support the view that the positively charged particles1 expelled from the emanation1 are directly responsible for the phenomena of excited activity2 for no excited activity is produced in a body exposed to the rays of the emanation1 provided the emanation itself does not come in contact with it4 ,there has been a tendency to attach undue importance to this apparent discrepancy between theory and experiment4 ,the difficulty is not so much to offer a probable explanation of the results as to select from a number of possible causes4 ,while there can be little doubt that the main factor in the disintegration of the atom consists in the expulsion of an particle carrying a positive charge1 a complicated series of processes probably occurs before the residue of the atom is carried to the negative electrode4 ,the experimental evidence suggests that one or more negative electrons of slow velocity escape from the atom at the same time as the particle4 ,this is borne out by the recent discovery that the particle expelled from radium1 freed from the ordinary rays1 and also from polonium1 is accompanied by a number of slowly moving and consequently easily absorbed electrons4 ,if two negative electrons escaped at the same time as the particle1 the residue would be left with a positive charge and would be carried to the negative electrode4 ,there is also another experimental point which is of importance in this connection4 ,in the absence of an electric field1 the carriers remain in the gas for a considerable time and undergo their transformation in situ4 ,there is also some evidence 7section #bbg7 that1 even in an electric field1 the carriers of the active deposit are not swept to the electrode immediately after the break up of the emanation1 but remain some time in the gas before they gain a positive charge4 ,it must be remembered that the atoms of the active deposit do not exist as a gas and by the process of diffusion would tend to collect together to form aggregates4 ,these aggregates would act as small metallic particles1 and1 if they were electro-positive in regard to the gas1 would gain a positive charge from the gas4 ,there can be little doubt that the processes occurring between the break up of the emanation and the deposit of the residue in the cathode in an electric field are complicated1 and further careful experiment is required to elucidate the sequence of the phenomena4 ,whatever view is taken of the process by which these carriers obtain a positive charge1 there can be little doubt that the expulsion of an particle with great velocity from the atom of the emanation must set the residue in motion4 ,on account of the comparatively large mass of this residue1 the velocity acquired will be small compared with that of the expelled particle1 and the moving mass will rapidly be brought to rest at atmospheric pressure by collision with the gas molecules in its path4 ,at low pressures1 however1 the collisions will be so few that it will not be brought to rest until it strikes the boundaries of the vessel4 ,a strong electric field would have very little effect in controlling the motion of such a heavy mass1 unless it has been initially brought to rest by collision with the gas molecules4 ,this would explain why the active matter is not deposited on the cathode at low pressures in an electric field4 ,some direct evidence of a process of this character1 obtained by ,debierne on examination of the excited activity produced by actinium1 is discussed in section #aib4 #aia4 ,the following method has been employed by the writer#bia to determine the velocity of the positive carriers of excited activity of radium and thorium in an electric field4 ,suppose ,a and ,b 7,fig4 #ga7 are two parallel plates exposed to the influence of the emanation1 which is uniformly distributed between them4 ,if an alternating ,e4,m4,f4 ,e is applied between the plates1 the same amount of excited activity is produced on each electrode4 ,if1 in series with the source of the alternating ,e4,m4,f41 a battery of ,e4,m4,f4 ,e less than ,e is placed1 the positive carrier moves in a stronger electric field in one half alternation than in the other4 ,a carrier consequently moves over unequal distances during the two half alternations1 since the velocity of the carrier is proportional to the strength of the electric field in which it moves4 ,the excited activity will in consequence be unequally distributed over the two electrodes4 ,if the frequency of alternation is sufficiently great1 only the positive carriers within a certain small distance of one plate can be conveyed to it1 and the rest1 in the course of several succeeding alternations1 are carried to the other plate4 ,illustration3 ,fig4 #ga4 ,when the plate ,b is negatively charged1 the ,e4,m4,f4 between the plates is ,e ,e1 when ,b is positive the ,e4,m4,f4 is ,e ,e4 ,let d distance between the plates1 ,t time of a half alternation1 ratio of the excited radio-activity on the plate ,b to the sum of the radio-activities on the plates ,a and ,b1 ,k velocity of the positive carriers for a potential-gradient of #a volt per centimetre4 ,on the assumption that the electric field between the plates is uniform1 and that the velocity of the carrier is proportional to the electric field1 the velocity of the positive carrier towards ,b is ,e ,e ---------- ,k d and1 in the course of the next half alternation1 ,e ,e ---------- ,k d towards the plate ,a4 ,if x is less than d1 the greatest distances x1 x passed over by the positive carrier during two succeeding half alternations is thus given by ,e ,e x ---------- ,,kt d and ,e ,e x ---------- ,,kt d ,suppose that the positive carriers are produced at a uniform rate of q per second for unit distance between the plates4 ,the number of positive carriers which reach ,b during a half alternation consists of two parts3 7#a7 ,one half of those carriers which are produced within the distance x of the plate ,b4 ,this number is equal to #a --- x q,t #b 7#b7 ,all the carriers which are left within the distance x from ,b at the end of the previous half alternation4 ,the number of these can readily be shown to be #a x --- x ---- q,t #b x ,the remainder of the carriers1 produced between ,a and ,b during a complete alternation1 will reach the other plate ,a in the course of succeeding alternations1 provided no appreciable recombination takes place4 ,this must obviously be the case1 since the positive carriers travel further in a half alternation towards ,a than they return towards ,b during the next half alternation4 ,the carriers thus move backwards and forwards in the changing electric field1 but on the whole move towards the plate ,a4 ,the total number of positive carriers produced between the plates during a complete alternation is #b;dq,t4 ,the ratio of the number which reach ,b to the total number produced is thus given by rho frac frac #ab x#aq,t frac #ab x#a frac x#ax#b q,t #b;dq,t frac #ad frac x#a;d frac x#a x#b x#b 4 ,substituting the values of x and x1 we find that ,k frac #b 7,e ,e#a7 d#b ,e 7,e ,e#a7 ,t rho 4 ,in the experiments1 the values of ,e1 ,e1 d1 and ,t were varied1 and the results obtained were in general agreement with the above equation4 ,the following were the results for thorium3 ,plates #acj cms4 apart4 ,e ,e ,alternations ,k ,e ,e per second #aeb #aja #eg #bg #abe #bbe #aej #eg #ch #aag #cjj #bjj #eg #dd #abd ,plates #b cms4 apart4 ,e ,e ,alternations ,k ,e ,e per second #bgc #bjg #dd #cg #adg #cjj #bjj #ec #bhf #ade ,the average mobility ,k deduced from a large number of experiments was #ac cms4 per sec4 per volt per cm4 for atmospheric pressure and temperature4 ,this velocity is about the same as the velocity of the positive ion produced by ,rntgen rays in air1 viz4 #acg cms4 per sec4 ,the results obtained with the radium emanation were more uncertain than those for thorium on account of the distribution of some excited activity on the positive electrode4 ,the values of the velocities of the carriers were however found to be roughly the same for radium as for thorium4 ,these results show that the carriers of the active deposit travel in the gas with about the same velocity as the positive or negative ions produced by the radiations in the gas4 ,this indicates either that the active matter becomes attached to positive ions1 or that the active matter itself1 acquiring in some way a positive charge1 collects a cluster of neutral molecules which travel with it4 #aib4 ,carriers of the excited activity from actinium and 8emanium40 ,giesel#bib observed that 8emanium0 gave off a large quantity of emanation1 and that this emanation gave rise to a type of radiation which he termed the ,e rays4 ,a narrow metal cylinder containing the active substance was placed with the open end downwards1 about #e cms4 above the surface of a zinc sulphide screen4 ,the screen was charged negatively to a high potential by an electric machine1 and the cylinder connected with earth4 ,a luminous spot of light was observed on the screen1 which was brighter at the edge than at the centre4 ,a conductor1 connected with earth1 brought near the luminous spot apparently repelled it4 ,an insulator did not show such a marked effect4 ,on removal of the active substance1 the luminosity of the screen persisted for some time4 ,this was probably due to the excited activity produced on the screen4 ,the results obtained by ,giesel support the view that the carriers of excited activity of 8emanium0 have a positive charge4 ,in a strong electric field the carriers travel along the lines of force to the cathode1 and there cause excited activity on the screen4 ,the movement of the luminous zone on the approach of a conductor is due to the disturbance of the electric field4 ,debierne#bic found that actinium also gave off a large amount of emanation1 the activity of which decayed very rapidly with the time1 falling to half value in #ci seconds4 ,this emanation produces excited activity on surrounding objects1 and at diminished pressure the emanation produces a uniform distribution of excited activity in the enclosure containing the emanation4 ,the excited activity falls to half value in #da minutes4 ,debierne observed that the distribution of excited activity was altered by a strong magnetic field4 ,the experimental arrangement is shown in ,fig4 #ga;,a4 ,the active matter was placed at ,m1 and two plates ,a and ,b were placed symmetrically with regard to the source4 ,on the application of a strong magnetic field normal to the plane of the paper1 the excited activity was unequally distributed between the plates ,a and ,b4 ,the results showed that the carriers of excited activity were deviated by a magnetic field in the opposite sense to the cathode rays1 i4e4 the carriers were positively charged4 ,in some cases1 however1 the opposite effect was obtained4 ,debierne considers that the excited activity of actinium is due to 8ions activants10 the motion of which is altered by a magnetic field4 ,other experiments showed that the magnetic field acted on the 8ions activants0 and not on the emanation4 ,illustration3 ,fig4 #ga;,a4 ,the results of ,debierne thus lead to the conclusion that the carriers of excited activity are derived from the emanation and are projected with considerable velocity4 ,this result supports the view1 advanced in section #aij1 that the expulsion of particles from the emanation must set the part of the system left behind in rapid motion4 ,a close examination of the mode of transference of the excited activity by actinium and the emanation substance is likely to throw further light on the processes which give rise to the deposit of active matter on the electrodes4 ,footnote #bfi3 ,m4 and ,mme4 ,curie1 ,c4 ,r4 #abi1 p4 #gad1 #ahii4 ,footnote #bgj3 ,rutherford1 ,phil4 ,mag4 ,jan4 and ,feb4 #aijj4 ,footnote #bga3 ,as regards date of publication1 the priority of the discovery of 8excited activity0 belongs to ,m4 and ,mme4 ,curie4 ,a short paper on this subject1 entitled 8,sur la radioactivit provoque par les rayons de ,becquerel10 was communicated by them to the ,comptes ,rendus1 ,nov4 #f1 #ahii4 ,a short note was added to the paper by ,becquerel in which the phenomena of excited activity were ascribed to a type of phosphorescence4 ,on my part1 ,i had simultaneously discovered the emission of an emanation from thorium compounds and the excited activity produced by it1 in ,july1 #ahii4 ,i1 however1 delayed publication in order to work out in some detail the properties of the emanation and of the excited activity and the connection between them4 ,the results were published in two papers in the ,philosophical ,magazine 7,jan4 and ,feb4 #aijj7 entitled 8,a radio-active substance emitted from thorium compounds10 and 8,radio-activity produced in substances by the action of thorium compounds40 ,footnote #bgb3 ,rutherford1 ,phil4 ,mag4 ,feb4 #aijj4 ,footnote #bgc3 ,rutherford1 ,phys4 ,zeit4 #c1 ,no4 #ab1 p4 #bed1 #aijb4 ,phil4 ,mag4 ,jan4 #aijc4 ,footnote #bgd3 ,miss ,brooks1 ,phil4 ,mag4 ,sept4 #aijd4 ,footnote #bge3 ,rutherford and ,miss ,brooks1 ,phil4 ,mag4 ,july1 #aijb4 ,footnote #bgf3 ,curie and ,danne1 ,c4 ,r4 #acf1 p4 #cfd1 #aijc4 ,footnote #bgg3 ,mme ,curie1 ,thse1 ,paris1 #aijc1 p4 #aaf4 ,footnote #bgh3 ,debierne1 ,c4 ,r4 #ach1 p4 #daa1 #aijd4 ,footnote #bgi3 ,giesel1 ,ber4 d4 ,d4 ,chem4 ,ges4 ,no4 #c1 p4 #gge1 #aije4 ,footnote #bhj3 ,miss ,brooks1 ,phil4 ,mag4 ,sept4 #aijd4 ,footnote #bha3 ,rutherford1 ,phys4 ,zeit4 #c1 ,no4 #ab1 p4 #bed1 #aijb4 ,footnote #bhb3 ,f4 von ,lerch1 ,annal4 d4 ,phys4 #ab1 p4 #gde1 #aijc4 ,footnote #bhc3 ,pegram1 ,phys4 ,review1 p4 #dbd1 ,dec4 #aijc4 ,footnote #bhd3 ,miss ,gates1 ,phys4 ,review1 p4 #cjj1 #aijc4 ,footnote #bhe3 ,a more complete examination of the effect of temperature on the excited activity of thorium has been made by ,miss ,slater 7section #bjg74 ,footnote #bhf3 ,rutherford1 ,phil4 ,mag4 ,feb4 #aijj4 ,footnote #bhg3 ,henning1 ,annal4 d4 ,phys4 #g1 p4 #efb1 #aijb4 ,footnote #bhh3 ,rutherford1 ,phil4 ,mag4 ,feb4 #aijj4 ,footnote #bhi3 ,curie and ,debierne1 ,c4 ,r4 #acb1 p4 #gfh1 #aija4 ,footnote #bij3 ,fehrle1 ,phys4 ,zeit4 #c1 ,no4 #g1 p4 #acj1 #aijb4 ,footnote #bia3 ,rutherford1 ,phil4 ,mag4 ,jan4 #aijc4 ,footnote #bib3 ,giesel1 ,ber4 d4 ,d4 ,chem4 ,ges4 #cf1 p4 #cdb1 #aijc4 ,footnote #bic3 ,debierne1 ,c4 ,r4 #acf1 pp4 #ddf and #fga1 #aijc2 #ach1 p4 #daa1 #aijd4 ,,chapter ,,ix4 ,,theory ,,of ,,successive ,,changes4 #aic4 ,introduction4 ,we have seen in previous chapters that the radio-activity of the radio-elements is always accompanied by the production of a series of new substances with some distinctive physical and chemical properties4 ,for example1 thorium produces from itself an intensely radio-active substance1 ,th ,x1 which can be separated from the thorium in consequence of its solubility in ammonia4 ,in addition1 thorium gives rise to a gaseous product1 the thorium emanation1 and also to another substance which is deposited on the surface of bodies in the neighbourhood of the thorium1 where its presence is indicated by the phenomenon known as 8excited activity40 ,a close examination of the origin of these products shows that they are not produced simultaneously1 but arise in consequence of a succession of changes originating in the radio-element4 ,thorium first of all gives rise to the product ,th ,x4 ,the ,th ,x produces from itself the thorium emanation1 and this in turn is transformed into a non-volatile substance4 ,a similar series of changes is observed in radium1 with the exception that there is no product in radium corresponding to the ,th ,x in the case of thorium4 ,radium first of all produces an emanation1 which1 like thorium1 is transformed into a non-volatile substance4 ,in uranium only one product1 ,ur ,x1 has been observed1 for uranium does not give off an emanation and in consequence does not produce excited activity on bodies4 ,as a typical example of the evidence1 from which it is deduced that one substance is the parent of another1 we will consider the connection of the two products ,th ,x and the thorium emanation4 ,it has been shown 7section #aed7 that after the separation of ,th ,x from a thorium solution1 by precipitation with ammonia1 the precipitated thorium hydroxide has lost to a large extent its power of emanating4 ,this cannot be ascribed to a prevention of escape of the emanation produced in it1 for very little emanation is observed when a current of air is drawn through the hydroxide in a state of solution1 when most of the emanation present would be carried off4 ,on the other hand1 the solution containing the ,th ,x gives off a large quantity of emanation1 showing that the power of giving off an emanation belongs to the product ,th ,x4 ,now it is found that the quantity of emanation given off by the separated ,th ,x decreases according to an exponential law with the time1 falling to half value in four days4 ,the rate of production of emanation thus falls off according to the same law and at the same rate as the activity of the ,th ,x measured in the ordinary manner by the rays4 ,now this is exactly the result to be expected if the ,th ,x is the parent of the emanation1 for the activity of ,th ,x at any time is proportional to its rate of change1 i4e41 to the rate of production of the secondary type of matter by the emanation in consequence of a change in it4 ,since the rate of change of the emanation 7half transformed in #a minute7 is very rapid compared with the rate of change of ,th ,x1 the amount of emanation present will be practically proportional to the activity of the ,th ,x at any instant1 i4e41 to the amount of unchanged ,th ,x present4 ,the observed fact that the hydroxide regains its power of emanating in the course of time is due to the production of fresh ,th ,x by the thorium1 which in turn produces the emanation4 ,in a similar way1 excited activity is produced on bodies over which the emanation is passed1 and in amount proportional to the activity of the emanation1 i4e41 to the amount of the emanation present4 ,this shows that the active deposit1 which gives rise to the phenomenon of excited activity1 is itself a product of the emanation4 ,the evidence thus seems to be conclusive that ,th ,x is the parent of the emanation and that the emanation is the parent of the deposited matter4 #aid4 ,chemical and ,physical properties of the active products4 ,each of these radio-active products is marked by some distinctive chemical and physical properties which differentiate it from the preceding and succeeding products4 ,for example1 ,th ,x behaves as a solid4 ,it is soluble in ammonia1 while thorium is not4 ,the thorium emanation behaves as a chemically inert gas and condenses at a temperature of #abj ,c4 ,the active deposit from the emanation behaves as a solid and is readily soluble in sulphuric and hydrochloric acids and is only slightly soluble in ammonia4 ,the striking dissimilarity which exists in many cases between the chemical and the physical properties of the parent matter and the product to which it gives rise is very well illustrated by the case of radium and the radium emanation4 ,radium is an element so closely allied in chemical properties to barium that1 apart from a slight difference in the solubility of the chlorides and bromides1 it is difficult to distinguish chemically between them4 ,it has a definite spectrum of bright lines similar in many respects to the spectra of the alkaline earths4 ,like barium1 it is non-volatile at ordinary temperature4 ,on the other hand1 the emanation which is continually produced from radium is a radio-active and chemically inert gas1 which is condensed at a temperature of #aej ,c4 ,both in its spectrum and in the absence of definite chemical properties1 it resembles the argon-helium group of inert gases1 but differs from these gases in certain marked features4 ,the emanation must be considered to be an unstable gas which breaks down into a non-volatile type of matter1 the disintegration being accompanied by the expulsion of heavy atoms of matter 7 particles7 projected with great velocity4 ,this rate of breaking up is not affected by temperature over the considerable range which has been examined4 ,after a month's interval1 the volume of the emanation has shrunk to a small portion of its initial value4 ,but the most striking property of the emanation1 which1 as we shall see later 7chapter ,,xii71 is a direct consequence of its radio-activity1 is the enormous amount of energy emitted from it4 ,the emanation in breaking up through its successive stages emits about #c million times as much energy as is given out by the explosion of an equal volume of hydrogen and oxygen1 mixed in the proper proportions to form water2 and yet1 in this latter chemical reaction more heat is emitted than in any other known chemical change4 ,we have seen that the two emanations and the products ,ur ,x1 ,th ,x lose their activity with the time according to a simple exponential law1 and at a rate that is independent--as far as observation has gone--of the chemical and physical agents at our disposal4 ,the time taken for each of these products to fall to half its value is thus a definite physical constant which serves to distinguish it from all other products4 ,on the other hand1 the variation of the excited activity produced by these emanations does not even approximately obey such a law4 ,the rate of decay depends not only on the time of exposure to the respective emanations1 but also1 in the case of radium1 on the type of radiation which is used as a means of comparative measurement4 ,it will be shown1 in succeeding chapters1 that the complexity of the decay is due to the fact that the matter in the active deposits undergoes several successive transformations1 and that the peculiarities of the curves of decay1 obtained under different conditions1 can be explained completely on the assumption that two changes occur in the active deposit from both thorium and actinium and six in the active deposit from radium4 #aie4 ,nomenclature4 ,the nomenclature to be applied to the numerous radio-active products is a question of great importance and also one of considerable difficulty4 ,since there are at least seven distinct substances produced from radium1 and probably five from thorium and actinium1 it is neither advisable nor convenient to give each a special name such as is applied to the parent elements4 ,at the same time1 it is becoming more and more necessary that each product should be labelled in such a way as to indicate its place in the succession of changes4 ,this difficulty is especially felt in discussing the numerous changes in the active deposits from the different emanations4 ,many of the names attached to the products were given at the time of their discovery1 before their position in the scheme of changes was understood4 ,in this way the names ,ur ,x1 ,th ,x were applied to the active residues obtained by chemical treatment of uranium and thorium4 ,since1 in all probability1 these substances are the first products of the two elements1 it may be advisable to retain these names1 which certainly have the advantage of brevity4 ,the name 8emanation0 was originally given to the radio-active gas from thorium1 and has since been applied to the similar gaseous products of radium and actinium4 ,finding the name 8radium emanation0 somewhat long and clumsy1 ,sir ,william ,ramsay#bid has recently suggested 8ex-radio0 as an equivalent4 ,this name is certainly brief and is also suggestive of its origin2 but at least six other ex-radios1 whose parentage is as certain as that of the emanation1 remain unnamed4 ,a difficulty arises in applying the corresponding names ex-thorio1 ex-actinio to the other gaseous products1 for1 unlike radium1 the emanations of thorium and actinium are probably the second1 not the first1 disintegration product of the radio-elements in question4 ,another name thus has to be applied to the first product in these cases4 ,it may be advisable to give a special name to the emanation1 since it has been the product most investigated and was the first to be isolated chemically2 but1 on the other hand1 the name 8radium emanation0 is historically interesting1 and suggests a type of volatile or gaseous matter4 ,since the term 8excited0 or 8induced0 activity refers only to the radiations from the active body1 a name is required for the radiating matter itself4 ,the writer in the first edition of this book suggested the name 8emanation ,x40#bie ,this title was given from analogy to the names ,ur ,x and ,th ,x1 to indicate that the active matter was product of the emanation4 ,the name1 however1 is not very suitable1 and1 in addition1 can only be applied to the initial product deposited1 and not to the further products of its decomposition4 ,it is very convenient in discussing mathematically the theory of successive changes to suppose that the deposited matter called ,a is changed into ,b1 ,b into ,c1 ,c into ,d1 and so on4 ,i have therefore discarded the name emanation ,x1 and have used the terms radium ,a1 radium ,b1 and so on1 to signify the successive products of the decomposition of the emanation of radium4 ,a similar nomenclature is applied to thorium and actinium4 ,this system of notation is elastic and simple1 and ,i have found it of great convenience in the discussion of successive products4 ,in speaking generally of the active matter1 which causes excited activity1 without regard to its constituents1 ,i have used the term 8active deposit40 ,the scheme of nomenclature employed in this book is clearly shown below3-- ,radium ,thorium ,uranium ,actinium ,radium ,th ,x ,ur ,x ,actinium ,x emanation ,radium ,a ,thorium ,final ,actinium 7,active7 emanation product emanation ,radium ,b ,thorium ,actinium 7,active7 ,a ,a 7,active7 7,active7 ,radium ,c ,thorium ,actinium 7,active7 ,b ,b 7,active7 7,active7 ,radium ,d ,thorium ,actinium 7,active7 ,c ,c 7final7 7final7 c4 ,each product on this scheme is the parent of the product below it4 ,since only two products have been observed in the active deposit of thorium and actinium1 thorium ,c and actinium ,c respectively refer to their final inactive products4 ,it will be shown in the next chapter that1 as in the case of thorium1 an intermediate product exists between actinium and its emanation4 ,from analogy to the products ,th ,x and ,ur ,x1 this substance is termed 8actinium ,x40 #aif4 ,theory of ,successive ,changes4 ,before considering the evidence from which these changes are deduced1 the general theory of successive changes of radio-active matter will be considered4 ,it is supposed that the matter ,a changes into ,b1 ,b into ,c1 ,c into ,d1 and so on4 ,each of these changes is supposed to take place according to the same law as a monomolecular change in chemistry1 i4e41 the number ,n of particles unchanged after a time t is given by ,m ,n e-t 1 where ,n is the initial number and the constant of the change4 ,since d,n_/dt -,n1 the rate of change at any time is always proportional to the amount of matter unchanged4 ,it has previously been pointed out that this law of decay of the activity of the radio-active products is an expression of the fact that the change is of the same type as a monomolecular chemical change4 ,suppose that ,p1 ,q1 ,r represent the number of particles of the matter ,a1 ,b1 and ,c respectively at any time t4 ,let 1 1 be the constants of change of the matter ,a1 ,b1 and ,c respectively4 ,each atom of the matter ,a is supposed to give rise to one atom of the matter ,b1 one atom of ,b to one of ,c1 and so on4 ,the expelled 8rays0 or particles are non-radio-active1 and so do not enter into the theory4 ,it is not difficult to deduce mathematically the number of atoms of ,p1 ,q1 ,r1 444 of the matter ,a1 ,b1 ,c1 444 existing at any time t after this matter is set aside1 if the initial values of ,p1 ,q1 ,r1 444are given4 ,in practice1 however1 it is generally only necessary to employ three special cases of the theory which correspond1 for example1 to the changes in the active deposit1 produced on a wire exposed to a constant amount of radium emanation and then removed1 7#a7 when the time of exposure is extremely short compared with the period of the changes1 7#b7 when the time of exposure is so long that the amount of each of the products has reached a steady limiting value1 and 7#c7 for any time of exposure4 ,there is also another case of importance which is practically a converse of ,case #c1 viz4 when the matter ,a is supplied at a constant rate from a primary source and the amounts of ,a1 ,b1 ,c are required at any subsequent time4 ,the solution of this can1 however1 be deduced immediately from ,case #c without analysis4 #aig4 ,,case #a4 ,suppose that the matter initially considered is all of one kind ,a4 ,it is required to find the number of particles ,p1 ,q1 ,r of the matter ,a1 ,b1 ,c respectively present after any time t4 ,then ,p ne-#a t 1 if n is the number of particles of ,a initially present4 ,now d,q1 the increase of the number of particles of the matter ,b per unit time1 is the number supplied by the change in the matter ,a1 less the number due to the change of ,b into ,c1 thus d,q_/dt ,p ,q 7#a74 ,similarly d,r_/dt ,q ,r 7#b74 ,substituting in 7#a7 the value of ,p in terms of n1 frac d,q dt #a ne-#a t #b ,q 4 ,the solution of this equation is of the form ,q n 7ae-#a t be-#b t7 4444447#c74 ,by substitution it is found that a _/7 74 ,since ,q #j when t #j1 b -7 74 ,thus ,q frac n#a #a #b 7e-#b t - e-#a t7 4444 7#d74 ,substituting this value of ,q in 7#b71 it can readily be shown that ,r n 7ae-#a t be-#b t ce-#c t7 44444 7#e74 where a frac #a #b 7#a #b7 7#a - #c7 1 b frac - #a #b 7#a #b7 7#b #c7 1 c frac #a #b 7#a #c7 7#b - #c7 1 ,illustration3 ,fig4 #gb4 ,the variation of the values of ,p1 ,q1 ,r with the time t1 after removal of the source1 is shown graphically in ,fig4 #gb1 curves ,a1 ,b1 and ,c respectively4 ,in order to draw the curves for the practical case which will be considered later corresponding to the first three changes in radium ,a1 the values of 1 1 were taken as #che #aj1 #ech #aj1 #dac #aj respectively1 i4e41 the times required for each successive type of matter to be half transformed are about #c1 #ba1 and #bh minutes respectively4 ,the ordinates of the curves represent the relative number of atoms of the matter ,a1 ,b1 and ,c existing at any time1 and the value of n1 the original number of atoms of the matter ,a deposited1 is taken as #ajj4 ,the amount of matter ,b is initially zero1 and in this particular case1 passes through a maximum about #aj minutes later1 and then diminishes with the time4 ,in a similar way1 the amount of ,c passes through a maximum about #cg minutes after removal4 ,after an interval of several hours the amount of both ,b and ,c diminishes very approximately according to an exponential law with the time1 falling to half value after intervals of #ba and #bh minutes respectively4 #aih4 ,,case #b4 ,a primary source supplies the matter ,a at a constant rate and the process has continued so long that the amount of the products ,a1 ,b1 ,c1 444 has reached a steady limiting value4 ,the primary source is then suddenly removed4 ,it is required to find the amounts of ,a1 ,b1 ,c1 444 remaining at any subsequent time t4 ,in this case1 the number n of particles of ,a1 deposited per second from the source1 is equal to the number of particles of ,a which change into ,b per second1 and of ,b into ,c1 and so on4 ,this requires the relation n ,p ,q ,r 7#f71 where ,p1 ,q1 ,r are the maximum numbers of particles of the matter ,a1 ,b1 and ,c when a steady state is reached4 ,the values of ,p1 ,q1 ,r at any time t after removal of the source are given by equations of the same form as 7#c7 and 7#e7 for a short exposure4 ,remembering the condition that initially ,p ,p n_/1 ,q ,q n_/1 ,r ,r n_/1 it can readily be shown that ,p frac n #a e-#a t 4444 7#g71 ,q frac n #a #b 7frac #a #b e-#b t e-#a t7 4444 7#h71 ,r n 7ae-#a t be-#b t ce-#c t7 4444 7#i71 where a frac #b 7#a #b7 7#a - #c7 1 b frac -#a 7#a #b7 7#b - #c7 1 c frac #a #b #c 7#a #c7 7#b #c7 4 ,illustration3 ,fig4 #gc4 ,the relative numbers of atoms of ,p1 ,q1 ,r existing at any time are shown graphically in ,fig4 #gc1 curves ,a1 ,b1 ,c respectively4 ,the number of atoms ,r is taken as #ajj for comparison1 and the values of 1 1 are taken corresponding to the #c1 #ba1 and #bh-minute changes in the active deposit of radium4 ,a comparison with ,fig4 #gb for a short exposure brings out very clearly the variation in the relative amounts of ,p1 ,q1 ,r in the two cases4 ,initially the amount of ,r decreases very slowly4 ,this is a result of the fact that the supply of ,c due to the breaking up of ,b at first1 nearly compensates for the breaking up of ,c4 ,the values of ,q and ,r after several hours decrease exponentially1 falling to half value in #bh minutes4 #aii4 ,,case #c4 ,suppose that a primary source has supplied the matter ,a at a constant rate for any time ,t and is then suddenly removed4 ,required the amounts of ,a1 ,b1 ,c at any subsequent time4 ,suppose that n particles of the matter ,a are deposited each second4 ,after a time of exposure ,t1 the number of particles ,p,t of the matter ,a present is given by ,p,t n int,t e-#a t dt frac n #a 7#a - e-#a ,t7 4 ,at any time t1 after removal of the source1 the number of particles ,p of the matter ,a is given by ,p ,p,t e-#a t frac n #a 7#a e-#a ,t7 e-#a t 4 ,consider the number of particles ndt of the matter ,a produced during the interval dt4 ,at any later time t1 the number of particles d,q of the matter ,b1 which result from the change in ,a1 is given 7see equation #d7 by d,q frac n #a #a #b 7e-#b t e-#a t7 dt n f7t7 dt 4444 7#aj74 ,after a time of exposure ,t1 the number of particles ,q,t of the matter ,b present is readily seen to be given by ,q,t n 7f7,t7 dt f7,t dt7 dt 444 f7#j7 dt7 n int,t f7t7 dt 4 ,if the body is removed from the emanation after an exposure ,t1 at any later time t the number of particles of ,b is in the same way given by ,q n intt,tt f7t7 dt 4 ,it will be noted that the method of deduction of ,q,t and ,q is independent of the particular form of the function f7t74 ,substituting the particular value of f7t7 given in equation 7#aj7 and integrating1 it can readily be deduced that frac ,q ,q,t frac ae-#b t be-#a t a - b 4444 7#aa71 where a frac 7#a e-#b ,t #b 1 b frac 7#a e-#a ,t #a 1 ,in a similar way1 the number of particles ,r of the matter ,c present at any time can be deduced by substitution of the value of f7t7 in equation 7#e74 ,these equations are1 however1 too complicated in form for simple application to experiment1 and will not be considered here4 #bjj4 ,,case #d4 ,the matter ,a is supplied at a constant rate from a primary source4 ,required to find the number of particles of ,a1 ,b1 ,c at any subsequent time t1 when initially ,a1 ,b1 ,c are absent4 ,the solution can be simply obtained in the following way4 ,suppose that the conditions of ,case #b are fulfilled4 ,the products ,a1 ,b1 ,c are in radio-active equilibrium and let ,p1 ,q1 ,r be the number of particles of each present4 ,suppose the source is removed4 ,the values of ,p1 ,q1 ,r at any subsequent time are given by equations 7#g71 7#h7 and 7#i7 respectively4 ,now suppose the source1 which has been removed1 still continues to supply ,a at the same constant rate and let ,p1 ,q1 ,r be the number of particles of ,a1 ,b1 ,c again present with the source at any subsequent time4 ,now we have seen1 that the rate of change of any individual product1 considered by itself1 is independent of conditions and is the same whether the matter is mixed with the parent substance or removed from it4 ,since the values of ,p1 ,q1 ,r represent a steady state where the rate of supply of each kind of matter is equal to its rate of change1 the sum of the number of particles ,a1 ,b1 ,c present at any time with the source1 and in the matter from which it was removed1 must at all times be equal to ,p1 ,q1 ,r1 4441 that is ,p ,p ,p1 ,q ,q ,q1 ,r ,r ,r4 ,this must obviously be the case1 for otherwise there would be a destruction or creation of matter by the mere process of separation of the source from its products2 but1 by hypothesis1 neither the rate of supply from the source1 nor the law of change of the products1 has been in any way altered by removal4 ,substituting the values of ,p1 ,q1 ,r from equations 7#g71 7#h71 and 7#i71 we obtain frac ,p#a ,p #a e-#a t 1 frac ,q#a ,q #a frac 7#a e-#b t - #b;e-#a t7 #a #b 1 frac ,r#a ,r #a #c 7ae-#a t be-#b t ce-#c t7 1 where a1 b1 and c have the values given after equation 7#i74 ,the curves representing the increase of ,p1 ,q1 ,r1 are thus1 in all cases1 complementary to the curves shown in ,fig4 #gc4 ,the sum of the ordinates of the two curves of rise and decay at any time is equal to #ajj4 ,we have already seen examples of this in the case of the decay and recovery curves of ,ur ,x and ,th ,x4 #bja4 ,activity of a mixture of products4 ,in the previous calculations we have seen how the number of particles of each of the successive products varies with the time under different conditions4 ,it is now necessary to consider how this number is connected with the activity of the mixture of products4 ,if ,n is the number of particles of a product1 the number of particles breaking up per second is ,n1 where is the constant of change4 ,if each particle of each product1 in breaking up1 emits one particle1 we see that the number of particles expelled per second from the mixture of products at any time is equal to ,p ,q ,r 4441 where ,p1 ,q1 ,r1 444 are the numbers of particles of the successive products ,a1 ,b1 ,c1 4444 ,substituting the values of ,p1 ,q1 ,r already found from any one of the four cases previously considered1 the variation of the number of particles expelled per second with the time can be determined4 ,the ideal method of measuring the activity of any mixture of radio-active products would be to determine the number of or particles expelled from it per second4 ,in practice1 however1 this is inconvenient and also very difficult experimentally4 ,certain practical difficulties arise in endeavouring to compare the activity of one product with another4 ,we shall see later that1 in many cases1 all of the successive products do not emit rays4 ,some give out and rays alone1 while there are several 8rayless0 products1 that is1 products which do not emit either 1 1 or rays4 ,in the case of radium1 for example1 radium ,a gives out only rays1 radium ,b no rays at all1 while radium ,c gives out 1 1 and rays4 ,in practice1 the relative activity of any individual product at any time is usually determined by relative measurements of the saturation ionization current produced between the electrodes of a suitable testing vessel4 ,let us consider1 for example1 the case of a product which gives out only rays4 ,the passage of the particles through the gas produces a large number of ions in its path4 ,since the particles from any individual product are projected with the same average velocity under all conditions1 the relative amount of the ionization produced per second in the testing vessel serves as an accurate means of determining the variation of its activity4 ,no two products1 however1 emit particles with the same average velocity4 ,we have seen that the rays from some products are more readily stopped in the gas than others4 ,thus the relative saturation current1 due to two different products in a testing vessel1 does not serve as an accurate method of comparing the relative number of particles expelled per second4 ,the ratio of the currents will in general depend upon the distance between the plates of the testing vessel1 and1 unless the relative ionization due to the average particle from the two products is known from other data1 the comparison of the currents can1 at best1 be only an approximate guide to the relative number of particles escaping into the gas4 #bjb4 ,some examples will now be considered to show how the factors1 above considered1 influence the character of the curves of activity obtained under different experimental conditions4 ,for the purpose of illustration1 we shall consider the variation after removal of the excited activity on a body exposed for different times to a constant supply of the radium emanation4 ,the active deposit on removal consists in general of a mixture of the products radium ,a1 ,b1 and ,c4 ,the nature of the rays from each product1 the time for each product to be transformed1 and the value of are tabulated below for convenience3-- ,product ,rays ,t4 7sec7 ,radium ,a rays #c min4 #che #aj ,radium ,b no rays #ba min4 #ech #aj ,radium ,c 1 1 #bh min4 #dac rays #aj ,since only the product ,c gives rise to and rays1 the activity measured by either of these types of rays will be proportional to the amount of ,c present at any time1 i4e4 to the value of ,r at any time4 ,for a long exposure1 the variation of activity with time measured by the and rays will thus be represented by the upper curve ,,cc of ,fig4 #gc1 where the ordinates represent activity4 ,this curve will be seen to be very similar in shape to the experimental curve for a long exposure which is given in ,fig4 #fh4 ,since radium ,b does not give out rays1 the number of particles expelled from the active deposit per second is proportional to ,p ,r4 ,the activity measured by the rays1 using the electrical method1 is thus proportional at any time to ,p ,k,r1 where ,k is a constant which represents the ratio of the number of ions1 produced in the testing vessel1 by an particle from ,c compared with that from an particle emitted by ,a4 ,it will be seen later that1 for this particular case1 ,k is nearly unity4 ,taking ,k #a1 the activity at any time after removal is proportional to ,p ,r4 ,,case #a4 ,we shall first consider the activity curve for a short exposure to the radium emanation4 ,the relative values of ,p1 ,q1 and ,r at any time corresponding to this case are graphically shown in ,fig4 #gd4 ,the activity measured by the rays at any time will be the sum of the activities due to ,a and ,c separately4 ,let curve ,,aa 7,fig4 #gd7 represent the activity due to ,a4 ,this decreases exponentially1 falling to half value in #c minutes4 ,in order to show the small activity due to ,c clearly in the ,figure1 the activity due to ,a is plotted after an interval of #f minutes1 when the activity has been reduced to #be per cent4 of its maximum value4 ,the activity due to ,c is proportional to ,r1 and in order to represent the activity due to ,c to the same scale as ,a1 it is necessary to reduce the scale of the ordinates of curve ,,cc in ,fig4 #gb in the ratio _/4 ,illustration3 ,fig4 #gd4 ,the activity due to ,c is thus represented by the curve ,,ccc1 ,fig4 #gd4 ,the total activity is thus represented by a curve ,a ,c whose ordinates are the sum of the ordinates of ,a and ,c4 ,this theoretical activity curve is seen to be very similar in its general features to the experimental curve shown in ,fig4 #ff1 where the activity from a very short exposure is measured by the rays4 ,,case #b4 ,the activity curve for a long exposure to the emanation will now be considered4 ,the activity after removal of ,a and ,c is proportional to ,p ,r1 where the values of ,p and ,r are graphically shown in ,fig4 #ge by the curves ,,aa1 ,,cc4 ,initially after removal1 ,p ,r1 since ,a and ,c are in radio-active equilibrium1 and the same number of particles of each product break up per second4 ,the activity due to ,a alone is shown in curve ,,aa1 ,fig4 #ge4 ,the activity decreases exponentially1 falling to half value in #c minutes4 ,the activity due to ,c at any time is proportional to ,r1 and is initially equal to that of ,a4 ,the activity curve due to ,c is thus represented by the curve ,,cc1 which is the same curve as the upper curve ,,cc of ,fig4 #gc4 ,the activity of ,a and ,c together is represented by the upper curve ,a ,c 7,fig4 #ge71 where the ordinates are equal to the sum of the ordinates of the curves ,a and ,c4 ,this theoretical curve is seen to be very similar in shape to the experimental curve 7,fig4 #fg7 showing the decay of activity of the active deposit from a long exposure measured by the rays4 ,illustration3 ,fig4 #ge4 #bjc4 ,effect of a rayless change on the activity curves4 ,certain important cases occur in the analysis of radio-active changes1 when one of the products does not give rise to rays and so cannot be detected directly4 ,the presence of this rayless change can1 however1 be readily observed by the variations which occur in the activity of the succeeding product4 ,let us consider1 for example1 the case where the inactive matter ,a1 initially all of one kind1 changes into the matter ,b which gives out rays4 ,the inactive matter ,a is supposed to be transformed according to the same law as the radio-active products4 ,let 1 be the constants of the change of ,a and ,b respectively4 ,if n is the number of particles of ,a1 initially present1 we see from the equation 7#d71 section #aig1 that the number of particles of the matter ,b present at any time is given by ,q frac n#a #a #b 7e-#b t - e#a t7 4 ,differentiating and equating to zero1 it is seen that the value of ,q passes through a maximum at a time ,t given by the equation #b e-#b ,t #a e-#a ,t 4 ,for the sake of illustration1 we shall consider the variation of the activity of the active deposit of thorium1 due to a very short exposure to the emanation4 ,thorium ,a gives out no rays1 and thorium ,b gives out 1 1 and rays1 while thorium ,c is inactive4 ,the matter ,a is half transformed in #aa hours1 and ,b is half transformed in #ee minutes4 ,the value of #age x #aj7sec47 and #bjh x #aj7sec474 ,the activity of the mixture of products ,a ,b is due to ,b alone1 and will1 in consequence1 be always proportional to the amount of ,b present1 that is1 to the value of ,q4 ,illustration3 ,fig4 #gf4 ,the variation of activity with time is shown graphically in ,fig4 #gf4 ,the activity rises from zero to a maximum in #bbj minutes and then decays1 finally decreasing1 according to an exponential law1 with the time1 falling to half value in #aa hours4 ,this theoretical curve is seen to agree closely in shape with the experimental curve 7,fig4 #fe71 which shows the variation of the activity of the active deposit of thorium1 produced by a short exposure in presence of the emanation4 ,there are several points of interest in connection with an activity curve of this character4 ,the activity1 some hours after removal1 decays according to an exponential law1 not at the rate of the product ,b1 from which the activity rises1 but at the same rate as the first rayless transformation4 ,this will also be the case if the rayless product has a slower rate of change than the succeeding active product4 ,given an activity curve of the character of ,fig4 #gf1 we can deduce from it that the first change is not accompanied by rays and also the period of the two changes in question4 ,we are1 however1 unable to determine from the curve which of the periods of change refers to the rayless product4 ,it is seen that the activity curve is unaltered if the values of 1 1 that is1 if the periods of the products are interchanged1 for the equation is symmetrical in 1 4 ,for example1 in the case of the active deposit of thorium1 without further data it is impossible to decide whether the period of the first change has a value of #ee minutes or #aa hours4 ,in such cases the question can only be settled by using some physical or chemical means in order to separate the product ,a from ,b1 and then testing the rate of decay of their activity separately4 ,in practice1 this can often be effected by electrolysis or by utilizing the difference in volatility of the two products4 ,if now a product is separated from the mixture of ,a and ,b which loses its activity according to an exponential law1 falling to half value in #ee minutes 7and such is experimentally observed71 we can at once conclude that the active product ,b has the period of #ee minutes4 ,the characteristic features of the activity curve shown in ,fig4 #gf becomes less marked with increase of the time of exposure of a body to the emanation1 that is1 when more and more of ,b is mixed with ,a at the time of removal4 ,for a long time of exposure1 when the products ,a and ,b are in radio-active equilibrium1 the activity after removal is proportional to ,q1 where ,q frac n #a #b 7frac #a #b e-#b t e#a t7 1 7see equation #h1 section #aih74 ,the value of ,q1 in this case1 does not increase after removal1 but at once commences to diminish4 ,the activity1 in consequence1 decreases from the moment of removal1 but more slowly than would be given by an exponential law4 ,the activity finally decays exponentially1 as in the previous case1 falling to half value in #aa hours4 ,in the previous case we have discussed the activity curve obtained when both the active and inactive product have comparatively rapid rates of transformation4 ,in certain cases which arise in the analysis of the changes in actinium and radium1 the rayless product has a rate of change extremely slow compared with that of the active product4 ,this corresponds to the case where the active matter ,b is supplied from ,a at a constant rate4 ,the activity curve will thus be identical in form with the recovery curves of ,th ,x and ,ur ,x1 that is1 the activity ,i at any time t will be represented by the equation frac ,it ,i #a e-#b t 1 where ,i is the maximum value of the activity and the constant of change of ,b4 #bjd4 ,in this chapter we have considered the variation with time1 under different conditions1 of the number of atoms of the successive products1 when the period and number of the changes are given4 ,it has been seen that the activity curves to be expected under various conditions can be readily deduced from the simple theory4 ,in practice1 however1 the investigator has been faced with the much more difficult inverse problem of deducing the period1 number1 and character of the products1 by analysis of the activity curves obtained under various conditions4 ,in the case of radium1 where at least seven distinct changes occur1 the problem has been one of considerable difficulty1 and a solution has only been possible by devising special physical and chemical methods of isolation of some of the products4 ,we shall see later that two rayless changes occur in radium and actinium and one in thorium4 ,it is at first sight a very striking fact that the presence of a substance which does not emit rays can be detected1 and its properties investigated4 ,this is only possible when the rayless product is transformed into another substance which emits rays2 for the variation of the activity of the latter may be such as to determine not only the period but also the physical and chemical properties of the parent product4 ,in the two following chapters the application of the theory of successive changes will be shown to account satisfactorily for the complicated processes occurring in the radio-elements4 ,footnote #bid3 ,ramsay1 ,proc4 ,roy4 ,soc4 p4 #dgj1 ,june1 #aijd2 ,c4 ,r4 #ach1 ,june #f1 #aijd4 ,footnote #bie3 ,phil4 ,mag4 ,february1 #aijd4 ,,chapter ,x4 ,,transformation ,,products ,,of ,,uranium1 ,,thorium1 ,,and ,,actinium4 #bje4 ,in the last chapter the mathematical theory of successive changes has been considered4 ,the results there obtained will now be applied to explain the radio-active phenomena observed with uranium1 thorium1 actinium1 radium1 and their products4 ,transformation products of ,uranium4 ,it has been shown in sections #abg and #abi that a radio-active constituent ,ur ,x can be separated from uranium by several different processes4 ,the activity of the separated ,ur ,x decays with the time1 falling to half value in about #bb days4 ,at the same time the uranium1 from which the ,ur ,x has been separated1 gradually regains its lost activity4 ,the laws of decay of ,ur ,x and of the recovery of the lost activity of the uranium are expressed by the equations frac ,i ,i e-t 1 and frac ,it ,i #a e-t 1 where is the radio-active constant of ,ur ,x4 ,the substance ,ur ,x is produced from uranium at a constant rate1 and the constant radio-activity observed in uranium represents a state of equilibrium1 where the rate of production of new active matter is balanced by the rate of change of the ,ur ,x already produced4 ,the radio-active processes occurring in uranium present several points of difference from the processes occurring in thorium and radium4 ,in the first place1 uranium does not give off an emanation1 and in consequence does not produce any excited activity on bodies4 ,so far only one active product ,ur ,x has been observed in uranium4 ,this active product ,ur ,x differs from ,th ,x and the emanations1 inasmuch as the radiation from it consists almost entirely of rays4 ,this peculiarity of the radiations from ,ur ,x initially led to some confusion in the interpretation of observations on ,ur ,x and the uranium from which it had been separated4 ,when examined by the photographic method1 the uranium freed from ,ur ,x showed no activity1 while the ,ur ,x possessed it to an intense degree4 ,with the electric method1 on the other hand1 the results obtained were exactly the reverse4 ,the uranium freed from ,ur ,x showed very little loss of activity1 while the activity of the ,ur ,x was very small4 ,the explanation of these results was given by ,soddy#bif and by ,rutherford and ,grier#big4 ,the rays of uranium are photographically almost inactive1 but produce most of the ionization in the gas4 ,the rays1 on the other hand1 produce a strong photographic action1 but very little ionization compared with the rays4 ,when the ,ur ,x is separated from the uranium1 the uranium does not at first give out any rays4 ,in the course of time fresh ,ur ,x is produced from the uranium1 and rays begin to appear1 gradually increasing in intensity until they reach the original value shown before the separation of the ,ur ,x4 ,in order to determine the recovery curves of uranium after the separation of ,ur ,x1 it was thus necessary to measure the rate of increase of the rays4 ,this was done by covering the uranium with a layer of aluminium of sufficient thickness to absorb all the rays1 and then measuring the ionization due to the rays in an apparatus similar to ,fig4 #ag4 ,uranium has not yet been obtained inactive when tested by the electric method4 ,becquerel#bih has stated that he was able to obtain inactive uranium1 but in his experiments the uranium was covered with a layer of black paper1 which would entirely absorb the rays4 ,there is no evidence that the radiation of uranium has been altered either in character or amount by any chemical treatment4 ,the rays appear to be inseparable from the uranium1 and it will be shown later that thorium and radium as well as uranium also possess a non-separable activity consisting entirely of rays4 ,the changes occurring in uranium must then be considered to be of two kinds1 7#a7 the change which gives rise to the rays and the product ,ur ,x1 7#b7 the change which gives rise to the rays from ,ur ,x4 ,the possibility of separating the ,ur ,x1 which gives rise to the rays of uranium1 shows that the and rays are produced quite independently of one another1 and by matter of different chemical properties4 ,following the general considerations discussed in section #acf we may suppose that every second some of the atoms of uranium--a very minute fraction of the total number present will suffice--become unstable and break up1 expelling an particle with great velocity4 ,the uranium atom1 minus one particle1 becomes the atom of the new substance1 ,ur ,x4 ,this in turn is unstable and breaks up with the expulsion of the particle and the appearance of a ray4 ,the changes occurring in uranium are graphically shown in ,fig4 #gg4 ,illustration3 ,fig4 #gg4 ,on this view the ray activity of uranium should be an inherent property of the uranium1 and should be non-separable from it by physical or chemical means4 ,the and ray activity of uranium is a property of ,ur ,x1 which differs in chemical properties from the parent substance and can at any time be completely removed from it4 ,the final product1 after the decay of ,ur ,x1 is so slightly active that its activity has not yet been observed4 ,we shall see later 7chapter ,,xiii47 that there is some reason to believe that the changes in uranium do not end at this point but continue through one or more stages1 finally giving rise to radium1 or in other words that radium is a product of the disintegration of the uranium atom4 ,meyer and ,schweidler#bii1 in a recent paper1 state that the activity due to uranium preparations increases somewhat in a closed vessel4 ,on removing the uranium no residual activity1 however1 was observed4 ,they consider that this effect may be due to a very short-lived emanation emitted by uranium4 #bjf4 ,effect of crystallization on the activity of uranium4 ,meyer and ,schweidler#cjj recently observed that uranium nitrate1 after certain methods of treatment1 showed remarkable variations of its activity1 measured by the rays4 ,the ray activity1 on the other hand1 was unaltered4 ,some uranium nitrate was dissolved in water and then shaken up with ether1 and the ether fraction drawn off4 ,the early experiments of ,crookes showed that1 by this method1 the uranium in the ether portion was photographically inactive4 ,this is simply explained by supposing that the uranium ,x is insoluble in ether1 and consequently remained behind in the water fraction4 ,the ether fraction gradually regained its ray activity at the normal rate to be expected if ,ur ,x was produced by the uranium at a constant rate1 for it recovered half its final activity in about #bb days4 ,some of the uranium in the water fraction was crystallized and placed under an electroscope4 ,the ray activity fell rapidly at first to half its value in the course of four days4 ,the activity then remained constant1 and no further change was observed over an interval of one month4 ,other experiments were made with crystals of uranium nitrate1 which had not been treated with ether4 ,the nitrate was dissolved in water and a layer of crystals separated4 ,the ray activity of these crystals fell rapidly at first1 the rate varying somewhat in different experiments1 but reached a minimum value after about five days4 ,the ray activity then rose again at a slow rate for several months4 ,the rapid drop of activity of the crystals seemed1 at first sight1 to indicate that crystallization was able in some way to alter the activity of uranium4 ,dr ,godlewski1 working in the laboratory of the writer1 repeated the work of ,meyer and ,schweidler1 and obtained results of a similar character1 but the initial drop of activity was found to vary both in rate and amount in different experiments4 ,these results were at first very puzzling and difficult to explain1 for the mother liquor1 left behind after removal of the crystals1 did not show the corresponding initial rise1 which would be expected if the variation of activity were due to the partial separation of some new product of uranium4 ,the cause of this effect was1 however1 rendered very evident by a few well-considered experiments made by ,godlewski4 ,the uranium nitrate was dissolved in hot water in a flat dish1 and allowed to crystallize under the electroscope4 ,up to the moment of crystallization the ray activity remained constant1 but as soon as the crystals commenced to form at the bottom of the solution the ray activity rapidly rose in the course of a few minutes to five times the initial value4 ,after reaching a maximum1 the activity very gradually decreased again to the normal value4 ,if1 however1 the plate of crystals was reversed1 the ray activity was found at first to be much smaller than the normal1 but increased as fast as that of the other side diminished4 ,the explanation of this effect is simple4 ,ur ,x is very soluble in water and1 at first1 does not crystallize with the uranium1 but remains in the solution1 and1 consequently1 when the crystallization commences at the bottom of the vessel the upper layer of liquid becomes richer in uranium ,x4 ,since the rays arise only from the product ,ur ,x and not from the uranium itself1 and the ,ur ,x is mostly confined to the upper layer1 a much greater proportion of the rays escape than if the ,ur ,x were uniformly distributed throughout the thick layer of uranium4 ,when the amount of water added is just sufficient to supply the water of crystallization1 the ,ur ,x in the upper layer of crystals gradually diffuses back through the mass and1 in consequence1 the activity of the upper surface diminishes and of the lower surface rises4 ,a similar explanation applies to the effects observed by ,meyer and ,schweidler4 ,the water fraction1 left behind after treatment with ether1 contained all the ,ur ,x4 ,the first layer of crystals formed in it contained some ,ur ,x1 and this was for the most part confined to the top layer of crystals4 ,the amount of rays at first diminished owing to the gradual diffusion of the ,ur ,x from the surface4 ,in the first experiment1 the amount of ,ur ,x present was in radio-active equilibrium with the uranium1 and1 after the initial drop1 the ray activity remained constant4 ,in the second experiment1 the gradual rise is due to the fact that the crystals of uranium first formed contained less than the equilibrium amount of ,ur ,x4 ,after falling to a minimum1 the ray activity1 in consequence1 slowly rose again to the equilibrium value4 ,these effects exhibited by uranium are of great interest1 and illustrate in a striking manner the difference in properties of ,ur ,x and the uranium4 ,the gradual diffusion of the ,ur ,x throughout the mass of crystals is noteworthy4 ,by measurements of the variation with time of the ray activity1 it should be possible to deduce its rate of diffusion into the crystallized mass4 ,transformation products of ,thorium4 #bjg4 ,analysis of the active deposit4 ,the radio-active processes occurring in thorium are far more complicated than those in uranium4 ,it has already been shown in chapter vi that a radio-active product ,th ,x is continuously produced from the thorium4 ,this ,th ,x breaks up1 giving rise to the radio-active emanation4 ,the emanation produces from itself a type of active matter which is deposited on the surface of bodies1 where it gives rise to the phenomena of excited or induced activity4 ,this active deposit possesses some distinctive chemical and physical properties which distinguish it from the emanation and the ,th ,x4 ,we have seen 7section #ahj7 that the rate at which the active deposit loses its activity depends upon the time of exposure of the body made active to the emanation4 ,the explanation of the activity curves for different time of exposure will now be considered4 ,the curve of variation of activity for a short exposure of #aj minutes has already been given in ,fig4 #fe4 ,the activity is small at first but increases rapidly with the time2 it passes through a maximum about #d hours later1 and finally decays exponentially with the time1 falling to half value in #aa hours4 ,this remarkable effect can be explained completely#cja if it be supposed that the active deposit consists of two distinct substances4 ,the matter initially deposited from the emanation1 which will be called thorium ,a1 is supposed to be changed into thorium ,b4 ,thorium ,a is transformed according to the ordinary exponential law1 but the change is not accompanied by any ionizing rays4 ,in other words1 the change from ,a to ,b is a 8rayless0 change4 ,on the other hand1 ,b breaks up into ,c with the accompaniment of all three kinds of rays4 ,on this view the activity of the active deposit at any time represents the amount of the substance ,b present1 since ,c is inactive or active to a very minute extent4 ,if the variation of the activity imparted to a body exposed for a short interval in the presence of the thorium emanation1 is due to the fact that there are two successive changes in the deposited matter ,a1 the first of which is a 8rayless0 change1 the activity ,it at any time t after removal should be proportional to the number ,qt of particles of the matter ,b present at that time4 ,now1 from equation 7#d7 section #aig1 it has been shown that ,q frac n#a #a #b 7e-#b t - e-#a t7 4444 7#d74 ,the value of ,qt passes through a maximum ,q,t at the time ,t when frac #b #a e-7#a-b7 ,t 4 ,the maximum activity ,i,t is proportional to ,q,t and frac ,it ,i,t frac ,qt ,q,t frac e-#b t - e-#a t e-#b ,t e-#a ,t 4 ,it will be shown later that the variation with time of the activity1 imparted to a body by a short exposure1 is expressed by an equation of the above form4 ,it thus remains to fix the values of 1 4 ,since the above equation is symmetrical with regard to 1 1 it is not possible to settle from the agreement of the theoretical and experimental curve which value of refers to the first change4 ,the curve of variation of activity with time is unaltered if the values of and are interchanged4 ,it is found experimentally that the activity #e or #f hours after removal decays very approximately according to an exponential law with the time1 falling to half value in #aa hours4 ,this is the normal rate of decay of thorium for all times of exposure1 provided measurements are not begun until several hours after the removal of the active body from the emanation4 ,this fixes the value of the constants of one of the changes4 ,let us assume for the moment that this gives the value of 4 ,then #age #aj 7sec74 ,since the maximum activity is reached after an interval ,t #bbj minutes 7see ,fig4 #fe71 substituting the values of and ,t in the equation1 the value of comes out to be #bjh #aj 7sec74 ,this value of corresponds to a change in which half the matter is transformed in #ee minutes4 ,substituting now the values of 1 1 ,t1 the equation reduces to frac ,it ,i,t #a;cdot#cg 7e-#b t e-#b t7 4 ,the agreement between the results of the theoretical equation and the observed values is shown in the following table3 ,time in ,theoretical ,observed value minutes value of of ,it_/,i,t ,it_/,i,t #ae #bb #bc #cj #ch #cg #fj #fd #fc #abj #ij #ia #bbj #ajj #ajj #cje #ig #if ,after #e hours the activity decreased nearly exponentially with the time1 falling to half value in #aa hours4 ,it is thus seen that the curve of rise of activity for a short exposure is explained very satisfactorily on the supposition that two changes occur in the deposited matter1 of which the first is a rayless change4 ,further data are required in order to fix which of the time constants of the changes refers to the first change4 ,in order to settle this point1 it is necessary to isolate one of the products of the changes and to examine the variation of its activity with time4 ,if1 for example1 a product can be separated whose activity decays to half value in #ee minutes1 it would show that the second change is the more rapid of the two4 ,now ,pegram#cjb has examined the radio-active products obtained by electrolysis of thorium solutions4 ,the rates of decay of the active products depended upon conditions1 but he found that1 in several cases1 rapidly decaying products were obtained whose activity fell to half value in about #a hour4 ,allowing for the probability that the product examined was not completely isolated by the electrolysis1 but contained also a trace of the other product1 this result would indicate that the last change which gives rise to rays is the more rapid of the two4 ,this point is very clearly brought out by some recent experiments of ,miss ,slater#cjc1 who has made a detailed examination of the effect of temperature on the active deposit of thorium4 ,a platinum wire was made active by exposure for a long interval to the thorium emanation1 and then heated for a few minutes to any desired temperature by means of the electric current4 ,the wire1 while being heated1 was surrounded by a lead cylinder in order that any matter driven off from it should be collected on its surface4 ,the decay of activity both of the wire and of the lead cylinder was then tested separately4 ,after heating to a dull red heat1 no sensible diminution of the activity was observed at first1 but the rate of decay of the activity on the wire was found to be more rapid than the normal4 ,the activity of the lead cylinder was small at first but increased to a maximum after about #d hours and then decayed at the normal rate with the time4 ,these results are to be expected if some thorium ,a is volatilized from the wire2 for the rise of activity on the lead cylinder is very similar to that observed on a wire exposed for a short time in the presence of the thorium emanation1 i4e41 under the condition that only thorium ,a is initially present4 ,on heating the wire above #gjj ,c4 the activity was found to be reduced1 showing that some thorium ,b had also been removed4 ,by heating for a few minutes at about #ajjj ,c4 nearly all the thorium ,a was driven off4 ,the activity on the wire then decayed exponentially with the time1 falling to half value in about #a hour4 ,after heating for a minute at about #abjj ,c4 all the activity was removed4 ,these results show that thorium ,a is more volatile than ,b1 and that the product which gives out rays1 viz4 thorium ,b1 has a period of about #ee minutes4 ,another series of experiments was made1 in which an active aluminium disc was placed in an exhausted tube1 and exposed to the cathode ray discharge4 ,under these conditions1 a part of the activity of the disc was removed4 ,when the disc was made the anode1 the loss of activity was usually #bj to #fj per cent4 for half-an-hour's exposure4 ,if the disc was made the cathode1 the loss was much greater1 amounting to about #ij per cent4 in #aj minutes4 ,part of the active matter removed from the disc was collected on a second disc placed near it4 ,this second disc on removal lost its activity at a far more rapid rate than the normal4 ,the rate of decay on the first disc was also altered1 the activity sometimes even increasing after removal4 ,these results indicate that1 in this case1 the apparent volatility of the products is reversed4 ,thorium ,b is driven off from the disc more readily than thorium ,a4 ,the rates of decay obtained under different conditions were satisfactorily explained by supposing that the surfaces of the discs after exposure to the discharge were coated with different proportions of thorium ,a and ,b4 ,the escape of thorium ,b from the disc under the influence of the discharge seems rather to be the result of an action similar to the well-known 8sputtering0 of electrodes than to a direct influence of temperature4 ,the results obtained by von ,lerch#cjd on the electrolysis of a solution of the active deposit also admit of a similar interpretation4 ,products were obtained on the electrodes of different rates of decay1 losing half their activity in times varying from about #a hour to #e hours4 ,this variation is due to the admixture of the two products in different proportions4 ,the evidence1 as a whole1 thus strongly supports the conclusion that the active deposit from thorium undergoes two successive transformations as follows3 7#a7 ,a 8rayless0 change for which #age #aj1 i4e41 in which half the matter is transformed in #aa hours2 7#b7 ,a second change giving rise to 1 and rays1 for which #bjh #aj1 i4e41 in which half the matter is transformed in #ee minutes#cje4 ,it is1 at first sight1 a somewhat unexpected result that the final rate of decay of the active deposit from thorium gives the rate of change not of the last product itself1 but of the preceding product1 which does not give rise to rays at all4 ,a similar peculiarity is observed in the decay of the excited activity of actinium1 which is discussed in section #bab4 ,for a long exposure in the presence of a constant supply of thorium emanation1 the equation expressing the variation of activity with time is found from equation 7#h71 section #aih1 frac ,it ,i frac ,q ,q frac #b #b - #a e-#a t frac #a #a #b e-#b t frac #b e-#a t #b - #a 7#a cdot#jhc e#a;cdot#ij #ajd t 7 4 ,about #e hours after removal the second term in the brackets becomes very small1 and the activity after that time will decay nearly according to an exponential law with the time1 falling to half value in #aa hours4 ,for any time of exposure ,t1 the activity at time t after the removal 7see equation #aa1 section #aii7 is given by frac ,it ,i frac ,q ,q,t frac ae-#b t - be-#a t a b 1 where ,i is the initial value of the activity1 immediately after removal1 and a frac #a e-#b ,t #b 1 b frac #a e-#a ,t #a 1 ,by variation of ,t the curves of variation of activity for any time of exposure can be accurately deduced from the equation1 when the values of the two constants 1 are substituted4 ,miss ,brooks#cjf has examined the decay curves of excited activity for thorium for different times of exposure and has observed a substantial agreement between experiment and theory4 ,illustration3 ,fig4 #gh4 ,the results are shown graphically in ,fig4 #gh4 ,the maximum value of the activity is1 for each time of exposure1 taken as #ajj4 ,the theoretical and observed values are shown in the ,figure4 #bjh4 ,analysis of the decay and recovery curves of ,th ,x4 ,the peculiarities of the initial portions of the decay and recovery curves of ,th ,x and thorium respectively 7,curves ,a and ,b1 ,fig4 #dg1 p4 #bba71 will now be considered4 ,it was shown that when the ,th ,x was removed from the thorium by precipitation with ammonia1 the radiation increased about #ae per cent4 during the first day1 passed through a maximum1 and then fell off according to an exponential law1 decreasing to half value in four days4 ,at the same time the activity of the separated hydroxide decreased for the first day1 passed through a minimum1 and then slowly increased again1 rising to its original value after the lapse of about one month4 ,when a thorium compound is in a state of radio-active equilibrium1 the series of changes in which ,th ,x1 the emanation1 and thorium ,a and ,b are produced1 go on simultaneously4 ,since a state of equilibrium has been reached for each of these products1 the amount of each product changing in unit time is equal to the amount of that product supplied from the preceding change in unit time4 ,now the matter ,th ,x is soluble in ammonia1 while thorium ,a and ,b are not4 ,the ,th ,x is thus removed from the thorium by precipitation with ammonia1 but ,a and ,b are left behind with the thorium4 ,since the active deposit is produced from the emanation1 which in turn arises from ,th ,x1 on the removal of the parent matter ,th ,x1 the radiation due to this active deposit will decay1 since the rate of production of fresh matter no longer balances its own rate of change4 ,disregarding the initial irregularity in the decay curve of the active deposit1 its activity will have decayed to half value in about #aa hours1 and to one quarter value at the end of #bb hours4 ,as soon1 however1 as the ,th ,x has been separated1 new ,th ,x is produced in the thorium compound4 ,the activity of this new ,th ,x is not1 however1 sufficient to compensate at first for the loss of activity due to the change in the active deposit1 so that1 as a whole1 the activity will at first decrease1 then pass through a minimum1 then increase again4 ,the correctness of this point of view has been tested by ,rutherford and ,soddy#cjg as follows3 ,if the precipitated thorium hydroxide after the removal of ,th ,x is put through a series of precipitations with ammonia at short intervals1 the ,th ,x is removed almost as fast as it is formed1 and1 at the same time1 the activity of thorium ,b in the thorium decays4 ,the following table indicates the results obtained4 ,a portion of the precipitated hydroxide was removed after each series of precipitations and its activity tested in the usual way4 ,activity of hydroxide per cent4 ,after #a precipitation #df ,after #c precipitations at #ci intervals of #bd hours ,after #c more precipitations at #bb intervals of #bd hours and #c at intervals of #h hours ,after #c more each of #h hours #bd ,after #f more each of #d hours #be ,illustration3 ,fig4 #gi4 ,the differences in the last three numbers are not significant1 for it is difficult to make accurate comparisons of the activity of thorium compounds which have been precipitated under slightly different conditions4 ,it is thus seen that as a result of successive precipitations1 the activity is reduced to a minimum of about #be per cent4 ,the recovery curve of the activity of this #bc times precipitated hydroxide is shown in ,fig4 #gi4 ,the initial drop in the curve is quite absent1 and the curve1 starting from the minimum1 is practically identical with the curve shown in ,fig4 #dh1 which gives the recovery curve of thorium hydroxide after the first two days4 ,this residual activity--about #be per cent4 of the maximum--is non-separable from the thorium by any chemical process that has been tried4 ,the initial rise of activity of ,th ,x1 after it has been separated1 will now be considered4 ,in all cases it was found that the activity of the separated ,th ,x had increased about #ae per cent4 at the end of #bd hours1 and then steadily decayed1 falling to half value in about four days4 ,this peculiarity of the ,th ,x curve follows1 of necessity1 from the considerations already advanced to explain the drop in the recovery curve4 ,as soon as the ,th ,x is separated1 it at once produces from itself the emanation1 and this in turn produces thorium ,a and ,b4 ,the activity due to ,b at first more than compensates for the decay of activity of the ,th ,x itself4 ,the total activity thus increases to a maximum1 and then slowly decays to zero according to an exponential law with the time4 ,the curve expressing the variation of the activity of the separated ,th ,x with time can be deduced from the theory of successive changes already considered in chapter ,,ix4 ,in the present case there are four successive changes occurring at the same time1 viz4 the change of ,th ,x into the emanation1 of the emanation into thorium ,a1 of ,a into ,b1 and of ,b into an inactive product4 ,since1 however1 the change of the emanation into thorium ,a 7about half changed in one minute7 is far more rapid than the changes occurring in ,th ,x or thorium ,a and ,b1 for the purposes of calculation it may be assumed without serious error that the ,th ,x changes at once into the active deposit4 ,the #ee minute change will also be disregarded for the same reason4 ,let and be the constants of decay of activity of ,th ,x and of thorium ,a respectively4 ,since the activity of ,th ,x and of thorium ,a falls to half value in #d days and #aa hours respectively1 the value of #jjgb and of #jfc1 where #a hour is taken as the unit of time4 ,the problem reduces to the following3 ,given the matter ,a 7thorium ,x7 all of one kind1 which changes into ,b 7thorium ,b71 find the activity of ,a and ,b together at any subsequent time4 ,this corresponds to ,case ,i4 7section #aig74 ,the amount ,q of ,b at any time ,t is given by ,q frac #a n #a #b 7e-#b t - e-#a t7 1 and the activity ,i at any time of the two together is proportional to ,p ,k,q1 where ,k is the ratio of the ionization of ,b compared with that of ,a4 ,then frac ,it ,i frac #a ,p ,k#b ,q #a n e-#a t 7#a frac ,k#b #b #a 7#a e-7#b #a7 t77 1 where ,i is the initial activity due to n particles of ,th ,x4 ,by comparison of this equation with the curve of variation of the activity of ,th ,x with time1 shown in ,fig4 #dg1 it is found that ,k is almost #dd4 ,it must be remembered that the activity of the emanation and ,th ,x are included together1 so that the activity of thorium ,b is about half of the activity of the two preceding products4 ,the calculated values of ,it_/,i for different values of t are shown in the second column of the following table1 and the observed values in the third column4 ,time ,theoretical ,observed value value #j #ajj #ajj #be days #aji -- #e #aaf -- #a #aae #aag #ae #aaa -- #b #ajd -- #c #hge #hh #d #ge #gb #f #ec #ec #i #cae #bie #ac #aeg #aeb ,illustration3 ,fig4 #hj4 ,the theoretical and observed values thus agree within the limit of error in the measurements4 ,the theoretical curve is shown in ,curve ,a1 ,fig4 #hj 7with the observed points marked1 for comparison74 ,the curve ,b shows the theoretical curve of the decay of the activity of ,th ,x and the emanation1 supposing there is no further change into the active deposit4 ,curve ,c shows the difference curve between the curves ,a and ,b1 i4e4 the proportion of the activity at different times due to the active deposit4 ,the activity due to the latter thus rises to a maximum about two days after removal of the ,th ,x1 and then decays with the time at the same rate as the ,th ,x itself1 i4e4 the activity falls to half value every four days4 ,when t exceeds four days1 the term e-7#b #a7 t in the theoretical equation is very small4 ,the equation of decay after this time is therefore expressed by frac ,it ,i 7#a frac ,k#b #b #a7 e-#a t 1 i4e4 the activity decays according to an exponential law with the time4 #bji4 ,radiations from ,thorium products4 ,it has been shown in the last section that the activity of thorium1 by successive precipitations with ammonia1 is reduced to a limiting value of almost #be per cent4 of the initial activity4 ,this 8non-separable activity0 consists of rays1 the and rays being altogether absent4 ,according to the disintegration theory1 this is an expression of the fact that the initial break-up of the thorium atom is accompanied only by the expulsion of particles4 ,we have seen in section #aef that the thorium emanation also gives out only rays4 ,in the active deposit1 thorium ,a gives out no rays1 while thorium ,b emits all three types of rays4 ,some hours after separation1 ,th ,x gives out 1 1 and rays1 but the appearance of and rays is probably due to the thorium ,b associated with it4 ,the and ray activity of ,th ,x is much reduced if a current of air is continuously aspirated through a solution of ,th ,x to remove the emanation4 ,it seems likely that if the emanation could be removed as fast as it was formed1 so as to prevent the formation of thorium ,b in its mass1 ,th ,x itself would give out only rays3 but1 on account of the rapid rate of change of the thorium emanation1 it is difficult to realize this experimentally4 #baj4 ,transformation products of ,thorium4 ,the transformation products of thorium and the rays emitted by them are graphically shown below 7,fig4 #ha74 ,illustration3 ,fig4 #ha4 ,a table of the transformation products of thorium is shown below1 with some of their physical and chemical properties4 ,product ,time to be 7sec7 ,radiations ,physical and half chemical transformed properties ,thorium rays ,insoluble in ammonia ,th4 ,x #d days #bjj rays ,soluble in #aj ammonia ,emanation #ed secs4 #abh rays ,inert gas1 #aj condenses #abj ,c4 ,thorium ,a #aa hours #age no rays ,soluble in #aj strong acids4 ,volatile at a white heat4 ,b can be separated from ,a by electrolysis and by difference of volatility4 ,thorium ,b #ee mins4 #ba 1 1 rays ,same #aj 8 -- -- -- - #baa4 ,transformation products of ,actinium4 ,it has previously been pointed out 7sections #ag and #ah7 that the actinium of ,debierne and the emanium of ,giesel contain the same radio-active constituent4 ,both give out a short-lived emanation which imparts activity to the surface of bodies4 ,recently1 thanks to ,dr ,giesel of ,braunschweig1 preparations of 8emanium0 have been placed on the market1 and most of the investigations that are described later have been made with this substance4 ,actinium ,x4 ,actinium and thorium are very closely allied in radio-active properties4 ,both emit an emanation which is rapidly transformed1 but the rate of change of the actinium emanation is still more rapid than that of thorium1 the activity decreasing to half value in #cg seconds4 ,miss ,brooks#cjh has analysed the active deposit from the emanation of actinium1 and has shown that two successive changes occur in it1 very similar in character to those observed in the active deposit of thorium4 ,it thus seemed probable1 from analogy1 that an intermediate product1 corresponding to ,th ,x in thorium1 would be found in actinium#cji4 ,recent work has verified this supposition4 ,giesel#caj and ,godlewski#caa independently observed that a very active substance could be separated from 8emanium10 very similar in chemical and physical properties to ,th ,x in thorium4 ,this product will1 from analogy1 be called 8actinium ,x40 ,the same method1 which was used by ,rutherford and ,soddy to separate ,th ,x from thorium1 is also effective in separating actinium ,x from actinium4 ,after precipitation of the active solution with ammonia1 actinium ,x is left behind in the filtrate4 ,after evaporation and ignition1 a very active residue remains4 ,at the same time1 the precipitated actinium loses a large proportion of its activity4 ,giesel observed the separation of an active product1 using a fluorescent screen to detect the radiations4 ,a very complete examination of the product actinium ,x has been made by ,godlewski in the laboratory of the writer4 ,after separation of actinium ,x1 the activity1 whether measured by the or rays1 increases about #ae per cent4 during the first day1 and afterwards decays exponentially with the time1 falling to half value in #ajb days4 ,the activity of the separated actinium was small at first but steadily increased with the time1 reaching a practical maximum after an interval of sixty days4 ,after the first day1 the decay and recovery curves of activity are complementary to one another4 ,the curves of rise and decay are shown graphically in ,fig4 #hb1 curves ,i and ,,ii respectively4 ,godlewski observed that a solution of actinium1 freed from actinium ,x1 gave out very little emanation1 while a solution of actinium ,x gave off the emanation in large quantity4 ,the amount of emanation from the solution was measured by observing the activity produced in a testing vessel1 similar to that shown in ,fig4 #ea1 when a constant current of air was passed through the solution4 ,the emanating power of actinium ,x decreased exponentially with the time at the same rate as that at which the actinium ,x lost its activity4 ,at the same time the actinium solution increased in emanating power1 reaching its original value after about #fj days4 ,the behaviour of actinium and thorium is thus quite analogous1 and the explanation advanced to explain the decay and recovery curves of thorium applies equally well to the corresponding curves of actinium4 ,illustration3 ,fig4 #hb4 ,the actinium ,x is produced at a constant rate from the parent matter actinium1 and is transformed according to an exponential law with the time4 ,the constant of change #jfh 7day71 and this value is characteristic of the product actinium ,x4 ,as in the case of thorium1 the above experiments show that the emanation does not arise from actinium itself but from actinium ,x4 ,the emanation in turn breaks up and gives rise to an active deposit on the surface of bodies4 #bab4 ,analysis of the active deposit from the emanation4 ,debierne#cab observed that the excited activity produced by actinium decayed to half value in about #da minutes4 ,miss ,brooks#cac showed that the curves of decay of the excited activity after removal depended upon the duration of exposure to the emanation4 ,the curves for different times of exposure have already been shown in ,fig4 #fi4 ,bronson1 using the direct deflection method described in section #fi1 accurately determined the activity curve corresponding to a short exposure to the actinium emanation4 ,the curve obtained is shown in ,fig4 #hc4 ,illustration3 ,fig4 #hc4 ,this curve is similar in shape to the corresponding curve obtained for the active deposit from thorium1 and is explained in a similar way4 ,the activity ,it at any time t is given by frac ,it ,i,t frac e-#b t e-#a t e-#b ,t e-#a ,t 1 where and are two constants1 and ,i,t the maximum activity reached after an interval ,t4 ,after #bj minutes the activity decreased exponentially with the time1 falling to half value in #ceg minutes4 ,this gives the value #jaid 7min474 ,by comparison with the curve1 the value of was found to be #cag 7min474 ,this corresponds to a change in which half the matter is transformed in #bae minutes4 ,exactly as in the analogous curve for thorium1 it can be shown that the matter initially deposited undergoes two changes1 the first of which is a rayless one4 ,the same difficulty arises in fixing which of the values of refers to the first change4 ,an experiment made by ,miss ,brooks 7loc4 cit47 shows that the rayless product has the slower period of transformation4 ,the active deposit of actinium was dissolved off a platinum wire and then electrolysed4 ,the anode was found to be active1 and the activity fell off exponentially with the time1 decreasing to half value in about #ae minutes4 ,allowing for the difficulty of accurately measuring such a rapid rate of decay1 this result indicates that the product which gives out rays has the rapid period of #bae minutes4 ,the analysis of the active deposit of actinium thus leads to the following conclusions3 7#a7 ,the matter initially deposited from the emanation1 called actinium ,a1 does not give out rays1 and is half transformed in #ceg minutes4 7#b7 ,a changes into ,b1 which is half transformed in #bae minutes1 and gives out both and 7and probably 7 rays4 ,godlewski found that the active deposit of actinium was very easily volatilized4 ,heating for several minutes at a temperature of #ajj ,c4 was sufficient to drive off most of the active matter4 ,the active deposit is readily soluble in ammonia and in strong acids4 #bac4 ,radiations from actinium and its products4 ,actinium in radio-active equilibrium gives out 1 1 and rays4 ,godlewski found several points of distinction between the and rays of actinium and of radium4 ,the rays of actinium appear to be homogeneous1 for the activity measured by an electroscope was found to fall off accurately according to an exponential law with the thickness of matter traversed4 ,the rays were half absorbed in a thickness of #jba mm4 of aluminium4 ,this indicates that the particles are all projected from actinium with the same velocity4 ,in this respect actinium behaves very differently from radium1 for the latter gives out particles whose velocities vary over a wide range4 ,after the rays were absorbed1 another type of more penetrating rays was observed1 which probably corresponds to the rays from the other radio-elements4 ,the rays of actinium were1 however1 far less penetrating than those from radium4 ,the activity due to these rays was reduced to one-half after passing through #ai mms4 of lead1 while the thickness of lead required in order to absorb half the rays of radium is about #i mms4 ,the active deposit gave out and 7and probably 7 rays4 ,it was difficult to decide definitely whether actinium ,x gave out as well as rays4 ,when the actinium ,x was heated to a red heat1 the activity was temporarily reduced to about half its initial value4 ,this decrease was probably due to the removal of the active deposit1 which1 we have seen1 is readily volatilized by heat4 ,if the ray activity cannot be further reduced1 this would point to the conclusion that actinium ,x1 as well as actinium ,b1 gives out rays1 but the evidence so far obtained is not conclusive4 ,the ease with which the active deposit is volatilized by heat offers a very simple explanation of the initial peculiarities of the decay and recovery curves 7,fig4 #hb7 of actinium ,x4 ,the activity of actinium ,x rises at first1 but there is no corresponding decrease in the activity of the actinium left behind4 ,it has been shown that the active deposit is soluble in ammonia1 and1 in consequence1 is removed with the actinium ,x4 ,the products actinium ,a and ,b and actinium ,x1 immediately after separation1 are in radio-active equilibrium and we should not therefore expect to find any increase of activity after removal1 such as is observed in the case of thorium1 where thorium ,a and ,b are not removed with thorium ,x4 ,however1 in heating the actinium ,x to drive off the ammonium salts1 some of the active deposit is volatilized4 ,after cooling1 the amount of the active deposit increases to nearly its old value and there is a corresponding increase of the activity4 ,illustration3 ,fig4 #hd4 #bad4 ,products of ,actinium4 ,there is one very interesting point of distinction between the radio-active behaviour of thorium and actinium4 ,the latter after removal of actinium ,x1 shows only about #e per cent4 of the original activity1 while thorium1 after removal of ,th ,x1 always shows a residual activity of about #be per cent4 of the maximum value4 ,this very small residual activity indicates that actinium1 if completely freed from all its products1 would not give out rays at all1 in other words1 the first change in actinium is a rayless one4 ,the radio-active products of actinium are shown graphically in ,fig4 #hd4 ,some of their chemical and physical properties are tabulated below4 ,products ,time to be ,rays ,some ,physical and half ,chemical properties transformed ,actinium 8 ,no rays ,insoluble in ammonia ,actinium ,x #ajb days 1 7 and 7 ,soluble in ammonia ,emanation #ci secs4 rays ,behaves as a gas ,actinium ,a #ceg mins4 ,no rays ,soluble in ammonia and strong acids4 ,actinium ,b #bae mins4 1 and ,volatilized at #ajj;,c4 ,b can be separated from ,a by electrolysis ,footnote #bif3 ,soddy1 ,trans4 ,chem4 ,soc4 #ha1 p4 #dfj1 #aijb4 ,footnote #big3 ,rutherford and ,grier1 ,phil4 ,mag4 ,sept4 #aijb4 ,footnote #bih3 ,becquerel1 ,c4 ,r4 #aca1 p4 #acg1 #aijj4 ,footnote #bii3 ,meyer and ,schweidler1 ,wien ,ber4 ,dec4 #a1 #aijd4 ,footnote #cjj3 ,meyer and ,schweidler1 ,wien ,ber4 #aac1 ,july1 #aijd4 ,footnote #cja3 ,rutherford1 ,phil4 ,trans4 ,a4 #bjd1 pp4 #afi-bai1 #aijd4 ,footnote #cjb3 ,pegram1 ,phys4 ,rev4 p4 #dbd1 ,december1 #aijc4 ,footnote #cjc3 ,miss ,slater1 ,phil4 ,mag4 #aije4 ,footnote #cjd3 von ,lerch1 ,ann4 de d4 ,phys4 ,november1 #aijc4 ,footnote #cje3 ,the #8rayless change' certainly does not give out rays1 and special experiments showed that no appreciable amount of rays were present4 ,on the other hand1 the second change gives out all three types of rays4 ,footnote #cjf3 ,miss ,brooks1 ,phil4 ,mag4 ,sept4 #aijd4 ,footnote #cjg3 ,rutherford and ,soddy1 ,trans4 ,chem4 ,soc4 #ha1 p4 #hcg1 #aijb4 ,phil4 ,mag4 ,nov4 #aijb4 ,footnote #cjh3 ,miss ,brooks1 ,phil4 ,mag4 ,sept4 #aijd4 ,footnote #cji3 ,rutherford1 ,phil4 ,trans4 ,a4 p4 #afi1 #aijd4 ,footnote #caj3 ,giesel1 ,ber4 d4 ,d4 ,chem4 ,ges4 p4 #gge1 #aije4 ,footnote #caa3 ,godlewski1 ,nature1 p4 #bid1 ,jan4 #ai1 #aije4 ,footnote #cab3 ,debierne1 ,c4 ,r4 #ach1 p4 #daa1 #aijd4 ,footnote #cac3 ,miss ,brooks1 ,phil4 ,mag4 ,sept4 #aijd4 ,,chapter ,,xi4 ,,transformation ,,products ,,of ,,radium4 #bae4 ,radio-activity of radium4 ,notwithstanding the enormous difference in their relative activities1 the radio-activity of radium presents many close analogies to that of thorium and actinium4 ,both substances give rise to emanations which in turn produce 8excited activity0 on bodies in their neighbourhood4 ,radium1 however1 does not give rise to any intermediate product between the element itself and the emanation it produces1 or in other words there is no product in radium corresponding to ,th ,x in thorium4 ,giesel first drew attention to the fact that a radium compound gradually increased in activity after preparation1 and only reached a constant value after a month's interval4 ,if a radium compound is dissolved in water and boiled for some time1 or a current of air drawn through the solution1 on evaporation it is found that the activity has been diminished4 ,the same result is observed if a solid radium compound is heated in the open air4 ,this loss of activity is due to the removal of the emanation by the process of solution or heating4 ,consider the case of a radium compound which has been kept for some time in solution in a shallow vessel1 exposed to the open air1 and then evaporated to dryness4 ,the emanation which1 in the state of solution1 was removed as fast as it was formed1 is now occluded1 and1 together with the active deposit which it produces1 adds its radiations to that of the original radium4 ,the activity will increase to a maximum value when the rate of production of fresh emanation balances the rate of change of that already produced4 ,if now the compound is again dissolved or heated1 the emanation escapes4 ,since the active deposit is not volatile and is insoluble in water1 it is not removed by the process of solution or heating4 ,since1 however1 the parent matter is removed1 the activity due to the active deposit will immediately begin to decay1 and in the course of a few hours will have almost disappeared4 ,the activity of the radium measured by the rays is then found to be about #be per cent4 of its original value4 ,this residual activity of radium1 consisting entirely of rays1 is non-separable1 and has not been further diminished by chemical or physical means4 ,rutherford and ,soddy#cad examined the effect of aspiration for long intervals through a radium chloride solution4 ,after the first few hours the activity was found to be reduced to #be per cent41 and further aspiration for three weeks did not produce any further diminution4 ,the radium was then evaporated to dryness1 and the rise of its activity with time determined4 ,the results are shown in the following table4 ,the final activity in the second column is taken as one hundred4 ,in column #c is given the percentage proportion of the activity recovered4 ,time in ,activity ,percentage days ,activity recovered #j #bej #j #jgj #ccg #aag #agg #dbg #bcg #dge #fhe #ehj #ghc #hce #ghj #afj #ifj #iej #baj #ajjj #ajjj ,the results are shown graphically in ,fig4 #he4 ,the decay curve of the radium emanation is shown in the same figure4 ,the curve of recovery of the lost activity of radium is thus analogous to the curves of recovery of uranium and thorium which have been freed from the active products ,ur ,x and ,th ,x respectively4 ,the intensity ,it of the recovered activity at any time is given by frac ,it ,i #a e-t 1 where ,i is the final value1 and is the radio-active constant of the emanation4 ,the decay and recovery curves are complementary to one another4 ,illustration3 ,fig4 #he4 ,knowing the rate of decay of activity of the radium emanation1 the recovery curve of the activity of radium can thus at once be deduced1 provided all of the emanation formed is occluded in the radium compound4 ,when the emanation is removed from a radium compound by solution or heating1 the activity measured by the rays falls almost to zero1 but increases in the course of a month to its original value4 ,the curve showing the rise of and rays with time is practically identical with the curve1 ,fig4 #he1 showing the recovery of the lost activity of radium measured by the rays4 ,the explanation of this result lies in the fact that the and rays from radium only arise from the active deposit1 and that the non-separable activity of radium gives out only rays4 ,on removal of the emanation1 the activity of the active deposit decays nearly to zero1 and in consequence the and rays almost disappear4 ,when the radium is allowed to stand1 the emanation begins to accumulate1 and produces in turn the active deposit1 which gives rise to and rays4 ,the amount of and rays 7allowing for a period of retardation of a few hours7 will then increase at the same rate as the activity of the emanation1 which is continuously produced from the radium4 #baf4 ,effect of escape of emanation4 ,if the radium allows some of the emanation produced to escape into the air1 the curve of recovery will be different from that shown in ,fig4 #he4 ,for example1 suppose that the radium compound allows a constant fraction of the amount of emanation1 present in the compound at any time1 to escape per second4 ,if n is the number of emanation particles present in the compound at the time t1 the number of emanation particles changing in the time dt is ndt1 where is the constant of decay of activity of the emanation4 ,if q is the rate of production of emanation particles per second1 the increase of the number dn in the time dt is given by dn qdt ndt ndt1 or dn ----- q 7 7n4 dt ,the same equation is obtained when no emanation escapes1 with the difference that the constant is replaced by 4 ,when a steady state is reached1 dn_/dt is zero1 and the maximum value of n is equal to q_/7 74 ,if no escape takes place1 the maximum value of n is equal to q_/4 ,the escape of emanation will thus lower the amount of activity recovered in the proportion _/7 74 ,if n is the final number of emanation particles stored up in the compound1 the integration of the above equation gives frac n n #a e-7 alpha7 t 4 ,the curve of recovery of activity is thus of the same general form as the curve when no emanation escapes1 but the constant is replaced by 4 ,for example1 if 1 the equation of rise of activity is given by frac n n #a e#b t 1 and1 in consequence1 the increase of activity to the maximum will be far more rapid than in the case of no escape of emanation4 ,a very slight escape of emanation will thus produce large alterations both in the final maximum and in the curve of recovery of activity4 ,a number of experiments have been described by ,mme ,curie in her ,thse prsente la ,facult des ,sciences de ,paris on the effect of solution and of heat in diminishing the activity of radium4 ,the results obtained are in general agreement with the above view1 that #ge per cent4 of the activity of radium is due to the emanation and the excited activity it produces4 ,if the emanation is wholly or partly removed by solution or heating1 the activity of the radium is correspondingly diminished1 but the activity of the radium compound is spontaneously recovered owing to the production of fresh emanation4 ,a state of radio-active equilibrium is reached1 when the rate of production of fresh emanation balances the rate of change in the emanation stored up in the compound4 ,the differences observed in the rate of recovery of radium under different conditions were probably due to variations in the rate of escape of the emanation4 #bag4 ,it has been shown in section #aeb that the emanation is produced at the same rate in the solid as in the solution1 and all the results obtained point to the conclusion that the emanation is produced from radium at a constant rate1 which is independent of physical conditions4 ,radium1 like thorium1 shows a non-separable activity of #be per cent4 of the maximum activity1 and consisting entirely of rays4 ,the and rays arise only from the active deposit4 ,the emanation itself 7section #aef7 gives out only rays4 ,these results thus admit of the explanation given in the case of thorium 7section #acf74 ,the radium atoms break up at a constant rate with the emission of particles4 ,the residue of the radium atom becomes the atom of the emanation4 ,this in turn is unstable and breaks up with the expulsion of an particle4 ,the emanation is half transformed in four days4 ,we have seen that this emanation gives rise to an active deposit4 ,the results obtained up to this stage are shown diagrammatically below4 particle particle _/ _/ _/ _/ ,,radium ,,atom ---- ,,atom ,,of ,,emanation ---- ,,atom ,,of ,,active ,,deposit #bah4 ,analysis of the active deposit from radium4 ,we have seen in chapter ,,viii that the excited activity produced on bodies1 by the action of the radium emanation1 is due to a thin film of active matter deposited on the surface of bodies4 ,this active deposit is a product of the decomposition of the radium emanation1 and is not due to any action of the radiations on the surface of the matter4 ,the curves showing the variation of the excited activity with time are very complicated1 depending not only upon the time of exposure in the presence of the emanation1 but also upon the type of radiation used for measurement4 ,the greater portion of the activity of this deposit dies away in the course of #bd hours1 but a very small fraction still remains1 which then changes very slowly4 ,it will be shown in this chapter that at least six successive transformations occur in the active deposit4 ,the matter initially produced from the emanation is called radium ,a1 and the succeeding products ,b1 ,c1 ,d1 ,e1 ,f4 ,the equations expressing the quantity of ,a1 ,b1 ,c1444444 present at any time are very complicated1 but the comparison of theory with experiment may be much simplified by temporarily disregarding some unimportant terms3 for example1 the products ,a1 ,b1 ,c are transformed at a very rapid rate compared with ,d4 ,the activity due to ,d ,e ,f is1 in most cases1 negligible compared with that of ,a or ,c1 being usually less than of the initial activity observed for ,a or ,c4 ,the analysis of the active deposit of radium may thus be conveniently divided into two stages3 7#a7 ,analysis of the deposit of rapid change1 which is mainly composed of radium ,a1 ,b1 and ,c2 7#b7 ,analysis of the deposit of slow change1 which is composed of radium ,d1 ,e1 and ,f4 #bai4 ,analysis of the deposit of rapid change4 ,in the experiments described below1 a radium solution was placed in a closed glass vessel4 ,the emanation then collected in the air space above the solution4 ,the rod1 to be made active1 was introduced through an opening in the stopper and exposed in the presence of the emanation for a definite interval4 ,if the decay was to be measured by the rays1 the rod was made the central electrode in a cylindrical vessel such as is shown in ,fig4 #ah4 ,a saturating voltage was applied1 and the current between the cylinders measured by an electrometer4 ,if a very active rod is to be tested1 a sensitive galvanometer can be employed1 but1 in such a case1 a large voltage is required to produce saturation4 ,a slow current of dust-free air was continuously circulated through the cylinder1 in order to remove any emanation that may have adhered to the rod4 ,for experiments on the and rays1 it was found advisable to use an electroscope1 such as is shown in ,fig4 #ab1 instead of an electrometer4 ,for measurements with the rays1 the active rod was placed under the electroscope1 and before entering the vessel the rays passed through a sheet of metal of sufficient thickness to absorb all the rays4 ,for measurements with the rays1 the electroscope was placed on a lead plate #jf cms4 thick1 and the active rod placed under the lead plate4 ,the and rays were completely stopped by the lead1 and the discharge in the electroscope was then due to the rays alone4 ,the electroscope is very advantageous for measurements of this character1 and accurate observations can be made simply and readily4 ,the curve of decay of activity1 measured by the rays1 for an exposure of #a minute in the presence of the radium emanation is shown in ,fig4 #hf1 curve ,,bb4 ,the curve exhibits three stages3-- 7#a7 ,a rapid decay in the course of #ae minutes to less than #aj per cent4 of the value immediately after removal2 7#b7 ,a period of #cj minutes in which the activity varies very little2 7#c7 ,a gradual decrease almost to zero4 ,the initial drop decays very approximately according to an exponential law with the time1 falling to half value in about #c minutes4 ,three or four hours after removal the activity again decays according to an exponential law with the time1 falling to half value in about #bh minutes4 ,the family of curves obtained for different times of exposure have already been shown in ,fig4 #fg4 ,these results thus indicate3-- 7#a7 ,an initial change in which half the matter is transformed in #c minutes2 7#b7 ,a final change in which half the matter is transformed in #bh minutes4 ,illustration3 ,fig4 #hf4 ,before considering the explanation of the intermediate portion of the curve further experimental results will be considered4 ,the curve of decay of the excited activity for a long exposure 7#bd hours7 is shown graphically in ,fig4 #hf1 curve ,,aa4 ,there is at first a rapid decrease for the first #ae minutes to about #ej per cent4 of the initial value1 then a slower decay1 and1 after an interval of about #d hours1 a gradual decay nearly to zero1 according to an exponential law with the time1 falling to half value in #bh minutes4 ,the curves of variation with time of the excited activity when measured by the rays are shown graphically in ,figs4 #hg and #hh4 ,fig4 #hg is for a short exposure of #a minute4 ,fig4 #hh shows the decay for a long exposure of about #bd hours4 ,illustration3 ,fig4 #hg4 ,the curves obtained for the rays are quite different from those obtained for the rays4 ,for a short exposure1 the activity measured by the rays is at first small1 then passes through a maximum about #cf minutes after removal4 ,there is then a gradual decrease1 and after several hours the activity decays according to an exponential law1 falling1 as in the other cases1 to half value in #bh minutes4 ,the curve shown in ,fig4 #hh for the rays is very similar in shape to the corresponding curve1 ,fig4 #hf1 curve ,,aa1 for the rays1 with the exception that the rapid initial drop observed for the -ray curve is quite absent4 ,the later portions of the curve are similar in shape1 and1 disregarding the first #ae minutes after removal1 the activity decays at exactly the same rate in both cases4 ,the curves obtained by means of the rays are identical with those obtained for the rays4 ,this shows that the and rays always occur together and in the same proportion4 ,for increase of the time of exposure from #a minute to #bd hours the curves obtained are intermediate in shape between the two representative limiting curves1 ,figs4 #hg and #hh4 ,some of these curves have already been shown in ,fig4 #fh4 ,illustration3 ,fig4 #hh4 #bbj4 ,explanation of the curves4 ,it has been pointed out that the rapid initial drop for curves ,a and ,b1 ,fig4 #hf1 is due to a change giving rise to rays1 in which half of the matter is transformed in about #c minutes4 ,the absence of the drop in the corresponding curves1 when measured by the rays1 shows that the first #c-minute change does not give rise to rays2 for if it gave rise to rays1 the activity should fall off at the same rate as the corresponding -ray curve4 ,it has been shown that the activity several hours after removal decays in all cases according to an exponential law with the time1 falling to half value in about #bh minutes4 ,this is the case whether for a short or long exposure1 or whether the activity is measured by the 1 1 or rays4 ,this indicates that the final #bh-minute change gives rise to all three types of rays4 ,it will be shown that these results can be completely explained on the supposition that three successive changes occur in the deposited matter of the following character#cae3-- 7#a7 ,a change of the matter ,a initially deposited in which half is transformed in about #c minutes4 ,this gives rise only to rays4 7#b7 ,a second 8rayless0 change in which half the matter ,b is transformed in #ba minutes4 7#c7 ,a third change in which half the matter ,c is transformed in #bh minutes4 ,this gives rise to 1 1 and rays4 #bba4 ,analysis of the -ray curves4 ,the analysis of the changes is much simplified by temporarily disregarding the first #c-minute change4 ,in the course of #f minutes after removal1 three quarters of the matter ,a has been transformed into ,b and #bj minutes after removal all but #a per cent4 has been transformed4 ,the variation of the amount of matter ,b or ,c present at any time agrees more closely with the theory1 if the first change is disregarded altogether4 ,a discussion of this important point is given later 7section #bbh74 ,the explanation of the -ray curves 7see ,figs4 #hg and #hh71 obtained for different times of exposure1 will be first considered4 ,for a very short exposure1 the activity measured by the rays is small at first1 passes through a maximum about #cf minutes later1 and then decays steadily with the time4 ,the curve shown in ,fig4 #hg is very similar in general shape to the corresponding thorium and actinium curves4 ,it is thus necessary to suppose that the change of the matter ,b into ,c does not give rise to rays1 while the change of ,c into ,d does4 ,in such a case the activity 7measured by the rays7 is proportional to the amount of ,c present4 ,disregarding the first rapid change1 the activity ,it at any time t should be given by an equation of the same form 7section #bjg7 as for thorium and actinium1 viz41 frac ,it ,i,t frac e-#c t e-#b t e-#c ,t e-#b ,t 1 where ,i,t is the maximum activity observed1 which is reached after an interval ,t4 ,since the activity finally decays according to an exponential law 7half value in #bh minutes71 one of the values of is equal to #dac #aj4 ,as in the case of thorium and actinium1 the experimental curves do not allow us to settle whether this value of is to be given to or 4 ,from other data 7see section #bbf7 it will be shown later that it must refer to 4 ,thus #dac #aj 7sec74 ,the experimental curve agrees very closely with theory if #ech #aj 7sec74 ,the agreement between theory and experiment is shown by the table given below4 ,the maximum value ,i,t 7which is taken as #ajj7 is reached at a time ,t #cf minutes4 ,in order to obtain the -ray curve1 the following procedure was adopted4 ,a layer of thin aluminium was placed inside a glass tube1 which was then exhausted4 ,a large quantity of radium emanation was then suddenly introduced by opening a stop-cock communicating with the emanation vessel1 which was at atmospheric pressure4 ,the emanation was left in the tube for #ae minutes and then was rapidly swept out by a current of air4 ,the aluminium was then removed and was placed under an electroscope1 such as is shown in ,fig4 #ab4 ,the rays from the aluminium were cut off by an interposed screen of aluminium #a mm4 thick4 ,the time was reckoned from a period of #de seconds after the introduction of the emanation4 ,time in ,theoretical ,observed minutes value of value of activity activity #j #j #j #aj #eha #ee #bj #hhf #hf #cj #igc #ig #cf #ajj #ajj #dj #iih #iie #ej #icd #ib #fj #hcd #hb #hj #fcg #fae #ajj #ddh #dbe #abj #cjh #bi ,there is thus a good agreement between the calculated and observed values of the activity measured by the rays4 ,the results are satisfactorily explained if it is supposed3-- 7#a7 ,that the change ,b into ,c 7half transformed in #ba minutes7 does not give rise to rays2 7#b7 ,that the change ,c into ,d 7half transformed in #bh minutes7 gives rise to rays4 #bbb4 ,these conclusions are very strongly supported by observations of the decay measured by the rays for a long exposure4 ,the curve of decay is shown in ,fig4 #hh and ,fig4 #hi1 curve ,i4 ,illustration3 ,fig4 #hi4 ,p4 ,curie and ,danne made the important observation that the curve of decay ,c1 corresponding to that shown in ,fig4 #hh1 for a long exposure1 could be accurately expressed by an empirical equation of the form frac ,it ,i ae-#c t 7a #a7 e-#b t 1 where #ech #aj 7sec7 and #dac #aj 7sec71 and #dbj is a numerical constant4 ,i have found that within the limit of experimental error this equation represents the decay of excited activity of radium for a long exposure1 measured by the rays4 ,the equation expressing the decay of activity1 measured by the rays1 differs considerably from this1 especially in the early part of the curve4 ,several hours after removal the activity decays according to an exponential law with the time1 decreasing to half value in #bh minutes4 ,this fixes the value of 4 ,the constant and the value of are deduced from the experimental curve by trial4 ,now we have already shown 7section #bjg7 that in the case of the active deposit from thorium1 where there are two changes of constants and 1 in which only the second change gives rise to a radiation1 the intensity of the radiation is given by frac ,it ,i frac #b #b #c e-#c t frac #c #b #c e-#b t for a long time of exposure 7see equation #h1 section #aih74 ,this is an equation of the same form as that found experimentally by ,curie and ,danne4 ,on substituting the values 1 found by them1 frac #b #b #c #d;cdot#c 1 and frac #a #a #c #c;cdot#c 4 ,thus the theoretical equation agrees in form with that deduced from observation1 and the values of the numerical constants are also closely concordant4 ,if the first as well as the second change gave rise to a radiation1 the equation would be of the same general form1 but the value of the numerical constants would be different1 the values depending upon the ratio of the ionization in the first and second changes4 ,if1 for example1 it is supposed that both changes give out rays in equal amounts1 it can readily be calculated that the equation of decay would be frac ,it ,i frac cdot#eb #b #c e-#c t cdot#e 7frac #c #b #c - #a7 e-#b t 4 ,taking the values of and found by ,curie1 the numerical factor e-#b t becomes #bae instead of #dc and #aae instead of #cc4 ,the theoretical curve of decay in this case would be readily distinguishable from the observed curve of decay4 ,the fact that the equation of decay found by ,curie and ,danne involves the necessity of an initial rayless change can be shown as follows3-- ,curve ,i 7,fig4 #hi7 shows the experimental curve4 ,at the moment of removal of the body from the emanation 7disregarding the initial rapid change71 the matter must consist of both ,b and ,c4 ,consider the matter which existed in the form ,c at the moment of removal4 ,it will be transformed according to an exponential law1 the activity falling by one-half in #bh minutes4 ,this is shown in curve ,,ii4 ,curve ,,iii represents the difference between the ordinates of curves ,i and ,,ii4 ,it will be seen that it is identical in shape with the curve 7,fig4 #hg7 showing the variation of the activity for a short exposure1 measured by the rays4 ,it passes through a maximum at the same time 7about #cf minutes74 ,the explanation of such a curve is only possible on the assumption that the first change is a rayless one4 ,the ordinates of curve ,,iii express the activity added in consequence of the change of the matter ,b1 present after removal1 into the matter ,c4 ,the matter ,b present gradually changes into ,c1 and this1 in its change to ,d1 gives rise to the radiation observed4 ,since the matter ,b alone is considered1 the variation of activity with time due to its further changes1 shown by curve ,,iii1 should agree with the curve obtained for a short exposure 7see ,fig4 #hg71 and this1 as we have seen1 is the case4 ,the agreement between theory and experiment is shown in the following table4 ,the first column gives the theoretical curve of decay for a long exposure deduced from the equation frac ,it ,i frac #b #b #c e-#c t frac #c #b #c e-#b t taking the value of #ech #aj and #dac #aj4 ,time in ,calculated ,observed minutes values values #j #ajj #ajj #aj #ifh #igj #bj #hid #hhe #cj #ghf #gge #dj #fib #fge #ej #eii #egj #fj #dib #dhb #hj #cdb #cce #ajj #bbg #bbe #abj #adi #ade ,the second column gives the observed activity 7measured by means of an electroscope7 for a long exposure of #bd hours in the presence of the emanation4 ,in cases where a steady current of air is drawn over the active body1 the observed values are slightly lower than the theoretical4 ,this is probably due to a slight volatility of the product radium ,b at ordinary temperatures4 ,illustration3 ,fig4 #ij4 #bbc4 ,analysis of the -ray curves4 ,the analysis of the decay curves of the excited activity of radium1 measured by the rays1 will now be discussed4 ,the following table shows the variation of the intensity of the radiation after a long exposure in the presence of the radium emanation4 ,a platinum plate was made active by exposure for several days in a glass tube containing a large quantity of emanation4 ,the active platinum after removal was placed on the lower of two parallel insulated lead plates1 and a saturating electromotive force of #fjj volts was applied4 ,the ionization current was sufficiently large to be measured by means of a sensitive high-resistance galvanometer1 and readings were taken as quickly as possible after removal of the platinum from the emanation vessel4 ,the initial value of the current 7taken as #ajj7 was deduced by continuing the curves backwards to meet the vertical axis 7see ,fig4 #ij71 and was found to be #c #aj ampere4 ,time in ,current minutes #j #ajj #b #hj #d #fie #f #fbd #h #egf #aj #ebj #ae #dhd #bj #ded #cj #djd #dj #cef #ej #cjd #fj #bed #hj #agd #ajj #aaf #abj #gf ,these results are shown graphically in the upper curve of ,fig4 #ij4 ,the initial rapid decrease is due to the decay of the activity of the matter ,a4 ,if the slope of the curve is produced backwards from a time #bj minutes after removal1 it cuts the vertical axis at about #ej4 ,the difference between the ordinates of the curves ,a ,b ,c and ,,ll at any time is shown in the curve ,,aa4 ,the curve ,,aa represents the activity at any time supplied by the change in radium ,a4 ,the curve ,,ll starting from the vertical axis is identical with the curve already considered1 representing the decay of activity measured by the rays for a long exposure 7see ,fig4 #hh74 ,time in ,calculated ,observed minutes value of value of activity activity #j #ajj #ajj #aj #ifh #igj #bj #hid #hib #cj #ghf #hjh #dj #fib #gab #ej #eii #fjh #fj #dib #eja #hj #cdb #cdh #ajj #bbg #bcb #abj #adi #aeb ,this is shown by the agreement of the numbers in the above table4 ,the first column in the table above gives the theoretical values of the activity deduced from the equation frac ,it ,i frac #b #b #c e-#c t frac #c #b #c e-#b t for the values of 1 previously employed4 ,the second column gives the observed values of the activity deduced from the decay curve ,,ll4 ,the close agreement of the curve ,,ll with the theoretical curve deduced on the assumption that there are two changes1 the first of which does not emit rays1 shows that the change of radium ,b into ,c does not emit rays4 ,in a similar way1 as in the curve ,i1 ,fig4 #hi1 the curve ,,ll may be analysed into its two components represented by the two curves ,,cc and ,,bb4 ,the curve ,,cc represents the activity supplied by the matter ,c present at the moment of removal4 ,the curve ,,bb represents the activity resulting from the change of ,b into ,c and is identical with the corresponding curve in ,fig4 #hi4 ,using the same line of reasoning as before1 we may thus conclude that the change of ,b into ,c is not accompanied by rays4 ,it has already been shown that it does not give rise to rays1 and the identity of the and -ray curves shows that it does not give rise to rays4 ,the change of ,b into ,c is thus a 8rayless0 change1 while the change of ,c into ,d gives rise to all three kinds of rays4 ,an analysis of the decay of the excited activity of radium thus shows that three distinct rapid changes occur in the matter deposited1 viz43-- 7#a7 ,the matter ,a1 derived from the change in the emanation1 is half transformed in #c minutes and is accompanied by rays alone2 7#b7 ,the matter ,b is half transformed in #ba minutes and gives rise to no ionizing rays2 7#c7 ,the matter ,c is half transformed in #bh minutes and is accompanied by 1 1 and rays2 7#d7 ,a fourth very slow change will be discussed later4 #bbd4 ,equations representing the activity curves4 ,the equations representing the variation of activity with time are for convenience collected below1 where #ch #aj1 #ech #aj1 #dac #aj3-- 7#a7 ,short exposure3 activity measured by rays1 frac ,it ,i,t #aj;cdot#c 7e-#c t e-#b t7 where ,i,t is the maximum value of the activity2 7#b7 ,long exposure3 activity measured by rays1 frac ,it ,i #d;cdot#c 7e-#c t #c;cdot#c e-#b t7 1 where ,i is the initial value2 7#c7 ,any time of exposure ,t3 activity measured by the rays1 frac ,it ,i frac ae-#c t be-#b t a b 1 where a frac #a e-#c ,t #c 1 b frac #a e-#b ,t #b 2 7#d7 ,activity measured by rays3 long time of exposure1 frac ,it ,i frac #ab e-#a t frac #ab 7#d;cdot#c e-#c t #c;cdot#c e-#b t7 4 ,the equations for the rays for any time of exposure can be readily deduced1 but the expressions are somewhat complicated4 ,illustration3 ,fig4 #ia4 #bbe4 ,equations of rise of excited activity4 ,the curves expressing the gradual increase to a maximum of the excited activity produced on a body exposed in the presence of a constant amount of emanation are complementary to the curves of decay for a long exposure4 ,the sum of the ordinates of the rise and decay curves is at any time a constant4 ,this follows necessarily from the theory and can also be deduced simply from priori considerations4 7,see section #bjj47 ,the curves of rise and decay of the excited activity for both the and rays are shown graphically in ,fig4 #ia4 ,the thick line curves are for the rays4 ,the difference between the shapes of the decay curves when measured by the or rays is clearly brought out in the figure4 ,the equations representing the rise of activity to a maximum are given below4 ,for the and rays1 frac ,it ,imax #a 7#d;cdot#c e-#c t #c;cdot#c e-#b t7 4 ,for the rays1 frac ,it ,imax #a frac #ab e-#a t frac #ab 7#d;cdot#c e-#c t #c;cdot#c e-#b t7 4 #bbf4 ,effect of temperature4 ,we have so far not considered the evidence on which the #bh-minute rather than the #ba-minute change is supposed to take place in the matter ,c4 ,this evidence has been supplied by some recent important experiments of ,p4 ,curie and ,danne#caf on the volatilization of the active matter deposited by the emanation4 ,miss ,gates#cag showed that this active matter was volatilized from a platinum wire above a red heat and deposited on the surface of a cold cylinder surrounding the wire4 ,curie and ,danne extended these results by subjecting an active platinum wire for a short time to the action of temperatures varying between #ae ,c4 and #acej ,c41 and then examining at room temperatures the decay curves not only for the active matter remaining on the wire1 but also for the volatilized part4 ,they found that the activity of the distilled part always increased after removal1 passed through a maximum1 and finally decayed according to an exponential law to half value in #bh minutes4 ,at a temperature of about #fcj ,c4 the active matter left behind on the wire decayed at once according to an exponential law1 falling to half value in #bh minutes4 ,p4 ,curie and ,danne showed that the matter ,b is much more volatile than ,c4 ,the former is completely volatilized at about #fjj ,c41 while the latter is not completely volatilized even at a temperature of #acjj ,c4 ,the fact that the matter ,c1 left behind when ,b is completely volatilized1 decays at once to half value in #bh minutes shows that the matter ,c itself and not ,b is half transformed in #bh minutes4 ,curie and ,danne also found that the rate of decay of the active matter varied with the temperature to which the platinum wire had been subjected4 ,at #fcj ,c4 the rate of decay was normal1 at #aajj ,c4 the activity fell to half value in about #bj minutes1 while at #acjj ,c4 it fell to about half value in about #be minutes4 ,i have repeated the experiments of ,curie and ,danne and obtained very similar results4 ,it was thought possible that the measured rate of decay observed after heating might be due to a permanent increase in the rate of volatilization of ,c at ordinary temperatures4 ,this explanation1 however1 is not tenable1 for it was found that the activity decreased at the same rate whether the activity of the wire was tested in a closed tube or in the open with a current of air passed over it4 ,these results are of great importance1 for they indicate that the rate of change of the product ,c is not a constant1 but is affected by differences of temperature4 ,this is the first case where temperature has been shown to exert an appreciable influence on the rate of change of any radio-active product4 #bbg4 ,volatility of radium ,b at ordinary temperature4 ,miss ,brooks#cah has observed that a body1 made active by exposure to the radium emanation1 possesses the power of exciting secondary activity on the walls of a vessel in which it is placed4 ,this activity was usually about of the whole1 but the amount was increased to about if the active wire was washed in water and dried over a gas flame--the method often adopted to free the wire of any trace of the radium emanation4 ,this effect of producing activity was most marked immediately after removal of the wire from the emanation1 and was almost inappreciable ten minutes afterwards4 ,the effect was particularly noticeable in some experiments with a copper plate1 which was made active by leaving it a short time in a solution of the active deposit from radium4 ,this active solution was obtained by placing an active platinum wire in dilute hydrochloric acid4 ,on placing the copper plate in a testing vessel for a few minutes1 and then removing it1 activity was observed on the walls of the vessel amounting to about one per cent4 of the activity of the copper plate4 ,it was found that this effect was not due to the emission of an emanation from the active body1 but must be ascribed to a slight volatility of radium ,b at ordinary temperatures4 ,this was proved by observations on the variation of the activity of the matter deposited on the walls of the vessel4 ,the activity was small at first1 but rose to a maximum after about #cj minutes1 and then decayed with the time4 ,the curve of rise was very similar to that shown in ,fig4 #hg1 and shows that the inactive matter radium ,b was carried to the walls and there changed into ,c1 which gave rise to the radiation observed4 ,the product ,b only escapes from the body for a short time after removal4 ,this is a strong indication that its apparent volatility is connected with the presence of the rapidly changing product radium ,a4 ,since ,a breaks up with an expulsion of an particle1 some of the residual atoms constituting radium ,b may acquire sufficient velocity to escape into the gas1 and are then transferred by diffusion to the walls of the vessel4 ,miss ,brooks observed that the activity was not concentrated on the negative electrode in an electric field but was diffused uniformly over the walls of the vessel4 ,this observation is of importance in considering the explanation of the anomalous effects exhibited by the active deposit of radium1 which will be discussed in the following section4 #bbh4 ,effect of the first rapid change4 ,we have seen that the law of decay of activity1 measured by the or rays1 can be explained very satisfactorily if the first #c-minute change is disregarded4 ,the full theoretical examination of the question given in sections #aig and #aih and the curves of ,figs4 #gb and #gc show1 however1 that the presence of the first change should exercise an effect of sufficient magnitude to be detected in measurements of the activity due to the succeeding changes4 ,the question is of great interest1 for it involves the important theoretical point whether the substances ,a and ,b are produced independently of one another1 or whether ,a is the parent of ,b4 ,in the latter case1 the matter ,a which is present changes into ,b1 and1 in consequence1 the amount of ,b present after ,a is transformed should be somewhat greater than if ,b were produced independently4 ,since the change of ,a is fairly rapid1 the effect should be most marked in the early part of the curve4 ,in order to examine this point experimentally1 the curve of rise of activity1 measured by the rays1 was determined immediately after the introduction of a large quantity of the radium emanation into a closed vessel4 ,the curve of decay of activity on a body for a long exposure after removal of the emanation1 and the rise of activity after the introduction of the emanation1 are in all cases complementary to one another4 ,while1 however1 it is difficult to measure with certainty whether the activity has fallen in a given time1 for example1 from #ajj to #ii or #ihe1 it is easy to be sure whether the corresponding rise of activity in the converse experiment is #a or #ae per cent4 of the final amount4 ,fig4 #ib1 curve ,i1 shows the rise of activity 7measured by the rays7 obtained for an interval of #bj minutes after the introduction of the emanation4 ,the ordinates represent the percentage amount of the final activity regained at any time4 ,curve ,,iii shows the theoretical curve obtained on the assumption that ,a is a parent of ,b4 ,this curve is calculated from equation 7#i7 discussed in section #aih1 and 1 1 are the values previously found4 ,curve ,,ii gives the theoretical activity at any time on the assumption that the substances ,a and ,b arise independently4 ,this is calculated from an equation of the same form as 7#h71 section #aih4 ,illustration3 ,fig4 #ib4 ,it is seen that the experimental results agree best with the view that ,a and ,b arise independently4 ,such a conclusion1 however1 is of too great importance to be accepted before examining closely whether the theoretical conditions are fulfilled in the experiments4 ,in the first place1 it is assumed that the carriers which give rise to excited activity are deposited on the surface of the body1 to be made active immediately after their formation4 ,there is some evidence1 however1 that some of these carriers exist for a considerable interval in the gas before their deposit on the body4 ,for example1 it is found that if a body is introduced for a short interval1 about #a minute1 into a vessel containing the radium emanation1 which has remained undisturbed for several hours1 the activity after the first rapid decay 7see ,fig4 #hf1 curve ,b7 is in much greater proportion than if an electric field had been acting for some time previously4 ,this result indicates that the carriers of ,b and ,c both collect in the gas and are swept to the electrode when an electric field is applied4 ,i have also observed that if radium emanation1 which has stood undisturbed for some time1 is swept into a testing vessel1 the rise curve is not complementary to the decay curve1 but indicates that a large amount of radium ,b and ,c was present with the emanation4 ,the experiments of ,miss ,brooks1 previously referred to1 indicate that radium ,b does not obtain a charge and so will remain in the gas4 ,dr ,bronson1 working in the laboratory of the writer1 has obtained evidence that a large amount of radium ,d remains in the gas even in a strong electric field4 ,if the matter ,b exists to some extent in the gas1 the difference between the theoretical curves for three successive changes would be explained2 for1 in transferring the emanation to another vessel1 the matter ,b mixed with it would commence at once to change into ,c and give rise to a part of the radiation observed4 ,the equal division of the activity between the products ,a and ,c 7see ,fig4 #ij7 supports the view that ,c is a product of ,a1 for when radio-active equilibrium is reached1 the number of particles of ,a changing per second is equal to the number of ,b or ,c changing per second4 ,if each atom of ,a and ,c expels an particle of the same mass and with the same average velocity1 the activity due to the matter ,a should be equal to that due to the matter ,c2 and this1 as we have seen1 is the case4 ,while it is a matter of great difficulty to give a definite experimental proof that radium ,a and ,b are consecutive products1 ,i think there is little doubt of its correctness4 ,accurate determinations of the curves of rise and decay may throw further light on the complicated processes which undoubtedly occur between the breaking up of the atoms of the emanation and the appearance of the active deposit on the electrodes4 #bbi4 ,relative activity supplied by the -ray products of radium4 ,there are four products in radium which give out rays1 viz4 radium itself1 the emanation1 radium ,a and ,c4 ,if these products are in radio-active equilibrium1 the same number of particles of each product are transformed per second and1 if each atom breaks up with the emission of one particle1 the number of particles expelled per second should be the same for each product4 ,since1 however1 the particles from the different products are not projected with the same velocity1 the activity1 measured by the ionization current in the usual manner1 will not be the same for all products4 ,the activity1 when measured by the saturation current between parallel plates at sufficient distance apart to absorb all the rays in the gas1 is proportional to the energy of the particles escaping into the gas4 ,it has been shown that the minimum activity of radium after removal of the emanation1 measured by the rays1 is #be per cent4 of the maximum value4 ,the remaining #ge per cent4 is due to the particles from the other products4 ,now the activity supplied by radium ,a and ,c is nearly the same 7section #bbh74 ,if the emanation is introduced into a cylindrical vessel about #e cms4 in diameter1 the activity increases to about twice its initial value owing to the deposit of radium ,a and ,c on the surface of the vessel4 ,this shows that the activity of the emanation is of about the same magnitude as that supplied by radium ,a or ,c1 but an accurate comparison is beset with difficulty1 for the emanation is distributed throughout the gas1 while radium ,a and ,c are deposited on the walls of the vessel4 ,in addition1 the relative absorption of the emanation compared with that of radium ,a and ,c is not known4 ,the writer has made some experiments on the decrease of activity of radium immediately after heating to a sufficient temperature to drive off the emanation4 ,the results obtained by this method are complicated by the alteration of the radiating surface in consequence of the heating1 but indicate that the emanation supplies about #gj per cent4 of the activity of radium ,a or ,c4 ,this points to the conclusion that the particles from the emanation are projected with less velocity than those from radium ,c4 ,the following table shows approximately the activity supplied by the different products of radium in radio-active equilibrium4 ,product ,percentage proportion of total activity ,radium #be per cent4 ,emanation #ag ,radium ,a #bi ,radium ,b #j ,radium ,c #bi ,the products of radium and their radiation are graphically shown later in ,fig4 #ie4 #bcj4 ,active deposit of radium of slow transformation4 ,it has been pointed out 7section #ahc7 that a body1 exposed in the presence of the radium emanation1 does not lose all its activity for a long time after removal2 a small residual activity is always observed4 ,the magnitude of this residual activity is dependent not only upon the amount of emanation employed1 but also upon the time of exposure of the body in the presence of the emanation4 ,for an exposure of several hours in the presence of the emanation1 the residual activity is less than one-millionth of the activity immediately after removal4 ,an account will now be given of some investigations made by the writer#cai on the nature of this residual activity and the chemical properties of the active matter itself4 ,it is first of all necessary to show that the residual activity arises in consequence of a deposit of radio-active matter1 and is not due to some action of the intense radiations to which the body made active has been subjected4 ,the inside of a long glass tube was covered with equal areas of thin metal1 including aluminium1 iron1 copper1 silver1 lead1 and platinum4 ,a large amount of radium emanation was introduced into the tube1 and the tube closed4 ,after seven days the metal plates were removed1 and1 after allowing two days to elapse for the ordinary excited activity to disappear1 the residual activity of the plates was tested by an electrometer4 ,the activity of the plates was found to be unequal1 being greatest for copper and silver1 and least for aluminium4 ,the activity of copper was twice as great as that of aluminium4 ,after standing for another week the activity of the plates was again tested4 ,the activity of each had diminished in the interval to some extent1 but the initial differences observed had to a large extent disappeared4 ,after reaching a minimum value the activity of each plate slowly but steadily increased at the same rate4 ,after a month's interval the activity of each of the plates was nearly the same1 and more than three times the minimum value4 ,the initial irregularities in the decay curves of the different metals are1 in all probability1 due to slight but different degrees of absorption of the radium emanation by the metal plates1 the absorption being greatest for copper and silver and least for aluminium4 ,as the occluded emanation was slowly released or lost its activity1 the activity of the metal fell to a limiting value4 ,the absorption of the radium emanation by lead1 paraffin1 and caoutchouc has been noticed by ,curie and ,danne 7section #ahb74 ,the residual activity on the plates comprised both and rays1 the latter being present1 in all cases1 in a very unusual proportion4 ,the equality of the activity and the identity of the radiation emitted from each plate show that the residual activity is due to changes of some form of matter deposited on the plates1 and that it cannot be ascribed to an action of the intense radiations2 for if such were the case1 it would be expected that the activity produced on the different plates would vary not only in quantity1 but also in quality4 ,this result is confirmed by the observation that the active matter can be removed from a platinum plate by solution in sulphuric acid1 and has other distinctive chemical and physical properties4 ,the variation with time of the residual activity measured by the rays will first be considered4 ,a platinum plate was exposed in the presence of the radium emanation for seven days4 ,the amount of emanation initially present was equal to that obtained from about #c milligrams of pure radium bromide4 ,the plate immediately after removal gave a saturation-current1 measured between parallel plates by a galvanometer1 of #ae #aj ampere4 ,some hours after removal1 the activity decayed according to an exponential law with the time1 falling to half value in #bh minutes4 ,three days after removal the active plate gave a saturation-current1 measured by an electrometer1 of #e #aj ampere2 i4e4 #j1jjj of the initial activity4 ,the activity was observed to increase steadily with the time4 ,the results are shown in ,fig4 #ic1 where the time is reckoned from the middle of the time of exposure to the emanation4 ,the curve is initially nearly a straight line passing through the origin4 ,the activity increases with the time for the interval of eight months over which the observations have extended4 ,the latter portions of the curve1 however1 fall below the tangent to the curve drawn through the origin1 showing that the activity is not increasing proportionately with the time4 ,the active deposit1 obtained in a different manner1 has been examined for a still longer period4 ,the emanation from #cj milligrams of radium bromide was condensed in a glass tube and then sealed4 ,after a month's interval1 the tube was opened and dilute sulphuric acid introduced4 ,the acid dissolved off the active deposit in the tube and on driving off the acid by heat1 a radio-active residue was obtained4 ,the activity of this residue1 measured by the rays1 steadily increased for a period of #ah months1 but the curve of variation of activity with time plotted as in ,fig4 #ic tends to become more flattened1 and is obviously approaching a maximum value4 ,illustration3 ,fig4 #ic4 ,the explanation of this curve will be considered later in section #bcf4 #bca4 ,variation of the ray activity4 ,the residual activity consists of both and rays1 the latter being present initially in an unusually large proportion4 ,the proportion of to rays from the platinum plate1 one month after removal1 was at the most one-fiftieth of that from a thin film of radium bromide in radio-active equilibrium4 ,unlike the ray activity1 the activity measured by the rays remains constant after the active deposit is about one month old1 and1 in consequence1 the proportion of to rays steadily increases with the time4 ,the experiments showed that the intensity of the rays did not vary much1 if at all1 over a further period of eighteen months4 ,the want of proportionality between the and rays shows that the two types of rays arise from different products4 ,this conclusion is confirmed by experiments1 to be described later1 which show that the products giving rise to and rays can be temporarily separated from one another by physical and chemical means4 ,illustration3 ,fig4 #id4 ,if observations of the active deposit are begun shortly after its formation1 it is found that the activity1 measured by the rays1 is small at first1 but increases with the time1 reaching a practical maximum about #dj days later4 ,experiments were made on a platinum plate1 which was exposed for #cge days in a vessel containing the radium emanation4 ,the observations of the ray activity began #bd hours after removal4 ,the results are shown in ,fig4 #id1 where the time was measured from the middle of the time of exposure to the emanation4 ,similar results were obtained for a negatively charged wire exposed to the emanation4 ,the curve1 if produced back to the origin1 is seen to be very similar to the recovery curves of ,ur ,x1 and other active products1 and can be expressed by the equation frac ,it ,i #a e-t 1 where ,i is the maximum activity4 ,the activity reaches half its final value in about six days1 and the value of is equal to #aae 7day74 ,we have shown in section #bjc that a rising curve of this character indicates that the ray activity arises from a product which is supplied at a constant rate from a primary source4 ,before discussing in detail the explanation of these curves1 showing the rise with time of the and ray activity1 further experimental results will be considered4 #bcb4 ,effect of temperature on the activity4 ,a platinum plate1 made active in the manner described1 was exposed to varying temperatures in an electric furnace1 and the activity tested at atmospheric temperature after exposure4 ,four minutes' exposure in the furnace1 at first at #dcj ,c41 and afterwards at #hjj ,c41 had little1 if any1 effect on the activity4 ,after four minutes at about #ajjj ,c4 the activity decreased about #bj per cent41 and a further exposure of eight minutes at a temperature of about #ajej ,c4 almost completely removed the ray activity4 ,on the other hand1 the ray activity1 when measured immediately after removal1 was not altered by the heating1 but exposure to a still higher temperature caused it to decrease4 ,these results show that the active matter consists of two kinds4 ,the part which emits rays is not volatile at #ajjj ,c41 but the other part1 which emits rays1 is almost completely volatilized at that temperature4 ,it was found1 however1 that the ray activity after heating to about #ajjj was not permanent1 but decayed according to an exponential law with the time1 the activity decreasing to half value in about #de days4 ,from the recovery curve of the ray activity already considered1 it was to be expected that the activity would decay to half value in six days4 ,this difference in the periods is possibly due to an effect of the high temperature in altering the rate of decay of radium ,e4 ,the period of six days is more probably correct4 ,the results obtained on the rise and decay of the rays1 taken together1 show3-- 7#a7 ,that the product giving rays is supplied at a constant rate from some parent matter of very slow rate of change4 7#b7 ,that this parent matter is volatilized at or below #ajjj ,c41 and the ray product is left behind4 ,since the parent matter is removed1 the product immediately begins to lose its activity at its characteristic rate1 viz4 the activity falls to half value in about six days4 #bcc4 ,separation of the constituents by means of a bismuth plate4 ,the active matter of slow decay was obtained in solution by introducing dilute sulphuric acid into a glass tube in which the emanation from #cj milligrams of radium bromide had been stored for a month4 ,the solution showed strong activity and gave out both and rays1 the latter1 as in other cases1 being present in an unusually large proportion4 ,when a polished bismuth disk was kept for some hours in the solution1 it became strongly active4 ,the active matter deposited on the bismuth gave out rays1 but no trace of rays4 ,after several bismuth disks had been successively left in the solution1 the active matter1 which emits rays1 was almost completely removed4 ,this was shown by evaporating down the solution after treatment4 ,the ray activity remained unchanged1 but that of the rays had been reduced to about #aj per cent4 of its original value4 ,three bismuth disks1 made active in this way1 were set aside and their activity measured at regular intervals4 ,the activity fell off according to an exponential law with the time during the #bjj days since their removal1 while that of each fell to half value on an average in about #adc days4 ,at the same time it was observed that the solution1 from which the ray activity was removed1 gradually regained its activity1 showing that the active substance which gave out rays was continuously produced from the matter left behind in the solution4 #bcd4 ,explanation of the results4 ,we have seen that a close examination of the active deposit of slow change has disclosed1 7#a7 the presence of a ray product which loses half of its activity in about six days2 7#b7 the presence of an ray product1 which is deposited on bismuth and is volatilized at #ajjj ,c4 ,this product loses half of its activity in #adc days2 7#c7 the presence of a parent substance1 which produces the ray product at a constant rate4 ,this parent product must be transformed very slowly since the ray product1 which arises from it1 soon reaches an equilibrium value1 which does not change appreciably over a period of more than one year4 ,the experimental evidence points to the conclusion that the parent product does not give rise to rays1 but that the rays arise entirely from the next product4 ,this parent product cannot give rise to rays1 for we have seen that the initial ray activity is at first extremely small1 but increases steadily with the time for a period of at least eighteen months4 ,thus the parent product does not give rise to either or rays1 and must be a 8rayless0 product4 ,the first three transition products of the radium emanation1 viz4 radium ,a1 ,b and ,c1 have already been analysed1 and shown to be consecutive4 ,it thus seems probable that the active deposit of slow change must arise from the successive transformations of the last product radium ,c4 ,the results already obtained can be completely explained if it is supposed that three transition products1 viz4 radium ,d1 ,e and ,f1 are present in the active deposit of slow rate of change4 ,the properties of these products are summarized below4 ,radium ,d is a rayless product of very slow rate of change4 ,it will be shown later that it is half transformed in about #dj years4 ,it is volatile below #ajjj ,c4 and is soluble in strong acids4 ,radium ,e is produced from radium ,d4 ,in breaking up1 it emits 7and probably 7 rays but no rays4 ,it is half transformed in about #f days and is not so volatile as radium ,d and ,f4 ,radium ,f is produced from radium ,e4 ,it emits only rays and is half transformed in #adc days4 ,this substance in solution attaches itself to bismuth4 ,it is volatile at about #ajjj ,c4 ,apart from their value and interest in showing the stages of transformation of the radium atom1 the results of this analysis have an important bearing upon the origin of some of the well-known radio-active substances separated from pitchblende2 for it will be shown later that the product radium ,f is the radio-active substance present in radio-tellurium and probably also in polonium4 ,in addition1 there is very strong evidence that the radio-active lead obtained by ,hofmann contains the three products radium ,d1 ,e and ,f together4 ,the changes of radium as far as they are at present known1 are shown diagrammatically in ,fig4 #ie4 ,it is possible that further investigation will show that the transformation does not end with radium ,f4 ,illustration3 ,fig4 #ie4 ,while we have shown that radium ,d is the parent of ,e1 we have not given any conclusive evidence that ,e is the parent of ,f4 ,this evidence is1 however1 supplied by the following experiment4 ,a platinum plate1 made active in the manner already described1 was placed in an electric furnace and heated for four minutes at about #ajjj ,c4 ,most of the products ,d and ,f were volatilized1 but ,e was left behind4 ,since the parent matter ,d was removed1 ,e at once commenced to lose its ray activity4 ,at the same time it was observed that the small ray activity1 left behind on the platinum plate1 increased rapidly at first and then more slowly1 as the activity of ,e became smaller and smaller4 ,this experiment shows conclusively that ,e was the parent of ,f1 the ray product4 #bce4 ,rate of transformation of radium ,d4 ,it has been observed experimentally that each of the products of radium1 which emit rays1 supplies about an equal proportion of the activity of radium when in radio-active equilibrium4 ,since1 when equilibrium is reached1 the same number of particles of each of the successive products must break up per second1 this is an expression of the fact that every atom of each product breaks up with the expulsion of an equal number 7probably one7 of particles4 ,now radium ,d is directly derived from radium ,c1 and1 since the rate of change of ,d is very slow compared with that of ,c1 the number of particles of ,d initially present must be very nearly equal to the number of particles of radium ,c which break up during the time that radium ,d is being formed4 ,now ,d does not itself give out rays1 but the succeeding product ,e does4 ,the products ,d and ,e are practically in radio-active equilibrium one month after ,d is set aside1 and the variation of the ray activity of ,e then serves as a measure of the variation of the parent product ,d4 ,suppose that a vessel is filled with a large quantity of radium emanation4 ,after several hours1 the product radium ,c1 which emits rays1 reaches a maximum value1 and then decreases at the same rate as the emanation loses its activity1 i4e4 it falls to half value in #ch days4 ,if ,n is the number of particles expelled from radium ,c at its maximum value1 the total number ,q of particles expelled during the life of the emanation is given approximately by ,q#a intinfty ,n#a e-#a t dt frac ,n#a #a 1 where is the constant of change of the emanation4 ,after the emanation has disappeared1 and the final products ,d ,e are in radio-active equilibrium1 suppose that the number of particles ,n expelled per second by radium ,e is determined4 ,if ,q is the total number of particles expelled during the life of ,d ,e1 then ,q as before is approximately given by ,q ,n_/ where is the constant of change of radium ,d4 ,now we have seen that if each particle of ,c and of ,e gives rise to one particle1 it is to be expected that ,q ,q1 or ,n ---- ---- 4 ,n ,the ratio ,n_/,n was determined by measuring the activity due to the rays from ,c and ,e in the same testing-vessel4 ,then1 since ,n_/,n is known1 and also the value of 1 the value of the constant of change1 1 of radium ,d is obtained4 ,in this way it was calculated that ,d is half transformed in about #dj years4 ,in the above calculations it is assumed1 as a first approximation1 that the rays from ,c and ,e have the same average velocity4 ,this is probably not accurately the case1 but the above number certainly serves to fix the order of magnitude of the period of the product ,d4 ,this calculation is confirmed by observations to be given later on the amount of ,d and ,e in old radium4 ,it may be of interest to mention that the writer calculated the period of radium ,f by a similar method1 before its value was experimentally determined1 and found that ,f should be half transformed in about one year4 ,this is not very different from the experimental value of #adc days found later4 ,in addition1 it was assumed in the calculation that the particles from ,c and ,f were projected with the same velocity1 and in consequence produced the same amount of ionization4 ,in practice1 however1 it is found that the particle of ,f is absorbed in about half the distance of the particles of ,c1 and in consequence produces only about half of the ionization of the latter4 ,if this correction were made1 the calculated period for half transformation would be six months instead of one year4 ,a table of the transformation products of radium1 together with some of their physical and chemical properties1 is given below4 ,transformation ,time to be ,rays ,chemical and ,products half ,physical transformed ,properties ,radium #abjj years rays -- ,emanation #ch days rays ,chemically inert gas2 condenses at #aej ,c4 ,radium ,a #c mins4 rays ,behaves as 7active deposit solid2 of rapid deposited on change7 the surface of bodies2 concentrated on cathode in electric field4 ,soluble in strong acids2 volatile at a white heat4 ,b is more volatile than ,a or ,c4 33 ,b 7same7 #ba mins4 no rays ,same 33 ,c 7same7 #bh mins4 1 1 rays ,same 33 ,d 7active about #dj no rays ,soluble in deposit of slow years strong acids change7 and volatized below #ajjj ,c4 33 ,e 7same7 #f days 7and 7 ,non-volatile at #ajjj;,c4 33 ,f 7same7 #adc days rays ,volatile at #ajjj ,c2 deposited from solution on to bismuth plate4 8 -- -- -- #bcf4 ,variation of the activity over long periods of time4 ,we are now in a position to calculate the variation of the and ray activity of the active deposit over long periods of time4 ,if it is supposed that the matter initially deposited consists only of ,d1 the amounts ,p1 ,q and ,r of radium ,d1 ,e and ,f existing at any later time are given by the equations #c1 #d1 #e1 section #aig4 ,since1 however1 the intermediate product ,e has a much more rapid rate of change than ,d or ,f1 the equations can be simplified1 without much loss of accuracy1 by disregarding the change ,e1 and by supposing that ,d gives out rays and changes directly into the ray product ,f4 ,let 1 be the constants of change ,d and ,f respectively4 ,let n be the number of particles of ,d present initially4 ,then using the notation of section #aig1 the amount ,p of radium ,d at any time t is given by ,p n e-#a t 4 ,the amount ,q of radium ,f is given by ,q frac n #a #a #b 7e-#b t - e-#a t7 4 ,illustration3 ,fig4 #if4 ,the number of particles emitted by ,d ,e per second1 some months afterwards1 is #a n e-#a t 1 and the number of particles emitted by radium ,f is frac #a #b n #a #b 7e-#b t e-#a t 7 4 ,the results are shown graphically in ,fig4 #if1 by the curves ,,ee and ,,ff1 in which the ordinates represent the number of and particles expelled per second by the products ,d and ,f respectively4 ,the complete calculation for three changes shows that the number of particles soon reaches a practical maximum1 and then decays nearly exponentially with the time1 falling to half value in #dj years4 ,the number of particles expelled per second increases for several years1 but reaches a maximum after #bf years and then diminishes1 finally falling off exponentially with the time to half value in #dj years4 ,the experimental curve of the rise of ray activity1 shown in ,fig4 #ic1 as far as it has been determined1 lies accurately on this curve1 if the maximum is calculated from the above theory4 ,the observed activity after a period of #bej days is marked by the point ,x on the curve4 #bcg4 ,experiments with old radium4 ,since the substance radium ,d is produced from radium at a constant rate1 the amount present mixed with the radium will increase with its age4 ,the writer had in his possession a small quantity of impure radium chloride1 kindly presented by ,professors ,elster and ,geitel four years before4 ,the amount of radium ,d present in it was tested in the following way3--,the substance was dissolved in water and kept continuously boiling for a period of about six hours4 ,under these conditions the emanation is removed as rapidly as it is formed1 and the rays from the radium1 due to the product radium ,c1 practically disappear4 ,a newly prepared specimen of radium bromide under these conditions retains only a fraction of #a per cent4 of its original radiation4 ,the old radium1 however1 showed 7immediately after this treatment7 an activity measured by the rays of about #h per cent4 of its original amount4 ,the activity could not be reduced any lower by further boiling or aspiration of air through the solution4 ,this residual ray activity was due to the product radium ,e stored up in the radium4 ,the ray activity due to radium ,e was thus about #i per cent4 of that due to radium ,c4 ,disregarding the differences in the absorption of the rays1 when the activity of the product ,e in radium reaches a maximum value1 the ray activity due to it should be the same as that due to ,c4 ,since the parent product ,d is half transformed in forty years1 the amount present in the radium after four years should be about #g per cent4 of the maximum amount2 i4e4 it should show a ray activity of about #g per cent4 of that due to radium ,c4 ,the observed and calculated values 7#g and #i per cent4 respectively7 are thus of the same order of magnitude4 ,the amount of rays from radium ,e present in pure radium bromide about one year old was about #b per cent4 of the total4 ,the amount of radium ,f present in old radium was measured by observations of the activity imparted to a bismuth disk left for several days in the solution1 and was found to be of the same order as the theoretical value4 ,radium ,f is not deposited to an appreciable extent on the bismuth from a water solution of radium bromide4 ,if1 however1 a trace of sulphuric acid is added to the solution1 the radium ,f is readily deposited on the bismuth4 ,the addition of sulphuric acid to the radium solution practically effected a separation of radium ,d1 ,e and ,f from the radium proper2 for the latter was precipitated as sulphate and the products ,d1 ,e and ,f remained in solution4 ,after filtering1 the solution contained the greater proportion of the products ,d1 ,e1 and ,f and very little radium4 #bch4 ,variation of the activity of radium with time4 ,it has been shown that the activity of freshly prepared radium increases at first with the time and practically reaches a maximum value after an interval of about one month4 ,the results already considered show that there is a still further slow increase of activity with the time4 ,this is the case whether the activity is measured by the or rays4 ,it will be shown later that radium is probably half transformed in about #ajjj years4 ,from this it can readily be calculated that after a lapse of about #bjj years the amount of the products radium ,d1 ,e and ,f will have reached a maximum value4 ,the same number of atoms of each of the products ,c and ,e will then break up per second4 ,if each atom of these products in disintegrating throws off an equal number 7probably one7 of particles1 the number of particles thrown off per second will be twice as great as from radium a few months old4 ,the number will increase at first at the rate of about #b per cent4 a year4 ,similar considerations apply to the ray activity4 ,since1 however1 there are four other products of radium besides radium itself which expel particles1 the number of particles emitted per second from old radium will not be more than #be per cent4 greater than the number from radium a few months old4 ,the activity measured by the rays will thus not increase more than #be per cent41 and probably still less1 as the particles from radium ,f produce less ionization than the particles expelled from the other radium products4 ,the activity of radium will consequently rise to a maximum after #bjj years and then slowly die away with the time4 #bci4 ,presence of these products in pitchblende4 ,the products radium ,d1 ,e and ,f must be present in pitchblende in amounts proportional to the quantity of radium present1 and should be capable of separation from the mineral by suitable chemical methods4 ,the radio-active properties of these substances1 if obtained in the pure state1 are summarized below4 ,radium ,d when first separated1 should give out very little or radiation4 ,the ray activity will rapidly increase1 reaching half its maximum value in #f days4 ,the ray activity will at first increase nearly proportionately with the time1 and will reach a maximum value after an interval of about #c years4 ,the and ray activity1 after reaching a maximum1 will finally decay1 the activity falling to half value in about #dj years4 ,since radium ,d is half transformed in #dj years1 and radium in #abjj years1 the maximum ray activity of radium ,d1 weight for weight1 will be about #cjj times that of radium4 ,the ray activity1 at any time1 will be removed by placing a bismuth disk in the solution4 ,radium ,f1 after separation1 will give out only rays4 ,its activity1 after separation1 will decrease according to an exponential law1 falling to half value in #adc days4 ,since radium in radio-active equilibrium contains four products which emit rays1 the number of particles expelled per second from radium ,f will1 weight for weight1 be about #hjj times as numerous as from new radium in radio-active equilibrium4 ,since the particles from radium ,f produce only about half as much ionization as the particles from the other radium products1 the activity of radium ,f1 measured by the electric method1 will be about #djj times that of radium4 #bdj4 ,origin of radio-tellurium and polonium4 ,it is now necessary to consider whether these products of radium have been previously separated from pitchblende1 and known by other names4 ,we shall first consider the ray product1 radium ,f4 ,the radio-tellurium of ,marckwald and the polonium of ,mme ,curie both resemble radium ,f in giving out only rays1 and in being deposited on a bismuth disk from a solution4 ,if the active constituent present in radio-tellurium is the same as radium ,f1 its activity should decay at the same rate as the latter4 ,the writer#cbj has carefully compared the rates of decay of the activity of radium ,f and of the radio-tellurium of ,marckwald and found them to be the same within the limits of experimental error4 ,both lose half of their activity in about #adc days#cba4 ,a similar value of the rate of decay of radio-tellurium has been obtained by ,meyer and ,schweidler#cbb4 ,the experiments on radio-tellurium were made upon the active bismuth plates supplied by ,dr ,sthamer of ,hamburg1 which were prepared under ,marckwald's directions4 ,an additional proof#cbc of the identity of these two products was obtained by comparing the absorption of the rays by aluminium foil4 ,the rays from different products are projected with different velocities1 and1 in consequence1 are unequally absorbed by matter4 ,the absorption of the rays from the two products by aluminium foil agreed very closely1 indicating the probable identity of the substances from which they were emitted4 ,there can thus be no doubt that the active constituent present in the radio-tellurium of ,marckwald is identical with the product radium ,f4 ,this is a very interesting result1 and shows how the close examination of the successive transformations of the radio-active bodies may throw light on the origin of the various substances found in pitchblende4 ,we have already seen 7section #ba7 that ,marckwald1 by special chemical methods1 was able to obtain a few milligrams of very active substance by working over #b tons of pitchblende4 ,we have already seen 7section #bci7 that this substance1 if obtained in the pure state1 should be about #djj times as active as radium4 ,comparative measurements of the activity of this substance with radium will thus indicate the amount of impurity that is present with the former4 ,this method should be of value in purifying radium ,f for the purpose of determining its spectrum1 which has not yet been observed4 #bda4 ,polonium4 ,since the separation of the active substance by ,marckwald1 called by him radio-tellurium1 there has been some discussion as to whether the active constituent is the same as that present in the polonium of ,mme ,curie4 ,both of these substances have similar radio-active and chemical properties1 but the main objection to the view that the active constituents were identical has rested on an early statement of ,marckwald that the activity of one of his very active preparations did not decay appreciably in the course of six months4 ,this objection is now removed1 for we have seen that the activity of radio-tellurium does decay fairly rapidly4 ,it was early recognised that the activity of the polonium1 separated from pitchblende by the methods of ,mme ,curie1 was not permanent1 but decayed with the time4 ,observations on the rate of decay have not been very precise1 but ,mme ,curie states that some of her preparations lost half of their activity in about six months but in others the rate of decay was somewhat smaller4 ,it is possible that the initial differences observed in the rates of decay of different specimens of polonium may be due to the presence of some radium ,d with the polonium4 ,the polonium in my possession lost its activity fairly rapidly1 and was reduced to a small portion of its value in the course of about four years4 ,rough observations of its activity1 made from time to time1 showed that its activity diminished to half value in about six months4 ,if it is identical with radio-tellurium1 the activity should decay to half value in #adc days1 and ,i think there is little doubt that more accurate measurement will prove this to be the case4 ,while the proof of the identity of the active constituent in polonium is not so definite as for radio-tellurium1 ,i think there can be no reasonable doubt that these substances both contain the same active substance1 which is the seventh transformation product of radium4 ,marckwald has noticed some chemical differences in the behaviour of polonium and radio-tellurium1 but little weight can be attached to such observations1 for it must be remembered that the active constituent in both cases is present in minute quantity in the material under examination1 and that the apparent chemical properties of the active substance are much influenced by the presence of impurities4 ,the most important and trustworthy test rests upon the identity of the radiations and the period of decay4 #bda ,a4 ,origin of radio-active lead4 ,some experiments will now be discussed which show that the radio-lead first separated from pitchblende by ,hofmann 7section #bb7 contains the products radium ,d1 ,e and ,f4 ,hofmann has observed that the activity of this substance did not appreciably decay in the course of several years4 ,in some recent experiments1 ,hofmann1 ,gonder and ,wlfl#cbd have made a close chemical examination of the radio-active lead1 and have shown the presence of two radio-active constituents1 which are probably identical with the products radium ,e and ,f4 ,the radio-active measurements were unfortunately not very precise1 and the periods of change of the separated products have not been examined very closely4 ,experiments were made on the effect of adding substances to a solution of radio-lead1 and then removing them by precipitation4 ,small quantities of iridium1 rhodium1 palladium1 and platinum1 in the form of chlorides1 were left in the solution for three weeks1 and then precipitated by formalin or hydroxylamine4 ,all of these substances were found to give out both and rays1 the activity being greatest for rhodium and least for platinum4 ,a large proportion of the ray activity disappeared in the course of six weeks1 and of the ray activity in one year4 ,it is probable that the two products radium ,e and ,f were in part removed with the metals from the radio-lead4 ,we have seen that radium ,e gives out rays and loses half of its activity in about six days1 while radium ,f gives out only rays and its activity falls to half value in #adc days4 ,this conclusion is further confirmed by experiments on the effect of heat on the activity of these substances4 ,by heating to a full red heat1 the ray activity was lost in a few seconds4 ,this is in agreement with the results 7section #bcb7 where we have seen that radium ,f is volatilized at about #ajjj ,c4 and radium ,e is left behind4 ,salts of gold1 silver and mercury added to the radio-lead were found to show only ray activity on removal4 ,this is in accordance with the view that radium ,f alone is removed with these substances4 ,bismuth salts on the other hand showed initially and ray activity1 but the latter rapidly died away4 ,the presence of rays in freshly prepared polonium was early observed by ,mme ,curie4 ,the and ray activity of the radio-lead is much reduced by the precipitation of bismuth added to the solution4 ,the and ray activity of the radio-lead1 however1 recovers itself again4 ,this result is exactly what is to be expected if radio-lead contains radium ,d1 ,e and ,f4 ,radium ,e and ,f are removed with the bismuth1 but the parent substance1 radium ,d1 is left behind1 and1 in consequence1 a fresh supply of radium ,e and ,f is produced4 ,while further experiments are required to settle definitely whether the products separated from radio-lead are identical with radium ,e and ,f1 there can be little doubt that such is the case4 ,this conclusion is strengthened by some experiments which ,i have made on a specimen of radio-lead1 which was kindly forwarded to me by ,mr ,boltwood of ,new ,haven4 ,this active lead gave out and rays1 the latter being in unusually large proportion4 ,the active lead was four months old when first tested4 ,the ray activity in the following six months has remained sensibly constant1 but the ray activity has steadily increased4 ,these results are to be expected if the radio-lead contains radium ,d4 ,radium ,e will reach a practical maximum about #dj days after separation of the product radium ,d with the lead4 ,the ray activity due to radium ,f should increase to a maximum in about #bf years 7see section #bcf74 ,further experiments are required to settle whether the lead immediately after separation from pitchblende contains only radium ,d1 or whether radium ,e also appears with it4 ,it seems likely1 however1 that the bismuth1 which is initially present in solution at the time of separation of the lead1 will retain both radium ,e and ,f1 and that the presence of these products in radio-lead is due to their production1 after separation1 by the parent substance1 radium ,d4 ,it would be of scientific value to separate radium ,d from pitchblende and obtain it in the pure state1 for1 a month after removal1 the ray activity from it would be about #cjj times as great as from an equal weight of radium4 ,by placing a bismuth plate in a solution of this substance1 radium ,f 7polonium7 should be separated1 and1 provided a sufficient interval is allowed to elapse1 a fresh supply of radium ,f can at any time be obtained4 ,the rate of transformation of radium ,d 7half transformed in #dj years7 is sufficiently slow not to interfere seriously with its utility in most experiments4 ,the results of the comparison of the products of radium with those contained in polonium1 radio-tellurium and radio-lead are summarized below4 ,radium ,d product in new radio-lead1 no rays4 ,half transformed in #dj years4 ,radium ,e gives out rays1 separated with bismuth1 iridium and platinum4 ,half transformed in #f days4 ,radium ,f product in polonium and radio-tellurium4 ,gives out only rays4 ,half transformed in #adc days4 #bdb4 ,temporary activity of inactive matter separated from radio-active substances4 ,we have seen in the last section that the platinum metals and bismuth acquire temporary activity by their admixture with a solution of radio-lead1 and that these effects are very satisfactorily explained on the view that some of the products of change of radio-lead are removed with the inactive substances4 ,very similar effects have been observed by ,pegram and von ,lerch 7section #ahf71 when inactive substances were added to solutions of thorium and of the active deposit of thorium4 ,these results1 too1 are almost certainly due to the removal of one or more of the products of thorium with the inactive matter4 ,examples of this character may readily be multiplied1 and some of the more interesting and important of these will be briefly discussed later4 ,there have been two general points of view regarding the character of this activity which is temporarily acquired by inactive matter4 ,some people have supposed that the inactive molecules of the substance1 mixed with the solution1 acquire by 8radio-active induction0 temporary activity1 the underlying idea being that the close admixture of an inactive and an active substance has communicated the property of radiating to some of the molecules of the former4 ,according to the disintegration theory of radio-activity1 on the other hand1 the temporary activity of originally inactive matter is not due to any alteration of the inactive substance itself1 but to an admixture with it of one or more of the numerous radio-active products4 ,the idea of 8radio-active induction0 has no definite experimental evidence in support of it1 while there is much indirect evidence against it4 ,we shall now consider how these facts are interpreted according to the disintegration theory4 ,in a specimen of old radium1 for example1 there are present1 besides radium itself1 the seven successive products which arise from it4 ,each of these differs in chemical and physical properties from the others4 ,if now1 for example1 a bismuth rod is introduced into the solution1 one or more of these products are deposited on the bismuth4 ,this action is most probably electrolytic in nature1 and will depend upon the electro-chemical behaviour of the bismuth compared with that of the products in solution4 ,an electro-negative substance will tend to remove the product or products which are strongly electro-positive4 ,this point of view serves to explain why different metals are made active to different degrees1 depending upon their position in the electro-chemical series4 ,it seems probable that the activity communicated to inactive matter by precipitation from an active solution occurs only during the precipitation4 ,the correctness of this view could readily be tested by observing whether the time that the inactive substance is present in solution has any effect on the magnitude of the activity imparted to it4 ,when it is remembered that in pitchblende there are present the radio-elements uranium1 thorium1 radium and actinium and their numerous family of products1 it is not surprising that many of the inactive substances separated from it may show very considerable activity due to the mixture of products which may be removed with them4 ,in carrying out experiments on the separation of radium from pitchblende1 ,m4 and ,mme ,curie observed that the separation of the active substance is fairly complete if the stage of purification is not far advanced4 ,copper1 antimony and arsenic can be separated only slightly active1 but other substances like lead and iron always show activity4 ,when the stage of precipitation is more advanced1 every substance separated from the active solution shows activity4 ,one of the earliest observations in this direction was made by ,debierne1 who found that barium could be made active by solution with actinium4 ,the active barium removed from the actinium still preserved its activity after chemical treatment1 and1 in this way1 barium chloride was obtained whose activity was #fjjj times that of uranium4 ,although the activity of the barium chloride could be concentrated in the same way as the activity of radiferous barium chloride1 it did not show any of the spectroscopic lines of radium1 and could not have been due to the admixture of that element with the barium4 ,the activity of the barium was not permanent1 and ,debierne states that the activity fell to about one-third of its value in three months4 ,it seems probable that the precipitated barium carried down with it the product actinium ,x1 and also some of the actinium itself1 and that the decay observed was due to the transformation of actinium ,x4 ,it is interesting to note that barium is capable of removing a large number of products of the different radio-elements4 ,this effect is probably connected with its position in the electro-chemical series1 for barium is highly electro-positive4 ,giesel showed in #aijj that bismuth could be made active by placing it in a radium solution1 and considered that polonium was in reality bismuth made active by the process of induction4 ,in later experiments1 he found that the bismuth plate gave out only rays1 and that the activity of the bismuth could not be ascribed to radium1 since no rays were present4 ,we have seen that this activity of the bismuth is due to the product radium ,f deposited on its surface4 ,mme ,curie also found that bismuth was made active by solution with a radium compound1 and succeeded in fractionating the above bismuth in the same way as polonium4 ,in this way bismuth was obtained #bjjj times as active as uranium1 but the activity1 like that of polonium separated from pitchblende1 decreased with the time4 ,in the light of the experiments on the transformation products of radium1 it is seen that these early experiments of ,mme ,curie add additional confirmation to the view that the product 7radium ,f7 separated from radium itself is identical with the polonium obtained directly from pitchblende4 ,footnote #cad3 ,rutherford and ,soddy1 ,phil4 ,mag4 ,april1 #aijc4 ,footnote #cae3 ,rutherford1 ,phil4 ,trans4 ,a4 p4 #afi1 #aijd4 ,curie and ,danne1 ,c4 ,r4 p4 #gdh1 #aijd4 ,footnote #caf3 ,p4 ,curie and ,danne1 ,comptes ,rendus1 #ach1 p4 #gdh1 #aijd4 ,footnote #cag3 ,miss ,gates1 ,phys4 ,rev4 p4 #cjj1 #aijc4 ,footnote #cah3 ,miss ,brooks1 ,nature1 ,july #ba1 #aijd4 ,footnote #cai3 ,rutherford1 ,phil4 ,mag4 ,nov4 #aijd4 ,nature1 p4 #cda1 ,feb4 #i1 #aije4 ,footnote #cbj3 ,rutherford1 ,nature1 p4 #cda1 ,feb4 #i1 #aije4 ,footnote #cba3 ,marckwald 7,ber4 d4 ,d4 ,chem4 ,ges4 p4 #eia1 #aije7 has recently found that the activity of his radio-tellurium falls to half value in #aci days4 ,footnote #cbb3 ,meyer and ,schweidler1 ,wien ,ber4 ,dec4 #a1 #aijd4 ,footnote #cbc3 ,rutherford1 ,phil4 ,trans4 ,a4 p4 #afi1 #aijd4 ,footnote #cbd3 ,hofmann1 ,gonder and ,wlfl1 ,annal4 d4 ,phys4 #ae1 p4 #fae1 #aijd4 ,,chapter ,,xii4 ,,rate ,,of ,,emission ,,of ,,energy4 #bdc4 ,it was early recognised that a considerable amount of energy is emitted by the radio-active bodies in the form of their characteristic radiations4 ,most of the early estimates of the amount of this energy were based on the number and energy of the expelled particles1 and were much too small4 ,it has been pointed out 7section #aad7 that the greater part of the energy emitted from the radio-active bodies in the form of ionizing radiations is due to the rays1 and that the rays in comparison supply only a very small fraction4 ,rutherford and ,mc,clung#cbe made an estimate of the energy of the rays1 emitted by a thin layer of active matter1 by determining the total number of ions produced by the complete absorption of the rays4 ,the energy required to produce an ion was determined experimentally by observations of the heating effect of ,x rays1 and of the total number of ions produced when the rays were completely absorbed in air4 ,the energy required to produce an ion in air was found to be #aij #aj ergs4 ,this1 as will be shown in ,appendix ,a1 is probably an over-estimate1 but was of the right order of magnitude4 ,from this it was calculated that one gram of uranium oxide spread over a plate in the form of a thin powdered layer emitted energy into the air at the rate of #jjcb gram calories per year4 ,this is a very small emission of energy1 but in the case of an intensely radio-active substance like radium1 whose activity is about two million times that of uranium1 the corresponding emission of energy is #fijjj gram calories per year4 ,this is obviously an under-estimate1 for it includes only the energy radiated into the air4 ,the actual amount of energy released in the form of rays is evidently much greater than this on account of the absorption of the rays by the active matter itself4 ,it will be shown later that the heating effect of radium and of its products is a measure of the energy of the expelled particles4 #bdd4 ,heat emission of radium4 ,p4 ,curie and ,laborde#cbf first drew attention to the striking result that a radium compound kept itself continuously at a temperature several degrees higher than that of the surrounding atmosphere4 ,thus the energy emitted from radium can be demonstrated by its direct heating effect1 as well as by photographic and electric means4 ,curie and ,laborde determined the rate of the emission of heat in two different ways4 ,in one method the difference of temperature was observed by means of an iron-constantine thermo-couple between a tube containing one gram of radiferous chloride of barium1 of activity about of pure radium1 and an exactly similar tube containing one gram of pure barium chloride4 ,the difference of temperature observed was #ae ,c4 ,in order to measure the rate of emission of heat1 a coil of wire of known resistance was placed in the pure barium chloride1 and the strength of the electric current required to raise the barium to the same temperature as the radiferous barium was observed4 ,in the other method1 the active barium1 enclosed in a glass tube1 was placed inside a ,bunsen calorimeter4 ,before the radium was introduced1 it was observed that the level of the mercury in the stem remained steady4 ,as soon as the radium1 which had previously been cooled in melting ice1 was placed in the calorimeter1 the mercury column began to move at a regular rate4 ,if the radium tube was removed1 the movement of the mercury ceased4 ,it was found from these experiments that the heat emission from the #a gram of radiferous barium1 containing about of its weight of pure radium chloride1 was #ad gram-calories per hour4 ,measurements were also made with #jjh gram of pure radium chloride4 ,curie and ,laborde deduced from these results that #a gram of pure radium emits a quantity of heat equal to about #ajj gram-calories per hour4 ,this result was confirmed by the experiments of ,runge and ,precht#cbg and others4 ,as far as observation has gone at present1 this rate of emission of heat is continuous and unchanged with lapse of time4 ,therefore1 #a gram of radium emits in the course of a day #bdjj1 and in the course of a year #hgf1jjj gram-calories4 ,the amount of heat evolved in the union of hydrogen and oxygen to form #a gram of water is #cijj gram-calories4 ,it is thus seen that #a gram of radium emits per day nearly as much energy as is required to dissociate #a gram of water4 ,in some later experiments using #jg gram of pure radium bromide1 ,p4 ,curie#cbh found that the temperature of the radium indicated by a mercury thermometer was #c ,c4 above that of the surrounding air4 ,this result was confirmed by ,giesel1 who obtained a difference of temperature of #e ,c4 with #a gram of radium bromide4 ,the actual rise of temperature observed will obviously depend upon the size and nature of the vessel containing the radium4 ,during their visit to ,england in #aijc to lecture at the ,royal ,institution1 ,m4 and ,mme ,curie performed some experiments with ,professor ,dewar1 to test by another method the rate of emission of heat from radium at very low temperatures4 ,this method depended on the measurement of the amount of gas volatilized when a radium preparation was placed inside a tube immersed in a liquefied gas at its boiling point4 ,the arrangement of the calorimeter is shown in ,fig4 #ig4 ,illustration3 ,fig4 #ig4 ,the small closed ,dewar flask ,a contains the radium in a glass tube ,r1 immersed in the liquid to be employed4 ,the flask ,a is surrounded by another ,dewar bulb ,b1 containing the same liquid1 so that no heat is communicated to ,a from the outside4 ,the gas liberated in the tube ,a is collected in the usual way over water or mercury1 and its volume determined4 ,by this method1 the rate of heat emission of the radium was found to be about the same in boiling carbon dioxide and oxygen1 and also in liquid hydrogen4 ,especial interest attaches to the result obtained with liquid hydrogen1 for at such a low temperature ordinary chemical activity is suspended4 ,the fact that the heat emission of radium is unaltered over such a wide range of temperature indirectly shows that the rate of expulsion of particles from radium is independent of temperature1 for it will be shown later that the heating effect observed is due to the bombardment of the radium by the particles4 ,the use of liquid hydrogen is very convenient for demonstrating the rate of heat emission from a small amount of radium4 ,from #jg gram of radium bromide 7which had been prepared only #aj days previously7 #gc c4c4 of gas were given off per minute4 ,in later experiments ,p4 ,curie 7loc4 cit47 found that the rate of emission of heat from a given quantity of radium depended upon the time which had elapsed since its preparation4 ,the emission of heat was at first small1 but after a month's interval practically attained a maximum4 ,if a radium compound is dissolved and placed in a sealed tube1 the rate of heat emission rises to the same maximum as that of an equal quantity of radium in the solid state4 #bde4 ,connection of the heat emission with the radiations4 ,the observation of ,curie that the rate of heat emission depended upon the age of the radium preparation pointed to the conclusion that the phenomenon of heat emission of radium was connected with the radio-activity of that element4 ,it had long been known that radium compounds increased in activity for about a month after their preparation1 when they reached a steady state4 ,it has been shown 7section #bae71 that this increase of activity is due to the continuous production by the radium of the radio-active emanation1 which is occluded in the radium compound and adds its radiation to that of the radium proper4 ,it thus seemed probable that the heating effect was in some way connected with the presence of the emanation4 ,some experiments upon this point were made by ,rutherford and ,barnes#cbi4 ,in order to measure the small amounts of heat emitted1 a form of differential air calorimeter shown in ,fig4 #ih was employed4 ,two equal glass flasks of about #ejj c4c4 were filled with dry air at atmospheric pressure4 ,these flasks were connected through a glass ,u-tube filled with xylene1 which served as a manometer to determine any variation of pressure of the air in the flasks4 ,a small glass tube1 closed at the lower end1 was introduced into the middle of each of the flasks4 ,when a continuous source of heat was introduced into the glass tube1 the air surrounding it was heated and the pressure was increased4 ,the difference of pressure1 when a steady state was reached1 was observed on the manometer by means of a microscope with a micrometer scale in the eye-piece4 ,on placing the source of heat in the similar tube in the other flask1 the difference in pressure was reversed4 ,in order to keep the apparatus at a constant temperature1 the two flasks were immersed in a water-bath1 which was kept well stirred4 ,illustration3 ,fig4 #ih4 ,observations were first made on the heat emission from #cj milligrams of radium bromide4 ,the difference in pressure observed on the manometer was standardized by placing a small coil of wire of known resistance in the place of the radium4 ,the strength of the current through the wire was adjusted to give the same difference of pressure on the manometer4 ,in this way it was found that the heat emission per gram of radium bromide corresponded to #fe gram-calories per hour4 ,taking the atomic weight of radium as #bbe1 this is equivalent to a rate of emission of heat from one gram of metallic radium of #aaj gram-calories per hour4 ,the emanation from the #cj milligrams of radium bromide was then removed by heating the radium 7section #bae74 ,by passing the emanation through a small glass tube immersed in liquid air1 the emanation was condensed4 ,the tube was sealed off while the emanation was still condensed in the tube4 ,in this way the emanation was concentrated in a small glass tube about #d cms4 long4 ,the heating effects of the 8de-emanated0 radium and of the emanation tube were then determined at intervals4 ,it was found that1 after removal of the emanation1 the heating effect of the radium decayed in the course of a few hours to a minimum1 corresponding to about #be per cent4 of the original heat emission1 and then gradually increased again1 reaching its original value after about a month's interval4 ,the heating effect of the emanation tube was found to increase for the first few hours after separation to a maximum1 and then to decay regularly with the time according to an exponential law1 falling to half its maximum value in about four days4 ,the actual heat emission of the emanation tube was determined by sending a current through a coil of wire occupying the same length and position as the emanation tube4 ,the variation with time of the heating effect from #cj milligrams of radium and the emanation from it is shown in ,fig4 #ii4 ,illustration3 ,fig4 #ii4 ,curve ,a shows the variation with time of the heat emission of the radium and curve ,b of the emanation4 ,the sum total of the rate of heat emission of the radium and the emanation together1 was at any time found to be equal to that of the original radium4 ,the maximum heating effect of the tube containing the emanation from #cj milligrams of radium bromide was #abf gram-calories per hour4 ,the emanation together with the secondary products which arise from it1 obtained from one gram of radium1 would thus give out #db gram-calories per hour4 ,the emanation stored up in the radium is thus responsible for more than two-thirds of the total heat emission from radium4 ,it will be seen later that the decrease to a minimum of the heating effect of radium1 after removal of the emanation1 is connected with the decay of the excited activity4 ,in a similar way1 the increase of the heating effect of the emanation to a maximum some hours after removal is also a result of the excited activity produced by the emanation on the walls of the containing vessel4 ,disregarding for the moment these rapid initial changes in heat emission1 it is seen that the heating effect of the emanation and its further products1 after reaching a maximum1 decreases at the same rate as that at which the emanation loses its activity1 that is1 it falls to half value in four days4 ,if ,qmax4 is the maximum heating effect and ,qt the heating effect at any time t later1 then frac ,qt ,qmax e-t where is the constant of change of the emanation4 ,the curve of recovery of the heating effect of radium from its minimum value is identical with the curve of recovery of its activity measured by the rays4 ,since the minimum heating effect is #be per cent4 of the total1 the heat emission ,qt at any time t after reaching a minimum is given by frac ,qt ,qmax cdot#be cdot#ge 7#a e-t7 1 where ,qmax4 is the maximum rate of heat emission and 1 as before1 is the constant of change of the emanation4 ,the identity of the curves of recovery and fall of the heating effect of radium and its emanation respectively with the corresponding curves for the rise and fall of radio-activity shows that the heat emission of radium and its products is directly connected with their radio-activity4 ,the variation in the heat emission of both radium and its emanation is approximately proportional to their activity measured by the rays4 ,it is not proportional to the activity measured by the or rays1 for the intensity of these rays falls nearly to zero some hours after removal of the emanation1 while the ray activity1 like the heating effect1 is #be per cent4 of the maximum value4 ,these results are thus in accordance with the view that the heat emission of radium accompanies the expulsion of particles1 and is approximately proportional to the number expelled4 ,before such a conclusion can be considered established1 it is necessary to show that the heating effect of the active deposit from the emanation varies in the same way as its ray activity4 ,experiments made to test this point will now be considered4 #bdf4 ,heat emission of the active deposit from the emanation4 ,new radium in radio-active equilibrium contains four successive products which break up with the emission of particles1 viz4 radium itself1 the emanation1 radium ,a and ,c4 ,radium ,b does not emit rays at all4 ,the effect of the later products radium ,d1 ,e and ,f may be neglected1 if the radium has not been prepared for more than a year4 ,it is not easy to settle definitely the relative activity supplied by each of these products when in radio-active equilibrium1 but it has been shown in section #bbi that the activity is not very different for the four ray products4 ,the particles from radium ,a and ,c are more penetrating than those from radium itself and the emanation4 ,the evidence at present obtained points to the conclusion that the activity supplied by the emanation is less than that supplied by the other products4 ,this indicates that the particles from the emanation are projected with less velocity than in the other cases4 ,when the emanation is suddenly released from radium by heat or solution1 the products radium ,a1 ,b and ,c are left behind4 ,since the parent matter is removed1 the amount of the products ,a1 ,b1 ,c at once commences to diminish1 and at the end of about three hours reaches a very small value4 ,if the heating effect depends upon the ray activity1 it is thus to be expected that the heat emission of the radium should rapidly diminish to a minimum after the removal of the emanation4 ,when the emanation is introduced into a vessel1 the products radium ,a1 ,b and ,c at once appear and increase in quantity1 reaching a practical maximum about #c hours later4 ,the heating effect of the emanation tube should thus increase for several hours after the introduction of the emanation4 ,in order to follow the rapid changes in the heating effect of radium1 after removal of the emanation1 ,rutherford and ,barnes 7loc4 cit47 used a pair of differential platinum thermometers4 ,each thermometer consisted of #ce cms4 of fine platinum wire1 wound carefully on the inside of a thin glass tube #e mms4 in diameter1 forming a coil #c cms4 long4 ,the glass tube containing the radium and also the tube containing the emanation were selected to slide easily into the interior of the coils1 the wire thus being in direct contact with the glass envelope containing the source of heat4 ,the change in resistance of the platinum thermometers1 when the radium or emanation tube was transferred from one coil to the other1 was readily measured4 ,illustration3 ,fig4 #ajj4 ,the heating effect of the radium in radio-active equilibrium was first accurately determined4 ,the radium tube was heated to drive off the emanation1 which was rapidly condensed in a small glass tube #c cms4 long and #c mms4 internal diameter4 ,after allowing a short time for temperature conditions to become steady1 the heating effect of the radium tube was measured4 ,the results are shown in ,fig4 #ajj4 ,an observation could not be taken until about #ab minutes after the removal of the emanation1 and the heating effect was then found to have fallen to about #ee per cent4 of the maximum value4 ,it steadily diminished with the time1 finally reaching a minimum value of #be per cent4 several hours later4 ,it is not possible in experiments of this character to separate the heating effect of the emanation from that supplied by radium ,a4 ,since ,a is half transformed in three minutes1 its heating effect will have largely disappeared after #aj minutes1 and the decrease is then mainly due to changes in radium ,b and ,c4 ,the variation with time of the heating effect of the active deposit is still more clearly brought out by an examination of the rise of the heating effect when the emanation is introduced into a small tube1 and of the decrease of the heating effect after the emanation is removed4 ,the curve of rise is shown in the upper curve of ,fig4 #aja4 #dj minutes after the introduction of the emanation1 the heating effect had risen to #ge per cent4 of the maximum value which was reached after an interval of about #c hours4 ,illustration3 ,fig4 #aja4 ,after the heating effect of the emanation tube had attained a maximum1 the emanation was removed1 and the decay with time observed as soon as possible afterwards4 ,the results are shown in the lower curve of ,fig4 #aja4 ,it is seen that the two curves of rise and decay are complementary to one another4 ,the first observation was made #aj minutes after removal1 and the heating effect had then dropped to #dg per cent4 of the original value4 ,this sudden drop is due partly to the removal of the emanation1 and partly to the rapid transformation of radium ,a4 ,the lower curve is almost identical in shape with the corresponding ray curve for the decay of the excited activity after a long exposure 7see ,fig4 #hf7 and clearly shows that the heating effect is directly proportional to the activity measured by the rays over the whole range examined4 ,the heating effect decreases according to the same law and at the same rate as the activity measured by the rays4 ,twenty minutes after the removal of the emanation1 radium ,a has been almost completely transformed1 and the activity is then proportional to the amount of radium ,c present1 since the intermediate product ,b does not give out rays4 ,the close agreement of the activity and heat emission curves shows that the heating effect is proportional also to the amount of radium ,c4 ,we may thus conclude that the rayless product ,b supplies little if any of the heat emission observed4 ,if radium ,b supplied the same amount as radium ,c1 the curve of decrease of heating effect with time would differ considerably from the activity curve4 ,the conclusion that the transformation of radium ,b is not accompanied by the release of as much heat as the other changes is to be expected if the heating effect is mainly due to the energy of motion of the expelled particles4 ,the relative heating effect due to the radium products is shown in the following table4 ,the initial heating effect of ,c is deduced by comparison with the corresponding activity curve4 ,products ,radiation ,initial rate of heat emission ,radium rays #be per cent4 of total ,emanation ,radium ,a #dd ,radium ,b no rays #j ,radium ,c 1 1 #ca rays ,since radium ,a and ,c supply almost an equal proportion of activity1 it is probable that they have equal initial heating effects4 ,if this is the case1 the heating effect of the emanation alone is #ac per cent4 of the total4 #bdg4 ,heating effects of the and rays4 ,it has been shown in section #aad that the kinetic energy of the particles emitted from radium is probably not greater than one per cent4 of that due to the particles4 ,if the heat emission is a result of bombardment by the particles expelled from its mass1 it is to be expected that the heating effect of the rays will be very small compared with that due to the rays4 ,this anticipation is borne out by experiment4 ,curie measured the heating effect of radium 7#a7 when enclosed in a thin envelope1 and 7#b7 when surrounded by one millimetre of lead4 ,in the former case a large proportion of the rays escaped1 and1 in the latter1 nearly all were absorbed4 ,the increase of heating effect in case 7#b7 was not more than five per cent41 and this is probably an over-estimate4 ,in a similar way1 since the total ionization due to the rays is about equal to that produced by the rays1 we should expect that the heating effect of the rays will be very small compared with that arising from the rays4 ,paschen made some experiments on the heating effect of radium in a ,bunsen ice calorimeter where the radium was surrounded by a thickness of #aib cms4 of lead--a depth sufficient to absorb a large proportion of the rays4 ,in his first publication#ccj1 results were given which indicated that the heating effect of the rays was even greater than that of the rays4 ,this was not confirmed by later observations by the same method4 ,he concluded that the ice calorimeter could not be relied on to measure such very small quantities of heat4 ,after the publication of ,paschen's first paper ,rutherford and ,barnes#cca examined the question by a different method4 ,an air calorimeter of the form shown in ,fig4 #ih was employed which was found to give very satisfactory results4 ,the heat emission of radium was measured 7#a7 when the radium was surrounded by a cylinder of aluminium and 7#b7 when surrounded by a cylinder of lead of the same dimensions4 ,the aluminium absorbed only a small fraction of the rays while the lead stopped more than half4 ,no certain difference between the heating effect in the two cases was observed1 although from the earlier experiments of ,paschen a difference of at least #ej per cent4 was to be expected4 ,we must therefore conclude that the and rays together do not supply more than a small percentage of the total heat emission of radium--a result which is in accordance with the calculations based on the total ionization produced by the different types of rays4 #bdh4 ,source of the energy4 ,it has been shown that the heating effect of radium is closely proportional to the activity measured by the rays4 ,since the activity is generally measured between parallel plates such a distance apart that most of the particles are absorbed in the gas1 this result shows that the heating effect is proportional to the energy of the emitted particles4 ,the rapid heat emission of radium follows naturally from the disintegration theory of radio-activity4 ,the heat is supposed to be derived not from external sources1 but from the internal energy of the radium atom4 ,the atom is supposed to be a complex system consisting of charged parts in very rapid motion1 and in consequence contains a large store of latent energy1 which can only be manifested when the atom breaks up4 ,for some reason1 the atomic system becomes unstable1 and an particle1 of mass about twice that of the hydrogen atom1 escapes1 carrying with it its energy of motion4 ,since the particles would be practically absorbed in a thickness of radium of less than #jja cm41 the greater proportion of the particles1 expelled from a mass of radium1 would be stopped in the radium itself and their energy of motion would be manifested in the form of heat4 ,the radium would thus be heated by its own bombardment above the temperature of the surrounding air4 ,the energy of the expelled particles probably does not account for the whole emission of heat by radium4 ,it is evident that the violent expulsion of a part of the atom must result in intense electrical disturbances in the atom4 ,at the same time1 the residual parts of the disintegrated atom rearrange themselves to form a permanently or temporarily stable system4 ,during this process also some energy is probably emitted1 which is manifested in the form of heat in the radium itself4 ,the view that the heat emission of radium is due very largely to the kinetic energy possessed by the expelled particles is strongly confirmed by calculations of the magnitude of the heating effect to be expected on such an hypothesis4 ,it has been shown in section #ic that one gram of radium bromide emits about #add #aj particles per second4 ,the corresponding number for #a gram of radium 7,ra #bbe7 is #be #aj4 ,now it has been calculated from experimental data in section #id1 that the average kinetic energy of the particles expelled from radium is #ei #aj ergs4 ,since all of the particles are absorbed either in the radium itself or the envelope surrounding it1 the total energy of the particles emitted per second is #ae #aj ergs4 ,this corresponds to an emission of energy of about #acj gram calories per hour4 ,now the observed heating effect of radium is about #ajj gram calories per hour4 ,considering the nature of the calculation1 the agreement between the observed and experimental values is as close as would be expected1 and directly supports the view that the heat emission of radium is due very largely to the bombardment of the radium and containing vessel by the particles expelled from its mass4 #bdi4 ,heating effect of the radium emanation4 ,the enormous amount of heat liberated in radio-active transformations which are accompanied by the expulsion of particles is very well illustrated by the case of the radium emanation4 ,the heat emission of the emanation released from #a gram of radium is #ge gram calories per hour at its maximum value4 ,this heat emission is not due to the emanation alone1 but also to its further products which are included with it4 ,since the rate of heat emission decays exponentially with the time to about half value in four days1 the total amount of heat liberated during the life of the emanation from #a gram of radium is equal to intinfty #ge e-t dt frac #ge #aj1jjj gram calories approximately1 since #jjgb7;hour74 ,now the volume of the emanation from #a gram of radium is about #a cubic millimetre at standard pressure and temperature 7section #agb74 ,thus #a cubic centimetre of the emanation would during its transformation emit #aj gram calories4 ,the heat emitted during the combination of #a c4c4 of hydrogen and oxygen to form water is about #b gram calories4 ,the emanation thus gives out during its changes #e #aj times as much energy as the combination of an equal volume of hydrogen and oxygen to form water1 although this latter reaction is accompanied by a larger release of energy than any other known to chemistry4 ,the production of heat from #a c4c4 of the radium emanation is about #ba gram calories per second4 ,this generation of heat would be sufficient to heat to redness1 if not to melt down1 the walls of the glass tube containing the emanation4 ,the probable rate of heat emission from #a gram weight of the emanation can readily be deduced1 assuming that the emanation has about #ajj times the molecular weight of hydrogen4 ,since #ajj c4c4 of the emanation would weigh about #a gram1 the total heat emission from #a gram of the emanation is about #aj gram calories4 ,it can readily be calculated that one pound weight of the emanation would1 at its maximum1 radiate energy at the rate of about #aj1jjj horse-power4 ,this radiation of energy would fall off with the time1 but the total emission of energy during the life of the emanation would correspond to #fj1jjj horse-power days4 #bej4 ,heating effects of uranium1 thorium1 and actinium4 ,since the heat emission of radium is a direct consequence of its bombardment by the particles expelled from its mass1 it is to be expected that all the radio-elements which emit rays should also emit heat at a rate proportional to their ray activity4 ,since the activity of pure radium is probably about two million times that of uranium or thorium1 the heat emission from #a gram of thorium or uranium should be about #e #aj gram calories per hour1 or about #jdd gram calories per year4 ,this is a very small rate of generation of heat1 but it should be detectable if a large quantity of uranium or thorium is employed4 ,experiments to determine the heating effect of thorium have been made by ,pegram#ccb4 ,three kilograms of thorium oxide1 enclosed in a ,dewar bulb1 were kept in an ice-bath1 and the difference of temperature between the thorium and ice-bath determined by a set of iron-constantan thermo-electric couples4 ,the maximum difference of temperature observed was #jjd ,c41 and1 from the rate of change of temperature1 it was calculated that one gram of thorium oxide liberated #h #aj gram calories per hour4 ,a more accurate determination of the heat emission is in progress1 but the results obtained are of the order of magnitude to be expected4 #bea4 ,energy emitted by a radio-active product4 ,an important consequence follows from the fact that the heat emission is a measure of the energy of the expelled particles4 ,if each atom of each product emits particles1 the total emission of energy from #a gram of the product can at once be determined4 ,the particles from the different products are projected with about the same velocity1 and consequently carry off about the same amount of energy4 ,now it has been shown that the energy of each particle expelled from radium is about #ei #aj ergs4 ,most of the products probably have an atomic weight in the neighbourhood of #bjj4 ,since there are #cf #aj molecules in one cubic centimetre of hydrogen1 it can easily be calculated that there are about #cf #aj atoms in one gram of the product4 ,if each atom of the product expels one particle1 the total energy emitted from #a gram of the matter is about #b #aj ergs or #h #aj gram calories4 ,the total emission of energy from a product which emits only rays is probably about one-hundredth of the above amount4 ,in this case we have only considered the energy emitted from a single product independently of the successive products which may arise from it4 ,radium1 for example1 may be considered a radio-active product which slowly breaks up and gives rise to four subsequent ray products4 ,the total heat emission from one gram of radium and products is thus about five times the above amount1 or #d #aj gram calories4 ,the total emission of energy from radium is discussed later in section #bff from a slightly different point of view4 #beb4 ,number of ions produced by an particle4 ,in the first edition of this book it was calculated by several independent methods that #a gram of radium emitted about #aj particles per second4 ,since the actual number has later been determined by measuring the charge carried by the rays 7section #ic7 we can1 conversely1 use this number to determine with more certainty some of the constants whose values were assumed in the original calculation4 ,for example1 the total number of ions produced by an particle in the gas can readily be determined4 ,the method employed is as follows4 #jdhd mgr4 of radium bromide was dissolved in water and then spread uniformly over an aluminium plate4 ,after evaporation1 the saturation ionization current1 due to the radium at its minimum activity1 was found to be #hd #aj ampere4 ,the plates of the testing vessel were sufficiently far apart to absorb all the rays in the gas4 ,the number of particles expelled per second into the gas was found experimentally to be #hg #aj4 ,taking the charge on an ion as #aac #aj coulombs 7section #cf71 the total number of ions produced per second in the gas was #ge #aj4 ,thus each particle on an average produced #hf1jjj ions in the gas before it was absorbed4 ,now ,bragg 7section #ajd7 has shown that the particles from radium at its minimum activity are stopped in about #c cms4 of air4 ,the results obtained by him indicate that the ionization of the particles per cm4 of path is less near the radium than some distance away4 ,assuming1 however1 as a first approximation that the ionization is uniform along the path1 the number of ions produced per cm4 of path by the particle is #bi1jjj4 ,since the ionization varies directly as the pressure1 at a pressure of #a mm4 of mercury the number of ions per unit path would be about #ch4 ,now ,townsend 7section #ajc7 found that the maximum number of ions produced per unit path of air at #a mm4 pressure by an electron in motion was #bj1 and in this case a fresh pair of ions was produced at each encounter of the electron with the molecules in its path4 ,in the present case the particle1 which has a very large mass compared with the electron1 appears to have a larger sphere of influence than the electron and to ionize twice as many molecules4 ,in addition1 the particle produces many more ions per unit path than an electron moving with the same velocity1 for it has been shown 7section #ajc7 that the electron becomes a less efficient ionizer after a certain velocity is reached4 ,as ,bragg 7loc4 cit47 has pointed out1 this is to be expected1 since the particle consists of a large number of electrons and consequently would be a far more efficient ionizer than an isolated electron4 ,a calculation of the energy required to produce an ion by an particle is given in ,appendix ,a4 #bec4 ,number of particles expelled from one gram of radium4 ,it is of importance to compare the total number of particles expelled from one gram of radium in radio-active equilibrium1 as1 theoretically1 this number should bear a definite relation to the total number of particles emitted4 ,we have seen that new radium in radio-active equilibrium contains four products which emit rays1 viz4 radium itself1 the emanation1 radium ,a and radium ,c4 ,on the other hand1 rays are expelled from only one product1 radium ,c4 ,the same number of atoms of each of these successive products in equilibrium break up per second4 ,if the disintegration of each atom is accompanied by the expulsion of one particle and1 in the case of radium ,c1 also of one particle1 the number of particles emitted from radium in radio-active equilibrium will be four times the number of particles4 ,the method employed by ,wien to determine the number of particles emitted from a known quantity of radium has already been discussed in section #hj4 ,on account of the absorption of some of the particles in the radium envelope and in the radium itself1 the number found by him is far too small4 ,it has been shown in section #he that a number of easily absorbed rays are projected from radium1 many of which would be stopped in the radium itself or in the envelope containing it4 ,in order to eliminate as far as possible the error due to this absorption1 in some experiments made by the writer1 the active deposit obtained from the radium emanation rather than radium itself was used as a source of rays4 ,a lead rod1 #d cms4 long and #d mms4 in diameter1 was exposed as the negative electrode in a large quantity of the radium emanation for three hours4 ,the rod was then removed and the ray effect from it immediately measured by an electroscope and compared with the corresponding ray effect from a known weight of radium bromide in radio-active equilibrium4 ,since the active deposit contains the product radium ,c which alone emits rays1 and1 since the intensities of the and rays are always proportional to each other1 the number of particles expelled from the lead rod per second is equal to the corresponding number from the weight of radium bromide which gives the same ray effect as the lead rod4 ,the rod was then enveloped in a thickness of aluminium foil of #jjec cms4--a thickness just sufficient to absorb the rays--and made the insulated electrode in a cylindrical metal vessel which was rapidly exhausted to a low pressure4 ,the current in the two directions was measured at intervals by an electrometer1 and1 as we have seen in section #ic1 the algebraic sum of these currents is proportional to ne1 where n is the number of particles expelled per second from the lead rod1 and e the charge on each particle4 ,the activity of the radium ,c decayed with the time1 but1 from the known curve of decay1 the results could be corrected in terms of the initial value immediately after the rod was removed from the emanation4 ,taking into account that half of the particles emitted by the active deposit were absorbed in the radium itself1 and reckoning the charge on the particle as #aac #aj coulombs1 two separate experiments gave #gf #aj and #gj #aj as the total number of particles expelled per second from one gram of radium4 ,taking the mean value1 we may conclude that the total number of particles expelled per second from one gram of radium in radio-active equilibrium is about #gc #aj4 ,the total number of particles expelled from one gram of radium at its minimum activity has been shown to be #fb #aj 7section #ic74 ,the approximate agreement between these numbers is a strong indication of the correctness of the theoretical views previously discussed4 ,it is to be expected that the number of particles1 deduced in this way1 will be somewhat greater than the true value1 since the particles give rise to a secondary radiation consisting also of negatively charged particles moving at a high speed4 ,these secondary particles1 arising from the impact of the particles on the lead1 will pass through the aluminium screen and add their effect to the primary rays4 ,the results1 however1 indicate that four particles are expelled from radium in radio-active equilibrium for each particle and thus confirm the theory of successive changes4 ,footnote #cbe3 ,phil4 ,trans4 ,a4 p4 #be1 #aija4 ,footnote #cbf3 ,p4 ,curie and ,laborde1 ,c4 ,r4 #acf1 p4 #fgc1 #aijc4 ,footnote #cbg3 ,runge and ,precht1 ,sitz4 ,ak4 ,wiss4 ,berlin1 ,no4 #ch1 #aijc4 ,footnote #cbh3 ,p4 ,curie1 ,socit de ,physique1 #aijc4 ,footnote #cbi3 ,rutherford and ,barnes1 ,nature1 ,oct4 #bi1 #aijc4 ,phil4 ,mag4 ,feb4 #aijd4 ,footnote #ccj3 ,paschen1 ,phys4 ,zeit4 ,sept4 #ae1 #aijd4 ,footnote #cca3 ,rutherford and ,barnes1 ,nature1 ,dec4 #ah1 #aijd2 ,phil4 ,mag4 ,may1 #aije4 ,footnote #ccb3 ,pegram1 ,science1 ,may #bg1 #aijd4 ,,chapter ,,xiii4 ,,radio-,,active ,,processes4 #bed4 ,theories of radio-activity4 ,in previous chapters1 a detailed account has been given of the nature and properties of the radiations1 and of the complex processes taking place in the radio-active substances4 ,the numerous products arising from the radio-elements have been closely examined1 and have been shown to result from a transformation of the parent element through a number of well-marked stages4 ,in this chapter1 the application of the disintegration theory to the explanation of radio-active phenomena will be considered still further1 and the logical deductions to be drawn from the theory will be discussed briefly4 ,a review will first be given of the working hypotheses which have served as a guide to the investigators in the field of radio-activity4 ,these working theories have in many cases been modified or extended with the growth of experimental knowledge4 ,the early experiments of ,mme ,curie had indicated that radio-activity was an atomic and not a molecular phenomenon4 ,this was still further substantiated by later work1 and the detection and isolation of radium from pitchblende was a brilliant verification of the truth of this hypothesis4 ,the discovery that the rays of the radio-elements were similar to the cathode rays produced in a vacuum tube was an important advance1 and has formed the basis of several subsequent theories4 ,j4 ,perrin#ccc1 in #aija1 following the views of ,j4 ,j4 ,thomson and others1 suggested that the atoms of bodies consisted of parts and might be likened to a miniature planetary system4 ,in the atoms of the radio-elements1 the parts composing the atoms more distant from the centre might be able to escape from the central attraction and thus give rise to the radiation of energy observed4 ,in ,december #aija1 ,becquerel#ccd put forward the following hypothesis1 which1 he stated1 had served him as a guide in his investigations4 ,according to the view of ,j4 ,j4 ,thomson1 radio-active matter consists of negatively and positively charged particles4 ,the former have a mass about of the mass of the hydrogen atom1 while the latter have a mass about one thousand times greater than that of the negative particle4 ,the negatively charged particles 7the rays7 would be projected with great velocity1 but the larger positive particles with a much lower velocity forming a sort of gas 7the emanation7 which deposits itself on the surface of bodies4 ,this in turn would subdivide1 giving rise to rays 7excited activity74 ,in a paper communicated to the ,royal ,society in ,june #aijj1 ,rutherford and ,mc,clung#cce estimated that the energy1 radiated in the form of ionizing rays into the gas1 was #cjjj gram-calories per year for radium of activity #ajj1jjj times that of uranium4 ,taking the latest estimate of the activity of a pure radium compound as #b1jjj1jjj1 this would correspond to an emission of energy into the gas in the form of rays of about #ff1jjj gram-calories per gram per year4 ,the suggestion was made that this energy might be derived from a re-grouping of the constituents of the atom of the radio-elements1 and it was pointed out that the possible energy to be derived from a greater concentration of the components of the atom was large compared with that given out in molecular reactions4 ,in the original papers#ccf giving an account of the discovery of the emanation of thorium and the excited radio-activity produced by it1 the view was taken that both of these manifestations were due to radio-active material4 ,the emanation behaved like a gas1 while the matter which caused excited activity attached itself to solids and could be dissolved in some acids but not in others4 ,rutherford and ,miss ,brooks showed that the radium emanation diffused through air like a gas of heavy molecular weight4 ,at a later date ,rutherford and ,soddy showed that the radium and thorium emanations behaved like chemically inert gases1 since they were unaffected by the most drastic physical and chemical treatment4 ,on the other hand1 ,p4 ,curie1 who1 in conjunction with ,debierne1 had made a series of researches on the radium emanation1 expressed dissent from this view4 ,p4 ,curie#ccg did not consider that there was sufficient evidence that the emanation was material in nature1 and pointed out that no spectroscopic evidence of its presence had yet been obtained1 and also that the emanation disappeared when contained in a sealed vessel4 ,it was pointed out by the writer#cch that the failure to detect spectroscopic lines was probably a consequence of the minute quantity of the emanation present1 under ordinary conditions1 although the electrical and phosphorescent actions produced by this small quantity are very marked4 ,this contention is borne out by later work4 ,p4 ,curie at first took the view that the emanation was not material1 but consisted of centres of condensation of energy attached to the gas molecules and moving with them4 ,m4 and ,mme ,curie have throughout taken a very general view of the phenomena of radio-activity1 and have not put forward any definite theory4 ,in ,jan4 #aijb1 they gave an account of the general working theory#cci which had guided them in their researches4 ,radio-activity is an atomic property1 and the recognition of this fact had created their methods of research4 ,each atom acts as a constant source of emission of energy4 ,this energy may either be derived from the potential energy of the atom itself1 or each atom may act as a mechanism which instantly regains the energy which is lost4 ,they suggested that this energy may be borrowed from the surrounding air in some way not accounted for by the principle of ,carnot4 ,in the course of a detailed study of the radio-activity of thorium1 ,rutherford and ,soddy#cdj found that it was necessary to suppose that thorium was continuously producing from itself new kinds of active matter1 which possess temporary activity and differ in chemical properties from the thorium itself4 ,the constant radio-activity of thorium was shown to be the result of equilibrium between the processes of production of active matter and the change of that already produced4 ,at the same time1 the theory was advanced that the production of active matter was a consequence of the disintegration of the atom4 ,the work of the following year was devoted to an examination of the radio-activity of uranium and radium on similar lines1 and it was found that the conclusions already advanced for thorium held equally for uranium and radium#cda4 ,the discovery of a condensation of the radio-active emanations#cdb gave additional support to the view that the emanations were gaseous in character4 ,in the meantime1 the writer#cdc had found that the rays consisted of positively charged bodies atomic in size1 projected with great velocity4 ,the discovery of the material nature of these rays served to strengthen the theory of atomic disintegration1 and at the same time to offer an explanation of the connection between the rays and the changes occurring in the radio-elements4 ,in a paper entitled 8,radio-active ,change10 ,rutherford and ,soddy#cdd put forward in some detail the theory of atomic disintegration as an explanation of the phenomena of radio-activity1 and at the same time some of the more important consequences which follow from the theory were discussed4 ,in a paper announcing the discovery of the heat emission of radium1 ,p4 ,curie and ,laborde#cde state that the heat energy may be equally well supposed to be derived from a breaking up of the radium atom or from energy absorbed by the radium from some external source4 ,j4 ,j4 ,thomson in an article on 8,radium10 communicated to ,nature#cdf1 put forward the view that the emission of energy from radium is probably due to some change within the atom1 and pointed out that a large store of energy would be released by a contraction of the atom4 ,sir ,william ,crookes#cdg1 in #ahii1 proposed the theory that the radio-active elements possess the property of abstracting energy from the gas4 ,if the moving molecules1 impinging more swiftly on the substance1 were released from the active substance at a much lower velocity1 the energy released from the radio-elements might be derived from the atmosphere4 ,this theory was advanced again later on to account for the large heat emission of radium1 discovered by ,p4 ,curie and ,laborde4 ,f4 ,re#cdh recently advanced a very general theory of matter with a special application to radio-active bodies4 ,he supposes that the parts of the atom were originally free1 constituting a nebula of extreme tenuity4 ,these parts have gradually become united round centres of condensation1 and have thus formed the atoms of the elements4 ,on this view an atom may be likened to an extinct sun4 ,the radio-active atoms occupy a transitional stage between the original nebula and the more stable chemical atoms1 and in the course of their contraction give rise to the heat emission observed4 ,lord ,kelvin in a paper to the ,british ,association meeting1 #aijc1 has suggested that radium may obtain its energy from external sources4 ,if a piece of white paper is put into one vessel and a piece of black paper into an exactly similar vessel1 on exposure of both vessels to the light the vessel containing the black paper is found to be at a higher temperature4 ,he suggests that radium in a similar manner may keep its temperature above the surrounding air by its power of absorption of unknown radiations4 ,richarz and ,schenck#cdi have suggested that radio-activity may be due to the production and breaking up of ozone which is known to be produced by radium salts4 #bee4 ,discussion of ,theories4 ,from the survey of the general hypotheses advanced as possible explanations of radio-activity1 it is seen that they may be divided broadly into two classes1 one of which assumes that the energy emitted from the radio-elements is obtained at the expense of the internal energy of the atom1 and the other that the energy is derived from external sources1 but that the radio-elements act as mechanisms capable of transforming this borrowed energy into the special forms manifested in the phenomena of radio-activity4 ,of these two sets of hypotheses the first appears to be the more probable1 and to be best supported by the experimental evidence4 ,up to the present not the slightest experimental evidence has been adduced to show that the energy of radium is derived from external sources4 ,j4 ,j4 ,thomson 7loc4 cit47 has discussed the question in the following way3-- 8,it has been suggested that the radium derives its energy from the air surrounding it1 that the atoms of radium possess the faculty of abstracting the kinetic energy from the more rapidly moving air molecules while they are able to retain their own energy when in collision with the slowly moving molecules of air4 ,i cannot see1 however1 that even the possession of this property would explain the behaviour of radium2 for imagine a portion of radium placed in a cavity in a block of ice2 the ice around the radium gets melted2 where does the energy for this come from8 ,by the hypothesis there is no change in the air-radium system in the cavity1 for the energy gained by the radium is lost by the air1 while heat cannot flow into the cavity from the outside1 for the melted ice round the cavity is hotter than the ice surrounding it40 ,the writer has recently found that the activity of radium is not altered by surrounding it with a large mass of lead4 ,a cylinder of lead was cast #aj cms4 in diameter and #aj cms4 high4 ,a hole was bored in one end of the cylinder to the centre1 and the radium1 enclosed in a small glass tube1 was placed in the cavity4 ,the opening was then hermetically closed4 ,the activity was measured by the rate of discharge of an electroscope by the rays transmitted through the lead1 but no appreciable change was observed during a period of one month4 ,m4 and ,mme ,curie early made the suggestion that the radiation of energy from the radio-active bodies might be accounted for by supposing that space is traversed by a type of ,rntgen rays1 and that the radio-elements possess the property of absorbing them4 ,recent experiments 7section #bgi7 have shown that there is present at the surface of the earth a very penetrating type of rays1 similar to the rays of radium4 ,even if it were supposed that the radio-elements possessed the power of absorbing this radiation1 the energy of the rays is far too minute to account even for the energy radiated from an element of small activity like uranium4 ,in addition1 all the evidence so far obtained points to the conclusion that the radio-active bodies do not absorb the type of rays they emit to any greater extent than would be expected from their density4 ,it has been shown 7section #hf7 that this is true in the case of uranium4 ,even if it were supposed that the radio-elements possess the property of absorbing the energy of some unknown type of radiation1 which is able to pass through ordinary matter with little absorption1 there still remains the fundamental difficulty of accounting for the peculiar radiations from the radio-elements1 and the series of changes that occur in them4 ,it is not sufficient for us to account for the heat emission only1 for it has been shown 7chapter ,,xii7 that the emission of heat is directly connected with the radio-activity4 ,in addition1 the distribution of the heat emission of radium amongst the radio-active products which arise from it is extremely difficult to explain on the hypothesis that the energy emitted is borrowed from external sources4 ,it has been shown that more than two-thirds of the heat emitted by radium is due to the emanation together with the active deposit which is produced by the emanation4 ,when the emanation is separated from the radium1 its power of emitting heat1 after reaching a maximum1 decreases with the time according to an exponential law4 ,it would thus be necessary on the absorption hypothesis to postulate that most of the heat emission of radium1 observed under ordinary conditions1 is not due to the radium itself but to something produced by the radium1 whose power of absorbing energy from external sources diminishes with time4 ,a similar argument also applies to the variation with time of the heating effect of the active deposit produced from the emanation4 ,it has been shown in the last chapter that most of the heating effect observed in radium and its products must be ascribed to the bombardment of the particles expelled from these substances4 ,it has already been pointed out 7section #acf7 that it is difficult to imagine any mechanism1 either internal or external1 whereby such enormous velocity can suddenly be impressed upon the particles4 ,we are forced to the conclusion that the particle did not suddenly acquire this energy of motion1 but was initially in rapid motion in the atom1 and for some reason1 was suddenly released with the velocity which it previously possessed in its orbit4 ,the strongest evidence against the hypothesis of absorption of external energy is that such a theory ignores the fact1 that1 whenever radio-activity is observed1 it is always accompanied by some change which can be detected by the appearance of new products having chemical properties distinct from those of the original substances4 ,this leads to some form of 8chemical0 theory1 and other results show that the change is atomic and not molecular4 #bef4 ,theory of radio-active change4 ,the processes occurring in the radio-elements are of a character quite distinct from any previously observed in chemistry4 ,although it has been shown that the radio-activity is due to the spontaneous and continuous production of new types of active matter1 the laws which control this production are different from the laws of ordinary chemical reactions4 ,it has not been found possible in any way to alter either the rate at which the matter is produced or its rate of change when produced4 ,temperature1 which is such an important factor in altering the rate of chemical reactions1 is1 in these cases1 almost entirely without influence4 ,in addition1 no ordinary chemical change is known which is accompanied by the expulsion of charged atoms with great velocity4 ,it has been suggested by ,armstrong and ,lowry#cej that radio-activity may be an exaggerated form of fluorescence or phosphorescence with a very slow rate of decay4 ,but no form of phosphorescence has yet been shown to be accompanied by radiations of the character of those emitted by the radio-elements4 ,whatever hypothesis is put forward to explain radio-activity must account not only for the production of a series of active products1 which differ in chemical and physical properties from each other and from the parent element1 but also for the emission of rays of a special character4 ,besides this1 it is necessary to account for the large amount of energy continuously radiated from the radio-elements4 ,the radio-elements1 besides their high atomic weights1 do not possess in common any special chemical characteristics which differentiate them from the other elements1 which do not possess the property of radio-activity to an appreciable degree4 ,of all the known elements1 uranium1 thorium1 and radium possess the greatest atomic weights1 viz43 radium #bbe1 thorium #bcbe1 and uranium #bdj4 ,if a high atomic weight is taken as evidence of a complicated structure of the atom1 it might be expected that disintegration would occur more readily in heavy than in light atoms4 ,at the same time1 there is no reason to suppose that the elements of the highest atomic weight must be the most radio-active2 in fact1 radium is far more active than uranium1 although its atomic weight is less4 ,this is seen to be the case also in the radio-active products2 for example1 the radium emanation is enormously more active weight for weight than the radium itself1 and there is every reason to believe that the emanation has an atom lighter than that of radium4 ,in order to explain the phenomena of radio-activity1 ,rutherford and ,soddy have advanced the theory that the atoms of the radio-elements suffer spontaneous disintegration1 and that each disintegrated atom passes through a succession of well-marked changes1 accompanied in most cases by the emission of rays4 ,a preliminary account of this hypothesis has already been given in section #acf1 while the mathematical theory of successive changes1 which is based upon it1 has been discussed in chapter ,,ix4 ,the general theory has been utilized in chapters ,x and ,,xi to account for the numerous active substances found in uranium1 thorium1 actinium and radium4 ,the theory supposes that1 on an average1 a definite small proportion of the atoms of each radio-active substance becomes unstable at a given time4 ,as a result of this instability1 the atoms break up4 ,in most cases1 the disintegration is explosive in violence and is accompanied by the ejection of an particle with great velocity2 in a few cases1 and particles are expelled together1 while in others a particle alone escapes4 ,in a few cases1 the change in the atom appears to be less violent in character1 and is not accompanied by the expulsion of either an or particle4 ,the explanation of these rayless changes is considered in section #bei4 ,the expulsion of an particle1 of mass about twice that of the hydrogen atom1 leaves behind it a new system lighter than the original one1 and possessing chemical and physical properties quite different from those of the original element4 ,this new system again becomes unstable1 and expels another particle4 ,the process of disintegration1 once started1 proceeds from stage to stage at a definite measurable rate in each case4 ,at any time after the disintegration has commenced1 there exists a proportion of the original matter1 which is unchanged1 mixed with the part which has undergone change4 ,this is in accordance with the observed fact that the spectrum of radium1 for example1 does not change progressively with time4 ,the radium breaks up so slowly that only a small fraction has been transformed in the course of a few years4 ,the unchanged part still shows its characteristic spectrum1 and will continue to do so as long as any radium exists4 ,at the same time it is to be expected that1 in old radium1 the spectrum of those products which exist in any quantity should also appear4 ,the term metabolon has been suggested as a convenient expression for each of these changing atoms1 derived from the successive disintegration of the atoms of the radio-elements4 ,each metabolon1 on an average1 exists only for a limited time4 ,in a collection of metabolons of the same kind the number ,n1 which are unchanged at a time t after production1 is given by ,n ,n e-t 1 where ,n is the original number4 ,now d,n_/dt -,n1 or the fraction of the metabolons present1 which change in unit time1 is equal to 4 ,the value #a_/ may be taken as the average life of each metabolon4 ,this may be simply shown as follows3--,at any time t after ,n metabolons have been set aside1 the number which change in the time dt is equal to ,ndt or ,n e-t dt 4 ,each metabolon has a life t1 so that the average life of the whole number is given by intinfty te-t dt frac #a ,the various metabolons from the radio-elements are distinguished from ordinary matter by their great instability and consequent rapid rate of change4 ,since a body which is radio-active must ipso facto be undergoing change1 it follows that none of the active products1 for example1 the emanations and ,th ,x1 can consist of any known kind of matter2 for there is no evidence to show that inactive matter can be made radio-active1 or that two forms of the same element can exist1 one radio-active and the other not4 ,for example1 half of the matter constituting the radium emanation has undergone change after an interval of four days4 ,after the lapse of about one month the emanation as such has nearly disappeared1 having been transformed through several stages into other and more stable types of matter1 which are in consequence difficult to detect by their radio-activity4 ,the striking difference in chemical and physical properties which exists in many cases between the various products themselves1 and also between the primary active substance and its products1 has already been drawn attention to in chapter ,,ix4 ,some of the products show distinctive electro-chemical behaviour and can be removed from a solution by electrolysis4 ,others show differences in volatility which have been utilized to effect a partial separation4 ,there can be no doubt that each of these products is a definite new chemical substance1 and if it could be collected in sufficient quantity to be examined by ordinary chemical means1 would be found to behave like a distinct chemical element4 ,it would differ1 however1 from the ordinary chemical element in the shortness of its life1 and the fact that it is continuously changing into another substance4 ,we shall see later 7section #bfa7 that there is every reason to believe that radium itself is a metabolon in the true sense of the term1 since it is continuously changing1 and is itself produced from another substance4 ,the main point of difference between it and the other products lies in the comparative slowness of its rate of change4 ,it is for this reason that radium exists in pitchblende in greater quantity than the other more rapidly changing products4 ,by working up a large amount of the mineral1 we have seen that a sufficient quantity of the pure product has been obtained for chemical examination4 ,on account of the short life of the emanation1 it exists in pitchblende in much less quantity than radium1 but it1 too1 has been isolated chemically and its volume measured4 ,the extraordinary properties of this emanation1 or gas1 have already been discussed1 and there can be no doubt that1 while it exists1 it must be considered a new element allied in chemical properties to the argon-helium group of gases4 ,there can be no doubt that in the radio-elements we are witnessing the spontaneous transformation of matter1 and that the different products which arise mark the stages or halting-places in the process of transformation1 where the atoms are able to exist for a short time before again breaking up into new systems4 #beg4 ,radio-active products4 ,the following table gives the list of the active products or metabolons known to result from the disintegration of the three radio-elements4 ,in the second column is given the value of the radio-active constant for each active product1 i4e4 the proportion of the active matter undergoing change per second2 in the third column the time ,t required for the activity to fall to one-half1 i4e4 the time taken for half the active product to undergo change2 in the fourth column1 the nature of the rays from each active product1 not including the rays from the products which result from it2 in the fifth column1 a few of the more marked physical and chemical properties of each metabolon4 ,products 7sec7 ,t ,nature of ,chemical and the rays ,physical properties of the product ,uranium -- -- ,soluble in excess of ammonium carbonate1 soluble in ether4 ,uranium ,x #cf #aj #bb days and ,insoluble in excess of ammonium carbonate1 soluble in ether and water4 ,thorium -- -- ,insoluble in ammonia4 ,thorium ,x #bj #aj #d days ,soluble in ammonia and water4 ,emanation #ac #aj #ec secs4 ,chemically inert gas of heavy molecular weight4 ,condenses at #abj ,c4 ,thorium ,a #agd #aj #aa hours no rays ,deposited on bodies2 concentrated on the cathode in an electric field4 ,soluble in some acids2 ,th ,a more volatile than ,th ,b2 shows definite electro-chemical behaviour4 ,thorium ,b #bb #aj #ee mins4 1 1 ,same 8 -- -- -- ,actinium -- -- no rays ,insoluble in ammonia4 ,actinium ,x #gh #aj #ajb days 7and 87 ,soluble in ammonia4 ,emanation #ag #ci secs4 ,behaves like a gas4 ,actinium ,a #cb #aj #cf mins4 no rays ,deposited on bodies2 concentrated on the cathode in an electric field1 soluble in ammonia and strong acids2 volatilized at a temperature of #ajj ,c41 ,a and ,b can be separated by electrolysis4 ,actinium ,b #ed #aj #bae mins4 1 1 ,same 8 -- -- -- ,radium -- #acjj years ,allied chemically to barium4 ,emanation #ba #aj #ch days ,chemically inert gas of heavy molecular weight2 condenses at #aej ,c4 ,radium ,a #che #aj #c mins4 ,deposited on 7active surface of deposit of bodies2 rapid concentrated on change7 cathode in electric field2 soluble in strong acids2 ,b volatized at about #gjj ,c41 ,a and ,c at about #ajjj ,c4 ,radium ,b #ech #aj #ba mins4 no rays ,same 7same7 ,radium ,c #dac #aj #bh mins4 1 1 ,same 7same7 ,radium ,d -- about #dj no rays ,soluble in 7active acids2 volatile deposit of below #ajjj ,c4 slow change7 ,radium ,e #ac #aj #f days and ,non-volatile at 7same7 #ajjj ,c4 ,radium ,f #ef #aj #adc days ,deposited on 7same7 bismuth from solution2 volatile at about #ajjj ,c41 same properties as radio-tellurium and polonium4 ,the products and their radiations are indicated graphically in ,fig4 #ajb on page #dej4 ,illustration3 ,fig4 #ajb4 ,one product has been observed in uranium1 four in thorium1 four in actinium and seven in radium4 ,it is not improbable that a closer examination of the radio-elements may reveal still further changes4 ,if any very rapid transformations exist1 they would be very difficult to detect4 ,the change of thorium ,x into the emanation1 for example1 would probably not have been discovered if the product of the change had not been gaseous in character4 ,the electrolysis of solutions is1 in many cases1 a very powerful method of separating active products from one another1 and its possibilities have not yet been exhausted4 ,the main family of changes of the radio-elements1 as far as they are known1 have been investigated closely1 and it is not likely that any product of comparatively slow rate of change has been overlooked4 ,there is a possibility1 however1 that two radio-active products may in some cases arise from the disintegration of a single substance4 ,this point is discussed further in section #bfj4 ,the remarkable way in which the disintegration theory can be applied to unravel the intricacies of the succession of radio-active changes is very well illustrated in the case of radium4 ,without its aid1 it would not have been possible to disentangle the complicated processes which occur4 ,we have already seen that this analysis has been instrumental in showing that the substances polonium1 radio-tellurium and radio-lead are in reality products of radium4 ,after the radio-active substances have undergone the succession of changes traced above1 a final stage is reached where the atoms are either permanently stable1 or change so slowly that it is difficult to detect their presence by means of their radio-activity4 ,it is probable1 however1 that the process of transformation still continues through further slow stages4 ,there is now considerable evidence that the elements uranium1 radium and actinium are intimately connected together4 ,the two latter probably result from the breaking up of uranium4 ,the evidence in support of this idea is given in section #bfb1 but there still remains much work to be done to bridge over the gaps which at present appear to separate these elements from one another4 ,after the series of transformations have come to an end1 there will probably remain a product or products which will be inactive1 or active only to a minute extent4 ,in addition1 since the particles1 expelled during the transformation1 are material in nature1 and are non-radio-active1 they must collect in some quantity in radio-active matter4 ,the probability that the particles consist of helium is considered later in section #bfh4 ,the value of ,t1 the time for a product to be half-transformed1 may be taken as a comparative measure of the stability of the different metabolons4 ,the stability of the products varies over a very wide range4 ,for example1 the value of ,t for radium ,d is #dj years1 and for the actinium emanation #ci secs4 ,this corresponds to a range of stability measured by #ch #aj4 ,the range of stability is still further extended1 when it is remembered that the atoms of the radio-elements themselves are very slowly changing4 ,the only two metabolons of about the same stability are thorium ,x and the radium emanation4 ,in each case1 the transformation is half completed in about four days4 ,i consider that the approximate agreement of the numbers is a mere coincidence1 and that the two types of matter are quite distinct from one another2 for1 if the metabolons were identical1 it would be expected that the changes which follow would take place in the same way and at the same rate1 but such is not the case4 ,moreover1 ,th ,x and the radium emanation have chemical and physical properties quite distinct from one another4 ,it is very remarkable that the three radio-active substances1 radium1 thorium and actinium1 should exhibit such a close similarity in the succession of changes which occur in them4 ,each of them at one stage of its disintegration emits a radio-active gas1 and in each case this gas is transformed into a solid which is deposited upon the surface of bodies4 ,it would appear that1 after disintegration of an atom of any of these has once begun1 there is a similar succession of changes1 in which the resulting systems have allied chemical and physical properties4 ,such a connection is of interest as indicating a possible origin of the recurrence of properties in the atoms of the elements1 as exemplified by the periodic law4 ,the connection between thorium and actinium is especially close both as regards the number and nature of the products4 ,the period of transformation of the successive products1 though differing in magnitude1 rises and falls in a very analogous manner4 ,this indicates that the atoms of these two elements are very similarly constituted4 #beh4 ,amount of the products4 ,by application of the theory of successive changes1 the probable amount of each of the products present in radium and the other radio-elements can readily be estimated4 ,since each radio-atom expels one particle of atomic weight about that of hydrogen or helium1 the atoms of the intermediate products will not differ much in weight from the parent atom4 ,the approximate weight of each product present in a gram of radium can be readily deduced4 ,let ,n,a1 ,n,b1 ,n,c be the number of atoms of the products ,a1 ,b1 ,c present per gram in radio-active equilibrium4 ,let ,a1 ,b1 ,c be the corresponding constants of change4 ,then if q is the number of the parent atoms breaking up per second1 per gram1 q ,a,n,a ,b,n,b ,c,n,c4 ,consider the case of the radium products1 where the value of q is #fb #aj 7section #ic74 ,knowing the value of and q1 the value of ,n can at once be calculated4 ,the corresponding weight can be deduced1 since in one gram of matter of atomic weight about #bjj1 there are about #d #aj atoms 7section #ci74 ,the results are shown in the following table3-- ,product ,value of ,number of ,weight of 7sec7 atoms1 ,n1 product gram present per of radium gram ,radium #bj #aj #cb #aj #h #aj emanation ,radium ,a #ch #aj #ag #aj #d #aj ,radium ,b #ed #aj #ac #aj #c #aj ,radium ,c #da #aj #af #aj #d #aj ,with the small quantities of radium available1 the amounts of the products radium ,a1 ,b and ,c are too small to weigh4 ,it may be possible1 however1 to detect their presence by means of the spectroscope4 ,in the case of thorium1 the weight of the product ,th ,x1 which is present in greatest quantity1 is far too small to be detected4 ,since the value of for ,th ,x is about the same as for the radium emanation1 the maximum weight present per gram is about #d #aj of a gram1 remembering that q for radium is about #b #aj times the value for thorium4 ,even with a kilogram of thorium1 the amount of ,th ,x is far too small to be detected by its weight4 ,this method can be used generally to calculate the relative amounts of any successive products in radio-active equilibrium1 provided the value of for each product is known4 ,for example1 it will be shown later that uranium is the parent of radium and is half transformed in about #f #aj years1 while radium and radium ,d are half transformed in #acjj and #dj years respectively4 ,the weight of radium present in one gram of uranium1 when equilibrium is established1 is thus #b #aj grams1 and the weight of radium ,d is #g #aj grams4 ,in a mineral containing a ton of uranium there should be about #ah grams of radium and #jfc grams of radium ,d4 ,some recent experiments described in section #bfb show that these theoretical estimates are about twice too great4 #bei4 ,rayless ,changes4 ,the existence of well-marked changes in radium1 thorium1 and actinium1 which are not accompanied by the expulsion of or particles1 is of great interest and importance4 ,since the rayless changes are not accompanied by any appreciable ionization of the gas1 their presence cannot be detected by direct means4 ,the rate of change of the substance can1 however1 be determined indirectly1 as we have seen1 by measurement of the variation with time of the activity of the succeeding product4 ,the law of change has been found to be the same as for the changes which give rise to rays4 ,the rayless changes are thus analogous1 in some respects1 to the monomolecular changes observed in chemistry1 with the difference that the changes are in the atom itself1 and are not due to the decomposition of a molecule into simpler molecules or into its constituent atoms4 ,it must be supposed that a rayless change is not of so violent a character as one which gives rise to the expulsion of or particles4 ,the change may be accounted for either by supposing that there is a rearrangement of the components of the atom1 or that the atom breaks up without the expulsion of its parts with sufficient velocity to produce ionization by collision with the gas4 ,the latter point of view1 if correct1 at once indicates the possibility that undetected changes of a similar character may be taking place slowly in the non-radio-active elements2 or1 in other words1 that all matter may be undergoing a slow process of change4 ,the changes taking place in the radio-elements have been observed only in consequence of the expulsion with great velocity of the parts of the disintegrated atom4 ,some recent experiments described in ,appendix ,a show that the particle from radium ceases to ionize the gas when its velocity falls below about #aj cms4 per second4 ,it is thus seen that particles may be projected with a great velocity1 and yet fail to produce ionization in the gas4 ,in such a case1 it would be difficult to follow the changes by the electrical method1 as the electrical effects would be very small in comparison with those produced by the known radio-active bodies4 #bfj4 ,radiations from the products4 ,we have seen that the great majority of the radio-active products break up with the expulsion of particles1 and that the particle with its accompaniment of the ray appears in most cases only in the last rapid change4 ,in the case of radium1 for example1 which has been most closely investigated on account of its great activity1 radium itself1 the emanation and radium ,a emit only particles2 radium ,b emits no rays at all2 while radium ,c emits all three kinds of rays4 ,it is difficult to settle with certainty whether the products thorium ,x and actinium ,x emit particles or not1 but the and rays certainly appear in each case in the last rapid change in the active deposit1 and1 in this respect1 behave in a similar manner to radium4 ,the very slow moving electrons which accompany the particles emitted from radium 7section #ic7 are not taken into account1 for they appear to be liberated as a result of the impact of particles on matter1 and are expelled with a speed insignificant compared with that of the particles emitted from radium ,c4 ,the appearance of and rays only in the last rapid changes of the radio-elements is very remarkable1 and cannot be regarded as a mere coincidence4 ,the final expulsion of a particle results in the appearance of a product of great stability1 or1 in the case of radium1 of a product 7radium ,d7 which has far more stability than the preceding one4 ,it would appear that the initial changes are accompanied by the expulsion of an particle1 and that once the particle is expelled1 the components of the residual atom fall into an arrangement of fairly stable equilibrium1 where the rate of transformation is very slow4 ,it thus appears probable that the particle1 which is finally expelled1 may be regarded as the active agent in promoting the disintegration of the radio-atom through the successive stages4 ,a discussion of this question will be given with more advantage later 7section #bgj71 when the general question of the stability of the atom is under consideration4 ,it is significant that the change in which the three types of rays appear is far more violent in character than the preceding changes4 ,not only are the particles expelled with greater velocity than in any other change1 but the particles are projected with a velocity very closely approaching that of light4 ,there is always a possibility that1 in such a violent explosion in the atom1 not only may the and particles be expelled1 but the atom itself may be disrupted into several fragments4 ,if the greater proportion of the matter resulting from the disintegration is of one kind1 it would be difficult to detect the presence of a small quantity of rapidly changing matter from observations of the rate of decay2 but1 if the products have distinctive electro-chemical behaviour1 a partial separation should1 in some cases1 be effected by electrolysis4 ,it has already been pointed out that the results of ,pegram and von ,lerch 7section #bjg7 on the electrolysis of thorium solutions may be explained on the supposition that thorium ,a and ,b have distinctive electro-chemical behaviour4 ,pegram1 however1 in addition observed the presence of a product which decayed to half value in six minutes4 ,this active product was obtained by electrolysing a solution of pure thorium salt1 to which a small quantity of copper nitrate had been added4 ,the copper deposit was slightly active and lost half of its activity in about six minutes4 ,the presence of such radio-active products1 which do not come under the main scheme of changes1 indicates that1 at some stage of the disintegration1 more than one substance results4 ,in the violent disintegration which occurs in radium ,c and thorium ,b1 such a result is to be expected1 for it is not improbable that there are several arrangements whereby the constituents of the atom form a system of some slight stability4 ,the two products resulting from the disintegration would probably be present in unequal proportion1 and1 unless they gave out different kinds of rays1 would be difficult to separate from each other4 #bfa4 ,life of radium4 ,since the atoms of the radio-elements are continuously breaking up1 they must also be considered to be metabolons1 the only difference between them and metabolons such as the emanations ,th ,x and others being their comparatively great stability and consequent very slow rate of change4 ,there is no evidence that the process of change1 traced above1 is reversible under present conditions1 and in the course of time a quantity of radium1 uranium1 or thorium left to itself must gradually be transformed into other types of matter4 ,there seems to be no escape from this conclusion4 ,let us consider1 for example1 the case of radium4 ,the radium is continuously producing from itself the radium emanation1 the rate of production being always proportional to the amount of radium present4 ,all the radium must ultimately be changed into emanation1 which in turn is transformed through a succession of stages into other kinds of matter4 ,there is no doubt that the emanation is chemically quite different from radium itself4 ,the quantity of radium must diminish1 to compensate for the emanation which is formed2 otherwise it is necessary to assume that matter in the form of emanation is created from some unknown source4 ,an approximate estimate of the rate of change of radium can easily be made by two different methods depending upon 7#a7 the number of atoms of radium breaking up per second1 and 7#b7 the amount of emanation produced per second4 ,it has been shown experimentally 7section #ic7 that #a gram of radium at its minimum activity expels #fb #aj particles per second4 ,the heating effect of radium and also its volume agree closely with calculation1 if it is supposed that each atom of each product in breaking up emits one particle4 ,on this supposition it is seen that #fb #aj atoms of radium break up per second4 ,now it has been shown experimentally 7section #ci7 that one cubic centimetre of hydrogen at standard pressure and temperature contains #cf #aj molecules4 ,taking the atomic weight of radium as #bbe1 the number of atoms in #a gram of radium is equal to #cf #aj4 ,the fraction of radium which breaks up is thus #aie #aj per second1 or #ed #aj per year4 ,it follows that in each gram of radium about half a milligram breaks up per year4 ,the average life of radium is about #ahjj years1 and half of the radium is transformed in about #acjj years4 ,we shall now consider the calculation1 based on the observed result of ,ramsay and ,soddy1 that the volume of emanation to be obtained from one gram of radium is about #a cubic millimetre4 ,the experimental evidence based on diffusion results indicates that the molecular weight of the emanation is about #ajj4 ,if the disintegration theory is correct1 the emanation is an atom of radium minus one particle1 and therefore must have a molecular weight of at least #bjj4 ,this high value is more likely to be correct than the experimental number1 which is based on evidence that must necessarily be somewhat uncertain4 ,now the rate of production of emanation per second is equal to ,n1 where ,n is the equilibrium amount4 ,taking the molecular weight as #bjj1 the weight of emanation produced per second from #a gram of radium #hif #aj #ai #aj gram4 ,now the weight of emanation produced per second is very nearly equal to the weight of radium breaking up per second4 ,thus the fraction of radium breaking up per second is about #ai #aj1 which is in agreement with the number previously calculated by the first method4 ,we may thus conclude that radium is half transformed in about #acjj years4 ,taking the activity of pure radium as about two million times that of uranium1 and remembering that only one change1 which gives rise to rays1 occurs in uranium and four in radium1 it can readily be calculated that the fraction of uranium changing per year is about #aj4 ,from this it follows that uranium should be half transformed in about #f #aj years4 ,if thorium is a true radio-active element1 the time for half transformation is about #bd #aj years1 since thorium has about the same activity as uranium but contains four products which emit rays4 ,if the activity of thorium is due to some radio-active impurity1 no estimate of the length of its life can be made until the primary active substance has been isolated and its activity measured4 #bfb4 ,origin of radium4 ,the changes in radium are thus fairly rapid1 and a mass of radium if left to itself should in the course of a few thousand years have lost a large proportion of its radio-activity4 ,taking the above estimate of the life of radium1 the value of is #ed #aj1 with a year as the unit of time4 ,a mass of radium left to itself should be half transformed in #acjj years and only one-millionth part would remain after #bf1jjj years4 ,thus supposing1 for illustration1 that the earth was originally composed of pure radium1 its activity per gram #bf1jjj years later would not be greater than the activity observed to-day in a good specimen of pitchblende4 ,even supposing this estimate of the life of radium is too small1 the time required for the radium practically to disappear is short compared with the probable age of the earth4 ,we are thus forced to the conclusion that radium is being continuously produced in the earth1 unless the very improbable assumption is made1 that radium was in some way suddenly formed at a date recent in comparison with the age of the earth4 ,it was early suggested by ,rutherford and ,soddy#cea that radium might be a disintegration product of one of the radio-elements found in pitchblende4 ,both uranium and thorium fulfil the conditions required in a possible source of production of radium4 ,both are present in pitchblende1 have atomic weights greater than that of radium1 and have rates of change which are slow compared with that of radium4 ,in some respects1 uranium fulfils the conditions required better than thorium2 for it has not been observed that minerals rich in thorium contain much radium1 while on the other hand1 the pitchblendes containing the most radium contain a large proportion of uranium4 ,if radium is not produced from uranium1 it is certainly a remarkable coincidence that the greatest activity of pitchblende yet observed is about five or six times that of uranium4 ,since radium has a life short compared with that of uranium1 the amount of radium produced should reach a maximum value after a few thousand years1 when the rate of production of fresh radium--which is also a measure of the rate of change of uranium--balances the rate of change of that product4 ,in this respect the process would be exactly analogous to the production of the emanation by radium1 with the difference that the radium changes much more slowly than the emanation4 ,but since radium itself in its disintegration gives rise to at least five changes with the corresponding production of rays1 the activity due to the radium 7measured by the rays71 when in a state of radio-active equilibrium with uranium1 should be about five times that of the uranium that produces it2 for it has been shown that only one change has so far been observed in uranium in which rays are expelled4 ,taking into account the presence of actinium in pitchblende1 the activity observed in the best pitchblende is about the same as would be expected if the radium were a disintegration product of uranium4 ,if this hypothesis is correct1 the amount of radium in any pitchblende should be proportional to the amount of uranium present1 provided the radium is not removed from the mineral by percolating water4 ,this question has been experimentally attacked by ,boltwood#ceb1 ,mc,coy#cec and ,strutt#ced4 ,mc,coy measured the relative activities of different minerals in the form of powder by means of an electroscope1 and determined the amount of uranium present by chemical analysis4 ,his results indicated that the activity observed in the minerals was very approximately proportional to their content of uranium4 ,since actinium is present as well as uranium and its products1 this would indicate that the amount of radium and actinium taken together is proportional to the amount of uranium4 ,this problem has been attacked more directly by ,boltwood and ,strutt by measuring the relative amount of the radium emanation evolved by different minerals4 ,by dissolving the mineral and then setting it aside in a closed vessel1 the amount of emanation present reaches a maximum value after about a month's interval4 ,the emanation is then introduced into a closed vessel containing a gold-leaf electroscope similar to that shown in ,fig4 #ab4 ,the rate of movement of the gold-leaf is proportional to the amount of emanation from the solution1 and this in turn is proportional to the amount of radium4 ,boltwood has made in this way a very complete and accurate comparison of the radium content of different varieties of pitchblende and other ores containing radium4 ,it was found that many of the minerals in the solid state allowed a considerable fraction of the emanation to escape into the air4 ,the percentage fraction of the total amount of emanation lost in this way is shown in ,column ,,ii of the following table4 ,column ,i gives the maximum amount of emanation present in #a gram of the mineral in arbitrary units when none of the emanation escapes2 ,column ,,iii the weight in grams of uranium contained in #a gram2 and ,column ,,iv the ratio obtained by dividing the quantity of emanation by the quantity of uranium4 ,the numbers in ,column ,,iv should be constant1 if the amount of radium is proportional to the amount of uranium4 ,locality ,i ,,ii ,,iii ,,iv ,substance ,uraninite ,north #agjj #aac #jgdfe #bbh ,carolina ,uraninite ,colorado #aeea #eb #jfifa #bbc ,gummite ,north #adgj #acg #jfech #bbe ,carolina ,uraninite ,joachimsthal #acif #ef #jfagd #bbf ,uranophane ,north #aagg #hb #jeafh #bbh ,carolina ,uraninite ,saxony #aaef #bg #jejfd #bbh ,uranophane ,north #aace #bbh #jdihd #bbh ,carolina ,thorogummite ,north #gbi #afb #jccag #bbj ,carolina ,carnotite ,colorado #dig #afc #jbbfa #bbj ,uranothorite ,norway #beb #ac #jaach #bba ,samarskite ,north #bcd #jg #jajdd #bbd ,carolina ,orangite ,norway #bca #aa #jajcd #bbc ,euxenite ,norway #aii #je #jjhga #bbh ,thorite ,norway #aff #fb #jjged #bbj ,fergusonite ,norway #abj #je #jjeeg #bae ,aeschynite ,norway #ajj #jb #jjdeb #bba ,xenotine ,norway #aed #bfj #jjjgj #bbj ,monazite ,north #jhh #jjjdc #bje 7sand7 ,carolina ,monazite ,norway #jhd #ab #jjjda #bjg 7crys47 ,monazite ,brazil #jgf #jjjca #bde 7sand7 ,monazite ,conn4 #jfc #jjjcj #baj 7massive7 ,with the exception of some of the monazites1 the numbers show a surprisingly good agreement1 and1 taking into consideration the great variation of the content of uranium in the different minerals1 and the wide range of locality from which they were obtained1 the results afford a direct and satisfactory proof that the amount of radium in the minerals is directly proportional to the amount of uranium4 ,in this connection1 it is of interest to note that ,boltwood found that a considerable quantity of radium existed in various varieties of monazite1 although most of the previous analyses agreed in stating that no uranium was present4 ,a careful examination was in consequence made to test this point1 and it was found by special methods that uranium was present1 and in about the amount to be expected from the theory4 ,the ordinary methods of analysis failed to give correct results on account of the presence of phosphates4 ,results of a similar character have recently been given by ,strutt#cee4 ,the weight of radium in a mineral per gram of uranium is thus a definite constant of considerable practical importance4 ,its value was recently determined by ,boltwood by comparison of the emanation1 liberated from a known weight of uraninite1 with that liberated from a known quantity of pure radium bromide1 supplied for the purpose by the writer4 ,a measured weight of radium bromide was taken from a stock which gave out heat at a rate of slightly over #ajj gram calories per hour per gram1 and was thus probably pure4 ,this was dissolved in water1 and1 by successive dilutions1 a standard solution was made up containing #aj gram of radium bromide per c4c4 ,taking the constitution of radium bromide as ,ra,br1 it was deduced that the weight of radium per gram of uranium in any mineral was #hj #aj gram4 ,the amount of radium in a mineral per ton of uranium is thus #jgb gram4 ,strutt 7loc4 cit47 obtained a value nearly twice as great1 but he had no means of ascertaining the purity of his radium bromide4 ,this amount of radium per gram of uranium is of the right order of magnitude to be expected on the disintegration theory1 if uranium is the parent of radium4 ,the activity of pure radium1 compared with uranium1 is not known with sufficient accuracy to determine with accuracy the theoretical proportion of radium to uranium4 ,the production of radium from uranium1 while very strongly supported by these experiments1 cannot be considered definitely established until direct experimental evidence is obtained of the growth of radium in uranium4 ,the rate of production of radium to be expected on the disintegration theory can readily be estimated4 ,the fraction of uranium breaking up per year has been calculated 7section #bfa7 and shown to be about #aj per year4 ,this number represents the weight of radium produced per year from #a gram of uranium4 ,the emanation1 released from the amount of radium produced in one year from #a gram of uranium1 would cause an ordinary gold-leaf electroscope to be discharged in about half-an-hour4 ,if a kilogram of uranium is used1 the amount of radium produced in a single day should be easily detectable4 ,experiments to detect the growth of radium in uranium have been made by several observers4 ,soddy#cef examined the amount of emanation given off at different times from one kilogram of uranium nitrate in solution1 which was originally freed from the small trace of radium present by a suitable chemical process4 ,the solution was kept stored in a closed vessel1 and the amount of emanation which collected in the solution was measured at regular intervals4 ,preliminary experiments showed that the actual rate of production of radium was far less than the amount to be expected theoretically1 and at first very little indication was obtained that radium was produced at all4 ,after allowing the uranium to stand for eighteen months1 ,soddy states that the amount of emanation was distinctly greater than at first4 ,the solution after this interval contained about #ae #aj gram of radium4 ,this gives the value of about #b #aj for the fraction of uranium changing per year1 while the theoretical value is about #aj4 ,whetham#ceg also found that a quantity of uranium nitrate which had been set aside for a year showed an appreciable increase in the content of radium1 and considers that the rate of production is faster than that found by ,soddy4 ,in his case1 the uranium was not originally completely freed from radium4 ,observations extending over years will be required before the question can be considered settled1 for the accurate estimation of small quantities of radium by the amount of emanation is beset with difficulties4 ,this is especially the case where observations are made over wide intervals of time4 ,the writer has made an examination to see if radium is produced from actinium or thorium4 ,it was thought possible that actinium might prove to be an intermediate product between uranium and radium4 ,the solutions1 freed from radium1 have been set aside for a year1 but no certain increase in the content of radium has been observed4 ,there is little doubt that the production of radium by uranium first proceeds at only a small fraction of the rate to be expected from theory4 ,this is not surprising when we consider that probably several changes intervene between the product ,ur ,x and the radium4 ,in the case of radium1 for example1 it has been shown that a number of slow changes follow the rapid changes ordinarily observed4 ,on account of the feeble activity of uranium1 it would not be easy to detect directly the occurrence of such changes4 ,if1 for example1 one or more rayless products occurred between ,ur ,x and radium1 which were removed from the uranium by the same chemical process used to free it from radium1 the rate of production of radium would be very small at first1 but would be expected to increase with time as more of the intermediary products were stored up in the uranium4 ,the fact that the contents of uranium and radium in radio-active minerals are always proportional to each other1 coupled with definite experimental evidence that radium is produced from uranium1 affords an almost conclusive proof that uranium is in some way the parent of radium4 ,the general evidence which has been advanced to show that radium must be continuously produced from some other substance applies also to actinium1 which has an activity of the same order of magnitude as that of radium4 ,the presence of actinium with radium in pitchblende would indicate that this substance also is in some way derived from uranium4 ,it is possible that actinium may prove to be produced either from radium or to be the intermediary substance between uranium and radium4 ,if it could be shown that the amount of actinium in radio-active minerals is1 like radium1 proportional to the amount of uranium1 this would afford indirect proof of such a connection4 ,it is not so simple to settle this point for actinium as for radium1 since actinium gives out a very short-lived emanation1 and the methods adopted to determine the content of radium in minerals cannot be applied without considerable modifications to determine the amount of actinium present4 ,the experimental data1 so far obtained1 do not throw much light upon the origin of the primary active matter in thorium4 ,hofmann and others 7section #bc7 have shown that thorium separated from minerals containing uranium is always more active the greater the quantity of uranium present4 ,this would indicate that the active substance in thorium also may be derived from uranium4 ,while much work still remains to be done1 a promising beginning has already been made in determining the origin and relation of the radio-elements4 ,we have seen that the connection between polonium1 radio-tellurium1 and radio-lead with radium has already been established4 ,radium itself is now added to the list1 and it is probable that actinium will soon follow4 ,while the experiments undoubtedly show that there is a definite relation between the amount of uranium and radium present in the ordinary radio-active minerals1 ,danne#ceh has recently called attention to a very interesting apparent exception4 ,considerable quantities of radium were found in certain deposits in the neighbourhood of ,issy-l',evque in the ,sane-,loire district1 although no trace of uranium was present4 ,the active matter is found in pyromorphite 7phosphate of lead71 in clays containing lead1 and in pegmatite1 but the radium is usually present in greater quantities in the former4 ,the pyromorphite is found in veins of the quartz and feldspar rocks4 ,the veins are always wet owing to the presence of a number of springs in the neighbourhood4 ,the content of uranium in the pyromorphite varies considerably1 but ,danne considers that about a centigram of radium is present per ton4 ,it seems probable that the radium found in this locality has been deposited from water flowing through it1 possibly in past times4 ,the presence of radium is not surprising1 since crystals of autunite have been found about #dj miles distant1 and probably there are deposits containing uranium in that region4 ,this result is of interest1 as suggesting that radium may be removed with water and deposited by physical or chemical action some distance away4 ,it will be shown in the next chapter that radium has been found very widely distributed over the surface of the earth1 but generally in very small quantities4 #bfc4 ,does the radio-activity of radium depend upon its concentration8 ,we have seen that the radio-active constant of any product is independent of the concentration of the product4 ,this result has been established over a very wide range for some substances1 and especially for the radium emanation4 ,no certain difference in the rate of decay of the emanation has been observed1 although the amount present in unit volume of the air has been varied a millionfold4 ,it has been suggested by ,j4 ,j4 ,thomson#cei that the rate of disintegration of radium may be influenced by its own radiations4 ,this1 at first sight1 appears very probable1 for a small mass of a pure radium compound is subjected to an intense bombardment by the radiations arising from it1 and the radiations are of such a character that they might be expected to produce a breaking up of the atoms of matter which they traverse4 ,if this be the case1 the radio-activity of a given quantity of radium should be a function of its concentration1 and should be greater in the solid state than when disseminated through a large mass of matter4 ,the writer has made an experiment to examine this question4 ,two glass tubes were taken1 in one of which was placed a few milligrams of pure radium bromide in a state of radio-active equilibrium1 and in the other a solution of barium chloride4 ,the two tubes were connected near the top by a short cross tube1 and the open ends sealed off4 ,the activity of the radium in the solid state was tested immediately after its introduction by placing it in a definite position near an electroscope made of thin metal of the type shown in ,fig4 #ab4 ,the increased rate of discharge of the electroscope due to the and rays from the radium was observed4 ,when a lead plate #f mms4 in thickness was placed between the radium and the electroscope1 the rate of discharge observed was due to the rays alone4 ,by slightly tilting the apparatus1 the barium solution flowed into the radium tube and dissolved the radium4 ,the tube was well shaken1 so as to distribute the radium uniformly throughout the solution4 ,no appreciable change of the activity measured by the rays was observed over the period of one month4 ,the activity measured by the and rays was somewhat reduced1 but this was not due to a decrease of the radio-activity1 but to an increased absorption of the rays in their passage through the solution4 ,the volume of the solution was at least #ajjj times greater than that of the solid radium bromide1 and1 in consequence1 the radium was subjected to the action of a much weaker radiation4 ,i think we may conclude from this experiment that the radiations emitted by radium have little if any influence in causing the disintegration of the radium atoms4 ,voller#cfj recently published some experiments which appeared to show that the life of radium varied enormously with its concentration4 ,in his experiments1 solutions of radium bromide of known strengths were evaporated down in a platinum vessel #ab sq4 cms4 in area1 and their activity tested from time to time4 ,the activity of the radium1 so deposited1 at first showed the normal rise to be expected on account of the production of the emanation1 but after reaching a maximum1 it rapidly decayed4 ,for a weight of #aj mgrs4 of radium bromide1 the activity for example1 practically disappeared in #bf days after reaching its maximum4 ,the time taken for the activity to disappear increased rapidly with the amount of radium present4 ,in another set of experiments1 he states that the activity observed on the vessel was not proportional to the amount of radium present4 ,for example1 the activity only increased #bd times for a millionfold increase of the radium present1 from #aj mgrs4 to #aj mgrs4 ,these results1 however1 have not been confirmed by later experiments made by ,eve4 ,he found that1 over the range examined1 the activity was directly proportional to the amount of radium present1 within the limits of experimental error4 ,the following table illustrates the results obtained4 ,the radium was evaporated down in platinum vessels #di sq4 cms4 in area4 ,weight of ,activity in radium in arbitrary units milligrams #aj #ajjj #aj #ajf #aj #aah #aj #abe ,for an increase of one-thousandfold of the quantity of radium1 the activity increased #hjj times1 while ,voller states that the activity1 in his experiments1 only increased #c to #d times4 ,in the experiments of ,eve1 the activity was measured by observing the increased rate of discharge of a gold-leaf electroscope when the platinum vessel containing the active deposit was placed inside the electroscope4 ,the activity of #aj mgrs4 was too small to be measured with accuracy in the electroscope employed1 while #aj mgrs4 gave too rapid a rate of discharge4 ,on the other hand1 the method of measurement employed by ,voller was unsuitable for the measurement of very weak radio-activity4 ,eve also found that a small quantity of radium kept in a closed vessel did not lose its activity with time4 ,a silvered glass vessel contained a gold-leaf system1 such as is shown in ,fig4 #ab4 ,a solution containing #aj mgrs4 of radium bromide was evaporated over the bottom of the vessel of area #gf sq4 cms4 ,the activity1 after reaching a maximum1 has remained constant over the #ajj days during which observations have so far been made4 ,these experiments of ,eve1 as far as they go1 show that the activity of radium is proportional to the amount of radium present1 and that radium1 kept in a closed vessel1 shows no signs of decreasing in activity4 ,on the other hand1 ,i think there is no doubt that a very small quantity of radium deposited on a plate and left in the open air does lose its activity fairly rapidly4 ,this loss of activity has nothing whatever to do with the shortness of life of the radium itself1 but is due to the escape of the radium from the plate into the surrounding gas4 ,suppose1 for example1 that a solution containing #aj mgrs4 of radium bromide is evaporated in a vessel of one sq4 cm4 in area4 ,this amount of radium is far too small to form even a layer of molecular thickness4 ,it seems likely that1 during the process of evaporation1 the radium would tend to collect in small masses and be deposited on the surface of the vessel4 ,these would very readily be removed by slow currents of air and so escape from the plate4 ,the disappearance of such minute amounts of radium is to be expected1 and would probably occur with all kinds of matter present in such minute amount4 ,such an effect has nothing to do with an alteration of the life of radium and must not be confused with it4 ,the result that the total radiation from a given quantity of radium depends only on the quantity of radium and not on the degree of its concentration is of great importance1 for it allows us to determine with accuracy the content of radium in minerals and in soils in which the radium exists in a very diffused state4 #bfd4 ,constancy of the radiations4 ,the early observations on uranium and thorium had shown that their radio-activity remained constant over the period of several years during which they were examined4 ,the possibility of separating from uranium and thorium the active products ,ur ,x and ,th ,x respectively1 the activity of which decayed with the time1 seemed at first sight to contradict this4 ,further observation1 however1 showed that the total radio-activity of these bodies was not altered by the chemical processes1 for it was found that the uranium and thorium from which the active products were removed1 spontaneously regained their radio-activity4 ,at any time after removal of the active product1 the sum total of the radio-activity of the separated product together with that of the substance from which it has been separated is always equal to that of the original compound before separation4 ,in cases where active products1 like ,ur ,x and the radium emanation1 decay with time according to an exponential law1 this follows at once from the experimental results4 ,if ,i is the activity of the product at any time t after separation1 and ,i the initial value1 we know that frac ,i#a ,i e-t 4 ,at the same time the activity ,i recovered during the same interval t is given by frac ,i#b ,i #a e-t 1 where is the same constant as before4 ,it thus follows that ,i ,i ,i1 which is an expression of the above result4 ,the same is also true whatever the law of decay of activity of the separated product 7see section #bjj74 ,for example1 the activity of ,th ,x after separation from thorium at first increases with the time4 ,at the same time1 the activity of the residual thorium compound at first decreases1 and at such a rate that the sum of the activities of the thorium and its separated product is always equal to that of the original thorium4 ,this apparent constancy of the total radiation follows from the general result that the radio-active processes cannot in any way be changed by the action of known forces4 ,it may be recalled that the constant of decay of the activity of a radio-active product has a definite fixed value under all conditions4 ,it is independent of the concentration of the active matter1 of the pressure1 and of the nature of the gas in which the substance is placed1 and it is not affected by wide ranges of temperature4 ,the only observed exception is the product radium ,c4 ,its value of increases with temperature to some extent at about #ajjj ,c41 but at #abjj ,c4 returns nearly to the normal value4 ,in the same way1 it has not been found possible to alter the rate of production of active matter from the radio-elements4 ,in addition1 there is not a single well-authenticated case where radio-activity has been altered or destroyed in any active body or created in an inactive element4 ,certain cases have been observed1 which at first sight seem to indicate a destruction of radio-activity4 ,for example1 the excited radio-activity is removed from a platinum wire when heated above a red heat4 ,it has been shown1 however1 by ,miss ,gates 7section #ahg7 that the radio-activity is not destroyed1 but is deposited in unaltered amount on the colder bodies surrounding it4 ,thorium oxide has been shown to lose to a large extent its power of emanating by ignition to a white heat4 ,but a close examination shows that the emanation is still being produced at the same rate1 but is occluded in the compound4 ,the total radio-activity of a given mass of a radio-element1 measured by the peculiar radiations emitted1 is a quantity which can neither be increased nor diminished1 although it may be manifested in a series of products which are capable of separation from the radio-element4 ,the term 8conservation of radio-activity0 is thus a convenient expression of the facts known at the present time4 ,it is quite possible1 however1 that further experiments at very high or very low temperatures may show that the radio-activity does vary4 ,although no difference has been observed in the radio-activity of uranium over an interval of five years1 it has been shown 7section #bfa7 that on theoretical grounds the radio-activity of a given quantity of a radio-element should decrease with the time4 ,the change will1 however1 be so slow in uranium1 that probably millions of years must elapse before a measurable change can take place1 while the total radio-activity of a given quantity of matter left to itself should thus decrease1 but it ought to be constant for a constant mass of the radio-element4 ,it has already been pointed out 7section #bch7 that the activity of radium1 measured by the and rays1 will probably increase for several hundred years after its separation4 ,this is due to the appearance of fresh products in the radium4 ,ultimately1 however1 the activity must decrease according to an exponential law with the time1 falling to half value in about #acjj years4 ,the conservation of radio-activity applies not only to the radiations taken as a whole1 but also to each specific type of radiation4 ,if the emanation is removed from a radium compound1 the amount of radiation of the radium at once commences to decrease1 but this is compensated by the appearance of rays in the radiations from the vessel in which the separated emanation is stored4 ,at any time the sum total of the radiations from the radium and the emanation vessel is always the same as that from the radium compound before the emanation was removed4 ,similar results have also been found to hold for the rays4 ,this was tested by the writer in the following way4 ,the emanation from some solid radium bromide was released by heat1 and condensed in a small glass tube which was then sealed off4 ,the radium so treated1 and the emanation tube1 were placed together under an electroscope1 with a screen of lead #a cm4 thick interposed in order to let through only the rays4 ,the experiments were continued over three weeks1 but the sum total of the rays from the radium and the emanation tube was1 over the whole interval1 equal to that of the original radium4 ,during this period the amount of rays from the radium at first decreased to only a few per cent4 of the original value1 and then slowly increased again1 until at the end of the three weeks it had nearly regained its original value1 before the emanation was removed4 ,at the same time the amount of rays from the emanation tube rose from zero to a maximum and then slowly decreased again at the same rate as the decay of the activity of the emanation in the tube4 ,this result shows that the amount of rays from radium was a constant quantity over the interval of observation1 although the amount of rays from the radium and emanation tube had passed through a cycle of changes4 ,there is one interesting possibility in this connection that should be borne in mind4 ,the rays from the active substances carry off energy in a very concentrated form1 and this energy is dissipated by the absorption of the rays in matter4 ,the rays might be expected to cause a disintegration of the atoms of inactive matter on which they fall and thus give rise to a kind of radio-activity4 ,this effect has been looked for by several observers4 ,ramsay and ,w4 ,t4 ,cooke#cfa state that they have noticed such an action1 using about a decigram of radium as a source of radiation4 ,the radium1 sealed in a glass vessel1 was surrounded by an external glass tube and exposed to the action of the and rays of radium for several weeks4 ,the inside and outside of the glass tube were found to be active1 and the active matter was removed by solution in water4 ,the radio-activity observed was very minute1 corresponding to only about #a milligram of uranium4 ,the writer has1 at various times1 tried experiments of this character but with negative results4 ,the greatest care is necessary in such experiments to ensure that the radio-activity is not due to other causes besides the rays from the radium4 ,this care is especially necessary in laboratories where considerable quantities of the radium emanation have been allowed to escape into the air4 ,the surface of every substance becomes coated with the slow transformation products of radium1 viz4 radium ,d1 ,e1 and ,f4 ,the activity communicated in this way to originally inactive matter is often considerable4 ,this infection by the radium emanation extends throughout the whole laboratory1 on account of the distribution of the emanation by convection and diffusion4 ,for example1 ,eve#cfb found that every substance which he examined in the laboratory of the writer showed much greater activity than the normal4 ,in this case the radium had been in use in the building for about two years4 #bfe4 ,loss of weight of the radio-elements4 ,since the radio-elements are continually throwing off particles atomic in size1 an active substance1 enclosed in a vessel sufficiently thin to allow the particles to escape1 must gradually lose in weight4 ,this loss of weight will be small under ordinary conditions1 since the greater proportion of the rays produced are absorbed in the mass of the substance4 ,if a very thin layer of a radium compound were spread on a very thin sheet of substance1 which did not appreciably absorb the particles1 a loss of weight due to the expulsion of particles might be detectable4 ,since e_/m #f #aj for the particle and e #aa #aj electromagnetic units and #be #aj particles are expelled per second per gram of radium1 the proportion of the mass expelled is #dh #aj per second and #aj per year4 ,there is one condition1 however1 under which the radium should lose in weight fairly rapidly4 ,if a current of air is slowly passed over a radium solution1 the emanation produced would be removed as fast as it was formed4 ,since the atom of the emanation has a mass probably not much smaller than the radium atom1 the fraction of the mass removed per year should be nearly equal to the fraction of the radium which changes per year1 i4e4 one gram of radium should diminish in weight about half a milligram 7section #bfa7 per year4 ,if it is supposed that the particles have weight1 the loss of weight due to their expulsion is very small compared with that due to the emission of particles4 ,the writer has shown 7section #bec7 that about #g #aj particles are projected per second from #a gram of radium4 ,the consequent loss of weight would only be about #aj grams per year4 ,except under very special experimental conditions1 it would thus be difficult to detect the loss of weight of radium due to the expulsion of particles from its mass4 ,there is1 however1 a possibility that radium might change in weight even though none of the radio-active products were allowed to escape4 ,for example1 if the view is taken that gravitation is the result of forces having their origin in the atom1 it is possible that1 if the atom were disintegrated1 the weight of the parts might not be equal to that of the original atom4 ,a large number of experiments have been made to see if radium preparations1 kept in a sealed tube1 alter in weight4 ,with the small quantities of radium available to the experimenter1 no difference of weight of radium preparations with time has yet been established with certainty4 ,heydweiller stated that he had observed a loss of weight of radium and ,dorn also obtained a slight indication of change in weight4 ,these results have not1 however1 been confirmed4 ,forch1 later1 was unable to observe any appreciable change4 ,j4 ,j4 ,thomson#cfc has made experiments to see if the ratio of weight to mass for radium is the same as for inactive matter4 ,we have seen in section #dh that a charge in motion possesses an apparent mass which is constant for slow speeds but increases as the speed of light is approached4 ,now radium emits some electrons at a velocity comparable with the velocity of light1 and presumably these electrons were in rapid motion in the atom before their expulsion4 ,it might thus be possible that the ratio for radium would differ from that for ordinary matter4 ,the pendulum method was used1 and the radium was enclosed in a small light tube suspended by a silk fibre4 ,within the limit of experimental error the ratio of weight to mass was found to be the same as for ordinary matter1 so that we may conclude that the number of electrons moving with a velocity approaching that of light is small compared with the total number present4 #bff4 ,total emission of energy from the radio-element4 ,it has been shown that #a gram of radium emits energy at the rate of #ajj gram-calories per hour or #hgf1jjj gram-calories per year4 ,if #a gram of radium in radio-active equilibrium be set apart1 its radio-activity and consequent heat emission is given at a time t by qe-t 1 where is the constant of decay of activity of radium and of the initial heating effect2 the total heat emission from #a gram of radium is given by intinfty qe-t dt frac q 4 ,now on the estimate of the life of radium given in section #bfa the value of is when #a year is taken as the unit of time4 ,the total heat emission from #a gram of radium during its life is thus #af #aj gram-calories4 ,the heat emitted in the union of hydrogen and oxygen to form #a gram of water is about #d #aj gram-calories1 and in this reaction more heat is given out for equal weights than in any other chemical reaction known4 ,it is thus seen that the total energy emitted from #a gram of radium during its changes is about one million times greater than in any known molecular change4 ,that matter is able1 under special conditions1 to emit an enormous amount of energy1 is well exemplified by the case of the radium emanation4 ,calculations of the amount of this energy have already been given in section #bdi4 ,since the other radio-elements only differ from radium in the slowness of their change1 the total heat emission from uranium and thorium must be of a similar high order of magnitude4 ,there is thus reason to believe that there is an enormous store of latent energy resident in the atoms of the radio-elements4 ,this store of energy could not have been recognized if the atoms had not been undergoing a slow process of disintegration4 ,the energy emitted in radio-active changes is derived from the internal energy of the atoms4 ,the emission of this energy does not disobey the law of the conservation of energy1 for it is only necessary to suppose that1 when the radio-active changes have ceased1 the energy stored up in the atoms of the final products is less than that of the original atoms of the radio-elements4 ,the difference between the energy originally possessed by the matter which has undergone the change1 and the final inactive products which arise1 is a measure of the total amount of energy released4 ,there seems to be every reason to suppose that the atomic energy of all the elements is of a similar high order of magnitude4 ,with the exception of their high atomic weights1 the radio-elements do not possess any special chemical characteristics which differentiate them from the inactive elements4 ,the existence of a latent store of energy in the atoms is a necessary consequence of the modern view developed by ,j4 ,j4 ,thomson1 ,larmor1 and ,lorentz1 of regarding the atom as a complicated structure consisting of charged parts in rapid oscillatory or orbital motion in regard to one another4 ,the energy may be partly kinetic and partly potential1 but the mere concentration of the charged particles1 which probably constitute the atom1 in itself implies a large store of energy in the atom1 in comparison with which the energy emitted during the changes of radium is insignificant4 ,the existence of this store of latent energy does not ordinarily manifest itself1 since the atoms cannot be broken up into simpler forms by the physical or chemical agencies at our disposal4 ,its existence at once explains the failure of chemistry to transform the atoms1 and also accounts for the rate of change of the radio-active processes being independent of all external agencies4 ,it has not so far been found possible to alter in any way the rate of emission of energy from the radio-elements4 ,if it should ever be found possible to control at will the rate of disintegration of the radio-elements1 an enormous amount of energy could be obtained from a small quantity of matter4 #bfg4 ,production of helium from radium and the radium emanation4 ,since the final products1 resulting from a disintegration of the radio-elements1 are not radio-active1 they should in the course of geologic ages collect in some quantity1 and should always be found associated with the radio-elements4 ,now the inactive products resulting from the radio-active changes are the particles expelled at each stage1 and the final inactive product or products which remain1 when the process of disintegration can no longer be traced by the property of radio-activity4 ,pitchblende1 in which the radio-elements are mostly found1 contains in small quantity a large proportion of all the known elements4 ,in searching for a possible disintegration product common to all the radio-elements1 the presence of helium in the radio-active minerals is noteworthy2 for helium is only found in the radio-active minerals1 and is an invariable companion of the radio-elements4 ,moreover1 the presence in minerals of a light1 inert gas like helium had always been a matter of surprise4 ,the production by radium and thorium of the radio-active emanations1 which behave like chemically inert gases of the helium-argon family1 suggested the possibility that one of the final inactive products of the disintegration of the radio-elements might prove to be a chemically inert gas4 ,the later discovery of the material nature of the rays added weight to the suggestion2 for the measurement of the ratio e_/m of the particle indicated that if the particle consisted of any known kind of matter1 it must either be hydrogen or helium4 ,for these reasons1 it was suggested in #aijb by ,rutherford and ,soddy#cfd that helium might be a product of the disintegration of the radio-elements4 ,sir ,william ,ramsay and ,mr ,soddy in #aijc undertook an investigation of the radium emanation1 with the purpose of seeing if it were possible to obtain any spectroscopic evidence of the presence of a new substance4 ,first of all1 they exposed the emanation to very drastic treatment 7section #aeh71 and confirmed and extended the results previously noted by ,rutherford and ,soddy that the emanation behaved like a chemically inert gas1 and in this respect possessed properties analogous to the gases of the helium-argon group4 ,on obtaining #cj milligrams of pure radium bromide 7prepared about three months previously7 ,ramsay and ,soddy#cfe examined the gases1 liberated by solution of the radium bromide in water1 for the presence of helium4 ,a considerable quantity of hydrogen and oxygen was released by the solution 7see section #abd74 ,the hydrogen and oxygen were removed by passing the liberated gases over a red-hot spiral of partially oxidized copper-wire and the resulting water vapour was absorbed in a phosphorus pentoxide tube4 ,the gas was then passed into a small vacuum tube which was in connection with a small ,u tube4 ,by placing the ,u tube in liquid air1 most of the emanation present was condensed1 and also most of the ,,co present in the gas4 ,on examining the spectrum of the gas in the vacuum tube1 the characteristic line ,d of helium was observed4 ,this experiment was repeated with #cj milligrams of radium bromide about four months old1 lent for the purpose by the writer4 ,the emanation and ,,co were removed by passing them through a ,u tube immersed in liquid air4 ,a practically complete spectrum of helium was observed1 including the lines of wave-lengths #ffgg1 #ehgf1 #ejaf1 #digb1 #dgac and #ddgb4 ,there were also present three other lines of wave-lengths about #fahj1 #efie1 #edee which have not yet been identified4 ,in later experiments1 the emanation from #ej milligrams of the radium bromide was conveyed with oxygen into a small ,u tube1 cooled in liquid air1 in which the emanation was condensed4 ,fresh oxygen was added1 and the ,u tube again pumped out4 ,the small vacuum tube1 connected with the ,u tube1 showed at first no helium lines when the liquid air was removed4 ,the spectrum obtained was a new one1 and ,ramsay and ,soddy considered it to be probably that of the emanation itself4 ,after allowing the emanation tube to stand for four days1 the helium spectrum appeared with all the characteristic lines1 and in addition1 three new lines present in the helium obtained by solution of the radium4 ,these results have since been confirmed4 ,the experiments1 which have led to such striking and important results1 were by no means easy of performance1 for the quantity of helium and of emanation released from #ej mgrs4 of radium bromide is extremely small4 ,it was necessary1 in all cases1 to remove almost completely the other gases1 which were present in sufficient quantity to mask the spectrum of the substance under examination4 ,the success of the experiments has been largely due to the application1 to this investigation1 of the refined methods of gas analysis1 previously employed by ,sir ,william ,ramsay with so much skill in the separation of the rare gases xenon and krypton1 which exist in minute proportions in the atmosphere4 ,the fact that the helium spectrum was not present at first1 but appeared after the emanation had remained in the tube for some days1 shows that the helium must have been produced from the emanation4 ,the emanation cannot be helium itself1 for1 in the first place1 helium is not radio-active1 and in the second place1 the helium spectrum was not present at first1 when the quantity of emanation in the tube was at its maximum4 ,moreover1 the diffusion experiments1 already discussed1 point to the conclusion that the emanation is of high molecular weight4 ,there can thus be no doubt that the helium is derived from the emanation of radium in consequence of changes of some kind occurring in it4 ,these results were confirmed later by other observers4 ,curie and ,dewar#cff performed the following experiment3 ,a weight of about #db gr4 of radium bromide was placed in a quartz tube1 and the tube exhausted until no further gas came off4 ,the radium was then heated to fusion1 about #bf c4c4 of gas being liberated in the process4 ,the tube was then sealed1 and some weeks afterwards the spectrum of the gas liberated in the tube by the radium was examined by ,deslandres and found to give the entire spectrum of helium4 ,the gas1 liberated during the initial heating of the radium1 was collected and found to contain a large amount of emanation1 although the gas had been passed through two tubes immersed in liquid air4 ,the tube containing these gases was very luminous and rapidly turned violet1 while more than half of the gases was absorbed4 ,the spectrum of the phosphorescent light was found to be discontinuous1 consisting of three nitrogen bands4 ,no sign of the helium spectrum was observed1 although helium must have been present4 ,himstedt and ,meyer#cfg placed #ej mgrs4 of radium bromide in a ,u tube connected with a small vacuum tube4 ,the tube was carefully exhausted and then sealed off4 ,the spectrum of hydrogen and carbon dioxide alone was observed for three months1 but after four months the red1 yellow1 green and blue lines of the helium spectrum were visible4 ,the slow appearance of the helium spectrum was probably due to the presence in the tube of a considerable quantity of hydrogen4 ,in another experiment1 some radium sulphate which had been heated to a bright red heat in a quartz tube was connected with a small vacuum tube4 ,after three weeks1 some of the lines of helium were clearly seen1 and increased in brightness with time4 #bfh4 ,connection between helium and the particles4 ,the appearance of helium in a tube containing the radium emanation may indicate either that the helium is one of the final products1 which appear at the end of the series of radio-active changes1 or that the helium is in reality the expelled particle4 ,the evidence at present points to the latter as being the more probable explanation4 ,in the first place1 the emanation diffuses like a gas of heavy molecular weight1 and it appears probable that after the expulsion of a few particles1 the atomic weight of the final product is comparable with that of the emanation4 ,on the other hand1 the value of e_/m determined for the projected particle points to the conclusion that1 if it consists of any known kind of matter1 it is either hydrogen or helium4 ,there has been a tendency to assume that the helium produced from the radium emanation is the last transformation product of that substance4 ,the evidence1 however1 does not support this view4 ,we have seen that the emanation1 after the initial rapid changes1 is transformed very slowly4 ,if the helium were the final product1 the amount present in the emanation tube after a few days or weeks would be insignificant1 since the product radium ,d intervenes1 which takes #dj years to be half transformed4 ,since the helium cannot be the final product of the series of changes1 and since all the other products are radio-active1 and almost certainly of high atomic weight1 it is difficult to see what position the helium atom occupies in the scheme of transformation1 unless it be the particle expelled during the successive changes4 ,it is a matter of great difficulty to settle definitely whether the particle is a projected helium atom or not4 ,on account of the very small deflection of the rays in an electric field1 and the complex nature of the radiation from radium1 an accurate determination of the value e_/m for the particle is beset with difficulties4 ,it may be possible to settle the question by accurate measurements of the volume of gas in a tube1 filled originally with the radium emanation4 ,since the emanation itself1 and two of the rapidly changing products which result from it1 emit particles1 the final volume of the particles1 if they can exist in the gaseous state1 would be three times the volume of the emanation4 ,ramsay and ,soddy 7section #agb7 have made experiments of this kind1 but the results obtained were very contradictory1 depending upon the kind of glass employed4 ,in one case1 the volume of the residual gases shrank almost to zero1 in another the initial volume increased to about ten times its initial value4 ,in the latter experiment a brilliant spectrum of helium was observed in the residual gas4 ,this difference of behaviour is probably due to different degrees of absorption of helium by the glass tubes4 ,if the particles are helium atoms1 we may expect that a large proportion of the helium1 which is produced in a tube containing the radium emanation1 will be buried in the wall of the glass tube2 for the particles are projected with sufficient velocity to penetrate some distance into the glass4 ,this helium may either remain in the glass1 or in some cases rapidly diffuse out again4 ,in any case1 a fraction of the helium would be liberated when an intense electric discharge is passed through the tube4 ,ramsay and ,soddy have in some instances observed that a slight amount of helium is liberated on heating the walls of the tube in which the emanation had been stored for some time4 ,the volume of helium produced per year by #a gram of radium can easily be calculated on the assumption that the particle is in reality a helium atom4 ,it has been shown that #be #aj particles are projected per second from #a gram of radium4 ,since there are #cf #aj molecules in one cubic centimetre of any gas at standard pressure and temperature1 the volume of the particles released per second is #g #aj c4c4 and per year #jbd c4c4 ,it has already been pointed out that1 on this hypothesis1 the volume of helium released by the emanation is three times the volume of the latter4 ,the amount of helium to be obtained from the emanation released from #a gram of radium in radio-active equilibrium is thus about #c cubic mms4 ,ramsay and ,soddy have tried to estimate experimentally the probable volume of helium produced per second by one gram of radium4 ,the helium1 obtained from #ej mgrs4 of radium bromide1 which had been kept in solution in a closed vessel for #fj days1 was introduced into a vacuum tube4 ,another similar tube was placed in series with it1 and the amount of the helium in the latter adjusted until on passing a discharge through the two tubes in series the helium lines in each tube were of about the same brightness4 ,in this way they calculated that the amount of helium present was #ja cubic mm4 ,on this estimate1 the amount of helium produced per year per gram of radium is about #bj cubic mms4 ,we have seen that the calculated amount is about #bdj cubic mms41 on the assumption that the particle is a helium atom4 ,ramsay and ,soddy consider that the presence of argon in one of the tubes may have seriously interfered with the correctness of the estimation4 ,on account of the great uncertainty attaching to estimates of the above character1 the value deduced by ,ramsay and ,soddy does not exclude the probability that the calculated volume may be of the right order of magnitude4 ,in order to explain the presence of helium in radium on ordinary chemical lines1 it has been suggested that radium is not a true element1 but a molecular compound of helium with some substance known or unknown4 ,the helium compound gradually breaks down1 giving rise to the helium observed4 ,it is at once obvious that this postulated helium compound is of a character entirely different from that of any other compound previously observed in chemistry4 ,weight for weight1 it emits during its change an amount of energy at least one million times greater than any molecular compound known 7see section #bdi74 ,in addition1 it must be supposed that the rate of breaking up of the helium compound is independent of great ranges of temperature--a result never before observed in any molecular change4 ,the helium compound in its breaking up must give rise to the peculiar radiations and also pass through the successive radio-active changes observed in radium4 ,thus in order to explain the production of helium and radio-activity on this view1 a unique kind of molecule must be postulated--a molecule1 in fact1 which is endowed with every single property which on the disintegration theory is ascribed to the atom of the radio-elements4 ,on the other hand1 radium as far as it has been examined1 has fulfilled every test required for an element4 ,it has a well-marked and characteristic spectrum1 and there is no reason to suppose that it is not an element in the ordinarily accepted sense of the term4 ,on the theory that the radio-elements are undergoing atomic disintegration1 the helium must be considered to be a constituent of the radium atom1 or1 in other words1 the radium atom is built up of parts1 one of which1 at least1 is the atom of helium4 ,the theory that the heavy atoms are all built up of some simple fundamental unit of matter or protyle has been advanced at various times by many prominent chemists and physicists4 ,prout's hypothesis that all elements are built up out of hydrogen is an example of this point of view of regarding the subject4 ,on the disintegration theory1 the changes occurring in the radio-atoms involve an actual transformation of the atoms through successive changes4 ,this change is so slow in uranium and thorium that at least a million years would be required before the amount of change could be measured by the balance4 ,in radium it is a million times faster1 but even in this case it is doubtful whether any appreciable change would have been observed by ordinary chemical methods for many years had not the possibility of such a change been suggested from other lines of evidence4 ,the similarity of the particles from the different radio-elements indicates that they consist of expelled particles of the same kind4 ,on this view1 helium should be produced by each of the radio-elements4 ,its presence in minerals containing thorium1 for example in monazite sand and the ,ceylon mineral described by ,ramsay1 indicates that helium may be a product of thorium as well as of radium4 ,strutt#cfh has recently suggested that most of the helium observed in radio-active minerals may be a decomposition product of thorium rather than of uranium and radium2 for he finds that minerals rich in helium always contain thorium1 while many uranium minerals nearly free from thorium contain little helium4 ,the evidence in support of this view is1 however1 not altogether satisfactory1 for some of the uranium minerals in question are secondary uranium minerals 7see ,appendix ,b71 deposited by the action of water or other agencies at a comparatively late date1 and are also1 in many cases1 highly emanating1 and consequently could not be expected to retain more than a fraction of the helium produced in them4 ,taking the view that the particles are projected helium atoms1 we must regard the atoms of the radio-elements as compounds of some known or unknown substance with helium4 ,these compounds break up spontaneously1 and at a very slow rate even in the case of radium4 ,the disintegration takes place in successive stages1 and at most of the stages a helium atom is projected with great velocity4 ,this disintegration is accompanied by an enormous emission of energy4 ,the liberation of such a large amount of energy in the radio-active changes at once explains the constancy of the rate of change under the action of any of the physical and chemical agencies at our command4 ,on this view1 uranium1 thorium and radium are in reality compounds of helium4 ,the helium1 however1 is held in such strong combination that the compound cannot be broken up by chemical or physical forces1 and1 in consequence1 these bodies behave as chemical elements in the ordinary accepted chemical sense4 ,it appears not unlikely that many of the so-called chemical elements may prove to be compounds of helium1 or1 in other words1 that the helium atom is one of the secondary units with which the heavier atoms are built up4 ,in this connection it is of interest to note that many of the elements differ in their atomic weight by four--the atomic weight of helium4 ,if the particle is a helium atom1 at least three particles must be expelled from uranium 7#bche7 to reduce its atomic weight to that of radium 7#bbe74 ,it is known that five particles are expelled from radium during its successive transformations4 ,this would make the atomic weight of the final residue #bbe #bj #bje4 ,this is very nearly the atomic weight of lead1 #bjfe4 ,i have1 for some time1 considered it probable that lead is the end or final product of radium4 ,the same suggestion has recently been made by ,boltwood#cfi4 ,this point of view is supported by the fact that lead is always found in small quantity in all uranium minerals1 and that the relative proportions of lead and helium in the radio-active minerals are about the same as would be expected if lead and helium were both decomposition products of radium4 ,dr ,boltwood has drawn my attention to the fact that the proportion of lead in many radio-active minerals varies with the content of helium4 ,a mineral rich in helium in nearly all cases contains more lead than a mineral poor in helium4 ,this cannot be considered1 at present1 more than a speculation1 but the facts as they stand are very suggestive4 #bfi4 ,age of radio-active minerals4 ,helium is only found in the radio-active minerals1 and this fact1 taken in conjunction with the liberation of helium by radium1 indicates that the helium must have been produced as a result of the transformation of radium and the other radio-active substances contained in the minerals4 ,now in a mineral about half the helium is1 in many cases1 released by heat and the residue by solution4 ,it seems probable that the helium produced throughout the mass of the mineral is mechanically imprisoned in it4 ,moss#cgj found that1 by grinding pitchblende in vacuo1 helium is evolved1 apparently showing that the helium exists in cavities of the mineral4 ,travers#cga has suggested that1 since helium is liberated on heating1 the effect may be due to the heat generated by grinding4 ,the escape of the helium from the heated mineral is probably connected with the fact observed by ,jaquerod#cgb that helium passes through the walls of a quartz tube1 heated above #ejj ,c4 ,the substance of the mineral probably possesses a similar property4 ,travers considers that helium is present in the mineral in a state of supersaturated solid solution1 but the facts are equally well explained by assuming that the helium is mechanically imprisoned in the mass of the mineral4 ,the sudden rise of temperature observed in the mineral fergusonite1 at the time the helium is released1 has been found to have nothing to do with the presence of helium1 for it also takes place in minerals not containing helium4 ,the old view that helium was in a state of chemical combination with the mineral must be abandoned in the light of these more recent experiments4 ,since the helium is only released from some minerals by the action of high temperatures and solution1 it appears probable that a large proportion of the helium found in the minerals is unable to escape under normal conditions4 ,thus if the rate of production of helium by the radio-active substance were definitely known1 it should be possible to calculate the age of the mineral by observing the volume of helium liberated from it by solution4 ,in the absence of such definite information1 an approximate calculation will be made to indicate the order of magnitude of the time that has elapsed since the mineral was formed or was at a temperature low enough to prevent the escape of the helium4 ,let us take1 for example1 the mineral fergusonite1 which was found by ,ramsay and ,travers#cgc to evolve #aha c4c4 of helium4 ,the fergusonite contained about #g per cent4 of uranium4 ,now uranium in old minerals probably contains about #h #aj of its weight of radium 7see section #bfb74 ,one gram of the mineral thus contained about #ef #aj grams of radium4 ,now if the particle is helium1 it has been shown that #a gram of radium produces #jbd c4c4 of helium per year4 ,the volume of helium produced per year in #a gram of fergusonite is thus #ac #aj c4c4 ,assuming that the rate of production of helium has been uniform1 the time required to produce #aha c4c4 per gram is about #adj million years4 ,if the calculated rate of production of helium by radium is an over-estimate1 the time is correspondingly lengthened4 ,i think that1 when the constants required for these calculations are more definitely fixed1 this method will probably give fairly trustworthy information as to the probable age of some of the radio-active minerals of the earth's crust1 and indirectly as to the age of the strata in which they are found4 ,in this connection it is of interest to note that ,ramsay#cgd found that a ,ceylon mineral1 thorianite1 contained as much as #ie c4c4 of helium per gram4 ,according to the analysis by ,dunstan1 this mineral contains about #gf per cent4 of thorium and #ab per cent4 of uranium4 ,the unusually large amount of helium evolved from this mineral would indicate that it was formed at an earlier date than the fergusonite previously considered4 #bgj4 ,possible causes of disintegration4 ,in order to explain the phenomena of radio-activity1 it has been supposed that a certain small fraction of the radio-atoms undergoes disintegration per second1 but no assumptions have been made as to the cause which produces the instability and consequent disintegration4 ,the instability of the atoms may be supposed to be brought about either by the action of external forces or by that of forces inherent in the atoms themselves4 ,it is conceivable1 for example1 that the application of some slight external force might cause instability and consequent disintegration1 accompanied by the liberation of a large amount of energy1 on the same principle that a detonator is necessary to start some explosives4 ,it has been shown that the number of atoms of any radio-active product which break up per second is always proportional to the number present4 ,this law of change does not throw any light on the question1 for it would be expected equally on either hypothesis4 ,it has not been found possible to alter the rate of change of any product by the application of any known physical or chemical forces1 unless possibly it is assumed that the force of gravitation which is not under our control may influence in some way the stability of the radio-atoms4 ,it seems likely therefore that the cause of the disruption of the atoms of the radio-elements and their products resides in the atoms themselves4 ,according to the modern views of the constitution of the atom1 it is not so much a matter of surprise that some atoms disintegrate as that the atoms of the elements are so permanent as they appear to be4 ,in accordance with the hypothesis of ,j4 ,j4 ,thomson1 it may be supposed that the atoms consist of a number of small positively and negatively charged particles in rapid internal movement1 and held in equilibrium by their mutual forces4 ,in a complex atom1 where the possible variations in the relative motion of the parts are very great1 the atom may arrive at such a phase that one part acquires sufficient kinetic energy to escape from the system1 or that the constraining forces are momentarily neutralised1 so that the part escapes from the system with the velocity possessed by it at the instant of its release4 ,sir ,oliver ,lodge#cge has advanced the view that the instability of the atom may be a result of radiation of energy by the atom4 ,larmor has shown that an electron1 subject to acceleration1 radiates energy at a rate proportional to the square of its acceleration4 ,an electron moving uniformly in a straight line does not radiate energy1 but an electron1 constrained to move in a circular orbit with constant velocity1 is a powerful radiator1 for in such a case the electron is continuously accelerated towards the centre4 ,lodge considered the simple case of a negatively charged electron revolving round an atom of mass relatively large but having an equal positive charge and held in equilibrium by electrical forces4 ,this system will radiate energy1 and1 since the radiation of energy is equivalent to motion in a resisting medium1 the particle tends to move towards the centre1 and its speed consequently increases4 ,the rate of radiation of energy will increase rapidly with the speed of the electron4 ,when the speed of the electron becomes very nearly equal to the velocity of light1 according to ,lodge1 another effect supervenes4 ,it has been shown 7section #hb7 that the apparent mass of an electron increases very rapidly as the speed of light is approached1 and is theoretically infinite at the speed of light4 ,there will be at this stage a sudden increase of the mass of the revolving atom1 and1 on the supposition that this stage can be reached1 a consequent disturbance of the balance of forces holding the system together4 ,lodge considers it probable that1 under these conditions1 the parts of the system will break asunder and escape from the sphere of one another's influence4 ,it seems probable that the primary cause of the disintegration of the atom must be looked for in the loss of energy of the atomic system due to electromagnetic radiation 7section #eb74 ,larmor#cgf has shown that the condition to be fulfilled in order that a system of rapidly moving electrons may persist without loss of energy is that the vector sum of the accelerations towards the centre should be permanently zero4 ,while a single electron moving in a circular orbit is a powerful radiator of energy1 it is remarkable how rapidly the radiation of energy diminishes if several electrons are revolving in a ring4 ,this has recently been shown by ,j4 ,j4 ,thomson#cgg1 who examined mathematically the case of a system of negatively electrified corpuscles1 situated at equal intervals round the circumference of a circle1 and rotating in one plane with uniform velocity round its centre4 ,for example1 he found that the radiation from a group of six particles moving with a velocity of of the velocity of light is less than one-millionth part of the radiation from a single particle describing the same orbit with the same velocity4 ,when the velocity is of that of light the amount of radiation is only #aj that of a single particle moving with the same velocity in the same orbit4 ,results of this kind indicate that an atom consisting of a large number of revolving electrons may radiate energy extremely slowly1 and yet1 finally1 this minute but continuous drain of energy from the atom must result either in a rearrangement of its component parts into a new system1 or of an expulsion of electrons or groups of electrons from the atom4 ,simple models of atoms to imitate the behaviour of polonium in shooting out particles1 and of radium in shooting out particles have been discussed by ,lord ,kelvin#cgh4 ,it is possible to devise certain stable arrangements of the positively and negatively electrified particles1 supposed to constitute an atom1 which1 on the application of some disturbing force1 break up with the expulsion of a part of the system with great velocity4 ,j4 ,j4 ,thomson#cgi has mathematically investigated the possible stable arrangements of a number of electrons moving about in a sphere of uniform positive electrification4 ,the properties of such a model atom are very striking1 and indirectly suggest a possible explanation of the periodic law in chemistry4 ,he has shown that the electrons1 if in one plane1 arrange themselves in a number of concentric rings2 and generally1 if they are not constrained to move in one plane1 in a number of concentric shells like the coats of an onion4 ,the mathematical problem is much simplified if the electrons are supposed to rotate in rings in one plane1 the electrons in each ring being arranged at equal angular intervals4 ,the ways in which the number of electrons group themselves1 for numbers ranging from #fj to #e at intervals of #e1 are shown in the following table3-- ,number #fj #ee #ej #de #dj #ce of electrons ,number in #bj #ai #ah #ag #af #af successive rings #af #af #ae #ad #ac #ab #ac #ab #aa #aj #h #f #h #g #e #d #c #a #c #a #a ,number #cj #be #bj #ae #aj #e of electrons ,number in #ae #ac #ab #aj #h #e successive rings #aj #i #g #e #b #e #c #a ,in the next table is given the possible series of arrangements of electrons which can have an outer ring of #bj3-- ,number #ei #fj #fa #fb #fc #fd #fe #ff #fg of electrons ,number in #bj #bj #bj #bj #bj #bj #bj #bj #bj successive rings #af #af #af #ag #ag #ag #ag #ag #ag #ac #ac #ac #ac #ac #ac #ad #ad #ae #h #h #i #i #aj #aj #aj #aj #aj #b #c #c #c #c #d #d #e #e ,the smallest number of electrons which can have an outer ring of #bj is #ei1 while #fg is the greatest4 ,the various arrangements of electrons can be classified into families1 in which the groupings of the electrons have certain features in common4 ,thus the group of #fj electrons consists of the same arrangement of electrons as the group of #dj with the addition of an outer ring of #bj electrons2 the group of #dj is the same as the group of #bd with an additional ring outside2 and the group of #bd in turn is the same as the group of #aa with an extra ring4 ,a series of model atoms may be formed in this way1 in which each atom is derived from the preceding member by an additional ring of electrons4 ,such atoms would be expected to possess many properties in common1 and would correspond to the elements in the same vertical column of the periodic table of ,mendelef4 ,different arrangements of electrons vary widely in stability4 ,some may acquire an extra electron or two and yet remain stable1 others readily lose an electron without disturbing their stability4 ,the former would correspond to an electro-negative atom1 the latter to an electro-positive4 ,certain arrangements of electrons are stable if the electrons move with an angular velocity greater than a certain value1 but become unstable when the velocity falls below this value4 ,four electrons in motion1 for example1 are stable in one plane1 but when the velocity falls below a certain critical value1 the system is unstable1 and the electrons tend to arrange themselves at the corners of a regular tetrahedron4 ,j4 ,j4 ,thomson 7loc4 cit47 applies this property to explain why an atom of radio-active matter breaks up1 as follows3-- 8,consider now the properties of an atom containing a system of corpuscles 7electrons7 of this kind4 ,suppose the corpuscles were originally moving with velocities far exceeding the critical velocity2 in consequence of the radiation from the moving corpuscles1 their velocity will slowly--very slowly--diminish2 when1 after a long interval1 the velocity reaches the critical velocity1 there will be what is equivalent to an explosion of the corpuscles1 the corpuscles will move far away from their original position1 their potential energy will decrease1 while their kinetic energy will increase4 ,the kinetic energy gained in this way might be sufficient to carry the system out of the atom1 and we should have1 as in the case of radium1 a part of the atom shot off4 ,in consequence of the very slow dissipation of energy by radiation the life of the atom would be very long4 ,we have taken the case of the four corpuscles as the type of a system which1 like a top1 requires for its stability a certain amount of rotation4 ,any system possessing this property would1 in consequence of the gradual dissipation of energy by radiation1 give to the atom containing it radio-active properties similar to those conferred by the four corpuscles40 #bga4 ,heat of the sun and earth4 ,it was pointed out by ,rutherford and ,soddy#chj that the maintenance of the sun's heat for long intervals of time did not present any fundamental difficulty if a process of disintegration1 such as occurs in the radio-elements1 were supposed to be taking place in the sun4 ,in a letter to ,nature 7,july #i1 #aijc7 ,w4 ,e4 ,wilson showed that the presence of #cf grams of radium in each cubic metre of the sun's mass was sufficient to account for the present rate of emission of energy by the sun4 ,this calculation was based on the estimate of ,curie and ,laborde that #a gram of radium emits #ajj gram-calories per hour1 and on the observation of ,langley that each square centimetre of the sun's surface emits #hbh #aj gram-calories per hour4 ,since the average density of the sun is #add1 the presence of radium in the sun1 to the extent of #be parts by weight in a million1 would account for its present rate of emission of energy4 ,an examination of the spectrum of the sun has not so far revealed any of the radium lines4 ,it is known1 however1 from spectroscopic evidence that helium is present1 and this indirectly suggests the existence of radio-active matter also4 ,it can readily be shown#cha that the absence of penetrating rays from the sun at the surface of the earth does not imply that the radio-elements are not present in the sun4 ,even if the sun were composed of pure radium1 it would hardly be expected that the rays emitted would be appreciable at the surface of the earth1 since the rays would be almost completely absorbed in passing through the atmosphere1 which corresponds to a thickness of #gf centimetres of mercury4 ,in the ,appendix ,e of ,thomson and ,tait's ,natural ,philosophy1 ,lord ,kelvin has calculated the energy lost in the concentration of the sun from a condition of infinite dispersion1 and concludes that it seems 8on the whole probable that the sun has not illuminated the earth for #ajj1jjj1jjj years and almost certain that he has not done so for #ejj1jjj1jjj years4 ,as for the future we may say1 with equal certainty1 that inhabitants of the earth cannot continue to enjoy the light and heat essential to their life for many million years longer1 unless sources now unknown to us are prepared in the great storehouses of creation40 ,the discovery that a small mass of a substance like radium can emit spontaneously an enormous quantity of heat renders it possible that this estimate of the age of the sun's heat may be much increased4 ,in a letter to ,nature 7,sept4 #bd1 #aijc7 ,g4 ,h4 ,darwin drew attention to this probability1 and at the same time pointed out that1 on ,kelvin's hypotheses1 his estimate of the duration of the sun's heat was probably much too high1 and stated that1 8,the lost energy of the sun1 supposed to be a homogeneous sphere of mass ,m and radius a1 is 77,m_/a where is the constant of gravitation4 ,on introducing numerical values for the symbols in this formula1 ,i find the lost energy to be #bg #aj ,m calories where ,m is expressed in grams4 ,if we adopt ,langley's value of the solar constant1 this heat suffices to give a supply for #ab million years4 ,lord ,kelvin used ,pouillet's value for that constant1 but if he had been able to use ,langley's1 his #ajj million would have been reduced to #fj million4 ,the discrepancy between my results of #ab million and his of #fj million is explained by a conjectural augmentation of the lost energy to allow for the concentration of the solar mass towards its central parts40 ,now it has been shown 7section #bff7 that one gram of radium emits during its life an amount of heat corresponding to #af #aj gram-calories4 ,it has also been pointed out that there is every reason to suppose that a similar amount of energy is resident in the chemical atoms of the inactive elements4 ,it is not improbable that1 at the enormous temperature of the sun1 the breaking up of the elements into simpler forms may be taking place at a more rapid rate than on the earth4 ,if the energy resident in the atoms of the elements is thus available1 the time during which the sun may continue to emit heat at the present rate may be at least #ej times longer than the value computed from dynamical data4 ,similar considerations apply to the question of the age of the earth4 ,a full discussion of the probable age of the earth1 computed from its secular cooling from a molten mass1 is given by ,lord ,kelvin in ,appendix ,d of ,thomson and ,tait's ,natural ,philosophy4 ,he has there shown that about #ajj million years after the earth was a molten mass1 the gradual cooling due to radiation from its surface would account for the average temperature gradient of ,f4 per foot1 observed to-day near the earth's surface4 ,some considerations will now be discussed which point to the probability that the present temperature gradient observed in the earth cannot be used as a guide to estimate the length of time that has elapsed since the earth has been at a temperature capable of supporting animal and vegetable life2 for it will be shown that probably there is sufficient radio-active matter on the earth to supply as much heat to the earth as is lost by radiation from its surface4 ,taking the average conductivity ,k of the materials of the earth as #jjd 7,c4,g4,s4 units7 and the temperature gradient ,t near the surface as #jjjcg ,c4 per cm41 the heat ,q in gram-calories conducted to the surface of the earth per second is given by ,q #d,r,,kt1 where ,r is the radius of the earth4 ,let ,x be the average amount of heat liberated per second per cubic centimetre of the earth's volume owing to the presence of radio-active matter4 ,if the heat ,q radiated from the earth is equal to the heat supplied by the radio-active matter in the earth1 ,x 4 77,r #d,r,,kt1 or #c,,kt ,x ------ 4 ,r ,substituting the values of these constants1 ,x #g #aj gram-calories per second #bb #aj gram-calories per year4 ,since #a gram of radium emits #hgf1jjj gram-calories per year1 the presence of #bf #aj grams of radium per unit volume1 or #df #aj grams per unit mass1 would compensate for the heat lost from the earth by conduction4 ,now it will be shown in the following chapter that radio-active matter seems to be distributed fairly uniformly through the earth and atmosphere4 ,in addition1 it has been found that all substances are radio-active to a feeble degree1 although it is not yet settled whether this radio-activity may not be due mainly to the presence of a radio-element as an impurity4 ,for example1 ,strutt#chb observed that a platinum plate was about as active as a crystal of uranium nitrate1 or about #b #aj as active as radium4 ,this corresponds to a far greater activity than is necessary to compensate for the loss of heat of the earth4 ,a more accurate deduction1 however1 can be made from data of the radio-activity exhibited by matter dug out of the earth4 ,elster and ,geitel#chc filled a dish of volume #cc #aj c4c4 with clay dug up from the garden1 and placed it in a vessel of #cj litres capacity in which was placed an electroscope to determine the conductivity of the enclosed gas4 ,after standing for several days1 they found that the conductivity of the air reached a constant maximum value1 corresponding to three times the normal4 ,it will be shown later 7section #bhd7 that the normal conductivity observed in sealed vessels corresponds to the production of about #cj ions per c4c4 per second4 ,the number of ions produced per second in the vessel by the radio-active earth was thus about #b #aj4 ,this would give a saturation current through the gas of #bb #aj electromagnetic units4 ,now the emanation from #a gram of radium stored in a metal cylinder gives a saturation current of about #cb #aj electromagnetic units4 ,elster and ,geitel considered that most of the conductivity observed in the gas was due to a radio-active emanation1 which gradually diffused from the clay into the air in the vessel4 ,the increased conductivity in the gas observed by ,elster and ,geitel would thus be produced by the emanation from #g #aj gram of radium4 ,taking the density of clay as #b1 this corresponds to about #aj gram of radium per gram of clay4 ,but it has been shown that if #df #aj gram of radium were present in each gram of earth1 the heat emitted would compensate for the loss of heat of the earth by conduction and radiation4 ,the amount of activity observed in the earth is thus about the right order of magnitude to account for the heat emission required4 ,in the above estimate1 the presence of uranium and thorium minerals in the earth has not been considered4 ,moreover1 it is probable that the total amount of radio-activity in the clay was considerably greater than that calculated1 for it is likely that other radio-active matter was present which did not give off an emanation4 ,if the earth is supposed to be in a state of thermal equilibrium in which the heat lost by radiation is supplied from radio-active matter1 there must be an amount of radio-active matter in the earth corresponding to about #bgj million tons of radium4 ,if there were more radium than this in the earth1 the temperature gradient would be greater than that observed to-day4 ,this may appear to be a very large quantity of radium1 but recent determinations 7section #bha7 of the amount of radium emanation in the atmosphere strongly support the view that a large quantity of radium must exist in the surface soil of the earth4 ,eve found1 on a minimum estimate1 that the amount of emanation always present in the atmosphere is equivalent to the equilibrium amount derived from #ajj tons of radium4 ,there is every reason to believe that the emanation found in the atmosphere is supplied both by the diffusion of the emanation from the soil and by the action of springs4 ,since the emanation loses half its activity in four days1 it cannot diffuse from any great depth4 ,assuming that the radium is uniformly distributed throughout the earth1 the quantity of the radium emanation produced in a thin shell of earth about thirteen metres in depth1 is sufficient to account for the amount ordinarily observed in the atmosphere4 ,i think we may conclude that the present rate of loss of heat of the earth might have continued unchanged for long periods of time in consequence of the supply of heat from radio-active matter in the earth4 ,it thus seems probable that the earth may have remained for very long intervals of time at a temperature not very different from that observed to-day1 and that1 in consequence1 the time during which the earth has been at a temperature capable of supporting the presence of animal and vegetable life may be very much longer than the estimate made by ,lord ,kelvin from other data4 #bgb4 ,evolution of matter4 ,although the hypothesis that all matter is composed of some elementary unit of matter or protyle has been advanced as a speculation at various times by many prominent physicists and chemists1 the first definite experimental evidence showing that the chemical atom was not the smallest unit of matter was obtained in #ahig by ,j4 ,j4 ,thomson in his classic research on the nature of the cathode rays produced by an electric discharge in a vacuum tube4 ,we have seen that ,sir ,william ,crookes1 who was the first to demonstrate the remarkable properties of these rays1 had suggested that they consisted of streams of projected charged matter and represented--as he termed it--a new or 8fourth state of matter40 ,j4 ,j4 ,thomson showed by two distinct methods 7section #ej71 that the cathode rays consisted of a stream of negatively charged particles projected with great velocity4 ,the particles behaved as if their mass was only about of the mass of the atom of hydrogen1 which is the lightest atom known4 ,these corpuscles1 as they were termed by ,thomson1 were found at a later date to be produced from a glowing carbon filament and from a zinc plate exposed to the action of ultra-violet light4 ,they acted as isolated units of negative electricity1 and1 as we have seen1 may be identified with the electrons studied mathematically by ,larmor and ,lorentz4 ,not only were these electrons produced by the action of light1 heat1 and the electric discharge1 but similar bodies were also found to be emitted spontaneously from the radio-elements with a velocity far greater than that observed for the electrons in a vacuum tube4 ,the electrons produced in these various ways were all found to carry a negative charge1 and to be apparently identical2 for the ratio e_/m of the charge of the electron to its mass was in all cases the same within the limits of experimental error4 ,since electrons1 produced from different kinds of matter and under different conditions1 were in all cases identical1 it seemed probable that they were a constituent part of all matter4 ,j4 ,j4 ,thomson suggested that the atom is built up of a number of these negatively charged electrons combined in some way with corresponding positively charged bodies4 ,on this view the atoms of the chemical elements differ from one another only in the number and arrangement of the component electrons4 ,the removal of an electron from the atom in the case of ionization does not appear to affect permanently the stability of the system1 for no evidence has so far been obtained to show that the passage of an intense electric discharge through a gas results in a permanent alteration of the structure of the atom4 ,on the other hand1 in the case of the radio-active bodies1 a positively charged particle of mass about twice that of the hydrogen atom escapes from the heavy radio-atom4 ,this loss appears to result at once in a permanent alteration of the atom1 and causes a marked change in its physical and chemical properties4 ,in addition there is no evidence that the process is reversible4 ,the expulsion of a particle with great velocity from an atom of radio-active matter also results in a transformation of the atom4 ,for example radium ,e emits a particle1 and1 in consequence1 gives rise to a distinct substance radium ,f 7polonium74 ,a case of this kind1 where the expulsion of a particle with great velocity causes a complete rearrangement of the parts of an atom1 is probably quite distinct from the process which occurs during ionization1 where a slow speed electron escapes from the atom without apparently affecting the stability of the atom left behind4 ,the only direct experimental evidence of the transformation of matter has been derived from a study of the radio-active bodies4 ,if the disintegration theory1 advanced to account for the phenomena of radio-activity1 is correct in the main essentials1 then the radio-elements are undergoing a spontaneous and continuous process of transformation into other and different kinds of matter4 ,the rate of transformation is slow in uranium and thorium1 but is fairly rapid in radium4 ,it has been shown that the fraction of a mass of radium which is transformed per year is about of the total amount present4 ,in the case of uranium and thorium probably a million years would be required to produce a similar amount of change4 ,thus the process of transformation in uranium and thorium is far too slow to be detected within a reasonable time by the use of the balance or spectroscope1 but the radiations which accompany the transformation can easily be detected4 ,although the process of change is slow it is continuous1 and in the course of ages the uranium and thorium present in the earth must be transformed into other types of matter4 ,those who have considered the possibility of atoms undergoing a process of transformation have generally thought that the matter as a whole would undergo a progressive change1 with a gradual alteration of physical and chemical properties of the whole mass of substance4 ,on the theory of disintegration this is not the case4 ,only a minute fraction of the matter present breaks up in unit time1 and in each of the successive stages through which the disintegrated atoms pass1 there is in most cases a marked alteration in the chemical and physical properties of the matter4 ,the transformation of the radio-elements is thus a transformation of a part per saltum1 and not a progressive change of the whole4 ,at any time after the process of transformation has been in progress there will thus remain a part of the matter which is unchanged1 and1 mixed with it1 the products which have resulted from the transformation of the remainder4 ,the question naturally arises whether the process of degradation of matter is confined to the radio-elements or is a universal property of matter4 ,it will be shown in chapter ,,xiv that all matter1 so far examined1 exhibits the property of radio-activity to a slight degree4 ,it is very difficult1 however1 to make certain that the observed radio-activity is not due to the presence in the matter of a slight trace of a radio-element4 ,if ordinary matter is radio-active1 it is certain that its activity is much less than that of uranium1 and consequently that its rate of transformation must be excessively slow4 ,there is1 however1 another possibility to be considered4 ,the changes occurring in the radio-elements would probably never have been detected if the change had not been accompanied by the expulsion of charged particles with great velocity4 ,it does not seem unlikely that an atom may undergo disintegration without projecting a part of its system with sufficient velocity to ionize the gas4 ,in fact1 we have seen that1 even in the radio-elements1 several of the series of changes in both thorium1 radium1 and actinium are unaccompanied by ionizing rays4 ,the experimental results given in ,appendix ,a strongly support this point of view4 ,it may thus be possible that all matter is undergoing a slow process of transformation1 which has so far only been detected in the radio-elements on account of the expulsion of charged particles with great velocity during the change4 ,this process of degradation of matter continuing for ages must reduce the constituents of the earth to the simpler and more stable forms of matter4 ,the idea that helium is a transformation product of radium suggests the probability that helium is one of the more elementary substances of which the heavier atoms are composed4 ,sir ,norman ,lockyer1 in his interesting book on 8,inorganic ,evolution10 has pointed out that the spectra of helium and of hydrogen predominate in the hottest stars4 ,in the cooler stars the more complex types of matter appear4 ,sir ,norman ,lockyer has based his theory of evolution of matter on evidence of a spectroscopic examination of the stars1 and considers that temperature is the main factor in breaking up matter into its simpler forms4 ,the transformation of matter occurring in the radio-elements is on the other hand spontaneous1 and independent of temperature over the range examined4 ,footnote #ccc3 ,perrin1 ,revue ,scientifique1 ,april #ac1 #aija4 ,footnote #ccd3 ,becquerel1 ,c4 ,r4 #acc1 p4 #igi1 #aija4 ,footnote #cce3 ,rutherford and ,mc,clung1 ,phil4 ,trans4 ,a1 p4 #be1 #aija4 ,footnote #ccf3 ,rutherford1 ,phil4 ,mag4 ,jan4 and ,feb4 #aijj4 ,footnote #ccg3 ,p4 ,curie1 ,c4 ,r4 #acf1 p4 #bbc1 #aijc4 ,footnote #cch3 ,rutherford1 ,phil4 ,mag4 ,april1 #aijc4 ,footnote #cci3 ,m4 and ,mme ,curie1 ,c4 ,r4 #acd1 p4 #he1 #aijb4 ,footnote #cdj3 ,rutherford and ,soddy1 ,trans4 ,chem4 ,soc4 #ha1 pp4 #cba1 #hcg1 #aijb4 ,phil4 ,mag4 ,sept4 and ,nov4 #aijb4 ,footnote #cda3 ,rutherford and ,soddy1 ,phil4 ,mag4 ,april1 #aijc4 ,footnote #cdb3 ,rutherford and ,soddy1 ,phil4 ,mag4 ,may1 #aijc4 ,footnote #cdc3 ,rutherford1 ,phys4 ,zeit4 #d1 p4 #bce1 #aijb4 ,phil4 ,mag4 ,feb4 #aijc4 ,footnote #cdd3 ,rutherford1 ,phil4 ,mag4 ,may1 #aijc4 ,footnote #cde3 ,curie and ,laborde1 ,c4 ,r4 #acf1 p4 #fgc1 #aijc4 ,footnote #cdf3 ,j4 ,j4 ,thomson1 ,nature1 p4 #fja1 #aijc4 ,footnote #cdg3 ,crookes1 ,c4 ,r4 #abh1 p4 #agf1 #ahii4 ,footnote #cdh3 ,f4 ,re1 ,c4 ,r4 p4 #acf1 p4 #acic1 #aijc4 ,footnote #cdi3 ,richarz and ,schenck1 ,berl4 ,ber4 p4 #aajb1 #aijc4 ,schenck1 ,berl4 ,ber4 p4 #cg1 #aijd4 ,footnote #cej3 ,armstrong and ,lowry1 ,proc4 ,roy4 ,soc4 #aijc4 ,chem4 ,news1 #hh1 p4 #hi1 #aijc4 ,footnote #cea3 ,rutherford and ,soddy1 ,phil4 ,mag4 ,may1 #aijc4 ,footnote #ceb3 ,boltwood1 ,nature1 ,may #be1 p4 #hj1 #aijd4 ,phil4 ,mag4 ,april1 #aije4 ,footnote #cec3 ,mc,coy1 ,ber4 d4 ,d4 ,chem4 ,ges4 ,no4 #aa1 p4 #bfda1 #aijd4 ,footnote #ced3 ,strutt1 ,nature1 ,march #ag and ,july #g1 #aijd4 ,proc4 ,roy4 ,soc4 ,march #b1 #aije4 ,footnote #cee3 ,strutt1 ,proc4 ,roy4 ,soc4 ,march #b1 #aije4 ,footnote #cef3 ,soddy1 ,nature1 ,may #ab1 #aijd2 ,jan4 #ai1 #aije4 ,footnote #ceg3 ,whetham1 ,nature1 ,may #e1 #aijd2 ,jan4 #bf1 #aije4 ,footnote #ceh3 ,danne1 ,c4 ,r4 ,jan4 #bc1 #aije4 ,footnote #cei3 ,j4 ,j4 ,thomson1 ,nature1 ,april #cj1 p4 #fja1 #aijc4 ,footnote #cfj3 ,voller1 ,phys4 ,zeit4 #e1 ,no4 #bd1 p4 #gha1 #aijd4 ,footnote #cfa3 ,ramsay and ,cooke1 ,nature1 ,aug4 #aa1 #aijd4 ,footnote #cfb3 ,eve1 ,nature1 ,march #af1 #aije4 ,footnote #cfc3 ,j4 ,j4 ,thomson1 ,international ,electrical ,congress1 ,st ,louis1 ,sept4 #aijd4 ,footnote #cfd3 ,rutherford and ,soddy1 ,phil4 ,mag4 p4 #ehb1 #aijb2 pp4 #dec and #egi1 #aijc4 ,footnote #cfe3 ,ramsay and ,soddy1 ,nature1 ,july #af1 p4 #bdf1 #aijc4 ,proc4 ,roy4 ,soc4 #gb1 p4 #bjd1 #aijc2 #gc1 p4 #cdf1 #aijd4 ,footnote #cff3 ,curie and ,dewar1 ,c4 ,r4 #ach1 p4 #aij1 #aijd4 ,chem4 ,news1 #hi1 p4 #he1 #aijd4 ,footnote #cfg3 ,himstedt and ,meyer1 ,ann4 d4 ,phys4 #ae1 p4 #ahd1 #aijd4 ,footnote #cfh3 ,strutt1 ,proc4 ,roy4 ,soc4 ,march #b1 #aije4 ,footnote #cfi3 ,boltwood1 ,phil4 ,mag4 ,april1 #aije4 ,footnote #cgj3 ,moss1 ,trans4 ,roy4 ,soc4 ,dublin1 #aijd4 ,footnote #cga3 ,travers1 ,nature1 p4 #bdh1 ,jan4 #ab1 #aije4 ,footnote #cgb3 ,jaquerod1 ,c4 ,r4 p4 #ghi1 #aijd4 ,footnote #cgc3 ,ramsay and ,travers1 ,zeitsch4 ,physik4 ,chem4 #be1 p4 #efh1 #ahih4 ,footnote #cgd3 ,ramsay1 ,nature1 ,april #g1 #aijd4 ,footnote #cge3 ,lodge1 ,nature1 ,june #aa1 p4 #abi1 #aijc4 ,footnote #cgf3 ,larmor1 ,aether and ,matter1 p4 #bcc4 ,footnote #cgg3 ,j4 ,j4 ,thomson1 ,phil4 ,mag4 p4 #fha1 ,dec4 #aijc4 ,footnote #cgh3 ,lord ,kelvin1 ,phil4 ,mag4 ,oct4 #aijd4 ,footnote #cgi3 ,thomson1 ,phil4 ,mag4 ,march1 #aijd4 ,footnote #chj3 ,rutherford and ,soddy1 ,phil4 ,mag4 ,may1 #aijc4 ,footnote #cha3 ,see ,strutt and ,joly1 ,nature1 ,oct4 #ae1 #aijc4 ,footnote #chb3 ,strutt1 ,phil4 ,mag4 ,june1 #aijc4 ,footnote #chc3 ,elster and ,geitel1 ,phys4 ,zeit4 #d1 ,no4 #ai1 p4 #ebb1 #aijc4 ,chem4 ,news1 ,july #ag1 p4 #cj1 #aijc4 ,,chapter ,,xiv4 ,,radio-,,activity ,,of ,,the ,,atmosphere ,,and ,,of ,,ordinary ,,materials4 #bgc4 ,radio-activity of the atmosphere4 ,the experiments of ,geitel#chd and ,c4 ,t4 ,r4 ,wilson#che in #aijj showed that a positively or negatively charged conductor placed inside a closed vessel gradually lost its charge4 ,this loss of charge was shown to be due to a small ionization of the air inside the vessel4 ,elster and ,geitel also found that a charged body exposed in the open air lost its charge rapidly1 and that the rate of discharge was dependent on the locality and on atmospheric conditions4 ,a more detailed description and discussion of these results will be given later in section #bhd4 ,in the course of these experiments1 ,geitel observed that the rate of discharge increased slightly for some time after the vessel had been closed4 ,he considered that this might possibly be due to the existence of some radio-active substances in the air1 which produced excited activity on the walls of the vessel and so increased the rate of dissipation of the charge4 ,in #aija ,elster and ,geitel#chf tried the bold experiment of seeing whether it were possible to extract a radio-active substance from the air4 ,the experiments of the writer had shown that the excited radio-activity from the thorium emanation could be concentrated on the negative electrode in a strong electric field4 ,this result indicated that the carriers of the radio-activity had a positive charge of electricity4 ,elster and ,geitel therefore tried an experiment to see whether positively charged carriers1 possessing a similar property1 were present in the atmosphere4 ,for this purpose a cylinder of wire-netting1 charged negatively to #fjj volts1 was exposed for several hours in the open air4 ,the cylinder was then removed1 and quickly placed in a large bell-jar1 inside which was placed an electroscope to detect the rate of discharge4 ,it was found that the rate of discharge was increased to a slight extent4 ,in order to multiply the effect a wire about #bj metres in length was exposed at some height from the ground1 and was kept charged to a high potential by connecting it to the negative terminal of an influence machine4 ,after exposure for some hours1 this wire was removed and placed inside the dissipation vessel4 ,the rate of discharge was found to be increased many times by the presence of the wire4 ,no increase was observed when the wire was charged positively instead of negatively4 ,the results also showed that the radio-active matter could be removed from the wire in the same way as from a wire made active by exposure in the presence of the thorium emanation4 ,a piece of leather moistened with ammonia was rubbed over the active wire4 ,on testing the leather1 it was found to be strongly radio-active4 ,when a long wire was used1 the amount of activity obtained on the leather was comparable with that possessed by a gram of uranium oxide4 ,the activity produced on the wire was not permanent1 but disappeared to a large extent in the course of a few hours4 ,the amount of activity produced on a wire of given size1 exposed under similar conditions1 was independent of the material of the wire4 ,lead1 iron and copper wires gave about equal effects4 ,the amount of activity obtained was greatly increased by exposing a negatively charged wire in a mass of air which had been undisturbed for a long time4 ,experiments were made in the great cave of ,wolfenbttel1 and a very large amount of activity was observed4 ,by transferring the activity to a piece of leather it was found that the rays could appreciably light up a screen of barium platinocyanide in the dark#chg4 ,the rays also darkened a photographic plate through a piece of aluminium #ja mm4 in thickness4 ,these remarkable experiments show that the excited radio-activity obtained from the atmosphere is very similar in character to the excited activity produced by the emanations of radium and thorium4 ,no investigators have contributed more to our knowledge of the radio-activity and ionization of the atmosphere than ,elster and ,geitel4 ,the experiments here described have been the starting-point of a series of researches by them and others on the radio-active properties of the atmosphere1 which have led to a great extension of our knowledge of that important subject4 ,rutherford and ,allan#chh determined the rate of decay of the excited activity produced on a negatively charged wire exposed in the open air4 ,a wire about #ae metres long was exposed in the open air1 and kept charged by an influence machine to a potential of about #aj1jjj volts4 ,an hour's exposure was sufficient to obtain a large amount of excited activity on the wire4 ,the wire was then rapidly removed and wound on a framework which formed the central electrode in a large cylindrical metal vessel4 ,the ionization current for a saturation voltage was measured by means of a sensitive ,dolezalek electrometer4 ,the current1 which is a measure of the activity of the wire1 was found to diminish according to an exponential law with the time1 falling to half value in about #de minutes4 ,the rate of decay was independent of the material of the wire1 of the time of exposure1 and of the potential of the wire4 ,an examination was also made of the nature of the rays emitted by the radio-active wire4 ,for this purpose a lead wire was made radio-active in the manner described1 and then rapidly wound into the form of a flat spiral4 ,the penetrating power of the rays was tested in a vessel similar to that shown in ,fig4 #ag4 ,most of the ionization was found to be due to some very easily absorbed rays1 which were of a slightly more penetrating character than the rays emitted from a wire made active by the radium or thorium emanations4 ,the intensity of the rays was cut down to half value by about #jjja cm4 of aluminium4 ,the photographic action observed by ,elster and ,geitel through #ja mm4 of aluminium showed that some penetrating rays were also present4 ,this was afterwards confirmed by ,allan1 who used the electric method4 ,these penetrating rays are probably similar in character to the rays from the radio-elements4 #bgd4 ,the excited activity produced on the negatively charged wire cannot be due to an action of the strong electric field on the surface of the wire2 for very little excited activity is produced if the wire is charged to the same potential inside a closed cylinder4 ,we have seen that the excited activity produced on the wire can be partially removed by rubbing and by solution in acids1 and1 in this respect1 it is similar to the excited activity produced in bodies by the emanations of radium and thorium4 ,the very close similarity of the excited activity obtained from the atmosphere to that obtained from the radium and thorium emanations suggests the probability that a radio-active emanation exists in the atmosphere4 ,this view is confirmed by a large amount of indirect evidence discussed in sections #bgf1 #bgg and #bhj4 ,assuming the presence of a radio-active emanation in the atmosphere1 the radio-active effects observed receive a simple explanation4 ,the emanation in the air gradually breaks up1 giving rise in some way to positively charged radio-active carriers4 ,these are driven to the negative electrode in the electric field1 and there undergo a further change1 giving rise to the radiations observed at the surface of the wire4 ,the matter which causes excited activity will thus be analogous to the active deposit of radium and thorium4 ,since the earth is negatively electrified with regard to the upper atmosphere1 these positive radio-active carriers produced in the air are continuously deposited on the surface of the earth4 ,everything on the surface of the earth1 including the external surface of buildings1 the grass1 and leaves of trees1 must be covered with an invisible deposit of radio-active material4 ,a hill1 or mountain peak1 or any high mass of rock or land1 concentrates the earth's electric field at that point and consequently will receive more excited radio-activity per unit area than the plain4 ,elster and ,geitel have pointed out that the greater ionization of the air observed in the neighbourhood of projecting peaks receives a satisfactory explanation on this view4 ,if the radio-active carriers are produced at a uniform rate in the atmosphere1 the amount of excited activity ,it1 produced on a wire exposed under given conditions1 will1 after exposure for a time t1 be given by ,it ,i 7#a e-t 7 1 where ,i is the maximum activity on the wire and is the constant of decay of the excited activity4 ,since the activity of a wire after removal falls to half value in about #de minutes1 the value of is #jib 7hour74 ,some experiments made by ,allan#chi are in rough agreement with the above equation4 ,accurate comparative results are difficult to obtain on account of the inconstancy of the radio-activity of the open air4 ,after an exposure of a wire for several hours1 the activity reached a practical maximum1 and was not much increased by continued exposure4 ,we have seen 7section #aia7 that the carriers of the active deposit of radium and thorium move in an electric field with about the same velocity as the ions4 ,we should expect therefore that a long wire charged to a high negative potential would abstract the active carriers from the atmosphere for a considerable distance4 ,this does not appear to be the case1 for ,eve 7see section #bha7 has found that the carriers are only abstracted from the air for a radius of less than one metre1 for a potential of the wire of #aj1jjj volts4 ,it seems probable that the carriers of the active matter are deposited on the numerous fine dust particles present in the air and thus move very slowly even in a strong electric field4 ,the amount of excited activity produced on a wire1 supported some distance from the surface of the earth1 should increase steadily with the voltage1 for the greater the potential1 the greater the volume of air from which the radio-active carriers are abstracted4 ,the presence of radio-active matter in the atmosphere will account for a considerable portion of the ionization of the air observed near the earth4 ,this important question is discussed in more detail in section #bha4 #bge4 ,radio-activity of freshly fallen rain and snow4 ,c4 ,t4 ,r4 ,wilson#cij tried experiments to see if any of the radio-active material from the air was carried down by rain4 ,for this purpose a quantity of freshly fallen rain was collected1 rapidly evaporated to dryness in a platinum vessel1 and the activity of the residue tested by placing the vessel in an electroscope4 ,in all cases1 the rate of discharge of the electroscope was considerably increased4 ,from about #ej c4c4 of rain water1 an amount of activity was obtained sufficient to increase the rate of discharge of the electroscope four or five times1 after the rays had traversed a thin layer of aluminium or gold-leaf4 ,the activity disappeared in the course of a few hours1 falling to half value in about #cj minutes4 ,rain water1 which had stood for some hours1 showed no trace of activity4 ,tap water1 when evaporated1 left no active residue4 ,the amounts of activity obtained from a given quantity of rain water were all of the same order of magnitude1 whether the rain was precipitated in fine or in large drops1 by night or by day1 or whether the rain was tested at the beginning or at the end of a heavy rainfall lasting several hours4 ,the activity obtained from rain is not destroyed by heating the platinum vessel to a red heat4 ,in this and other respects it resembles the excited activity obtained on negatively charged wires exposed in the open air4 ,c4 ,t4 ,r4 ,wilson#cia obtained a radio-active precipitate from rain water by adding a little barium chloride and precipitating the barium with sulphuric acid4 ,an active precipitate was also obtained when alum was added to the water1 and the aluminium precipitated by ammonia4 ,the precipitates obtained in this way showed a large activity4 ,the filtrate when boiled down was quite inactive1 showing that the active matter had been completely removed by precipitation4 ,this effect is quite analogous to the production of active precipitates from a solution containing the active deposit of thorium 7see section #ahe74 ,the radio-activity of freshly fallen snow was independently observed by ,c4 ,t4 ,r4 ,wilson#cib in ,england1 and ,allan#cic and ,mc,lennan#cid in ,canada4 ,in order to obtain a large amount of activity1 the surface layer of snow was removed1 and evaporated to dryness in a metal vessel4 ,an active residue was obtained with radio-active properties similar to those observed for freshly fallen rain4 ,both ,wilson and ,allan found that the activity of rain and snow decayed at about the same rate1 the activity falling to half value in about #cj minutes4 ,mc,lennan states that he found a smaller amount of radio-activity in the air after a prolonged fall of snow4 ,schmauss#cie has observed that drops of water falling through air ionized by ,rntgen rays acquire a negative charge4 ,this effect is ascribed to the fact that the negative ions in air diffuse faster than the positive4 ,on this view the drops of rain and flakes of snow would acquire a negative charge in falling through the air4 ,they would in consequence act as collectors of the positive radio-active carriers from the air4 ,on evaporation of the water the radio-active matter would be left behind4 #bgf4 ,radio-active emanations from the earth4 ,elster and ,geitel observed that the air in caves and cellars was1 in most cases1 abnormally radio-active1 and showed very strong ionization4 ,this action might possibly be due to an effect of stagnant air1 by which it produced a radio-active emanation from itself1 or to a diffusion of a radio-active emanation from the soil4 ,to test whether this emanation was produced by the air itself1 ,elster and ,geitel shut up the air for several weeks in a large boiler1 but no appreciable increase of the activity or ionization was observed4 ,to see whether the air imprisoned in the capillaries of the soil was radio-active1 ,elster and ,geitel#cif put a pipe into the earth and sucked up the air into a testing vessel by means of a water pump4 ,the apparatus employed to test the ionization of the air is shown in ,fig4 #ajc4 ,c is an electroscope connected with a wire net1 ,z4 ,the active air was introduced into a large bell-jar of #bg litres capacity1 the inside of which was covered with wire netting1 ,,mm4 ,the bell-jar rested on an iron plate ,,ab4 ,the electroscope could be charged by the rod ,s4 ,the rate of discharge of the electroscope1 before the active air was introduced1 was noted4 ,on allowing the active air to enter1 the rate of discharge increased rapidly1 rising in the course of a few hours in one experiment to #cj times the original value4 ,they found that the emanation produced excited activity on the walls of the containing vessel4 ,the air sucked up from the earth was even more active than that observed in caves and cellars4 ,there can thus be little doubt that the abnormal activity observed in caves and cellars is due to a radio-active emanation1 present in the earth1 which gradually diffuses to the surface and collects in places where the air is not disturbed4 ,results similar to those obtained by ,elster and ,geitel for the air removed from the earth at ,wolfenbttel were also obtained later by ,ebert and ,ewers#cig at ,munich4 ,they found a strongly active emanation in the soil1 and1 in addition1 examined the variation with time of the activity due to the emanation in a sealed vessel4 ,after the introduction of the active air into the testing vessel1 the activity was observed to increase for several hours1 and then to decay1 according to an exponential law1 with the time1 falling to half value in about #cb days4 ,this rate of decay is more rapid than that observed for the radium emanation1 which decays to half value in a little less than four days4 ,the increase of activity with time is probably due to the production of excited activity on the walls of the vessel by the emanation4 ,in this respect it is analogous to the increase of activity observed when the radium emanation is introduced into a closed vessel4 ,no definite experiments were made by ,ebert and ,ewers on the rate of decay of this excited activity4 ,in one experiment the active emanation1 after standing in the vessel for #adj hours1 was removed by sucking ordinary air of small activity through the apparatus4 ,the activity rapidly fell to about half value1 and this was followed by a very slow decrease of the activity with time4 ,this result indicates that about half the rate of discharge observed was due to the radiation from the emanation and the other half to the excited activity produced by it4 ,the apparatus employed by ,ebert and ,ewers in these experiments was very similar to that employed by ,elster and ,geitel1 shown in ,fig4 #ajc4 ,ebert and ,ewers observed that1 when the wire net attached to the electroscope was charged negatively1 the rate of discharge observed was always greater than when it was charged positively4 ,the differences observed between the two rates of discharge varied between #aj and #bj per cent4 ,a similar effect has been observed by ,sarasin1 ,tommasina and ,micheli#cih for a wire made active by exposure to the open air4 ,this difference in the rates of discharge for positive and negative electricity is probably connected with the presence of particles of dust or small water globules suspended in the gas4 ,the experiments of ,miss ,brooks 7section #aha7 have shown that the particles of dust present in the air containing the thorium emanation become radio-active4 ,a large proportion of these dust particles acquire a positive charge and are carried to the negative electrode in an electric field4 ,this effect would increase the rate of discharge of the electroscope when charged negatively4 ,in later experiments1 ,ebert and ,ewers noticed that1 in some cases1 when the air had been kept in the vessel for several days1 the effect was reversed1 and the electroscope showed a great rate of discharge when charged positively4 ,illustration3 ,fig4 #ajc4 ,j4 ,j4 ,thomson#cii has observed that the magnitude of the ionization current depends on the direction of the electric field1 if fine water globules are suspended in the ionized gas4 ,in later experiments1 ,ebert#djj found that the radio-active emanation could be removed from the air by condensation in liquid air4 ,this property of the emanation was independently discovered by ,ebert before he was aware of the results of ,rutherford and ,soddy on the condensation of the emanations of radium and thorium4 ,to increase the amount of radio-active emanation in a given volume of air1 a quantity of the active air1 obtained by sucking the air from the soil1 was condensed by a liquid air machine4 ,the air was then allowed partially to evaporate1 but the process was stopped before the point of volatilization of the emanation was reached4 ,this process was repeated with another quantity of air and the residues added together4 ,proceeding in this way1 he was able to concentrate the emanation in a small volume of air4 ,on allowing the air to evaporate1 the ionization of the air in the testing vessel increased rapidly for a time and then slowly diminished4 ,ebert states that the maximum for the emanation which had been liquefied for some time was reached earlier than for fresh air4 ,the rate of decay of activity of the emanation was not altered by keeping it at the temperature of liquid air for some time4 ,in this respect it behaves like the emanations of radium and thorium4 ,j4 ,j4 ,thomson#dja found that air bubbled through ,cambridge tap water showed much greater conductivity than ordinary air4 ,the air was drawn through the water by means of a water pump into a large gasometer1 when the ionization current was tested with a sensitive electrometer4 ,when a rod charged negatively was introduced into this conducting air it became active4 ,after an exposure for a period of #ae to #cj minutes in the conducting gas1 the rod1 when introduced into a second testing vessel1 increased the saturation current in the vessel to about five times the normal amount4 ,very little effect was produced when the rod was uncharged or charged positively for the same time4 ,the activity of the rod decayed with the time1 falling to half value in about #dj minutes4 ,the amount of activity produced on a wire under constant conditions was independent of the material of the wire4 ,the rays from the rod were readily absorbed in a few centimetres of air4 ,these effects were1 at first1 thought to be due to the action of the small water drops suspended in the gas1 for it was well known that air rapidly drawn through water causes a temporary increase in its conductivity4 ,later results1 however1 showed that there was a radio-active emanation present in ,cambridge tap water4 ,this led to an examination of the waters from deep wells in various parts of ,england1 and ,j4 ,j4 ,thomson found that1 in some cases1 a large amount of emanation could be obtained from the well water4 ,the emanation was released either by bubbling air through the water or by boiling the water4 ,the gases obtained by boiling the water were found to be strongly active4 ,a sample of air mixed with the radio-active emanation was condensed4 ,the liquefied gas was allowed to evaporate1 and the earlier and later portions of the gas were collected in separate vessels4 ,the final portion was found to be about #cj times as active as the first portion4 ,an examination of the radio-active properties of the active gases so obtained has been made by ,adams#djb4 ,he found that the activity of the emanation decayed1 according to an exponential law1 with the time1 falling to half value in about #cd days4 ,this is not very different from the rate of decay of the activity of the radium emanation1 which falls to half value in a little less than four days4 ,the excited activity produced by the emanation decayed to half value in about #ce minutes4 ,the decay of the excited activity from radium is at first irregular1 but after some time falls off1 according to an exponential law1 diminishing to half value in #bh minutes4 ,taking into account the uncertainty attaching to measurements of the very small ionization observed in these experiments1 the results indicate that the emanation obtained from well water in ,england is similar to1 if not identical with1 the radium emanation4 ,adams observed that the emanation was slightly soluble in water4 ,after well water had been boiled for a while and then put aside1 it was found to recover its power of giving off an emanation4 ,the amount obtained after standing for some time was never more than #aj per cent4 of the amount first obtained4 ,thus it is probable that the well water1 in addition to the emanations mixed with it1 has also a slight amount of a permanent radio-active substance dissolved in it4 ,ordinary rain water or distilled water does not give off an emanation4 ,bumstead and ,wheeler#djc have made a very careful examination of the radio-activity of the emanation obtained from the surface water and soil at ,new ,haven1 ,connecticut4 ,the emanation1 obtained from the water by boiling1 was passed into a large testing cylinder1 and measurements of the current were made by means of a sensitive electrometer4 ,the current gradually rose to a maximum1 after the introduction of the emanation1 in exactly the same way as the current increases in a vessel after the introduction of the radium emanation4 ,the decay of activity of the emanations obtained from the water and soil was carefully measured1 and1 within the limits of experimental error1 agreed with the rate of decay of activity observed for the radium emanation4 ,the identity of the emanations from the water and soil with the radium emanation was still further established by experiments on the rate of diffusion of the emanation through a porous plate4 ,by comparative tests it was found that the coefficient of diffusion of the emanations from the water and soil was the same as for the radium emanation4 ,also1 by comparison of the rate of diffusion of carbonic acid1 it was found that the density of the emanation was about four times that of carbonic acid1 a result in good agreement with that found for the radium emanation 7sections #afa and #afb74 ,bumstead#djd has found that a considerable amount of thorium as well as radium emanation exists in the air of ,new ,haven4 ,for a three hour exposure in the open air1 #c to #e per cent4 of the excited activity on the wire is due to thorium4 ,for a twelve hour exposure1 the thorium activity was sometimes #ae per cent4 of the whole4 ,on account of the comparatively slow decay of the excited activity of thorium1 the activity on the wire after removal for three or four hours was due almost entirely to thorium4 ,the rate of decay could then be measured accurately1 and was found to be the same as for a wire exposed in the presence of the thorium emanation4 ,dadourian#dje has made an examination of the underground air in ,new ,haven1 and has found that this too contains a large quantity of the thorium emanation4 ,a circular hole about #ej cms4 in diameter and #b metres deep was dug in the ground4 ,a number of wires were wound on an insulated frame and suspended in the hole1 the top of the hole then being covered over4 ,the wire was charged negatively by a ,wimshurst machine4 ,after a long exposure the excited activity on the wire diminished at a rate that showed it to be a mixture of the excited activities of thorium and radium4 ,a very large amount of work has been done in examining various hot and mineral springs for the presence of the radium emanation1 and it is not possible here to refer more than briefly to a few of the very numerous papers that have been published on this subject both in ,europe and ,america4 ,h4 ,s4 ,allen and ,lord ,blythswood#djf have observed that the hot springs at ,bath and ,buxton gave off a radio-active emanation4 ,this was confirmed by ,strutt#djg1 who found that the escaping gases contained the radium emanation1 and also that the mud deposited from the springs contained a trace of radium salts4 ,these results are of considerable interest1 for ,lord ,rayleigh has observed that helium is contained among the gases evolved by the springs4 ,it appears probable that the helium observed is produced from the radium or radio-active deposits through which the water flows4 ,many mineral and hot springs which are famous for their curative properties have been found to contain traces of radium and also considerable amounts of radium emanation4 ,it has been suggested that the curative properties may be due to some extent to the presence of these minute quantities of radium4 ,himstedt#djh found that the thermal springs at ,baden ,baden contained the radium emanation1 while ,elster and ,geitel#dji examined the deposits formed by these springs and found them to contain small quantities of radium salts4 ,results of a similar character were obtained for a number of waters in ,germany by ,dorn#daj1 ,schenck#daa1 and ,h4 ,mache#dab4 ,curie and ,laborde#dac have tested the waters of a large number of mineral springs and found that the great majority contain the radium emanation4 ,in this connection1 it is of interest to note that ,curie and ,laborde found very little emanation in the waters of ,salins-,moutiers1 while ,blanc#dad observed1 on the other hand1 that the sediment from the spring was very active4 ,a closer examination of this deposit by ,blanc revealed the fact that it contained a considerable quantity of thorium4 ,this was proved by finding that it gave out an emanation1 which lost half of its activity in one minute1 and produced excited activity1 which fell to half value in about #aa hours4 ,boltwood#dae has tested a number of samples of spring water from different sources in ,america and has found that many of them contain the radium emanation4 ,most of the results upon the amount of radium emanation from different sources have been expressed in arbitrary units without1 in many cases1 any comparative standard being given4 ,boltwood 7loc4 cit47 has described a satisfactory method for collecting and testing the emanation from different waters1 and has suggested that the rate of discharge observed by the electroscope or the electrometer should be expressed in terms of the effect due to the emanation liberated on solution of a definite weight of the mineral uraninite4 ,since in every mineral so far examined1 the amount of radium present is proportional to the amount of uranium1 such a standard would be sufficiently definite for practical purposes4 ,the emanation liberated from a few centigrams of the mineral is sufficient to give a convenient rate of discharge of an electroscope4 ,such a method is preferable to using a known quantity of a radium compound as a standard1 since it is difficult to know with certainty the activity of the preparations of radium which may be in the possession of the different experimenters4 #bgg4 ,radio-activity of constituents of the earth4 ,elster and ,geitel#daf observed that1 although in many cases the conductivity of the air was abnormally high in underground enclosures1 the conductivity varied greatly in different places4 ,in the ,baumann ,cave1 for example1 the conductivity of the air was nine times the normal1 but in the ,iberg ,cave only three times the normal4 ,in a cellar at ,clausthal the conductivity was only slightly greater than the normal1 but the excited radio-activity obtained on a negatively charged wire exposed in it was only of the excited radio-activity obtained when the wire was exposed in the free air4 ,they concluded from these experiments that the amount of radio-activity in the different places probably varied with the nature of the soil4 ,observations were then made on the conductivity of the air sucked up from the earth at different parts of the country4 ,the clayey and limestone soils at ,wolfenbttel were found to be strongly active1 the conductivity varying from four to sixteen times the normal amount4 ,a sample of air from the shell limestone of ,wrzburg and from the basalt of ,wilhelmshhe showed very little activity4 ,experiments were made to see whether any radio-active substance could be detected in the soil itself4 ,for this purpose some earth was placed on a dish and introduced under a bell-jar1 similar to that shown in ,fig4 #ajc4 ,the conductivity of the air in the bell-jar increased with the time1 rising to three times the normal value after several days4 ,little difference was observed whether the earth was dry or moist4 ,the activity of the soil seemed to be permanent1 for no change in the activity was observed after the earth had been laid aside for eight months4 ,attempts were then made to separate the radio-active constituent from the soil by chemical treatment4 ,for this purpose a sample of clay was tested4 ,by extraction with hydrochloric acid all the calcium carbonate was removed4 ,on drying the clay the activity was found to be reduced1 but it spontaneously regained its original activity in the course of a few days4 ,it seems probable1 therefore1 that an active product had been separated from the soil by the acid4 ,elster and ,geitel consider that an active substance was present in the clay1 which formed a product more readily soluble in hydrochloric acid than the active material itself4 ,there seemed to be a process of separation analogous to that of ,th ,x from thorium by precipitation with ammonia4 ,experiments were also made to see whether substances placed in the earth acquired any radio-activity4 ,for this purpose samples of potter's clay1 whitening1 and heavy spar1 wrapped in linen1 were placed in the earth #ej cms4 below the surface4 ,after an interval of a month1 these were dug up and their activity examined4 ,the clay was the only substance which showed any activity4 ,the activity of the clay diminished with the time1 showing that activity had been excited in it by the emanations present in the soil4 ,elster and ,geitel#dag have found that a large quantity of the radio-active emanation can be obtained by sucking air through clay4 ,in some cases1 the conductivity of the air in the testing vessel was increased over #ajj times4 ,they have also found that the so-called 8fango0--a fine mud obtained from hot springs in ,battaglia1 ,northern ,italy--gives off three or four times as much emanation as clay4 ,by treating the fango with acid1 the active substance present was dissolved4 ,on adding some barium chloride to the solution1 and precipitating the barium as sulphate1 the active substance was removed1 and in this way a precipitate was obtained over #ajj times as active1 weight for weight1 as the original fango4 ,comparisons were made of the rate of decay of the excited activity1 due to the emanation from fango1 with that due to the radium emanation1 and within the limits of error1 the decay curves obtained were found to be identical4 ,there can thus be no doubt that the activity observed in fango is due to the presence of a small quantity of radium4 ,elster and ,geitel calculate that the amount of radium1 contained in it1 is only about one-thousandth of the amount to be obtained from an equal weight of pitchblende from ,joachimsthal4 ,vincenti and ,levi ,da ,zara#dah have found that the waters and sediments of a number of hot springs in ,northern ,italy contain the radium emanation4 ,elster and ,geitel observed that natural carbonic acid obtained from great depths of old volcanic soil was radio-active1 while ,burton#dai found that the petroleum from a deep well in ,ontario1 ,canada1 contained a large quantity of emanation1 probably of radium1 since its activity fell to half value in #ca days1 while the excited activity produced by the emanation fell to half value in about #ce minutes4 ,a permanently active deposit was left behind after volatilization of the oil1 indicating that probably one or more of the radio-elements were present in minute quantity4 ,elster and ,geitel#dbj have found that the active sediments obtained from springs at ,nauheim and ,baden ,baden showed abnormal rates of decay of the excited activity4 ,this was finally traced to the presence in the deposit of both thorium and radium4 ,by suitable chemical methods1 the two active substances were separated from each other and were then tested separately4 #bgh4 ,effect of meteorological conditions upon the radio-activity of the atmosphere4 ,the original experiments of ,elster and ,geitel on the excited radio-activity derived from the atmosphere were repeated by ,rutherford and ,allan#dba in ,canada4 ,it was found that a large amount of excited radio-activity could be derived from the air1 and that the effects were similar to those observed by ,elster and ,geitel in ,germany4 ,this was the case even on the coldest day in winter1 when the ground was covered deeply with snow and wind was blowing from the north over snow-covered lands4 ,the results showed that the radio-activity present in the air was not much affected by the presence of moisture1 for the air during a ,canadian winter is extremely dry4 ,the greatest amount of excited activity on a negatively charged wire was obtained in a strong wind4 ,in some cases the amount produced for a given time of exposure was ten to twenty times the normal amount4 ,a cold bright day of winter usually gave more effect than a warm dull day in summer4 ,elster and ,geitel#dbb have made a detailed examination of the effect of meteorological conditions on the amount of excited radio-activity to be derived from the atmosphere4 ,for this purpose a simple portable apparatus was devised by them and used for the whole series of experiments4 ,a large number of observations were taken1 extending over a period of twelve months4 ,they found that the amount of excited activity obtained was subject to great variations4 ,the extreme values obtained varied in the ratio of #af to #a4 ,no direct connection could be traced between the amount of ionization in the atmosphere and the amount of excited activity produced4 ,they found that the greatest amount of excited activity was obtained during a fog1 when the amount of ionization in the air was small4 ,this result1 however1 is not necessarily contradictory to the view that the ionization and activity of the air are to a certain extent connected4 ,from the experiments of ,miss ,brooks on the effect of dust in acting as carriers of excited activity1 more excited activity should be obtained during a fog than in clear air4 ,the particles of water become centres for the deposit of radio-active matter4 ,the positive carriers are thus anchored and are not removed from the air by the earth's field4 ,in a strong electric field1 these small drops will be carried to the negative electrode and manifest their activity on the surface of the wire4 ,on the other hand1 the distribution of water globules throughout the air causes the ions in the air to disappear rapidly in consequence of their diffusion to the surface of the drops 7see section #ca74 ,for this reason the denser the fog1 the smaller will be the conductivity observed in the air4 ,lowering the temperature of the air had a decided influence4 ,the average activity observed below #j ,c4 was #add times the activity observed above #j ,c4 ,the height of the barometer was found to exert a marked influence on the amount of excited activity to be derived from the air4 ,the lower the barometer the greater was the amount of excited activity in the air4 ,the effect of variation of the height of the barometer is intelligible1 when it is considered that probably a large proportion of the radio-activity observed in the air is due to the radio-active emanations which are continuously diffusing from the earth into the atmosphere4 ,elster and ,geitel have suggested that a lowering of the pressure of the air would cause the air from the ground to be drawn up from the capillaries of the earth into the atmosphere4 ,this1 however1 need not necessarily be the case if the conditions of the escape of the emanation into the atmosphere are altered by the variation of the position of underground water or by a heavy fall of rain4 ,the amount of excited activity to be derived from the air on the ,baltic ,coast was only one-third of that observed inland at ,wolfenbttel4 ,experiments on the radio-activity of the air in mid-ocean would be of great importance in order to settle whether the radio-activity observed in the air is due to the emanations from the soil alone4 ,it is probable that the radio-activity of the air at different points of the earth may vary widely1 and may largely depend on the nature of the soil4 ,saake#dbc has found that the amount of emanation present in the air at high altitudes in the valley of ,arosa in ,switzerland is much greater than the normal amount at lower levels4 ,elster and ,geitel have observed that there is also a larger number of ions in the air at high altitudes1 and suggest that the curative effect of thermal springs and the physiological actions of the air at high levels may be connected with the presence of an unusual amount of radio-active matter in the atmosphere4 ,simpson#dbd made experiments on the amount of excited activity at ,karasjoh1 ,norway1 at a height of about #aej feet above sea level4 ,the sun did not rise above the level of the horizon during the time the observations were taken4 ,the average amount of excited activity obtained from the air was considerably greater than the normal amount observed by ,elster and ,geitel in ,germany4 ,this was the more surprising as the ground was frozen hard and covered with deep snow4 ,allan1 working in ,montreal1 ,canada1 early observed that the amount of activity to be obtained from the air was about the same in summer as in winter1 although1 in the latter case1 the whole earth was deeply frozen and covered with snow1 and the winds blew from the north over snow-covered lands4 ,under such conditions1 a diminution of the amount of activity is to be expected since the diffusion of the emanation must be retarded1 if not altogether stopped1 by the freezing of the soil4 ,on the other hand1 it appears difficult to escape from the conclusion of ,elster and ,geitel that the emanation present in the atmosphere is evolved from the earth itself4 ,some interesting experiments have been made by ,mc,lennan#dbe on the amount of excited radio-activity to be derived from the air when filled with fine spray4 ,the experiments were made at the foot of the ,american ,fall at ,niagara4 ,an insulated wire was suspended near the foot of the ,fall1 and the amount of excited activity on the wire compared with the amount to be obtained on the same wire for the same exposure in ,toronto4 ,the amount of activity obtained from the air at ,toronto was generally five or six times that obtained from the air at the ,falls4 ,in these experiments it was not necessary to use an electric machine to charge the wire negatively1 for the falling spray kept the insulated wire permanently charged to a potential of about #gejj volts4 ,these results indicate that the falling spray had a negative charge and electrified the wire4 ,the small amount of the excited radio-activity at the ,falls was probably due to the fact that the negatively charged drops abstracted the positively charged radio-active carriers from the atmosphere1 and in falling carried them to the river below4 ,on collecting the spray and evaporating it1 no active residue was obtained4 ,such a result is1 however1 to be expected on account of the minute proportion of the spray tested compared with that present in the air4 #bgi4 ,a very penetrating radiation from the earth's surface4 ,mc,lennan#dbf1 and ,rutherford and ,cooke#dbg independently1 observed the presence of a very penetrating radiation inside buildings4 ,mc,lennan measured the natural conductivity of the air in a large closed metal cylinder by means of a sensitive electrometer4 ,the cylinder was then placed inside another and the space between filled with water4 ,for a thickness of water between the cylinders of #be cms4 the conductivity of the air in the inner cylinder fell to about #fc per cent4 of its initial value4 ,this result shows that part of the ionization in the inner cylinder was due to a penetrating radiation from an external source1 which radiation was partially or wholly absorbed in water4 ,rutherford and ,cooke observed that the rate of discharge of a sealed brass electroscope was diminished by placing a lead screen around the electroscope4 ,a detailed investigation of the decrease of the rate of discharge in the electroscope1 when surrounded by metal screens1 was made later by ,cooke#dbh4 ,a thickness of #e cms4 of lead round the electroscope decreased the rate of discharge about #cj per cent4 ,further increase of the thickness of the screen had no effect4 ,when the apparatus was surrounded by #e tons of pig-lead the rate of discharge was about the same as when it was surrounded by a plate about #c cms4 thick4 ,an iron screen also diminished the rate of discharge to about the same extent as the lead4 ,by suitably arranging lead screens it was found that the radiation came equally from all directions4 ,it was of the same intensity by night as by day4 ,in order to be sure that this penetrating radiation did not arise from the presence of radio-active substances in the laboratory1 the experiments were repeated in buildings in which radio-active substances had never been introduced1 and also on the open ground far removed from any building4 ,in all cases a diminution of the rate of discharge of the electroscope1 when surrounded by lead screens1 was observed4 ,these results show that a penetrating radiation is present at the surface of the earth1 arising partly from the earth itself and partly from the atmosphere4 ,the result is not surprising when the radio-activity of the earth and atmosphere is taken into account4 ,the writer has found that bodies made active by exposure to the emanations from thorium and radium give out rays4 ,we may expect then that the very similar excited radio-activity which is present in the earth and atmosphere should also give rise to rays of a similar character4 ,more recent work1 however 7section #bhf71 indicates that this explanation is not sufficient to explain all the facts observed4 #bhj4 ,comparison of the radio-activity of the atmosphere with that produced by the radio-elements4 ,the radio-active phenomena observed in the earth and atmosphere are very similar in character to those produced by thorium and radium4 ,radio-active emanations are present in the air of caves and cellars1 in natural carbonic acid1 and in deep well water1 and these emanations produce excited radio-activity on all bodies in contact with them4 ,the question now arises whether these effects are due entirely to known radio-elements present in the earth or to unknown kinds of radio-active matter4 ,the simplest method of testing this point is to compare the rate of decay of the radio-active product in the atmosphere with those of the known radio-active products of thorium and radium4 ,a cursory examination of the facts at once shows that the radio-activity of the atmosphere is much more closely allied to effects produced by radium than to those due to thorium4 ,the activity of the emanation released from well water1 and also that sucked up from the earth1 decays to half value in about #cc days1 while the activity of the radium emanation decays to half value in an interval of #cg to #d days4 ,considering the difficulty of making accurate determinations of these quantities1 the rates of decay of the activity of the emanations from the earth and from radium agree within the limits of experimental error4 ,a large number of observers have found that the radium emanation is present in the water of thermal springs and in the sediment deposited by them4 ,bumstead and ,wheeler have shown that the emanation from the soil and surface water of ,new ,haven is identical with that from radium4 ,if the emanations from the earth and from radium are the same1 the excited activities produced should have the same rate of decay4 ,the emanation from well water in ,england approximately fulfils this condition 7section #bgf71 but an observation recorded by ,ebert and ,ewers 7section #bgf7 seems to show that the excited activity due to the emanation sucked up from the earth decays at a very slow rate compared with that due to radium4 ,bumstead has given undoubted evidence that the thorium as well as the radium emanation is also present in the atmosphere at ,new ,haven1 while ,dadourian has shown that it is emitted by ,new ,haven soil4 ,blanc1 and ,elster and ,geitel1 have also found that thorium is present in the sediment from some thermal springs4 ,if the active matter in the atmosphere consists mainly of the radium emanation1 the active deposit on a negatively charged wire1 exposed in the open air1 should initially consist of radium ,a1 ,b and ,c4 ,the curve of decay should be identical with the decay curve of the excited activity of radium1 measured by the rays1 that is1 there should be a rapid initial drop corresponding to the initial #c minute change1 then a slow rate of variation1 the activity after several hours decaying to half value in about #bh minutes 7see section #bbb74 ,the rapid initial drop has been observed by ,bumstead for the air at ,new ,haven4 ,allan#dbi did not observe this initial drop in ,montreal1 but found the activity fell to half value in about #de minutes1 reckoning from a time about #aj minutes after the removal of the active wire4 ,this is about the rate of decay to be expected for the active deposit of radium over the same interval4 ,allan obtained evidence that there were several kinds of active matter deposited on the wire4 ,for example1 the activity transferred from the active wire to a piece of leather1 moistened with ammonia1 fell to half value in #ch minutes2 for a piece of absorbent felt treated similarly1 the activity fell to half value in #fj minutes1 the normal time for the untreated wire being #de minutes4 ,it is probable that this variation of the rate of decay is due to the fact that unequal proportions of radium ,b and ,c were transferred from the wire to the rubber4 ,if a greater proportion of ,b than of ,c were removed1 the decay would be slower and vice versa4 ,the fact that the activity of rain and snow falls to half value in about #cj minutes is a strong indication that the radium emanation is present in the atmosphere4 ,the active matter with the rain and snow after standing some time would consist mainly of radium ,c and this should decay exponentially with the time1 falling to half value in #bh minutes4 ,on account of the rapid decay of the thorium emanation--half value in one minute--it is not likely that much of the activity of the atmosphere can be ascribed to it4 ,its effect would be most marked near the surface of the soil4 ,there can be little doubt1 that a large part of the radio-activity of the atmosphere is due to the radium emanation1 which is continually diffusing into the atmosphere from the pores of the earth4 ,since radio-activity has been observed in the atmosphere at all points at which observations have1 so far1 been made1 radio-active matter must be distributed in minute quantities throughout the soil of the earth4 ,the volatile emanations escape into the atmosphere by diffusion1 or are carried to the surface in spring water or by the escape of underground gases1 and cause the radio-active phenomena observed in the atmosphere4 ,the observation of ,elster and ,geitel that the radio-activity of the air is much less near the sea than inland is explained at once1 if the radio-activity of the atmosphere is due mainly to the diffusion of emanations from the soil into the air above it4 ,the rare gases helium and xenon which exist in the atmosphere have been tested and found to be non-radio-active4 ,the radio-activity of the air cannot be ascribed to a slight radio-activity possessed by either of these gases4 #bha4 ,amount of the radium emanation in the atmosphere4 ,it is a matter of great interest to form an estimate of the amount of radium emanation present in the atmosphere1 for since it comes from the earth1 it indirectly serves as a means of estimating the amount of radium which is distributed over a thin crust of the earth4 ,some experiments in this direction have been made by ,eve in the laboratory of the writer4 ,the experiments are not yet completed but the results so far obtained allow us to calculate the probable amount of emanation per cubic kilometre of the atmosphere near the earth4 ,experiments were first made with a large iron tank #aed cms4 square and #gcj cms4 deep1 in a building in which no radium or other radio-active material had ever been introduced4 ,the saturation ionization current for the air in the tank was first measured by means of an electroscope1 connected with an insulated electrode passing up the centre of the closed tank4 ,assuming that the ionization in the tank was uniform1 the number of ions produced per c4c4 of the air in the tank was found to be #aj4 ,this is a considerably lower value than has usually been observed in a small closed vessel 7see section #bhd74 ,cooke obtained the value #aj for a well cleaned brass electroscope1 surrounded by lead1 while ,schuster obtained a value about #ab for the air in the laboratory of ,owens ,college1 ,manchester4 ,in order to measure the amount of the excited activity from the tank1 a central insulated wire was charged negatively to about #aj1jjj volts by a ,wimshurst machine4 ,after two hours1 the wire was removed and wound on an insulated frame connected with a gold-leaf electroscope4 ,the rate of decay of the activity on the wire was found to be about the same as for the excited activity produced by the radium emanation4 ,in order to estimate the amount of radium emanation present in the large tank1 special experiments were made with a smaller tank in which a known quantity of the radium emanation was introduced by employing a solution of pure radium bromide of known concentration4 ,a central wire was made the negative electrode as before1 and1 after removal1 it was wound on the frame and its activity tested4 ,in this way it was found that the amount of radium emanation present in the large tank1 in order to produce the excited activity observed1 must have been equal to the equilibrium or maximum amount to be obtained from #ie #aj grams of pure radium bromide4 ,the volume of the large tank was #ag cubic metres1 so that the amount of emanation present per cubic metre was equivalent to that liberated from #ef #aj grams of radium bromide in radio-active equilibrium4 ,if the amount of the emanation in the tank is taken as the average amount existing in the outside air1 the amount of radium emanation present per cubic kilometre of the air is equivalent to that supplied by #jef grams of radium bromide4 ,for the purpose of calculation1 suppose the emanation is uniformly distributed over the land portion of the earth 7 of the total surface71 and to extend to an average height of #e kilometres4 ,the air over the sea is not taken into account as its radio-activity has not been examined4 ,the total amount of emanation present in the atmosphere under these conditions corresponds to that supplied by about #djj tons of radium bromide4 ,in order to maintain this amount of emanation in the atmosphere1 it must be supplied at a constant rate from the earth's surface4 ,since the greater amount of the emanation probably escapes into the air by transpiration and diffusion through the soil1 the emanation cannot reach the surface except from a very thin layer of the earth4 ,the probable thickness of this layer can be estimated if it is assumed that the present loss of heat from the earth is supplied from the radio-active matter contained in it4 ,we have seen 7section #bga7 that1 on this hypothesis1 there must be an amount of active matter in the earth corresponding to about #cjj million tons of radium4 ,if this is supposed to be uniformly distributed1 a thickness of layer of about #ac metres will suffice to maintain the calculated amount of emanation in the atmosphere4 ,this thickness of layer is about the order of magnitude to be expected from general considerations4 ,these results lead indirectly to the conclusion that a large amount of emanation does undoubtedly exist in the surface crust of the earth4 ,experiments were also made by ,eve with a large zinc cylinder exposed in the open air4 ,volume for volume1 the average amount of excited activity derived from it was only about one-third of that obtained from the large iron tank4 ,this would reduce the amount of emanation1 previously deduced1 to about one-third4 ,before such calculations can be considered at all definite1 it will be necessary to make comparative measurements of the amount of emanation in the atmosphere at various parts of the earth4 ,the air at ,montreal is not abnormally active1 so that the calculations probably give the right order of magnitude of the quantities4 ,eve also observed that the amount of activity to be obtained per unit length of the wire in the zinc cylinder of about #gj cms4 in diameter was about the same as for a wire #e mms4 in diameter charged to #aj1jjj volts in the open air1 supported #bj feet from the ground4 ,this shows that such a potential does not draw in the carriers of excited activity which are more than half a metre away1 and probably the range is even less4 ,it is of great importance to find how large a proportion of the number of ions produced in the atmosphere is due to the radio-active matter distributed throughout it4 ,the results of ,eve with the large iron tank1 already referred to1 indicate that a large proportion of the ionization in the tank was due to the radio-active matter contained in it1 for the ratio of the excited activity on the central electrode to the total ionization current in the tank was about of the corresponding ratio for a smaller tank into which a supply of the radium emanation had been introduced4 ,this result requires confirmation by experiments at other parts of the earth1 but the results point to the conclusion that a large part1 if not all1 of the ionization at the earth's surface is due to radio-active matter distributed in the atmosphere4 ,a constant rate of production of #cj ions per second per c4c4 of air1 which has been observed in the open air at the surface of the earth in various localities1 would be produced by the presence in each c4c4 of the air of the amount of emanation liberated from #bd #aj grams of radium bromide in radio-active equilibrium4 ,it is not likely1 however1 that the ionization of the upper part of the atmosphere is due to this cause alone4 ,in order to explain the maintenance of the large positive charge1 which generally exists in the upper atmosphere1 there must be a strong ionization of the upper air1 which may possibly be due to ionizing radiations emitted by the sun4 #bhb4 ,ionization of atmospheric air4 ,a large number of measurements have been made during the last few years to determine the relative amount of ionization in the atmosphere in different localities and at different altitudes4 ,measurements of this character were first undertaken by ,elster and ,geitel with a special type of electroscope4 ,a charged body exposed to the air was attached to a portable electroscope1 and the rate of loss of charge was observed by the movement of the gold or aluminium leaf4 ,the rates of discharge of the electroscope for positive and negative electricity were generally different1 the ratio depending on the locality and the altitude1 and on the meteorological conditions4 ,this apparatus is not suitable for quantitative measurements and the deductions to be drawn from the observations are of necessity somewhat indefinite4 ,ebert#dcj has designed a portable apparatus in which the number of ions per c4c4 of the air can be determined easily4 ,a constant current of air is drawn between two concentric cylinders by means of a fan actuated by a falling weight4 ,the inner cylinder is insulated and connected with an electroscope4 ,knowing the capacity of the apparatus1 and the velocity of the current of air1 the rate of movement of the gold-leaf affords a measure of the number of ions present in unit volume of the air drawn between the cylinders4 ,in this way ,ebert found that the number of ions in the air was somewhat variable1 but on an average corresponded to about #bfjj per c4c4 in the particular locality where the measurements were made4 ,this is the equilibrium number of ions present per c4c4 when the rate of production balances the rate of recombination4 ,if q is the number of ions produced per second per unit volume of the air and n is the equilibrium number1 then q n where is the constant of recombination 7section #cj74 ,by a slight addition to the apparatus of ,ebert1 ,schuster#dca has shown that the constant of recombination for the particular sample of air under investigation can be determined4 ,the value so obtained for air in the neighbourhood of ,manchester was variable1 and two or three times as great as for dust-free air4 ,the results of some preliminary measurements showed that the number of ions present per c4c4 of the air in different localities varied from #bcgj to #cffj1 while the value of q1 the number of ions produced per c4c4 per second1 varied between #ab and #che4 ,rutherford and ,allan and ,eberts showed that the ions in the air had about the same mobility as the ions produced in air by ,rntgen rays and radio-active substances4 ,in some recent determinations by ,mache and ,von ,schweidler#dcb1 the velocity of the positive ion was found to be about #ajb cms4 per second1 and that of the negative #abe cms41 for a potential gradient of one volt per cm4 ,langevin#dcc has recently shown that in addition to these swift moving ions1 there are also present in the atmosphere some ions which travel extremely slowly in an electric field4 ,the number of these slowly moving ions in the air in ,paris is about #dj times as great as the number of the swifter ions4 ,this result is of great importance1 for in the apparatus of ,ebert these ions escape detection1 since the electric field is not strong enough to carry them to the electrodes during the time of their passage between the cylinders4 #bhc4 ,radio-activity of ordinary materials4 ,it has been shown that radio-active matter seems to be distributed fairly uniformly over the surface of the earth and in the atmosphere4 ,the very important question arises whether the small radio-activity observed is due to known or unknown radio-elements present in the earth and atmosphere1 or to a feeble radio-activity of matter in general1 which is only readily detectable when large quantities of matter are present4 ,the experimental evidence is not yet sufficient to answer this question1 but undoubted proof has been obtained that many of the metals show a very feeble radio-activity4 ,whether this radio-activity is due to the presence of a slight trace of the radio-elements or is an actual property of the metals themselves will be discussed in more detail in section #bhf4 ,schuster#dcd has pointed out that every physical property hitherto discovered for one element has been found to be shared by all the others in varying degrees4 ,for example1 the property of magnetism is most strongly marked in iron1 nickel1 and cobalt1 but all other substances are found to be either feebly magnetic or diamagnetic4 ,it might thus be expected on general principles that all matter should exhibit the property of radio-activity in varying degrees4 ,on the view developed in chapter ,x41 the presence of this property is an indication that the matter is undergoing change accompanied by the expulsion of charged particles4 ,it does not1 however1 by any means follow that because the atom of one element in the course of time becomes unstable and breaks up1 that1 therefore1 the atoms of all the other elements pass through similar phases of instability4 ,it has already been mentioned 7section #h71 that ,mme ,curie made a very extensive examination of most of the elements and their compounds for radio-activity4 ,the electric method was used1 and any substance possessing an activity of of that of uranium would certainly have been detected4 ,with the exception of the known radio-elements and the minerals containing uranium and thorium1 no other substances were found to be radio-active even to that degree4 ,certain substances like phosphorus#dce possess the property of ionizing a gas under special conditions4 ,the air which is drawn over the phosphorus is conducting1 but it has not yet been settled whether this conductivity is due merely to ions formed at the surface of the phosphorus or to ions produced by the phosphorus nuclei or emanations1 as they have been termed1 which are carried along with the current of air4 ,it does not however appear that the ionization of the gas is in any way due to the presence of a penetrating type of radiation such as is emitted by the radio-active bodies4 ,le ,bon 7section #h7 observed that quinine sulphate1 after being heated to a temperature below the melting point and then allowed to cool1 showed for a time strong phosphorescence and was able rapidly to discharge an electroscope4 ,the discharging action of quinine sulphate under varying conditions has been very carefully examined by ,miss ,gates#dcf4 ,the ionization could not be observed through thin aluminium foil or gold-leaf1 but appeared to be confined to the surface of the sulphate4 ,the current observed by an electrometer was found to vary with the direction of the electric field1 indicating that the positive and negative ions had very different mobilities4 ,the discharging action appears to be due either to an ionization of the gas very close to the surface by some short ultra-violet light waves1 accompanying the phosphorescence1 or to a chemical action taking place at the surface4 ,thus1 neither phosphorus nor quinine sulphate can be considered to be radio-active1 even under the special conditions when they are able to discharge an electrified body4 ,no evidence in either case has been found that the ionization is due to the emission of a penetrating radiation4 ,no certain evidence has yet been obtained that any body can be made radio-active by exposure to ,rntgen rays or cathode rays4 ,a metal exposed to the action of ,rntgen rays gives rise to a secondary radiation which is very readily absorbed in a few centimetres of air4 ,it is possible that this secondary radiation may prove to be analogous in some respects to the rays from the radio-elements4 ,the secondary radiation1 however1 ceases immediately the ,rntgen rays are cut off4 ,villard#dcg stated that a piece of bismuth produced a feeble photographic action after it had been exposed for some time to the action of the cathode rays in a vacuum4 ,it has not however been shown that the bismuth gives out rays of a character similar to those of the radio-active bodies4 ,the experiments of ,ramsay and ,cooke on the production of apparent activity in inactive matter by the radiations from radium have already been discussed in section #bfd4 ,the existence of a very feeble radio-activity of ordinary matter has been deduced from the study of the conductivity of gases in closed vessels4 ,the conductivity is extremely minute1 and special methods are required to determine it with accuracy4 ,a brief account will now be given of the gradual growth of our knowledge on this important question4 #bhd4 ,conductivity of air in closed vessels4 ,since the time of ,coulomb onwards several investigators have believed that a charged conductor placed inside a closed vessel lost its charge more rapidly than could be explained by the conduction leak across the insulating support4 ,matteucci1 as early as #ahej1 observed that the rate of loss of charge was independent of the potential4 ,boys1 by using quartz insulators of different lengths and diameters1 arrived at the conclusion that the leakage must in part take place through the air4 ,this loss of charge in a closed vessel was believed to be due in some way to the presence of dust particles in the air4 ,on the discovery that gases become temporary conductors of electricity under the influence of ,rntgen rays and the rays from radio-active substances1 attention was again drawn to this question4 ,geitel#dch and ,c4 ,t4 ,r4 ,wilson#dci independently attacked the problem1 and both came to the conclusion that the loss of charge was due to a constant ionization of the air in the closed vessel4 ,geitel employed in his experiments an apparatus similar to that shown in ,fig4 #ajc4 ,the loss of charge of an ,exner electroscope1 with the cylinder of wire netting ,z attached1 was observed in a closed vessel containing about #cj litres of air4 ,the electroscope system was found to diminish in potential at the rate of about #dj volts per hour1 and this leakage was shown not to be due to a want of insulation of the supports4 ,wilson1 on the other hand1 used a vessel of very small volume1 in order to work with air which could be completely freed from dust4 ,in the first experiments a silvered glass vessel with a volume of only #afc c4c4 was employed4 ,the experimental arrangement is shown in ,fig4 #ajd4 ,illustration3 ,fig4 #ajd4 ,the conductor1 of which the loss of charge was to be measured1 was placed near the centre of the vessel ,a4 ,it consisted of a narrow strip of metal with a gold-leaf attached4 ,the strip of metal was fixed to the upper rod by means of a small sulphur bead4 ,the upper rod was connected with a sulphur condenser with an ,exner electroscope ,b attached to indicate its potential4 ,the gold-leaf system was initially charged to the same potential as the upper rod and condenser by means of a fine steel wire which was caused to touch the gold-leaf system by the attraction of a magnet brought near it4 ,the rate of movement of the gold-leaf was measured by means of a microscope provided with a micrometer eye-piece4 ,by keeping the upper rod at a slightly higher potential than the gold-leaf system1 it was ensured that the loss of charge of the gold-leaf system should not be due in any way to a conduction leakage across the sulphur bead4 ,the method employed by ,wilson in these experiments is very certain and convenient when an extremely small rate of discharge is to be observed4 ,in this respect the electroscope measures with certainty a rate of loss of charge much smaller than can be measured by a sensitive electrometer4 ,both ,geitel and ,wilson found that the leakage of the insulated system in dust-free air was the same for a positive as for a negative charge1 and was independent of the potential over a considerable range4 ,the leakage was the same in the dark as in diffuse daylight4 ,the independence of leakage of the potential is strong evidence that the loss of charge is due to a constant ionization of the air4 ,when the electric field acting on the gas exceeds a certain value1 all the ions are carried to the electrodes before recombination occurs4 ,a saturation current is reached1 and it will be independent of further increase of the electric field1 provided1 of course1 a potential sufficiently high to cause a spark to pass is not applied4 ,c4 ,t4 ,r4 ,wilson has recently devised a striking experiment to show the presence of ions in dust-free air which is not exposed to any external ionizing agency4 ,two large metal plates are placed in a glass vessel connected with an expansion apparatus similar to that described in section #cd4 ,on expanding the air1 the presence of the ions is shown by the appearance of a slight cloud between the plates4 ,these condensation nuclei carry an electric charge1 and are apparently similar in all respects to the ions produced in gases by ,x rays1 or by the rays from active substances4 ,wilson found that the loss of charge of the insulated system was independent of the locality4 ,the rate of discharge was unaltered when the apparatus was placed in a deep tunnel1 so that it did not appear that the loss of charge was due to an external radiation4 ,from experiments already described1 however 7section #bgi71 it is probable that about #cj per cent4 of the rate of discharge observed was due to a very penetrating radiation4 ,this experiment of ,wilson's indicates that the intensity of the penetrating radiation was the same in the tunnel as at the earth's surface4 ,wilson found that the ionization of the air was about the same in a brass vessel as in one of glass1 and came to the conclusion that the air was spontaneously ionized4 ,using a brass vessel of volume about #dga c4c41 ,wilson determined the number of ions that must be produced in air per unit volume per second1 in order to account for the loss of charge of the insulated system4 ,the leakage system was found to have a capacity of about #aa electrostatic units1 and lost its charge at the rate of #da volts per hour for a potential of #baj volts1 and #dj volts per hour for a potential of #abj volts4 ,taking the charge on an ion as #cd #aj electrostatic units1 this corresponds to a production of #bf ions per second4 ,rutherford and ,allan#ddj repeated the results of ,geitel and ,wilson1 using an electrometer method4 ,the saturation current was observed between two concentric zinc cylinders of diameter #bee and #ge cms4 respectively and length #aed cms4 ,it was found that the saturation current could practically be obtained with a potential of a few volts4 ,saturation was however obtained with a lower voltage after the air had remained undisturbed in the cylinders for several days4 ,this was probably due to the gradual settling of the dust originally present in the air4 ,later observations of the number of ions produced in air in sealed vessels have been made by ,patterson#dda1 ,harms#ddb1 and ,cooke#ddc4 ,the results obtained by different observers are shown in the following table4 ,the value of the charge on an ion is taken as #cd #aj electrostatic units3 ,material of ,number of ions ,observer vessel produced per c4c4 per second ,silvered #cf ,c4 ,t4 ,r4 ,wilson glass ,brass #bf ,zinc #bg ,rutherford and ,allan ,glass #ec to #fc ,harms ,iron #fa ,patterson ,cleaned #aj ,cooke brass ,it will be shown later that the differences in these results are probably due to differences in the radio-activity of the containing vessel4 #bhe4 ,effect of pressure and nature of gas4 ,c4 ,t4 ,r4 ,wilson 7loc4 cit47 found that the rate of leakage of a charged conductor varied approximately as the pressure of the air between the pressures examined1 viz4 #dc mms4 and #gdc mms4 of mercury4 ,these results point to the conclusion that1 in a good vacuum1 a charged body would lose its charge extremely slowly4 ,this is in agreement with an observation of ,crookes1 who found that a pair of gold-leaves retained their charge for several months in a high vacuum4 ,wilson#ddd at a later date investigated the leakage for different gases4 ,the results are included in the following table1 where the ionization produced in air is taken as unity3 ,gas ,relative 7,relative ionization ionization7 _/ 7density7 ,air #ajj #ajj ,hydrogen #jahd #bg ,carbon dioxide #afi #aaj ,sulphur #bfd #aba dioxide ,chloroform #dg #aji ,with the exception of hydrogen1 the ionization produced in different gases is approximately proportional to their density4 ,the relative ionization is very similar to that observed by ,strutt 7section #de7 for gases exposed to the influence of the and rays from radio-active substances1 and points to the conclusion that the ionization observed may be due either to a radiation from the walls of the vessel or from external sources4 ,jaff#dde has made a careful examination of the natural ionization in the very heavy gas nickel-carbonyl1 ,ni7,,co71 in a small silvered glass vessel4 ,the ionization of this gas was #ea times that of air at normal pressure while its density is #ei times that of air4 ,the leak of the electroscope was nearly proportional to the pressures except at low pressure1 when the leak was somewhat greater than would be expected if the pressure law held4 ,the fact that a gas of such high density and complicated structure behaves like the simpler and lighter gases is a strong indication that the ionization itself is due to a radiation from the walls of the vessel and not to a spontaneous ionization of the gas4 ,patterson#ddf examined the variation of the ionization of air with pressure in a large iron vessel of diameter #cj cms4 and length #bj cms4 ,the current between a central electrode and the cylinder was measured by means of a sensitive ,dolezalek electrometer4 ,he found that the saturation current was practically independent of the pressure for pressures greater than #cjj mms4 of mercury4 ,below a pressure of #hj mms4 the current varied directly as the pressure4 ,for air at atmospheric pressure1 the current was independent of the temperature up to #dej ,c4 ,with further increase of temperature1 the current began to increase1 and the increase was more rapid when the central electrode was charged negatively than when it was charged positively4 ,this difference was ascribed to the production of positive ions at the surface of the iron vessel4 ,the results obtained by ,patterson render it very improbable that the ionization observed in air is due to a spontaneous ionization of the enclosed air3 for we should expect the amount of this ionization to depend on the temperature of the gas4 ,on the other hand1 these results are to be expected if the ionization of the enclosed air is mainly due to an easily absorbed radiation from the walls of the vessel4 ,if this radiation had a penetrating power about equal to that observed for the rays of the radio-elements1 the radiation would be absorbed in a few centimetres of air4 ,with diminution of pressure1 the radiations would traverse a greater distance of air before complete absorption1 but the total ionization produced by the rays would still remain about the same1 until the pressure was reduced sufficiently to allow the radiation to traverse the air space in the vessel without complete absorption4 ,with still further diminution of pressure1 the total ionization produced by the radiation1 and in consequence the current observed1 would vary directly as the pressure4 #bhf4 ,examination of ordinary matter for radio-activity4 ,strutt#ddg1 ,mc,lennan and ,burton#ddh1 and ,cooke#ddi1 independently observed about the same time that ordinary matter is radio-active to a slight degree4 ,strutt1 by means of an electroscope1 observed that the ionization produced in a closed vessel varied with the material of the vessel4 ,a glass vessel with a removable base was employed and the vessel was lined with the material to be examined4 ,the following table shows the relative results obtained4 ,the amount of leakage observed is expressed in terms of the number of scale divisions of the eye-piece passed over per hour by the gold-leaf3 ,material of ,leakage in lining of scale divisions vessel per hour ,tinfoil #cc another #bc sample ,glass coated #ac with phosphoric acid ,silver #af chemically deposited on glass ,zinc #ab ,lead #bb ,copper 7clean7 #bc 7oxidized7 #ag ,platinum #bj1 #bi1 #ci 7various samples7 ,aluminium #ad ,there are thus marked differences in the leakage observed for different materials and also considerable differences in different samples of the same metal4 ,for example1 one specimen of platinum caused nearly twice the leakage of another sample from a different stock4 ,mc,lennan and ,burton1 on the other hand1 measured by means of a sensitive electrometer the ionization current produced in the air in a closed iron cylinder #be cms4 in diameter and #acj cms4 in length1 in which an insulated central electrode was placed4 ,the open cylinder was first exposed for some time at the open window of the laboratory4 ,it was then removed1 the top and bottom closed1 and the saturation current through the gas determined as soon as possible4 ,in all cases it was observed that the current diminished for two or three hours to a minimum and then very slowly increased again4 ,in one experiment1 for example1 the initial current observed corresponded to #cj on an arbitrary scale4 ,in the course of four hours the current fell to a minimum of #ff1 and #dd hours later had risen to a practical maximum of #bd4 ,the initial decrease observed is probably due to a radio-activity of the enclosed air or walls of the vessel1 which decayed rapidly with the time4 ,the decay of the excited activity produced on the interior surface of the cylinder when exposed to the air was probably responsible for a part of the decrease observed4 ,mc,lennan ascribes the increase of current with time to a radio-active emanation which is given off from the cylinder1 and ionizes the enclosed air4 ,on placing linings of lead1 tin1 and zinc in the iron cylinder1 considerable differences were observed both for the minimum current and also for the final maximum4 ,lead gave about twice the current due to zinc1 while tin gave an intermediate value4 ,these results are similar in character to those obtained by ,strutt4 ,mc,lennan and ,burton also investigated the effect of diminution of pressure on the current4 ,the cylinder was filled with air to a pressure of #g atmospheres1 and allowed to stand until the current reached a constant value4 ,the air was then allowed to escape and the pressure reduced to #dd mms4 of mercury4 ,the current was found to vary approximately as the pressure over the whole range4 ,these results are not in agreement with the results of ,patterson already described1 nor with some later experiments of ,strutt4 ,mc,lennan's results however point to the conclusion that the ionization was mainly due to an emanation emitted from the metal4 ,since the air was rapidly removed1 a proportionate amount of the emanation would be removed also1 and it might thus be expected that the current would vary directly as the pressure4 ,if this is the case the current through the gas at low pressures should increase again to a maximum if time is allowed for a fresh emanation to form4 ,h4 ,l4 ,cooke1 using an electroscopic method1 obtained results very similar to those given by ,strutt4 ,cooke observed that a penetrating radiation was given out from brick4 ,when a brass vessel containing the gold-leaf system was surrounded by brick1 the discharge of the electroscope was increased by #dj to #ej per cent4 ,this radiation was of about the same penetrating power as the rays from radio-active substances4 ,the rays were completely absorbed by surrounding the electroscope with a sheet of lead #b mms4 in thickness4 ,this result is in agreement with the observation of ,elster and ,geitel1 already mentioned1 that radio-active matter was present in clay freshly dug up from the earth4 ,cooke also observed that the ionization of the air in a brass electroscope could be reduced to about one-third of its usual value if the interior surface of the brass was carefully cleaned4 ,by removing the surface of the brass he was able to reduce the ionization of the enclosed air from #cj to #aj ions per c4c4 per second4 ,this is an important observation1 and indicates that a large proportion of the radio-activity observed in ordinary matter is due to a deposit of radio-active matter on its surface4 ,it has already been shown that bodies which have been exposed in the presence of the radium emanation retain a residual activity which decays extremely slowly4 ,there can be no doubt that the radium emanation is present in the atmosphere1 and the exposed surface of matter1 in consequence1 will become coated with an invisible film of radio-active matter1 deposited from the atmosphere4 ,on account of the slow decay of this activity it is probable that the activity of matter exposed in the open air would steadily increase for a long interval4 ,metals1 even if they are originally inactive1 would thus acquire a fairly permanent activity1 but it should be possible to get rid of this by removing the surface of the metal or by chemical treatment4 ,the rapid increase of activity of all matter left in a laboratory in which a large quantity of emanation has been released has been drawn attention to by ,eve#dej4 ,this superficial activity1 due to the products radium ,d1 ,e1 and ,f1 was mainly removed by placing the metal in strong acid4 ,a number of experiments have been made by ,j4 ,j4 ,thomson1 ,n4 ,r4 ,campbell1 and ,a4 ,wood in the ,cavendish laboratory to examine whether the radio-activity observed in ordinary matter is a specific property of such matter or is due to the presence of some radio-active impurity4 ,an account of these experiments was given by ,professor ,j4 ,j4 ,thomson in a discussion on the ,radio-activity of ,ordinary ,matter at the ,british ,association meeting at ,cambridge1 #aijd4 ,the results#dea1 as a whole1 support the view that each substance gives out a characteristic type or types of radiation and that the radiation is a specific property of the substance4 ,j4 ,j4 ,thomson#deb has made experiments to observe the action of different substances in cutting off the external very penetrating radiation 7section #bgi7 observed by ,cooke and ,mc,lennan4 ,he found that some substances cut off this external radiation1 while others had little if any effect4 ,for example1 the ionization in a closed vessel was reduced #ag per cent4 by surrounding it with a thick lead envelope2 but1 on surrounding it with an equivalent absorbing thickness of water1 or water mixed with sand1 no sensible diminution was observed4 ,in other experiments ,wood#dec found that the diminution of the ionization by a given screen depended upon the material of the vessel4 ,for example1 the ionization in a lead vessel1 surrounded by a lead screen1 was reduced #aj per cent41 while in an iron vessel it was reduced #bd per cent4 ,he concludes from his experiments that the ionization observed in a closed vessel has a threefold origin4 ,part of it is due to an external penetrating radiation1 part to a secondary radiation set up by it1 while the remainder is due to an intrinsic radiation from the walls1 altogether independent of the external radiation4 ,in some experiments of ,campbell#ded1 the variation of the ionization current between two parallel plates was observed for a progressive increase of the distance between them4 ,the effects observed are shown in ,fig4 #aje4 ,the curves at first rise rapidly1 then bend over and finally become a straight line4 ,the knee of the curve is at a different distance for the different substances4 ,the shape of these curves indicates that two types of radiation are present1 one of which is readily absorbed in the gas while the other1 a more penetrating type of radiation1 extends over the whole distance between the plates4 ,in another series of experiments1 one side of the testing vessel was of thin aluminium1 and the ionization current was observed when an exterior screen was brought up to it4 ,lead gave a considerable increase1 but the radiation from it was readily absorbed by an interposed screen4 ,the radiation emitted by carbon and zinc was more than twice as penetrating as from lead4 ,illustration3 ,fig4 #aje4 ,attempts were made to see whether a radio-active emanation was given off by dissolving solid substances and then keeping the solutions in a closed vessel and afterwards testing the activity of the air drawn from them4 ,in some cases an emanation was observed1 but the amount varied with different specimens of the same material2 in others no effect was detected4 ,when linings of different substances were placed in a closed testing vessel1 the ionization current in most cases fell at first1 passed through a minimum1 and then slowly increased to a maximum4 ,for lead the maximum was reached in #i hours1 for tin in #ad and for zinc in #ah hours4 ,these results indicate that an emanation is given off from the metal1 and that the amount reaches a maximum value at different intervals in the various cases4 ,this was confirmed by an examination of a piece of lead which was left in radium-free nitric acid4 ,twenty times the normal effect was observed after this treatment4 ,this is probably due to the increase of porosity of the lead which allows a greater fraction of the emanation produced in the metal to diffuse out with the gas4 ,the activity observed in ordinary matter is extremely small4 ,the lowest rate of production of ions yet observed is #aj per cubic centimetre per second in a brass vessel4 ,suppose a spherical brass vessel is taken of capacity #a litre4 ,the area of the interior surface would be about #dhj sq4 cms4 and the total number of ions produced per second would be about #aj4 ,now it has been shown1 in section #beb1 that an particle projected from radium itself gives rise to #hf #aj ions before it is absorbed in the gas4 ,an expulsion of one particle every #h seconds from the whole vessel1 or of one particle from each square centimetre of surface per hour would thus account for the minute conductivity observed4 ,even if it were supposed that this activity is the result of a breaking up of the matter composing the vessel1 the disintegration of one atom per second per gram1 provided it was accompanied by the expulsion of an particle1 would fully account for the conductivity observed4 ,while the experiments1 already referred to1 afford strong evidence that ordinary matter does possess the property of radio-activity to a feeble degree1 it must not be forgotten that the activity observed is excessively minute1 compared even with a weak radio-active substance like uranium or thorium4 ,the interpretation of the results is complicated1 too1 by the presence of the radium emanation in the atmosphere1 for we have seen that the surface of every body exposed to the open air must become coated with the slowly changing transformation products of the radium emanation4 ,the distribution of radio-active matter throughout the constituents of the earth renders it difficult to be certain that any substance1 however carefully prepared1 is freed from radio-active impurities4 ,if matter in general is radio-active1 it must be undergoing transformation at an excessively slow rate1 unless it be supposed 7see ,appendix ,a7 that changes of a similar character to those observed in the radio-elements may occur without the appearance of their characteristic radiations4 ,footnote #chd3 ,geitel1 ,phys4 ,zeit4 #b1 p4 #aaf1 #aijj4 ,footnote #che3 ,c4 ,t4 ,r4 ,wilson1 ,proc4 ,camb4 ,phil4 ,soc4 #aa1 p4 #cb1 #aijj4 ,proc4 ,roy4 ,soc4 #fh1 p4 #aea1 #aija4 ,footnote #chf3 ,elster and ,geitel1 ,phys4 ,zeit4 #b1 p4 #eij1 #aija4 ,footnote #chg3 ,elster and ,geitel1 ,phys4 ,zeit4 #c1 p4 #gf1 #aija4 ,footnote #chh3 ,rutherford and ,allan1 ,phil4 ,mag4 ,dec4 #aijb4 ,footnote #chi3 ,allan1 ,phil4 ,mag4 ,feb4 #aijd4 ,footnote #cij3 ,c4 ,t4 ,r4 ,wilson1 ,proc4 ,camb4 ,phil4 ,soc4 #aa1 p4 #dbh1 #aijb4 ,footnote #cia3 ,c4 ,t4 ,r4 ,wilson1 ,proc4 ,camb4 ,phil4 ,soc4 #aa1 p4 #dbh1 #aijb2 #ab1 p4 #ag1 #aijc4 ,footnote #cib3 ,c4 ,t4 ,r4 ,wilson1 ,proc4 ,camb4 ,phil4 ,soc4 #ab1 p4 #he1 #aijc4 ,footnote #cic3 ,allan1 ,phys4 ,rev4 #af1 p4 #ajf1 #aijc4 ,footnote #cid3 ,mc,lennan1 ,phys4 ,rev4 #af1 p4 #ahd1 #aijc4 ,footnote #cie3 ,schmauss1 ,annal4 d4 ,phys4 #i1 p4 #bbd1 #aijb4 ,footnote #cif3 ,elster and ,geitel1 ,phys4 ,zeit4 #c1 p4 #egd1 #aijb4 ,footnote #cig3 ,ebert and ,ewers1 ,phys4 ,zeit4 #d1 p4 #afb1 #aijb4 ,footnote #cih3 ,sarasin1 ,tommasina and ,micheli1 ,c4 ,r4 #aci1 p4 #iag1 #aije4 ,footnote #cii3 ,j4 ,j4 ,thomson1 ,phil4 ,mag4 ,sept4 #aijb4 ,footnote #djj3 ,ebert1 ,sitz4 ,akad4 d4 ,wiss4 ,munich1 #cc1 p4 #acc1 #aijc4 ,footnote #dja3 ,j4 ,j4 ,thomson1 ,phil4 ,mag4 ,sept4 #aijb4 ,footnote #djb3 ,adams1 ,phil4 ,mag4 ,nov4 #aijc4 ,footnote #djc3 ,bumstead and ,wheeler1 ,amer4 ,journ4 ,science1 #ag1 p4 #ig1 ,feb4 #aijd4 ,footnote #djd3 ,bumstead1 ,amer4 ,journ4 ,science1 #ah1 ,july1 #aijd4 ,footnote #dje3 ,dadourian1 ,amer4 ,journ4 ,science1 #ai1 ,jan4 #aije4 ,footnote #djf3 ,h4 ,s4 ,allen and ,lord ,blythswood1 ,nature1 #fh1 p4 #cdc1 #aijc2 #fi1 p4 #bdg1 #aijd4 ,footnote #djg3 ,strutt1 ,proc4 ,roy4 ,soc4 #gc1 p4 #aia1 #aijd4 ,footnote #djh3 ,himstedt1 ,ann4 d4 ,phys4 #ac1 p4 #egc1 #aijd4 ,footnote #dji3 ,elster and ,geitel1 ,phys4 ,zeit4 #e1 ,no4 #ab1 p4 #cba1 #aijd4 ,footnote #daj3 ,dorn1 ,abhandl4 d4 ,natur4 ,ges4 ,halle1 #be1 p4 #ajg1 #aijd4 ,footnote #daa3 ,schenck1 ,thesis ,univ4 ,halle1 #aijd4 ,footnote #dab3 ,mache1 ,wien4 ,ber4 #aac1 p4 #acbi1 #aijd4 ,footnote #dac3 ,curie and ,laborde1 ,c4 ,r4 #ach1 p4 #aaej1 #aijd4 ,footnote #dad3 ,blanc1 ,phil4 ,mag4 ,jan4 #aije4 ,footnote #dae3 ,boltwood1 ,amer4 ,journ4 ,science1 #ah1 ,nov4 #aijd4 ,footnote #daf3 ,elster and ,geitel1 ,phys4 ,zeit4 #d1 p4 #ebb1 #aijc4 ,footnote #dag3 ,elster and ,geitel1 ,phys4 ,zeit4 #e1 ,no4 #a1 p4 #aa1 #aijc4 ,footnote #dah3 ,vincenti and ,levi ,da ,zara1 ,atti d4 ,r4 ,instit4 ,veneto d4 ,scienze1 #ed1 p4 #ie1 #aije4 ,footnote #dai3 ,burton1 ,phil4 ,mag4 ,oct4 #aijd4 ,footnote #dbj3 ,elster and ,geitel1 ,phys4 ,zeit4 #f1 ,no4 #c1 p4 #fg1 #aije4 ,footnote #dba3 ,rutherford and ,allan1 ,phil4 ,mag4 ,dec4 #aijb4 ,footnote #dbb3 ,elster and ,geitel1 ,phys4 ,zeit4 #d1 p4 #ach1 #aijb2 #d1 p4 #ebb1 #aijc4 ,footnote #dbc3 ,saake1 ,phys4 ,zeit4 #d1 p4 #fbf1 #aijc4 ,footnote #dbd3 ,simpson1 ,proc4 ,roy4 ,soc4 #gc1 p4 #bji1 #aijd4 ,footnote #dbe3 ,mc,lennan1 ,phys4 ,rev4 #af1 p4 #ahd1 #aijc1 and ,phil4 ,mag4 #e1 p4 #dai1 #aijc4 ,footnote #dbf3 ,mc,lennan1 ,phys4 ,rev4 ,no4 #d1 #aijc4 ,footnote #dbg3 ,rutherford and ,cooke1 ,americ4 ,phys4 ,soc4 ,dec4 #aijb4 ,footnote #dbh3 ,cooke1 ,phil4 ,mag4 ,oct4 #aijc4 ,footnote #dbi3 ,allan1 ,phil4 ,mag4 ,feb4 #aijd4 ,footnote #dcj3 ,ebert1 ,phys4 ,zeit4 #b1 p4 #fbb1 #aija4 ,zeitschr4 f4 ,luftschiffahrt1 #d1 ,oct4 #aijb4 ,footnote #dca3 ,schuster1 ,proc4 ,manchester ,phil4 ,soc4 p4 #dhh1 ,no4 #ab1 #aijd4 ,footnote #dcb3 ,mache and ,von ,schweidler1 ,phys4 ,zeit4 #f1 ,no4 #c1 p4 #ga1 #aije4 ,footnote #dcc3 ,langevin1 ,c4 ,r4 #adj1 p4 #bcb1 #aije4 ,footnote #dcd3 ,schuster1 ,british ,assoc4 #aijc4 ,footnote #dce3 ,j4 ,j4 ,thomson1 ,conduction of ,electricity through ,gases1 p4 #cbd1 #aijc4 ,footnote #dcf3 ,miss ,gates1 ,phys4 ,rev4 #ag1 p4 #dii1 #aijc4 ,footnote #dcg3 ,villard1 ,socit de ,physique1 ,july1 #aijj4 ,footnote #dch3 ,geitel1 ,phys4 ,zeit4 #b1 p4 #aaf1 #aijj4 ,footnote #dci3 ,c4 ,t4 ,r4 ,wilson1 ,proc4 ,camb4 ,phil4 ,soc4 #aa1 p4 #eb1 #aijj4 ,proc4 ,roy4 ,soc4 #fh1 p4 #aeb1 #aija4 ,footnote #ddj3 ,rutherford and ,allan1 ,phil4 ,mag4 ,dec4 #aijb4 ,footnote #dda3 ,patterson1 ,phil4 ,mag4 ,august1 #aijc4 ,footnote #ddb3 ,harms1 ,phys4 ,zeit4 #d1 ,no4 #a1 p4 #aa1 #aijb4 ,footnote #ddc3 ,cooke1 ,phil4 ,mag4 ,oct4 #aijc4 ,footnote #ddd3 ,wilson1 ,proc4 ,roy4 ,soc4 #fi1 p4 #bgg1 #aija4 ,footnote #dde3 ,jaff1 ,phil4 ,mag4 ,oct4 #aijd4 ,footnote #ddf3 ,patterson1 ,phil4 ,mag4 ,aug4 #aijc4 ,footnote #ddg3 ,strutt1 ,phil4 ,mag4 ,june1 #aijc4 ,nature1 ,feb4 #ai1 #aijc4 ,footnote #ddh3 ,mc,lennan and ,burton1 ,phys4 ,rev4 ,no4 #d1 #aijc4 ,j4 ,j4 ,thomson1 ,nature1 ,feb4 #bf1 #aijc4 ,footnote #ddi3 ,cooke1 ,phil4 ,mag4 ,aug4 #f1 #aijc4 ,rutherford1 ,nature1 ,april #b1 #aijc4 ,footnote #dej3 ,eve1 ,nature1 ,march #af1 #aije4 ,footnote #dea3 ,see article in ,le ,radium1 ,no4 #c1 p4 #ha1 ,sept4 #ae1 #aijd4 ,footnote #deb3 ,j4 ,j4 ,thomson1 ,proc4 ,camb4 ,phil4 ,soc4 #ab1 p4 #cia1 #aijd4 ,footnote #dec3 ,wood1 ,phil4 ,mag4 ,april1 #aije4 ,footnote #ded3 ,campbell1 ,nature1 p4 #eaa1 ,march #ca1 #aijd4 ,phil4 ,mag4 ,april1 #aije4 ,,appendix ,a4 ,,properties ,,of ,,the ,,rays4 ,a brief account is given here of some investigations made by the writer on the properties of the rays from radium--investigations which were not completed in time for the results to be incorporated in the text4 ,the experiments were undertaken primarily with a view of determining accurately the value of e_/m of the particle from radium1 in order to settle definitely whether or not it is an atom of helium4 ,in the previous experiments of the writer1 ,becquerel1 and ,des ,coudres1 on this subject 7sections #hi1 #ij1 and #ia71 a thick layer of radium in radio-active equilibrium has been used as a source of rays4 ,bragg 7section #ajc7 has shown that the rays emitted from radium under such conditions are complex1 and consist of particles projected over a considerable range of velocity4 ,in order to obtain a homogeneous pencil of rays it is necessary to use a very thin layer of a simple radio-active substance as a source of rays4 ,in the experiments that follow1 this condition was fulfilled by using a fine wire which was made active by exposure for several hours in the presence of a large quantity of radium emanation4 ,by charging the wire negatively the active deposit was concentrated upon the wire1 which was made intensely active4 ,the active deposit initially contains radium ,a1 ,b1 and ,c4 ,the activity of radium ,a practically disappears in about fifteen minutes1 and the radiation is then due entirely to the single product radium ,c1 since radium ,b is a rayless product4 ,the activity of radium ,c decreases to about #ae per cent4 of its initial value after two hours4 ,magnetic deflection of the rays4 ,the photographic method was employed to determine the deviation of the pencil of rays in a magnetic field4 ,the experimental arrangement is shown in ,fig4 #ajf4 ,the rays from the active wire1 which was placed in a slot1 passed through a narrow slit and fell normally on a photographic plate1 placed at a known distance above the slit4 ,the apparatus was enclosed in a brass tube which could be exhausted rapidly to a low pressure by means of a ,fleuss pump4 ,the apparatus was placed in a strong uniform magnetic field parallel to the plane of the slit4 ,the magnetic field was reversed every ten minutes1 so that on developing the plate two narrow bands were observed1 the distance between which represented twice the deviation from the normal of the pencil of rays by the magnetic field4 ,the width of the band was found to be the same whether the magnetic field was applied or not1 showing that the pencil of rays was homogeneous and consisted of particles projected with the same velocity4 ,illustration3 ,fig4 #ajf4 ,by placing the photographic plate at different distances from the slit it was found that the rays1 after entering the magnetic field1 described the arc of a circle of radius equal to #dbj cms4 ,the strength of field ,h was #idgj ,c4,g4,s4 units1 so that the value of ,h for the particles expelled from radium ,c is #cih1jjj4 ,this is in good agreement with the maximum values of ,h1 previously found for radium rays 7see section #ib74 ,the electric deviation of the rays from radium ,c has not yet been accurately measured1 but an approximate determination of e_/m for the particles can be obtained by assuming that the heating effect of radium ,c is a measure of the kinetic energy of the particles expelled from it4 ,we have seen in section #bdf that the heating effect of the radium ,c present in one gram of radium in radio-active equilibrium is #ca gram calories per hour1 which corresponds to an emission of energy of #cf #aj ergs per second4 ,now when radio-active equilibrium is reached1 the number of particles expelled from radium ,c per second is equal to the number of particles expelled per second from radium at its minimum activity4 ,this number1 n1 is #fb #aj 7section #ic74 ,then mnv #cf #aj1 or 7m_/e7v #ajc #aj1 substituting the value of n1 and the value of the ionic charge e4 ,the value of e in this case has not been assumed1 since n i_/e1 where i was the measured current due to the charge carried by the rays4 ,from the magnetic deflection1 it is known that 7m_/e7v #cih #aj4 ,from these two equations we obtain v #bf #aj cms4 per second4 e_/m #fe #aj electromagnetic units4 ,these values are in surprisingly good agreement with the previous values of the writer and ,des ,coudres 7section #ia74 ,on account of the uncertainty attaching to the value of n1 not much weight can be attached to the determination by this method of the constants of the particles4 ,decrease of velocity of the particles in passing through matter4 ,some experiments were made to determine the velocity of the particles from radium ,c after passing through known thicknesses of aluminium4 ,the previous apparatus was employed1 and the distance between the photographic bands was observed for successive layers of aluminium foil1 each #jjjca cms4 thick1 placed over the active wire4 ,the photographic plate was placed #b cms4 above the slit1 and the magnetic field extended #a cm4 below the slit4 ,the amount of deviation of the rays is inversely proportional to their velocity after traversing the aluminium screens4 ,the impressions on the plate were clear and distinct1 and about the same in all cases1 showing that the rays were still homogeneous after passing through the aluminium4 ,a clear photographic impression was obtained for #ab layers of foil1 but it was not found possible to obtain any effect through #ac layers4 ,this result shows that the photographic action of the rays1 like the ionizing action1 ceases very abruptly4 ,the results obtained are shown in the following table4 ,assuming that the value of e_/m is constant1 the third column gives the velocity of the particles after traversing the aluminium4 ,this is expressed in terms of ,v1 the velocity of the particle when the screens are removed4 ,number of ,distance ,velocity layers of between of aluminum bands on the particles foil plate #j #adf mms4 #ajj ,v #e #aga #he #h #aia #gf #aj #bja #gc #ab #bbi #fd #ac ,no photographic effect ,the velocity of the particle is thus reduced only about #cf per cent4 of its initial value when it fails to produce any action on the photographic plate4 ,now ,bragg has shown 7section #ajd7 that the particle produces approximately the same number of ions per cm4 of path in air over its whole range4 ,consequently1 the simplest assumption to make is that the energy of the particle is diminished by a constant amount in traversing each layer of foil4 ,after passing through #ab layers the kinetic energy is reduced to #da per cent4 of the maximum4 ,each layer of foil thus absorbs #di per cent4 of the maximum energy4 ,the observed kinetic energy of the particle after passing through successive layers of foil1 and the value calculated on the above assumptions1 are shown in the following table4 ,number of ,observed ,calculated layers of energy energy aluminum foil #j #ajj #ajj #e #gc #ge #h #eh #fa #aj #ec #ea #ab #da #da ,the experimental and theoretical values agree within the limits of experimental error4 ,we may thus conclude1 as a first approximation1 that the same proportion of the total energy is abstracted from the particles in passing through equal distances of the absorbing screen4 ,range of ionization and photographic action in air4 ,the abrupt falling off of the photographic impression after the rays had passed through #ab layers of foil suggested that it might be directly connected with the corresponding abrupt falling off of the ionization in air1 so clearly brought out by ,bragg4 ,this was found to be the case4 ,it was found experimentally that the absorption in each layer of aluminium foil was equivalent to that produced by a distance of #ed cms4 of air4 ,twelve layers of foil thus corresponded to #fe cms4 of air4 ,now ,bragg found that the rays from radium ,c ionize the air for a distance #fg cms41 and that the ionization then falls off very rapidly4 ,we may thus conclude that the rays cease to affect the photographic plate at the same velocity as that at which they cease to ionize the gas4 ,this is a very important result1 and1 as we shall see later1 suggests that the action on the photographic plate is due to an ionization of the photographic salts4 ,the velocity of the particles from the different radio-active products can at once be calculated1 knowing the maximum range in air of the rays from each product4 ,the latter have been experimentally determined by ,bragg4 ,the velocity is expressed in terms of ,v1 the initial velocity of the particles from radium ,c4 ,the rays from radium ,c are projected with a greater velocity than the rays from the other products of radium4 ,product ,maximum ,velocity range of of particles particles in air ,radium #c cms4 #hb ,v ,emanation #ch or #dd #hg or #ij cms4 ,v ,rad4 ,a #dd or #ch #ij or #hg ,v ,rad4 ,c #fg #ajj ,v ,it is difficult to determine from the experiments whether the range #ch cms4 belongs to the rays from the emanation or from radium ,a4 ,the mean velocity of the particles is thus #ij ,v1 and the maximum variation for the individual products does not vary more than #aj per cent4 from the mean value4 ,the results of ,becquerel1 discussed in section #ib1 at once receive an explanation on the above results4 ,the particles1 expelled from radium in radio-active equilibrium1 have all ranges lying between #j and #fg cms4 of air4 ,the velocity of the particles which are able to produce a photographic impression varies between #fd ,v and ,v4 ,the particles which have only a short range in air are projected with a smaller velocity than those which have a greater range4 ,the former are in consequence more bent by a magnetic field4 ,it is thus to be expected that the apparent curvature of the path of rays in a uniform magnetic field will be greater close to the radium than at some distance away4 ,range of phosphorescent action in air4 ,some experiments were also made to see whether the action of the rays in producing luminosity in substances like zinc sulphide1 barium platinocyanide1 and willemite1 ceased at the same distance as the ionizing action4 ,a very active wire was placed on a movable plate1 the distance of which from a fixed screen of phosphorescent substance could be varied4 ,the distance at which the phosphorescent action ceased could be determined fairly accurately4 ,different thicknesses of aluminium foil were then placed over the active wire1 and the corresponding distance at which the luminosity disappeared was measured4 ,the results are shown graphically in ,fig4 #ajg1 where the ordinates represent the distance of the phosphorescent screen from the active wire1 and the abscissae the number of layers of aluminium foil1 each #jjjca cms4 thick4 ,illustration3 ,fig4 #ajg4 ,it is seen that the curve joining the points is a straight line4 #abe thicknesses of foil absorbed the rays to the same extent as #fh cms4 of air1 so that each thickness of aluminium corresponded in absorbing power to #ed cms4 of air4 ,for a screen of zinc sulphide1 the phosphorescent action ceased at a distance of air of #fh cms41 showing that the photographic and phosphorescent ranges of the rays in air were practically identical4 ,the experiments with barium platinocyanide and willemite were more difficult1 as the and rays from the active wire produced a luminosity comparable with that produced by the rays4 ,fairly concordant results1 however1 were obtained by introducing a thin sheet of black paper between the active wire and the screen4 ,if the luminosity was sensibly changed1 it was concluded that the rays still produced an effect1 and in this way the point of cessation of phosphorescent action could be approximately determined4 ,for example1 with eight thicknesses of foil over the active wire the additional thickness of air required to cut off the phosphorescent effect of the a rays was #be cms4 for willemite1 and #ba cms4 for barium platinocyanide4 ,the corresponding distance for zinc sulphide was #bdj cms41 a value intermediate between the other two4 ,since eight layers of foil are equivalent to #dc cms4 of air1 the ranges in air of phosphorescent action for zinc sulphide1 barium platinocyanide1 and willemite correspond to #fg1 #fh1 and #fd cms4 respectively4 ,the differences observed are quite likely to be due to experimental error4 ,discussion of results4 ,we have seen that the ionizing1 phosphorescent1 and photographic actions of the rays emitted from radium ,c cease after traversing very nearly the same distance of air4 ,this is a surprising result when it is remembered that the particle1 after passing through this depth of air1 still possesses a velocity of at least #fj per cent4 of its initial value4 ,taking the probable value of the initial velocity of the particle from radium ,c as #be #aj cms4 per sec41 the ionizing1 phosphorescent1 and photographic actions cease when the velocity of the particle falls below #ae #aj cms4 per second1 that is1 a velocity of about of that of light4 ,the particle still possesses nearly #dj per cent4 of its initial energy of projection at this stage4 ,these results show that the property of the rays of producing ionization in gases1 of producing luminosity in some substances1 and of affecting a photographic plate1 ceases when the velocity of the particle falls below a certain fixed value which is the same in each case4 ,it seems reasonable1 therefore1 to suppose that these three properties of the rays must be ascribed to a common cause4 ,now the absorption of the rays in gases is mainly a consequence of the energy absorbed in the production of ions in the gas4 ,when the particles are completely absorbed in the gas1 the same total amount of ionization is produced1 showing that the energy required to produce an ion is the same for all gases4 ,on the other hand1 for a constant source of radiation1 the ionization per unit volume of the gas is approximately proportional to its density4 ,since the absorption of the rays in solid matter is approximately proportional to the density of the absorbing medium compared with air1 it is probable that this absorption is also a result of the energy used up in producing ions in the solid matter traversed1 and that about the same amount of energy is required to produce an ion in matter whether solid1 liquid1 or gaseous4 ,it is probable1 therefore1 that the production of ions in the phosphorescent material and in the photographic film would cease at about the same velocity for which the particle is unable to ionize the gas4 ,on this view1 then1 the experimental results receive a simple explanation4 ,the action of the rays in producing photographic and phosphorescent actions is primarily a result of ionization4 ,this ionization may possibly give rise to secondary actions which influence the effects observed4 ,this point of view is of interest in connection with the origin of the 8scintillations0 observed in zinc sulphide and other substances when exposed to the action of the rays4 ,this effect is ascribed by ,becquerel to the cleavage of the crystals under the bombardment of the particles4 ,these results1 however1 show that we must look deeper for the explanation of this phenomenon4 ,the effect is primarily due to the production of ions in the phosphorescent material and not to direct bombardment1 for we have seen that the particle produces no scintillations when it still possesses a large amount of kinetic energy4 ,it seems not unlikely that the scintillations produced by the rays must be ascribed to the recombination of the ions which are produced by the particle in the crystalline mass4 ,it is difficult to see how this ionization could result in a cleavage of the crystals4 ,this close connection of the photographic and phosphorescent actions of the rays with their property of producing ions1 raises the question whether photographic and phosphorescent actions in general may not1 in the first place1 be due to a production of ions in the substance4 ,ionization curve for the rays from radium ,c4 ,mr ,mc,clung1 working in the laboratory of the writer1 has recently determined the relative ionization per unit path of the particles projected from radium ,c1 using the method first employed by ,bragg and discussed in section #ajd4 ,an active wire1 exposed for several hours to the emanation from radium1 was used as a source of rays4 ,the particles were homogeneous1 since the film of radio-active matter was extremely thin4 ,the relation between the ionization observed over the cross section of the narrow cone of rays and the distance from the source of rays is shown in ,fig4 #ajh4 ,illustration3 ,fig4 #ajh4 ,the curve exhibits the same peculiarities as those given by ,bragg for a thin film of matter of one kind4 ,the ionization of the particle per unit path increases slowly for about #d cms4 ,there is then a more rapid increase just before the particle ceases to ionize the gas1 and then a rapid falling off4 ,the ionization does not appear to end so abruptly as is really the case1 since there is a correction to be applied for the angle subtended by the cone of rays4 ,the maximum range of the rays in air was #fg cms41 a number in agreement with that obtained by ,bragg by measurements on the range of the rays from radium4 ,these results show that the ionization per unit path of the particle increases at first slowly and then rapidly with decrease of velocity until the rays cease to ionize the gas4 ,energy required to produce an ion4 ,from the above results the energy required to produce an ion by collision of the particle with the gas molecules can readily be deduced4 ,the particles1 emitted from radium itself1 are initially projected with a velocity #hh,v where ,v is the initial velocity of projection of the particles from radium ,c4 ,the particles cease to ionize the gas at a velocity #fd,v4 ,from this it can at once be deduced that #dh of the total energy of the particle1 shot out by radium itself1 is absorbed when it ceases to ionize the gas4 ,assuming that the heating effect of radium at its minimum activity--#be gram calories per hour per gram--is a measure of the kinetic energy of the expelled particles1 it can be calculated that the kinetic energy of each particle is #dg #aj ergs4 ,the amount of energy absorbed when the particle just ceases to ionize the gas is #bc #aj ergs4 ,assuming that this energy is used up in ionization1 and remembering that the particle from radium itself produces #hfjjj ions in its path 7section #beb71 the average energy required to produce an ion is #bg #aj ergs4 ,this is equivalent to the energy acquired by an ion moving freely between two points differing in potential by #bd volts4 ,townsend found that fresh ions were produced by an electron for a corresponding difference of potential of #aj volts4 ,stark1 from other data1 obtained a value #de volts1 while ,langevin considers that #fj volts is an average value4 ,the value obtained by ,rutherford and ,mc,clung for ionization by ,x-rays was #age volts1 and is probably too high4 ,rayless changes4 ,we have seen that the particles from the radio-active substances are projected with an average velocity not more than #cj per cent4 greater than the minimum velocity1 below which the particles are unable to produce any ionizing1 photographic1 or phosphorescent action4 ,such a conclusion suggests that the property of the radio-active substances of emitting particles has been detected because the particles were projected slightly above this minimum velocity4 ,a similar disintegration of matter may be taking place in other substances at a rate much greater than in uranium without producing much electrical effect1 provided the particles are projected below the critical velocity4 ,the particle1 on an average1 produces about #ajj1jjj ions in the gas before it is absorbed1 so that the electrical effect observed is about #ajj1jjj times as great as that due to the charge carried by the particles alone4 ,it is not unlikely that the numerous rayless products which have been observed may undergo disintegration of a similar character to the products which obviously emit rays4 ,in the rayless product the particle may be expelled with a velocity less than #ae #aj cms4 per second and so fail to produce much electrical effect4 ,these considerations have an important bearing on the question whether matter in general is radio-active4 ,the property of emitting particles above the critical velocity may well be a property only of a special class of substances1 and need not be exhibited by matter in general4 ,at the same time the results suggest that ordinary matter may be undergoing transformation accompanied by the expulsion of particles at a rate much greater than that shown by uranium1 without producing appreciable electrical or photographic action4 ,,appendix ,b4 ,,radio-,,active ,,minerals4 ,those natural mineral substances which possess marked radio-active properties have been found to contain either uranium or thorium1 one of these elements being always present in sufficient proportion readily to permit its chemical separation and identification by the ordinary analytical methods#dee4 ,a large number of uranium and thorium minerals are known at the present time1 but they are for the most part found very sparingly1 and some of them have been observed to occur only in a single locality4 ,the chief commercial sources of uranium are uraninite1 gummite1 and carnotite1 while thorium is obtained almost exclusively from monazite4 ,rutherford and ,soddy 7,phil4 ,mag4 #fe1 #efa 7#aijc771 were the first to call attention to the important fact that the relations between the various radio-active substances and the other elements could best be determined from the study of the natural minerals in which these bodies occur1 since these minerals represent mixtures of extreme antiquity1 which have remained more or less undisturbed for almost countless ages4 ,in dealing with these matters1 however1 it is highly important that we bring to our aid the data furnished by geology and mineralogy1 from which it is often possible to determine the relative ages of the different substances with at least a rough degree of approximation4 ,thus1 for example1 if a certain mineral occurs as a primary constituent of a rock of remote geological period1 it can safely be assumed that its age is greater than that of a similar or different mineral occurring in a later formation4 ,it is1 moreover1 quite evident that those minerals which are obviously produced by the decomposition and alteration of the primary minerals1 through the action of percolating water and other agencies acting from the surface downward1 are of less antiquity than the primary minerals from which they originated4 ,through the application of these considerations it should1 in general1 be possible to arrange the various minerals roughly in the order of their probable ages4 ,the most familiar and widely known uranium mineral is uraninite1 commonly called pitchblende1 which consists essentially of uranium dioxide 7,,uo71 uranium trioxide 7,,uo71 and lead oxide 7,pb,o71 present in varying proportions4 ,the uraninites can be distinguished as primary1 namely1 those which occur as a primary constituent of pegmatitic dikes and coarse granites1 and secondary1 when they occur in metalliferous veins associated with the sulphides of silver1 lead1 copper1 nickel1 iron1 and zinc4 ,the former varieties are quite frequently crystalline in character1 contain a larger proportion of the rare earths and helium1 and have a higher specific gravity than the latter1 which are always massive and botryoidal4 ,the following are the most prominent localities in which primary uraninites occur3 #a4 ,north ,carolina1 ,u4,s4,a4 7especially in ,mitchell and ,yancey counties74 ,the uraninite is found in a coarse pegmatitic dike which is mined for the mica constituent4 ,the associated feldspar of the dike is considerably decomposed through the action of meteoric waters and gases1 and the uraninite itself is largely altered into the secondary minerals gummite and uranophane through the same agencies4 ,among the associated primary minerals are allanite1 zircon1 columbite1 samarskite1 fergusonite and monazite1 while the secondary minerals include gummite1 thorogummite1 uranophane1 autunite1 phosphuranylite1 hatchettolite1 and cyrtolite4 ,the geological period of this formation is difficult to establish with certainty1 but is stated to be perhaps ,archean1 or possibly to correspond with the close of the ,ordovician or with the ,permian4 #b4 ,connecticut1 ,u4,s4,a4 ,the best known localities are ,glastonbury1 where the uraninite is found in the feldspar quarries1 and ,branchville1 where it occurs in an albitic granite4 ,both of these localities have furnished fine crystals4 ,the geological period probably corresponds with the close of the ,ordovician or ,carboniferous eras1 and is stated to be certainly ,post-,cambrian and ,pre-,triassic4 ,among the associated minerals are 7primary7 columbite1 7secondary7 torbernite and autunite4 #c4 ,southern ,norway1 particularly in the neighbourhood of ,moss4 ,here uraninite occurs in the augite-syenite and pegmatite4 ,the varieties found are known as cleveite and brggerite1 and among the primary associated minerals are orthite1 fergusonite1 monazite1 and thorite4 ,the period is stated to be ,post-,devonian4 #d4 ,llano ,county1 ,texas4 ,the variety of uraninite known as nivenite is found here in a quartzose pegmatite1 associated with the primary minerals gadolinite1 allanite and fergusonite1 and the secondary minerals cyrtolite1 yttrialite1 gummite1 and thorogummite4 ,secondary uraninite is found at ,johanngeorgenstadt1 ,marienberg and ,schneeberg in ,saxony1 at ,joachimsthal and ,pribam in ,bohemia1 at ,cornwall in ,england1 and at ,black ,hawk1 ,colorado1 and in the ,black ,hills1 ,south ,dakota1 in the ,united ,states4 ,the exact geological period of most of these secondary occurrences is somewhat uncertain1 but they are undoubtedly very much later than the primary occurrences mentioned above4 ,as a matter of general interest the analysis of a typical primary uraninite 7,no4 #a7 and of a typical secondary uraninite 7,no4 #b7 is given below#def3 ,no4 #a ,no4 #b ,glastonbury1 ,johanngeorgenstadt1 ,conn4 ,saxony ,sp4 ,gr4 #iei #fhi ,,uo #bfdh #fjje ,,uo #egdc #bbcc ,th,o #igi 444 ,ce,o #jbe 444 ,la,o #jac 444 ,y,o #jbj 444 ,pb,o #cbf #fci ,ca,o #jjh #ajj ,he und4 und4 ,h,o #jfa #cag ,fe,o #jdj #jba ,si,o #jbe #jej ,al,o 444 #jbj ,bi,o 444 #jge ,cu,o 444 #jag ,mn,o 444 #jji ,mg,o 444 #jag ,na,o 444 #jca ,p,o 444 #jjf ,,so 444 #jai ,as,o 444 #bcd ,insoluble #jgj 444 ,the following list comprises the more important radio-active minerals1 with their approximate chemical composition and some notes on their occurrence and probable origin4 ,name ,composition ,remarks ,uraninite1 ,oxides of ,occurs primary ,cleveite1 uranium and as a ,brggerite1 lead4 ,usually constituent of ,nivenite1 contains rocks and ,pitchblende thorium1 secondary in other rare veins with earths and metalliferous helium4 sulphides ,uranium #ej-hj4 ,thorium #j-aj ,gummite 7,pb1 ,ca7 ,an alteration ,u,si,o product of 4 #f;,h,o8 uraninite4 ,uranium ,formed by the #ej-fe action of percolating waters ,uranophane1 ,ca,o 4 #b,,uo ,an alteration ,uranotil 4 #b,si,o 4 product of #f;,h,o uraninite ,uranium through #dd-ef gummite ,carnotite ,a vanadate of ,occurs as a uranium and secondary potassium4 mineral ,uranium impregnating a #db-ea porous1 sedimentary sandstone4 ,found in ,colorado and ,utah ,uranosphaerite ,bi,o 4 ,alteration #b,,uo 4 product of #c;,h,o4 other uranium ,uranium #da minerals ,torbernite1 ,cu,o 4 #b,,uo ,cuprouranite 4 ,p,o 4 #h;,h,o4 ,uranium #dd-ea ,autunite1 ,ca,o 4 #b,,uo ,calciouranite 4 ,p,o 4 #h;,h,o4 ,uranium #de-ea ,uranocircite ,ba,o 4 #b,,uo 4 ,p,o 4 #h;,h,o4 ,uranium #df ,phosphuranylite #c,,uo 4 ,p,o 4 #f;,h,o4 ,uranium #eh-fd ,zunerite ,cu,o 4 #b,,uo 4 ,as,o 4 #h;,h,o4 ,uranium #df ,uranospinite ,ca,o 4 #b,,uo 4 ,as,o 4 #h;,h,o4 ,uranium #di ,walpurgite #e;,bi,o 4 #c,,uo 4 ,as,o 4 #ab;,h,o4 ,uranium #af ,thorogummite ,,uo 4 ,a variety of #c,th,o 4 gummite #c,si,o 4 #f;,h,o8 ,uranium #da ,thorite1 ,th,si,o4 ,a primary ,orangite1 ,uranium constituent of ,uranothorite #a-aj4 pegmatite ,thorium oxide dikes #dh-ga ,thorianite ,oxide of ,occurs as a thorium1 primary uranium1 the constituent of rare earths a pegmatite and lead4 dike in ,contains a ,ceylon4 relatively ,geological age large probably proportion of ,archean helium4 ,uranium #i-aj4 ,thorium oxide #gc-gg ,samarskite ,niobate and ,primary tantalate of constituent of rare earths4 pegmatite ,uranium #h-aj dikes ,fergusonite ,metaniobate and tantalate of rare earths4 ,uranium #a-f ,euxenite ,niobate and titanate of rare earths4 ,uranium #c-aj ,monazite ,phosphate of the rare earths1 chiefly cerium4 ,uranium #jc-jd ,footnote #dee3 ,an apparent exception has been observed by ,danne in the case of certain lead minerals which occur under peculiar conditions at d',issy-l'vque1 ,france4 ,see p4 #dfe4 ,footnote #def3 ,hillebrand1 ,am4 ,j4 ,sci4 #dj1 #chd 7#ahij72 ibid4 #db1 #cij 7#ahia74 ,,index4 ,the numbers refer to the pages4 rays discovery of1 #ada nature of1 #ada magnetic deviation of1 #adb et seq4 electrostatic deviation of1 #adf velocity of1 #adh value of e_/m for1 #adh charge carried by1 #aea et seq4 number of particles expelled from one gram of radium1 #aee mass and energy of1 #aef origin of1 in atomic disintegration1 #aeg scintillations produced by1 #aeh et seq4 absorption of1 by matter1 #afa et seq4 increase of absorption with thickness of matter traversed1 #afc relative absorption of rays from radio-elements1 #afd absorption of1 by gases1 #afe et seq4 connection between absorption and density1 #afi relation between ionization and absorption1 #agj theory of absorption of1 #agj et seq4 range of ionization of1 #agb et seq4 complexity of rays from radium1 #agd et seq4 effect of thickness of layer of radiating matter on emission of1 #aie relative ionization produced by and rays1 #aif et seq4 phosphorescence by rays1 #bjb et seq4 connection of1 with radio-active changes1 #bce1 #ddd et seq41 #dee from the emanations1 #bfc emission of energy from radio-elements in form of rays1 #dai et seq4 connection of heat emission of radium with rays1 #dba et seq4 number of ions produced by an particle1 #dcc absence of1 in rayless changes1 #ded emission from active products1 #ded et seq4 loss of weight due to expulsion of1 #dgc particles consist of helium1 #dgi et seq4 magnetic deflection of1 from radium ,c1 #edc velocity and e_/m for1 from radium ,c1 #edc et seq4 diminution of velocity of1 in passing through matter1 #ede diminution in velocity of1 in passing through aluminium1 #ede velocity of1 when ionization ceases1 #ede et seq4 connection of phosphorescent1 photographic1 and ionization effects produced by1 #edf et seq4 energy required to produce an ion by rays1 #eea ,abraham apparent mass of moving charged body1 #ga1 #abg ,absorption law of1 in gases1 #fd et seq4 relative absorption of 1 and rays by matter1 #aaa connection between absorption and ionization1 #acd et seq41 #agj et seq4 of rays by solids1 #acd et seq4 connection between absorption and density for rays1 #acg of rays in radio-active matter1 #adj of rays by solids1 #afa et seq4 of rays in gases1 #afg1 #agj et seq4 connection between absorption and density for rays1 #afi theory of1 #agj et seq4 of rays by solids1 #agi et seq4 connection between absorption and density for rays1 #aha of rays from the emanations1 #bfc of penetrating rays from the earth1 #ebj1 #edj ,actinium methods of separation of1 #bj et seq4 properties of1 #ba similarity to 8emanating substance0 of ,giesel1 #ba possible connection with radio-activity of thorium1 #bh emanation from1 #bdi excited activity produced by1 #caa effect of magnetic field on excited activity from1 #cbd separation of actinium ,x1 #cfe decay of actinium ,x1 #cfe source of actinium emanation1 #cfe analysis of active deposit of1 #cff radiations from products of1 #cfh penetrating power of and rays from1 #cfh products of1 #cfi table of products of1 #ddi possible origin of1 #dfd ,actinium ,a separation and period of1 #cfg et seq4 ,actinium ,b period of1 #cfh properties of1 #cfh ,actinium ,x separation and decay of1 #cfd et seq4 production of emanation by1 #cfe ,adams decay of activity of emanation from well water1 #eaa decay of excited activity from the emanation1 #eaa ,age of radium1 #deg of sun and earth1 #dib et seq4 ,allan1 ,s4 ,j4 increase with time of excited activity from atmosphere1 #eje radio-activity of snow1 #ejf effect of conditions on decay of activity from air1 #eai1 #ebc ,allan and ,rutherford decay of excited activity from the atmosphere1 #ejc ionization of air in closed vessels1 #ecd ,allen1 ,h4 ,s4 and ,lord ,blythswood radium emanation in ,bath springs1 #eac ,anderson and ,hardy action of radium rays on the eye1 #bag ,armstrong and ,lowry radio-activity and phosphorescence1 #ddd ,arnold rays from phosphorescent substances1 #d ,aschkinass and ,caspari action of radium rays on microbes1 #baf ,atmosphere excited activity from1 #eja et seq4 radio-activity of1 due to emanations1 #ejd diffusion of emanations into1 from the earth1 #ejg effect of temperature1 pressure1 c4 on radio-activity of1 #eag et seq4 presence of very penetrating radiation in1 #ebj comparison of radio-activity of1 with radio-elements1 #eba et seq4 amount of radium emanation in1 #ebd et seq4 ionization of1 due to radium emanation1 #ebf ,atom number of per c4c41 #ed disintegration of1 #bcd et seq4 complex nature of1 #bce changing atoms1 #ddd et seq4 possible causes of disintegration of1 #dhf evolution of1 #dif ,atomic weight of radium by chemical methods1 #ag from spectroscopic evidence1 #ah emanations1 #bgc of radio-elements and connection with radio-activity1 #dde rays discovery of1 #aac magnetic deflection of1 #aad complexity of1 #aaf examination by the electrical method1 #aah effect of1 on a fluorescent screen1 #aai charge carried by the1 #abj et seq4 electrostatic deviation of1 #abd velocity of1 and value of e_/m for1 #abf variation of e_/m with velocity of1 #abg et seq4 distribution of velocity amongst particles1 #aca absorption of1 #acd et seq4 variation of absorption with density1 #acf et seq4 number of particles stopped by matter1 #acg et seq4 variation of intensity of1 with thickness of layer1 #adj secondary rays1 #ahi et seq4 relative ionization produced by and rays1 #aif relative energy emitted in form of and rays1 #aif et seq4 phosphorescent action of1 #bja et seq4 physical action produced by1 #bjg et seq4 chemical action of1 #bac physiological action of1 #baf from ,ur ,x1 #cdg from active deposit of radium1 #cgg et seq4 significance of appearance of1 only in last radio-active changes1 #dee change of weight due to expulsion of1 #dgc ,barium platinocyanide phosphorescence of1 under radium rays1 #bjc change of colour due to radium rays1 #bje ,barkla polarization of ,x rays1 #hj ,barnes and ,rutherford heating effect of radium emanation1 #dba1 #dbi connection of heating effect with radio-activity1 #dba heating effect of active deposit1 #dbe heating effect of rays1 #dbi heating effect of emanation1 #dca division of heating effect among active products1 #dcc ,bary phosphorescence under radium rays1 #bjb ,baskerville activity of thorium1 #bi phosphorescence of kunzite under radium rays1 #bjc ,baskerville and ,kunz phosphorescence of substances under radium rays1 #bjd ,beattie1 ,smolan and ,kelvin discharging power of uranium rays1 #g ,becquerel rays from calcium sulphide1 #d rays from uranium1 #e et seq4 permanence of uranium rays1 #f discharging power of uranium rays1 #f magnetic deflection of radium rays by photographic method1 #aad et seq4 curvature of radium rays in a magnetic field1 #aae et seq4 complexity of radium rays1 #aaf et seq4 electrostatic deflection of rays of radium1 #abd et seq4 value of e_/m for rays of radium1 #abf et seq4 magnetic deviation of rays of radium and polonium1 #ade trajectory of rays of radium in magnetic field1 #adh scintillations due to cleavage of crystals1 #afj rays from radium1 #agi secondary rays produced by active substances1 #ahg phosphorescence produced by radium rays1 #bja conductivity of paraffin under radium radiation1 #baj effect of temperature on uranium rays1 #baj chemical action of radium rays1 #bad removal of activity from uranium by precipitation with barium1 #bai theory of radio-activity1 #dch ,bemont et ,m4 et ,mme ,curie discovery of radium1 #ac ,berndt spectrum of polonium1 #bc ,blanc thorium in sediments from hot springs1 #ead ,blythswood1 ,lord and ,allen1 ,h4 ,s4 radium emanation in ,bath springs1 #eac ,bdlander and ,runge evolution of gases from radium1 #bae ,boltwood origin of radium1 #dfj amount of radium in minerals1 #dfj proportionality of uranium and radium in minerals1 #dfa production of lead by uranium1 #dhd radium emanation in spring water1 #ead method of standardization of amount of emanation in waters1 #ead ,boys rate of dissipation of charge1 #eca ,bragg and ,kleeman theory of absorption of rays1 #agb et seq4 relation between ionization and absorption1 #agd et seq4 range of rays in air1 #agd four sets of rays from radium1 #agd et seq4 ,bronson use of steady deflection method with an electrometer1 #ajd decay of thorium emanation1 #bdb decay of excited activity from actinium1 #cab ,brooks1 ,miss variation of excited activity from thorium for short exposures1 #cjd effect of dust on distribution of excited activity1 #cje decay curves of excited activity of radium measured by and rays1 #cjg et seq4 decay curves of excited activity from actinium1 #cab ,brooks and ,rutherford absorption of rays by matter1 #afa comparison of absorption of rays from radio-elements1 #afd diffusion of radium emanation1 #bgj decay of excited activity from radium1 #cjf ,bumstead presence of thorium emanation in atmosphere1 #eab ,bumstead and ,wheeler diffusion of radium emanation1 #bgc emanation from surface water and the soil1 #eab1 #ebb identity of emanation from soil with radium emanation1 #eab1 #ebb ,burton radium emanation in petroleum1 #eaf ,burton and ,mc,lennan penetrating radiation from the earth1 #ebj radio-activity of ordinary materials1 #ecg emanation from ordinary matter1 #ech ,campbell radio-activity of ordinary materials1 #edj ,canal rays discovery of1 #gh magnetic and electric deflection of1 #gh value of e_/m for1 #gh similarity of1 to rays1 #aaj ,capacity of electroscopes1 #hg of electrometers1 #id1 #ajb standards of1 #ajb ,carbonic acid radio-activity of natural1 #eaf ,caspari and ,aschkinass action of radium rays on microbes1 #baf ,cathode rays discovery of1 #gc magnetic and electric deflection of1 #gd value of e_/m for1 #ge radiation of energy from1 #gi comparison of1 with rays1 #abj absorption of1 by matter1 #acf1 #acg see also rays ,caves radio-active matter present in air of1 #ead et seq4 radio-activity of air of1 due to emanation from the soil1 #eae ,changes 7see ,transformations7 ,charge carried by the ions1 #ej et seq4 negative charge carried by rays1 #abj measurement of charge carried by rays1 #aba et seq4 positive charge carried by rays1 #ade measurement of charge carried by rays1 #aea et seq4 ,chemical nature of emanation1 #bfg of active deposit1 #cab ,chemical actions of radium rays production of ozone1 #bac coloration of glass and rock-salt1 #bac on phosphorus1 #bad on iodoform1 #bad on globulin1 #bad evolution of hydrogen and oxygen1 #bae ,child potential gradient between electrodes1 #fe variation of current with voltage for surface ionization1 #ff ,clouds formation of1 by condensation of water round ions1 #df et seq4 difference between positive and negative ions in formation of1 #di ,collie and ,ramsay spectrum of emanation1 #bib ,collision ionization by1 #ci1 #eg number of ions produced by rays per cm4 of path1 #dcd total number of ions produced by collisions of particles1 #dcd ,coloration of crystals of radiferous barium1 #ae of bunsen flame by radium1 #ae of glass by radium rays1 #bac of rock-salt1 fluor-spar and potassium sulphate by radium rays1 #bac ,concentration of excited activity on negative electrode1 #big activity of radium independent of1 #dff ,condensation of water round the ions1 #df et seq4 of emanations1 #bgg experimental illustration of1 #bgi temperature of1 #bhj difference between point of1 for emanations of thorium and radium1 #bhc from air sucked up from the earth1 #eaj ,conductivity of gases exposed to radiations1 #ca et seq4 variation of1 with pressure1 #fa et seq4 variation of1 with nature of gas1 #fd comparison of1 for gases exposed to 1 1 and rays1 #fd comparison of1 when exposed to rays and to hard ,x rays1 #ahd of insulators1 #bji of air in caves and cellars1 #ejg et seq4 of air in closed vessels1 #eca et seq4 variation of1 in closed vessels with pressure and nature of gas1 #ecd variation of1 with temperature for air in closed vessels1 #ecf increase of1 with time1 in a closed vessel1 #ecg ,conservation of radio-activity examples of1 #dfi et seq4 ,cooke1 ,h4 ,l4 penetrating rays from the earth1 #ebj number of ions per c4c4 in closed vessels1 #ecd radio-activity from ordinary matter1 #ecf ,cooke1 ,w4 ,t4 and ,ramsay radio-activity produced by radiations of radium1 #dgb ,corpuscle 7see ,electron7 ,crookes1 ,sir ,w4 spectrum of radium1 #ag spectrum of polonium1 #bc nature of cathode rays1 #gc nature of rays1 #adb scintillations produced by radium1 #aeh spinthariscope1 #aeh number of scintillations independent of pressure and temperature1 #aei phosphorescence of diamond1 #bjd separation of ,ur ,x1 #bai theory of radio-activity1 #dda ,crookes and ,dewar absence of nitrogen spectrum in phosphorescent light of radium at low pressures1 #bjf ,crystallization effect of1 on activity of uranium1 #cdi ,curie1 ,mme permanence of uranium rays1 #f discovery of radio-activity of thorium1 #aj radio-activity of uranium and thorium minerals1 #aa relative activity of compounds of uranium1 #ab coloration of radium crystals1 #ae spectrum of radium1 #af discovery of polonium1 #bb nature of a rays1 #adb absorption of rays from polonium1 #afc secondary radiation tested by electric method1 #ahh slowly decaying excited activity from radium1 #caa recovery of activity of radium1 #cge bismuth made active by solution of barium1 #dag ,curie1 ,p4 magnetic deviation of radium rays by electric method1 #aad conductivity of dielectrics under radium rays1 #bji radio-activity of radium unaffected by temperature1 #baj decay of activity of radium emanation1 #bdg discovery of excited radio-activity from radium1 #bie heat emission of radium at low temperature and variation of heat emission with age of radium1 #dai nature of the emanation1 #dci ,curie1 ,m4 et ,mme discovery of radium1 #ac charge carried by rays1 #aba luminosity of radium compounds1 #bje production of ozone by radium rays1 #bac coloration of glass by radium rays1 #bac theory of radio-activity1 #dci possible absorption by radio-elements of unknown radiations1 #ddb ,curie1 ,j4 et ,p4 quartz piezo-lectrique1 #aje et seq4 ,curie1 ,p4 et ,danne diffusion of radio-active emanation1 #bgb decay of excited activity from radium1 #cji decay curves of radium and equation1 #cji occlusion of radium emanation in solids1 #caj changes in radium1 #cha effect of temperature on active deposit1 #cij ,curie1 ,p4 and ,debierne evolution of gas from radium1 #bae active gases evolved from radium1 #bea phosphorescence produced by radium emanation1 #beb distribution of luminosity1 #beb rate of production of emanation independent of pressure1 #bff effect of pressure on amount of excited activity1 #bff1 #cag ,curie and ,dewar production of helium by radium1 #dgi ,curie1 ,p4 and ,laborde heat emission of radium1 #dai origin of heat from radium1 #ddj radium emanation in waters of hot springs1 #ead ,current through gases1 #ca et seq4 variation of1 with distance between the plates1 #ei et seq4 variation of1 with pressure of gas1 #fa et seq4 variation of1 with nature of gas1 #fd measurement of1 by galvanometer1 #hd measurement of1 by electroscope1 #he et seq4 measurement of1 by electrometer1 #ij et seq4 measurement of1 by quartz piezo-lectrique1 #aje ,dadourian presence of thorium emanation in the earth1 #eab ,danne on deposit of radium not containing uranium1 #dfe ,danne et ,curie diffusion of radio-active emanation1 #bgb decay of excited activity from radium1 #cji decay curves of radium and equation1 #cji occlusion of radium emanation in solids1 #caj changes in radium1 #cha effect of temperature on active deposit1 #cij ,danysz action of radium rays on skin1 #baf ,darwin1 ,g4 ,h4 age of sun1 #dib ,debierne actinium1 #ba emanation from actinium1 #bdi decay of excited activity from actinium1 #caa effect of magnetic field on activity excited from actinium1 #cbd barium made active by actinium1 #dag ,debierne and ,curie evolution of gas from radium1 #bae active gases evolved from radium1 #bea phosphorescence produced by radium emanation1 #beb distribution of luminosity1 #beb rate of production of emanation independent of pressure1 #bff effect of pressure on amount of excited activity1 #bff1 #cag ,decay of activity of ,th ,x1 #bba of activity of ,ur ,x1 #bbc significance of law of1 #bbi effect of conditions on the rate of1 #bcb of activity of thorium emanation1 #bda of activity of radium emanation1 #bdg of activity of actinium emanation1 #bdi of excited activity due to thorium for long exposure1 #cjb of excited activity due to thorium for short exposure1 #cjd of excited activity due to radium1 #cjf et seq4 excited activity of slow decay due to radium1 #caa of excited activity from actinium1 #caa of radium ,a1 ,b and ,c1 #cgg et seq4 of radium ,d1 ,e and ,f1 #cig et seq4 of heating effect of emanation1 #dbc of excited activity from atmosphere1 #ejb of activity of rain and snow1 #ejf of emanation from the earth1 #ejh differences in1 of excited activity from atmosphere1 #eba et seq4 ,demaray spectrum of radium1 #af ,deposit1 active connection of1 with excited activity1 #cja physical and chemical properties of1 #cab electrolysis of1 #cac effect of temperature on1 #cae effect of pressure on distribution of1 #cag transmission of1 by positive carriers1 #cah et seq4 nomenclature of1 #cbh theory of changes in1 #cca et seq4 theory of activity due to1 #ccg et seq4 theory of rayless change in1 #cda et seq4 of thorium1 #cjb et seq41 #cea et seq4 analysis of1 #cea rayless change in1 #ceb effect of temperature on1 #ced period of products of1 #cee of actinium1 #caa et seq4 decay curves of1 #caa analysis of1 #cfg rayless change in1 #cfg period of products of1 #cfh radiations from1 #cfh of radium1 #cgf et seq4 connection of excited activity with1 #cjf general analysis of1 #cgf et seq4 analysis of1 of rapid change1 #cgg et seq4 analysis of ray curves1 #cgg ray curves of1 #cgh ray curves of1 #cgi analysis of ray curves1 #cha equations of activity curves1 #chi effect of temperature on1 #cij volatility of1 #cia of slow transformation1 #caa1 #cig variation of ray activity of1 #cih variation of ray activity of1 #cii separation of constituents of1 #dja et seq4 successive products in1 #djb variation of activity of1 for long periods1 #djg presence in old radium1 #djh effect of1 on variation of activity of radium with time1 #dji presence in pitchblende1 #daj connection with radio-tellurium1 #daa connection with polonium1 #daa1 #dab connection with radio-lead1 #dac connection of1 with radio-active induction1 #dae et seq4 heat emission of1 #dbe et seq4 use of1 to determine number of particles from radium1 #dce use of1 as source of rays1 #edc ,des ,coudres magnetic and electric deviation of rays1 #adh determination of e_/m for rays1 #adh ,dewar emission of heat from radium in liquid hydrogen1 #dbj ,dewar and ,crookes absence of nitrogen spectrum in phosphorescent light of radium at low pressures1 #bjf ,dewar and ,curie production of helium by radium1 #dgi ,dielectrics condition of1 under radium rays1 #bji ,diffusion of ions1 #ea et seq4 of radium emanation into gases1 #bgj of thorium emanation into gases1 #bge of radium emanation into liquids1 #bgf ,discharge action of rays on spark and electrodeless1 #bjh ,disintegration account of theory of1 #bcd1 #cbe1 #dde list of products of1 #ddi rate of1 in radio-elements1 #deg emission of energy in consequence of1 #dgd et seq4 helium a product of1 #dgf et seq4 possible causes of1 #dhf et seq4 of matter in general1 #dif et seq4 ,dissipation of charge in caves and cellars1 #ead et seq4 in closed vessels1 #eca effect of pressure and nature of gas on1 #ecd et seq4 effect of material of vessel on1 #ecf et seq4 ,dolezalek electrometer1 construction of1 #id et seq4 ,dorn charge carried by rays1 #abb electrostatic deflection of rays from radium1 #abd discovery of radium emanation1 #bdf effect of moisture on emanating power of thorium1 #bee electrolysis of radium solution1 #cac loss of weight of radium1 #dgd radium emanation in springs1 #eac ,dreyer and ,salomonsen coloration of quartz by radium rays1 #bac ,dunston analysis of thorianite1 #dhf ,durack ionization by collision of electrons of great velocity1 #aga ,dust effect of1 on recombination of ions1 #db effect of1 on distribution of excited activity1 #cje ,earth amount of radium in1 #dic et seq4 age of1 #dif excited activity deposited on1 #ejd activity concentrated on peaks of1 #ejd emanation from1 #ejg very penetrating radiation from1 #ebj ,ebert condensation of emanation from the earth1 #eaj apparatus for determining number of ions per c4c4 in air1 #ebg velocity of ions in air1 #ebh ,ebert and ,ewers emanation from the earth1 #ejh ,electrolysis separation of radio-tellurium by1 #be of solutions of active deposit1 #cac of radium solutions1 #cac of thorium solutions1 #cad ,electrometer description of1 #ij et seq4 use of1 in measurements1 #ij construction of1 #ia et seq4 ,dolezalek1 #id adjustment and screening of1 #ie special key for1 #ig application of1 to measurements of radio-activity1 #ig et seq4 measurement of current by1 #ajj capacity of1 #aja use with steady deflection1 #ajc use with quartz piezo-lectrique1 #aje ,electron definition of1 #ef production of1 under different conditions1 #gf et seq4 identity of rays with electrons1 #abj et seq4 variation of apparent mass of electron with velocity1 #abg et seq4 evidence that mass of electron is electromagnetic1 #abi et seq4 diameter of1 #aca ,electroscope description of1 used by ,curie1 #he construction of1 for accurate measurements1 #hf use of1 in measurements of minute currents1 #hf of ,c4 ,t4 ,r4 ,wilson1 #hi use of1 in measuring conductivity of air in closed vessels1 #eca et seq4 use of1 for determining radio-activity of ordinary matter1 #ecg ,elster and ,geitel radio-active lead1 #bg effect of magnetic field on conductivity produced in air by rays1 #aac scintillations produced by active substances1 #aeh action of radium rays on spark1 #bjh photo-electric action of body1 coloured by radium rays1 #bad radio-active matter in earth1 #did discovery of excited activity in atmosphere1 #eja emanations from the earth1 #ejg radio-activity of air in caves1 #ejg radio-activity of the soil1 #eae radio-activity of fango1 #eaf variation of radio-activity in atmosphere with meteorological conditions1 #eag effect of temperature and pressure on atmospheric radio-activity1 #eah ,emanation of thorium1 discovery and properties of1 #bch methods of measurement of1 #bdj decay of activity of1 #bda effect of thickness of layer on amount of1 #bdc increase of1 with time to a maximum1 #bde of radium1 #bdf decay of activity of1 #bdg of actinium1 properties of1 #bdi of radium1 phosphorescence produced by1 #bea rate of emission of1 #bed effect of conditions on rate of emission of1 #bee regeneration of emanating power1 #bef continuous rate of production of1 #beg source of thorium emanation1 #bfa source of radium and actinium emanation1 #bfc radiations from1 #bfc effect of pressure on production of1 #bfe chemical nature of1 #bfg experiments to illustrate gaseous nature of1 #bfh rate of diffusion of radium emanation1 #bfi rate of diffusion of thorium emanation1 #bge diffusion of1 into liquids1 #bgf condensation of1 #bgg temperature of condensation of1 #bhj volume of1 from one gram of radium and thorium1 #bhh measurement of volume of1 from radium1 #bhi diminution of volume of1 #bij spectrum of emanation1 #bib connection between emanation and excited activity1 #bih effect of removal of1 on activity of radium1 #cga et seq4 fraction of activity of radium due to1 #cgd effect of rate of escape of1 on activity of radium1 #cgd heat emission of1 #dbj1 #dca variation of heat emission with time1 #dba et seq4 enormous emission of energy from emanation1 #dca radio-activity of atmosphere due to emanations1 #ejd sucked up from the earth1 #ejg in caves1 #ejg et seq4 rate of decay of activity of1 from the earth1 #ejh condensation of1 from the atmosphere1 #eaj in well water and springs1 #eaj et seq4 from 8fango10 #eaf effect of meteorological conditions on amount of1 in atmosphere1 #eag et seq4 from metals1 #ech ,emanating power measurement of1 #bed effect of conditions on1 #bee regeneration of1 #bef ,emanium or 8emanating substance0 of ,giesel 7see ,actinium7 discovery of1 #ba separation and properties of1 #ba similarity of1 to actinium1 #ba emanation from1 #bdi excited activity produced by1 #caa action of an electric field on1 #cbc ,energy of particle1 #aef of particle1 #aif comparison of1 for and particles1 #aif emitted from radium in form of heat1 #dai et seq4 emission of1 from the emanation1 #dca emission of1 from radio-active products of radium1 #dcc total emission of1 from #a gram of radio-elements1 #dgd et seq4 latent store of1 in matter1 #dge ,eve conductivity of gases exposed to ,x rays1 #fd conductivity of gases exposed to ,x rays and rays1 #ahc1 #ahd secondary rays produced by and rays1 #ahi et seq4 magnetic deflection of secondary rays from rays1 #aic variation of activity of radium with concentration1 #dfg amount of radium emanation in the atmosphere1 #ebd et seq4 ionization due to emanation in atmosphere1 #ebf et seq4 ,evolution of matter evidence of1 #dig ,ewers and ,ebert emanation from the earth1 #ejh ,excited radio-activity discovery and properties of1 #bie et seq4 concentration of1 on negative electrode1 #big connection of1 with the emanations1 #bih removal of1 by acids1 #cjj decay of1 due to thorium1 #cjb decay of1 for short exposure to thorium1 #cjd effect of dust on distribution of1 #cje decay curves for different times of exposure1 #cjf et seq4 decay of1 from radium1 #cjf decay curves of1 measured by rays1 #cjh decay curves of1 measured by rays1 #cji decay curves of1 from actinium1 #caa of radium1 of very slow decay1 #caa effect of solution on1 #cab electrolysis of active solutions1 #cac effect of temperature on1 #cae variation with electric field1 of amount of1 #caf effect of pressure on distribution of1 #cag transmission of1 #cah from actinium and emanium1 #cbc heat emission due to1 #dbe et seq4 from the atmosphere1 #eja et seq4 decay of1 #ejb due to emanation in atmosphere1 #ejd distribution of1 on surface of the earth1 #ejd concentration of1 on prominences of the earth1 #ejd of rain and snow1 #ejf produced by emanation from tap water1 #eaj effect of meteorological conditions on amount of1 #eag et seq4 amount of1 at ,niagara ,falls1 #ebj rate of decay of1 dependent on conditions1 #ebb et seq4 ,exner and ,haschek spectrum of radium1 #ag ,eye action of radium rays on1 #bag ,fehrle distribution of excited activity on a plate in electric field1 #cah ,fluorescence produced in substances by radium rays1 #ah produced in substances by radium and polonium rays1 #bja et seq4 ,fog large amount of excited activity during1 #eah ,forch loss of weight of radium1 #dgd rays relative conductivity of gas exposed to and hard ,x rays1 #fd1 #ahd discovery of1 #agi absorption of1 by matter1 #agi et seq4 connection between absorption of1 and density1 #ahb discussion of nature of rays1 #ahb et seq4 secondary rays produced by rays1 #ahi measurement of radio-activity by means of1 #ddb1 #dfg conservation of radio-activity measured by1 #dga ,gases evolved by radium1 #bae presence of helium in gases from radium1 #baf ,gates1 ,miss ,f4 effect of temperature on excited activity1 #cae discharge of quinine sulphate1 #ecj ,geitel natural conductivity of air in closed vessels1 #eja1 #eca ,geitel and ,elster radio-active lead1 #bg effect of magnetic field on conductivity produced by radium rays1 #aac scintillations produced by active substances1 #aeh action of radium rays on spark1 #bjh photo-electric action of bodies coloured by radium rays1 #bad radio-active matter in earth1 #did discovery of radio-active matter in atmosphere1 #eja emanations from the earth1 #ejg radio-activity of air in caves1 #ejg radio-activity of the soil1 #eae radio-activity of fango1 #eaf variation of radio-activity of air with meteorological conditions1 #eag effect of temperature and pressure on radio-activity in atmosphere1 #eah ,giesel coloration of bunsen flame by radium1 #ae separation of radium by crystallization of bromide1 #ae emanating substance1 #ba radio-active lead1 #bg magnetic deviation of rays1 #aac decrease with time of luminosity of radio-active screen1 #bje spectrum of phosphorescent light of emanium due to didymium1 #bjf1 #bjg coloration of bodies by radium rays1 #bac evolution of gases from radium1 #bae action of radium rays on the eye1 #bag emanation from the emanating substance1 #bej luminosity produced by radium emanation1 #bea decay of excited activity of emanium1 #cab activity of radium dependent on age1 #cga bismuth made active by radio-active solution1 #dag temperature of radium bromide above air1 #dbj ,gimingham and ,rossignol decay of thorium emanation1 #bdb ,glass coloration produced in1 by radium rays1 #bac phosphorescence produced in1 by emanation1 #beb ,glew simple form of spinthariscope1 and scintillations1 #aei ,globulin action of radium rays on1 #bad ,godlewski effect of crystallization on activity of uranium1 #cdi diffusion of uranium ,x1 #cej separation of actinium ,x1 #cfe source of actinium emanation1 #cfe recovery and decay curves of actinium1 #cff penetrating power of and rays from actinium1 #cfh radiations from active products1 #cfh ,goldstein canal rays1 #gh coloration of bodies by radium rays1 #bac ,gonder1 ,hofmann1 and ,wlfl properties of radio-active lead1 #bg1 #dac ,grier and ,rutherford magnetic deviation of rays of thorium1 #aad relative current due to and rays1 #aie nature of rays from ,ur ,x1 #cdg ,hardy coagulation of globulin by radium rays1 #bad ,hardy and ,miss ,willcock coloration of iodoform solutions by radium rays1 #bad ,hardy and ,anderson action of radium rays on the eye1 #bag ,harms number of ions per c4c4 in closed vessel1 #ecd ,hartmann spectrum of phosphorescent light of emanium1 #bjf ,haschek and ,exner spectrum of radium1 #ag ,heat rate of emission of1 from radium1 #dai et seq4 emission of1 from radium at low temperatures1 #dbj connection of heat emission with the radio-activity1 #dba et seq4 source of heat energy1 #dba et seq4 rate of emission of1 after removal of the emanation1 #dbb et seq4 rate of emission of1 by emanation1 #dbc1 #dca variation with time of heat emission of radium1 and of its emanation1 #dbc heating effect of the emanation1 #dbc1 #dca heating effect of active deposit1 #dbe proportion of heating effect due to radio-active products1 #dcc origin of1 in radium1 #ddb et seq4 total heat emission during life of radio-elements1 #dgd et seq4 heating of earth by radio-active matter1 #dic ,heaviside apparent mass of moving charged body1 #ga1 #abg ,helium produced by radium and its emanation1 #dgf et seq4 amount of1 from radium1 #dhj origin of1 #dhj ,helmholtz and ,richarz action of ions on steam jet1 #dg ,hemptinne action of rays on spark and electrodeless discharge1 #bjh ,henning resistance of radium solutions1 #bjh effect of voltage on amount of excited activity1 #caf ,henning and ,kohlrausch conductivity of solutions of radium bromide1 #bjh ,hertz electric deviation of cathode rays1 #gc ,heydweiler loss of weight of radium1 #dgd ,himstedt action of radium rays on selenium1 #bjh radium emanation in springs of ,baden1 #eac ,himstedt and ,meyer production of helium by radium1 #dgi ,himstedt and ,nagel action of radium rays on the eye1 #bag ,hofmann1 ,gonder1 and ,wlfl properties of radio-active lead1 #bg1 #dac ,hofmann and ,strauss radio-active lead1 #bg ,hofmann and ,zerban connection of activity of thorium with uranium1 #bi ,huggins1 ,sir ,w4 and ,lady spectrum of phosphorescent light of radium bromide1 #bje ,hydrogen production of1 by radium rays1 #bae ,induced radio-activity 7see ,excited radio-activity7 ,induction radio-active1 #bd meaning and examples of1 #dae et seq4 ,insulators conduction of1 under radium rays1 #bji ,iodoform coloration produced in1 by radium rays1 #bad ,ionization theory of1 to explain conductivity of gases1 #ca et seq4 by collision1 #ci1 #eg variation of1 with pressure of gas1 #fa et seq4 variation of1 with nature of gas1 #fd comparison of1 produced by rays1 #aaa1 #aid production of1 in insulators1 #bji total1 produced by #a gram of radium1 #dcc et seq4 natural ionization of gases1 #eca et seq4 connection of1 with phosphorescent and photographic actions1 #edi ,ions in explanation of conductivity of gases1 #ca et seq4 production of1 by collision1 #ci1 #eg rate of recombination of1 #dj et seq4 mobility of1 #db et seq4 difference between mobility of positive and negative1 #dc et seq4 condensation of water around1 #df et seq4 difference between positive and negative1 #di charge carried by1 #ej diffusion of1 #ea et seq4 charge on an ion same as on hydrogen atom1 #ed number of1 produced per c4c41 #ed size and nature of1 #ee et seq4 definition of1 #ef et seq4 velocity acquired by1 between collisions1 #eh energy required to produce1 #eh1 #eea comparative number of1 produced in gases1 #fe disturbance of potential gradient by movement of1 #fe production of1 in insulators1 #bji number of1 produced by particle1 #dcc number produced per c4c4 in closed vessels1 #ecc et seq4 ,joly motion of radium in an electric field1 #baa absorption of radium rays by atmosphere1 #dib 7see foot-note7 ,kaufmann velocity of cathode rays1 #ge variation of e_/m with velocity of electron1 #abg et seq4 ,kelvin theory of radio-activity1 #dda age of sun and earth1 #dib1 #dic ,kelvin1 ,smolan and ,beattie discharging power of uranium rays1 #g ,kleeman and ,bragg theory of absorption of rays1 #agb et seq4 relation between ionization and absorption1 #agd et seq4 range of rays in air1 #agd four sets of rays from radium1 #agd et seq4 ,kohlrausch conductivity of water altered by radium rays1 #bjh ,kohlrausch and ,henning conductivity of solutions of radium bromide1 #bjh ,kunz phosphorescence of willemite and kunzite1 #bjc ,kunz and ,baskerville phosphorescence of substance under radium rays1 #bjd ,kunzite phosphorescence of1 under radium rays1 #bjc ,laborde and ,curie heat emission of radium1 #dai origin of heat from radium1 #ddj radium emanation in waters of hot springs1 #ead ,langevin coefficient of recombination of ions1 #da velocity of ions1 #de et seq4 energy required to produce an ion1 #eh secondary radiation produced by ,x rays1 #ahg slow moving ions in air1 #ebh ,larmor radiation theory1 #gg radiation of energy from moving electron1 #gi structure of the atom1 #aeg ,lead1 radio-active preparation of1 #bf radiations from1 #bf ,le ,bon rays from bodies exposed to sunlight1 #e discharging power of quinine sulphate1 #i1 #ecj ,lenard ionization of gases by ultra-violet light1 #i action of ions on a steam jet1 #dg penetrating power of cathode rays1 #gc negative charge carried by ,lenard rays1 #abj absorption of cathode rays proportional to density1 #acf1 #acg ,lerch1 von chemical properties of active deposit of thorium1 #cac electrolysis of solution of active deposit1 #cac effect of temperature on excited activity1 #cae temporary activity of active deposit from thorium1 #dae ,lockyer inorganic evolution1 #dii ,lodge1 ,sir ,oliver electronic theory1 #fi instability of atoms1 #dhg ,lorentz structure of atoms1 #aeg ,lowry and ,armstrong radio-activity and phosphorescence1 #ddd ,luminosity of radium compounds1 #bje change of1 in radium compounds with time1 #bje spectrum of phosphorescent light from radium bromide1 #bjf of radium compounds unaffected by temperature1 #baj ,mache radium emanation in hot springs1 #eac ,mache and von ,schweidler velocity of ions in air1 #ebh ,makower diffusion of radium emanation1 #bgd diffusion of thorium emanation1 #bgf ,marckwald preparation of radio-tellurium1 #be rate of decay of radio-tellurium1 #daa ,mass apparent mass of electron1 #ga1 #abg variation of mass of electron with speed1 #abg et seq4 of particle1 #adg et seq4 ,materials radio-activity of ordinary1 #ebh1 #ecf et seq4 ,matteucci rate of dissipation of charge in closed vessels1 #eca ,mc,clelland absorption of rays1 #aha secondary rays from and rays from radium1 #aib ,mc,clung coefficient of recombination of ions1 #da conductivity of gases exposed to ,x rays1 #fd ionization by rays from radium ,c1 #eej ,mc,clung and ,rutherford energy required to produce an ion1 #eh variation of current with thickness of layer of uranium1 #aie estimate of energy radiated from radio-elements1 #dah radiation of energy from radium1 #dch ,mc,lennan absorption of cathode rays1 #fe radio-activity of snow1 #ejf excited radio-activity at ,niagara ,falls1 #eai ,mc,lennan and ,burton penetrating radiation from the earth1 #ebj radio-activity of ordinary materials1 #ecg emanation from ordinary matter1 #ech ,metabolon definition of1 #ddf table of metabolons1 #ddi radio-elements as metabolons1 #deg ,meteorological conditions effect of1 on radio-activity of atmosphere1 #eag ,methods of measurement in radio-activity1 #hb et seq4 comparison of photographic and electrical1 #hc et seq4 description of electrical1 #hd et seq4 ,meyer and ,himstedt production of helium by radium1 #dgi ,meyer and ,schweidler magnetic deviation of rays by electrical method1 #aac absorption of rays of radium by matter1 #acf activity proportional to amount of uranium1 #aie emanation from uranium1 #cdh effect of crystallization on activity of uranium1 #cdi rate of decay of radio-tellurium1 #daa ,minerals1 radio-active constant ratio of radium to uranium1 #dei et seq4 list of minerals1 #dfa age of1 #dhe composition of1 #eed et seq4 ,mobility of ions1 #dc et seq4 ,moisture effect of1 on velocity of ions1 #dc1 #de effect of1 on emanating power1 #bee ,molecule number of1 in #a c4c4 of hydrogen1 #ed ,molecular weight of radium emanation1 #bgc of thorium emanation1 #bge ,nagel and ,himstedt action of radium rays on the eye1 #bag ,niewenglowski rays from sulphide of calcium1 #d ,nomenclature of successive products1 #cbh et seq4 ,number of molecules per c4c4 of hydrogen1 #ed of ions produced in gas by active substances1 #ee of particles expelled from #a gram of radium1 #abd of particles emitted per gram of radium1 #aee of ions produced per c4c4 in closed vessels1 #ecd ,occlusion of emanation in thorium and radium1 #beh of radium emanation by solids1 #caj ,owens saturation current affected by dust1 #db penetrating power of rays independent of compound1 #afd absorption of rays varies directly as the pressure of gas1 #afi effect of air currents on conductivity produced by thorium1 #bch ,oxygen change into ozone1 by radium rays1 #bac production of1 from radium solutions1 #bae ,ozone production of1 by radium rays1 #bac ,paraffin objection to1 as an insulator1 #if conductivity of1 under radium rays1 #baj ,paschen distribution of velocity amongst particles1 #aca et seq4 absence of magnetic deflection of rays1 #ahc rays and electrons1 #ahe heating effect of rays1 #ahf1 #dbi ,patterson number of ions per c4c4 in closed vessel1 #ecd natural conductivity of air due to an easily absorbed radiation1 #ecf effect of temperature on natural conductivity of air1 #ecf ,peck and ,willows action of radium rays on spark1 #bjh ,pegram electrolysis of thorium solutions1 #cad temporary activity of substances separated from thorium1 #dae ,penetrating power comparison of1 for 1 and rays1 #aaa variation in1 of rays1 #acd et seq4 variation of1 with density for rays1 #acg comparison of1 for rays from radio-elements1 #afd variation of1 with density for rays1 #afi variation of1 with density for rays1 #ahb ,penetrating radiation from the earth and atmosphere1 #ebj ,perrin charge carried by cathode rays1 #gc theory of radio-activity1 #dcg ,phosphorescence production of1 by radium1 #ai production of1 by radium and polonium rays1 #bja et seq4 comparison of1 produced by and rays1 #bjb of zinc sulphide1 #bjb of barium platinocyanide1 #bjc of willemite and kunzite1 #bjc produced by radium emanation in substances1 #bjc1 #beb diminution of1 with time1 #bje of radium compounds1 #bje spectrum of phosphorescent light of radium bromide1 #bje spectrum of phosphorescent light of 8emanium10 #bjf production of by heat 7thermo-luminescence71 #bjg use of1 to illustrate condensation of emanations1 #bgi connection of with ionization1 #edg et seq4 ,phosphorus action of radium rays on1 #bad ionization produced by1 #ebi ,photo-electric action produced by radium rays in certain substances1 #bad ,photographic method1 advantages and disadvantages of1 #hc relative photographic action of rays1 #hc connection of photographic action with ionization1 #edf ,physical action of radium rays on sparks1 #bjh on electrodeless discharge1 #bjh on selenium1 #bjh on conductivity of insulators1 #bji ,physiological action of radium rays production of burns1 #baf effect on bacteria1 #baf effect on eye1 #bag ,piezo-lectrique of quartz description of1 #aje ,pitchblendes comparison of radio-activity of1 #aa radio-elements separated from1 #ac et seq4 radium continually produced from1 #dei constitution of1 #eeg ,polarization of uranium rays absence of1 #g ,polonium methods of separation of1 #bb rays from1 #bc decay of activity of1 #bc discussion of nature of1 #bd similarity to radio-tellurium1 #bf magnetic deviation of rays from1 #adf1 #aej slow moving electrons1 #aec increase of absorption with thickness of matter traversed1 #afc connection of1 with radium ,f1 #daa ,potential required to produce saturation1 #cb et seq4 fall of potential needed to produce ions at each collision1 #eh gradient due to movement of ions1 #fe ,precht and ,runge spectrum of radium1 #ag atomic weight of radium1 #ah heating effect of radium1 #dbj ,pressure effect of1 on velocity of ions1 #df effect of1 on current through gases1 #fa et seq4 production of emanation independent of1 #bfe effect of1 on distribution of excited activity1 #cag effect of1 on natural conductivity of air in closed vessels1 #ecd ,products1 radio-active list of1 from radio-elements1 #ddi properties of1 #ddi amount of in radium1 #deb et seq4 radiations from1 #dee ,quartz piezo-lectrique use of1 in measurement of current1 #aje ,quinine sulphate discharging power of1 #ecj phosphorescence of1 #ecj ,radiations emitted by uranium1 #h emitted by thorium1 #aj emitted by radium1 #ah emitted by actinium1 #ba emitted by polonium1 #bc method of measurement of1 #hb et seq4 methods of comparison of1 #ajh three kinds of1 #aji analogy to rays from a ,crookes tube1 #aaj relative ionizing and penetrating power of1 #aaa difficulties of comparative measurement of1 #aab rays1 #aac rays1 #ada rays1 #agi secondary rays1 #ahg comparison of ionization of and rays1 #aid phosphorescent effect of1 #bja et seq4 physical actions of1 #bjg et seq4 chemical actions of1 #bac et seq4 physiological actions of1 #baf from the emanation1 #bfc from ,ur ,x1 #cdg connection of1 with heat emission1 #dba et seq4 from different active products1 #dee conservation of energy of each specific type of1 #dfi et seq4 ,radio-lead connection of1 with polonium1 #daa et seq4 connection of1 with radium ,d1 #dac ,radio-tellurium rate of decay of1 #daa connection of with radium ,f1 #daa ,radium discovery of1 #ac separation of1 #ac spectrum of1 #af atomic weight of1 #ag radiations from1 #ah compounds of1 #ai nature of radiations from1 #aji rays from1 #aac rays from1 #ada rays from1 #agi secondary rays from1 #ahg production of phosphorescence by1 #bja et seq4 spectrum of phosphorescent light of1 #bjf physical actions of1 #bjg et seq4 chemical actions of1 #bac et seq4 physiological actions of1 #baf emanation from1 #bdf properties of emanation from1 #bdg et seq4 chemical nature of emanation from1 #bfg diffusion of emanation from1 #bfi condensation of emanation from1 #bgg amount of emanation from1 #bhh volume of emanation from1 #bhi spectrum of emanation from1 #bib excited radio-activity from1 #bie et seq4 decay of excited activity from1 #cjf et seq4 difference in properties of radium and the emanation1 #cbg nomenclature of products1 #cbh theory of successive changes in1 #ccj alteration of activity of1 by removal of emanation1 #cga et seq4 recovery of activity of1 after removal of emanation1 #cgb effect of escape of emanation on recovery of activity of1 #cgd non-separable activity of1 #cge period and properties of radium ,a1 ,b and ,c1 #cgf et seq4 analysis of active deposit of rapid changes of radium1 #cgg analysis of ray curves1 #cha et seq4 analysis of ray curves1 #chf et seq4 equations of activity curves1 #chi effect of temperature on active deposit of1 #cij relative activity due to products of1 #cie active deposit of slow transformation1 #cig physical and chemical properties of radium ,d1 ,e and ,f1 #cih et seq4 effect of temperature on active deposit of slow change1 #dja separation of radium ,f by bismuth1 #djb products of1 #djb et seq4 rate of transformation of radium ,d1 #djd et seq4 variation of the activity of the active deposit over long periods of time1 #djg amounts of radium ,d1 ,e and ,f in old radium1 #djh variation of activity of1 with time1 #dji products of in pitchblende1 #daj origin of radio-tellurium1 #daa origin of polonium1 #daa1 #dab origin of radio-lead1 #dac temporary activity of inactive matter separated from pitchblende1 #dae et seq4 heat emission of1 #dai et seq4 heat emission of emanation from1 #dbj1 #dca heating effects due to products of1 #dcc theories of radio-activity of1 #dcg et seq4 discussion of theories of radio-activity of1 #dda et seq4 energy of radiations1 not derived from external source1 #ddb et seq4 theory of radio-active change1 #ddd et seq4 list of active products of1 #ddi amount of products of1 #deb rate of change of1 #deg life of radium1 #deg origin of1 #dei et seq4 production of1 by uranium1 #dei et seq4 amount of in #a gram of uranium1 #dfa amount of1 in minerals1 #dfa radio-activity of1 independent of concentration1 #dff et seq4 disappearance of1 #dfg life of1 independent of concentration1 #dfh conservation of radio-activity of1 #dfi et seq4 loss of weight of1 #dgc experiments to determine loss of weight of1 #dgd total emission of energy from #a gram of1 #dgd et seq4 production of helium from1 #dgf helium1 disintegration product of1 #dgi et seq4 amount of helium from1 #dhj possible causes of disintegration of1 #dhf et seq4 amount of1 to account for heat of sun1 #dia possible connection of with heat of sun1 #dia possible connection of with heat of earth1 #dic probable amount of1 in earth1 #die amount of1 in atmosphere1 #die1 #ebd presence of1 in atmosphere1 #eba et seq4 ,radium ,a decay curve of1 #cgh radiation from1 #cha effect of1 on activity curves1 #chf et seq4 connection with later changes1 #cib activity supplied by1 #cic ,radium ,b absence of rays in1 #cha effect of1 on activity curves1 #cha et seq4 effect of temperature on1 #cij volatility of1 #cij absence of heating effect of1 #dcc nature of rayless change in1 #ded1 #eeb ,radium ,c radiations from1 #cha analysis of ray curves of1 #cha et seq4 analysis of ray curves of1 #chf et seq4 effect of temperature on1 #cij activity supplied by1 #cid et seq4 heating effect of1 #dbe use of1 as a source of rays1 #dce explosive nature of change in1 #def magnetic deflection of rays from1 #edc velocity and value of e_/m for rays from1 #edd ,radium ,d origin of name of1 #cgf connection of1 with active deposit1 #djc period of transformation of1 #djf effect of1 on variation of activity1 #djg presence in old radium1 #djh effect of1 on activity of old radium1 #dji presence in pitchblende1 #daj connection with radio-lead1 #dac amount of1 in #a ton of uranium1 #ded ,radium ,e effect of temperature on1 #dja connection of1 with ray activity active deposit1 #djc1 #djj connection with radio-lead1 #dac ,radium ,f variation of activity due to1 #cih effect of temperature on1 #dja separation of1 on bismuth plate1 #djb connection with active deposit1 #djc variation of activity of1 over long periods of time1 #djg presence in old radium1 #dji effect of1 on activity of old radium1 #dji presence in pitchblende1 #daj connection with radio-tellurium1 #daa connection with polonium1 #daa1 #dab connection with radio-lead1 #dac ,rain radio-activity of1 #eje decay of activity of1 #ejf ,ramsay1 ,sir ,w4 amount of helium in thorianite1 #dhf ,ramsay and ,collie spectrum of emanation1 #bib ,ramsay and ,cooke radio-activity produced by radiation from radium1 #dgb ,ramsay and ,soddy evolution of gas from radium1 #bae production of hydrogen and oxygen from radium1 #bae chemical nature of the emanation1 #bfh gaseous nature of the emanation1 #bfh volume of emanation1 and change with time1 #bhi helium from radium emanation1 #bia amount of helium produced by radium1 #dhj ,ramsay and ,travers amount of helium in fergusonite1 #dhf ,rayless changes discussion of1 #ded1 #eeb ,re1 ,f4 theory of radio-activity1 #dda ,recombination of ions1 #dj et seq4 constant of1 #db ,recovery of activity of thorium after removal of ,th ,x1 #bba of activity of uranium after removal of ,ur ,x1 #bbc significance of law of1 #bbd effect of conditions on rate of1 #bcb of activity of radium after removal of emanation1 #cgb of heating effect of radium1 #dbc ,reflection no evidence of direct reflection for uranium rays1 #g diffuse reflection of rays1 #g ,refraction no evidence of1 for uranium rays1 #g ,regeneration of emanating power1 #bef ,richarz and von ,helmholtz action of ions on steam jet1 #dg ,richarz and ,schenck theory of radio-activity1 #dda ,rossignol and ,gimingham decay of thorium emanation1 #bdb ,runge spectrum of radium1 #ag ,runge and ,bdlander evolution of gas from radium1 #bae ,runge and ,precht spectrum of radium1 #ag atomic weight of radium1 #ah heating effect of radium1 #dbj ,russel photographic action of substances1 #hc ,saake amount of emanation in air at high altitudes1 #eai ,salomonsen and ,dreyer coloration of quartz by radium rays1 #bac ,saturation current meaning of1 #cc et seq4 application of1 to measurements of radio-activity1 #hd measurement of1 #ajj et seq4 ,schenck radium emanation in springs1 #eac ,schenck and ,richarz theory of radio-activity1 #dda ,schmidt discovery of radio-activity of thorium1 #aj ,schmidt and ,wiedemann thermo-luminescence1 #bjg ,schuster number of ions per c4c4 in air of ,manchester1 #ebh radio-activity of matter1 #ebi ,schweidler and ,mache velocity of ions in air1 #ebh ,schweidler and ,meyer magnetic deviation of rays by electrical method1 #aac absorption of rays of radium by matter1 #acf activity proportional to amount of uranium1 #aie emanation from uranium1 #cdh effect of crystallization on activity of uranium1 #cdi rate of decay of radio-tellurium1 #daa ,scintillations discovery of1 in zinc sulphide screen1 #aeh connection of1 with rays1 #aeh illustration of1 by spinthariscope1 #aeh cause of1 #afj production of1 by action of electric field1 #afj ,searle apparent mass of moving charged body1 #ga1 #abg ,secondary rays examination of1 by photographic method1 #ahg examination of1 by electrical method1 #ahh production of1 by and rays1 #ahi et seq4 from different materials1 #aia amount of1 depends upon atomic weight1 #aib magnetic deflection of1 #aic ,seitz absorption of electrons by matter1 #acg et seq4 ,selenium action of radium rays on1 #bjh ,simon value of e_/m for cathode rays1 #ge1 #abi ,simpson amount of excited activity in north of ,norway1 #eai ,slater1 ,miss effect of temperature on active deposit of thorium1 #ced ,smolan1 ,beattie and ,kelvin discharging power of uranium rays1 #g ,snow radio-activity of1 #ejf decay of activity of1 #ejg ,soddy comparison of photographic and electrical action of uranium rays1 #hc nature of rays from ,ur ,x1 #cdg production of radium from uranium1 #dfc ,soddy and ,ramsay evolution of gas from radium1 #bae production of hydrogen and oxygen from radium1 #bae chemical nature of the emanation1 #bfh gaseous nature of the emanation1 #bfh volume of the emanation1 and change with time1 #bhi helium from radium emanation1 #bia amount of helium produced by radium1 #dhj ,soddy and ,rutherford separation of ,th ,x1 #bbj decay of activity of ,th ,x1 #bba recovery of activity of thorium freed from ,th ,x1 #bba decay of activity of ,ur ,x1 #bbc recovery of activity of uranium freed from ,ur ,x1 #bbc explanation of decay and recovery curves1 #bbd rate of production of ,th ,x1 #bbg theory of decay of activity1 #bbi influence of conditions on rate of decay and recovery of activity1 #bcc disintegration hypothesis1 #bcd decay of activity of radium emanation1 #bdg measurements of emanating power1 #bed effect of temperature1 moisture1 and solution1 on emanating power1 #bee regeneration of emanating power1 #bef constant rate of production of emanation of radium and thorium1 #beg source of thorium emanation1 #bfa radiations from the emanation1 #bfd chemical nature of emanation1 #bfg condensation of emanations of radium and thorium1 #bgg temperature of condensation of emanation1 #bgh effect of successive precipitations on activity of thorium1 #ceh recovery of activity of radium1 #cgb theory of radio-activity1 #dci theory of radio-active change1 #dde conservation of radio-activity1 #dfi ,soil radio-activity of1 #ejg et seq4 difference in activity of1 #ejh et seq4 ,solution coloration of1 by radium1 #ae of active deposit in acids1 #cab electrolysis of active1 #cac ,source of thorium emanation1 #bfa of radium and actinium emanations1 #bfc ,spark action of radium rays on1 #bjh ,spectrum spark spectrum of radium1 #ae1 #af flame spectrum of radium1 #ag effect of a magnetic field on spectrum of radium1 #ag of polonium1 #bc of phosphorescent light of radium bromide1 #bjf of emanation1 #bib of helium in radium gases and emanation1 #dgg ,spinthariscope description of1 #aeh ,springs emanation from water of1 #eac ,stark energy to produce an ion1 #eh ,stoney1 ,johnstone use of term electron1 #gf ,strauss and ,hofmann radio-active lead1 #bg ,strutt conductivity of gases for radiation1 #fc1 #fd conductivity of gases produced by rays1 #fd1 #ahc negative charge carried by rays1 #abb et seq4 absorption of rays proportional to density1 #acf nature of rays1 #adb attempt to measure charge of rays1 #aec constant ratio of uranium to radium in minerals1 #dfb connection of thorium with helium1 #dhc absorption of radium rays from sun by atmosphere1 #dib presence of radium in ,bath waters1 #eac radio-activity of ordinary matter1 #ecf ,sun effect of radium in1 #dia age of1 #dib ,temperature effect of1 on intensity of radiations from uranium and radium1 #baj effect of1 on luminosity1 #baj rate of decay of radium emanation unaffected by1 #bdi of condensation of emanations1 #bhc rate of decay of thorium emanation unaffected by1 #bhg effect of1 on excited activity1 #cae effect of1 on active deposit of thorium1 #ced effect of1 on active deposit of actinium1 #cfh effect of1 on active deposit of rapid change of radium1 #cij effect of1 on active deposit of slow change1 #dja of radium above surrounding space1 #dai effect of1 on amount of excited activity in atmosphere1 #eah effect of1 on natural ionization of air1 #ecf ,theories of radio-activity1 review of1 #dcg et seq4 discussion of1 #dda et seq4 disintegration theory1 #dde et seq4 ,thermo-luminescence1 #bjg ,thomson1 ,j4 ,j4 relation between current and voltage for ionized gases1 #cd difference between ions as condensation nuclei1 #di charge on ion1 #ej magnetic field produced by an ion in motion1 #fi apparent mass of electron1 #ga action of magnetic field on moving ion1 #gb determination of e_/m for cathode stream1 #gc origin of ,x rays1 #hj slow velocity electrons from radio-tellurium1 #aec charge carried by rays1 #aed theory of radio-activity1 #ddj cause of heat emission from radium1 #ddb structure of atom1 #dhg possible causes of disintegration of radium1 #dhg nature of electrons1 #dif emanation from tap-water and deep wells1 #eaj radio-activity of ordinary materials1 #eci ,thomson1 ,j4 ,j4 and ,rutherford ionization theory of gases1 #ca et seq4 ,thorium discovery of radio-activity of1 #aj emanation from1 #aa preparation of non-radio-active thorium1 #bi nature of radiations from1 #aji rays from1 #aad rays from1 #ada rays from1 #ahj separation of ,th ,x from1 #bbj recovery of activity of1 #bba disintegration of1 #bcd emanation from1 #bch properties of emanation from1 #bci diffusion of emanation from1 #bge condensation of emanation from1 #bgg excited radio-activity from1 #bie et seq4 analysis of active deposit of1 #cea et seq4 rayless change in1 #ceb explanation of initial portion of decay curve1 #ceh explanation of initial portion of recovery curve1 #ceh effect of successive precipitations on1 #ceh recovery curve after large number of precipitations1 #cei products of1 #cfc non-separable activity of1 #cfc radiations from active products of1 #cfc division of activity amongst active products of1 #cfc rate of emission of energy by1 #dcb theories of radio-activity of1 #dch discussion of theories of radio-activity1 #dda et seq4 source of energy of radiations1 #ddb et seq4 theory of radio-active change1 #ddd et seq4 table of radio-active products of1 #ddi rate of change of1 #deh life of1 #deh conservation of radio-activity of1 #dfi total emission of energy from #a gram of1 #dge possible causes of disintegration of1 #dhf et seq4 ,thorium ,a period and properties of1 #ceb et seq4 absence of rays in1 #ceb effect of temperature on1 #ced ,thorium ,b period and properties of1 #ceb et seq4 effect of temperature on1 #ced radiations from1 #cfc ,thorium ,x methods of separation of1 #bbj law of decay of activity of1 #bba law of recovery of activity of1 #bba theory to explain production of1 #bbd material nature of1 #bbf continuous production of1 #bbg explanation of decay of activity of1 #bbi effect of conditions on the rate of change of1 #bcc disintegration hypothesis to explain production of1 #bcd minute amount of1 produced1 #bcg effect of successive separations of1 on activity of thorium1 #ceh et seq4 analysis of decay and recovery curves of1 #ceh radiations from1 #cfc ,tommasina scintillations produced by electrification1 #afj ,townsend ions by collision1 #ci1 #eg coefficient of recombination1 #da diffusion of ions1 #ea et seq4 comparison of charge on ion with that on hydrogen atom in electrolysis1 #ec number of molecules per c4c4 of gas1 #ed ionization by collision for different speeds1 #aga ,transformations1 successive theory of1 #cbe et seq4 nomenclature of1 #cbh activity due to1 #ccg detection of a rayless change in1 #cda in uranium1 #cdf et seq4 in thorium1 #cea et seq4 in actinium1 #cfd et seq4 in radium1 #cga et seq4 list of1 #ddi origin of radium in1 #dei helium1 a result of1 #dgf et seq4 possible cause of1 #dhf et seq4 application of1 to evolution of matter1 #dig et seq4 ,transmission of excited radio-activity of radium and thorium1 #cah et seq4 of excited radio-activity of actinium1 #cbc ,travers and ,ramsay amount of helium in fergusonite1 #dhf ,troost rays from hexagonal blende1 #d ,uranium discovery of radio-activity of1 #e persistence of radiations of1 #f discharging power of rays1 #g absence of reflection1 refraction and polarization1 #g examination of uranium minerals1 #aa et seq4 relative activity of compounds of uranium1 #ab nature of radiations from1 #aji rays from1 #aad rays from1 #ada rays from1 #ahj separation of ,ur ,x from1 #bai recovery of activity of1 #bai changes in1 #cdf et seq4 non-separable activity of1 #cdg radiations from ,ur ,x1 #cdg et seq4 method of measurement of activity of ,ur ,x1 #cdg emission of energy by1 #dah theories of radio-activity of1 #dcg et seq4 discussion of theories of radio-activity1 #dda et seq4 source of energy of radiation1 #ddb et seq4 theory of radio-active change1 #ddd et seq4 table of active products1 #ddi rate of change of1 #deh life of1 #deh radium probable product of1 #dei et seq4 amount of radium in1 #dfj et seq4 amount of1 in radio-active minerals1 #dfa growth of radium in1 #dfc conservation of radio-activity of1 #dfi total emission of energy from #a gram of1 #dge possible causes of disintegration of1 #dhf et seq4 ,uranium ,x separation of1 by ,crookes1 #bai separation of1 by ,becquerel1 #bai decay of activity of1 #bbc recovery of activity of1 #bbc theory to explain production of1 #bbd et seq4 material nature of1 #bbf explanation of decay of activity of1 #bbi changes in1 #cdf et seq4 radiations from1 #cdg et seq4 method of measurement of radiations from1 #cdg effect of crystallization on activity of1 #cdi diffusion of1 #cej ,velocity of ions in electric field1 #db et seq4 difference between1 of positive and negative ions1 #dc et seq4 of particle or electron1 #abf et seq4 variation of mass of electron with1 #abg of particle1 #adh of transmission of carriers of excited activity1 #cbj et seq4 of ions in atmosphere1 #ebh ,villard discovery of rays from radium1 #agi alteration of ,x ray screen with time1 #bje activity produced by cathode rays1 #ecj ,vincenti and ,levi ,da ,zara radium emanation in spring waters1 #eaf ,voller variation of activity of radium with concentration1 #dfg ,volume of radium emanation1 calculation of1 #bhi decrease of1 of radium emanation1 #bij ,walker1 ,g4 ,w4 theory of electrometer1 #ij ,walkhoff action of radium rays on skin1 #baf ,wallstabe diffusion of radium emanation into liquids1 #bgf ,water emanation from1 #eaj et seq4 decay of activity of emanation from1 #eaa et seq4 ,water-falls amount of excited activity produced at ,niagara1 #ebj electrification produced near1 #ebj ,watts1 ,marshall atomic weight of radium1 #ah ,weichert velocity of cathode rays1 #gf ,weight loss of by radio-elements1 #dgc attempts to measure loss of in radium1 #dgd ,wheeler and ,bumstead diffusion of radium emanation1 #bgc emanation from surface water and the soil1 #eab1 #ebb identity of emanation from soil with radium emanation1 #eab1 #ebb ,whetham effect of valency of ion on colloidal solutions1 #bae production of radium from uranium1 #dfc ,wiedemann thermo-luminescence1 #bjg ,wiedemann and ,schmidt thermo-luminescence1 #bjg ,wien value of e_/m for canal rays1 #gh positive charge of canal rays1 #gh amount of charge carried by rays1 #abd ,willcock1 ,miss and ,hardy coloration of iodoform solution by radium rays1 #bad ,willemite phosphorescence of1 under radium rays1 #bjc use of1 to show condensation of emanation1 #bgi ,willows and ,peck action of radium rays on spark1 #bjh ,wilson1 ,c4 ,t4 ,r4 ions as nuclei of condensation1 #dg et seq4 difference between positive and negative ions as condensation nuclei1 #di equality of charges carried by positive and negative ions1 #ej construction of electroscope1 #hf1 #hh natural ionization of air in vessels1 #eja radio-activity of rain and snow1 #eje1 #ejf loss of charge in closed vessels1 #eca et seq41 #ecd presence of ions in dust-free air shown by condensation1 #ecc number of ions produced per c4c41 #ecc effect of pressure and nature of gas on ionization in sealed vessels1 #ecd ,wilson1 ,h4 ,a4 charge on ion1 #ea ,wilson1 ,w4 ,e4 radium in sun1 #dia ,wlfl1 ,hofmann and ,gonder properties of radio-active lead1 #bg1 #dac ,wood1 ,a4 radio-activity of ordinary materials1 #edj ,zara1 ,levi ,da and ,vincenti radium emanation in spring waters1 #eaf ,zeeman action of magnetic field on light1 #gg ,zeleny velocity of ions1 #db et seq4 difference of velocity of ions1 #de potential gradient between electrodes1 #fe ,zerban and ,hofmann connection of activity of thorium with uranium1 #bi ,zinc ,sulphide scintillations produced in by rays1 #aeh cause of luminosity of1 #afj1 #edi ,,cambridge3 ,,printed ,,by ,,john ,,clay1 ,m4,a4 ,,at ,,the ,,university ,,press4 ,,cambridge ,,physical ,,series4 ,conduction of ,electricity through ,gases4 ,by ,j4 ,j4 ,,thomson1 ,d4,sc41 ,,ll4,d41 ,ph4,d41 ,f4,r4,s41 ,fellow of ,trinity ,college and ,cavendish ,professor of ,experimental ,physics4 ,demy #hvo4 viii #efh pp4 #afs4 ,,contents4 ,i4 ,electrical ,conductivity of ,gases in a normal state4 ,,ii4 ,properties of a ,gas when in the conducting state4 ,,iii4 ,mathematical ,theory of the ,conduction of ,electricity through a ,gas containing ,ions4 ,,iv4 ,effect produced by a ,magnetic ,field on the ,motion of the ,ions4 ,v4 ,determination of the ,ratio of the ,charge to the ,mass of an ,ion4 ,,vi4 ,determination of the ,charge carried by the ,negative ,ion4 ,,vii4 ,on some ,physical ,properties of ,gaseous ,ions4 ,,viii4 ,ionisation by ,incandescent ,solids4 ,,ix4 ,ionisation in ,gases from ,flames4 ,x4 ,ionisation by ,light4 ,photo-,electric ,effects4 ,,xi4 ,ionisation by ,rntgen ,rays4 ,,xii4 ,becquerel ,rays4 ,,xiii4 ,spark ,discharge4 ,,xiv4 ,the ,electric ,arc4 ,,xv4 ,discharge through ,gases at ,low ,pressures4 ,,xvi4 ,theory of the ,discharge through ,vacuum ,tubes4 ,,xvii4 ,cathode ,rays4 ,,xviii4 ,rntgen ,rays4 ,,xix4 ,properties of ,moving ,electrified ,bodies4 ,supplementary ,notes4 ,index4 ,times4--8,it is difficult to think of a single branch of the physical sciences in which these advances are not of fundamental importance4 ,the physicist sees the relations between electricity and matter laid bare in a manner hardly hoped for hitherto4444 ,the workers in the field of ,science are to-day reaping an unparalleled harvest1 and we may congratulate ourselves that in this field at least we more than hold our own among the nations of the world40 ,a ,treatise on the ,theory of ,solution1 including the ,phenomena of ,electrolysis4 ,by ,,william ,,cecil ,,dampier ,,whetham1 ,m4,a41 ,f4,r4,s41 ,fellow of ,trinity ,college4 ,demy #hvo4 x #dhh pp4 #ajs4 net4 ,athenum4--8,the choice and arrangement of the matter included are alike excellent4444 ,mr ,whetham's book is probably the most complete and satisfactory treatise on the subject in any language and as such is sure to enjoy a wide popularity40 ,nature4--8,the treatment throughout is characterised by great clearness1 especially in the physical and mathematical portions1 so that the volume may be warmly recommended to students of chemistry40 ,,cambridge ,,physical ,,series4 ,,contents ,,of ,,mr ,,whetham',s #8,,solution ,,and ,,electrolysis4' ,i4 ,thermodynamics4 ,,ii4 ,the ,phase ,rule4 ,,iii4 ,the ,phase ,rule4 ,two ,components4 ,solutions4 ,,iv4 ,solubility4 ,v4 ,osmotic ,pressure4 ,,vi4 ,vapour ,pressures and ,freezing ,points4 ,,vii4 ,theories of ,solution4 ,,viii4 ,electrolysis4 ,,ix4 ,conductivity of ,electrolytes4 ,x4 ,galvanic ,cells4 ,,xi4 ,contact ,electricity and ,polarization4 ,,xii4 ,the ,theory of ,electrolytic ,dissociation4 ,,xiii4 ,diffusion in ,solutions4 ,,xiv4 ,solutions of ,colloids4 ,additions4 ,table of ,electro-chemical ,properties of ,aqueous ,solutions4 ,electricity and ,magnetism3 an ,elementary ,text-book1 ,theoretical and ,practical4 ,by ,r4 ,t4 ,,glazebrook1 ,m4,a41 ,f4,r4,s41 ,director of the ,national ,physical ,laboratory and ,fellow of ,trinity ,college1 ,cambridge4 ,crown #hvo4 ,cloth4 #a-ddj pp4 #gs4 #f;d4 ,athenum4--8,if the nature of the book be taken into consideration1 it will be found unusually free from the influence of the examination spirit4 ,the writing is bright and interesting1 and will stimulate a desire1 we think1 for further study40 ,guardian4--8,every schoolmaster and teacher who has under consideration the selection of a text-book for his better students should most certainly look into this book4 ,the information is everywhere absolutely sound and reliable40 ,,preface4 ,some words are perhaps necessary to explain the publication of another book dealing with ,elementary ,electricity4 ,a considerable portion of the present work has been in type for a long time2 it was used originally as a part of the practical work in ,physics for ,medical ,students at the ,cavendish ,laboratory in connexion with my lectures1 and was expanded by ,mr ,wilberforce and ,mr ,fitzpatrick in one of their ,laboratory ,note-books of ,practical ,physics4 ,when ,i ceased to deliver the first year course ,i was asked to print my lectures for the use1 primarily1 of the ,students attending the practical classes2 the lectures on ,mechanics1 ,heat and ,light have been in type for some years4 ,other claims on my time have prevented the issue of the present volume until now1 when it appears in response to the promise made several years ago4 ,meanwhile the subject has changed2 but while this is the case the elementary laws and measurements on which the science is based remain unaltered1 and ,i trust the book may be found of service to others besides my successors at the ,cavendish ,laboratory4 ,the book is to be used in the same way as its predecessors4 ,the apparatus for most of the ,experiments is of a simple character and can be supplied at no great expense in considerable quantities4 ,thus the ,experiments should all1 as far as possible1 be carried out by the members of the class1 the teacher should base his reasoning on the results actually obtained by his pupils4 ,ten or twelve years ago this method was far from common2 the importance to a ,school of a ,physical ,laboratory is now more generally recognized2 it is with the hope that the book may be of value to those who are endeavouring to put the method in practice that it is issued now4 ,heat and ,light4 ,an ,elementary ,text-book1 ,theoretical and ,practical1 for ,colleges and ,schools4 ,by ,r4 ,t4 ,,glazebrook1 ,m4,a4 ,crown #hvo4 #es4 ,also in separate volumes3 ,heat4 #bcj pp4 #cs4 ,light4 #bac pp4 #cs4 ,mechanics and ,hydrostatics4 ,an ,elementary ,text-book1 ,theoretical and ,practical1 for ,colleges and ,schools4 ,by ,r4 ,t4 ,,glazebrook1 ,m4,a4 ,crown #hvo4 #hs4 #f;d4 ,also in separate volumes3 ,part ,i4 ,dynamics4 #bef pp4 #ds4 ,part ,,ii4 ,statics4 #ahb pp4 #cs4 ,part ,,iii4 ,hydrostatics4 #baf pp4 #cs4 ,,extracts ,,from ,,press ,,notices4 8,schools and ,colleges will certainly benefit by adopting this book for their students40 ,nature4 8,mr ,glazebrook's volumes on ,heat and ,light deal with these subjects from the experimental side and it is difficult to admire sufficiently the ingenuity and simplicity of many of the experiments without losing sight of the skill and judgment with which they are arranged40 ,saturday ,review4 8,the books almost cover the advanced stages of the ,south ,kensington prospectus and their use can certainly be recommended to all who wish to study these subjects with intelligence and thoroughness40 ,schoolmaster4 8,mr ,glazebrook's great practical experience has enabled him to treat the experimental aspect of the book with unusual power and it is in this that the great value of the book as compared with most of the ordinary manuals consists40 ,educational ,review4 8,the book is very simply and concisely written1 is clear and methodic in arrangement4444 ,we recommend the book to the attention of all students and teachers of this branch of physical science40 ,educational ,news4 8,we wish ,mr ,glazebrook every success on the extension of his practical system to all the ,colleges and ,schools of the country4 ,it is the only way in which the interest of the student can be awakened and the study of the subject made popular and real40 ,technical ,world4 8,it will be especially appreciated by teachers who possess the necessary apparatus for experimental illustrations40 ,athenum4 8,text-books on this subject are generally too simple or too elaborate for a conception of elementary mechanical principles4 ,this book cannot fail to recommend itself therefore for a first course preliminary to the study of physical science4 ,no other book presents in the same space with the same clearness and exactness so large a range of mechanical principles40 ,physical ,review4 8,marked ability has been shewn in the development of the subject of ,statics in the present volume4444 ,the collected examples for students' exercises are excellent40 ,glasgow ,herald4 ,general ,editors3 ,f4 ,h4 ,,neville1 ,m4,a41 ,f4,r4,s4 and ,w4 ,c4 ,d4 ,,whetham1 ,m4,a41 ,f4,r4,s4 ,mechanics4 ,by ,,john ,,cox1 ,m4,a41 ,f4,r4,s4,c41 ,macdonald ,professor of ,experimental ,physics in ,mc,gill ,university1 ,montreal4 ,demy #hvo4 pp4 xiv #ccb4 #is4 ,net4 ,athenum4--8,it may reasonably be hoped that this endeavour to bridge over the gulf which has hitherto separated theory from practice in respect of the principles of mechanics1 by showing their intimate connexion1 and to present the subject in a more living and attractive form1 by drawing attention to the gradual stages and methods by which the early investigators discovered the laws which govern the science1 will meet with the success which it deserves40 ,the ,study of ,chemical ,composition4 ,an ,account of its ,method and ,historical ,development1 with illustrative quotations4 ,by ,,ida ,,freund1 ,staff ,lecturer and ,associate of ,newnham ,college4 ,demy #hvo4 xvi #fej pp4 #ahs4 ,net4 ,saturday ,review4--8,written from a broad1 philosophical standpoint1 we know of no book more suited for the student of chemistry who has attained a sound general knowledge of the science1 and is now ready to appreciate a critical discussion of the methods by which the results he has learnt have been built up1 thereby fitting himself for the real world of investigation on his own account40 ,a ,treatise on the ,theory of ,alternating ,currents4 ,by ,,alexander ,,russell1 ,m4,a41 ,m4,i4,e4,e4 ,vol4 ,i4 ,demy #hvo4 pp4 #djh4 #abs4 ,net4 ,vol4 ,,ii4 ,in the ,press4 ,scotsman4--8,the volume is not only rich in its own substantive teaching1 but well supplied with references to the more remote authorities upon its subject4 ,it opens an important and valuable contribution to the theoretical literature of electrical engineering40 ,radio-activity4 ,by ,e4 ,,rutherford1 ,d4,sc41 ,f4,r4,s41 ,f4,r4,s4,c41 ,macdonald ,professor of ,physics1 ,mc,gill ,university1 ,montreal4 ,demy #hvo4 pp4 x #djj4 #ajs4 #f;d4 ,net4 ,athenum4--8,english students have had to wait till now for any connected and detailed account of this new branch of physics from the pen of one who has a first hand knowledge of it40 ,nature4--8,the arrangement of the matter and its treatment are throughout admirable40 ,the ,theory of ,experimental ,electricity4 ,by ,w4 ,c4 ,d4 ,,whetham1 ,m4,a41 ,f4,r4,s41 ,fellow of ,trinity ,college4 ,demy #hvo4 #hs4 ,net4 ,,cambridge ,,university ,,press ,,warehouse1 ,c4 ,f4 ,,clay1 ,,manager1 ,london3 ,,ave ,,maria ,,lane1 ,glasgow3 #ej1 ,,wellington ,,street4 ,,also ,london3 ,h4 ,k4 ,,lewis1 #acf1 ,,gower ,,street1 ,w4,c4 ,transcriber's ,notes3 ,inconsistent spelling and hyphenation were made consistent only when a predominant form was found in this book4 ,text that was in italics is enclosed by underscores 7italics74 ,text that was in bold face is enclosed by equals signs 7bold74 ,footnotes have been moved to follow the chapters in which they are referenced4 ,formulae in the text have been rendered in text form1 and in some cases there are unusual conventions used4 ,the convention nameexpression means that the expression is a subscript to the name1 and nameexpression means that expression is a subscript to the name4 ,many parentheses are added to allow an expression to be transcribed on one line4