,transcriber's ,notes ,obvious typographical errors have been silently corrected4 ,variations in hyphenation have been standardised but all other spelling and punctuation remains unchanged4 ,italics are represented thus italic1 bold thus bold and superscripts thus yen4 ,,worlds ,,in ,,the ,,making ,,the ,,evolution ,,of ,,the ,,universe ,,by ,,svante ,,arrhenius ,,director ,,of ,,the ,,physico-,,chemical ,,nobel ,,institute1 ,,stockholm ,,translated ,,by ,,dr4 ,h4 ,,borns ,,illustrated colophon ,,new ,,york ,,and ,,london ,,harper ,,brothers ,,publishers ,,mcmviii ,copyright1 #aijh1 by ,,harper ,,brothers4 ,all rights reserved4 ,published ,march1 #aijh4 ,,table ,,of ,,contents ,i4 ,,volcanic ,,phenomena ,,and ,,earthquakes #a ,destruction caused by volcanism and by earthquakes4--,different kinds of volcanoes4--,vesuvius4--,products of eruption4--,volcanic activity diminishing4--,structure of volcanoes4--,geographical distribution of volcanoes4--,temperature in the interior of the earth4--,significance of water for volcanism4--,composition of the earth's interior4--,geographical distribution of earthquakes4--,fissures in the earth's crust4--,groups of earthquakes4--,waves in the sea and in the air accompanying earthquakes4--,their connection with volcanism4--,systems of fissures4--,seismograms4 ,,ii4 ,,the ,,celestial ,,bodies1 ,,in ,,particular ,,the ,,earth1 ,,as ,,abodes ,,of ,,living ,,beings #ci ,manifold character of the worlds4--,the earth probably at first a ball of gases4--,formation of the earth crust and its rapid cooling4--,balance between heat received and heat lost by radiation4--,life already existing on the earth for a milliard of years4--,the waste of solar heat4--,temperature and habitability of the planets4--,heat-preserving influence of the atmosphere4--,significance of carbon dioxide in the atmosphere4--,warm and cold geological ages4--,fluctuations in the percentage of carbon dioxide of the air4--,combustion1 decay1 and growth4--,atmospheric oxygen4--,vegetable life more ancient than animal life4--,the atmospheres of planets4--,chances of an improvement in the climate4 ,,iii4 ,,radiation ,,and ,,constitution ,,of ,,the ,,sun #fd ,stability of the solar system4--,losses and possible gains of heat by the sun4--,theses of ,mayer and of ,helmholtz4--,temperatures of the white1 yellow1 and reddish stars1 and of the sun4--,sun-spots and sun facul4--,prominences4--,spectra of the parts of the sun4--,temperature of the sun4--,the interior of the sun4--,its composition according to the mechanical theory of heat4--,the losses of heat by the sun probably covered by the enormous solar energy4 ,,iv4 ,,the ,,radiation ,,pressure #id ,newton's law of gravitation4--,kepler's observation of comets' tails4--,the thesis of ,euler4--,proof of ,maxwell4--,the radiation pressure4--,electric charges and condensation4--,comets' tails and radiation pressure4--,constituents and properties of comets' tails4--,weight of the solar corona4--,loss and gain of matter by the sun4--,nature of meteorites4--,electric charge of the sun4--,electrons drawn into the sun4--,magnetic properties of the sun and appearance of the corona4--,constituents of the meteors4--,nebul and their heat and light4 ,v4 ,,the ,,solar ,,dust ,,in ,,the ,,atmosphere4 ,,polar ,,lights ,,and ,,the ,,variations ,,of ,,terrestrial ,,magnetism #aah ,the supply of dust from the sun rather insignificant4--,polarization of the light of the sky4--,the upper clouds4--,different kinds of auror4--,their connection with the corona of the sun4--,polar lights and sun-spots4--,periodicity of polar lights4--,polar lights and magnetic disturbances4--,velocity of solar dust4--,fixation of atmospheric nitrogen4--,the ,zodiacal ,light4 ,,vi4 ,,end ,,of ,,the ,,sun4--,,origin ,,of ,,nebul #adh ,the extinction of the sun4--,collision between two celestial bodies4--,the new star in ,perseus4--,formation of nebul4--,the appearance of nebul4--,the nebul catch wandering meteors and comets4--,the ring nebula in ,lyra4--,variable stars4--,eta in ,argus4--,mira ,ceti4--,lyra and ,algol stars4--,evolution of the stars4 ,,vii4 ,,the ,,nebular ,,and ,,the ,,solar ,,states #aia ,the energy of the universe4--,the entropy of the universe4--,the entropy increases in the suns1 but decreases in the nebul4--,temperature and constitution of the nebul4--,schuster's calculations of the condition of a celestial body consisting of gases4--,action of the loss of heat on nebul and on suns4--,development of a rotating nebula into a planetary system4--,the hypothesis of ,kant-,laplace4--,objections to it4--,the views of ,chamberlin and ,moulton4--,the radiation pressure balances the effect of ,newtonian gravitation4--,the emission of gases from the nebul balances the waste of heat characteristic to the solar systems4 ,,viii4 ,,the ,,spreading ,,of ,,life ,,through ,,the ,,universe #bab ,stability of the species4--,theory of mutation4--,spontaneous generation4--,bathybius4--,panspermia4-- ,the stand-points of ,richter1 ,ferdinand ,cohn1 and ,lord ,kelvin4--,the radiation pressure enables spores to escape4--,the effect of strong sunlight and of cold on the germinating power4--,transport of spores through the atmosphere into universal space and through it to other planets4--,general conclusions4 ,,explanation ,,of ,,abbreviations1 ,,etc4 ,the temperatures are stated in degrees centigrade 7 ,c471 either on the ,celsius scale1 on which the freezing-point of water is #j1 or on the absolute scale1 whose zero lies #bgc degrees below the freezing-point of water1 at -#bgc ,c4 ,the equivalent temperatures on the ,fahrenheit scale 7freezing-point of water #cb ,f47 are added in brackets 7 ,f474 #a metre 7m47 #aj decimetres 7dm47 #ajj centimetres 7cm47 #ajjj millimetres 7mm47 #c4bh ft42 #a kilometre 7km47 #ajjj metres 7m47 #j4fb miles2 #a mile #a4f kilometres 7km474 ,light travels in vacuo at the rate of #cjj1jjj km4 7nearly #bjj1jjj miles7 per second4 ,,illustrations ,,fig4 ,,page #a4 ,,vesuvius1 ,,as ,,seen ,,from ,,the ,,island ,,of ,,nisida1 ,,in ,,moderate ,,activity #b #b4 ,,eruption ,,of ,,vesuvius ,,in #ahhb #d #c4 ,,eruption ,,of ,,vesuvius ,,in #ahgb #f #d4 ,,photograph ,,of ,,vesuvius1 #aijf4 ,,chiefly ,,clouds ,,of ,,ashes #h #e4 ,,block ,,lava ,,on ,,mauna ,,loa #aj #f4 ,,the ,,excelsior ,,geyser ,,in ,,yellowstone ,,park1 ,u4 ,s4 ,a4 ,,remnant ,,of ,,the ,,powerful ,,volcanic ,,activity ,,in ,,the ,,tertiary ,,age #aa #g4 ,,mato ,,tepee ,,in ,,wyoming1 ,u4 ,s4 ,a4 ,,typical ,,volcanic 8,,neck0 #ab #h4 ,,clefts ,,filled ,,with ,,lava ,,and ,,volcanic ,,cone ,,of ,,ashes1 ,,torowheap ,,ca,,on1 ,,plateau ,,of ,,colorado #ac #i4 ,,the ,,kilauea ,,crater ,,on ,,hawaii #ae #aj4 ,,chief ,,earthquake ,,centres1 ,,according ,,to ,,the ,,british ,,association ,,committee #bb #aa4 ,,clefts ,,in ,,valentia ,,street1 ,,san ,,francisco1 ,,after ,,the ,,earthquake ,,of #aijf #be #ab4 ,,sand ,,craters ,,and ,,fissures1 ,,produced ,,by ,,the ,,corinth ,,earthquake ,,of #ahfa4 ,,in ,,the ,,water1 ,,branches ,,of ,,flooded ,,trees #bg #ac4 ,,earthquake ,,lines ,,in ,,lower ,,austria #cj #ad4 ,,liberty ,,building ,,of ,,leland ,,stanford ,,junior ,,university1 ,,in ,,california1 ,,after ,,the ,,earthquake ,,of #aijf #cb #ae4 ,,earthquake ,,lines ,,in ,,the ,,tyrrhenian ,,depression #cd #af4 ,,seismogram ,,recorded ,,at ,,shide1 ,,isle ,,of ,,wight1 ,,on ,,august #ca1 #ahih #ce #ag4 ,,photograph ,,of ,,the ,,surface ,,of ,,the ,,moon1 ,,in ,,the ,,vicinity ,,of ,,the ,,crater ,,of ,,copernicus #fb #ah4 ,,sun-,,spot ,,group ,,and ,,granulation ,,of ,,the ,,sun #gd #ai4 ,,part ,,of ,,the ,,solar ,,spectrum ,,of ,,january #c1 #ahgb #ge #bj4 ,,metallic ,,prominences ,,in ,,vortex ,,motion #gf #ba4 ,,fountain-,,like ,,metallic ,,prominences #gf #bb4 ,,quiet ,,prominences ,,of ,,smoke-,,column ,,type #gg #bc4 ,,quiet ,,prominences1 ,,shape ,,of ,a ,,tree #gg #bd4 ,,diagram ,,illustrating ,,the ,,differences ,,in ,,the ,,spectra ,,of ,,sun-,,spots ,,and ,,of ,,the ,,photosphere #gh #be4 ,,spectrum ,,of ,a ,,sun-,,spot1 ,,the ,,central ,,band ,,between ,,the ,,two ,,portions ,,of ,,the ,,photosphere ,,spectrum #gh #bf4 ,,the ,,great ,,sun-,,spot ,,of ,,october #i1 #aijc #gi #bg4 ,,the ,,great ,,sun-,,spot ,,of ,,october #i1 #aijc #hj #bh4 ,,the ,,great ,,sun-,,spot ,,of ,,october #i1 #aijc #ha #bi4 ,,the ,,great ,,sun-,,spot ,,of ,,october #i1 #aijc #hb #cj4 ,,photograph ,,of ,,the ,,solar ,,corona ,,of #aijj #hc #ca4 ,,photograph ,,of ,,the ,,solar ,,corona ,,of #ahgj #hd #cb4 ,,photograph ,,of ,,the ,,solar ,,corona ,,of #ahih #he #cc4 ,,photograph ,,of ,,roerdam',s ,,comet 7#ahic ,,ii471 ,,suggesting ,,several ,,strong ,,nuclei ,,in ,,the ,,tail #ajj #cd4 ,,photograph ,,of ,,swift',s ,,comet 7#ahib ,i47 #aja #ce4 ,,donati',s ,,comet ,,at ,,its ,,greatest ,,brilliancy ,,in #aheh #ajb #cf4 ,,imitation ,,of ,,comets' ,,tails #ajd #cg4 ,,granular ,,chondrum ,,from ,,the ,,meteorite ,,of ,,sexes4 ,,enlargement #a3gj #aji #ch4 ,,arch-,,shaped ,,auror ,,borealis1 ,,observed ,,by ,,nordenskiold ,,during ,,the ,,wintering ,,of ,,the ,,vega ,,in ,,bering ,,strait #ahgi #abd #ci4 ,,aurora ,,borealis1 ,,with ,,radial ,,streamers #abe #dj4 ,,aurora ,,with ,,corona1 ,,observed ,,by ,,gyllenski,,ld ,,on ,,spitzbergen1 #ahhc #abf #da4 ,,polar-,,light ,,draperies1 ,,observed ,,in ,,finnmarken1 ,,northern ,,norway #abg #db4 ,,curve ,,of ,,magnetic ,,declination ,,at ,,kew1 ,,near ,,london1 ,,on ,,november #ae ,,and #af1 #aije #ach #dc4 ,,curve ,,of ,,horizontal ,,intensity ,,at ,,kew ,,on ,,november #ae ,,and #af1 #aije #aci #dd4 ,,zodiacal ,,light ,,in ,,the ,,tropics #adf #de4 ,,spectrum ,,of ,,nova ,,aurig1 #ahib #aed #df4 ,,diagram ,,indicating ,,the ,,consequences ,,of ,a ,,collision ,,between ,,two ,,extinct ,,suns #aeg #dg4 ,,spiral ,,nebula ,,in ,,the ,,canes ,,venatici #aei #dh4 ,,spiral ,,nebula ,,in ,,the ,,triangle #afa #di4 ,,the ,,great ,,nebula ,,in ,,andromeda #afc #ej4 ,,ring-,,shaped ,,nebula ,,in ,,lyra #afd #ea4 ,,central ,,portion ,,of ,,the ,,great ,,nebula ,,in ,,orion #afe #eb4 ,,nebular ,,stri ,,in ,,the ,,stars ,,of ,,the ,,pleiades #afg #ec4 ,,nebular ,,stri ,,in ,,the ,,swan #afi #ed4 ,,nebula ,,and ,,star ,,rift ,,in ,,the ,,swan1 ,,in ,,the ,,milky ,,way #aga #ee4 ,,great ,,nebula ,,near ,,rho1 ,,in ,,ophiuchus #agb #ef4 ,,star ,,cluster ,,in ,,hercules #agc #eg4 ,,star ,,cluster ,,in ,,pegasus #age #eh4 ,,cone-,,shaped ,,star ,,cluster ,,in ,,gemini #agf #ei4 ,,comparison ,,of ,,spectra ,,of ,,stars ,,of ,,classes #b1 #c1 #d #ahe #fj4 ,,comparison ,,of ,,spectra ,,of ,,stars ,,of ,,classes #b1 #c1 #d #ahf ,,preface ,when1 more than six years ago1 ,i was writing my ,treatise of ,cosmic ,physics1 ,i found myself confronted with great difficulties4 ,the views then held would not explain many phenomena1 and they failed in particular in cosmogonic problems4 ,the radiation pressure of light1 which had not1 so far1 been heeded1 seemed to give me the key to the elucidation of many obscure problems1 and ,i made a large use of this force in dealing with those phenomena in my treatise4 ,the explanations which ,i tentatively offered could1 of course1 not claim to stand in all their detail2 yet the scientific world received them with unusual interest and benevolence4 ,thus encouraged1 ,i tried to solve more of the numerous important problems1 and in the present volume ,i have added some further sections to the complex of explanatory arguments concerning the evolution of the ,universe4 ,the foundation to these explanations was laid in a memoir which ,i presented to the ,academy of ,sciences at ,stockholm in #aijj4 ,the memoir was soon afterwards printed in the ,physikalische ,zeitschrift1 and the subject was further developed in my ,treatise of ,cosmic ,physics4 ,it will be objected1 and not without justification1 that scientific theses should first be discussed and approved of in competent circles before they are placed before the public4 ,it cannot be denied that1 if this condition were to be fulfilled1 most of the suggestions on cosmogony that have been published would never have been sent to the compositors2 nor do ,i deny that the labor spent upon their publication might have been employed for some better purpose4 ,but several years have elapsed since my first attempts in this direction were communicated to scientists4 ,my suggestions have met with a favorable reception1 and ,i have1 during these years1 had ample opportunity carefully to re-examine and to amend my explanations4 ,i therefore feel justified in submitting my views to a larger circle of readers4 ,the problem of the evolution of the ,universe has always excited the profound interest of thinking men4 ,and it will1 without doubt1 remain the most eminent among all the questions which do not have any direct1 practical bearing4 ,different ages have arrived at different solutions to this great problem4 ,each of these solutions reflected the stand-point of the natural philosophers of its time4 ,let me hope that the considerations which ,i offer will be worthy of the grand progress in physics and chemistry that has marked the close of the nineteenth and the opening of the twentieth century4 ,before the indestructibility of energy was understood1 cosmogony merely dealt with the question how matter could have been arranged in such a manner as to give rise to the actual worlds4 ,the most remarkable conception of this kind we find in ,herschel's suggestion of the evolution of stellar nebul1 and in the thesis of ,laplace concerning the formation of the solar system out of the universal nebula4 ,observations more and more tend to confirm ,herschel's view4 ,the thesis of ,laplace1 for a long time eulogized as the flower of cosmogonic speculations1 has more and more had to be modified4 ,if we attempt1 with ,kant1 to conceive how wonderfully organized stellar systems could originate from absolute chaos1 we shall have to admit that we are attacking a problem which is insoluble in that shape4 ,there is a contradiction in those very attempts to explain the origin of the ,universe in its totality1 as ,stallo#a emphasizes3 8,the only question to which a series of phenomena gives legitimate rise relates to their filiation and interdependence40 ,i have1 therefore1 only endeavored to show how nebul may originate from suns and suns from nebul2 and ,i assume that this change has always been proceeding as it is now4 ,footnote #a3 ,stallo3 ,concepts and ,theories of ,modern ,physics4 ,london1 #aijj1 p4 #bgf4 ,the recognition of the indestructibility of energy seemed to accentuate the difficulties of the cosmogonic problems4 ,the theses of ,mayer and of ,helmholtz1 on the manner in which the ,sun replenishes its losses of heat1 have had to be abandoned4 ,my explanation is based upon chemical reactions in the interior of the ,sun in accordance with the second law of thermodynamics4 ,the theory of the 8degradation0 of energy appeared to introduce a still greater difficulty4 ,that theory seems to lead to the inevitable conclusion that the ,universe is tending towards the state which ,clausius has designated as 8,wrme ,tod0 7heat death71 when all the energy of the ,universe will uniformly be distributed through space in the shape of movements of the smallest particles4 ,that would imply an absolutely inconceivable end of the development of the ,universe4 ,the way out of this difficulty which ,i propose comes to this3 the energy is 8degraded0 in bodies which are in the solar state1 and the energy is 8elevated10 raised to a higher level1 in bodies which are in the nebular state4 ,finally1 ,i wish to touch upon one cosmogonical question which has recently become more actual than it used to be4 ,some kind of 8spontaneous generation10 origination of life from inorganic matter1 had been acquiesced in4 ,but just as the dreams of a spontaneous generation of energy--i4e41 of a perpetuum mobile--have been dispelled by the negative results of all experiments in that direction1 just in the same way we shall have to give up the idea of a spontaneous generation of life after all the repeated disappointments in this field of investigation4 ,as ,helmholtz#b says1 in his popular lecture on the growth of the planetary system 7#ahga73 8,it seems to me a perfectly just scientific procedure1 if we1 after the failure of all our attempts to produce organisms from lifeless matter1 put the question1 whether life has had a beginning at all1 or whether it is not as old as matter1 and whether seeds have not been carried from one planet to another and have developed everywhere where they have fallen on a fertile soil40 ,footnote #b3 ,helmholtz1 ,populr ,wissenschaftliche ,vortrge4 ,braunschweig1 #ahgf1 vol4 iii41 p4 #aja4 ,this hypothesis is called the hypothesis of panspermia1 which ,i have modified by combining it with the thesis of the radiation pressure4 ,my guiding principle in this exposition of cosmogonic problems has been the conviction that the ,universe in its essence has always been what it is now4 ,matter1 energy1 and life have only varied as to shape and position in space4 ,,the ,,author4 ,,stockholm1 ,december1 #aijg4 ,,worlds ,,in ,,the ,,making ,i ,,volcanic ,,phenomena ,,and ,,earthquakes ,the ,interior of the ,earth ,the disasters which have recently befallen the flourishing settlements near ,vesuvius and in ,california have once more directed the attention of mankind to the terrific forces which manifest themselves by volcanic eruptions and earthquakes4 ,the losses of life which have been caused in these two last instances are1 however1 insignificant by comparison with those which various previous catastrophes of this kind have produced4 ,the most violent volcanic eruption of modern times is no doubt that of ,august #bf and #bg1 #ahhc1 by which two-thirds of the island of ,krakatoa1 #cc square kilometres 7#ac square miles7 in area1 situated in the ,east ,indian ,archipelago1 were blown into the air4 ,although this island was itself uninhabited1 some #dj1jjj people perished on that occasion1 chiefly by the ocean wave which followed the eruption and which caused disastrous inundations in the district4 ,still more terrible was the destruction wrought by the ,calabrian earthquake of ,february and ,march1 #aghc1 which consisted of several earthquake waves4 ,the large town of ,messina was destroyed on ,february #eth1 and the number of people killed by this event has been estimated at #ajj1jjj4 ,the same region1 especially ,calabria1 has1 moreover1 frequently been visited by disastrous earthquakes--again in #aije and #aijg4 ,another catastrophe upon which history dwells1 owing to the loss of life 7not less than #ij1jjj71 was the destruction of the capital of ,portugal on ,november #a1 #agee4 ,two-thirds of the human lives which this earthquake claimed were destroyed by a wave #e m4 in height rushing in from the sea4 ,illustration3 ,fig4 #a4--,vesuvius1 as seen from the ,island of ,nisida1 in moderate activity ,vesuvius is undoubtedly the best studied of all volcanoes4 ,during the splendor of ,rome this mountain was quite peaceful--known as an extinct volcanic cone so far as history could be traced back4 ,on the extraordinarily fertile soil about it had arisen big colonies of such wealth that the district was called ,great ,greece 7,grcia ,magna74 ,then came1 in the year #gi ,a4,d41 the devastating eruption which destroyed1 among others1 the towns of ,herculaneum and ,pompeii4 ,the volumes of gas1 rushing forth with extreme violence from the interior of the earth1 pushed aside a large part of the volcanic cone whose remnant is now called ,monte ,somma1 and the falling masses of ashes1 mixed with streams of lava1 built up the new ,vesuvius4 ,this mountain has repeatedly changed its appearance during later eruptions1 and was provided with a new cone of ashes in the year #aijf4 ,the outbreak of the year #gi was succeeded by new eruptions in the years #bjc1 #dgb1 #eab1 #fhe1 #iic1 #ajcf1 #aaci1 #aejj1 #afca1 and #affj1 at quite irregular intervals4 ,since that time ,vesuvius has been in almost uninterrupted activity1 mostly1 however1 of a harmless kind1 so that only the cloud of smoke over its crater indicated that the internal glow was not yet extinguished4 ,very violent eruptions took place in the years #agid1 #ahbb1 #ahgb1 and #aijf4 ,other volcanoes behave quite differently from these violent volcanoes1 and do hardly any noteworthy damage4 ,among these is the crater-island of ,stromboli1 situated between ,sicily and ,calabria4 ,this volcano has been in continuous activity for thousands of years4 ,its eruptions succeed one another at intervals ranging from one minute to twenty minutes1 and its fire serves the sailors as a natural light-house4 ,the force of this volcano is1 of course1 unequal at different periods4 ,in the summer of #aijf it is said to have been in unusually violent activity4 ,very quiet1 as a rule1 are the eruptions of the great volcanoes on ,hawaii4 ,foremost among the substances which are ejected from volcanoes is water vapor4 ,the cloud floating above the crater is1 for this reason1 the surest criterion of the activity of the volcano4 ,violent eruptions drive the masses of steam up into the air to heights of #h km4 7#e miles71 as the illustrations 7,figs4 #a to #d7 will show4 ,the height of the cloud may be judged from the height of ,vesuvius1 #acjj metres 7nearly #dcjj ft47 above sea-level4 ,the illustration on page #d 7,fig4 #b7 is a reproduction of a drawing by ,poulett ,scrope1 representing the ,vesuvius eruption of the year #ahbb4 ,there seems to have been no wind on this day2 the masses of steam formed a cloud of a regular shape which reminds us of a pine-tree4 ,according to the description of ,plinius1 the cloud noticed at the eruption of ,vesuvius in the year #gi must have been of the same kind4 ,when the air is not so calm the cloud assumes a more irregular shape 7,fig4 #c74 ,clouds which rise to such elevations as we have spoken of are distinguished by strong electric charges4 ,the vivid flashes of lightning which shoot out of the black clouds add to the terror of the awful spectacle4 ,illustration3 ,fig4 #b4--;,eruption of ,vesuvius in #ahhb4 7,after a contemporaneous drawing by ,poulett ,scrope7 ,the rain which pours down from this cloud is often mixed with ashes and is as black as ink4 ,the ashes have a color which varies between light-gray1 yellow-gray1 brown1 and almost black1 and they consist of minute spherules of lava ejected by the force of the gases and rapidly congealed by contact with the air4 ,larger drops of lava harden to volcanic sand--the so-called 8lapilli0 7that is1 little stones71 or to 8bombs10 which are often furrowed by the resistance offered by the air1 and turn pear-shaped4 ,these solid products1 as a rule1 cause the greatest damage due to volcanic eruptions4 ,in the year #aijf the weight of these falling masses 7,fig4 #d7 crushed in the roofs of houses4 ,a layer of ashes #g m4 7#bc ft47 in thickness buried ,pompeii under a protective crust which had covered it up to days of modern excavations4 ,the fine ashes and the muddy rain clung like a mould of plaster to the dead bodies4 ,the mud hardened afterwards into a kind of cement1 and as the decomposition products of the dead bodies were washed away1 the moulds have provided us with faithful casts of the objects that had once been embedded in them4 ,when the ashes fall into the sea1 a layer of volcanic tuffa is formed in a similar manner1 which enshrines the animals of the sea and alg4 ,of this kind is the soil of the ,campagna ,felice1 near ,naples4 ,larger lumps of solid stones with innumerable bubbles of gases float as pumice-stone on the sea1 and are gradually ground down into volcanic sand by the action of the waves4 ,the floating pumice-stone has sometimes become dangerous or1 at any rate1 an obstacle to shipping1 through its large masses2 that was1 at least1 the case with the ,krakatoa eruption of #ahhc4 ,illustration3 ,fig4 #c4--;,eruption of ,vesuvius in #ahgb4 7,after a photograph47 ,among the gases which are ejected in addition to water vapor1 carbonic acid should be mentioned in the first instance2 also vapors of sulphur and sulphuretted hydrogen1 hydrochloric acid1 and chloride of ammonium1 as well as the chlorides of iron and copper1 boric acid1 and other substances4 ,a large portion of these bodies is precipitated on the edges of the volcano1 owing to the sudden cooling of the gases4 ,the more volatile constituents1 such as carbonic acid1 sulphuretted hydrogen1 and hydrochloric acid1 may spread over large areas1 and destroy all living beings by their heat and poison4 ,it was these gases1 for example1 which caused the awful devastation at ,st4 ,pierre1 where #cj1jjj human lives were destroyed on ,may #h1 #aijb1 by the eruption of ,mont ,pele4 ,the ejection of hydrogen gas1 which1 on emerging from the lava1 is burned to water by the oxygen of the air1 has been observed in the crater of ,kilauea4 ,the ashes of the volcanoes are sometimes carried to vast distances by the air currents--e4g41 from the western coast of ,south ,america to the ,antilles2 from ,iceland to ,norway and ,sweden2 from ,vesuvius 7#aijf7 to ,holstein4 ,best known in this respect is the eruption of the ,krakatoa1 which drove the fine ashes up to an elevation of #cj km4 7#ah miles74 ,the finest particles of these ashes were slowly carried by the winds to all parts of the earth1 where they caused1 during the following two years1 the magnificent sunrises and sunsets which were spoken of as 8the red glows40 ,this glow was also observed in ,europe after the eruption of ,mont ,pele4 ,the dust of ,krakatoa further supplied the material for the so-called 8luminous clouds of the night10 which were seen in the years #ahhc to #ahib floating at an elevation of about #hj km4 7#ej miles71 and hence illuminated by the light of the sun long after sunset4 ,the crater of ,kilauea1 on the high volcano of ,mauna ,loa1 in ,hawaii--this volcano is about of the same height as ,mont ,blanc--has excited special interest4 ,the crater forms a large lake of lava having an area of about #ab sq4 km4 7nearly #e sq4 miles71 which1 however1 varies considerably with time4 ,the lava boiling at red glow is constantly emitting masses of gas under slight explosions1 spurting out fiery fountains to a height of #bj m4 7#fe ft47 into the air4 ,here and there lava flows out from crevices in the wall of the crater down the slope of the mountain1 until the surface of the lake of lava has descended below these cracks4 ,as a rule1 this lava is of a thin fluid consistency1 and it spreads1 therefore1 rather uniformly over large areas4 ,of a similar kind are also the floods of lava which are sometimes poured over thousands of square kilometres on ,iceland4 ,the so-called ,laki eruption of the year #aghc was of a specially grand nature4 ,though occurring in an uninhabited district1 it did a great amount of damage4 ,in the more ancient geological periods1 especially in the ,tertiary age1 similar sheets of lava of vast extensions have been spread over ,england and ,scotland 7more than #ajj1jjj sq4 km41 roughly1 #dj1jjj sq4 miles72 over ,deccan1 in ,india1 #djj1jjj sq4 kms4 7#aej1jjj sq4 miles71 up to heights of #bjjj m4 7#fejj ft472 and over ,wyoming1 ,yellowstone ,park1 ,nevada1 ,utah1 ,oregon1 and other districts of the ,united ,states1 as well as over ,british ,columbia4 ,illustration3 ,fig4 #d4--,photograph of ,vesuvius1 #aijf4 ,chiefly clouds of ashes ,in other cases the slowly ejected lava is charged with large volumes of gases1 which escape when the lava congeals and burst it up into rough1 unequal blocks1 forming the so-called block lava 7,fig4 #e74 ,the streams of lava can likewise produce terrible devastation when they descend into inhabited districts2 on account of their slow motion1 they rarely cause loss of life1 however4 ,where the volcanic activity gradually lessens or ceases1 we can still trace it by the exhalations of gas and the springs of warm water which we find in many districts where1 during the ,tertiary age1 powerful volcanoes were ejecting their streams of lava4 ,to this class belong the famous geysers of ,iceland1 of ,yellowstone ,park 7,fig4 #f71 and of ,new ,zealand2 also the hot springs of ,bohemia1 so highly valued therapeutically 7e4g41 the ,karlsbad ,sprudel72 the ,fumaroli of ,italy1 ,greece1 and other countries1 exhaling water vapor2 the ,mofett1 with their exhalations of carbonic acid 7of frequent occurrence in the district of the ,eifel and on both sides of the middle ,rhine1 in the ,dogs ,grotto near ,naples1 and in the ,valley of ,death in ,java72 the ,solfatara1 exhaling vapors of sulphur--sulphuretted hydrogen and sulphur dioxide 7they are found near ,naples on the ,phlegran ,fields and in ,greece72 as well as many of the so-called mud volcanoes1 which eject mud1 salt water1 and gases 7as a rule1 carbonic acid and hydrocarbons7--for example1 the mud volcanoes near ,parma and ,modena1 in ,italy1 and those near ,kronstadt1 in ,transylvania4 ,illustration3 ,fig4 #e4--;,block lava on ,mauna ,loa ,the extinct volcanoes1 of which some1 like the ,aconcagua1 #figj m4 7#bb1hgj ft471 in ,south ,america1 and the ,kilimanjaro1 in ,africa1 #fjaj m4 7#ai1gej ft471 rank among the highest mountains1 are exposed to a rapid destruction by the rain1 because they consist largely of loose materials--volcanic ashes with interposed layers of lava4 ,where these lava streams expand gradually1 they protect the ground underneath from erosion by water1 and in this way proper cuts are formed on the edges of the lava streams1 passing through the old volcano and through the sedimentary strata at deeper levels4 ,illustration3 ,fig4 #f4--,the ,excelsior ,geyser in ,yellowstone ,park1 ,u4 ,s4 ,a4 ,remnant of powerful volcanic activity in the ,tertiary age ,the old volcano of ,monte ,venda1 near ,padua1 affords an interesting example of this type4 ,we can observe there how the sedimentary limestone has been changed by the lava1 which was flowing over it1 into marble to a depth of about #a m4 7#c ft47 ,sometimes the limestone which is lying over the lava has also undergone the same transformation1 which would indicate that lava has not only been flowing above the edge of the crater1 but has also forced itself out on the sides through the fissures between two layers of limestone4 ,massive subterranean lava streams of this kind are found in the so-called lakkolithes of ,utah and in the ,caucasus4 ,there the superior layers have been forced upward by the lava pressing from below2 the lava froze1 however1 before it reached the surface of the earth1 where it might have formed a volcano4 ,quite a number of granites1 the so-called batholithes1 chiefly occurring in ,norway1 ,scotland1 and ,java1 are of similar origin4 ,occasionally it is only the core of congealed lava that has remained of the whole volcano4 ,these cores1 which originally filled the pipe of the crater1 are frequent in ,scotland and in ,north ,america1 where they are designated 8necks0 7,fig4 #g74 ,illustration3 ,fig4 #g4--,mato ,tepee in ,wyoming1 ,u4 ,s4 ,a4 ,typical volcanic 8,neck0 ,the so-called caons of the ,colorado ,plateau1 with their almost vertical walls1 are the results of the erosive action of rivers4 ,a drawing by ,dutton shows a wall of this kind more than #hjj m4 7#bfjj ft47 in height1 through four fissures of which lava streams have forced their way up to the surface 7,fig4 #h74 ,over one of these fissures a small cone of volcanic ashes is still visible1 while the cones which probably overtopped the three other fissures have been washed away1 so that the veins end in small 8necks40 ,evidently a very fluid lava--strong percentages of magnesia and of oxide of iron render the lava more fluid than an admixture of silicic acid1 and the fluidity is further increased by the presence of water--has been forced into the fissures which were already present1 and has reached the surface of the earth before it froze4 ,the driving force behind them must have been pretty strong2 else the lava streams could not have attained the necessary velocity of flow4 ,illustration3 ,fig4 #h4--;,clefts filled with lava and volcanic cone of ashes1 ,torowheap ,caon1 ,plateau of ,colorado4 ,diagram4 ,when the ,krakatoa was blown into the air in #ahhc half of the volcano remained behind4 ,this half clearly shows the section of the cone of ashes1 which has been but very slightly affected by the destructive action of the water4 ,we find there in the central part the light-colored stopper of lava in the volcano pipe1 and issuing from it more light-colored beds of lava1 between which darker strata of ashes can be seen4 ,the distribution of volcanoes over the surface of the earth is marked by striking regularities4 ,almost all the volcanoes are situated near the shores of the sea4 ,a few are found in the interior of ,east ,africa2 but they are1 at any rate1 near the ,great ,lakes of the equatorial regions4 ,the few volcanoes which are supposed to be situated in ,central ,asia must be regarded as doubtful4 ,we miss1 however1 volcanoes on some sea-coasts1 as in ,australia and along the long coast-lines of the ,northern ,arctic ,ocean to the north of ,asia1 ,europe1 and ,america4 ,volcanoes occur only where great cracks occur in the crust of the earth along the sea-coast4 ,where such fissures are found1 but where the sea or large inland lake basins are not near--as1 for instance1 in the ,austrian ,alps--we do not meet with any volcanoes2 such districts are1 however1 renowned for their earthquakes4 ,since ancient ages the belief has been entertained that the molten masses of the interior of the earth find an outlet through the volcanoes4 ,attempts have been made to estimate the depth of the hearths of volcanoes1 but very different values have been deduced4 ,thus1 the hearth under the volcano of ,monte ,nuovo1 which was thrown up in the year #aech on the ,phlegran ,fields1 near ,naples1 has been credited with depths varying from #a4c km4 to #fj km4 7#a mile to #dj miles72 for the ,krakatoa1 estimates of more than #ej km4 7#cj miles7 have been made4 ,all these calculations are rather aimless2 for the volcanoes are probably situated on folds of the earth-crust1 through which the fluid mass 7the magma7 rushes forth in wedges from the interior of the earth1 and it will presumably be very difficult to say where the hearth of magma ends and where the volcanic pipe commences4 ,the ,kilauea gives the visitor the impression that he is standing over an opening in the crust of the earth1 through which the molten mass rushes forth directly from the interior of the earth4 7,fig4 #i47 ,illustration3 ,fig4 #i4--,the ,kilauea ,crater on ,hawaii ,as regards the earth-crust1 we know from observations in bore-holes made in different parts of the world that the temperature increases rather rapidly with the depth1 on an average by about thirty degrees ,cent4 per kilometre 7about #a4f ,f4 per #ajj feet74 ,it must be remarked1 however1 that the depth of our deepest bore-holes hardly exceeds #b km4 7,paruchowitz1 in ,silesia1 #bjjc m41 or #fegj ft42 ,schladebach1 near ,merseburg1 ,prussian ,saxony1 #agbj m474 ,if the temperature should go on increasing at the rate of #cj degrees ,cent4 for each further kilometre1 the temperature at a depth of #dj kilometres should attain degrees at which all the common rocks would melt4 ,but the melting-point certainly rises at the same time as the pressure4 ,the importance of this circumstance was1 however1 much exaggerated when it was believed that for this reason the interior of the earth might possibly be solid4 ,tammann has shown by direct experiments that the temperature of fusion only rises up to a certain pressure1 and that it begins to decrease again on a further increase of pressure4 ,the depths indicated above are therefore not quite correct4 ,if we assume1 however1 that other kinds of rock behave like diabase--the melting-point of which1 according to the determinations of ,barus1 rises by #a ,cent4 for each #dj atmospheres of pressure corresponding to a depth of #aee m4--we should conclude that the solid crust of the earth could not have a greater thickness than #ej or #fj km4 7#dj miles74 ,at greater depths we should therefore penetrate into the fused mass4 ,on account of its smaller density the silicic acid will be concentrated in the upper strata of the molten mass1 while the basic portions of the magma1 which are richer in iron oxide1 will collect in the lower strata1 owing to their greater density4 ,this magma we have to picture to ourselves as an extremely viscid liquid resembling asphalt4 ,the experiments of ,day and ,allen show that rods1 supported at their ends1 of #cj #b #a mm4 of different minerals1 like the feldspars microcline and albite1 could retain their shape for three hours without curving noticeably1 although their temperature was about a hundred degrees above their melting-point1 and although they appeared completely fused1 or1 more correctly1 completely vitrified when taken out of the furnace4 ,these molten silicates behave very differently from other liquids like water and mercury1 with which we are more accustomed to deal4 ,the motion and diffusion in the magma1 and especially in the very viscous and sluggish acid portions of the upper strata1 will therefore be exceedingly small1 and the magma will behave almost like a solid body1 like the minerals of the experiments of ,day and ,allen4 ,the magmas of volcanoes like ,etna1 ,vesuvius1 and ,pantellaria may1 therefore1 have quite different compositions1 as we should conclude from their lavas without our being forced to believe1 with ,stbel1 that these three hearths of volcanoes are completely separated1 though not far removed from one another4 ,in the lava of ,vesuvius a temperature of #ajjj or #aajj degrees has been found at the lower extremity of the stream4 ,from the occurrence in the lava of certain crystals like leucite and olivin1 which we have reason to assume must have been formed before the lava left the crater1 it has been concluded that the lava temperature cannot have been higher than #adjj degrees before it left the volcanic pipe4 ,it would1 however1 be erroneous to deduce from the temperature of the lava of ,vesuvius that the hearth of the volcano must be situated at a depth of approximately #ej kilometres4 ,most likely its depth is much smaller1 perhaps not even #aj kilometres4 ,for there1 as everywhere where volcanoes occur1 the crust of the earth is strongly furrowed1 and the magma will just at the spots where we find volcanoes come much nearer to the surface of the earth than elsewhere4 ,the importance of water for the formation of volcanoes probably lies in the fact that1 in the neighborhood of cracks under the bottom of the sea1 the water penetrates down to considerable depths4 ,when the water reaches a stratum of a temperature of #cfe degrees--the so-called critical temperature of water--it can no longer remain in the liquid state4 ,that would not prevent1 however1 its penetrating still farther into the depths1 in spite of its gaseous condition4 ,as soon as the vapor comes in contact with magma1 it will eagerly be absorbed by the magma4 ,the reason is that water of a temperature of more than #cjj degrees is a stronger acid than silicic acid2 the latter is therefore expelled by it from its compounds1 the silicates1 which form the main constituents of the magma4 ,the higher the temperature1 the greater the power of the magma to absorb water4 ,owing to this absorption the magma swells and becomes at the same time more fluid4 ,the magma is therefore pressed out by the action of a pressure which is analogous to the osmotic pressure by virtue of which water penetrates through a membrane into a solution of sugar or salt4 ,this pressure may become equivalent to thousands of atmospheres1 and this very pressure would raise the magma up the volcanic pipe even to a height of #fjjj m4 7#bj1jjj feet7 above the sea-level4 ,as the magma is ascending in the volcanic pipe it is slowly cooled1 and its capacity for binding water diminishes with falling temperature4 ,the water will hence escape under violent ebullition1 tearing drops and larger lumps of lava with it1 which fall down again as ashes or pumice-stone4 ,after the lava has flown out of the crater and is slowly cooling1 it continues to give off water1 breaking up under the formation of block lava 7see ,fig4 #e74 ,if1 on the other hand1 the lava in the crater of the volcano is comparatively at rest1 as in ,kilauea1 the water will escape more slowly2 owing to the long-continued contact of the surface layer of lava with the air1 little water will remain in it1 the water being1 so to say1 removed by aeration1 and the lava streams will therefore1 when congealing1 form more smooth surfaces4 ,in some cases volcanoes have been proved 7,stbel and ,branco7 not to be in connection with any fractures in the crust of the earth4 ,that holds1 for instance1 for several volcanoes of the early ,tertiary age in ,swabia4 ,we may imagine that the pressure produced by the swelling of the magma became so powerful as to be able to break through the earth-crust at thinner spots1 even in the absence of previous fissures4 ,if1 in our consideration1 we follow the magma farther into the depths1 we shall not find any reason for assuming that the temperature will not rise farther towards the interior of the earth4 ,at depths of #cjj or #djj km4 7#bej miles7 the temperature must finally attain degrees such that no substance will be able to exist in any other state than the gaseous4 ,within this layer the interior of the earth must1 therefore1 be gaseous4 ,from our knowledge of the behavior of gases at high temperatures and pressures1 we may safely conclude that the gases in the central portions of the earth will behave almost like an extremely viscid magma4 ,in certain respects they may probably be compared to solid bodies2 their compressibility1 in particular1 will be very small4 ,we might think that we could not possibly learn anything concerning the condition of those strata4 ,earthquakes have1 however1 supplied us with a little information4 ,such gaseous masses must fill by far the greatest part of the earth1 and they must have a very high specific gravity2 for the average density of the earth is #e4eb1 and the outer strata1 the ocean and the masses of the surface which are known to us1 have smaller densities4 ,the ordinary rocks possess a density ranging from #b4e to #c4 ,it must1 therefore1 be assumed that the materials of the innermost portions of the earth must be metallic1 and ,wiechert1 in particular1 has advocated this view4 ,iron will presumably form the chief constituent of this gas of the central earth4 ,spectrum analysis teaches us that iron is a very important constituent of the sun4 ,we know1 further1 that the metallic portions of the meteorites consist essentially of iron2 and finally terrestrial magnetism indicates that there must be large masses of iron in the interior of the earth4 ,we have also reason to believe that the native iron occurring in nature--e4g41 the well-known iron of ,ovifak1 in ,greenland--is of volcanic origin4 ,the materials in the gaseous interior of the earth will1 owing to their high density1 behave in chemical and physical respects like liquids4 ,as substances like iron will1 also at very high temperatures1 have a far higher specific gravity than their oxides1 and these again have a higher gravity than their silicates1 we have to assume that the gases in the core of the earth will almost exclusively be metallic1 that the outer portions of the core will contain essentially oxides1 and those farther out again mostly silicates4 ,the fused magma will1 on penetrating in the shape of batholithes into the upper layers1 probably be divided into two portions1 of which one1 the lighter and gaseous1 will contain water and substances soluble in it2 while the other1 heavier portion1 will essentially consist of silicates with a lower percentage of water4 ,the more fluid portion1 richer in water1 will be secreted in the higher layers1 will penetrate into the surrounding sedimentary strata1 especially into their fissures1 and will fill them with large crystals1 often of metallurgical value--e4g41 of the ores of tin1 copper1 and other metals1 while the water will slowly evaporate through the superposed strata4 ,the more viscid and sluggish mass of silicates1 on the other hand1 will congeal1 thanks to its great viscosity1 to glass1 or1 when the cooling is very slow1 to small crystals4 ,we now turn to earthquakes4 ,no country has been absolutely spared by earthquakes4 ,in the districts bounding upon the ,baltic1 and especially in northern ,russia1 they have1 however1 been of a quite harmless type4 ,the reason is that the earth-crust there has been lying undisturbed for long geological epochs and has never been fractured4 ,the comparatively severe earthquake which shook the west coast of ,sweden on ,october #bc1 #aijd1 to an unusually heavy degree1 without1 however1 causing any noteworthy damage 7a few chimneys were knocked over71 was caused by a fault of relatively pronounced character for those districts in the ,skager-,rack--a continuation of the deepest fold in the bottom of the ,north ,sea1 the so-called ,norwegian ,trough1 which runs parallel to the ,norwegian coast4 ,in ,germany1 the ,vogtland and the districts on both sides of the middle ,rhine have frequently been visited by earthquakes4 ,of other ,european countries1 ,switzerland1 ,spain1 ,italy1 and the ,balkan ,peninsula1 as well as the ,karst districts of ,austria1 have often suffered from earthquakes4 ,illustration3 ,fig4 #aj4--;,chief earthquake centres1 according to the ,british ,association ,committee ,according to the committee appointed by the ,british ,association for the investigation of earthquakes--a committee which has contributed a great deal to our knowledge of these great natural phenomena--earthquakes of some importance emanate from certain centres which have been indicated on the subjoined map 7,fig4 #aj74 ,the most important among these regions comprises ,farther ,india1 the ,sunda ,isles1 ,new ,guinea1 and ,northern ,australia2 it is marked on the map by the letter ,f4 ,from this district have emanated in the six-year period #ahii-aijd no fewer than #bdi earthquakes1 which have been recorded in many observatories far removed from one another4 ,this earthquake centre ,f is closely related to the one marked ,e1 in ,japan1 from which #ahi earthquakes have proceeded4 ,next to this comes the extensive district ,k with #agd earthquakes1 comprising the most important folds in the crust of the ,old ,world1 including the mountain chains from the ,alps to the ,himalaya4 ,this district is interesting1 because it has been disturbed by a great many earthquakes1 although it is almost entirely situated on the ,continent4 ,after that we have the districts ,a1 ,b1 ,c1 with #abe1 #ih1 and #ie earthquakes4 ,they are situated near lines of fracture in the earth-crust along the ,american coast of the ,pacific ,ocean and the ,caribbean ,sea4 ,district ,d1 with #gh earthquakes1 is similarly situated4 ,the three last-mentioned districts1 ,b1 ,c1 ,d1 as well as ,g1 between ,madagascar and ,india1 with #he earthquakes1 all seem to be surpassed by the district ,h in the eastern ,atlantic1 with its #ajg earthquakes4 ,these latter are1 however1 relatively feeble1 and we owe their accurate records probably to the circumstances that a great many earthquake observatories are situated within the immediate surroundings of this district4 ,the same may be said of the district ,i1 or ,newfoundland1 which is not characterized by many earthquakes1 and of the district ,j1 between ,iceland and ,spitzbergen1 with #ca and #ai earthquakes respectively4 ,the last on the list used to be the district ,l1 situated about the ,south ,pole1 with only eight earthquakes4 ,this small number is probably merely due to the want of observatories in those parts of the earth4 ,another district1 ,m1 has finally been added1 which extends to the southwest from ,new ,zealand4 ,no fewer than #ge intense earthquakes were recorded between ,march #ad and ,november #bc1 #aijc1 by the ,discovery ,expedition1 in #gj southern latitude and #agh eastern longitude4 ,earthquakes commonly occur in swarms or groups4 ,thus1 more than #bjjj shocks were counted on ,hawaii in ,march1 #ahfh4 ,during the earthquakes which devastated the district of ,phokis1 in ,greece1 in #ahgj-gc1 shocks succeeded one another for a long time at intervals of three seconds4 ,during the whole period of three and a half years about half a million shocks were counted1 and1 further1 a quarter of a million subterranean reports which were not accompanied by noticeable concussions4 ,yet of all these shocks only about #cjj did noteworthy damage1 and only #ce were considered worth being reported in the newspapers4 ,the concussion of ,october #bc1 #aijd1 belonged to a group which lasted from ,october #aj to ,october #bh1 and in which numerous small tremors were noticed1 especially on ,october #bd and #be4 ,the earthquake of ,san ,francisco commenced on ,april #ah1 #aijf1 at #e hrs4 #ab min4 #f sec4 ,a4,m4 7,pacific ,ocean time71 and ended at #e hrs4 #ac min4 #aa sec1 lasting therefore #a minute and #e seconds4 ,twelve smaller shocks succeeded in the following hour4 ,before #f hrs4 #eb min4 ,p4,m41 nineteen further concussions were counted1 and various smaller shocks succeeded in the following days4 ,with such groups of earthquakes weaker tremors usually precede the violent destructive shocks and give a warning4 ,unfortunately this is not always so1 and no warning was given by the earthquakes which destroyed ,lisbon in #agee and ,caracas in #ahab1 nor by those which devastated ,agram in #ahhj1 nor1 finally1 in the case of the ,san ,francisco disaster4 ,a not very severe earthquake without feebler precursors befell ,ischia in #ahha1 while the violent catastrophe which devastated this magnificent island in #ahhc was heralded by several warnings4 ,as in ,san ,francisco and ,chili in #aijf1 less violent concussions generally succeed the destructive shocks4 ,earthquakes like that of ,lisbon in #agee1 consisting of a single shock1 are very rare4 ,the violent concussions often produce large fissures in the ground4 ,such were noticed in several places at ,san ,francisco4 ,one of the largest fissures known1 that of ,midori1 in ,japan1 was caused by the earthquake of ,october #bj1 #ahia4 ,it left a displacement of the ground ranging up to #f m4 7#bj ft47 in the vertical and #d m4 7#ac ft47 in the horizontal direction4 ,this crack had a length of not less than #fe km4 7#dj miles74 ,extensive fissures were also formed by the earthquakes of ,calabria1 in #aghc1 at ,monte ,san ,angelo1 and in the sandstones of the ,blpakrm ,plateau in ,india1 in #ahig4 ,in mountainous districts falls of rock are a frequent consequence of the formation of fissures and earthquakes4 ,a large number of rocks fell in the neighborhood of ,delphi during the ,phokian earthquake4 ,on ,january #be1 #acdh1 an earthquake sent down a large portion of ,mount ,dobratsch 7in the ,alps of ,villach1 in ,carinthia1 which is now much frequented by tourists7 and buried two towns and seventeen villages4 ,the earthquake of ,april #ah1 #aijf1 in ,california started from a crack which extends from the mouth of ,alder ,creek1 near ,point ,arena1 running parallel with the coast-line mostly inland1 then entering the sea near ,san ,francisco1 and turning again inland between ,santa ,cruz and ,san ,jos1 finally proceeding via ,chittenden up to ,mount ,pinos1 a distance of about #fjj km4 7#djj miles71 in the direction of ,n4 #ce ,w4 to ,s4 #ce ,e4 ,along this crack the two masses of the earth have been displaced so that the ground situated to the southwest of the fissure has been moved by about #c m4 7#aj ft471 and in some spots even by #f m4 7#bj ft47 towards the northwest4 ,in some localities in ,sonoma and ,mendocino counties the southwestern part has been raised1 but nowhere by more than #a4b m4 7#d ft474 ,this is the longest crack which has ever been noticed in connection with an earthquake4 ,illustration3 ,fig4 #aa4--;,clefts in ,valentia ,street1 ,san ,francisco1 after the earthquake of #aijf ,the earthquake over1 the ground does not always return to its original position1 but remains in a more or less wavy condition4 ,this can most easily be observed in districts where streets or railways cross the ground4 ,it is reported1 for instance1 that the track of the tramway-lines in ,market ,street1 the chief thoroughfare of ,san ,francisco1 formed large wavelike curves after the earthquake4 ,as a consequence of the displacements in the interior of the earth and of the formation of fissures1 river courses are changed1 springs become exhausted1 and new springs arise4 ,that was the case1 for instance1 in ,california in #aijf4 ,the ground water often rushes out with considerable violence1 tearing with it sand and mud and stones1 and piling them up1 occasionally forming little craters 7,fig4 #ab74 ,extensive floods may also be caused on such occasions4 ,by such a flood the ancient ,olympia was submerged under a layer of river sand which for some time preserved from destruction the ancient ,greek masterpieces of art--among them the famous statue of ,hermes4 ,the floods afterwards receded1 and the treasures of ancient ,olympia could be excavated4 ,like the natural water channels and arteries in the interior of the earth1 water mains are displaced by the concussions4 ,the direct damage caused by the floods is often less important than the damage due to the impossibility of extinguishing the fires which follow the destruction of the buildings4 ,it was the fires that did most of the enormous material damage in the destruction of ,san ,francisco4 ,still greater devastation is wrought by the ocean waves thrown up by earthquakes4 ,we have already referred to the flood of ,lisbon in #agee1 which was felt on the western coast of ,norway and ,sweden4 ,another wave1 in #aeaj1 devoured #aji mosques and #ajgj houses in ,constantinople4 ,another wave1 again1 invaded ,kamashi1 in ,japan1 on ,june #ae1 #ahif1 swept away #gfjj houses and killed #bg1jjj people4 ,we have repeatedly alluded to the disastrous flood-wave of ,krakatoa of #ahhc4 ,this wave traversed the whole of the ,indian ,ocean1 passing to the ,cape of ,good ,hope and ,cape ,horn1 and travelled round half the globe afterwards4 ,even more remarkable was the aerial wave1 which spread like an explosion wave4 ,illustration3 ,fig4 #ab4--,sand craters and fissures1 produced by the ,corinth earthquake of #ahfa4 ,in the water1 branches of flooded trees ,while the most violent cannonades are rarely heard for more than #aej km4 7#ie miles7--in a single case at a distance of #bgj km4 7#agj miles7--the eruption of ,krakatoa was heard at ,alice ,springs1 at a distance of #cfjj kilometres1 and on the island of ,rodriguez1 at almost #dhjj km4 7#cjjj miles74 ,the barographs of the meteorological stations first marked a sudden rise and then a decided sinking of the air pressure1 succeeded by a few smaller fluctuations4 ,these air pulses were repeated in some places as many as seven times4 ,we may therefore assume that the aerial wave passed these places three times in the one direction1 and three times in the other1 travelling round the earth4 ,the velocity of propagation of this wave was #cad4b m4 7#ajcj ft47 per second1 corresponding to a temperature of -#bg ,cent4 7#ag ,f47 which prevails at an altitude of about #h km4 7#e miles7 above the earth's surface1 at which altitude this wave may have travelled4 ,within the last decade a peculiar phenomenon 7leading to what is designated variation of latitudes7 has been studied4 ,the poles of the axis of the earth appear to move in a very irregular curve about their mean axis4 ,the movement is exceedingly small4 ,the deviation of the ,north ,pole from its mean position does not amount to more than #aj m4 7about #cc ft474 ,it has been believed that these motions of the ,north ,pole are subject to sudden fluctuations after unusually violent earthquakes1 especially when such concussions follow at rapid intervals4 ,that would give us1 perhaps more than any other observation1 an idea of the force of earthquakes1 since they would appear to be able to disturb the equilibrium of the whole mass of our globe4 ,a severely felt effect of earthquakes1 though most people perhaps pay little attention to it1 is the destruction of submarine cables4 ,the gutta-percha sheaths of cables are frequently found in a fused condition1 suggesting volcanic eruptions under the bottom of the sea4 ,we take care now to avoid earthquake centres in laying telegraphic cables4 ,their positions have been ascertained by the most modern investigations 7see ,fig4 #aj74 ,people have always been inclined to look for a connection between earthquakes and volcanic eruptions4 ,the connection is unquestionable in a large number of violent earthquakes4 ,in order to establish it1 the above-mentioned committee of the ,british ,association has compiled the following table of the history of the earthquakes of the ,antilles3 #afib4--,port ,royal1 ,jamaica1 destroyed by an earthquake2 land sinking into the sea4 ,eruption on ,st4 ,kitts4 #agah4--,terrible earthquake on ,st4 ,vincent1 followed by an eruption4 #agff-fg4--;,great shocks in northeastern ,south ,america1 in ,cuba1 ,jamaica1 and the ,antilles4 ,eruption on ,santa ,lucia4 #agig4--;,earthquake in ,quito1 loss of #dj1jjj lives4 ,concussions in the ,antilles1 eruption on ,guadeloupe4 #ahjb4--,violent shocks in ,antigua4 ,eruption on ,guadeloupe4 #ahab4--;,caracas1 capital of ,venezuela1 totally destroyed by earthquake4 ,violent shocks in the ,southern ,states of ,north ,america1 commencing on ,november #aa1 #ahaa4 ,eruptions on ,st4 ,vincent and ,guadeloupe4 #ahce-cf4--,violent concussions in ,chili and ,central ,america4 ,eruption on ,guadeloupe4 #aijb4--;,april #ai4 ,violent shocks1 destroying many towns of ,central ,america4 ,mont ,pele1 on ,martinique1 in activity4 ,eruption on ,may #c4 ,submarine cables break1 sea recedes4 ,renewed violent movements of the sea on ,may #h1 #ai1 #bj4 ,eruption on ,st4 ,vincent1 cable destroyed on ,may #g4 ,violent eruption of ,mont ,pele on ,may #h4 ,destruction of ,st4 ,pierre4 ,numerous smaller earthquakes4 ,this table distinctly marks the restless state of affairs in that part of the earth1 and how quiet and safe matters are comparatively in old ,europe1 especially in the north4 ,some parts of ,central ,america are so persistently visited by earthquakes that one of them1 ,salvador1 has been christened 8,schaukelmatte40 ,it is not saying too much to assert that the earth is there incessantly trembling4 ,other districts which are very frequently visited are the ,kuriles and ,japan1 as well as the ,east ,indian islands4 ,in all these countries the crust of the earth has been broken and folded within comparatively recent epochs 7chiefly in the ,tertiary age7 by numerous fissures1 and their compression is still going on4 ,illustration3 ,fig4 #ac4--;,earthquake lines in lower ,austria ,the smaller earthquakes1 of which not less than #cj1jjj are counted in the course of a year1 do not stand in any closer relation to volcanic eruptions4 ,this is also the case for a number of large earthquakes1 among which we have to count the ,san ,francisco earthquake4 ,it is assured with good reason that earthquakes are often produced at the bottom of the sea1 where there is a strong slope1 by slips of sedimentary strata which have been washed down from the land into the sea in the course of centuries4 ,milne believes that the seaquake of ,kamashi of ,june #ae1 #ahif1 was of this character4 ,concussions may even be promoted by the different loading of the earth resulting from the fluctuations in the pressure of the air above it4 ,smaller1 though occasionally rather violent1 earthquakes are not infrequent in the neighborhood of ,vienna4 ,on the map 7,fig4 #ac7 we see three lines4 ,the line ,a ,b is called the thermal line1 because along it a number of hot springs1 the therm of ,meidling1 ,baden1 ,vslau1 etc41 are located1 which are highly valued2 the other line ,b ,c is called the ,kamp line1 because it is traversed by the river ,kamp2 and the third ,b ,f is called the ,mrz line1 after the river ,mrz4 ,the main railway-track between ,vienna and ,bruck follows the valleys of ,a ,b and ,e ,f4 ,these lines1 which probably correspond to large fissures in the earth-crust1 are known as sources of numerous earthquakes4 ,the district about ,wiener ,neustadt1 where the three lines intersect1 is often shaken by violent earthquakes2 some of their dates have been marked on the map4 ,the curve which is indicated by the letters ,x ,x on the map marks the outlines of an earthquake which started on ,january #c1 #ahgc1 from both sides of the ,kamp line4 ,it is striking to see how the earthquake spread in the loose ground of the plain between ,st4 ,plten and ,tulln1 while the masses of rock situated to the northwest and southeast formed obstacles to the propagation of the earthquake waves4 ,illustration3 ,fig4 #ad4--,library building of ,leland ,stanford ,junior ,university1 in ,california1 after the earthquake of #aijf4 ,the photograph shows the great strength of iron structures in comparison to the strength of brickwork4 ,the effect of the earthquake on wooden structures can be seen in ,fig4 #aa ,similar conclusions have been deduced from the study of the spreading of the waves which destroyed ,charleston1 ,south ,carolina1 in #ahhf4 ,twenty-seven lives were destroyed by this shock4 ,it was the most terrible earthquake that ever visited the ,united ,states before the year #aijf4 ,in the ,charleston concussion the ,alleghany ,mountains proved a powerful bar against the further propagation of the shocks1 which all the more easily travelled in the loose soil of the ,mississippi ,valley4 ,in ,san ,francisco1 likewise1 the worst devastation fell upon those parts of the town which had been built upon the loose1 partly made ground in the neighborhood of the harbor1 while the buildings erected on the famous mountain ridges of ,san ,francisco suffered comparatively little damage1 in so far as they were not reached by the destructive fires4 ,as regards the destructive effects of the earthquake in ,san ,francisco1 the building-ground of that city has been divided into four classes 7the first is the safest1 the last the most unsafe7--namely3 #a4 ,rocky soil4 #b4 ,valleys situated between rocks and filled up by nature in the course of time4 #c4 ,sand-dunes4 #d4 ,soil created by artificial filling up4 ,this latter soil 8behaved like a semiliquid jelly in a dish10 according to the report of the ,earthquake ,commission4 ,for similar reasons the sky-scrapers1 constructed of steel on deep foundations1 stood firmest4 ,after them came brick houses1 with well-joined and cemented walls on deep foundations4 ,the weakness of wooden houses proved mainly due to the poor connection of the beams1 a defect which might easily be remedied4 ,the superiority of the steel structure will be apparent from the illustrations 7,figs4 #aa and #ad74 ,the spots situated just over the crack1 of which we spoke on page #be1 suffered the most serious damage4 ,next to them1 devastation befell especially localities which1 like ,santa ,rosa1 ,san ,jos1 and ,palo ,alto with ,leland ,stanford ,junior ,university1 are situated on the loose soil of the valley1 whose deepest portions are covered by the bay of ,san ,francisco4 ,the splendidly endowed ,california ,university1 in ,berkeley1 and the famous ,lick ,observatory1 both erected on rocky ground1 fortunately escaped without any notable damage4 ,the map sketch 7,fig4 #ae7 by ,suess represents the earthquake lines of ,sicily and ,calabria4 ,these districts have1 as mentioned before1 been devastated by severe earthquakes1 of which the most terrible occurred in the year #aghc1 and again in #aije and #aijg4 ,they have also been the scene of many smaller concussions4 ,illustration3 ,fig4 #ae4--;,earthquake lines in the ,tyrrhenian depression ,the bottom of the ,tyrrhenian ,sea--between ,italy1 ,sicily1 and ,sardinia--has been lowered in rather recent ages and is still sinking4 ,we notice on the map five dotted lines1 corresponding to cracks in the crust of the earth4 ,these lines would intersect in the volcanic district of the ,lipari ,islands4 ,we further see a dotted circular arc corresponding to a fissure which is regarded as the source of the ,calabrian earthquakes of #aghc1 #aije1 and #aijg4 ,the earth-crust behaved somewhat after the manner of a windowpane which was burst by a heavy impact from a point corresponding to the ,island of ,lipari4 ,from this point radiate lies of fracture1 and fragments have been broken off from the earth-crust by arc-shaped cracks4 ,the volcano ,etna is situated on the intersection of the radial and circular fissures4 ,illustration3 ,fig4 #af4--,seismogram recorded at ,shide1 ,isle of ,wight1 on ,august #ca1 #ahih ,in recognition of the high practical importance of earthquake observations1 seismological stations have in recent days been erected in many localities4 ,at these observatories the earthquakes are recorded by pendulums whose styles draw lines on tapes of paper moved by clock-work4 ,as long as the earth is quiet the drawn line is straight4 ,when earthquakes set in1 the line passes into a wavy curve4 ,as long as the movement of the paper is slow1 the curve merely looks like a widened straight line4 ,the subjoined illustration 7,fig4 #af7 represents a seismogram taken at the station of ,shide1 on the ,isle of ,wight1 on ,august #ca1 #ahih4 ,the earthquake recorded originated in the ,centre ,g1 in the ,indian ,ocean4 ,the origin has been deduced from the moments of arrival of the different waves at different stations4 ,we notice on the seismogram a faint widening of the straight line at #bj hrs4 #e min4 #b sec4 7#h hrs4 #e min4 #b sec4 ,p4,m474 ,the amplitude of the oscillations then began to widen1 and the heaviest concussions were noticed at #bj hrs4 #cf min4 #be sec41 and #bj hrs4 #db min4 #di sec41 after which the amplitudes slowly decreased with smaller shocks4 ,the first shock of #bj hrs4 #e min4 #b sec4 is called the preliminary tremor4 ,this tremor passes through the interior of the earth at a velocity of propagation of #i4b km4 7#e-c_/d miles7 per second4 ,it would require twenty-three minutes to pass through the earth along a diameter4 ,the tremor is very feeble1 which is ascribed to the extraordinarily great friction characteristic of the strongly heated gases which are confined in the interior of the earth4 ,the principal violent shock at #bj hrs4 #cf min4 #be sec4 was caused by a wave travelling through the solid crust of the earth4 ,the intensity of this shock is much less impaired than that of the just-mentioned tremor1 and it travels with the smaller velocity of about #c4d km4 7#b4a miles7 along the earth's surface4 ,the velocity of propagation of concussion pulses has been calculated for a mountain of quartz1 in which it would be #c4f km4 7#b4b miles7 per second1 very nearly the same as the last-mentioned figure4 ,we should expect this1 since the firm crust of the earth consists essentially of solid silicates--i4e41 compounds of quartz endowed with similar properties4 ,measured at small distances from the origin1 the velocity of propagation of the wave appears smaller1 and the first preliminary tremor is frequently not observed4 ,the velocity may be diminished to #b km4 7#a-a_/d miles7 per second4 ,the reason is that the pulse partly describes a curve in the more solid portions of the crust1 and partly passes through looser strata1 through which the wave travels at a much slower rate than in firm ground2 for instance1 at #a4b km4 through loose sandstones1 at #a4d km4 through the water of the ocean1 and at #j4c km4 through loose sand4 ,we recognize that it should be possible to calculate the distance between the point of observation and the origin of the earthquake from the data relating to the arrivals of the first preliminary tremor and of the principal shock of maximum amplitude4 ,the violent shock is sometimes repeated after a certain time1 though with decreased intensity4 ,it has often been observed that this secondary1 less violent1 shock seems to have travelled all round the earth via the longest road between the origin and the point of observation1 just like one portion of the aerial waves in the eruption of ,krakatoa 7compare page #bg74 ,the velocity of propagation of this secondary shock is the same as that of the principal shock4 ,milne has deduced from his observations that1 when the line joining the origin of the earthquake and the point of observation does not at its lowest level descend deeper than #ej km4 below the surface of the earth1 the pulse will travel undivided through the solid crust of the earth4 ,for this reason we estimate the thickness of the solid crust at #ej km4 ,the value is in almost perfect agreement with the one which we had 7on page #af7 derived from the increase of temperature with greater depths4 ,it should further be mentioned1 perhaps1 that the density of the earth in the vicinity has been determined from pendulum observation1 and that this density seems to be rather variable down to the depths of #ej or #fj km41 but to become more uniform at greater depths4 ,these #ej or #fj km4 7#ca or #cg miles7 would belong to the solid crust of the earth4 ,the movement of earthquake shocks through the earth thus teaches us that the solid earth-crust cannot be very thick1 and that the core of the earth is probably gaseous4 ,the similar conclusions1 to which these various considerations had led us1 may therefore come very near the truth4 ,a careful study of seismograms may1 we hope1 help us to learn more about the central portions of the earth1 which at first sight appear to be absolutely inaccessible to scientific research4 ,,ii ,,the ,,celestial ,,bodies1 ,,in ,,particular ,,the ,,earth1 ,,as ,,abodes ,,of ,,organisms ,there is no more elevating spectacle than to contemplate the sky with its thousands of stars on a clear night4 ,when we send our thoughts to those lights glittering in infinite distance1 the question forces itself upon us1 whether there are not out there planets like our own that will sustain organic life4 ,how little interest do we take in a barren island of the ,arctic ,circle1 on which not a single plant will grow1 compared to an island in the tropics which is teeming with life in its most wonderful variety6 ,the unknown worlds occupy our minds much more when we may fancy them inhabited than when we have to regard them as dead masses floating about in space4 ,we have to ask ourselves similar questions with regard to our own little planet1 the earth4 ,was it always covered with verdure1 or was it once sterile and barren8 ,and if that be so1 what are the conditions under which the earth can fulfil its actual part of harboring organic life8 ,that 8the earth was without form0 in the beginning is unquestionable4 ,it does not matter whether we assume that it was once all through an incandescent liquid1 which may be the most probable assumption1 or that it was1 as ,lockyer and ,moulton think1 formed by the accumulation of meteoric stones which became incandescent when arrested in their motion4 ,we have seen that the earth probably consists of a mass of gas encased within a shell which is solid on the outside and remains a viscid liquid on the inner side4 ,we presume with good reason that the earth was originally a mass of gas separated from the sun1 which is still in the same state4 ,by radiation into cold space the sphere of gas which1 on the whole1 would behave as our sun does now1 would gradually lose its high temperature1 and finally a solid crust could form on its surface4 ,lord ,kelvin has calculated that it would not require more than one hundred years before the temperature of this crust would sink to #ajj4 ,supposing1 even1 that ,kelvin's calculations should not quite be confirmed1 we may yet maintain that not many thousands of years would have elapsed from the time when the earth assumed its first crust at about #ajjj till the age when this temperature had fallen below #ajj 7#bab ,f474 ,living beings certainly could not exist so long1 since the albumen of the cells would at once coagulate at the temperature of boiling water1 like the white of an egg4 ,yet it has been reported that some of the hot springs of ,new ,zealand contain alg1 although at a temperature of over #hj4 ,when ,i went to ,yellowstone ,park to inquire into the correctness of this statement1 ,i found that the alg existed only at the edge of the hot springs1 where the temperature did not exceed #fj 7#adj ,f474 ,the famous ,american physiologist ,loeb states that we do not meet with alg in hot springs at temperatures above #ee4 ,since1 now1 the temperature of the earth-crust would much more quickly sink from #ajj to #ee than it had fallen from #ajjj to #ajj1 we may imagine that only a few thousands of years may have intervened between the formation of the first crust of the earth and the cooling down to a temperature such as would sustain life4 ,since that time the temperature has probably never been so low that the larger portion of the earth's surface would not have been able to support organisms1 although there have been several glacial ages in which the arctic districts inaccessible to life must have extended much farther than at present4 ,the ocean will also have been free of ice over much the greatest portion of its surface at all times1 and may therefore have been inhabited by organisms in all ages4 ,the interior of the earth cools continually1 though slowly1 because heat passes from the inner1 warmer portions to the other1 cooler portions through the crust of the earth4 ,the earth is able to serve as the abode of living beings because its outer portions are cooled to a suitable temperature 7below #ee7 by radiation1 and because the cooling does not proceed so far that the open sea would continually be frozen over1 and that the temperature on the ,continent would always remain below freezing-point4 ,we owe this favorable intermediate stage to the fact that the radiation from the sun balances the loss of heat by radiation into space1 and that it is capable of maintaining the greater portion of the surface of the earth at a temperature above the freezing-point of water4 ,the temperature conditioning life on a planet is therefore maintained only because1 on the one side1 light and heat are received by radiation from the sun in sufficient quantities1 while on the other side an equivalent radiation of heat takes place into space4 ,if the heat gain and the heat loss were not to balance each other1 the term of suitable conditions would not last long4 ,the temperature of the earth-crust could sink in a few hundreds or thousands of years from #ajjj to #ajj1 because when the earth was at this high temperature its radiation into space predominated over the radiation received from the sun4 ,on the other hand1 about a hundred million years have passed1 according to ,joly1 since the age when the ocean originated4 ,the temperature of the earth1 therefore1 required this long space of time in order to cool down from #cfe 7at which temperature water vapor can first be condensed to liquid water7 to its present temperature4 ,the cooling afterwards proceeded at a slower rate1 because the difference between the radiations inward and outward was lessened with the diminishing temperature of the earth4 ,various methods have been applied in estimating these periods4 ,joly based his estimate on the percentage of salt in the sea and in the rivers4 ,if we calculate how much salt there is in the sea1 and how much salt the rivers can supply to it in the course of a year1 we arrive at the result that the quantity of salt now stored in the ocean might have been supplied in about a hundred million years4 ,we arrive at still higher numbers when we calculate the time which must have elapsed during the deposition of all the stratified and sedimentary layers4 ,sir ,archibald ,geikie estimates the total thickness of those strata1 supposing them to have been undisturbed1 at #cj1jjj m4 7nearly #bj miles74 ,he concludes1 further1 from the examination of more recent strata1 that every stratum one metre in thickness must have required from #cjjj to #bj1jjj years for its formation4 ,we should1 therefore1 have to allow a space of from ninety to six hundred million years for the deposition of all the sedimentary strata4 ,the ,finnish geologist ,sederholm even fixes the time at a thousand million years4 ,another method again starts from the consideration that1 while the temperature of the surface of the earth remains fairly steady owing to the heat exchange between solar radiation and terrestrial radiation into space1 the interior of the earth must have shrunk with the cooling4 ,how far this shrinkage extends we may estimate from the formation of the mountain chains which1 according to ,rudzki1 cover #a4f per cent4 of the earth's surface4 ,the earth's radius should consequently have contracted by about #j4h per cent41 corresponding to a cooling through about #cjj1 which would require two thousand million years4 ,quite recently the renowned physical chemist ,rutherford has expounded a most original method of estimating the age of minerals4 ,uranium and thorium are supposed to produce helium by their slow dissociation1 and we know how much helium is produced from a certain quantity of uranium or thorium in a year4 ,now ,ramsay has determined the percentage of helium in the uranium mineral fergusonite and in thorianite4 ,rutherford then calculates the time which would have passed since the formation of these minerals4 ,he demands at least four hundred million years1 8for very probably some helium has escaped from the minerals during that time40 ,although this estimate is very uncertain1 it is interesting to find that it leads to an age for the solid earth-crust of the same order of magnitude as the other methods4 ,during this whole epoch of almost inconceivable length of between one hundred million and two thousand million years1 organisms have existed on the surface of the earth and in the sea which do not differ so very much from those now alive4 ,the temperature of the surface may have been higher than it is at present2 but the difference cannot be very great1 and will amount to #bj ,cent4 7#cf ,f47 at the highest4 ,the actual mean temperature of the surface of the earth is #af ,cent4 7#fa ,f474 ,it varies from about -#bj ,cent4 7-#d ,f47 at the ,north ,pole1 and -#aj ,cent4 7#ad ,f47 at the ,south ,pole to #bf ,cent4 7#gi ,f47 in the tropical zone4 ,the main difference between the temperatures of the earth's surface in the most remote period from which fossils are extant and the actual state rather seems to be that the different zones of the earth are now characterized by unequal temperatures1 while in the remote epochs the heat was almost uniformly distributed over the whole earth4 ,the condition for this prolonged1 almost stationary state was that the gain of heat of the earth's surface by radiation from the sun and the loss of heat by radiation into space nearly balanced each other4 ,that the replenishing supply by radiation from an intensely hot body--in our case the sun--is indispensable for the existence of life will be evident to everybody4 ,not everybody may1 however1 have considered that the loss of heat into cold space or into colder surroundings is just as indispensable4 ,to some people1 indeed1 the assumption that the earth as well as the sun should waste the largest portions of their vital heat as radiation into cold space appears so unsatisfactory that they prefer to believe radiation to be confined to radiation between celestial bodies2 there is no radiation into space1 in their opinion4 ,all the solar heat would thus benefit the planets and the moons in the solar system1 and only a vanishing portion of it would fall upon the fixed stars1 because their visual angles are so small4 ,if that were really correct1 the temperature of the planets would rise at a rapid rate until it became almost equal to that of the sun1 and all life would become impossible4 ,we are therefore constrained to admit that 8things are best as they are10 although the great waste of solar heat certainly weakens the solar energy4 ,the opinion that all the solar heat radiated into infinite space is wasted1 starts moreover from a hypothesis which is not proved1 and which is highly improbable--namely1 that only an extremely small portion of the sky is covered with celestial bodies4 ,that might certainly be correct if we assumed1 as has formerly been done1 that the majority of the celestial bodies must be luminous4 ,we do not possess1 however1 any reliable knowledge of the number and size of the dark celestial bodies4 ,in order to account for the observed movements of different stars1 it has been thought that there must be in the neighborhood of some of them dark stars of enormous size whose masses would surpass the mass of our sun1 or1 at least1 be equal to it4 ,but the largest number of the dark celestial bodies which hide the rays from the stars behind them probably consist of smaller particles1 such as we observe in meteors and in comets1 and to a large extent of so-called cosmical dust4 ,the observations of later years1 by the aid of most powerful instruments1 have shown that so-called nebul and nebulous stars abound throughout the heavens4 ,in their interior we should probably find accumulations of dark masses4 ,the light intensity of most of the nebul is1 moreover1 far too weak to permit of their being perceived4 ,we have1 therefore1 to imagine that there are bodies all through infinite space1 and about as numerous as they are in the immediate neighborhood of our solar system4 ,thus every ray from the sun1 of whatever direction1 would finally hit upon some celestial body1 and nothing would be lost of the solar radiation1 nor of the stellar radiation4 ,as regards the radiation-heat exchange1 the earth might be likened to a steam-engine4 ,in order that the steam-engine shall perform useful work1 it is necessary not only that the engine be supplied with heat of high temperature from a furnace and a boiler1 but also that the engine be able to give its heat up again to a heat reservoir of lower temperature--a condenser or cooler4 ,it is only by transferring heat from a body of higher temperature to another body of lower temperature that the engine can do work4 ,in a similar way no work can be done on the earth1 and no life can exist1 unless heat be conferred by the intermediation of the earth from a hot body1 the sun1 to the colder surroundings of universal space--i4e41 to the cold celestial bodies in it4 ,to a certain extent the temperature of the earth's surface1 as we shall presently see1 is conditional by the properties of the atmosphere surrounding it1 and particularly by the permeability of the latter for the rays of heat4 ,if the earth did not possess an atmosphere1 or if this atmosphere were perfectly diathermal--i4e41 pervious to heat radiations--we should be able to calculate the mean temperature of the earth's surface1 given the intensity of the solar radiation1 from ,stefan's law of the dependence of heat radiation on its temperature4 ,starting from the not improbable assumption that1 at a mean distance of the earth from the sun1 the solar rays would send #b4e gramme-calories per minute to a body of cross section of #a sq4 centimetre at right angles to the rays of the sun1 ,christiansen has calculated the mean temperatures of the surfaces of the various planets4 ,the following table gives his figures1 and also the mean distances of the planets from the sun1 in units of the mean distance of the earth from the sun1 #adi4e million km4 7nearly #ic million miles73 ,radius ,mass ,mean ,mean ,density ,planet distance temperature according ,according to ,see to ,see ,mercury #j4cda #j4jbbd #j4ci #agh7ccb7 #j4efd ,venus #j4iee #j4hae #j4gb #fe #j4icf ,earth #a #a #a #f4e #a ,moon #j4bgc #j4jabbh #a #f4e7aje7 #j4fjd ,mars #j4ec #j4ajgg #a4eb - #cg #j4gbi ,jupiter #aa4ac #cag4g #e4b - #adg #j4bcj ,saturn #i4ce #ie4a #i4ee - #ahj #j4aaf ,uranus #c4ce #ad4f #ai4bb - #bjg #j4chh ,neptune #c4dc #ag4b #cj4ab - #bba #j4dbi ,sun #aji4a #ccb1gej #j #fbjj #j4bef ,in the case of ,mercury1 ,i have added another figure1 #ccb4 ,mercury always turns the same side to the sun1 and the hottest point of this side would reach a temperature of #cig2 its mean temperature1 according to my calculation1 is #ccb1 while the other side1 turned away from the sun1 cannot be at a temperature much above absolute zero1 -#bgc4 ,i have made a similar calculation for the moon1 which turns so slowly about its axis 7once in twenty-seven days7 that the temperature on the side illuminated by the sun remains almost as high 7#ajf7 as if the moon were always turning the same face to the sun4 ,the hottest point of this surface would attain a temperature of #aej1 while the poles of the moon and that part of the other side which remains longest without illumination can1 again1 not be much above absolute zero temperature4 ,this estimate is in fair agreement with the measurements made of the lunar radiation and the temperature estimate based upon it4 ,the first measurement of this kind was made by the ,earl of ,rosse4 ,he ascertained that the moon disk as illuminated by the sun--that is to say1 the full moon--would radiate as much heat as a black body of the temperature #aaj ,cent4 7#bcj ,f474 ,a later measurement by the ,american ,very seems to indicate that the hottest point of the moon is at about #ahj1 which would be #cj higher than my estimate4 ,in the cases of the moon and of ,mercury1 which do not possess any atmosphere to speak of1 this calculation may very fairly agree with the actual state of affairs4 ,the temperature of the planet ,venus would be about #fe ,cent4 7#adi ,f47 if its atmosphere were perfectly transparent4 ,we know1 however1 that dense clouds1 probably of water drops1 are floating in the atmosphere of this planet1 preventing us from seeing its land and water surfaces4 ,according to the determinations made by ,zllner and others1 ,venus would reflect not less than #gf per cent4 of the incident light of the sun1 and the planet would thus be as white as a snow-ball4 ,the rays of heat are not reflected to the same extent4 ,we may estimate that the portion of heat absorbed by the planet is about half the incident heat4 ,the temperature of ,venus will therefore be reduced considerably1 but it is partly augmented again by the protective action of this atmosphere4 ,the mean temperature of ,venus may1 hence1 not differ much from the calculated temperature1 and may amount to about #dj 7#ajd ,f474 ,under these circumstances the assumption would appear plausible that a very considerable portion of the surface of ,venus1 and particularly the districts about the poles1 would be favorable to organic life4 ,passing to the earth1 we find that the temperature-reducing influence of the clouds must be strong4 ,they protect about half of the earth's surface 7#eb per cent47 from solar radiation4 ,but even with a perfectly clear sky1 not all the light from the sun really reaches the earth's surface2 for finely distributed dust is floating even in the purest air4 ,i have estimated that this dust would probably absorb #ag per cent4 of the solar heat4 ,clouds and dust would therefore together deprive the earth of #cd per cent4 of the heat sent to it1 which would lead to a reduction of the temperature by about #bh4 ,dust and the water-bubbles in the clouds also prevent the radiation of heat from the earth1 so that the total loss of heat to be charged to clouds and dust will amount to about #bj 7#cf ,f474 ,it has now been ascertained that the mean temperature of the earth is #af 7#fa ,f471 instead of the calculated #f4e 7#dc4g ,f474 ,deducting the #bj due to the influence of dust and clouds1 we obtain -#ad 7#g ,f471 and the observed temperature would therefore be higher than the calculated by no less than #cj 7#ed ,f474 ,the discrepancy is explained by the heat-protecting action of the gases contained in the atmosphere1 to which we shall presently refer 7page #ea74 ,there are but few clouds on ,mars4 ,this planet is endowed with an atmosphere of extreme transparency1 and should therefore have a high temperature4 ,instead of the temperature of -#cg 7#ce ,f471 calculated1 the mean temperature seems to be #aj 7#ej ,f474 ,during the winter large white masses1 evidently snow1 collect on the poles of ,mars1 which rapidly melt away in spring and change into water that appears dark to us4 ,sometimes the snow-caps on the poles of ,mars disappear entirely during the ,mars summer2 this never happens on our terrestrial poles4 ,the mean temperature of ,mars must therefore be above zero1 probably about #aj4 ,organic life may very probably thrive1 therefore1 on ,mars4 ,it is1 however1 rather sanguine to jump at the conclusion that the so-called canals of ,mars prove its being inhabited by intelligent beings4 ,many people regard the 8canals0 as optical illusions2 ,lowell's photographs1 however1 do not justify this opinion4 ,as regards the other large planets1 the temperatures which we have calculated for them are very low4 ,this calculation is1 however1 rather illusory1 because these planets probably do not possess any solid or liquid surface1 but consist altogether of gases4 ,their densities1 at least1 point in this direction4 ,in the case of the inner planets1 ,mars and our moon included1 the density is rather less than that of the earth4 ,mercury stands last among them1 with its specific gravity of #j4efd4 ,there follows a great drop in the specific gravities of the outer large planets4 ,saturn1 with a density of #j4aaf1 is last in this order2 the densities of the two outermost planets lie somewhat higher--by #j4c or #j4d about--but these last data are very uncertain4 ,yet these figures are of the same order of magnitude as that assumed for the sun--#j4be--;and we believe that the sun1 apart from the small clouds1 is wholly a gaseous body4 ,it is therefore probable that the outer planets1 including ,jupiter1 will also be gaseous and be surrounded by dense veils of clouds which prevent our looking down into their interior4 ,that view would contend against the idea that these planets can harbor any living beings4 ,we could rather imagine their moons to be inhabited4 ,if these moons received no heat from their planets1 they would assume the above-stated temperatures of their central bodies4 ,looked at from our moon1 the earth appears under a visual angle1 #c4g times as large as that of the sun4 ,as the temperature of the sun has1 from its radiation1 been estimated at #fbjj ,cent41 or #fejj absolute1 the moon would receive as much heat from the earth as from the sun1 if the earth had a temperature of about #cajj ,cent41 or #cchj absolute4 ,when the first clouds of water vapor were being formed in the terrestrial atmosphere1 the earth's temperature was about #cfj1 and the radiation from the earth to the moon only about #a4be-thousandth of that of the sun4 ,the present radiation from the earth does not even attain one-twentieth of this value4 ,it is thus manifest that the radiation from the earth does not play any part in the thermal household of the moon4 ,the relations would be quite different if the earth had the #aa4f times greater diameter of ,jupiter1 or the diameter of ,saturn1 which is #i4c times greater than its own4 ,the radiation from the earth to the moon would then make up about a sixth or a ninth of the actual solar radiation1 taking the temperature of the earth's surface at #cfj4 ,we can easily calculate1 further1 that ,jupiter and ,saturn would radiate as much heat against a moon at a distance of #bdj1jjj or #aia1jjj km4 respectively 7since the distance of the moon from the earth amounts to #chd1jjj km47 as the sun sends to ,mars--taking the temperature of those planets at #cfj ,cent4 ,now we find1 near ,jupiter as well as near ,saturn1 moons at the distances of #abf1jjj and #ahf1jjj km4 respectively1 which are smaller than those mentioned1 and it is not inconceivable that these moons receive from their central bodies sufficient heat to render life possible1 provided that they be enveloped by a heat-absorbing atmosphere4 ,the conditions appear to be less favorable for the innermost satellites of ,jupiter and ,saturn4 ,when their planets are shining at the maximum brilliancy1 their light intensity is only a sixth or a ninth of the solar light intensity1 which upon these satellites is itself only one-twenty-seventh or one-ninetieth of the intensity on the earth4 ,during the incandescence epoch of these planets their moons will certainly for some time have been suitable for the development of life4 ,that the atmospheric envelopes limit the heat losses from the planets had been suggested about #ahjj by the great ,french physicist ,fourier4 ,his ideas were further developed afterwards by ,pouillet and ,tyndall4 ,their theory has been styled the hot-house theory1 because they thought that the atmosphere acted after the manner of the glass panes of hot-houses4 ,glass possesses the property of being transparent to heat rays of small wave lengths belonging to the visible spectrum2 but it is not transparent to dark heat rays1 such1 for instance1 as are sent out by a heated furnace or by a hot lump of earth4 ,the heat rays of the sun now are to a large extent of the visible1 bright kind4 ,they penetrate through the glass of the hot-house and heat the earth under the glass4 ,the radiation from the earth1 on the other hand1 is dark and cannot pass back through the glass1 which thus stops any losses of heat1 just as an overcoat protects the body against too strong a loss of heat by radiation4 ,langley made an experiment with a box1 which he packed with cotton-wool to reduce loss by radiation1 and which he provided1 on the side turned towards the sun1 with a double glass pane4 ,he observed that the temperature rose to #aac 7#bce ,f471 while the thermometer only marked #ad or #ae 7#eg or #ei ,f47 in the shade4 ,this experiment was conducted on ,pike's ,peak1 in ,colorado1 at an altitude of #dbjj m4 7#ac1hjj ft471 on ,september #i1 #ahha1 at #a hr4 #d min4 ,p4,m41 and therefore at a particularly intense solar radiation4 ,fourier and ,pouillet now thought that the atmosphere of our earth should be endowed with properties resembling those of glass1 as regards permeability of heat4 ,tyndall later proved this assumption to be correct4 ,the chief invisible constituents of the air which participate in this effect are water vapor1 which is always found in a certain quantity in the air1 and carbonic acid1 also ozone and hydrocarbons4 ,these latter occur in such small quantities that no allowance has been made for them so far in the calculations4 ,of late1 however1 we have been supplied with very careful observations on the permeability to heat of carbonic acid and of water vapor4 ,with the help of these data ,i have calculated that if the atmosphere were deprived of all its carbonic acid--of which it contains only #j4jc per cent4 by volume--the temperature of the earth's surface would fall by about #ba4 ,this lowering of the temperature would diminish the amount of water vapor in the atmosphere1 and would cause a further almost equally strong fall of temperature4 ,the examples1 so far as they go1 demonstrate that comparatively unimportant variations in the composition of the air have a very great influence4 ,if the quantity of carbonic acid in the air should sink to one-half its present percentage1 the temperature would fall by about #d2 a diminution to one-quarter would reduce the temperature by #h4 ,on the other hand1 any doubling of the percentage of carbon dioxide in the air would raise the temperature of the earth's surface by #d2 and if the carbon dioxide were increased fourfold1 the temperature would rise by #h4 ,further1 a diminution of the carbonic acid percentage would accentuate the temperature differences between the different portions of the earth1 while an increase in this percentage would tend to equalize the temperature4 ,the question1 however1 is whether any such temperature fluctuations have really been observed on the surface of the earth4 ,the geologists would answer3 yes4 ,our historical era was preceded by a period in which the mean temperature was by #b 7#c4f ,f47 higher than at present4 ,we recognize this from the former distribution of the ordinary hazel-nut and of the water-nut 7,trapa natans74 ,fossil nuts of these two species have been found in localities where the plants could not thrive in the present climate4 ,this age1 again1 was preceded by an age which1 we are pretty certain1 drove the inhabitants of northern ,europe from their old abodes4 ,the glacial age must have been divided into several periods1 alternating with intervals of milder climates1 the so-called inter-glacial periods4 ,the space of time which is characterized by these glacial periods1 when the temperature--according to measurements based upon the study of the spreading of glaciers in the ,alps--must have been about #e 7#h ,f47 lower than now1 has been estimated by geologists at not less than #ajj1jjj years4 ,this epoch was preceded by a warmer age1 in which the temperature1 to judge from fossilized plants of those days1 must at times have been by #h or #i 7#ad or #af ,f47 higher than at present1 and1 moreover1 much more uniformly distributed over the whole earth 7,eocene74 ,pronounced fluctuations of this kind in the climate have also occurred in former geological periods4 ,are we now justified in supposing that the percentage of carbon dioxide in the air has varied to an extent sufficient to account for the temperature changes8 ,this question has been answered in the affirmative by ,hgbom1 and1 in later times1 by ,stevenson4 ,the actual percentage of carbonic acid in the air is so insignificant that the annual combustion of coal1 which has now 7#aijd7 risen to about #ijj million tons and is rapidly increasing1#c carries about one-seven-hundredth part of its percentage of carbon dioxide to the atmosphere4 ,although the sea1 by absorbing carbonic acid1 acts as a regulator of huge capacity1 which takes up about five-sixths of the produced carbonic acid1 we yet recognize that the slight percentage of carbonic acid in the atmosphere may by the advances of industry be changed to a noticeable degree in the course of a few centuries4 ,that would imply that there is no real stability in the percentage of carbon dioxide in the air1 which is probably subject to considerable fluctuations in the course of time4 ,footnote #c3 ,it amounted in #ahij to #eaj million tons2 in #ahid1 to #eej2 in #ahii1 to #fij2 and in #aijd1 to #hij million tons4 ,volcanism is the natural process by which the greatest amount of carbonic acid is supplied to the air4 ,large quantities of gases originating in the interior of the earth are ejected through the craters of the volcanoes4 ,these gases consist mostly of steam and of carbon dioxide1 which have been liberated during the slow cooling of the silicates in the interior of the earth4 ,the volcanic phenomena have been of very unequal intensity in the different phases of the history of the earth1 and we have reason to surmise that the percentage of carbon dioxide in the air was considerably greater during periods of strong volcanic activity than it is now1 and smaller in quieter periods4 ,professor ,frech1 of ,breslau1 has attempted to demonstrate that this would be in accordance with geological experience1 because strongly volcanic periods are distinguished by warm climates1 and periods of feeble volcanic intensity by cold climates4 ,the ice age in particular was characterized by a nearly complete cessation of volcanism1 and the two periods at the commencement and at the middle of the ,tertiary age 7,eocene and ,miocene7 which showed high temperatures were also marked by an extraordinarily developed volcanic activity4 ,this parallelism can be traced back into more remote epochs4 ,it may possibly be a matter of surprise that the percentage of carbon dioxide in the atmosphere should not constantly be increased1 since volcanism is always pouring out more carbon dioxide into our atmosphere4 ,there is1 however1 one factor which always tends to reduce the carbon dioxide of the air1 and that is the weathering of minerals4 ,the rocks which were first formed by the congelation of the volcanic masses 7the so-called magma7 consist of compounds of silicic acid with alumina1 lime1 magnesia1 some iron and sodium4 ,these rocks were gradually decomposed by the carbonic acid contained in the air and in the water1 and it was especially the lime1 the magnesia1 and the alkalies1 and1 in some measure also the iron1 which formed soluble carbonates4 ,these carbonates were carried by the rivers down into the seas4 ,there lime and magnesia were secreted by the animals and by the alg1 and their carbonic acid became stored up in the sedimentary strata4 ,hgbom estimates that the limestones and dolomites contain at least #be1jjj times more carbonic acid than our atmosphere4 ,chamberlin has arrived at nearly the same figure--from #bj1jjj to #cj1jjj2 he does not allow for the precambrian limestones4 ,these estimates are most likely far too low4 ,all the carbonic acid that is stored up in sedimentary strata must have passed through the atmosphere4 ,another process which withdraws carbonic acid from the air is the assimilation of plants4 ,plants absorb carbonic acid under secretion of carbon compounds and under exhalation of oxygen4 ,like the weathering1 the assimilation increases with the percentage of carbonic acid4 ,the ,polish botanist ,e4 ,godlewski showed as early as #ahgb that various plants 7he studied ,typha latifolia and ,glyceria spectabilus with particular care7 absorb from the air an amount of carbonic acid which increases proportionally with the percentage of carbonic acid in the atmosphere up to #a per cent41 and that the assimilation then attains1 in the former plant1 a maximum at #f per cent41 and in the latter plant at #i per cent4 ,the assimilation afterwards diminishes if the carbonic acid percentage is further augmented4 ,if1 therefore1 the percentage of carbon dioxide be doubled1 the absorption by the plants would also be doubled4 ,if1 at the same time1 the temperature rises by #d1 the vitality will increase in the ratio of #a3 #a4e1 so that the doubling of the carbon dioxide percentage will lead to an increase in the absorption of carbonic acid by the plant approximately in the ratio of #a3 #c4 ,the same may be assumed to hold for the dependence of the weathering upon the atmospheric percentage of carbonic acid4 ,an increase of the carbon dioxide percentage to double its amount may hence be able to raise the intensity of vegetable life and the intensity of the inorganic chemical reactions threefold4 ,according to the estimate of the famous chemist ,liebig1 the quantity of organic matter 7freed of water7 which is produced by one hectare 7#b4e acres7 of soil1 meadowland1 or forest is nearly the same1 approximately #b4e tons per year in central ,europe4 ,in many parts of the tropics the growth is much more rapid2 in other places1 in the deserts and arctic regions1 much more feeble4 ,we may be justified in accepting ,liebig's figure as an average for the firm land on our earth4 ,of the organic substances to which we have referred1 and which mainly consist of cellulose1 carbon makes up #dj per cent4 ,thus the actual annual carbon production by plants would amount to #ac1jjj million tons--i4e41 not quite fifteen times more than the consumption of coal1 and about one-fiftieth of the quantity of the carbon dioxide in the air4 ,if1 therefore1 all plants were to deposit their carbon in peat-bogs1 the air would soon be depleted of its carbon dioxide4 ,but it is only a fraction of one per cent4 of the coal which is produced by plants that is stored up for the future in this way4 ,the rest is sent back into the atmosphere by combustion or by decay4 ,chamberlin relates that1 together with five other ,american geologists1 he attempted to estimate how long a time would be required before the carbon dioxide of the air would be consumed by the weathering of rocks4 ,their various estimates yielded figures ranging from #ejjj to #ah1jjj years1 with a probable average of #aj1jjj years4 ,the loss of carbonic acid by the formation of peat may be estimated at the same figure4 ,the production of carbonic acid by the combustion of coal would therefore suffice to cover the loss of carbonic acid by weathering and by peat formation seven times over4 ,those are the two chief factors deciding the consumption of carbonic acid1 and we thus recognize that the percentage of carbonic acid in the air must be increasing at a constant rate as long as the consumption of coal1 petroleum1 etc41 is maintained at its present figure1 and at a still more rapid rate if this consumption should continue to increase as it does now4 ,this consideration enables us to picture to ourselves the possibility of the enormous plant-growth which must have characterized certain geological periods of our earth--for instance1 the carboniferous period4 ,this period is known to us from the extraordinarily large number of plants which we find embedded in the clay of the swamps of those days4 ,those plants were slowly carbonized afterwards1 and their carbon is in our age returned to its original place in the household of nature in the shape of carbonic acid4 ,a great portion of the carbonic acid has disappeared from the atmosphere of the earth1 and has been stored up as coal1 lignite1 peat1 petroleum1 or asphalt in the sedimentary strata4 ,oxygen was liberated at the same time1 and passed into our atmospheric sea4 ,it has been calculated that the amount of oxygen in the air--#abaf billion tons--approximately corresponds to the mass of fossil coal which is stored up in the sedimentary strata4 ,the supposition appears natural1 therefore1 that all the oxygen of the air may have been formed at the expense of the carbonic acid in the air4 ,this view was first advanced by ,khne1 of ,brussels1 in #ahef1 and later discussions have strengthened its probability4 ,part of the oxygen is certainly consumed by weathering processes1 and absorbed--e4g41 by sulphides and by ferro-salts2 without this oxidation the actual quantity of oxygen in the air would be greater4 ,on the other hand1 there are in the sedimentary strata many oxidizable compounds--e4 g41 especially iron sulphides--which have probably been reduced by the interaction of carbon 7by organic compounds74 ,a large number of the substances which consume oxygen during their decomposition and decay have also been produced by the intermediation of the coal which had previously been deposited under liberation of oxygen1 so that these substances are1 by their oxidation1 restored to their original state4 ,we may hence take it as established that the masses of free oxygen in the air and of free carbon in the sedimentary strata approximately correspond to each other1 and that probably all the oxygen of the atmosphere owes its existence to plant life4 ,this appears plausible also for another reason4 ,we know for certain that there is some free oxygen in the atmosphere of the sun1 and that hydrogen abounds in the sun4 ,the earth's atmosphere may originally have been in the same condition4 ,when the earth cooled gradually1 hydrogen and oxygen combined to water1 but an excess of hydrogen must have remained4 ,the primeval atmosphere of the earth may also have contained hydrocarbons1 as they play an important part in the gases of comets4 ,to these gases there were added carbonic acid and water vapor1 coming from the interior of the earth4 ,thanks to its chemical inertia1 the nitrogen of the air may not have undergone much change in the course of the ages4 ,an ,english chemist1 ,phipson1 claims to have shown that both higher plants 7the corn-bind7 and lower organisms 7various bacteria7 can live and develop in an atmosphere devoid of oxygen when it contains carbonic acid and hydrogen4 ,it is also possible that simple forms of vegetable life existed before the air contained any oxygen1 and that these plants liberated the oxygen from the carbonic acid exhaled by the craters4 ,this oxygen gradually 7possibly under the influence of electric discharges7 converted the hydrogen and the hydrocarbons of the air into water and carbonic acid until those elements were consumed4 ,the oxygen remained in the air1 whose composition gradually approached more the actual state4#d ,footnote #d3 ,according to the opinion of a colleague of mine1 a botanist1 the results of the experiments of ,phipson must be regarded as very doubtful1 and some oxygen would appear to be indispensable for the growth of plants4 ,we have to imagine the development somewhat as follows3 ,as the earth separated from the solar nebula1 its temperature was very high at first in its outer portions4 ,at this temperature it was not able to retain the lighter gases1 like hydrogen and helium1 for a long period2 the heavy gases1 like nitrogen and oxygen1 remained4 ,the original excess of hydrogen and helium disappeared1 therefore1 before the crust of the earth had been formed1 and the atmosphere of the earth immediately after the formation of the crust contained some oxygen1 besides much nitrogen1 carbonic acid1 and water vapor4 ,the main bulk of the actual atmospheric oxygen would therefore have been reduced from carbon dioxide by the intermediation of plants4 ,the view that celestial bodies may lose part of their atmosphere is due to ,johnstone ,stoney4 ,the atmospheric gases escape the more rapidly the lighter their molecules and the smaller the mass of the celestial bodies4 ,on these lines we explain that the smaller celestial bodies like the moon and ,mercury1 have lost almost all their atmosphere1 while the earth has only lost hydrogen and helium1 which again have been retained by the sun4 ,this oxygen is an essential element for the production of animal life4 ,as animal life stands above vegetable life1 so animal life could only originate at a later stage than plant life4 ,plants require1 in addition to suitable temperature1 only carbonic acid and water1 and these gases will probably be found in the atmospheres of all the planets as exhalations of their inner incandescent masses which are slowly cooling4 ,the presence of water vapor has directly been established1 by means of the spectroscope1 in the atmospheres of other planets--,venus1 ,jupiter1 and ,saturn--and indirectly by the observation of a snow-cap on ,mars4 ,the spectroscope further gives us indication of the presence of other gases4 ,there is an intense band in the red part of the spectra of ,jupiter and ,saturn1 of wave-length #j4jjjfah mm4 ,other new constituents of unknown nature have been discerned in the spectra of ,uranus and ,neptune4 ,on the other hand1 there is hardly any1 or at any rate only a quite insignificant1 atmosphere on the moon and on ,mercury4 ,this is easily understood4 ,the temperature on that side of ,mercury which is turned away from the sun is near absolute zero4 ,all the gases of the planetary atmosphere would collect and condense there4 ,if1 then1 ,mercury had originally an atmosphere1 it must have lost it as it lost its own rotation1 compelling it to turn always the same face towards the sun4 ,similar reasons may account for the absence of a lunar atmosphere4 ,if ,venus should likewise always turn the same side towards the sun1 as many astronomers assert1 ,venus should not have any notable atmosphere1 nor clouds either4 ,we know1 however1 that this planet is surrounded by a very marked developed atmosphere4#e ,footnote #e3 ,that results from the very strong refraction which light undergoes in the atmosphere of ,venus when this planet is seen in front of the sun's edge during the so-called ,venus transits4 ,and that is the strongest objection to the assumption that ,venus follows the example of ,mercury as regards the rotation about its own axis4 ,since1 now1 warm ages have alternated with glacial periods1 even after man appeared on the earth1 we have to ask ourselves3 ,is it probable that we shall in the coming geological ages be visited by a new ice period that will drive us from our temperate countries into the hotter climates of ,africa8 ,there does not appear to be much ground for such an apprehension4 ,the enormous combustion of coal by our industrial establishments suffices to increase the percentage of carbon dioxide in the air to a perceptible degree4 ,volcanism1 whose devastations-- on ,krakatoa 7#ahhc7 and ,martinique 7#aijb7--;have been terrible in late years1 appears to be growing more intense4 ,it is probable1 therefore1 that the percentage of carbonic acid increases at a rapid rate4 ,another circumstance points in the same direction2 that is1 that the sea seems to withdraw carbonic acid from the air4 ,for the carbonic acid percentage above the sea and on islands is on an average #aj per cent4 less than the above continents4 ,illustration3 ,fig4 #ag4--,photograph of the surface of the moon1 in the vicinity of the crater of ,copernicus4 ,taken at the ,yerkes ,observatory1 ,chicago1 ,u4 ,s4 ,a4 ,scale3 ,diameter of moon1 #j4ee m4 #ba4g in4 ,owing to the absence of an atmosphere and of atmospheric precipitations1 the precipitous walls of the crater and other elevations do not indicate any signs of decay4 ,if the carbonic acid percentage of the air had kept constant for ages1 the percentage of the water would have found time to get into equilibrium with it2 but the sea actually absorbs carbonic acid from the air4 ,thus the sea-water must have been in equilibrium with an atmosphere which contained less carbonic acid than the present atmosphere4 ,hence the carbonic acid percentage has been increasing of late4 ,we often hear lamentations that the coal stored up in the earth is wasted by the present generation without any thought of the future1 and we are terrified by the awful destruction of life and property which has followed the volcanic eruptions of our days4 ,we may find a kind of consolation in the consideration that here1 as in every other case1 there is good mixed with the evil4 ,by the influence of the increasing percentage of carbonic acid in the atmosphere1 we may hope to enjoy ages with more equable and better climates1 especially as regards the colder regions of the earth1 ages when the earth will bring forth much more abundant crops than at present1 for the benefit of rapidly propagating mankind4 ,,iii ,,radiation ,,and ,,constitution ,,of ,,the ,,sun ,the question has often been discussed in past ages1 and again in the last century1 in how far the position of our earth within the solar system may be regarded as secure4 ,one might apprehend two things4 ,either the distance of the earth from the sun might increase or decrease1 or the rotation of the earth about its axis might be arrested2 and either of these possibilities would threaten the continuance of life on the earth4 ,the problem of the stability of the solar system has been investigated by the astronomers1 and their patrons have offered high prizes for a solution of the problem4 ,if the solar system consisted merely of the sun and the earth1 the earth's existence would be secure for ages2 but the other planets exercise a certain1 though small1 influence upon the movements of the earth4 ,that this influence can only be of slight importance is due to the fact that the total mass of all the planets does not aggregate more than one-seven-hundred-and-fiftieth of the mass of the sun1 and1 further1 to the fact that the planets all move in nearly circular orbits around the centre1 the sun1 so that they never approach one another closely4 ,the calculations of the astronomers demonstrate that the disturbances of the earth's orbit are merely periodical1 representing long cycles of from #ej1jjj to #b1jjj1jjj years4 ,thus the whole effect is limited to a slight vacillation of the orbits of the planets about their mean positions4 ,so far everything is well and good4 ,but our solar system is traversed by other celestial bodies1 mostly of unknown1 but certainly not of circular orbits--namely1 the comets4 ,the fear of a collision with a comet still alarmed the thinkers of the past century4 ,experience has1 however1 taught us that collisions between the earth and comets do not lead to any serious consequence4 ,the earth has several times passed through the tails of comets--for instance1 in #ahai and #ahfa--;and it was only the calculating astronomer who became aware of the fact4 ,once on such an occasion we have thought that we observed a glow like that of an aurora in the sky4 ,when the earth was drawing near the denser parts of the comet1 particles fell on the earth in the shape of showers of shooting-stars1 without doing any appreciable damage4 ,the mass of comets is too small perceptibly to disturb the paths of the planets4 ,the rotation of the earth about its axis should slowly be diminished by the effects of the tides1 since they act like a brake applied to the surface of the earth4 ,this retardation is1 however1 so unimportant that the astronomers have not been able to establish it in historical times4 ,the slow shrinkage of the earth somewhat counteracts this effect4 ,laplace believed that we were able to deduce1 from an analysis of the observations of solar eclipses in ancient centuries1 that the length of the day had not altered by more than #j4ja second since the year #gbi ,b4,c4 ,we know that the sun1 unaccompanied by its planets1 is moving in space towards the constellation of ,hercules with a velocity of #bj km4 7#ac miles7 per second1 which is amazing to our terrestrial conceptions4 ,possibly the constituents of our solar system might collide with some other unknown celestial body on this journey4 ,but as the celestial bodies are sparsely distributed1 we may hope that many billions of years will elapse before such a catastrophe will take place4 ,in mechanical respects the stability of our system appeared to be well established4 ,since the modern theory of heat has made its triumphant entry into natural science1 however1 the aspect of matters has changed4 ,we are convinced that all life and all motion on the earth can be traced back to solar radiation4 ,the tidal motions alone make a rather unimportant exception4 ,we have to ask ourselves3 ,will not the store of energy in the sun1 which goes out1 not only to the planets1 but to a far greater extent into unknown domains of cold space1 come to an end1 and will not that be the end of all the joys and sorrows of earthly existence8 ,the position appears desperate when we consider that only one part in #bcjj millions of the solar radiation benefits the earth1 and perhaps ten times as much the whole system1 with all its moons4 ,the solar radiation is so powerful that every gramme of the mass of the sun loses two calories in the course of a year4 ,if1 therefore1 the specific heat of the sun were the same as that of water1 which in this respect surpasses most other substances1 the solar temperature would fall by #b ,cent4 7#c4f ,f47 every year4 ,as1 now1 the temperature of the sun in its outer portion has been estimated at from #fjjj to #gjjj1 the sun should have cooled completely within historical times4 ,and though the interior of the sun most probably has a vastly higher temperature than the outer portions which we can observe1 we should1 all the same1 have to expect that the solar temperature and radiation would noticeably have diminished in historical times4 ,but all the documents from ancient ,babylon and ,egypt seem to point out that the climate at the dawn of historical times was in those countries nearly the same as at present1 and that1 therefore1 the sun shone over the most ancient representatives of culture in the same way as it shines on their descendants now4 ,the thesis has frequently been advanced1 therefore1 that the sun has in its heat balance not only an expenditure side1 but also an almost equally substantial income side4 ,the ,german physician ,r4 ,mayer1 who has the immortal merit of first having given expression to the conception of a relation between heat and mechanical work1 directed his attention also to the household of the sun4 ,he suggested that swarms of meteorites1 rushing into the sun with an amazing velocity 7of over #fjj km4 per second71 would1 when stopped in their motion1 generate heat at the rate of #de million calories per gramme of meteorites4 ,in future ages it would be the turn of the planets to sustain for some time longer the spark of life in the sun1 by the sacrifice of their own existences4 ,the sun would therefore1 like the god ,saturn1 have to devour its own children in order to continue its existence4 ,of how little avail that would be we learn from the consideration that the fall of the earth into the sun would not be able to prolong the heat expenditure of the sun by as many as a hundred years4 ,by their rush into the sun1 almost uniformly from all sides1 the meteorites would1 moreover1 long since have put a stop to the rotation of the sun about its axis4 ,further1 by virtue of the increasing mass and the hence augmenting attraction of the sun1 the length of our year would have had to diminish by about #b4h seconds per year1 which is in absolute contradiction to the observations of the astronomers4 ,according to ,mayer's thesis1 a corresponding number of meteorites would1 finally1 also have to tumble upon the surface of the earth1 and 7according to data which will be furnished in ,chapter ,,iv47 they should raise the surface temperature to about #hjj4 ,the thesis is therefore misleading4 ,we must look for another explanation4 ,it occurred to ,helmholtz1 one of the most eminent investigators in the domain of the mechanical theory of heat1 that1 instead of the meteorites1 parts of the sun itself might fall towards its centre1 or1 in other words1 that the sun was shrinking4 ,owing to the high gravitation of the sun 7#bg4d times greater than on the surface of the earth71 the shrinkage would liberate a great amount of heat4 ,helmholtz calculated that1 in order to cover the heat expenditure of the sun1 a shrinkage of its diameter by #fj m4 annually would be required4 ,if the sun's diameter should only be diminished by one-hundredth of one per cent4--a change which we should not be able to establish--the heat loss would be covered for more than #bjjj years4 ,that seems at first satisfactory4 ,but if we proceed with our estimate1 we find that if the sun went on losing as much heat as at present for seventeen million years it would have to contract within this period to a quarter of its present volume1 and would therefore acquire a density like that of the earth4 ,long before that1 however1 the radiation from the sun would have been decreased so powerfully that the temperature on the earth's surface would no longer rise above freezing-point4 ,helmholtz1 on this argument1 limited the further existence of the earth to about six million years4 ,that is less satisfactory4 ,but we know nothing of the future and must be content with possibilities4 ,not so1 however1 if we calculate back with the aid of ,helmholtz's theory4 ,according to this theory1 and according to ,helmholtz's own data1 a state like the present cannot have existed for more than ten million years4 ,since1 now1 geologists have come to the conclusion that the petrefactions which we find in the fossil-bearing strata of the earth have needed at least a hundred million years for their formation1 and more probably a thousand million years1 and since1 moreover1 the still more ancient formations--the so-called precambrian strata--have been deposited in equally long or still longer periods1 we see that the theory of ,helmholtz is unsatisfactory4 ,a somewhat peculiar way out of the dilemma has been suggested by a few scientists4 ,we know that one gramme of the wonderful element radium emits about #abj calories per hour1 or in the course of a year1 in round numbers1 a million calories4 ,this radiation seems to continue unimpaired for years4 ,if we now assume that each kilogramme of the mass of the sun contains only two milligrammes of radium1 that amount would be sufficient to balance the heat expenditure of the sun for all future ages4 ,without some further auxiliary hypothesis1 we can1 however1 not listen to this suggestion4 ,it presupposes that heat is created out of nothing4 ,some scientists1 indeed1 believe that radium may absorb a radiation1 coming from space1 in some unknown manner and convert it into heat4 ,before we enter seriously into a discussion of this explanation we shall have to answer the questions where that radiation comes from and where it takes its store of energy4 ,we must1 therefore1 again search for another source of heat energy for the sun4 ,before we can hope to find it1 we had better study the sun itself a little4 ,all scientists are agreed that the sun is of the same constitution as the thousands of luminous stars which we see in the sky4 ,according to the color of the light which they emit1 stars are classified as white1 yellow1 and red stars4 ,the differences in their light become much more distinct when we examine them spectroscopically4 ,in the white stars the helium and hydrogen lines predominate decidedly2 the helium stars contain1 in addition1 oxygen4 ,metals are comparatively little represented2 but they play a main part in the spectra of the yellow stars1 in which1 further1 some bands become visible4 ,in the spectra of the red stars we notice many bands which indicate that chemical compounds are present in the outer portions4 ,everybody knows that the platinum wire or the filament of an incandescent lamp which has been heated to incandescence by the electric current first shines reddish1 then yellow when the current is increased1 and finally more and more white4 ,at the same time the temperature rises4 ,we can estimate the temperature from the brightness of the glow4 ,if we know the wave-length of the radiations of that color which emits the greatest amount of heat in the spectrum 7it should be a normal spectrum71 it is easy to calculate the temperature of the star from ,wien's law of displacements4 ,we need only divide #b4hi by the respective wave-length expressed in mm4 to find the absolute temperature of the star2 by deducting #bgc from the result1 we obtain the temperature in degrees ,cent4 on the ordinary scale4 ,for the sun the maximum of heat radiation lies near wave-length #j4jjjee 7in the greenish-yellow light71 and therefore the absolute temperature of the radiating disk of the sun1 the so-called photosphere1 should be #ebee absolute1 or nearly #ejjj ,cent4 ,but our atmosphere weakens the sunlight1 and it also causes a displacement of the maximum radiation in the spectrum4 ,the same applies to the sun's own atmosphere1 so that we have to adopt a higher estimate than #ejjj ,cent4 ,by means of ,stefan's law of radiation1 the solar temperature has been estimated at about #fbjj1 which would correspond to a wave-length of about #j4jjjde mm4 ,this correction is therefore significant4 ,about half of it has to be ascribed to the influence of the solar atmosphere1 the other half to the terrestrial atmosphere4 ,a ,hungarian astronomer1 ,harknyi1 has determined in the same way the temperature of several white stars 7,vega and ,sirius71 and found it to be about #ajjj higher than that of the sun1 while the red star ,betelgeuse1 the most prominent star in ,orion1 would have a temperature by #bejj lower than that of the sun4 ,it must expressly be stated that in making these estimates we understand by the temperature of the star in this case the temperature of a radiating body which emits the same light as that which reaches us from the star4 ,but the stellar light undergoes important changes on its way to us4 ,we learn from observing new stars that a star may be surrounded by a cloud of cosmical dust which sifts the blue rays out and permits the red ones to pass4 ,the star then shines with a less brilliantly white light than in the absence of the cloud4 ,the consequence is that we estimate the temperature lower than it really is4 ,in the red stars bands have been noticed1 indicating1 as we have already said1 the presence of chemical compounds4 ,the most interesting of these are the compounds of cyanogen and of carbon1 probably with hydrogen1 which appear to resemble those observed by ,swan in the spectrum of gas flames and which were named after him4 ,it was formerly thought that the presence of these compounds implied lower temperature4 ,but we shall see that this conclusion is not firmly established4 ,hale has found during eclipses of the sun that exactly the same compounds occur immediately above the luminous clouds of the sun4 ,they are probably more numerous below the clouds1 where the temperature is no doubt higher1 than above them4 ,however that may be1 we have reason to assume that the now yellow sun was once a white star like the brilliant ,sirius1 that it has slowly cooled down to its present appearance1 and that it will some day shine with the reddish light of ,betelgeuse4 ,the sun will then only radiate a seventh of the heat which it emits now1 and it is very likely that the earth will have been transformed into a glacial desert long before that time4 ,it has already been pointed out that the atmospheres of both the sun and of the earth produce a strong absorption of the solar rays1 and especially of the blue and white rays4 ,it is for this reason that the light of the sun appears more red in the evening than at noon1 because in the former case it has to pass through a thicker layer of air1 which absorbs the blue rays4 ,for the same reason the limb of the sun appears more red in spectroscopic examinations than the centre of the sun4 ,this weakening of the sun's light is due to the fine dust pervading the atmospheres of the earth and the sun4 ,when the products of strong volcanic eruptions1 like the eruptions of ,krakatoa in #ahhc and of ,mont ,pele in #aijb1 filled the atmosphere with a fine volcanic dust1 the sun appeared distinctly red when standing low in the horizon4 ,it was this dust that caused the red glow4 ,when we examine an image of the sun which has been thrown on a screen by the aid of a lens or a system of lenses1 we notice on the sun's disk a mottling of characteristic darker spots4 ,these spots struck the attention of ,galileo1 and they were discovered almost simultaneously by him1 by ,fabricius1 and by ,scheiner 7#afaj-afaa74 ,these spots have since been the most diligently studied features of the sun4 ,we carefully determine their number and sizes1 and combine these two data to make the so-called sun-spot numbers4 ,these numbers change from year to year in a rather irregular way1 the period amounting on an average to #aa4a years4 ,the spots appear in two belts on the sun1 and they glide over the disk in the course of thirteen or fourteen days4 ,sometimes they reappear after another thirteen or fourteen days4 ,it is therefore believed that they lie comparatively quiet on the surface of the sun1 and that the sun rotates about its own axis in about twenty-seven days1 so that after that period the same points are again opposite the earth4 ,this is the so-called synodical period4 ,the great interest which attaches to the study of these features lies in the fact that simultaneously with these spots several other phenomena seem to vary which attain their maxima at the same time4 ,such are1 in the first instance1 the polar lights and the magnetic variations1 and1 to a lesser degree1 the cirrus clouds and temperature changes1 as well as several other meteorological phenomena 7compare ,chapter ,v474 ,about the sun-spots we notice the so-called facul--portions which are much brighter than their surroundings4 ,when we carefully examine a strongly magnified image of the sun1 we find that it has a granulated appearance 7,fig4 #ah74 ,langley compares the disk to a grayish-white cloth almost hidden by flakes of snow4 ,the less bright portions are designated 8pores10 the brighter portions 8granules40 ,it is generally assumed that the granules correspond to clouds which rise like the clouds of our atmosphere on the top of ascending convection currents4 ,but while the terrestrial clouds are formed of drops of rain or of crystals of ice1 the granules consist probably of soot--that is to say1 condensed carbon--and of drops of metals1 iron1 and others4 ,the smallest granule which we are able to discern has a diameter of about #bjj km4 7#acj miles74 ,illustration3 ,fig4 #ah4--,sun-spot group and granulation of the sun4 7,photographed at the ,meudon ,observatory1 near ,paris1 ,april #a1 #ahhd7 ,the facul are formed by very large accumulations of clouds which are carried up by strong ascending currents and spread over large areas1 as in our cyclones4 ,the spots correspond to descending masses of gas with rising temperatures1 which are therefore 8dry0 and do not carry any clouds1 as in terrestrial anticyclones4 ,through these holes in the walls of solar clouds we peep a little farther into the gigantic masses of gas1 and we obtain an idea of the state of affairs in the deeper strata of the sun4 ,the depth of the wall of cloud is1 of course1 not large compared to the radius of the sun4 ,illustration3 ,fig4 #ai4--,part of the solar spectrum of ,january #c1 #ahgb4 ,after ,langley4 ,the bright horizontal bands are due to prominences4 ,in the middle 7at #bjh7 the hydrogen line ,f1 strongly distorted by violent agitation ,the study of the spectra affords us the best insight into the nature of the different parts of the sun4 ,the spectra teach us not only the constituents of these parts1 but also the velocities with which they move4 ,we have learned in this way that1 lying above the luminous clouds of the sun which are radiating to us1 there are great masses of gas containing most of our terrestrial elements4 ,we distinguish particularly in them iron1 magnesium1 calcium1 sodium1 helium1 and hydrogen4 ,the two last-mentioned constituents1 being the least dense1 are found particularly in the outermost strata of the atmosphere4 ,the solar atmosphere becomes visible when1 during an eclipse of the sun1 the disk of the moon has proceeded so far as to cover the intensely luminous clouds in the so-called photosphere4 ,owing to its strong percentage of hydrogen1 the gaseous atmosphere generally shines in the purple hue which is characteristic of this element4 ,this stratum of gas is also called the chromosphere 7from the ,greek word 1 meaning color74 ,its thickness is estimated at from #gjjj to #ijjj km4 7#ejjj to #fjjj miles74 ,from it rise rays of fire over the surrounding surface like blades of grass on meadows1 to which their appearance has been likened4 ,illustration3 ,fig4 #bj4--,metallic prominences in vortex motion4 ,the white spot marks the size of the earth ,illustration3 ,fig4 #ba4--;,fountain-like metallic prominences ,when these flames rise still higher1 to about #ae1jjj km4 7#icjj miles7 or more1 they are called protuberances or prominences4 ,their number as well as their altitude grow with the number of sun-spots4 ,they are distinguished as metallic and as quiet prominences4 ,the former are characterized by particularly violent motion1 as will become apparent from ,figs4 #bj and #ba1 and they contain large amounts of metallic vapor4 ,they appear only within the belt of sun-spots which are most pronounced at a distance of about #bj from the solar equator4 ,their movements are so violent that they often traverse several hundreds of kilometres in a second4 ,the ,hungarian ,fnyi observed1 indeed1 on ,july #ae1 #ahie1 a prominence whose greatest velocity in the line of sight1 measured spectroscopically1 amounted to #hfb km4 7#ecf miles71 and whose maximum velocity at right angles to this direction was #hdj km4 per second4 ,these colossal velocities distinguish the highest parts1 while the lower portions1 which are the most dense and which contain most metallic vapor1 are less mobile1 as might be expected4 ,their altitude above the sun's surface may reach exceedingly high figures1 and this applies also to the quiet prominences4 ,the above-mentioned prominence of ,july #ae1 #ahie1 reached a height of #ejj1jjj km41 and ,langley observed1 on ,october #g1 #ahhj1 one at an altitude of #efj1jjj km41 whose tip1 therefore1 nearly attained an elevation equal to that of a radius of the sun1 #fij1jjj km4 above the limb of the sun's photosphere4 ,the mean altitude of these prominences is #dj1jjj km4 ,after their discovery by ,lector ,vassenius1 of ,gtheborg1 in #agcc1 they could only be studied during total solar eclipses1 until ,lockyer and ,janssen taught us1 in the year #ahfh1 how to observe them in full sunlight by means of the spectroscope4 ,illustration3 ,fig4 #bb4--,quiet prominences of smoke-column type ,illustration3 ,fig4 #bc4--,quiet prominences1 shape of a tree4 ,the white spot indicates the size of the earth ,illustration3 ,fig4 #bd4--;,diagram illustrating the differences in the spectra of sun-spots and of the photosphere4 ,some lines in the spot spectrum are stronger1 others fainter1 than in the photosphere spectrum4 ,in the central portion1 two reversals2 to the right1 two bands4 ,after ,mitchell ,illustration3 ,fig4 #be4--,spectrum of a sun-spot1 the central band between the two portions of the photosphere spectrum4 ,the spot spectrum is bordered with the half-shadows of the edge of the spot4 ,after ,mitchell ,the quiet prominences consist almost exclusively of hydrogen and helium2 sometimes they contain also traces of metallic gases4 ,they resemble clouds floating quietly in the solar atmosphere1 or masses of smoke coming from a chimney4 ,they may appear anywhere on the sun1 and their stability is so great that they have sometimes been watched during a complete solar rotation 7for about forty days72 this is possible only when they occur in the neighborhood of the poles1 where they always remain visible outside the sun's limb4 ,figs4 #bb and #bc show several such prominences according to ,young4 ,illustration3 ,fig4 #bf4--,the great sun-spot of ,october #i1 #aijc4 ,taken with the photo-heliograph of ,greenwich in the usual manner4 ,the spot is shown at mean level of the calcium facul4 ,the two following photographs show a lower-level and a higher-level section through the calcium facul ,sometimes the matter of the prominences seems to fall back upon the surface of the sun between the smaller flames of fire which we have likened to blades of grass 7,fig4 #ba74 ,in most cases1 however1 the prominences appear slowly to dissolve4 ,when their brilliant glow fades owing to their intense radiation1 they can no longer be observed4 ,the quiet prominences1 which seem to float at heights of about #ej1jjj km4 and at still greater heights1 must there be almost in a vacuum4 ,their particles cannot be supported by any surrounding gases1 after the manner of the drops of water in terrestrial clouds4 ,in order that they may remain floating they must be pushed away from the sun by a peculiar force--the radiation pressure 7see ,chapter ,,iv474 ,illustration3 ,fig4 #bg4--,the great sun-spot of ,october #i1 #aijc4 ,photograph of the low-level calcium facul with the aid of the light of the calcium line ,h4 ,the spot is not obscured by the facul--at least1 not so much as in the following illustrations ,the facul can be studied in the same way as the prominences1 and of late ,deslandres and ,hale have used for this purpose a special instrument1 the heliograph 7compare ,figs4 #bf to #bi74 ,when the facul approach the limb of the sun they appear particularly brilliant by comparison with their surroundings4 ,that seems to indicate that they are lying at a great altitude1 and that their light is hence not weakened by the superposed hazy stratum4 ,when they reach the sun's limb they appear to us like raised portions of the photosphere4 ,the clouds which form these facul are carried upward by powerful ascending streams of gas whose expansion is due to the diminution of the gaseous pressure4 ,illustration3 ,fig4 #bh4--,the great sun-spot of ,october #i1 #aijc4 ,photograph of the higher-level calcium facul1 taken with the light of the central portion of the line ,h 7calcium74 ,the higher-level facul hide the spot1 indicating that the facul spread considerably during their ascent ,illustration3 ,fig4 #bi4--,the great sun-spot of ,october #i1 #aijc4 ,photograph of the hydrogen facul1 taken with the light of the spectral line ,f 7hydrogen74 ,only the darkest portions of the spot are visible4 ,the other portions are obscured by masses of the hydrogen1 which were evidently in a restless state ,sun-spots display many peculiarities in their spectra 7,figs4 #bd and #be74 ,very prominent is always the helium line2 prominent likewise the dark sodium lines1 which are markedly widened and which show in their middle portions a bright line--the so-called reversal of lines 7,fig4 #bd74 ,this occurrence indicates that the metal is lying in a deeper stratum4 ,in the red portion of the spectrum we find bands1 just as in the spectra of the red stars4 ,these bands1 which appear to be resolved into crowds of lines by the aid of powerful instruments1 indicate the presence of chemical compounds4 ,since the spot is comparatively of feeble intensity1 its spectrum appears superposed like a less bright ribbon upon the background of the spectrum of the more luminous photosphere4 ,the violet end of the sun-spot spectrum is particularly weakened4 ,although the spot has the appearance of a pit in the photosphere1 and when on the sun's limb makes it look as if a piece had been cut out of the edge1 it yet does not appear darker than the sun's edge4 ,that points to the conclusion that the light emitted by the spot emanates chiefly from its upper1 cold portions4 ,illustration3 ,fig4 #cj4--,photograph of the solar corona of #aijj4 7,after ,langley and ,abbot47 ,illustrating the appearance of the corona in years of minimum sun-spot frequency ,the light coming from the deeper portions is distinctly absorbed to a large degree by the higher-lying strata4 ,the sun-spots also appear to become narrower in their lower parts1 owing to the compression of the gases at greater depths1 and one may regard their funnel-shaped cloud-walls as 8half-shadows10 which appear darker than the surroundings1 but brighter than the so-called core of the spot4 ,the weakening of the violet end of the spectrum is probably due to the presence of fine particles of dust in the solar gases1 just as they cause the corresponding weakening of the violet end of the spectrum of the sun's limb4 ,the bands in the red parts of the sun-spot spectrum may originate from the deeper portions of the spot1 because all the higher parts of the solar atmosphere yield simple1 sharp lines4 ,the bands suggest that chemical compounds can exist at the higher pressure of the inner portions of the sun1 and that these compounds are decomposed in the outer parts of the sun1 to give the line spectra of chemical elements4 ,illustration3 ,fig4 #ca4--,photograph of the solar corona of #ahgj4 7,after ,davis47 ,the year #ahgj was one of maximum sun-spot frequency ,the enigmatical corona lies farther out in the atmosphere of the sun4 ,it consists of streamers which may extend beyond the disk of the sun to the length of several solar diameters4 ,the corona can only be observed at total eclipses of the sun4 ,figs4 #cj to #cb illustrate the appearance of this very peculiar phenomenon4 ,illustration3 ,fig4 #cb4--,photograph of the solar corona of #ahih4 7,after ,maunder47 #ahih was a year of average solar activity ,when the number of sun-spots is small1 the corona streamers extend like huge brooms from the equatorial parts1 and the feebler rays of the corona near the solar poles are then bent downward to the equator1 just like the lines of force about the poles of a magnet 7,fig4 #cj74 ,we suppose1 for this reason1 that the sun acts like a strong magnet1 whose poles are situated near the geographical poles of the sun4 ,in years which are richer in sun-spots the distribution of the streamers of the corona is more uniform4 ,at moderate sun-spot frequency1 large numbers of rays seem to emanate from the neighborhood of the maximum belt of sun-spots1 so that the corona often assumes a quadrangular shape 7compare ,fig4 #cb74 ,these remarks hold for the 8outer corona10 while the inner portion1 the so-called 8inner corona10 shines in a more uniform light4 ,the spectroscopic examination demonstrates that the light consists mainly of hydrogen gas and of an unknown gas designated coronium1 which particularly seems to occur in the higher parts of the inner corona4 ,the outer streamers of the corona1 on the contrary1 yield a continuous spectrum which shows that the light is radiated by solid or liquid particles4 ,in the spectrum of the coronal rays at an extreme distance from the disk1 astronomers have sometimes fancied that they discerned dark lines on a bright ground1 just as in the spectrum of the photosphere4 ,it has been assumed that this light is reflected sunlight1 originating from small solid or liquid particles of the outer corona4 ,it must be reflected1 because it is partly polarized4 ,the radiating disposition of the outer corona indicates the action of a force1 the radiation pressure1 which drives the smaller particles away from the centre of the sun4 ,as regards the temperature of the sun1 we have already seen that the two methods applied for its determination have yielded somewhat unequal results4 ,from the intensity of the radiation1 ,christiansen1 and afterwards ,warburg1 calculated a temperature of about #fjjj ,cent4 ,wilson and ,gray found for the centre of the sun #fbjj1 which they afterwards corrected into #hjjj4 ,owing to the absorption of light by the terrestrial and the solar atmospheres1 we always find too low values4 ,that applies1 to a still greater extent1 to any estimate based upon the determination of that wave-length for which the heat emission from the solar spectrum is maximum4 ,le ,chatelier compared the intensity of sunlight filtered through red glass with the intensities of light from several terrestrial sources of fairly well-known temperatures treated in the same way4 ,these estimates yielded to him a solar temperature of #gfjj ,cent4 ,most scientists reckon with an absolute temperature of #fejj1 corresponding to about #fbjj ,celsius4 ,that is what is known as the 8effective temperature0 of the sun4 ,if the solar rays were not partially absorbed1 this temperature would correspond to that of the clouds of the photosphere4 ,since red light is little absorbed comparatively1 ,le ,chatelier's value of #gfjj1 and the almost equal value of ,wilson and ,gray of #hjjj1 should approximately represent the average temperature of the outer portions of the clouds of the photosphere4 ,the higher temperature of the facul is evident from their greater light intensity1 which1 however1 may partly be due to their greater height4 ,carrington and ,hodgson saw1 on ,september #a1 #ahei1 two facul break out from the edge of a sun-spot4 ,their splendor was five or six times greater than that of the surrounding parts of the photosphere4 ,that would correspond to a temperature of about #aj1jjj or #ab1jjj ,cent4 ,the deeper parts of the sun which broke out on these occasions evidently have a higher temperature1 and this is not unnatural1 since the sun is losing heat by radiation from its outer portions4 ,we know that the temperature of our atmosphere decreases with greater heights4 ,the movements of the air are concerned in this change4 ,a sinking mass of air is compressed by the increased pressure to which it is being exposed1 and its temperature rises1 therefore1 just as the temperature rises in a pneumatic gas-lighter when the piston is pressed down4 ,if the air were dry and in strong vertical motion1 its temperature would change by #aj ,cent4 7#ah ,f47 per km4 ,if it stood still1 it would assume an almost uniform temperature2 that is to say1 there would be no lowering of the temperature as we proceed upward4 ,the actual value lies between the two extremes4 ,as the gravitation in the photosphere of the sun is #bg4d times greater than on the surface of the earth1 we can deduce that1 if the air on the sun were as dense as on the earth1 the temperature on the sun would vary #bg4d times as much as on the earth with the increasing height--that is to say1 by #bgj degrees per kilometre1 provided its atmosphere were in violent agitation4 ,now1 the outer portions of the solar atmosphere are1 indeed1 in violent motion1 so that this latter assumption seems to be justified4 ,but this part consists essentially of hydrogen1 which is #bi times lighter than the air4 ,we must1 therefore1 reduce the value at which we arrive to one-twenty-ninth4 ,as a result1 the final temperature gradient per kilometre would only be #i ,cent4 7#af4b ,f474 ,but the radiation is extremely powerful on the sun1 and it tends to equalize the conditions4 ,nine degrees per kilometre is therefore1 without doubt1 too high a value4 ,further1 in the interior of the sun the gases are much heavier4 ,at a small depth1 however1 they will be so strongly compressed by the upper strata that their further compressibility will be limited1 and the calculation which we have just made loses its validity4 ,yet1 in any case1 the temperature of the sun must increase as we penetrate nearer to its centre4 ,if we accept a temperature gradient per kilometre of the value above indicated1 #i--;it is three times greater in the solid earth-crust--we should obtain for the centre of the sun a temperature of more than six million degrees4 ,all substances melt and evaporate as their temperature is raised4 ,if the temperature exceeds a certain limit1 the 8critical temperature10 the substance can no longer be condensed to a liquid1 however high the pressure may be pushed1 and the substance will only exist as a gas4 ,if we start from -#bgc as absolute zero1 this critical temperature is nearly one and a half times as high as the ebullition temperature of the substance under atmospheric pressure4 ,so far as our experience goes1 it does not appear probable that the critical temperature of any substance could be higher than #aj1jjj or #ab1jjj ,cent41 the highest values which we have calculated for the temperature of the facul4 ,the inner portions of the sun must hence be gaseous1 and the whole sun be a strongly compressed mass of gas of extremely high temperature1 which1 owing to the high pressure1 is at a density #a4d times as great as that of water1 and which in many respects1 therefore1 will resemble a liquid4 ,it must1 for instance1 be extremely viscid1 and that accounts for the relatively great stability of the sun-spots 7one sun-spot held out for a year and a half in #ahdj and #ahda74 ,the sun would thus have to be regarded as a sphere of gas1 in the outer portions of which a certain amount of condensations of cloud character have taken place1 owing to radiation and to the outward movements of the gaseous masses4 ,the pressure in the photosphere--that is1 in those parts in which these clouds are floating--has been averaged at five or six atmospheres1 a figure which1 considering the very high gravitation1 would suggest a layer of superposed gas above it corresponding to not more than a fifth of our terrestrial atmosphere4 ,at an approximately corresponding height1 #aa1ejj m4 7#ch1jjj ft471 there are floating in the terrestrial atmosphere the highest cirrus clouds1 to which the clouds of the photosphere may in many respects be compared4 ,we turn back to the unanswered question whence the sun takes the compensation for the heat which it constantly radiates into space4 ,the most powerful source of heat known to us is that of chemical reactions4 ,the most familiar reaction of daily life is the combustion of coal4 ,by burning one gramme of carbon we obtain #hjjj calories4 ,if the sun consisted of pure carbon1 its energy would not hold out more than #djjj years4 ,it is not to be wondered at1 therefore1 that most scientists soon abandoned the hope of solving the problem in this way4 ,the ,french astronomer ,faye attempted to explain the replenishment of the losses of heat by radiation from the sun by arguments in which he resorted to the heat of a combination of the constituents of the sun4 ,he said3 8,so high a temperature must prevail in the interior of the sun that everything there will be decomposed into its elementary constituents4 ,when the atoms afterwards penetrate into the outer layers1 they are again united1 and they liberate heat40 ,faye thus imagined that new masses of elements would constantly rise from the interior of the sun and would be reunited in chemical combination on the surface4 ,but if new masses are to penetrate upward to the surface1 those which were at first above must go back to the centre of the sun1 in order to be re-decomposed by the great heat there2 and this re-decomposition would consume just as much heat as was gained by the rising of the same masses to the surface4 ,this convection can therefore only help to transport the store of heat from the interior to the surface4 ,the total amount of heat stored in the sun would in this way1 supposing the mean temperature to be six million degrees1 be able to cover the heat expenditure for about three million years4 ,we have1 moreover1 seen that the highest strata of the sun are distinguished by line spectra1 suggestive of simple chemical compounds1 while at greater depth in the sun-spots chemical combinations occur which are characterized by band spectra4 ,it is quite incorrect to assert that high temperatures must necessarily decompose all chemical compounds into their elements4 ,the mechanical theory of heat teaches us only that at rising temperatures products are formed whose formation goes hand in hand with an absorption of heat4 ,thus1 at a high temperature1 ozone is formed from oxygen1 although ozone is more complex in composition than oxygen1 and by this reaction #gej calories are consumed when one gramme of oxygen is transformed into one gramme of ozone4 ,we likewise know that in the electric arc1 at a temperature of about #cjjj1 a compound is formed under consumption of heat by the oxygen and nitrogen of the atmosphere4 ,a new method for the technical preparation of nitric acid from the nitrogen of the air is based upon this reaction4 ,again1 the well-known compounds benzene and acetylene are formed from their elements1 carbon and hydrogen1 under absorption of heat4 ,all these bodies can only be synthesized from their elementary constituents at high temperatures4 ,we further know from experience that the higher the temperature at which a reaction takes place1 the greater1 in general1 the amount of heat which it absorbs4 ,a similar law applies to the influence of pressure4 ,when the pressure is increased1 such processes will be favored as will yield products of a smaller volume4 ,if we imagine that a mass of gas rushes down from a higher stratum of the sun into the depths of the sun's interior1 as gases do in sun-spots1 complex compounds will be produced by virtue of the increased pressure4 ,this pressure must increase at an immense rate towards the interior of the sun1 by about #cejj atmospheres per kilometre4 ,the gases which dissociate into atoms at the lower pressures and the higher temperatures of the extreme solar strata above the photosphere clouds enter into chemical combination in the depths of the spots1 as we learn from spectroscopic examination4 ,owing to their high temperatures1 these compounds absorb enormous quantities of heat in their building up1 and these quantities of heat are to those which are concerned in the chemical processes of the earth in the same ratio as the temperature of the sun is to that at which the chemical reactions are proceeding on the earth4 ,as these gases penetrate farther into the sun1 temperature and pressure are still more and more increased1 and there will result products more and more abounding in energy and concentration4 ,we may1 therefore1 imagine the interior of the sun charged with compounds which1 brought to the surface of the sun1 would dissociate under an enormous evolution of heat and an enormous increase of volume4 ,these compounds have to be regarded as the most powerful blasting agents1 by comparison with which dynamite and gun-cotton would appear like toys4 ,in confirmation of this view1 we observe that gases when penetrating into the photosphere clouds are able to eject prominences at a stupendous velocity1 obtaining several hundred kilometres per second4 ,this velocity surpasses that of the swiftest rifle-bullet about a thousandfold4 ,we may hence ascribe to the explosives which are confined in the interior of the sun energies which must be a million times greater than the energy of our blasting agents4 7,for the energy increases with the square of the velocity47 ,and yet these solar blasting agents have already given up a large part of their energy during their passage from the sun's interior4 ,it thus becomes conceivable that the solar energy--instead of holding out for #djjj years1 as it would if it depended upon the combustion of a solar sphere made out of carbon--will last for something like four thousand million years4 ,perhaps we may further extend this period to several billions4 ,that there are such energetic compounds we have learned from the discovery of the heat evolution of radium4 ,according to ,rutherford1 radium is decomposed by one-half in the space of about #acjj years4 ,in this decomposition a quantity of about a million calories is evolved per gramme and per year1 and we thus find that the decomposition of radium into its final products is accompanied by a heat evolution of about two thousand millions of calories per gramme--about a quarter of a million times more heat than the combustion of one gramme of carbon would yield4 ,in chemical respects as well1 then1 the earth is a dwarf compared to the sun1 and we have every reason to presume that the chemical energy of the sun will be sufficient to sustain the solar heat during many thousand millions and possibly billions of years to come4 ,,iv ,,the ,,radiation ,,pressure ,next to simple measuring and simple calculations1 astronomy appears to be the most ancient science4 ,yet1 though man has worshipped the sun from the most remote ages1 it was not fully comprehended before the middle of the past century that the sun is the source of all life and of all motion4 ,part of the veneration for the sun was transferred to the moon1 with its mild light1 and to the smaller celestial lights4 ,it did not escape notice that their positions in the sky were always changing simultaneously with the annual variations in the weather1 and all human undertakings depended upon the weather and the seasons4 ,the moon and the stars were worshipped--we know now1 without any justification whatever--as ruling over the weather1 and consequently over man's fate4#f ,before anything was undertaken people attempted first to assure themselves of the favorable aspect of the constellations1 and since the most remote ages astrologers have exercised a vast influence over the ignorant and superstitious multitude4 ,footnote #f3 ,the moon strongly1 and more than any other agent1 influences the tides4 ,apart from this effect the position of the moon has only a feeble influence upon the air pressure and upon atmospheric electricity and terrestrial magnetism4 ,the influence of the stars is imperceptible4 ,in spite of the vehement enunciation of ,giordano ,bruno 7#aedh-afjj71 this superstition was still deeply rooted when ,newton succeeded in proving1 in #afhf1 that the movements of the so-called wandering stars1 or planets1 and of their moons could be calculated with the aid of one very simple law3 that all these celestial bodies are attracted by the sun or by their respective central bodies with a force which is proportional to their own mass and to the mass of the central body and inversely proportional to the square of their distance from that central body4 ,newton's contemporary1 ,halley1 applied the law of gravitation also to the mysterious comets1 and calculating astronomy has since been based upon this1 its firmest law1 to which there has not been found any exception4 ,the world was thus at once rid of the paralyzing superstition which exacted belief in a mysterious ruling of the stars4 ,the contemporaries of ,newton1 as well as their descendants1 have rightly valued this discovery more highly than any other scientific triumph of this hero's4 ,according to ,newton's law1 all material bodies would tend to become more and more concentrated and united1 and the development of the universe would result in the sucking up of the smaller celestial bodies--the meteorites1 for instance--by the larger bodies4 ,it must1 however1 be remarked that ,newton's great precursor1 ,kepler1 observed in #afah that the matter of the comets is repelled by the sun4 ,like ,newton1 he believed in the corpuscular theory of light4 ,the sun and all other luminous bodies radiated light1 they thought1 because they ejected minute corpuscles of light matter in all directions4 ,if1 now1 these small corpuscles hit against the dust particles in the comets' tails1 the dust particles would be carried away with them1 and their repulsion by the sun would become intelligible4 ,it is characteristic that ,newton would not admit this explanation of ,kepler's1 although he shared ,kepler's opinion on the nature of light4 ,according to ,newton1 the deviation of the tails of comets from his law of general attraction was only apparent4 ,the tails of comets1 he argued1 behaved like the columns of smoke rising from a chimney1 which1 although the gases of combustion are attracted by the earth1 yet ascend because they are lighter than the surrounding air4 ,this view1 which has been characterized by ,newcomb as no longer to be seriously taken into consideration1 demonstrates the strong tendency of ,newton to explain everything with the aid of his law4 ,the astronomers followed faithfully in the footsteps of their inimitable master1 ,newton1 and they brushed aside every phenomenon which would not fit into his system4 ,an exception was made by the famous ,euler1 who1 in #agdf1 expressed the opinion that the waves of light exerted a pressure upon the body upon which they fell4 ,this opinion1 however1 could not prevail against the criticisms with which others1 and especially ,de ,mairan1 assailed it4 ,that ,euler was right1 however1 was proved by ,maxwell's great theoretical treatise on the nature of electricity 7#ahgc74 ,he showed that rays of heat--and the same applies1 as ,bartoli established in #ahgf1 to radiations of any kind--must exercise a pressure just as great as the amount of energy contained in a unit volume1 by virtue of their radiation4 ,maxwell calculated the magnitude of this pressure1 and he found it so small that it could hardly have been demonstrated with the experimental means then at our disposal4 ,but this demonstration has since been furnished1 with the aid of measurements obtained in a vacuum1 by the ,russian ,lebedeff and by the ,americans ,nichols and ,hull 7#aijj1 #aija74 ,they have found that this pressure1 the so-called radiation pressure1 is exactly as great as ,maxwell predicted4 ,in spite of ,maxwell's great authority1 astronomers quite overlooked this important law of his4 ,lebedeff1 indeed1 tried in #ahib to apply it to the tails of comets1 which he regarded as gaseous2 but the law is not applicable in this case4 ,as late as the year #aijj1 shortly before ,lebedeff was able to publish his experimental verification of this law1 ,i attempted to prove its vast importance for the explanation of several celestial phenomena4 ,the magnitude of the radiation pressure of the solar atmosphere must be equivalent to #b4ge milligrammes if the rays strike vertically against a black body one square centimetre in area4 ,i also calculated the size of a spherule of the same specific gravity as water1 such that the radiation pressure to which it would be exposed in the vicinity of the sun would balance the attraction by the sun4 ,it resulted that equilibrium would be established if the diameter of the sphere were #j4jjae mm4 ,a correction supplied by ,schwarzschild showed that the calculation was only valid when the sphere completely reflects all the rays which fall upon it4 ,if the diameter of the spherule be still smaller1 the radiation pressure will prevail over the attraction1 and such a sphere would be repelled by the sun4 ,owing to the refraction of light1 this will1 according to ,schwarzschild1 further necessitate that the circumference of the spherule should be greater than #j4c times the wave-length of the incident rays4 ,when the sphere becomes still smaller1 gravitation will once more predominate4 ,but spherules whose sizes are intermediate between these two limits will be repelled4 ,it results1 therefore1 that molecules1 which have far smaller dimensions than those mentioned1 will not be repelled by the radiation pressure1 and that therefore ,maxwell's law does not hold for gases4 ,when the circumference of the spherule becomes exactly equal to the wave-length of the radiation1 the radiation pressure will act at its maximum1 and it will then surpass gravity not less than nineteen times4 ,these calculations apply to all spheres1 totally reflecting the light1 of a specific gravity like water1 and to a radiation and attraction corresponding to that of the sun4 ,since the sunlight is not homogeneous1 the maximum effect will somewhat be diminished1 and it is nearly equal to ten times the gravity for spheres of a diameter of about #j4jjjaf mm4#g ,footnote #g3 ,one centimetre of water contains #dgj billions of these spheres4 ,such a little drop of water1 again1 contains #if millions of molecules1 and there are probably organisms which are smaller than these drops4 ,compare the experiments with ultra-microscopic organisms by ,e4 ,raehlmann1 ,n4 ,gaidukow1 and others4 ,before we had recourse to the radiation pressure for the explanation of the repulsion phenomena such as have been observed in the tails of comets1 it was generally believed with ,zllner that the repulsion was due to electrical forces4 ,electricity undoubtedly plays an important part in these phenomena1 as we shall see4 ,the way in which it acts in these instances was explained by a discovery of ,c4 ,t4 ,r4 ,wilson in #ahii4 ,gases can in various ways be transformed into conductors of electricity which as a rule they do not conduct4 ,the conducting gases are said to be ionized--that is to say1 they contain free ions1 minute particles charged with positive or negative electricity4 ,gases can be ionized1 among other ways1 by being radiated upon with ,rntgen rays1 kathode rays1 or ultra-violet light1 as well as by strong heat4 ,since the light of the sun contains a great many ultra-violet radiations1 it is indisputable that the masses of gases in the neighborhood of the sun 7e4g41 probably in comets when they come near the sun7 will partly be ionized1 and will contain both positive and negative ions4 ,ionized gases are endowed with the remarkable capability of condensing vapors upon themselves4 ,wilson showed that this property is possessed to a higher degree by the negative ions than by the positive ions 7in the condensation of water vapor74 ,if there are1 therefore1 water vapors in the neighborhood of the sun which can be condensed by cooling1 drops of water will1 in the first instance1 be condensed upon the negative ions4 ,when these drops are afterwards repelled by the radiation pressure1 or when they sink1 owing to gravity1 as drops of rain sink in the terrestrial atmosphere1 they will carry with them the charge of the negative ions1 while the corresponding positive charge will remain behind in the gas or in the air4 ,in this way the negative and positive charges will become separated from each other1 and electric discharges may ensue if sufficiently large quantities of opposite electricity have been accumulated4 ,by reason of these discharges the gases will become luminescent1 although their temperature may be very low4 ,stark has even shown that low temperatures are favorable for the display of a strong luminosity in electric discharges4 ,we have stated that ,kepler1 as early as the beginning of the seventeenth century1 came to the conclusion that the tails of comets were repelled by the sun4 ,newton indicated how we might1 from the shape of the comets' tails1 calculate their velocity4 ,the best way1 however1 is to determine this velocity by direct observation4 ,the comets' tails are not so uniform in appearance as they are generally represented in illustrations1 but they often contain several luminous nuclei 7,fig4 #cc71 whose motions can be directly ascertained4 ,illustration3 ,fig4 #cc4--,photograph of ,roerdam's comet 7#ahic ,,ii471 suggesting several strong nuclei in the tail ,from a study of the movements of comets' tails1 ,olbers concluded1 about the beginning of the last century1 that the repulsion of the comets' tails by the sun is inversely proportional to the square of their distance--that is to say1 that the force of the repulsion is subject to the same law as the force of gravitation4 ,we can1 therefore1 express the repulsion effect in units of solar gravitation1 and this has generally been done4 ,that the radiation pressure will in the same manner change with the distance is only natural4 ,for the radiation against the same surface is also inversely proportional to the square of the distance from the radiating body1 the sun4 ,illustration3 ,fig4 #cd4--,photograph of ,swift's comet 7#ahib ,i47 ,in the latter part of the past century the ,russian astronomer ,bredichin conducted a great many measurements on the magnitude of the forces with which comets' tails are repelled by the sun4 ,he considered himself1 on the strength of these measurements1 justified in dividing comets' tails into three classes4 ,in the first class the repulsion was #ai times stronger than gravitation2 in the second class1 from #c4b to #a4e times stronger2 and in the third class1 from #a4c to #a times stronger4 ,still higher values have1 however1 been deduced for several comets4 ,thus ,hussey found for the comet of #ahic 7,roerdam's comet1 #ahic ,,ii41 ,fig4 #cc7 a repulsion #cg times as strong as gravitation2 and ,swift's comet 7#ahib ,i47 yields the still higher value of #dj4e 7,fig4 #cd74 ,some comets show several tails of different kinds1 as the famous comet of ,donati 7,fig4 #ce74 ,its two almost straight tails would belong to the first class1 and the more strongly developed and curved third tail to the second class4 ,illustration3 ,fig4 #ce4--;,donati's comet at its greatest brilliancy in #aheh ,schwarzschild1 as already stated1 calculated that small spherules reflecting all the incident light and of the specific gravity of water would be repelled by the sun with a force that might balance ten times their weight4 ,for a spherule absorbing all the light falling upon it this figure would be reduced to five times the weight4 ,the small particles of comets which1 according to spectroscopic observations1 probably consist of hydrocarbons are not perfectly absorbing1 but they permit certain rays of the sun to pass4 ,a closer calculation shows that in this case forces of about #c4c times the gravity would result4 ,larger spherules yield smaller values4 ,bredichin's second and third classes would thus be well adapted to meet the requirements which the radiation pressure demands4 ,it is more difficult to explain how such great forces of repulsion as those of the first group of ,bredichin or of the peculiar comets of ,swift and of ,roerdam can occur4 ,when a particle or drop of some hydrocarbon is exposed to powerful radiation1 it may finally become so intensely heated that it will be carbonized4 ,it will yield a spongy coal1 because gases 7chiefly hydrogen7 will escape during the carbonization1 and the particles of coal will resemble the little grains of coal-dust which fall from the smokestacks of our steamboats1 and which afterwards float on the surface of the water4 ,it is quite conceivable that such spherules of coal 7consisting probably of so-called marguerites1 felted or pearly structures resembling chains of bacilli7 may have a specific gravity of #j4a1 if we make allowance for the gases they include 7compare page #ajf47 ,a light-absorbing drop of this density of #j4a might1 in the most favorable case1 experience a repulsion forty times as strong as the gravitation of the sun4 ,in this manner we can picture to ourselves the possibility of the greatest observed forces of repulsion4 ,the spectra of comets confirm in every respect the conclusions to which the theory of the radiation pressure leads up4 ,they display a faint1 continuous spectrum which is probably due to sunlight reflected by the small particles4 ,besides this1 we observe1 as already mentioned1 a spectrum of gaseous hydrocarbons and cyanogen4 ,these band spectra are due to electric discharges2 for they are observed in comets whose distance from the sun is so great that they cannot appear luminous owing to their own high temperatures4 ,in the tail of ,swift's comet banded spectra have been observed in portions which were about five million kilometres from the nucleus4 ,the electric discharges must chiefly be emitted from the outer parts of the tails1 where1 according to the laws of static electricity1 the electric forces would be strongest4 ,for this reason the larger tails of comets look as if they were enveloped in cloaks of light of a more intense luminosity4 ,illustration3 ,fig4 #cf4--;,imitation of comets' tails4 ,experiment by ,nichols and ,hull4 ,the light of an arc-lamp is concentrated by a lens upon the stream of finely powdered particles ,when a comet comes nearer to the sun1 other less volatile bodies also begin to evaporate4 ,we then find the lines of sodium and1 when the comet comes very close to the sun1 also the lines of iron in its spectrum4 ,these lines are evidently produced by substances which have been evaporated from the nucleus of the comet4 ,like the meteorites falling upon our earth1 the nucleus will consist essentially of silicates1 and particularly of the silicate of sodium1 and1 further1 of iron4 ,we can easily imagine how the tails of comets change in appearance4 ,when a comet draws near to the sun1 we observe that matter is ejected from that part of the nucleus which is turned towards the sun4 ,the case is analogous to the formation of clouds in the terrestrial atmosphere on a hot summer day4 ,the clouds are provided with a kind of hood which envelops like a thin1 semi-spherical veil that side of the nucleus which turns to the sun4 ,sometimes we observe two or more hoods corresponding to the different layers of clouds in the terrestrial atmosphere4 ,from the farther side of the hood matter streams away from the sun4 ,the tails of comets are usually more highly developed when they approach the sun than when they recede from it4 ,that may be1 as has been assumed for a long time1 because a large part of the hydrocarbons will become exhausted while the comet passes the sun4 ,we have also noticed that the so-called periodical comets1 which return to the sun at regular intervals1 showed at every reappearance a fainter development of the tail4 ,comets1 further1 shine at their greatest brilliancy in periods of strong solar-spot activity4 ,we may1 therefore1 assume that in those periods the surroundings of the sun are charged to a relatively high degree with the fine dust which can serve as a condensation nucleus for the matter of the comets' tails4 ,it is also probable that in such periods the ionizing radiation of the sun is more pronounced than usual1 owing to the simultaneous predominance of facul4 ,nichols and ,hull have attempted to imitate tails of comets4 ,they heated the spores of the fungus ,lycoperdon bovista1 which are almost spherical and of a diameter of about #j4jjb mm41 up to a red glow1 and they thus produced little spongy balls of carbon of an average density of #j4a4 ,these they mixed with emery-powder and introduced them into a glass vessel resembling an hour-glass 7,fig4 #cf7 from which the air had previously been exhausted as far as possible4 ,they then caused the powdered mass to fall in a fine stream into the lower part of the vessel while exposing it at the same time to the concentrated light of an arc-lamp4 ,the emery particles fell perpendicularly to the bottom1 while the little balls of carbon were driven aside by the radiation pressure of the light4 ,we also meet with the effects of the radiation pressure in the immediate neighborhood of the sun4 ,the rectilinear extension of the corona streamers to a distance which has been known to exceed six times the solar diameter 7about eight million km47 indicates that repelling forces from the sun are acting upon the fine dust4 ,astronomers have also compared the corona of the sun with the tails of comets1 and ,donitsch would class it with ,bredichin's comets' tails of the second class4 ,it is possible to calculate the mass of the corona from its radiation of heat and light4 ,the heat radiated has been measured by ,abbot4 ,at a distance of #cj1jjj km4 from the photosphere1 the corona radiated only as little heat as a body at -#ee ,cent4 ,the reason is that the corona in these parts consists of an extremely attenuated mist whose actual temperature can be estimated by ,stefan's law at #dcjj ,cent4 ,the corona must1 therefore1 be so attenuated that it would only cover a #aij1jjjth part of the sky behind it4 ,we arrive at the same result when we calculate the amount of light radiated by the corona2 this radiation is of the order of that of the full moon1 being sometimes smaller1 sometimes greater1 up to twice as great4 ,the considerations we have been offering apply to the most intense part of the corona1 the so-called inner corona4 ,according to ,turner1 its light intensity outward diminishes in the inverse ratio of the sixth power of its distance from the centre of the sun4 ,at the distance of a solar radius 7#fij1jjj km47 the light intensity would therefore be only #a4f per cent4 of the intensity near the surface of the sun4 ,let us assume that the matter of the corona consists of particles of just such a size that the radiation pressure would balance their weight 7other particles would be expelled from the inner corona72 then we find that the weight of the whole corona of the sun would not exceed twelve million metric tons4 ,that is not more than the weight of four hundred of our large ocean steamships 7e4g41 the ,oceanic71 and only about as much as the quantity of coal burned on the earth within one week4 ,that the mass of the corona must be extremely rarefied has already been concluded1 from the fact that comets have wandered through the corona without being visibly arrested in their motion4 ,in #ahdc a comet passed the sun's surface at a distance of only one-quarter the sun's radius without being disturbed in its progress4 ,moulton calculated that the great comet of #ahha1 which approached the sun within one-half its radius1 did not encounter a resistance of more than one-fifty-thousandth of its mass1 and that the nucleus of the comet was at least five million times denser than the matter of the corona4 ,newcomb has possibly expressed the degree of attenuation of the corona in a somewhat exaggerated way when he said that it contains perhaps one grain of dust per cubic kilometre 7a cube whose side has a length of three-fifths of a mile74 ,however small the quantity of matter in the corona may be1 and however unimportant a fraction of this mass may pass into the coronal rays1 it is yet certain that there is a constant loss of finely divided matter from the sun4 ,the loss1 however1 is not greater than the supply of matter 7compare below7--namely1 about #cjj thousand millions of tons in a year--so that during one billion years not even one-six-thousandth of the solar mass 7#b #ajbg tons7 will be scattered into space4 ,this number is very unreliable1 however4 ,we know that many meteorites fall upon the earth1 partly as compact stones1 partly as the finest dust of shooting-stars which flash up in the terrestrial atmosphere rapidly to be extinguished4 ,these masses may be estimated at about #bj1jjj tons per year4 ,according to this estimate1 the rain of meteorites which falls upon the sun may amount to #cjj thousand millions of tons in a year4 ,all the suns have emitted matter into space for infinite ages1 and it seems1 therefore1 a natural inference that many suns would no longer be in existence if there had not been a supply of matter to make up for this loss4 ,the cold suns undergo relatively small losses1 but receive just as large inflows of matter as the warm suns4 ,as1 now1 our sun belongs to the colder type of stars1 it is probable that the loss of matter from the sun has for this reason been overestimated by being presumed to be as great as the accession4 ,the presence of dark celestial bodies may compensate for this overestimation4 ,illustration3 ,fig4 #cg4--;,granular chondrum from the meteorite of ,sexes4 ,enlargement #a3 #gj4 ,after ,s4 ,tschermak ,whence do the meteorites come8 ,if they were not constantly being created1 their number should have diminished in the course of ages2 for they are gradually being caught up by the larger celestial bodies4 ,it is not at all improbable that they arise from the accrescence of small particles which the radiation pressure has been driving out of the sun4 ,the chondri1 which are so characteristic of meteorites1 display a structure as if they had grown together out of a multitude of extremely fine grains 7,fig4 #cg74 ,nordenskild says3 8,most meteoric iron consists of an extremely delicate texture of various alloys of metals4 ,this mass of meteoric iron is often so porous that it oxidizes on exposure to the air like spongy iron4 ,the ,pallas iron1 when cut through with a saw1 shows this property1 which is so distressing for the collector4 ,the iron of ,cranbourne1 of ,toluca1 and others--in fact1 almost all the meteorites with a few exceptions--display the same texture4 ,it all indicates that these cosmical masses of iron were built up in the universe by particle being piled upon particle1 of iron1 nickel1 phosphorus1 etc41 analogous to the manner in which one atom of a metal coalesces with another atom when the metal is galvanically deposited from a solution4 ,most of the stony meteorites present a similar appearance4 ,apart from the crust of slag on the surface1 the stone is often so porous and so loose that it might be used as a filtering material1 and it may easily be crumbled between the fingers40 ,when the electrically charged grains of dust coalesce1 their small electrical potential 7of about #j4jb volt7 may increase considerably4 ,under the influence of ultra-violet light these masses of meteorites are discharged when they approach the sun1 as ,lenard has shown4 ,their negative charge then escapes in the shape of so-called electrons4 ,since1 now1 the sun loses through the rays of the corona large multitudes of particles1 and these particles probably carry1 according to ,wilson1 negative electricity with them1 the positive charge must remain behind in the stratum from which the coronal rays were emitted1 and also on the sun itself4 ,if this charge were sufficiently powerful1 it would prevent the negatively charged particles in the corona from escaping from the sun1 and all the phenomena which we have ascribed to the radiation pressure would cease4 ,by the aid of the tenets of the modern theory of electrons1 ,i have calculated the maximum charge that the sun could bear1 if it is not to stop these phenomena4 ,the charge would amount to two hundred thousand millions of coulombs--not by any means too large a quantity of electricity1 as it would only be sufficient to decompose twenty-four tons of water4 ,by means of this positive charge the sun exerts a vast attractive power upon all negatively charged particles which come near it4 ,we have already remarked that the grains of sun-dust which have united to form meteorites lose under the influence of ultra-violet light their charge in the shape of negative electrons1 extremely minute particles1 of which perhaps one thousand weigh as much as one atom of hydrogen 7#a gramme of hydrogen contains about #ajbd atoms1 corresponding to #ajbg electrons74 ,these electrons wander about in space4 ,when they approach a positively charged celestial body they are attracted by it with great force4 ,if the electrons were moving with a velocity of #cjj km4 per second1 as in ,lenard's experiment1 and if the sun were charged to one-tenth the maximum amount just calculated1 it would be able to draw up all the electrons whose rectilinear path 7so far as not curved by the sun's attraction7 would lie at a distance from the sun #abe times as great as the distance between the sun and its most remote planet1 ,neptune1 and #chjj times as great as the distance between the sun and the earth1 which1 after all1 would only be one-sixtieth of the distance from our nearest fixed-star neighbor4 ,the sun drains1 so to speak1 its surroundings of negative electricity1 and this draining effect carries to the sun1 as could easily be proved1 a quantity of electricity which is directly dependent upon the positive charge of the sun4 ,thus1 so far as electricity is concerned1 ample provision has been made for maintaining equilibrium between the income and expenditure of the sun4 99 99 99 99 99 ,when an electrical particle enters into a magnetic field it describes a spiral about the so-called magnetic lines of force2 when at a greater distance1 the particles appear to move in the direction of the lines themselves4 ,the rays of the corona emanating from the solar poles show a distinct curvature like that of the lines of force about a magnet1 and for this reason the sun has been regarded as a big magnet whose magnetic poles nearly coincide with the geographical poles4 ,the coronal rays nearer the equator likewise show this curvature 7compare ,fig4 #cj74 ,the repelling force of the radiation pressure there is1 however1 at right angles to the lines of force and much stronger than the magnetic force1 so that the rays of the corona are compelled to form two big streams flowing in the equatorial direction4 ,this is especially noticeable at times of sun-spot minima4 ,during the times of sun-spot maxima the strength of the radiation pressure of the initial velocity of the grains of dust seem to predominate so markedly that the magnetic force is relatively small4 ,the astronomers tell us that the sun is only a star of small light intensity compared to the prominent stars which excite our admiration4 ,the sun further belongs to a group of relatively cold stars4 ,we may easily imagine1 therefore1 that the radiation pressure in the vicinity of these larger stars will be able to move much larger masses of matter than in our solar system4 ,if the different stars had at any time consisted of different chemical elements1 this difference would have been equalized in the course of ages4 ,the meteorites may be regarded as samples of matter collected and despatched from all possible divisions of space4 ,now1 what bodies do we find in them8 ,in the comets 7compare page #ajd7 iron1 sodium1 carbon1 hydrogen1 and nitrogen 7as cyanogen7 play the most important part4 ,we know1 especially from the researches of ,schiaparelli that meteorites often represent fragments of comets1 and must therefore be related to them4 ,thus ,biela's comet1 which had a period of #f4f years1 has disappeared since #aheb--;it had divided into two parts in #ahdd-ahde4 ,the comet was rediscovered in a belt of meteorites of the same period which approaches the orbit of the earth each year on ,november #bg4 ,similar relations have been observed with regard to several other swarms of meteorites4 ,we know also that the just-mentioned elements which spectrum analysis has proved to exist in comets are the main constituents of the meteorites1 which1 in addition1 contain the metals calcium1 magnesium1 aluminium1 nickel1 cobalt1 and chromium1 as well as the metalloids oxygen1 silicon1 sulphur1 phosphorus1 chlorine1 arsenic1 argon1 and helium4 ,their composition strongly recalls the volcanic products of so-called basic nature--that is to say1 those which contain relatively large proportions of metallic oxides1 and which have been thought for good reasons to hail from the deeper strata of the interior of the earth4 ,lockyer heated meteoric stones in the electric arc to incandescence and found their spectra to be very similar to the solar spectrum4 ,we therefore draw the conclusion that these messengers from other solar systems which bring us samples of their chemical elements are closely related to our sun and to the interior of our earth4 ,that other stars and comets are essentially composed of the same elements as our sun and earth1 spectrum analysis had already intimated to us4 ,but various metalloids1 like chlorine1 bromine1 sulphur1 phosphorus1 and arsenic1 which are of importance for the composition of the earth1 have so far not been traced in the spectra of the celestial bodies1 nor in that of the sun4 ,we find them in meteorites1 however1 and there is not the slightest doubt that we must likewise count them among the essential constituents of the sun and other celestial bodies4 ,it is with difficulty1 however1 that the metalloids can be made to exhibit their spectra1 and this is manifestly the reason why spectrum analysis has not yet succeeded in establishing their presence in the heavens4 ,as regards the recently discovered so-called noble gases helium1 argon1 neon1 krypton1 and xenon1 their presence in the chromosphere has been discovered on spectrograms taken during eclipses of the sun 7,stassano74 ,according to ,mitchell1 however1 these statements would appear to be somewhat uncertain as to krypton and xenon4 ,the small particles of dust which the radiation pressure drives out into space to all possible distances from the sun and the stars may hit against one another and may accumulate to larger or smaller aggregates in the shape of cosmical dust or meteorites4 ,these aggregates will partly fall upon other stars1 planets1 comets1 or moons1 and partly--and this in very great multitudes--they will float about in space4 ,there they may1 together with the larger dark celestial bodies1 form a kind of haze1 which partly hides from us the light of distant celestial bodies4 ,hence we do not see the whole sky covered with luminous stars1 which would be the case if1 as we may surmise1 the stars were uniformly distributed all through the infinite space of the universe1 and if there were no obstacle to their emission of light4 ,if there were no other celestial bodies of very low temperature and very large dimensions which absorbed the heat of the bright suns1 the dark celestial bodies1 the meteorites1 and the dark cosmical dust would soon be so strongly heated by solar radiation that they would themselves turn incandescent1 and the whole dome of the sky would appear to us like one glowing vault whose hot radiation down to the earth would soon burn every living thing4 ,these other cold celestial bodies which absorb the solar rays without themselves becoming hot are known as nebul4 ,more recent researches make us believe that these peculiar celestial bodies occur nearly everywhere in the sky4 ,the wonderful mechanism which enables them to absorb heat without raising their own temperature will be explained later 7in ,chapter ,,vii474 ,as these cold nebul occupy vast portions of space1 most of the cosmical dust must finally1 in its wanderings through infinite space1 stray into them4 ,this dust will there meet masses of gases which stop the penetration of the small corpuscles4 ,as the dust is electrically charged 7particularly with negative electricity71 these charges will also be accumulated in the outer layers of the nebul4 ,this will proceed until the electrical tension becomes so strong that discharges are started by the ejection of electrons4 ,the surrounding gases will therefore be rendered luminescent1 although their temperature may not much 7perhaps by #ej7 exceed absolute zero1 -#bgc ,cent41 and in this way we are enabled to observe these nebul4 ,most of the particles will be stopped before they have had time to penetrate very deeply into a nebula1 and it will therefore principally be the outer portions of the nebul which send their light to us4 ,that would conform to ,herschel's description of planetary nebul1 which display no greater luminosity in their centres1 but which shine as if they formed hollow spherical shells of nebulous matter4 ,it is very easy to demonstrate that only substances1 such as helium and hydrogen1 which are most difficult to condense1 can at this low temperature exist in gaseous form to any noticeable degree4 ,the nebul1 therefore1 shine almost exclusively in the light of these gases4 ,there occurs in the nebul1 in addition to these gases1 a mysterious substance1 nebulium1 whose peculiar spectrum has not been found on the earth nor in the light of stars4 ,formerly the character of the nebular spectrum was explained by the assumption either that no other bodies occurred in nebul than the substances mentioned1 or that all the other elements in them were decomposed into hydrogen--helium was not known then4 ,the simple explanation is that only the gases of the outer layer of the nebul are luminous4 ,how their interiors are constituted1 we do not know4 ,it has been objected to the view just expressed that the whole sky should glow in a nebulous light1 and that even the outer atmosphere of the earth should display such a glow4 ,but hydrogen and helium occur only very sparely in the terrestrial atmosphere4 ,we find1 however1 another light1 the so-called auroral line1 which may possibly be due to krypton in our atmosphere4 ,whichever way we turn the spectroscope on a very clear night1 especially in the tropics1 we observe this peculiar green line4 ,it was formerly considered to be characteristic of the ,zodiacal ,light1 but on a closer examination it has been traced all over the sky1 even where the ,zodiacal ,light could not be observed4 ,one of the objections to our view is therefore unjustified4 ,as regards the other objection1 we have to remark that any light emission must exceed a certain minimum intensity to become visible4 ,there may be nebul1 and they probably constitute the majority1 which we cannot observe because the number of electrically charged particles rushing into them is far too insignificant4 ,a confirmation of this view was furnished by the flashing-up of the new star in ,perseus on ,february #ba and #bb1 #aija4 ,this star ejected two different kinds of particles1 of which the one kind travelled with nearly double the velocity of the other4 ,the accumulations of dust formed two spherical shells around the new star1 corresponding in every respect to the two kinds of comets' tails of ,bredichin's first and second classes1 which we have sometimes observed together in the same comet 7,fig4 #ce74 ,when these dust particles1 on their road1 hit against nebular masses1 the latter became luminescent1 and we thereby obtained knowledge of the presence of large stellar nebul of whose existence we previously had not the faintest suspicion4 ,conditions1 no doubt1 are similar in other parts of the heavens where 8we have not so far discovered any nebul--we believe1 because of the small number of these charged particles straying about in those parts4 ,on the same grounds we may explain the variability of certain nebul which formerly appeared quite enigmatical40 ,v ,,the ,,solar ,,dust ,,in ,,the ,,atmosphere--,,polar ,,lights ,,and ,,the ,,variations ,,of ,,terrestrial ,,magnetism ,we have so far dwelt on the effects which the particles expelled from the sun and the stars exert on distant celestial bodies4 ,it may be asked whether this dust does not act upon our own earth4 ,we have already recognized the peculiar luminescence which on clear nights is diffused over the sky as a consequence of electrical discharges of this straying dust4 ,this leads to the question whether the magnificent polar lights1 which according to modern views are also caused by electric discharges in the higher strata of the atmosphere1 are not produced by dust which the sun sends to us4 ,it will1 indeed1 be seen that we can in this way explain quite a number of the peculiarities of these mysterious phenomena which have always excited man's imagination4 ,we know that meteorites and shooting-stars are rendered incandescent by the resistance which they encounter in the air at an average height of #abj km4 7#ge miles71 sometimes of #aej and #bjj km4 ,in isolated cases meteorites are supposed to have become visible even at still greater altitudes4 ,it would result that there must be appreciable quantities of air still at relatively high elevation1 and that the atmosphere cannot be imperceptible at an altitude of less than #ajj km41 as was formerly assumed4 ,bodies smaller than the meteorites as well as the solar dust we have spoken of--which1 owing to their minuteness and to the strong cooling by heat radiation and conduction that they undergo in passing through the atmosphere1 could never attain incandescence--would be stopped at greater heights4 ,we will assume that they are arrested at a mean height of about #djj km4 7#bej miles74 ,the masses of dust which are expelled by the sun are partly uncharged1 partly charged with positive or negative electricity4 ,only the latter can be connected with the polar lights2 the former would remain dark and slowly sink through our atmosphere to the surface of the earth4 ,they form the so-called cosmical dust1 of whose great importance ,nordenskild was so firmly convinced4 ,he estimated that the yearly increase in the weight of the earth by the addition of the meteorites was at least ten million tons1 or five hundred times more than we stated above 7page #ajh74 ,like ,lockyer and1 in more recent days1 ,chamberlin1 he believed that the planets were largely built up of meteorites4 ,the dust reaching the earth from the sun would not1 were it not electrically charged1 amount to more than #bjj tons in a year4 ,although this figure may be far too low1 yet the supply of matter by these means is certainly very small in comparison with the #bj1jjj tons which the earth receives in the shape of meteorites and shooting-stars4 ,but owing to its extremely minute distribution1 the effect of this dust is very important1 and it may constitute a much greater portion of the finely distributed cosmical dust in the highest strata of the atmosphere than the dust introduced by falling meteorites and shooting-stars4 ,that these particles exert a noticeable influence upon terrestrial conditions1 in spite of their relatively insignificant mass1 is due to two causes4 ,they are extremely minute and therefore remain suspended in our atmosphere for long periods 7for more than a year in the case of the ,krakatoa dust71 and they are electrically charged4 ,in order to understand their action upon the earth1 we will examine how the terrestrial conditions depend upon the position of the earth with regard to the various active portions of the sun1 and upon the change of the sun itself in regard to its emission of dust particles4 ,for this examination we have to avail ourselves of extensive statistical data2 for only a long series of observations can give us a clear conception of the action of solar dust4 ,these particles withdraw from the sun gases which they were able to condense on their surface1 and which had originally been in the chromosphere and in the corona of the sun4 ,the most important among these gases is hydrogen2 next to it come helium and the other noble gases which ,ramsay has discovered in the atmosphere1 in which they occur in very small quantities4 ,as regards hydrogen1 ,liveing and 7after him7 ,mitchell have maintained that it is not produced in the terrestrial atmosphere4 ,occasionally it is certainly found in volcanic gases4 ,thus hydrogen escapes1 for instance1 from the crater of ,kilauea1 on ,hawaii1 but it is burned at once in the atmosphere4 ,if hydrogen were present in the atmosphere1 it would gradually combine with the oxygen to water vapor2 and we have to assume1 therefore1 that the hydrogen must be introduced into our atmosphere from another source--namely1 from the sun4 ,mitchell finds in this view a strong support for the opinion that solar dust is always trickling down through our atmosphere4 ,the quantity of solar dust which reaches our atmosphere will naturally vary in proportion with the eruptive activity of the sun4 ,the quantity of dust in the higher strata influences the color of the light of the sun4 ,after the eruption of the volcano ,rakata on ,krakatoa1 in #ahhc1 and again1 though to a lesser degree1 after the eruption of ,mont ,pele on ,martinique1 red sunsets and sunrises were observed all over the globe4 ,at the same time1 another phenomenon was noticed which could be estimated quantitatively4 ,the light of the sky is polarized with the exception of the light coming from a few particular spots4 ,of these spots1 one called ,arago's ,point is situated a little above the antipode of the sun1 and another1 ,babinet's ,point1 is situated above the sun4 ,if we determine the elevation of these points above the horizon at sunset1 we find in accordance with the theoretical deduction that this elevation is greater when the higher strata of the atmosphere are charged with dust 7as after the eruption of ,rakata7 than under normal conditions4 ,busch1 a ,german scientist1 analyzed the mean elevation of these points 7stated in degrees of arc7 at sunset1 and found the following peculiar numbers3 #ahhf '#hg '#hh '#hi '#ij ,arago's ,point #bj4a #ai4g #ah4d #ag4h #ag4g ,babinet's ,point#bc4i #ba4i #ag4i #ef4h #ae4d ,sun-spot ,number#ba4a #ai4a #f4g #f4a #f4e '#ia '#ib '#ic '#id '#ie ,mean ,arago's ,point #bj4f #ai4f #bj4b #bj4gah4h #ai4d ,babinet's ,point#bc4c #ba4e #bd4b #bc4c #ai4j #bj4g ,sun-spot ,number#ce4f #gc4h #hd4i #gh4j #fc4i #dj4j ,there is a distinct parallelism in these series of figures4 ,almost simultaneously with the sun-spot maximum the height of the two so-called neutral points above the horizon attains its maximum at sunset1 and the same applies to the minimum4 ,that the phenomena in the atmosphere take place a little later than the phenomena on the sun which caused them is perhaps only natural4 ,when the air is rich in dust1 or when it is strongly ionized by kathode rays1 conditions are favorable for the formation of clouds4 ,this can be observed1 for instance1 with auroral lights4 ,they regularly give rise to a characteristic cloud formation1 so much so that ,adam ,paulsen was able to recognize polar lights by the aid of these clouds in full daylight4 ,klein has compiled a table on the connection between the frequency of the higher clouds1 the so-called cirrus clouds1 at ,cologne1 and the number of sun-spots during the period #ahej-aijj4 ,he demonstrates that during this half-century1 which comprises more than four sun-spot periods1 the sun-spot maxima fell in the years in which the greatest number of cirrus clouds had been observed4 ,the minima of the two phenomena are likewise in agreement4 ,a similarly intensified formation of clouds seems also to occur on ,jupiter when sun-spots are frequent4 ,vogel states that ,jupiter at such times shines with a whiter light1 while at sun-spot minima it appears of a deeper red4 ,the deeper we are able to peep into the atmosphere of ,jupiter1 the more reddish it appears4 ,during periods of strong solar activity the higher portions of ,jupiter's atmosphere therefore appear to be crowded with clouds4 ,the discharge of the charged solar dust in our atmosphere calls forth the polar lights4 ,the polar lights occur1 as the name indicates1 most frequently in the districts about the poles of the earth4 ,they are1 however1 not actually more frequent the nearer we come to the poles2 but they attain a maximum of frequency in circles which enclose the magnetic and the geographical poles4 ,the northern maximum belt passes1 via ,cape ,tscheljuskin1 north of ,novaja ,semlja1 along the northwestern coast of ,norway1 a few degrees to the south of ,iceland and ,greenland1 right across ,hudson ,bay and over the northwestern extension of ,alaska4 ,when we go to the south of this belt1 the auroras1 or boreal lights1 diminish markedly4 ,they are four times less frequent in ,edinburgh1 and fifteen times less frequent in ,london or ,new ,york1 than in the ,orkney ,islands or ,labrador4 ,paulsen divides the auroras into two classes1 which behave quite differently in several respects4 ,the great difficulties which the solution of the problems of polar lights has so far offered seem to a large extent to be due to the fact that all polar lights were treated as being of the same kind4 ,the polar lights of the first class do not display any streamers4 ,they cover a large portion of the sky in a horizontal direction4 ,they are very quiet1 and their light is strikingly constant4 ,as a rule1 they drift slowly towards the zenith1 and they do not give rise to any magnetic disturbances4 ,these polar lights generally have the shape of an arch whose apex is situated in the direction of the magnetic meridian 7,fig4 #ch74 ,sometimes several arches are grouped one above another4 ,nordenskild observed these arches quite regularly during the polar night when he was wintering near ,pitlekaj1 in the neighborhood of ,bering ,sound4 ,adam ,paulsen has often seen them on ,iceland and ,greenland1 which are situated within the maximum belt spoken of1 where northern lights are very common4 ,occasionally auroras are also seen farther from the poles1 as circular arches of a milky white1 which may be quite high in the heavens4 ,sometimes we perceive in the arctic regions that large areas of the heavens are covered by a diffused light which might best be compared to a luminous1 transparent cloud2 the darker portions in it probably appear dark by contrast4 ,this phenomenon was frequently observed during the ,swedish expedition of #ahhb-ahhc1 near ,cape ,thordsen4 ,illustration3 ,fig4 #ch4--;,arch-shaped auror borealis1 observed by ,nordenskild during the wintering of the ,vega in ,bering ,strait1 #ahgi ,masses of light at so low a level that the rocks behind them are obscured have frequently been observed to float in the air1 especially in the arctic districts4 ,thus ,lemstrm saw an aurora on the island of ,spitzbergen in front of a wall of rock only #cjj m4 7#ajjj ft47 in height4 ,in northern ,finland he observed the auroral line in the light of the air in front of a black cloth only a few metres distant4 ,adam ,paulsen counts these phenomena also as polar lights of the first class1 and he regards them as phosphorescent clouds which have been carried down by convection currents to an unusually low level of our atmosphere4 ,polar lights of the second class are distinguished by the characteristic auroral rays or streamers4 ,sometimes these streamers are quite separated from one another 7see ,fig4 #ci72 as a rule they melt into one another1 especially below1 so as to form draperies which are so easily moved and unsteady that they appear to flutter in the wind 7,fig4 #da47 ,the streamers run very approximately in the direction of the inclination 7magnetic dip7 needle1 and when they are fully developed around the celestial dome their point of convergence is distinctly discernible in the so-called corona 7,fig4 #dj74 ,when the light is at its greatest intensity the aurora is traversed by numerous waves of light4 ,illustration3 ,fig4 #ci4--;,aurora borealis1 with radial streamers ,the draperies are very thin4 ,paulsen watched them sometimes drifting over his head in ,greenland4 ,the draperies then appeared foreshortened1 in the shape of stri or ribbons of light in convolutions4 ,these polar lights influence the magnetic needle4 ,when they pass the zenith their influence changes sign1 so that the deviation of the magnetic needle changes from east to west when the ribbon is moving from north to south4 ,paulsen therefore concluded that negative electricity 7kathode rays7 was moving downward in these rays4 ,these polar lights correspond to violent displacements of negative electricity1 while polar lights of the first class appear to consist of a phosphorescent matter which is not in strong agitation4 ,the streamers may penetrate down into rather low atmospheric strata1 at least in districts which are near the maximum belt of the northern lights4 ,thus ,parry observed at ,port ,bowen an auroral streamer in front of a cliff only #bad m4 7#gjj ft47 in height4 ,illustration3 ,fig4 #dj4--;,aurora with corona1 observed by ,gyllenskild on ,spitzbergen1 #ahhc ,polar lights of the first order may pass into those of the second order1 and vice versa4 ,we frequently see rays suddenly flash out from the arch of the aurora1 mostly downward1 but1 when the display is very intense1 also upward4 ,on the other hand1 the violent agitation of a 8drapery light0 may cease1 and may give way to a diffused1 steady glow in the sky4 ,the polar light of the first class is chiefly observed in the arctic regions4 ,to it corresponds1 in districts farther removed from the pole1 the diffused light which appears to be spread uniformly over the heavens and which gives the auroral line4 ,illustration3 ,fig4 #da4--,polar-light draperies1 observed in ,finnmarken1 northern ,norway ,the usually observed polar lights 7speaking not only of those seen on arctic expeditions7 belong to the second class1 which comprises also all those included in the subjoined statistics1 with the exception of the auroral displays reported from ,iceland and ,greenland4 ,while the streamer lights distinctly conform to the #aa4a years' period1 and become more frequent at times of sun-spot maxima1 this is not the case1 according to ,tromholt1 with the auroras of ,iceland and ,greenland4 ,their frequency1 on the contrary1 seems to be rather independent of the sun-spot frequency4 ,not rarely auroral maxima corresponding to sun-spot maxima are subdivided into two by a secondary minimum4 ,this phenomenon is most evident in the polar regions1 but it can also be traced in the statistics from ,scandinavia and from other countries4 ,better to understand the nature of auroras1 we will consider the sun's corona during the time of a minimum year1 taking as an example the year #aijj 7compare ,fig4 #cj74 ,the rays of the corona in the neighborhood of the poles of the sun are laterally deflected by the action of the magnetic lines of force of the sun4 ,the small1 negatively charged particles have evidently only a low velocity1 so that they move quite close to the lines of force in the neighborhood of the solar poles and are concentrated near the equator4 ,there the lines of force are less crowded--that is to say1 the magnetic forces are weaker--and the solar dust can therefore be ejected by the radiation pressure and will accumulate to a large disk expanding in the equatorial plane4 ,to us this disk appears like two large streams of rays which project in the direction of the solar equator4 ,part of this solar dust will come near the earth and be deflected by the magnetic lines of force of the earth2 it will hence be divided into two streams which are directed towards the two terrestrial magnetic poles4 ,these poles are situated below the earth's crust1 and therefore not all the rays will be concentrated towards the apparent position of the magnetic poles upon the surface of the earth4 ,it is to be expected that the negatively charged particles coming from the sun will chiefly drift towards that district which is situated somewhat to the south of the magnetic north pole1 when it is noon at this pole4 ,when it is midnight at the magnetic pole1 most of the negatively charged particles will be caught by the lines of force before they pass the geographical north pole1 and the maximum belt of the auroras will for this reason surround the magnetic and the geographical poles1 as has already been pointed out 7compare page #abb74 ,the negatively charged solar dust will thus be concentrated in two rings above the maximum belts of the polar lights4 ,where the dust collides with molecules of the air1 it will produce a phosphorescent glow1 as if these molecules were hit by the electrically charged particles of radium4 ,this phosphorescent glow rises in the shape of a luminous arch to a height of about #djj km4 7#bej miles7--according to ,paulsen--and the apex of this arch will in every part seem to lie in the direction where the maximum belt is nearest to the station of the observer4 ,that will fairly coincide with the direction of the magnetic needle4 ,the solar corona of a sun-spot maximum year is of a very different appearance 7,fig4 #ca74 ,the streamers radiate straight from the sun in almost all directions2 and if there be some privileged directions1 it will be those above the sun-spot belts4 ,the velocity of the solar dust is evidently so great that the streamers are no longer visibly deflected by the magnetic lines of force of the sun4 ,nor is this charged dust influenced to any noticeable degree by the magnetic lines of force of the earth4 ,it will in the main fall straight down in that part of the atmosphere in which the radiation is most intense3 ,as these 8hard0 rays of the sun#h seem to issue from the facul of the sun which are most frequent in maximum sun-spot years1 some polar lights will also be seen in districts which are far removed from the maximum belt of the auroras1 especially when the number of sun-spots is large4 ,the opposite relation holds for the 8soft0 streams of solar dust which fall near the maximum belt of the polar lights4 ,these streams occur most frequently with low sun-spot frequency1 as we know from observations of the solar corona4 ,possibly they are carried along by the stream of harder rays in maximum years4 ,the polar lights corresponding to these rays therefore attain their maximum with few sun-spots4 ,hard and soft dust streams occur1 of course1 simultaneously2 but the former predominate in maximum sun-spot years1 the latter in minimum years4 ,footnote #h3 ,the designations 8hard0 and 8soft0 streams of solar dust correspond to the terms used with regard to kathode rays4 ,the soft rays have a smaller velocity1 and are therefore more strongly deflected by external forces1 as1 for instance1 magnetic forces4 ,that the periodicity of the polar lights in regions without the maximum belt follows very closely the periodicity of the sun-spots was shown by ,fritz as early as #ahfc4 ,the length of the period varies between #g and #af years1 the average being #aa4a years4 ,the years of maxima and minima for sun-spots and for northern auroras are the following3 ,,maximum ,,years ,sun-spots #agbh '#ci '#ej '#fb '#gj '#gh '#hh #ahjd '#af '#cj #ahcg '#dh '#fj '#ga '#hc '#ic #aije ,northern lights #agcj '#da '#di '#fa '#gc '#gh '#hh #ahje '#ai '#cj #ahdj '#ej '#fb '#ga '#hb '#ic #aije ,,minimum ,,years ,sun-spots #agcd '#de '#ee '#fg '#gf '#he '#ih #ahaa '#bc '#cd #ahdd '#ef '#fg '#gh '#hi #aijj ,northern lights #agce '#dd '#ee '#ff '#ge '#hc '#ii #ahaa '#bb '#cd #ahdd '#ef '#ff '#gh '#hi #aijj ,there are1 in addition1 as ,de ,mairan proved in his classical memoir of the year #agdf1 longer periods common to both the number of sun-spots and the number of auroras4 ,according to ,hansky1 the length of this period is #gb years2 according to ,schuster1 #cc years4 ,very pronounced maxima occurred at the beginning and the end of the eighteenth century1 the last in the year #aghh2 afterwards auroras became very rare in the years #ahjj-ahcj1 just as in the middle of the eighteenth century4 ,in #ahej1 and particularly in #ahga1 there were strong maxima2 they have been absent since then4 ,the estimates of the heights of the polar lights vary very considerably4 ,the height seems to be the greater1 on the whole1 the nearer the point of observation is to the equator1 which would well agree with the slight deflection of the kathode rays towards the surface of the earth in regions which are farther removed from the pole4 ,gyllenskild found on ,spitzbergen a mean height of #ee km42 ,bravais1 in northern ,norway1 #ajj to #bjj km42 ,de ,mairan1 in central ,europe1 #ijj km42 ,galle1 again1 #cjj km4 ,in ,greenland1 ,paulsen observed northern lights at very low levels4 ,in ,iceland he fixed the apex of the northern arch which may be considered as a point where the charged particles from the sun are discharged into the air at about #djj km4 ,not much reliance can be placed upon the earlier determinations2 but the heights given conform approximately to the order of magnitude which we may deduce from the height at which the solar dust will be stopped by the terrestrial atmosphere4 ,the polar lights possess1 further1 a pronounced yearly periodicity which is easily explicable by the aid of the solar dust theory4 ,we have seen that sun-spots are rarely observed near the solar equator1 and the same applies to solar facul4 ,they rapidly increase in frequency with higher latitudes of the sun1 and their maximum occurs at latitudes of about fifteen degrees4 ,the equatorial plane of the sun is inclined by about seven degrees towards the plane of the earth's orbit4 ,the earth is in the equatorial plane of the sun on ,december #fth and ,june #dth1 and most distant from it three months later4 ,we may1 therefore1 expect that the smallest number of solar-dust particles will fall on the earth when the earth is in the equator of the sun--that is1 in ,december and ,june--and the greatest number in ,march and ,september4 ,these relations are somewhat disturbed by the twilight1 which interferes with the observation of auroras in the bright summer nights of the arctic region1 while the dark nights of the winter favor the observation of these phenomena4 ,the distribution of the polar lights over the different seasons of the year will become clear from the subjoined table compiled by ,ekholm and myself3 ,iceland and ,united ,southern ,sweden ,norway ,greenland ,states auror 7#ahhc-if7 7#ahfa-ie7 7#ahgb-ib7 7#ahga-ic7 7#ahef-id7 ,january #ajef #bea #hjd #ajje #ef ,february #aagc #cca #gcd #adee #abf ,march #acab #cce #fac #acif #ahc ,april #efh #ij #abh #agbd #adh ,may #agj #f #a #abgj #ed ,june #aj #j #j #ajfa #dj ,july #ed #j #j #abcc #ce ,august #aia #ah #dj #abaj #ge ,september #ajee #bji #dee #agce #abj ,october #aaad #cec #gaf #afcj #aib ,november #ajgg #cbf #haa #abdj #aab ,december #idj #bfj #hfc #iab #ha ,average number #gbg #aha #dcj #acbb #ajb ,in zones where the difference between the lengths of day and night of the different seasons is not very great1 as in the ,united ,states1 and in districts in which the southern light is observed 7about latitude #dj ,s471 the chief minimum falls in winter3 on the northern hemisphere1 in ,december2 on the southern hemisphere1 in ,june or ,july4 ,a less pronounced minimum occurring in the summer4 ,twice in the course of the year the earth passes through the plane of the solar equator4 ,during these periods a minimum of solar dust trickles down upon the earth1 and that period is characterized by a larger number of polar lights which is distinguished by a higher elevation of the sun above the horizon4 ,we may expect this2 for most solar dust will fall upon that portion of the earth over which the sun is highest at noon4 ,the two maxima of ,march or ,april and of ,september or ,october1 when the earth is at its greatest distance from the plane of the solar equator1 are strongly marked in all the series1 except in those for the polar districts ,iceland and ,greenland4 ,there the auroral frequency is solely dependent upon the intensity of the twilight1 so that we find a single maximum in ,december and the corresponding minimum in ,june4 ,more recent statistics 7#ahia-aijc7 indicate1 however1 a minimum in ,december4 ,for the same reason the summer minimum in countries of high latitudes1 like ,sweden and ,norway1 is very much accentuated4 ,similar reasons render it difficult for most localities to indicate the daily periodicity of the polar lights4 ,most of the solar dust falls about noon2 and most polar lights should therefore be counted a few hours after noon1 just as the highest temperature of the day is reached a little after noon4 ,on account of the intense sunlight1 however1 this maximum can only be established in the wintry night of the polar regions1 and even there only when a correction has been made for the disturbing effect of the twilight4 ,in this way ,gyllenskild found a northern-light maximum at #b4dj ,p4,m4 for ,cape ,thordsen1 on ,spitzbergen1 the corresponding minimum being at #g4dj ,a4,m4 ,in other localities we can only ascertain that the polar lights are more intense and more frequent before than after midnight4 ,in central ,europe the maximum occurs at about #i ,p4,m42 in ,sweden and ,norway 7in latitude #fj ,n471 half an hour or an hour later4 ,a few other periods1 approximately of the length of a month1 have been suggested with regard to polar lights4 ,a period lasting #be4ic days predominates in the southern lights1 where the maximum exceeds the average by #dd per cent4 ,for the northern lights in ,norway the corresponding excess percentage is #bc2 for ,sweden1 only #aa4i ,footnote #i3 ,the reason is that in the southern district only very few1 and chiefly the most intense1 auroras are recorded4 ,if we observe very assiduously in a large country1 and conduct the observations at different spots1 we shall find polar light almost every night4 ,this consideration partly wipes out the just-mentioned differences4 ,the same period of nearly twenty-six days had already been pointed out for a long series of other especially magnetic phenomena which1 as we shall see1 are very closely connected with auroras1 and it had also been found in the frequency of thunder-storms and in the variations of the barometer4 ,this periodicity has often been thought to be connected with the axial rotation of the sun4 ,the ,austrian scientist ,hornstein has even gone so far as to propose that the length of this period should be carefully determined1 8because it would give a more accurate value for the rotation of the sun than the direct determinations40 ,we know now that the length of the solar revolution is different for different solar altitudes1 a circumstance with which observations of sun-spot movements at different latitudes had already made ,carrington and ,sprer familiar1 but which was not safely established before ,dunr's spectroscopical determination of the movement of the solar photosphere4 ,dunr found the following sidereal revolutions for different latitudes of the sun to which the subjoined synodical revolution would correspond4 7,by sidereal revolution of a point on the sun we understand the time which elapses between the two moments when a certain star passes1 on two consecutive occasions1 through the meridian plane of the point--that is to say1 through a plane laid through the poles of the sun and the point in question4 ,the synodical revolution is determined by the passage of the earth through this meridian4 ,on account of the proper motion of the earth the synodical period is longer than the sidereal period47 ,latitude on the sun 7degrees7 #j #ae #cj #de #fj #ge ,sidereal revolution 7days7 #be4d #bf4d #bg4f #cj4j #cc4i #ch4e ,synodical revolution 7days7 #bg4c #bh4e #bi4i #cb4g #cg4d #dc4j ,that the periods of rotation of the solar photosphere1 and1 in a similar way1 the periods of the spots1 the facul1 and the prominences1 should become so considerably longer with increasing latitudes is one of the most mysterious problems of the physics of the sun4 ,something similar applies to the clouds of ,jupiter1 but the difference in that case is much smaller--only about one per cent4 ,the clouds of our atmosphere behave quite differently1 a fact which is explained by our atmospheric circulation4#aj ,footnote #aj3 ,the very highest strata of our atmosphere 7at levels of from #bj to #hj km41 #ae to #ej miles7 may perhaps form an exception4 ,the luminous clouds which were observed in the years #ahhc-ahib at ,berlin 7after the eruption of ,krakatoa71 and which were floating at a very high level1 showed a drift with regard to the surface of the earth opposite to the drift of the cirrus clouds1 which are directed eastward4 ,in our case1 of course1 the position of the sun with regard to the earth--that is to say1 the synodical period--can alone be of importance4 ,we recognize that the period of #be4ic days does not at all agree with any period of the solar photosphere4 ,the solar equatorial zone differs least1 and it would be appropriate to reckon with this period1 since the earth never moves very far from the plane of the solar equator1 and returns to that plane1 at any rate1 twice in the course of a year4 ,but there is another peculiarity4 ,the higher a point is situated in the atmosphere of the sun1 the shorter is its period4 ,thus the synodical period of the facul near the equator is on an average #bf4jf1 the period of the spots #bf4hb1 of the photosphere #bg4c days4 ,facul situated at higher levels revolve still more rapidly1 and we are thus driven to the conclusion that the period to which we have alluded agrees with the period of the facul situated at higher levels in the equatorial zone of the sun1 and is probably dependent upon them4 ,that would conform to our ideas concerning the physics of the sun4 ,for the facul are produced in the ascending currents of gas and at rather lower levels than the spherules which are expelled by the radiation pressure4 ,this radiation pressure is strongest just in the neighborhood of the facul4 ,for the same reason the repulsion of the solar dust becomes particularly powerful when the facul are strongly developed--that is to say1 just in the time of pronounced eruptive activity of the sun which is characterized by many sun-spots4 ,we must thus imagine that the radiation of the sun will be stronger in times of strong eruptive activity than during the days of low sun-spot frequencies4 ,direct observations of the intensity of the solar radiation which have been made by ,saveljeff in ,kieff confirm this assumption4 ,it must be pointed out1 however1 that another phenomenon investigated by ,kppen seems to contradict this conclusion4 ,kppen ascertained that in our tropics the temperature was by #j4cb ,cent4 7nearly #j4f ,f47 lower during sun-spot maxima than the average1 and that five years later1 a year before the sun-spot minimum1 it reached its maximum value of #j4da ,cent4 7#j4g ,f47 above the average4 ,a similar peculiarity can be traced to other zones1 but on account of the less steady climates it is much less marked there than in the tropics4 ,a ,french physicist1 ,nordmann1 has fully confirmed the observations of ,kppen4 ,on the other hand1 ,very1 an ,american astronomer1 has found that the temperature in very dry 7desert7 districts of the tropics 7near ,port ,darwin1 #ab #bh ,s41 and near ,alice ,springs1 #bc #ch ,s41 both in ,australia7 is higher at sun-spot maxima than at minima2 but ,very was in this research guided merely by the records of maximum and minimum thermometers4 ,from ,very's investigation it would appear that the solar radiation is really more intense with larger sun-spot numbers4#aa ,this1 it must be remarked1 is only noticeable in exceedingly dry districts in which there is no cloud formation worth mentioning4 ,in other districts the cloud formation which accompanies sun-spot maxima interferes with the simplicity of the phenomena4 ,the cooling effect of the clouds seems in these cases by far to surpass the direct heating effect of the solar rays1 and in this manner ,kppen's conclusion would become explicable4 ,if we could observe the temperatures of the atmospheric strata above the clouds1 their variation would no doubt be in the same degree as that in the desert4 ,footnote #aa3 ,according to ,memery 7,bull4 ,soc4 ,astr41 ,march #g1 #aijf1 p4 #afh7 an instantaneous rise of temperature is observed immediately when a sun-spot is first seen1 and the temperature sinks again when the sun-spot disappears4 ,finally1 we have to note another period in the phenomena of the polar lights--the so-called tropical month1 whose length is #bg4c days4 ,the nature of this period is little understood4 ,it is possibly connected with the electric charge of the moon4 ,the peculiarity of this period is that it acts in an opposite way in the northern and southern hemispheres4 ,when the moon is above the horizon1 it seems to prevent the formation of polar lights2 but for this case the difficulties of observation caused by the moonlight must1 of course1 be taken into consideration4 ,illustration3 ,fig4 #db4--;,curve of magnetic declination at ,kew1 near ,london1 on ,november #ae and #af1 #aije4 ,the violent disturbance of ,november #ae1 #i ,p4,m41 corresponds to the maximum intensity of the aurora4 ,compare the following figure ,celsius and ,hiorter observed in #agda that the polar lights exercise an influence on the magnetic needle4 ,from this circumstance we have drawn the conclusion that the polar lights are in some way due to electric discharges which act upon the magnetic needle4 ,these magnetic effects1 the disturbances of the otherwise steady position of the magnetic needle1 are not influenced by the light of the sun and moon1 and can therefore be studied to greater advantage than auroras4 ,we have already pointed out that it is only the aurora of the radial1 streamer type which exerts this magnetic influence 7compare ,figs4 #db and #dc74 ,illustration3 ,fig4 #dc4--;,curve of horizontal intensity at ,kew on ,november #ae and #af1 #aije4 ,on ,november #ae a magnificent aurora was observed in ,galicia1 ,germany1 ,france1 ,norway1 ,england1 ,ireland1 and ,nova ,scotia1 with a maximum at #i ,p4,m4 ,the polar light was unusually brilliant as early as #f ,p4,m4 ,these magnetic variations show exactly the same periods as the northern lights and the sun-spots4 ,as regards1 first1 the long period of #aa4a years1 our observations prove that the so-called magnetic disturbances of the position of the magnetic needle faithfully reflect the variations in the sun-spots4 ,this connection was discovered in #aheb by ,sabine in ,england1 by ,wolf in ,switzerland1 and by ,gautier in ,france4 ,even the more irregular diurnal variations in the magnetic elements are subjected to a solar period4 ,the magnetic needle points in our districts with its north end towards the north--not exactly1 though1 being deflected towards the west4 ,this western deviation or declination is greatest soon after noon1 about one o'clock1 and this diurnal change is greater in summer than in winter1 and the fluctuation of the position of the magnetic needle greater in daytime than at night-time4 ,it is1 therefore1 manifest that we have to deal with some solar effect4 ,this becomes still more distinct when we study the change with reference to the daily variation in the number of sun-spots4 ,in the subjoined table the variation in the declination has been compiled for ,prague for the years #ahef to #ahhi4 ,only years with maxima and minima of sun-spots and of magnetic variations have been noticed in this table3 #ahef #ahfj #ahfg #ahga #ahgi #ahhd #ahhi ,sun-spot number #d4c #ie4g #g4c #aci4a #c4d #fc4g #f4c ,,daily ,,variations ,,in ,,declination #ahef #ahei #ahfg #ahga #ahgh #ahhc #ahhi ,observed #e4ih #aj4cf #f4ie #aa4dc #e4fe #h4cd #e4ii ,calculated #f4jh #aj4bj #f4bb #ab4ae #f4jd #h4gf #f4ag ,we see that the maxima and minima years of the two phenomena very nearly coincide4 ,the accord is so evident that we may calculate the diurnal variation as proportional to the increase in the number of sun-spots4 ,this is shown by the two last lines of the table4 ,the yearly variation is again exactly the same as that of polar lights1 as the following table indicates1 in which the disturbances of magnetic declination1 horizontal intensity1 and vertical intensity are compiled for ,toronto1 ,canada2 for comparison the means of these three magnitudes are added for ,greenwich4 ,the unit of this table is the average annual variation3 ,,toronto ,jan4 ,feb4 ,mar4 ,april ,may ,june ,july ,declination #j4eg #j4hd #a4aa #a4db #j4ih #j4ec #j4id ,horizontal #j4ef #j4id #j4id #a4ej #j4ij #j4cf #j4fa ,vertical #j4eg #j4gd #a4jh #a4di #a4ab #j4ej #j4ga ,aug4 ,sept4 ,oct4 ,nov4 ,dec4 ,declination #a4af #a4fb #a4ca #j4gh #j4gf ,horizontal #j4ge #a4ga #a4dh #j4ih #j4eh ,vertical #a4jh #a4fa #a4bi #j4ge #j4fa ,,greenwich ,jan4 ,feb4 ,mar4 ,april ,may ,june ,july ,means #j4ic #a4bc #a4bb #a4ji #j4ha #j4ga #j4ha ,aug4 ,sept4 ,oct4 ,nov4 ,dec4 ,means #j4ij #a4ae #a4ah #a4jb #j4hc ,the daily variation of the disturbances has been analyzed by ,van ,bemmelen for the period #ahhb-ahic and for the observatory of ,batavia1 on ,java4 ,the maximum occurs there about #a ,p4,m41 and is about #a4hf times as great as the average value for the day4 ,the minimum of #j4dh occurs at #aa ,p4,m4 ,between #h ,p4,m4 and #c ,a4,m4 the disturbances are almost as rare as about #aa o'clock at night4 ,the variation is greatest with that declination which has its maximum of #c4bf at #ab ,m41 and its minimum of #j4ad at #aa ,p4,m4 ,the period of almost #bf days first investigated by ,hornstein has also been refound in the magnetic variations and disturbances by ,broun1 ,liznr1 and ,c4 ,a4 ,mller4 ,it must be added1 however1 that ,schuster does not consider these data as in any way conclusive4 ,the moon has also a slight influence upon the magnetic needle1 as ,kreil proved as early as #ahda4 ,the effect is in a different sign in the northern and southern hemispheres1 and may be likened to a tidal phenomenon4 ,the ultra-violet rays of the sun are strongly absorbed by the atmosphere1 and they cause an ionization of the molecules of the air4 ,this ionization is1 on the whole1 more marked at higher altitudes4 ,the ascending air currents carry with them water vapor which is condensed on the ions1 particularly on the negative ions4 ,in this way most clouds become negatively charged2 this interesting fact--i4e41 that they are more frequently charged with negative than with positive electricity--was first proved by ,franklin in his kite experiments4 ,when the rain-drops have fallen1 the air above remains positively charged2 this has been observed during balloon ascensions4 ,the clouds which are formed at high levels are most strongly charged2 for this reason thunder-storms over land occur mostly in the summer-time4 ,the thunder-storms also show the #bf-;day period1 as ,bezold has proved for southern ,germany1 and ,ekholm and myself have shown for ,sweden4 ,a vast amount of material concerning these questions and magnetic phenomena in particular has been collected by the various meteorological observatories and is awaiting analysis4 ,although some observers like ,sidgreaves question the correlation of sun-spots and polar lights or magnetic disturbances1 because strong spots have been seen on the disk of the sun without any magnetic disturbances having been noticed1 yet the view predominates that the magnetic disturbances are caused by sun-spots when the sun-spots cross the central meridian of the sun which is opposite the earth4 ,thus ,maunder observed a magnetic storm and a northern light succeeding the passage of a large sun-spot through the central solar meridian on the #hth to the #ajth of ,september1 #ahih4 ,the magnetic effect attained its maximum about twenty-one hours after the passage through the meridian4 ,similarly ,ricc established in ten instances1 in which exact determination was possible1 a time interval of #de4e hours on an average between the meridian passage of a spot and the maximum magnetic effect4 ,ricc also submitted to an analysis the data which ,ellis had collected and which ,maunder had investigated4 ,he found for these instances1 on an average1 almost exactly the same numbers1 the time interval being #db4e hours4 ,that would correspond to a mean velocity of the solar dust of from #iaj to #ihj km4 per second4 ,on the other hand1 we have no difficulty at all in calculating the time which a spherule of a diameter of #j4jjjaf mm4 7those particles travel fastest7 and of the specific gravity of water would need in order to reach the earth1 under the influence of solar gravitation and of a mechanical radiation pressure #b4e times as large from the outside of the sun4 ,the time found1 #ef4a hours1 corresponds to a mean velocity of #gdj km4 per second4 ,in order that the solar dust may move with the velocity calculated by ,ricc1 its specific gravity should be less than #a--viz41 #j4ff and #j4eg4 ,this value looks by no means improbable1 when we assume that the spherules consist of hydrocarbons saturated with hydrogen1 helium1 and other noble gases4 ,we should also arrive at larger velocities for the solar dust1 as has already been pointed out with regard to the tails of comets1 when we presume that the particles consist of felted marguerites of carbon or silicates1 or of iron--materials which we regard as the main constituents of meteorites4 ,it should1 perhaps1 be mentioned that the most intense spectrum line of the polar lights has been found to be characteristic of the noble gas krypton4 ,as this gas is found only in very small quantities in our atmosphere1 it does not appear improbable that it has been carried to us together with the solar dust1 and that its spectrum becomes perceptible during the discharge phenomena4 ,the other spectrum lines of the polar lights belong to the spectra of nitrogen1 argon1 and of the other noble gases4 ,the volumes of noble gases which are brought into our terrestrial atmosphere in this manner would in any case be exceedingly small4 ,the electrical phenomena of our terrestrial atmosphere indirectly possess considerable importance for organic life and1 consequently1 for human beings4 ,by the electrical discharges part of the nitrogen is made to combine with the oxygen and hydrogen 7liberated by the electric decomposition of water vapor7 of our air1 and it thus forms the ammonia compounds1 as well as the nitrates and nitrites1 which are so essential to vegetable growth4 ,the ammonia compounds which play a most important part in the temperate zones appear especially to be formed by the so-called silent discharges which we connect with auroras4 ,the oxygen compounds of nitrogen1 on the other hand1 seem to be chiefly the products of the violent thunder-storms of the tropics4 ,the rains carry these compounds down into the soil1 where they fertilize the plants4 ,the supply of nitrogen thus fixed amounts in the course of a year to about #a4be gramme per square metre in ,europe1 and to almost fourfold that figure in the tropics4 ,if we accept three grammes as the average number for the whole firm land of the earth1 that would mean #c tons per square kilometre1 and about #djj million tons per year for the whole firm land of #acf million square kilometres4 ,a very small portion of this fixed nitrogen1 possibly one-twentieth1 falls on cultivated soil2 a larger portion will help to stimulate plant growth in the forests and on the steppes4 ,we may mention1 for comparison1 that the nitrogen contained in the saltpetre which the mines of ,chili yield to us has risen from #ej1jjj tons in #ahhj to #abj1jjj tons in #ahij1 to #baj1jjj tons in #aijj1 and to #bfj1jjj tons in #aije4 ,the nitrogen produced in the shape of ammonium salts 7sulphate7 by the gas-works of ,europe amounts to about one-quarter of the last-mentioned figure4 ,to this figure we have1 of course1 to add the production of coal-gas ammonia in the ,united ,states and elsewhere4 ,yet even allowing for this item1 we shall find that the artificial supply of combined nitrogen on the earth does not represent more than about one-thousandth of the natural supply4 ,as the nitrogen contents of the air may be estimated at #cihj billion tons1 we recognize that only one part in three millions of the nitrogen of the atmosphere is every year fixed by electric discharges1 presuming that the nitrogen supply to the sea is as great per square kilometre as to the land4 ,the nitrogen thus fixed benefits the plants of the sea and of the land1 and passes back into the atmosphere or into the water during the life of the plants or during their decay4 ,water absorbs some nitrogen1 and equilibrium between the nitrogen contents of the atmosphere and of the sea is thus maintained4 ,hence we need not fear any noteworthy depletion of the nitrogen contents of the air4 ,this conclusion is in accord with the fact that no notable accumulation of fixed nitrogen appears to have taken place in the solid and liquid constituents of the earth4 ,the reader may remember 7compare page #eg7 that during the annual cycle of vegetation not less than one-fiftieth of the atmospheric contents in the carbon dioxide is absorbed4 ,since oxygen is formed from this carbon dioxide1 and since the air contains about seven hundred times as many parts per volume of oxygen as of carbon dioxide1 the exchange of atmospheric oxygen is about one part in thirty-five thousand4 ,in other words1 the oxygen of the air participates about one hundred times more energetically in the processes of vegetation than the nitrogen1 and this is in accordance with the general high chemical activity of oxygen4 ,before we close this chapter we will briefly refer to the peculiar phenomenon known as the ,zodiacal ,light1 which can be seen in the tropics almost any clear night for a few hours after or before sunset or sunrise4 ,in our latitudes the light is rarely visible1 and is best seen about the periods of the vernal and autumnal equinoxes4 ,the phenomenon is generally described as a luminous cone whose basis lies on the horizon1 and whose middle line coincides with the zodiac4 ,hence the name4 ,according to ,wright and ,liais1 its spectrum is continuous4 ,it is stated that the ,zodiacal ,light is as strong in the tropics as that of the ,milky ,way4 ,illustration3 ,fig4 #dd4--,zodiacal ,light in the tropics ,there can be no doubt that this glow is due to dust particles illuminated by the sun4 ,it has therefore been assumed that this dust is floating about the sun in a ring1 and that it represents the rest of that primeval nebula out of which the solar system has been condensed1 according to the theory of ,kant and ,laplace 7compare ,chapter ,,vii474 ,sometimes a fairly luminous band seems to shoot out from the apex of the cone of the ,zodiacal ,light and to cross the sky in the plane of the ecliptic4 ,in that part of the sky which is just opposite the sun this band expands to a larger1 diffused1 not well-defined spot of light covering about #ab of arc in latitude and #ij in longitude4 ,this luminescence is called the counter-glow 7,gegenschein71 and was first described by ,pezenas in #aghj4 ,the most probable view concerning the nature of this counter-glow is that it is caused by small particles of meteorites or dust which fall towards the sun4 ,like the position of the corona of the aurora1 the position of this counter-glow seems to be an effect of perspective2 the orbits of the little particles are directed towards the sun1 and they therefore appear to radiate from a point opposite to it4 ,we know very little about this phenomenon4 ,even the position of the ,zodiacal ,light along the zodiac which has given rise to its name has been questioned1 and it would appear from recent investigations that the glow is situated in the plane of the solar equator4 ,however that may be1 the view is very generally held that the glow is due to particles which come from the sun or enter into it4 ,we have already adduced arguments to prove that the mass of solar dust cannot be unimportant2 this dust may therefore be the cause of the phenomenon which we have just been discussing4 ,,vi ,,end ,,of ,,the ,,sun--,,origin ,,of ,,nebul ,we have seen that the sun is dissipating and wasting almost inconceivable amounts of heat every year3 #c4h4 #ajcc gramme-calories1 corresponding to #b gramme-calories for each gramme of its mass4 ,we have also obtained an idea as to how the enormous storage of heat energy in the sun may endure this loss for ages4 ,finally1 however1 the time must come when the sun will cool down and when it will cover itself with a solid crust1 as the earth and the other planets--so far1 probably1 in a gaseous state--have done long since or will do before long4 ,no living being will be able to watch this extinction of the sun despairingly from one of the wandering planets2 for1 in spite of all our inventions1 all life will long before have ceased on the satellites of the sun for want of heat and light4 ,the further development of the cold sun will recall the actual progress of our earth1 except in so far as the sun will have no life-spending1 central source of light and heat near it4 ,in the beginning the thin1 solid crust will again and again be burst by gases1 and streams of lava will rush out from the interior of the sun4 ,after a while these powerful discharges will stop1 the lava will freeze1 and the fragments will close up more firmly than before4 ,only on some of the old fissures volcanoes will rise and allow the gases to escape from the interior--water vapor and1 to a less extent1 carbonic acid1 liberated by the cooling4 ,then water will be condensed4 ,oceans will flood the sun1 and for a short period it will resemble the earth in its present condition1 though with the one important difference4 ,the extinct sun1 unlike our earth1 will not receive life-giving heat from the outside1 excepting the small amount of radiation from universal space and the heat generated by the fall of meteorites4 ,the temperature fall will therefore be rapid1 and the vanishing clouds of the attenuated atmosphere will not long check radiation4 ,the ocean will become covered with a crust of ice4 ,then the carbonic acid will commence to condense1 and will be precipitated as a light snow in the solar atmosphere4 ,finally1 at a temperature of about -#bjj ,cent41 the gases of the atmosphere will be condensed1 and new oceans1 now principally of nitrogen1 will be produced4 ,let the temperature sink another #bj1 and the energy of the inrushing meteorites will just suffice to balance a further loss of heat by radiation4 ,the solar atmosphere will then consist essentially of helium and hydrogen--the two gases which are most difficult to condense--and of some nitrogen4 ,in this stage the heat loss of the sun will be almost imperceptible4 ,owing to the low thermal conductive power of the earth's crust1 there escapes through each square mile of this crust scarcely one-thousand-millionth part of the heat which the sun is radiating from an equal area of its surface4 ,in future days1 when the solar crust will have attained a thickness of #fj km4 7#dj miles71 its loss of heat will be diminished to the same degree4 ,the temperature on the surface of the sun may then still be some #ej or #fj above absolute zero1 and volcanic eruptions will raise the temperature only for short periods and over small areas4 ,yet in the interior the temperature will still be at nearly the same actual intensity1 something like several million degrees1 and the compounds of infinite explosive energy will be stored up there as today4 ,like an immense dynamite magazine1 the dark sun will float about in universal space without wasting much of its energy in the course of billions of years4 ,immutable1 like a spore1 it will retain its immense store of force until it is awakened by external forces into a new span of life similar to the old life4 ,a slow shrinkage of the surface1 due to the progressive loss of heat of the core and to the consequent contraction1 will in the meanwhile have covered the sun with the wrinkles of old age4 ,let us suppose that the crusts of the sun and the earth have the same thermal conductivity--namely1 that of granite4 ,according to ,homn1 a slab of granite one centimetre in thickness1 whose two surfaces are at a temperature difference of #a ,cent41 will permit #j4ehb calorie to pass per minute per square centimetre of surface4 ,by analogy1 the earth's crust1 with an increase of temperature of #cj per kilometre1 as we penetrate inward1 would allow #a4ge 4#aj-d calorie to pass per minute and per square centimetre 7this is #a_/cehj of the mean heat supply of the earth1 #j4fbe calorie per minute per square centimetre72 while the sun1 with a crust of the same thickness as the earth1 but with a diameter #ajh4f times larger1 would lose #c4c times more heat per minute than the earth receives from it at the present time4 ,at present the sun loses #bbfj million times more heat than the earth receives2 consequently1 the loss of heat would be reduced to #a_/fhf1jjj1jjj of the present amount4 ,if the thickness of the solar crust amounted to #a_/adj of the solar radius--that is to say1 to the same fraction that the thickness of the earth's crust represents of the terrestrial radius--the sun would in #gd1ejj million years not lose any more heat than it does now in a single year4 ,this number has to be diminished1 on account of the colder surface which the sun would have by that time1 to about #fj1jjj million years4 ,considering that the mean temperature of the sun may be as high as #e million degrees ,celsius1 the cooling down to the freezing-point of water might occupy #aej1jjj billion years1 assuming that its mean specific heat is as great as that of water4 ,during this time the crust of the sun would increase in thickness and the cooling would1 of course1 proceed at a decreasing rate4 ,in any case1 the total loss of energy during a period of a thousand billion years could1 under these circumstances1 only constitute a very small fraction of the total stored energy4 ,when an extinct star moves forward through infinite spaces of time1 it will ultimately meet another luminous or likewise extinct star4 ,the probability of such a collision is proportional to the angle under which the star appears--which1 though very small1 is not of zero magnitude--and to the velocity of the sun4 ,the probability is increased by the deflection which these celestial bodies will undergo in their orbits on approaching each other4 ,our nearest neighbors in the stellar universe are so far removed from us that light1 the light of our sun1 requires1 on an average1 perhaps ten years to reach them4 ,in order that the sun1 with its actual dimensions and its actual velocity in space--#bj km4 7#ac miles7 per second--should collide with another star of similar kind1 we should require something like a hundred thousand billion years4 ,suppose that there are a hundred times more extinct than luminous stars--an assumption which is not unjustifiable--the probable interval up to the next collision may be something like a thousand billion years4 ,the time during which the sun would be luminous would represent perhaps one-hundredth of this--that is to say1 ten billion years4 ,this conclusion does not look unreasonable4 ,for life has only been existing on the earth for about a thousand million years1 and this age represents only a small fraction of the time during which the sun has emitted light and will continue to emit light4 ,the probability of a collision between the sun and a nebula is1 of course1 much greater2 for the nebul extend over very large spaces4 ,in such a case1 however1 we need not apprehend any more serious consequences than result when a comet is passing through the corona of the sun4 ,owing to the very small amount of matter in the corona1 we have not perceived any noteworthy effects in these instances4 ,nevertheless1 the entrance of the sun into a nebula would increase the chance of a collision with another sun2 for we shall see below that dark and luminous celestial bodies appear to be aggregated in the nebul4 ,from time to time we see new stars suddenly flash up in the sky1 rapidly decrease in splendor again1 to become extinguished or1 at any rate1 to dwindle down to faint visibility once more4 ,the most remarkable of these exceedingly interesting events occurred in ,february1 #aija1 when a star of the first magnitude appeared in the constellation of ,perseus4 ,this star was discovered by ,anderson1 a ,scotchman1 on the morning of ,february #bb1 #aija4 ,it was then a star of the third magnitude4#ab ,on a photograph which had been taken only twenty-eight hours previous to the discovery of this star1 the star was not visible at all1 although the plate marked stars down to the twelfth magnitude4 ,the light intensity of this new star would hence appear to have increased more than five-thousand-fold within that short space of time4 ,on ,february #bc;d the new star surpassed all other stars except ,sirius in intensity4 ,by ,february #beth it was of the first magnitude1 by ,february #bgth of the second1 by ,march #fth of the third1 and by ,march #ahth of the fourth magnitude4 ,then its brightness began to fluctuate periodically up to ,june #bb;d1 with a period first of three1 then of five days1 while the average light intensity decreased4 ,by ,june #bc;d it was of the sixth magnitude4 ,the light intensity diminished then more uniformly4 ,by ,october1 #aija1 it was a star of the seventh magnitude2 by ,february1 #aijb1 of the eighth magnitude2 by ,july1 #aijb1 of the ninth magnitude2 by ,december1 #aijb1 of the tenth magnitude2 and since then it has gradually dwindled to the twelfth magnitude4 ,when this star was at its highest intensity it shone with a bluish-white light4 ,the shade then changed into yellow1 and by the beginning of ,march1 #aija1 into reddish4 ,during its periodical fluctuations the hue was whitish yellow at its maximum and reddish at its minimum intensity4 ,since then the color has gradually passed into pure white4 ,footnote #ab3 ,stars are classified in magnitude1 the order being such that the most luminous stars have the lowest numbers4 ,a star of the first magnitude is #b4eb times brighter than a star of the second magnitude2 this1 again1 #b4eb times brighter than a star of the third magnitude1 and so on4 ,all this from the point of view of an observer on the earth4 ,the spectrum of this star shows the greatest similarity to that of the new star in the constellation ,auriga 7,nova ,aurig7 of the year #ahib 7,fig4 #de74 ,illustration3 ,fig4 #de4--,spectrum of ,nova ,aurig1 #ahib ,on the whole1 it is characteristic of new stars that the spectrum lines appear double--dark on the violet and bright on the red side4 ,in the spectrum of ,nova ,aurig this peculiarity is1 among others1 striking in the three hydrogen lines ,c1 ,f1 and ,h1 in the sodium line1 in the nebula lines1 and also in the magnesium line4 ,in the spectrum of ,nova ,persei the displacement of the hydrogen lines towards the violet is so great that1 according to ,doppler's principle1#ac the hydrogen gas which absorbed the light must have been moving towards us with a velocity of #gjj or more kilometres 7#dej miles7 per second4 ,some calcium lines show a similar displacement1 which is less noticeable in the case of the other metals4 ,this would appear to indicate that relatively cold masses of gas are issuing from the stars and streaming with enormous velocities towards the earth4 ,the luminous parts of the stars were either at a stand-still or they were moving away from us4 ,the simplest explanation of these phenomena would be that a star when flashing up by virtue of its high temperature and high pressure shows enhanced 7widened7 spectral lines1 whose violet portion is absorbed by the strongly cooled masses of gas which are moving towards us and are cooled by their own strong expansion4 ,these gases stream1 of course1 in all directions from the star1 but we only become aware of those gases which absorb the light of the stars--that is to say1 those situated between the star and the earth1 and streaming in our direction4 ,footnote #ac3 ,when1 standing on a station platform1 we watch an express train rushing through the station1 the pitch of the engine whistle seems to become higher as long as the train is approaching us1 and deeper again when the train is moving away from us4 ,the pitch of a note depends upon the number of oscillations which our ear receives per second4 ,now1 when the train is fast approaching us1 more vibrations are sent into our ear than when the train is at a stand-still1 and the pitch1 therefore1 appears to become higher4 ,the same reasoning holds for light waves1 of which ,doppler1 of ,prague 7,bohemia71 was in fact thinking when first announcing his principle in #ahdb4 ,the wave-length of a particular color of the spectrum is fixed with the aid of some ,fraunhofer line characteristic of a certain metal4 ,if we compare the spectrum of a star and the spectrum of a glowing metal1 photographed on the same plate1 the stellar lines will appear shifted towards the violet end 7violet light is produced by nearly twice as many vibrations of the ether per second as red light7 when the star is moving towards us in the line of sight4 ,this principle has successfully been applied by ,huggins1 ,h4 ,c4 ,vogel1 and others1 for determining the motion of a star in our line of sight4 ,when a star is revolving about its own axis1 the equatorial belt will seem to come nearer to us 7or to recede from us71 while the polar regions will seem to be at a stand-still2 the lines will then appear oblique 7not vertical74 ,in this way ,keeler proved that the rings of ,saturn consists of swarms of meteorites moving at different velocities in the different rings4--,h4 ,b4 ,gradually the light of the metallic lines and of the continuous spectrum on which they were superposed began to fade1 first in the violet1 while the hydrogen lines and nebular lines still remained distinct2 like other new stars1 this star showed1 after a while1 the nebular spectrum4 ,this interesting fact was first noticed by ,h4 ,c4 ,vogel in the new star in the ,swan 7,nova ,cygni1 #ahgf74 ,the star ,p in the ,swan1 which flashed up in the year #afjj1 still offers us a spectrum which indicates the emission of hydrogen gas4 ,it is not impossible that this 8new0 star has not yet reached its equilibrium1 and is still continuing to emit cold streams of gases4 ,insignificant quantities of gas suffice for the formation of an absorption spectrum2 thus the emission of gas might continue for long periods without exhausting the supply4 ,we have already mentioned 7page #aaf7 the peculiar clouds of light which were observed around ,nova ,persei4 ,two annular clouds moved away from this star with velocities of #a4d and #b4h seconds of arc per day between ,march #bi1 #aija1 and ,february1 #aijb4 ,if we calculate backward from these dates the time which must have elapsed since those gases left the star1 we arrive at the date of the week--,february #h to #af1 #aija--;in satisfactory agreement with the period of greatest luminosity of the star of ,february #bc;d4 ,it would1 therefore1 appear that these emanations came from the star and were ejected by the radiation pressure4 ,their light did not mark any noticeable polarization1 and could not be reflected light for this reason4 ,we may suppose that the dust particles discharged their electric charges1 and that the gases became thereby luminous4 ,in this case we were evidently witnesses of the grand finale of the independent existence of a celestial body by collision with some other body of equal kind4 ,the two colliding bodies were both dark1 or they emitted so little light that their combined light intensities did not equal that of a star of the twelfth magnitude4 ,as1 now1 their splendor after the collision was greater than that of a star of the first magnitude1 although their distance has been estimated to be at least #abj light years1#ad their radiation intensity must have exceeded that of the sun several thousand times4 ,under these circumstances the mechanical radiation pressure must also have been many times more powerful than on the surface of the sun1 and the masses of dust which were ejected by the new star must have possessed a velocity very much greater than that of solar dust4 ,yet this velocity must have been smaller than that of light1 which1 indeed1 the effect of the radiation pressure can never equal4 ,footnote #ad3 ,one light year corresponds to #i4e billion kilometres1 and it is the distance which the light traverses in the course of a year4 ,illustration3 ,fig4 #df4--;,diagram indicating the consequences of a collision between two extinct suns1 ,a and ,b #8moving' in the direction of the straight arrows4 ,a rapid rotation in the direction of the curved arrows results1 and two powerful streamers are ejected by ,a ,b1 the explosive substances from the deeper strata of ,a and ,b being brought up to the surface by the collision ,it is not difficult to picture to ourselves the enormous violence with which this 8collision0 must have taken place4 ,a strange body--for instance1 a meteorite--which rushes from the infinite universe into the sun has at its collision a velocity of #fjj km4 7#djj miles7 per second1 and the velocity of the two colliding suns must have been of approximately that order4 ,the impact will in general be oblique1 and1 although part of the energy will of course be transformed into heat1 the rest of the kinetic energy must have produced a rotational velocity of hundreds of kilometres per second4 ,by comparison with this number the actual circumferential speed of the sun1 about #b km4 7#a-a_/d miles7 per second on the equator1 would vanish altogether2 and the difference is still more striking for the earth1 with its #j4dfe km4 per second at the equator4 ,we shall1 therefore1 not commit an error of any consequence if we presume the two bodies to have been practically devoid of circumferential speeds before their collision4 ,at the collision1 matter will have been ejected from both these celestial bodies at right angles to the relative directions of their motion in two powerful torrents1 which would be situated in the plane in which the two bodies were approaching each other 7compare ,fig4 #df74 ,the rotational speed of the double star1 which will be diminished by this ejection of matter1 will have contributed to increase the energy of ejection4 ,we remember1 now1 that when matter is brought up from the interior to the surface of the sun it will behave like an explosive of enormous power4 ,the ejected gases will be hurled in terrific flight about the rapidly revolving central portions4 ,we obtain an idea 7though a very imperfect one7 of these features when we look at a revolving pinwheel in a fireworks display4 ,two pinwheels have been attached to the ends of a diameter and belch forth fire in radial lines4 ,the farther removed from the wheel1 the smaller will be the actual velocity and also the angular velocity of these torrents of fire4 ,similarly with the star4 ,the streams are rapidly cooled1 owing to the rapid expansion of the gas4 ,they will also contain fine dust1 largely consisting of carbon1 probably1 which had been bound by the explosive materials4 ,the clouds of fine dust will obscure the new star more and more1 and will gradually change its white brilliancy into yellow and reddish1 because the fine dust weakens blue-and-green rays more than it does yellow-and-red rays4 ,at first the clouds were so near to the star that they possessed a high angular velocity of their own2 they then appeared to surround the star completely4 ,but after ,march #bb1 #aija1 the outer particles of the streams attained greater distances and assumed longer periods of revolution 7six days72 the star then became more obscured when the extreme dust clouds of the streams covering it happened to get between us and the star4 ,as the streams of particles were moving farther away1 their rotational periods increased gradually to ten days4 ,the star1 therefore1 became periodical with a slowly growing length of period1 and its glow turned more reddish at its minimum than at its maximum of intensity4 ,at the same time1 the absorptive power of the marginal particles decreased1 partly owing to their increasing expansion1 partly because the dust was slowly aggregating to coarser particles2 possibly1 also1 because the finest particles were being driven away by the radiation pressure4 ,the sifting influence which the dust exercised upon the light1 and owing to which the red-and-yellow rays were more readily transmitted than the blue-and-green1 gradually became impaired2 hence the color of the light turned more gray1 and after a certain time the star appeared once more of a whitish hue4 ,this white color indicates that the star must still have a very high temperature4 ,by the continued ejection of dust-charged masses of gas1 probably with gradually decreasing violence1 the light intensity of the star must slowly diminish 7as seen from the earth7 and the distribution of the layers of dust around the luminous core will more and more become uniform4 ,how violent the explosion must have been1 we recognize from the observation that the first ejected masses of hydrogen rushed out with an apparent velocity of at least #gjj km4 per second4 ,this velocity is of the same order as that of the most remarkable prominences of the sun4 ,illustration3 ,fig4 #dg4--,spiral nebula in the ,canes ,venatici4 ,messier #ea4 ,taken at the ,yerkes ,observatory on ,june #c1 #aijb4 ,scale1 #a mm4 #ac4b sec4 of arc ,it will be admitted that these arguments present us with a faithful simile even of the details of the observed course of events1 and it is therefore highly probable that our view is in the main correct4 ,but what has meanwhile become of the new star8 ,spectrum analysis tells us that it has been converted into a stellar nebula like other new stars4 ,the continuous light of the central body has more and more been weakened by the surrounding masses of dust4 ,by the radiation pressure these masses are driven towards the outer particles of the surrounding gaseous envelope consisting principally of hydrogen1 helium1 and 8nebular matter40 ,there the dust discharges its negative electricity1 and thus calls forth a luminescence which equals that of the nebul4 ,illustration3 ,fig4 #dh4--,spiral nebula in the ,triangle4 ,messier #cc4 ,taken at the ,yerkes ,observatory on ,september #d and #f1 #aijb4 ,scale1 #a mm4 #cj4g sec4 of arc ,we have to consider next that owing to the incredibly rapid rotation1 the central main mass of the two stars will1 in its outer portions1 be exposed to centrifugal forces of extraordinary intensity1 and will therefore become flattened out to a large revolving disk4#ae ,as the pressure in the outer portions will be relatively small1 the density of the gases will likewise be diminished there4 ,the energetic expansion and1 more still1 the great heat radiation will lower the temperature at a rapid rate4 ,we have thus to deal with a central body whose inner portion will possess a high density1 and which will resemble the mass of the sun1 while the outer portion will be attenuated and nebular4 ,distributed about the central body we shall find the rest of the two streams of gases which were ejected immediately after the violent collision between the two celestial bodies4 ,a not inconsiderable portion of the matter of these spirally arranged outer parts will probably travel farther away into infinite space1 finally to join some other celestial body or to form parts of the great irregular spots of nebular matter which are collected around the star clusters4 ,another portion1 not able to leave the central body1 will remain in circular movement about it4 ,in consequence of this circular movement1 which will be extremely slow1 the outlines of the two spirals will gradually become obliterated1 and the spirals will themselves more and more assume the shape of nebular rings about the central mass4 ,footnote #ae3 ,a4 ,ritter has calculated that when two suns of equal size collide with one another from an infinite distance1 the energy of the collision is not more than sufficient to enlarge the volume of the suns to four times the previous amount4 ,the largest portion of the mass will therefore probably remain in the centre1 and it will only be masses of light gases which will be ejected4 ,illustration3 ,fig4 #di4--,the great nebula in ,andromeda4 ,taken at the ,yerkes ,observatory on ,september #ah1 #aija4 ,scale1 #a mm4 #ed4f sec4 of arc ,illustration3 ,fig4 #ej4--,ring-shaped nebula in ,lyra4 ,taken at the ,yerkes ,observatory ,this spiral form 7,figs4 #dg and #dh7 of the outer portions of the nebul has for a long time excited the greatest attention4 ,in almost all the investigated instances it has been observed that two spiral branches are coiling about the central body4 ,this would indicate that the matter is in a revolving movement about the central axis of the spiral1 and that it has streamed away from the axis in two opposite directions4 ,sometimes the matter appears arranged as in a coil2 of this type the great nebula of ,andromeda is the best-known example 7,fig4 #di74 ,a closer inspection of this nebula with more powerful instruments indicates1 however1 that it is also spiral and that it appears coiled1 because we are looking at it from the side4 ,the late famous ,american astronomer ,keeler1 who has studied these nebul with greater success than any one else1 has catalogued a great many of them in all the divisions of the heavens which were accessible to his instruments1 and he has found that these formations are predominatingly of a spiral nature4 ,illustration3 ,fig4 #ea4--;,central portion of the great nebula in ,orion4 ,taken at the ,yerkes ,observatory4 ,scale1 #a mm4 #ab sec4 of arc ,some nebul1 like the so-called planetary nebul1 offer rather the appearance of luminous spheres4 ,we may assume in these cases that the explosions were less violent1 and that the spirals1 therefore1 are situated so closely together that they seem to merge into one another4 ,possibly the inequalities in their development have become equalized in the course of time4 ,a few nebul are ring-shaped1 as the well-known nebula of ,lyra 7see ,fig4 #ej74 ,these rings may1 again1 have been formed out of spiral nebul1 and the spirals may have gradually been obliterated by rotation1 while the central nebulous matter may have been concentrated on the planets travelling round the central star4 ,schaeberle1 an eminent ,american astronomer1 has discovered traces of spiral shape also in the ,lyra nebula4 ,another kind of nebula is the ordinary nebula of vast extension and irregular shape1 evidently formed out of most extremely attenuated matter2 well-known characteristic examples are found in ,orion1 about the ,pleiades1 and in the ,swan 7,figs4 #ea1 #eb1 and #ec74 ,in these nebul portions of a spiral structure have likewise often been discerned4 ,illustration3 ,fig4 #eb4--,nebular stri in the stars of the ,pleiades4 ,taken at the ,yerkes ,observatory on ,october #ai1 #aija4 ,scale1 #a mm4 #db4b sec4 of arc ,we have said that the collision between two celestial bodies would result in the formation of a spiral with two wings4 ,if the impact is such that the two centres of the celestial bodies move straight towards each other1 a disk will arise1 and not a spiral2 or if one star is much smaller than the other1 possibly a cone1 because the gases will uniformly be spread in all directions about the line of impact4 ,a perfectly central impact is obviously very rare2 but there may be cases which approach this limiting condition more or less1 especially when the relative velocity of the two bodies is small4 ,by slow diffusion a feebly developed spiral may also be converted into a rotating disklike structure4 ,the extension of these nebular structures will depend upon the ratio between the mass of the system and the velocity of ejection of the gases4 ,if1 for example1 two extinct suns of nearly equal dimensions and mass1 like our sun1 should collide1 some gas masses would travel into infinite space1 being hurled out with a velocity of more than #ijj km4 7#eej miles7 per second2 while other particles1 moving at a slower rate1 would remain in the neighborhood of the central body4 ,the nearer to that body1 the smaller was their velocity4 ,from their position they might fall back into the central body1 to be reincorporated in it1 if two circumstances did not prevent this4 ,the one circumstance is the enormous radiation pressure of the glowing central mass4 ,that pressure keeps masses of dust particles floating1 which by friction will carry the surrounding masses of gas with them4 ,owing to the absorption of the radiation by the dust particles1 only the finer particles will be supported farther outside1 and at the extreme margin of the nebula even the very finest dust will no longer be maintained in suspension by the greatly weakened radiation pressure4 ,thus we arrive at an outer limit for the nebula4 ,the other circumstance is the violent rotation which is set up by the impact of the central bodies4 ,the rotation and the centrifugal forces will produce a disk-shaped expansion of the whole central mass4 ,owing to molecular collisions and to tidal effects1 the angular velocity will in the denser portions tend to become uniform1 so that the whole will rotate like a flattened-out ball filled with gas1 and the spiral structure will gradually disappear in those parts4 ,in the more remote particles the velocity will only increase to such an extent as to equal that of a planet moving at the same distance--that is to say1 the gravitation towards the central body will be balanced by the centrifugal force1 and at the very greatest distances the molecular bombardments1 as well as gravitation towards the centre1 will become so insignificant that any masses collected there will retain their shape for an almost unlimited space of time4 ,in the centre of this system the main bulk of the matter would be concentrated in a sun of extreme brightness1 whose light intensity would1 however1 owing to strong radiation1 diminish with comparative rapidity4 ,illustration3 ,fig4 #ec4--,nebular stri in the ,swan4 ,new ,general ,catalogue1 #fiib4 ,taken at the ,yerkes ,observatory on ,october #e1 #aija4 ,scale1 #a mm4 #da sec4 of arc ,such an extensive nebular system1 in which gravitation1 on account of the enormous distances1 would act feebly and very slowly1 would yet1 in spite of the extraordinary attenuation of matter in its outer portions1 and just on account of its vast extension1 be able to stop the movement of the particles of dust penetrating into it4 ,if the gases of the nebula are not to escape into space1 notwithstanding the infinitesimal gravitation1 their molecules must be assumed to be almost at a stand-still1 and their temperature must not rise by more than #ej or #fj ,cent4 above absolute zero4 ,at such low temperatures the so-called adsorption plays an enormously important part 7,dewar74 ,the small dust particles form centres about which the gases are condensed to a remarkable degree4 ,the extremely low density of these gases does not prevent their condensation2 for the adsorption phenomenon follows a law according to which the mass of condensed gas will only be reduced by about one-tenth when the density of the surrounding gas has been decreased by one-ten-thousandth4 ,the mass of dust particles or dust grains will thus be augmented1 and when they collide they will be cemented together by the semiliquid films condensed upon them4 ,there must1 hence1 be a relatively energetic formation of meteorites in the nebul1 and especially in their interiors4 ,then stars and their satellites1 migrating through space1 will stray into these swarms of gases and meteorites within the nebul4 ,the larger and more rapidly moving celestial bodies will crush through this relatively less dense matter2 but thousands of years may yet be occupied in their passing through nebul of vast dimensions4 ,illustration3 ,fig4 #ed4--,nebula and star rift in the ,swan1 in the ,milky ,way4 ,taken by ,m4 ,wolf1 ,knigstuhl1 near ,heidelberg ,illustration3 ,fig4 #ee4--;,great nebula near ,rho1 in ,ophiuchus4 ,photograph by ,e4 ,e4 ,barnard1 ,lick ,observatory4 ,there are several empty spots and rifts near the larger stars of the nebula ,an extraordinarily interesting photograph obtained by the celebrated ,professor ,max ,wolf1 of ,heidelberg1 shows us a part of the nebula in the ,swan into which a star has penetrated from outside4 ,the intruder has collected about it the nebulous matter it met on its way1 and has thus left an empty channel behind it marking its track4 ,similar spots of vast extent1 relatively devoid of nebulous matter1 occur very frequently in the irregular nebul2 they are frequently called 8fissures10 or by the specifically ,english term 8rifts10 because they have generally a long-drawn-out appearance4 ,the presumption that these rifts represent the tracks of large celestial bodies which have cut their way through widely expanded nebular masses 7,fig4 #ed7 has been entertained for a long time4 ,illustration3 ,fig4 #ef4--,star cluster in ,hercules4 ,messier #ac4 ,taken at the ,yerkes ,observatory4 ,scale1 #a mm4 #i4bb sec4 of arc ,the smaller and more slowly moving immigrants1 on the other hand1 are stopped by the particles of the nebul4 ,we therefore see the stars more sparsely distributed in the immediate neighborhood of the nebul1 while in the nebul themselves they appear more densely crowded4 ,this fact had struck ,herschel in his observations of nebul2 in recent days it has been investigated by ,courvoisier and ,m4 ,wolf4 ,in this way several centres of attraction are created in a nebula2 they condense the gases surrounding the nebula1 and catch1 so to say1 any stray meteorites and collect them especially in the inner portions of the nebula4 ,we frequently observe1 further1 how the nebular matter appears attenuated at a certain distance from the luminous bright stars 7compare ,figs4 #eb and #ee74 ,finally1 the nebul change into star clusters which still retain the characteristic shapes of the nebul2 of these the spiral is the most usual1 while we also meet with conical shapes1 originating from conical nebul1 and spherical shapes 7compare ,figs4 #ef1 #eg1 and #eh74 ,this is1 broadly1 the type of evolution through which ,herschel1 relying upon his observations1 presumed a nebula to pass4 ,he was1 however1 under the impression that the nebulous matter would directly be condensed into star clusters without the aid of strange celestial immigrants4 ,illustration3 ,fig4 #eg4--,star cluster in ,pegasus4 ,messier #ae4 ,taken at the ,yerkes ,observatory4 ,scale1 #a mm4 #f4d sec4 of arc ,it has been known since the most ancient times1 and has been confirmed by the observations of ,herschel and others in a most convincing manner1 that the stars are strongly concentrated about the middle line of the ,milky ,way4 ,it is not improbable that there was originally a nebula of enormous dimensions in the plane of the ,milky ,way1 produced possibly by the collision of two such giant suns as ,arcturus4 ,this gigantic nebula has gathered up the smaller migrating celestial bodies which1 in their turn1 have condensed upon themselves nebular matter1 and have thereby become incandescent1 if they were not so before4 ,the rotational movement in those parts which were far removed from the centre of the ,milky ,way may be neglected4 ,at a later period collisions succeeded between the single stars which had been gathered up1 and it is for this reason that gaseous nebul1 as well as new stars1 are comparatively frequent phenomena in the plane of the ,milky ,way4 ,this view may some day receive confirmation1 when we succeed in proving the existence of a central body in the ,milky ,way1 evidence of which might possibly be deduced from the curvature of the orbits of the sun or of other stars4 ,illustration3 ,fig4 #eh4--;,cone-shaped star cluster in ,gemini4 ,as regards the ring-shaped nebula in the ,lyre 7,fig4 #ej71 the most recent measurements made by ,newkirk point to the result that the star visible in its centre is distant from us about thirty-two light-years4 ,as it appears probable that this star really forms the central core of the nebula1 the distance of the nebula itself must be thirty-two light-years4 ,from the diameter of the ring-shaped nebula which ,newkirk estimates at one minute of arc1 this astronomer has calculated that the distance of the ring from its central body is equal to about three hundred times the radius of the earth's orbit--that is to say1 the ring is about ten times as far from its sun as ,neptune is from our sun4 ,there is a faint luminescence within this ring4 ,the nebular matter may originally have been more concentrated at this spot than in the outer portions of the ring itself4 ,but this mass was probably condensed on meteors which immigrated from outside1 and when these meteors coalesced dark planets were produced which move about the central body1 and which have gathered about them most of the gases4 ,if that central body were as heavy as our sun1 the matter in the ring should revolve about it in five thousand years4 ,that rotation would suffice to wipe out the original spiral shape1 enough of which has yet been left to permit of our distinctly discerning the two wings of the spiral4 ,the central body of this ring-shaped nebula gives a continuous spectrum of bright lines which is particularly developed on the violet side4 ,the star would therefore appear to be much younger and much hotter than our sun1 and its radiation pressure would therefore be much more intense4 ,the period of rotation of the nebula may1 for this reason1 have to be estimated at a considerably higher figure4 ,the eminent ,dutch astronomer ,kapteyn has deduced from the proper motions of #afh nebul that their average distance from the earth is about seven hundred light-years and equal to that of stars of the tenth magnitude4 ,the old idea1 that the nebul must be infinitely farther removed from us than the fainter stars1 would therefore appear to be erroneous4 ,according to the measurements of ,professor ,bohlin1 the nebula in ,andromeda may indeed be at a distance of not more than forty light-years4 ,the 8new stars0 form a group among the peculiar celestial bodies which on account of their variable light intensity have been designated as 8variable stars10 and of which a few typical cases should be mentioned1 because a great scientific interest attaches to these problems4 ,the star ,eta1 in ,argus1 may be said to illustrate the strange fate that a star has to pass through when it has drifted into a nebula filled with immigrated celestial bodies4 ,it is one of the most peculiar variable stars4 ,the star shines through one of the largest nebular clouds in the heavens4 ,whether it stands in any physical connection with its surroundings cannot be stated without further examination4 ,the star might1 for instance1 be at a considerable distance in front of the nebula1 between the latter and ourselves4 ,its frequent change in light intensity suggests1 however1 a series of collisions1 which do not appear unnatural to us when we suppose that the star is within a nebula into which many celestial bodies have drifted4 ,as this star belongs to the southern hemisphere1 it was not observed before our astronomers commenced to visit that hemisphere4 ,in #afgg it was classed as a star of the fourth magnitude2 ten years later it was of the second magnitude2 the same in #agea4 ,in #ahbg it was of the first magnitude1 and it was found to be variable--that is to say1 it shone with variable brightness4 ,herschel observed that it fluctuated between the first and second magnitudes1 and that it increased in brightness after #ahcg1 so that it was by #ahch of magnitude #j4b4 ,after that it began to decrease in intensity up to ,april1 #ahci1 when it had the magnitude #a4a4 ,it remained for four years approximately at this intensity2 then it increased rapidly again in #ahdc1 and surpassed all stars except ,sirius 7magnitude -#a4g74af ,afterwards its intensity slowly diminished once more1 so that it remained just visible to the naked eye 7sixth magnitude72 by #ahfi it had become invisible4 ,since then it has been fluctuating between the sixth and seventh magnitudes4 ,footnote #af3 ,this figure1 -#a4g1 signifies that the brightness of ,sirius is #b4ebb4g #ab times greater than that of a star of magnitude #a4 ,next to ,sirius comes ,canopus1 with magnitude -#a4j1 being #f4c times brighter than a star of magnitude #a4 ,the last changes in the intensity of this star strongly recall the behavior of the new star in ,perseus1 only that the latter has been passing through its phases at a much more rapid rate4 ,it appears to be certain1 however1 that ,eta1 in ,argus1 was from the very beginning far brighter than ,nova ,persei1 and that at least once before the great collision in #ahdc 7after which it was surrounded by obscuring clouds of increasing opacity7--namely1 in ,january1 #ahch1 it had been exposed to a slight collision of quickly vanishing effect4 ,this lesser collision was probably of the kind which ,mayer imagined for the earth and sun4 ,it would give rise to heat development corresponding to the heat expenditure of the sun in about a hundred years4 ,as it had been observed that the star was variable in an irregular manner before that1 we may1 perhaps1 presume that it had already undergone another collision4 ,according to the observations of ,borisiak1 a student in ,kief1 the new star in ,perseus would have been1 on the evening of ,february #ba1 #aija1 of #a4e magnitude1 while a few hours previously it had been of magnitude #ab1 and the following evening of magnitude #b4g2 afterwards its intensity increased up to the following evening1 when it outshone all the other stars in the northern sky4 ,if this statement is not based on erroneous observations1 the new star must have been in collision with another celestial body two days before its great collision1 perhaps with a small planet in the neighborhood of the sun1 with which it later collided4 ,that would account for its temporary brilliancy4 ,new stars are by no means so rare as one might perhaps assume4 ,almost every year some new star is discovered4 ,by far most of these are seen in the neighborhood of the ,milky ,way1 where the visible stars are unusually crowded1 so that a collision which would become visible to us may easily occur4 ,for similar reasons we find there also most of the gaseous nebul4 ,most of the star clusters are also in the neighborhood of the ,milky ,way4 ,this is in consequence of the facts just alluded to4 ,the nebul which are produced by collisions between two suns are soon crossed by migrating celestial bodies such as meteorites or comets1 which there occur in large numbers2 by the condensing action of these intruders they are then transformed into star clusters4 ,in parts of the heavens where stars are relatively sparse 7as at a great distance from the ,milky ,way71 most of the nebul observed exhibit stellar spectra4 ,they are nothing but star clusters1 so far removed from us that the separate stars can no longer be distinguished4 ,that single stars and gaseous nebul are so rarely perceived in these regions is1 no doubt1 due to their great distance4 ,among the variable stars we find quite a number which display considerable irregularity in their fluctuations of brightness1 and which remind us of the new stars4 ,to this class belongs the just-mentioned star ,eta1 in ,argus4 ,another example 7the first one which was recognized as 8variable07 is ,mira ,ceti1 which may be translated1 8,the ,wonderful ,star in the ,constellation of the ,whale40 ,this mysterious body was discovered by the ,frisian priest ,fabricius1 on ,august #ab1 #aeif1 as a star of the second magnitude4 ,the priest1 an experienced astronomer1 had not previously noticed this star1 and he looked for it in vain in ,october1 #aeig4 ,in the years #afch and #afci the variability of the star was recognized1 and it was soon ascertained to be irregular4 ,the period has a length of about eleven months1 but it fluctuates irregularly about this figure as a mean value4 ,at its greatest intensity the star ranks with those of the first or second order4 ,sometimes it is weaker1 but it is always of more than the fifth magnitude4 ,ten weeks after a maximum the star is no longer visible1 and its brightness may diminish to magnitude #i4e4 ,in other words1 its intensity varies about in the ratio of #a3 #ajjj 7or possibly more74 ,after a minimum the brightness once more increases1 the star becomes visible again--that is to say1 it attains the sixth magnitude--and after another six weeks it will once more be at its maximum4 ,we have evidently to deal with several superposed periods4 ,the spectrum of this star is rather peculiar4 ,it belongs to the red stars with a band spectrum which is crossed by bright hydrogen lines4 ,the star is receding from us with a velocity of not less than #fc km4 7#ci miles7 per second4 ,the bright hydrogen lines which correspond to the spectrum of the nebula may sometimes be resolved into three components1 of which the middle one corresponds to a mean velocity of #fj km41 and the two others have variable receding velocities of #ce and #hb km4--that is to say1 velocities of #be or #bb km4 less or more than the mean velocity4 ,evidently the star is surrounded by three nebul2 one is concentrated about its centre2 the two others lie on a ring the matter of which has been concentrated on two opposite sides4 ,the ring1 which recalls the ring nebula in the ,lyre1 seems to move about the star with a velocity of #bc4e km4 per second4 ,as this revolution is accomplished within eleven--or1 more correctly1 within twenty-two months1 since there must be two maxima and two minima during one revolution--the total circumference of the ring will be #bc4e #hf1djj #fgj--acfa millions1 and the radius of its orbit #bag million km41 which is #a4de times greater than the radius of the earth's orbit4 ,now the velocity of the earth in its orbit is #bi4e km4 7#ah4c miles7 per second4 ,a planet at #a4de times that distance from the sun would have the 7#a4bjc times smaller7 velocity of #bd4e km4 per second1 which is approximately that of the hypothetical ring of ,mira ,ceti4 ,we conclude1 therefore1 that the mass of the central sun in ,mira ,ceti will nearly equal the mass of our sun4 ,the calculation really suggests that ,mira would be about eight per cent4 smaller2 but the difference lies within the range of the probable error4 ,chandler has directed attention to a striking regularity in these stars4 ,the longer the period of their variation1 the redder in general their color4 ,this is easily comprehended4 ,the denser the original atmosphere1 the more widely the gases will have extended outward from the star1 and the more dust will have been caught or secreted by it4 ,we have seen that the limb of the sun has a reddish light because of the quantities of dust in the solar atmosphere4 ,the effect is chiefly to be ascribed to the absorption of the blue rays by the dust2 but it may partly be explained on the assumption that the solar radiations render the dust incandescent1 though its temperature may be lower than that of the photosphere1 because the dust lies outside the sun1 and that it will therefore emit a relatively reddish light4 ,the more dust there is in a nebula1 the redder will be its luminescence2 and as the quantity of dust increases in general with the extension of the nebula1 that star which is surrounded by wider rings of nebul will in general be more red2 but the greater the radius of the ring1 the longer also will in general be its period4 ,the so-called red stars show1 in addition to the bright hydrogen lines1 banded spectra which indicate the presence of chemical compounds4 ,on this account such stars were formerly credited with a lower temperature4 ,but the same peculiarity is also observed in sun-spots1 although the latter1 on account of their position1 must have a higher temperature than the surrounding photosphere4 ,the presence of bands in the spectrum certainly suggests high pressure1 however4 ,the red stars are evidently surrounded by a very extensive atmosphere of gases1 in the inner portions of which the pressure is so high that the atoms enter into combination4 ,the spectra of the red stars display1 on the whole1 a striking resemblance to those of the sun-spots4 ,the violet portion of the spectrum is weakened1 because the masses of dust have extinguished this light4 ,owing to the large masses of dust which lie in our line of sight1 the spectrum lines are in both cases markedly widened and sometimes accompanied by bright lines4 ,another class of stars1 distinguished by bright lines1 comprises those studied by ,wolf and ,rayet1 and named after them4 ,these stars are characterized by a hydrogen atmosphere of enormous extension1 large enough in some cases1 it has been calculated1 to fill up the orbit of ,neptune4 ,these stars are evidently either hotter and more strongly radiating than the red stars1 or there is not so much dust in their neighborhood--the dust may possibly have been expelled by the strong radiating pressure4 ,they are1 therefore1 classed with the yellow1 and not with the red stars4 ,although there is every reason to suppose that their central bodies are at least as hot as those of the white stars1 the dust is yet able to reduce the color to yellow1 owing to the vast extensions of their atmospheres4 ,the unequal periods in stars like ,mira may be explained by the supposition that there are several rings of dust moving about them1 as in the case of the planet ,saturn4 ,in the case of the inner rings which have a short period1 there has probably been sufficient time during the uncounted number of revolutions to effect a uniform distribution of the dust4 ,hence we do not discern any noteworthy nuclei in them1 such as we observe in the tails of comets2 the dust rings only help to impart to the star a uniform reddish hue4 ,in the outer rings the distribution of dust will1 however1 not be uniform4 ,one of the rings may be responsible for the chief proper period4 ,by the co-operation of other less important dust rings1 the maximum or minimum1 we shall easily understand1 may be displaced1 and thus the time interval between the maxima and minima be altered4 ,this alteration of the period is so strong for some stars that we have not yet succeeded in establishing any simple periodicity4 ,the best-known star of this type is the bright-red star ,betelgeuse in the constellation of ,orion4 ,the brightness of this star fluctuates irregularly between the magnitudes #a4j and #a4d4 ,by far the largest number of variable stars belong to the type of ,mira4 ,others resemble the variable star ,beta in the constellation of the ,lyre1 and thus belong to the ,lyre type4 ,the variability of the spectra of a great many of these stars indicates that they must be moving about a dark star as companion1 or rather that they both move about a common centre of gravity4 ,the change in the light intensity is1 as a rule1 explained by the supposition that the bright star is partially obscured at times by its dark companion4 ,many irregularities1 however1 in their periods and other circumstances prove that this explanation is not sufficient4 ,the assumption of rings of dust circulating about the star and of larger condensation centres affords a better elucidation of the variability of these stars4 ,they are grouped with the white or yellow stars1 in whose surroundings the dust does not play so large a part as in that of ,mira ,ceti4 ,the period of their variability is1 as a rule1 very short1 moreover--generally only a few days 7the shortest known1 only four hours7--while the period of the ,mira stars amounts to at least sixty-five days1 and may attain two years4 ,there may be still longer periods so far not investigated4 ,nearly related to the ,lyre stars are the ,algol stars1 whose variability can be explained by the assumption that another bright or dark star is moving within their vicinity1 partially cutting off their light4 ,there is no dust in these cases1 and the spectrum characterizes these stars as stars of the first class--that is1 as white stars--so far as they have been studied up to the present4 ,we must presume for all the variable stars that the line of sight from the observer to the star falls in the plane of their dust rings or of their companions4 ,if this were not so1 they would appear to us like a nebula with a central condensation nucleus1 or1 so far as ,algol stars are concerned1 like the so-called spectroscopic doubles whose motion about each other is recognized from the displacement of their spectral lines4 ,the evolution of stars from the nebulous state has been depicted by the famous chief of the ,lick ,observatory1 in ,california1 ,w4 ,w4 ,campbell1 as follows 7compare the spectra of the stars of the #b;d1 #c;d1 and #dth class1 ,figs4 #ei and #fj73 ,illustration3 #gd ,my1 in ,sun #bhj ,schjellerup's ,gemini ,class #b ,schjellerup's ,catalogue ,class #c ,catalogue ,class #d ,class #d ,fig4 #ei4--;,comparison of spectra of stars of classes #b1 #c1 #d4 ,after photographs taken at the ,yerkes ,observatory4 ,blue portions of spectrum4 ,wave-lengths in millionths of a millimetre ,illustration3 #bhj ,sun ,my1 in #gd ,schjellerup's ,class #b ,gemini ,schjellerup's ,catalogue ,class #c ,catalogue ,class #d ,class #d ,fig4 #fj4--;,comparison of spectra of stars of classes #b1 #c1 #d4 ,after photographs taken at the ,yerkes ,observatory4 ,green and yellow portions of spectrum4 ,wave-lengths in millionths of a millimetre 8,it is not difficult to select a long list of well-known stars which cannot be far removed from nebular conditions4 ,these are the stars containing both the ,huggins and the ,pickering series of bright hydrogen lines1 the bright lines of helium1 and a few others not yet identified4 ,gamma ,argus and ,zeta ,puppis are of this class4 ,another is ,,dm #cj4cfci1 which is actually surrounded with a spherical atmosphere of hydrogen some five seconds of arc in diameter4 ,a little further removed from the nebular state are the stars containing both bright and dark hydrogen lines--caught1 so to speak1 in the act of changing from bright-line to dark-line stars4 ,gamma ,cassiopei1 ,pleione1 and ,my ,centauri are examples4 ,closely related to the foregoing are the helium stars4 ,their absorption lines include the ,huggins hydrogen series complete1 a score or more of the conspicuous helium lines1 frequently a few of the ,pickering series1 and usually some inconspicuous metallic lines4 ,the white stars in ,orion and in the ,pleiades are typical of this age4 8,the assignment of the foregoing types to an early place in stellar life was first made upon the evidence of the spectroscope4 ,the photographic discovery of nebulous masses in the regions of a large proportion of the bright-line and helium stars affords extremely strong confirmation of their youth4 ,who that has seen the nebulous background of ,orion 7,fig4 #ea7 or the remnants of nebulosity in which the individual stars of the ,pleiades 7,fig4 #eb7 are immersed can doubt that the stars in these groups are of recent formation8 8,with the lapse of time1 stellar heat radiates into space1 and1 so far as the individual star is concerned1 is lost4 ,on the other hand1 the force of gravity on the surface strata increases4 ,the inevitable contraction is accompanied by increasing average temperature4 ,changes in the spectrum are the necessary consequence4 ,the second hydrogen series vanishes1 the ordinary hydrogen absorption is intensified1 the helium lines become indistinct1 and calcium and iron absorptions begin to assert themselves4 ,vega and ,sirius are conspicuous examples of this period4 ,increasing age gradually robs the hydrogen lines of their importance1 the ,h and ,k lines broaden1 the metallic lines develop1 the bluish-white color fades in the direction of the yellow1 and1 after passing through types exemplified by many well-known stars1 the solar stage is reached4 ,the reversing layer in solar stars represents but four or five hydrogen lines of moderate intensity2 the calcium lines are commandingly permanent1 and some twenty thousand metallic lines are visible4 ,the solar type seems to be near the summit of stellar life4 ,the average temperature of the mass must be nearly a maximum2 for the low density indicates a constitution that is still gaseous compare ,chapter ,,vii44 8,passing time brings a lowering of the average temperature4 ,the color passes from yellow to red1 in consequence of lower radiation1 temperature1 and increasing general absorption by the atmosphere4 ,the hydrogen lines become indistinct1 metallic absorption remains permanent1 and broad absorption bands are introduced4 ,in one type 7,secchi's ,type ,,iii471 of which ,alpha ,herculis is an example1 these bands are of unknown origin4 ,in another class 7,secchi's ,type ,,iv471 illustrated by the star #ai ,piscium1 they have been definitely identified as of carbon origin4 8,there is scarcely room for doubt that these types of stars 7,type ,,iv47 are approaching the last stages of stellar development4 ,surface temperatures have been lowered to the point of permitting more complex chemical combinations than those in the sun4 8,secchi's ,type ,,iii4 includes the several numbered long-period variable stars of the ,mira ,ceti class1 whose spectra at maximum brilliancy show several bright lines of hydrogen and other chemical elements4#ag ,it is significant that the dull-red stars are all very faint2 there are none brighter than magnitude #e4e4 ,their effective radiatory power is undoubtedly very low40 ,footnote #ag3 ,this circumstance indicates that the red color of these stars1 as we have already remarked with regard to ,mira ,ceti1 is not to be traced back to a low temperature1 but rather to the dust surrounding them4 ,the most extraordinary brightness of some stars1 like ,arcturus and ,betelgeuse1 which are redder than the sun1 and whose spectra1 according to ,hale1 resemble those of the sun-spots1 presuppose a very high temperature4 ,the characteristic lines of their spectra are produced by the relatively cool vapors of their outer portions4 ,the state of evolution1 which succeeds that characterized as the ,secchi ,type ,,iv41 may be elucidated with the aid of the examples of ,jupiter and the earth1 with which we are more familiar4 ,these planets would be invisible if they were not shining in borrowed light4 ,jupiter has not advanced so far as the earth4 ,the specific gravity of ,jupiter is somewhat lower than that of the sun 7#a4bg against #a4ch71 and1 apart from the clouds in its atmosphere1 this planet is probably altogether in a gaseous condition1 while the earth1 with its mean density of #e4eb1 possesses a solid cold crust1 enclosing its incandescent interior4 ,this state of the earth corresponds to the last stage in the evolution of the stars4 ,of the streams of gaseous matter which are ejected when stars collide with one another1 the metallic vapors are rapidly condensed by cooling2 only helium and hydrogen will remain in the gaseous condition and form nebular masses about the central body4 ,these nebul yield bright lights4 ,their luminosity is due to the negative particles which are sent to them by the radiation pressure of near stars1 and especially by the central bodies of the nebula4 ,with the new stars which have so far been observed1 this pressure of radiation soon diminishes1 and the nebular light likewise decreases in such cases4 ,in other instances1 as with the stars characterized by bright hydrogen and helium lines1 the radiation of the central body or stars in their vicinity seems to be maintained at full force for long periods4 ,the nebulous accumulations of helium and hydrogen will gradually escape and be condensed in near-by stars under the formation of 8explosive0 compounds4 ,the tendency to enter into combination seems to be strongest in the case of helium2 it disappears first from the stellar atmosphere4 ,that helium enters into compounds at high temperatures seems to follow from the researches of ,ramsay1 ,cooke1 and ,kohlschtter4 ,hydrogen will afterwards be absorbed1 and the light of the central body will then show the predominating occurrence of the vapors of calcium and of other metals in its atmosphere4 ,simultaneously with these1 chemical compounds will be noticed1 among which the carbon compounds will play an important part--in the outer portions of the sun-spots1 in the stars of the ,secchi ,type ,,iv41 as well as in the gaseous envelopes of the comets4#ah ,footnote #ah3 ,the presence of carbon bands in the spectrum need not be taken as a mark of low temperature4 ,crew and ,hale have observed that these bands gradually vanished from an arc spectrum as the temperature was lowered by decreasing the current intensity4 ,finally a crust will form4 ,the star is extinct4 ,,vii ,,the ,,nebular ,,and ,,the ,,solar ,,states ,we will now proceed to a more intimate consideration of the chemical and physical conditions which probably characterize the nebul in distinction from the suns4 ,these properties differ in many respects essentially from those which we are accustomed to associate with matter as investigated by us1 which may1 from this point of view1 be styled relatively concentrated4 ,the differences must be fundamental4 ,for the motto of ,clausius1 which comprises the sum of our knowledge of the nature of heat1 cannot apply to nebul4 ,this motto reads3 8,the energy of the universe is constant4 ,the entropy of the universe tends to a maximum40 ,everybody understands what is meant by energy4 ,we know energy in many forms4 ,the most important are3 energy of position 7a heavy body has larger energy by virtue of its having been raised to a certain height above the surface of the earth than when it is lying on the surface72 energy of motion 7a discharged rifle-bullet has an energy which is proportional to the mass of the bullet and to the square of its velocity72 energy of heat1 which is regarded as the energy of the motion of the smallest particles of a body2 electrical energy1 such as can1 for instance1 be stored in an accumulator battery1 and which1 like all other modifications of energy1 may be converted into energy of heat2 and chemical energy1 such as is displayed by a mixture of eight grammes of oxygen with one gramme of hydrogen1 which can be transformed into water under a strong evolution of heat4 ,when we say that the energy of a system to which energy is not imparted from outside is constant1 we merely mean that the different forms of energy of the separate parts of this system may be transformed into other forms of energy1 but that the sum total of all the energies must always remain unchanged4 ,according to ,clausius this law is valid throughout the infinite space of the universe4 ,by entropy we understand the quantity of heat of a body divided by its absolute temperature4 ,if a quantity of heat1 of ,q calories1 of a body at a temperature of #ajj 7absolute temperature1 #cgc7 passes over to another body of #j 7absolute temperature1 #bgc71 the total entropy of the two will have been decreased by ,q_/#cgc1 and increased by ,q_/#bgc4 ,as the latter quantity is the greater1 the entropy of the whole will have increased4 ,by itself1 we know1 heat always passes1 either by radiation or by conduction1 from bodies of higher temperature to bodies of lower temperature4 ,that evidently implies an increase in entropy1 and it is in agreement with the law of ,clausius that entropy tends to increase4 ,the most simple case of heat equilibrium is that in which we place a number of bodies of unequal temperatures in an enclosure which neither receives heat from outside nor communicates heat to the outside4 ,in some way or other1 usually by conduction or radiation1 the heat will pass from the warmer to the colder bodies1 until at last equilibrium ensues and all the bodies have the same temperature4 ,according to ,clausius1 the universe tends to that thermal equilibrium4 ,if it be ever attained1 all sources of motion1 and hence of light1 will have been exhausted4 ,the so-called 8heat-death0 7,wrmetod7 will have come4 ,if ,clausius were right1 however1 this heat-death1 we may object1 should already have occurred in the infinitely long space of time that the universe has been in existence4 ,or we might argue that the world has not yet been in existence sufficiently long1 but that1 anyhow1 it had a beginning4 ,that would contradict the first part of the law of ,clausius1 that the energy of the universe is constant2 for in that case all the energy would have originated in the moment of creation4 ,that is quite inconceivable1 and we must hence look for conditions for which the entropy law of ,clausius does not hold4 ,the famous ,scotch physicist ,clerk-,maxwell has conceived of such a case4 ,imagine a vessel which is divided by a partition into two halves1 both charged with a gas of perfectly uniform temperature4 ,let the partition be provided with a number of small holes which would not allow more than one gas molecule to pass at a time4 ,in each hole ,maxwell places a small1 intelligent being 7one of his 8demons071 which directs all the molecules which enter into the hole1 and which have a greater velocity than the mean velocity of all the molecules1 to the one side1 and which sends to the other side all the molecules of a smaller velocity than the average4#ai ,all the undesirable molecules the demon bars by means of a little flap4 ,in this way all the molecules of a velocity greater than the average may be collected in the one compartment1 and all the molecules of a lesser velocity in the other compartment4 ,in other words1 heat--for heat consists in the movements of molecules--will pass from the one constantly cooling side to the other1 which is constantly raising its temperature1 and which must therefore become warmer than the former4 ,footnote #ai3 ,the kinetic theory of gases imagines all the molecules of a gas to be in constant motion4 ,the internal pressure of the gas depends upon the mean velocity of the particles2 but some particles will move at a greater1 and some at a smaller velocity than the average4--,h4,b4 ,in this instance heat would therefore pass from a colder to a warmer body1 and the entropy would diminish4 ,nature1 of course1 does not know any such intelligent beings4 ,nevertheless1 similar conditions may occur in celestial bodies in the gaseous state4 ,when the molecules of gas in the atmosphere of a celestial body have a sufficient velocity--which in the case of the earth would be #aa km4 7#g miles7 per second--and when they travel outward into the most extreme strata1 they may pass from the range of attraction out into infinite space1 after the manner of a comet1 which1 if endowed with sufficient velocity when near the sun1 must escape from the solar system4 ,according to ,stoney1 it is in this way that the moon has lost its original atmosphere4 ,this loss of gas is certainly imperceptible in the case of our sun and of large planets like the earth4 ,but it may play an important part in the household of the nebul1 where all the radiation from the hot celestial bodies is stored up1 and where1 owing to the enormous distances1 the restraining force of gravity is exceedingly feeble4 ,thus the nebul will lose their most rapid molecules from their outer portions1 and they will therefore be cooling in these outer strata4 ,this loss of heat is compensated by the radiation from the stars4 ,if1 now1 there were only nebul of one kind in the whole universe1 those escaped molecules would finally land on some other nebula1 heat equilibrium would thus be established between the different nebul1 and the 8heat-death0 be realized4 ,but we have already remarked that the nebul enclose many immigrated celestial bodies1 which are able to condense the gases from their neighborhood1 and which thereby assume a higher temperature4 ,the lost molecules of gases may also stray into the vast atmosphere of these growing stars1 and the condensation will then be hastened under a continuous lowering of the entropy4 ,by such processes the clock-work of the universe may be maintained in motion without running down4 ,about the bodies which have drifted into nebul1 and about the remnants of new stars which lie inside the nebul1 the gases will thus collect which had formerly been scattered through the outer portions of the nebula4 ,these gases originate from the explosive compounds which had been stored in the interior of the new stars4 ,hydrogen and helium are1 most likely1 the most important of these2 for they are the most difficult to be condensed1 and can exist in notable quantities at extremely low temperatures1 such as must prevail in the outermost portions of the nebul1 in which gases of other substances would be liquefied4 ,even if the nebul had an absolute temperature of #ej 7-#bbc ,c471 the vapor of the most volatile of all the metals1 mercury1 would even in the saturated state be present in such a small quantity that a single gramme would occupy the space of a cube whose side would correspond to about two thousand light-years--that is to say1 to #dej times the distance of the earth from the nearest fixed star4 ,one gramme of sodium1 likewise a very volatile metal1 and of a comparatively high importance in the constitution of the fixed stars1 would fill the side of a cube that would be a thousand million times as large4 ,still more inconceivable numbers result for magnesium and iron1 which are very frequent constituents of fixed stars1 and which are less volatile than the just-mentioned metals4 ,we thus recognize the strongly selective action of the low temperatures upon all the substances which are less difficult to condense than helium and hydrogen4 ,as we now know that there is another substance in the nebul1 which has been designated nebulium1 and which is characterized by two spectral lines not found in any terrestrial substance1 we must conclude that this otherwise unknown element nebulium must be almost as difficult to condense as hydrogen and helium4 ,its boiling-point will probably lie below #ej absolute1 like that of those gases4 ,that hydrogen and helium1 together with nebulium1 alone seem to occur in the vastly extended nebul is probably to be ascribed to their low boiling-points4 ,we need not look for any other explanation4 ,the supposition of ,lockyer that all the other elements would be transformed into hydrogen and helium at extreme rarefaction is quite unsupported4 ,in somewhat lower strata of the nebula1 where its shape resembles a disk1 other not easily condensable substances1 such as nitrogen1 hydrocarbons of simple composition1 carbon monoxide1 further1 at deeper levels1 cyanogen and carbon dioxide1 and1 near the centre1 sodium1 magnesium1 and even iron may occur in the gaseous state4 ,these less volatile constituents may exist as dust in the outermost strata4 ,this dust would not be revealed to us by the spectroscope4 ,in the strongly developed spiral nebul1 however1 the extreme layers1 which seem to hide the central body1 appear to be so attenuated that the dust floating in them is not able to obscure the spectrum of the metallic gases4 ,the spectrum of the nebula then resembles a star spectrum1 because the deepest strata contain incandescent layers of dust clouds1 whose light is sifted by the surrounding masses of gases4 ,it has been observed that the lines of the different elements are not uniformly distributed in the nebul4 ,thus ,campbell observed1 for instance1 when investigating a small planetary nebula in the neighborhood of the great ,orion nebula1 that the nebulium had not the same distribution as the hydrogen4 ,the nebulium1 which was concentrated in the centre of the nebula1 probably has a higher boiling-point than hydrogen1 therefore1 and occurs in noticeable quantities in the inner1 hotter parts of the nebula4 ,systematic investigations of this kind may help us to a more perfect knowledge of the temperature relations in these peculiar celestial objects4 ,ritter and ,lane have made some interesting calculations on the equilibrium in a gaseous celestial body of so low a density that the law of gases may be applied to it4 ,that is only permissive for gases or for mixtures of gases whose density does not exceed one-tenth of that of water or one-fourteenth of the actual density of the sun4 ,the pressure in the central portions of such a mass of gas would1 of course1 be greater than the pressure in the outer portions1 just as the pressure rises as we penetrate from above downward into our terrestrial atmosphere4 ,if we imagine a mass of the air of our atmosphere transferred one thousand metres higher up1 its volume will increase and its temperature will fall by #i4h ,c4 7#ah ,f474 ,if there were extremely violent vertical convection currents in the air1 its temperature would diminish in this manner with increasing altitude2 but internal radiation tends to equalize these temperature differences4 ,the following calculation by ,schuster concerning the conditions of a mass of gas of the size of the sun is based on ,ritter's investigation4 ,it has been made under the hypothesis that the thermal properties of this mass of gas are influenced only by the movements in it1 and not by radiation4 ,the calculation is applied to a star which has the same mass as the sun 7#a4i #ajcc grammes1 or #cbd1jjj times the mass of the earth71 and a radius of about ten times that of the sun 7#aj #fij1jjj km471 whose mean density would thus be #ajjj times smaller than that of the sun1 or #j4jjad times the density of water at #d ,c4 ,in the following table the first column gives the distance of a point from the centre of the star as a fraction of its radius2 the density 7second column7 is expressed in the usual scale1 water being the unit2 pressures are stated in thousands of atmospheres1 temperatures in thousands of degrees ,centigrade4 ,the temperature will vary proportionately to the molecular weight of the gas of which the star consists2 the temperatures1 in the fourth column of the table1 concern a gas of molecular weight #a--that is to say1 hydrogen gas dissociated into atoms1 as it will be undoubtedly on the sun and on the star4 ,if the star should consist of iron1 we should have to multiply these latter numbers by #ef1 the molecular weight of iron2 the corresponding figures will be found in the fifth column4 ,temperature in ,distance from ,density ,pressure in #ajc #ajc ,cent4 centre atmospheres ,hydrogen ,iron #j #j4jjhdd #heb #bdfj #acg1ejj #j4a #j4jjhag #hjg #bdjf #acd1fjj #j4b #j4jjgci #fhc #bbea #abf1ajj #j4c #j4jjfbc #eac #bjjg #aab1djj #j4d #j4jjdhh #cdb #agjg #ie1fjj #j4e #j4jjced #bjj #acgg #gg1ajj #j4f #j4jjbcc #ajj #ajdc #eh1djj #j4g #j4jjacf #dj #gbh #dh1hjj #j4h ,o4#jjjfe #ab #dde #bd1ijj #j4i #j4jjjbj #a4g #bjb #aa1cjj #a4j #j4jjjjj #j #j #j ,schuster's calculation was really made for the sun--that is to say1 for a celestial body whose diameter is ten times smaller1 and whose specific gravity is therefore a thousand times greater than the above-assumed values4 ,according to the laws of gravitation and of gases1 the pressure must there be #aj1jjj times greater1 and the temperature ten times higher1 than those in our table4 ,the density of the interior portions would1 however1 become far too large to admit of the application of the gas laws4 ,i have therefore modified the calculations so as to render them applicable to a celestial body of ten times the radius of the sun or of #ajhj times the radius of the earth2 the radius would then represent one-twenty-second of the distance from the centre of the sun to the earth's orbit1 and the respective celestial body would have very small dimensions indeed if compared to a nebula4 ,the extraordinarily high pressure in the interior portions of the celestial body is striking2 this is due to the great mass and to the small distances4 ,in the centre of the sun the pressure would amount to #hebj million atmospheres1 since the pressure increases inversely as the fourth power of the radius4 ,the pressure near the centre of the sun is1 indeed1 almost of that order4 ,if the sun were to expand to a spherical planetary nebula of a thousand times its actual linear dimensions 7when it would almost fill the orbit of ,jupiter71 the specific gravity at its centre would be diminished to one-millionth of the above-mentioned value--that is to say1 matter in this nebula would not1 even at the point of greatest concentration1 be any denser than in the highly rarefied vacuum tubes which we can prepare at ordinary temperatures4 ,the pressure would likewise be greatly diminished--namely1 to about six millimetres only1 near the centre of the gaseous mass4 ,the temperature1 however1 would be rather high near the centre--namely1 #bd1fjj ,c41 if the nebula should consist of atomatic hydrogen1 and fifty-six times as high again if consisting of iron gas4 ,such a nebula would restrain gases with #a4fc times the force which the earth exerts4 ,molecules of gases moving outward with a velocity of about #ah km4 7#aa miles7 per second would forever depart from this atmosphere4 ,the estimation of the temperature in such masses of gases is certainly somewhat unreliable4 ,we have to presume that neither radiation nor conduction exert any considerable influence4 ,that might be permitted for conduction2 but we are hardly justified in neglecting radiation4 ,the temperatures within the interior of the nebula will1 therefore1 be lower than our calculated values4 ,it is1 however1 difficult to make any definite allowance for this factor4 ,if the mass of the celestial body should not be as presumed--for instance1 twice as large--we should only have to alter the pressure and the density of each layer in the same proportion1 and thus to double the above values4 ,the temperature would remain unchanged4 ,we are hence in a position to picture to ourselves the state of a nebula of whatever dimensions and mass4 ,lane has proved1 what the above calculations also indicate1 that the temperature of such nebula will rise when it contracts in consequence of its losing heat4 ,if heat were introduced from outside1 the nebula would expand under cooling4 ,a nebula of this kind presumably loses heat and gradually raises its own temperature until it has changed into a star1 which will at first have an atmosphere of helium and of hydrogen like that of the youngest stars 7with white light74 ,by-and-by1 under a further rise of temperature1 the extremely energetic chemical compounds will be formed which characterize the interior of the sun1 because helium and hydrogen--which were liberated when the nebula was re-formed and which dashed out into space--will diffuse back into the interior of the star1 where they will be bound under the formation of the compounds mentioned4 ,the atmosphere of hydrogen and of helium will disappear 7helium first71 the star will contract more and more1 and the pressure and the convection currents in the gases will become enormous4 ,there will be a strong formation of clouds in the atmosphere of the star1 which will gradually become endowed with the properties which characterize our sun4 ,the sun behaves very differently from the gaseous nebul for which the calculations of ,lane1 ,ritter1 and ,schuster hold4 ,for when the contraction of a gas shall have proceeded to a certain limit1 the pressure will increase in the ratio #a3 #af1 while the volume will decrease in the ratio #h3 #a1 provided there be no change in the temperature4 ,when the gas has reached this point and is still further compressed1 the temperature will remain in steady equilibrium4 ,at still higher pressures1 however1 the temperature must fall if equilibrium is to be maintained4 ,according to ,amagat1 this will occur at #ag ,c4 7#bij absolute7 in gases like hydrogen and nitrogen1 which at this temperature are far above their critical points1 and at a pressure of #cjj or #bej atmospheres4 ,when the temperature is twice as high on the absolute scale1 or at #cjg ,c41 twice the pressure will be required4 ,we can now calculate when our nebula will pass through this critical stage1 to which a lowering of the temperature must succeed4 ,accepting the above figures1 we find that half the mass of the nebula will fill a sphere of a radius #j4ec of that of the nebula4 ,if the mass were everywhere of the same density1 half of it would fill a sphere of #j4hd of this radius4 ,when will the interior mass cross the boundary of the above stage1 while the exterior portions still remain below this stage8 ,that will be at about the time when the nebula in its totality will pass through its maximum temperature4 ,we will now base our calculations on the temperatures which apply to iron in the gaseous state2 for in the interior of the nebula the mean molecular weight will at least be #ef 7that of iron74 ,we shall find that the pressure at the distance #j4ec will be about #agg1jjj atmospheres1 and the temperature approximately #ga million degrees--i4e41 #bde1jjj times higher than the absolute temperature in the experiments of ,amagat4 ,the specified stage will then be reached when the pressure will be #bde1jjj times as large as #bej atmospheres--viz41 #fa million atmospheres4 ,as1 now1 the pressure is only #agg1jjj atmospheres1 our nebula will yet be far removed from that stage at which cooling will set in4 ,we can easily calculate that this will take place when the nebula has contracted to a volume about three times that of our sun4 ,the assertion which is so often made that the sun might possibly attain higher temperatures in the future is unwarranted4 ,this celestial body has long since passed through the culminating-point of its thermal evolution1 and is now cooling4 ,as the temperatures which ,schuster deduced were no doubt much too high1 the cooling must1 indeed1 have set in already in an earlier stage4 ,but stars like ,sirius1 whose density is probably not more than one per cent1 of the solar density1 are probably still in a rising-temperature stage4 ,their condition approximates that of the mass of gas of our example4 ,the planetary nebul are vastly more voluminous4 ,the immense space which these celestial bodies may occupy will be understood from the fact that the largest among them1 ,no4 #e in ,herschel's catalogue1 situated near the star ,b in the ,great ,bear1 has a diameter of #b4fg seconds of arc4 ,if it were as near to us as our nearest star neighbor1 its diameter would yet be more than three times that of the orbit of ,neptune2 doubtless it is many hundreds of times larger4 ,this consideration furnishes us with an idea of the infinite attenuation in such structures4 ,in their very densest portions the density cannot be more than one-billionth of the density of the air4 ,in the outer portions of such nebul the temperature must also be exceedingly low2 else the particles of the nebula could not be kept together1 and only hydrogen and helium can occur in them in the gaseous state4 ,yet we may regard the density and temperature of such celestial bodies as gigantic by comparison with those of the gases in the spirals of the nebul4 ,there never is equilibrium in these spirals1 and it is only because the forces in action are so extraordinarily small that these structures can retain their shapes for long periods without noticeable changes4 ,it is1 probably1 chiefly in those parts in which the cosmical dust is stopped in its motion that meteorites and comets are produced4 ,the dust particles wander into the more central portions of the nebul1 into which they penetrate deeply1 owing to their relatively large mass1 to form the nuclei for the growth of planets and moons4 ,by their collisions with the masses of gases which they encounter1 they gradually assume a circular movement about the axis of rotation of the nebula4 ,in this rotation they condense portions of the gases on their surface1 and hence acquire a high temperature--which they soon lose again1 however1 owing to the comparatively rapid radiation4 ,so far as we know1 spiral nebul are characterized by continuous spectra4 ,the splendor of the stars within them completely outshines the feeble luminosity of the nebula4 ,the stars in them are condensation products and undoubtedly in an early stage of their existence2 they may therefore be likened to the white stars1 like the new star in ,perseus and the central star in the ring nebula of the ,lyre4 ,nevertheless1 it has been ascertained that the spectrum of the ,andromeda nebula has about the same length as that of the yellow stars4 ,that may be due to the fact that the light of the stars in this nebula1 which we only seem to see from the side1 is partly extinguished by dust particles in its outer portion1 as was the case with the light of the new star in ,perseus during the period of its variability4 ,our considerations lead to the conclusion that there is rotating about the central body of the nebula an immense mass of gas1 and that outside this mass there are other centres of condensation moving about the central body together with the masses of gas concentrated about them4 ,owing to the friction between the immigrated masses and the original mass of gas which circulated in the equatorial plane of the central body1 all these masses will keep near the equatorial plane1 which will therefore deviate little from the ecliptic4 ,we thus obtain a proper planetary system1 in which the planets are surrounded by colossal spheres of gas like the stars in the ,pleiades 7,fig4 #eb74 ,if1 now1 the planets have very small mass by comparison with the central body--as in our solar system--they will be cooled at an infinitely faster rate than the sun4 ,the gaseous masses will soon shrink1 and the periods of rotation will be shortened2 but for those planets1 at least1 which are situated near the centre1 these periods will originally differ little from the rotation of the central body4 ,the dimensions of the central body will always be very large1 and the planets circulating about it will produce very strong tidal effects in its mass4 ,its period of rotation will be shortened1 while the orbital rotation of the planets will tend to become lengthened4 ,thus the equilibrium is disturbed2 it is re-established again1 because the planet is1 so to say1 lifted away from the sun1 as ,g4 ,h4 ,darwin has so ingeniously shown with regard to the moon and the earth4 ,similar relations will prevail in the neighborhood of those planets which will thus become provided with moons4 ,hence we understand the peculiar fact that all the planets move almost in the same plane1 the so-called ecliptic1 and in approximately circular orbits2 that they all move in the same direction1 and that they have the same direction of rotation in common with their moons and with the central body1 the sun4 ,it is only the outermost planets1 like ,uranus and ,neptune1 in whose cases the tidal effects were not of much consequence1 that form exceptions to this rule4 ,in explanation of these phenomena various philosophers and astronomers have advanced a theory which is known as the ,kant-,laplace theory1 after its most eminent advocates4 ,suggestions pointing in the same direction we find in ,swedenborg 7#agcd74 ,swedenborg assumed that our planetary system had been evolved under the formation of vortices from a kind of 8chaos solare10 which had acquired a more and more energetic circulating motion about the sun under the influence of internal forces1 possibly akin to magnetic forces4 ,finally a ring had been thrown off from the equator1 and had separated into fragments1 out of which the planets had been formed4 ,buffon introduced gravitation as the conservational principle4 ,in an ingenious essay1 8,formation des ,plantes0 7#agde71 he suggests that the planets may have been formed from a 8stream0 of matter which was ejected by the sun when a comet rushed into it4 ,kant started from an original chaos of stationary dust1 which under the influence of gravitation arranged itself as a central body1 with rings of dust turning around it2 the rings1 later on1 formed themselves into planets4 ,the laws of mechanics teach1 however1 that no rotation can be set up in a central body1 which is originally stationary1 by the influence of a central force like gravitation4 ,laplace1 therefore1 assumed with ,swedenborg that the primeval nebula from which our solar system was evolved had been rotating about the central axis4 ,according to ,laplace1 rings like those of ,saturn would split off1 as such a system contracted1 and planets and their moons and rings would afterwards be formed out of those rings4 ,it is generally believed at present1 however1 that only meteorites and small planets1 but not the larger planets1 could have originated in this way4 ,we have1 indeed1 such rings of dust rotating about ,saturn1 the innermost more rapidly1 the outer rings more slowly1 just as they would if they were crowds of little moons4 ,many further objections have later been raised against the hypothesis of ,laplace1 first by ,babinet1 later especially by ,moulton and ,chamberlin4 ,in its original shape this hypothesis would certainly not appear to be tenable4 ,i have therefore replaced it by the evolution thesis outlined above4 ,it is rather striking that the moons of the outermost planets1 ,neptune and ,uranus1 do not move in the plane of the ecliptic1 and that their moons further describe a 8retrograde0 movement--that is to say1 they move in the direction opposite to that conforming to the theory of ,laplace4 ,the same seems to hold for the moon of ,saturn1 which was discovered in #ahih by ,pickering4 ,all these facts were1 of course1 unknown to ,laplace in #aggf2 and if he had known them he would scarcely have advanced his thesis in the garb in which he offered it4 ,the explanation of these facts does not cause any difficulty4 ,we may assume that the matter in the outer portions of the primeval nebula was so strongly attenuated that the immigrating planet did not attain a sufficient volume to have the large common rotation in the equatorial plane of the sun impressed upon it by the tidal effects4 ,charged only with the small mass of matter which they met on their road1 the planet and its moon1 on the contrary1 remained victorious in the limited districts in which they were rotating4 ,only the slow orbital movement about the central body was influenced1 and that adapted itself to the common direction and the circular orbit4 ,it is not inconceivable that there may be1 farther out in space1 planets of our solar system1 unknown to us1 moving in irregular paths like the comets4 ,the comets1 ,laplace assumed1 probably immigrated at a later period into our solar system when the condensation had already advanced so far that the chief mass of the nebular matter had disappeared from interplanetary space4 ,chamberlin and ,moulton have attempted to show that the difficulties of the hypothesis of ,laplace may be obviated by the assumption that the solar system has evolved from a spiral nebula1 into which strange bodies intruded which condensed the nebular mass of their surroundings upon themselves4 ,we have pointed out examples of how the nebula seems to vanish in the vicinity of the stars1 which would correspond to growing planets1 located in nebul4 ,in concluding this consideration1 we may draw a comparison between the views which were still entertained a short time ago and the views and prospects which the discoveries of modern days open to our eyes4 ,up to the beginning of this century the gravitation of ,newton seemed to rule supreme over the motions and over the development of the material universe4 ,by virtue of this gravitation the celestial bodies should tend to draw together1 to unite in ever-growing masses4 ,in the infinite space of past time the evolution should have proceeded so far that some large suns1 bright or extinct1 could alone persist4 ,all life would be impossible under such conditions4 ,and yet we discern in the neighborhood of the sun quite a number of dark bodies1 our planets1 and we may surmise that similar dark companions or satellites exist in the vicinity of other suns and stars2 for we could not understand the peculiar to-and-fro motions of those stars on any other view4 ,we further observe that quite a number of small celestial bodies rush through space in the shapes of meteorites or shooting-stars which must have come to us from the most remote portions of the universe4 ,the explanation of these apparent deviations from what we may regard as a necessary consequence of the exclusive action of gravity will be found under two heads--in the action of the mechanical radiation pressure of light1 and in the collisions between celestial bodies4 ,the latter produce enormous vortices of gases about nebular structures in the gaseous condition2 the radiation pressure carries cosmical dust into the vortices1 and the dust collects into meteorites and comets and forms1 together with the condensation products of the gaseous envelope1 the planets and the moons accompanying them4 ,the scattering influence of the radiation pressure therefore balances the tendency of gravitation to concentrate matter4 ,the vortices of gases in the nebul only serve to fix the position of the dust1 which is ejected from the suns through the action of the radiation pressure4 ,the masses of gas within the nebul form the most important centres of concentration of the dust which is ejected from the sun and stars4 ,if the world were limited1 as people used to fancy--that is to say1 if the stars were crowded together in a huge heap1 and only infinite1 empty space outside of this heap1 the dust particles ejected from the suns during past ages by the action of the radiating pressure would have been lost in infinite space1 just as we imagined that the radiated energy of the sun was lost4 ,if that were so1 the development of the universe would long since have come to an end1 to an annihilation of all matter and of all energy4 ,herbert ,spencer1 among others1 has explained how thoroughly unsatisfactory this view is4 ,there must be cycles in the evolution of the universe1 he has emphasized4 ,that is manifestly indispensable if the system is to last4 ,in the more rarefied1 gaseous1 cold portions of the nebul we find that part of the machinery of the universe which checks the waste of matter and1 still more1 the waste of force from the suns4 ,the immigrating dust particles have absorbed the radiation of the sun and impart their heat to the separate particles of the gases with which they collide4 ,the total mass of gas expands1 owing to this absorption of heat1 and cools in consequence4 ,the most energetic molecules travel away1 and are replaced by new particles coming from the inner portions of the nebul1 which are in their turn cooled by expansion4 ,thus every ray emitted by a sun is absorbed1 and its energy is transferred1 through the gaseous particles of the nebul1 to suns that are being formed and which are in the neighborhood of the nebula or in its interior portions4 ,the heat is hence concentrated about centres of attraction that have drifted into the nebula or about the remnants of the celestial bodies which once collided there4 ,thanks to the low temperature of the nebula1 the matter can again accumulate1 while the radiation pressure1 as ,poynting has shown1 will suffice to keep bodies apart if their temperature is #ae ,c41 their diameter #c4d cm41 and their specific gravity as large as that of the earth1 #e4e4 ,at the distance of the orbit of ,neptune1 where the temperature is about #ej absolute and approximates1 therefore1 that of a nebula1 this limit of size is reduced to nearly one millimetre4 ,it has already been suggested 7compare page #aec7 that capillary forces1 which would prevail under the co-operation of the gases condensed upon the dust grains1 rather than gravity1 play a chief part in the accumulation and coalescence of the small particles4 ,in the same manner as matter is concentrated about centres of attraction energy may be accumulated there in contradiction to the law of the constant increase of entropy4 ,during this conservational activity the layers of gas are rapidly rarefied1 to be replaced by new masses from the inner parts of the nebula1 until this centre is depleted1 and the nebula has been converted into a star cluster or a planetary system which circulates about one or several suns4 ,when the suns collide once more new nebul are created4 ,the explosive substances1 consisting probably of hydrogen and helium 7and possibly of nebulium71 in combination with carbon and metals1 play a chief part in the evolution from the nebular to the stellar state1 and in the formation of new nebul after collisions between two dark or bright celestial bodies4 ,the chief laws of thermodynamics lead to the assumption that these explosive substances are formed during the evolution of the suns and are destroyed during their collisions4 ,the enormous stores of energy concentrated in these bodies perform1 in a certain sense1 the duty of powerfully acting fly-wheels interposed in the machinery of the universe in order to regulate its movements and to make certain that the cyclic transition from the nebular to the star stage1 and vice versa1 will occur in a regular rhythm during the immeasurable epochs which we must concede for the evolution of the universe4 ,by virtue of this compensating co-operation of gravity and of the radiation pressure of light1 as well as of temperature equalization and heat concentration1 the evolution of the world can continue in an eternal cycle1 in which there is neither beginning nor end1 and in which life may exist and continue forever and undiminished4 ,,viii ,,the ,,spreading ,,of ,,life ,,through ,,the ,,universe ,we have just recognized the probability of the assumption that solar systems have been evolved from nebul1 and that nebul are produced by the collision of suns4 ,we likewise consider it probable that there circulate about the newly formed suns smaller celestial bodies which cool more rapidly than the central sun4 ,when these satellites have provided themselves with a solid crust1 which will partly be covered by water1 they may1 under favorable conditions1 harbor organic life1 as the earth and probably also ,venus and ,mars do4 ,the satellites would thereby gain a greater interest for us than if we had to imagine them as consisting entirely of lifeless matter4 ,the question naturally arises whether we may believe that life can really originate on a celestial body as soon as circumstances are favorable for its evolution and propagation4 ,this question will occupy us in this last chapter4 ,men have been pondering over these problems since the remotest ages4 ,all living beings1 past ages recognized1 must have been generated and they had to die after a certain shorter or longer life4 ,somewhat later1 and yet still in a very early epoch1 experience must have taught men that organisms of one kind can only generate other organisms of the same kind2 that the species are invariable1 as we now express it4 ,the idea was that all species originally came from the hands of the ,creator endowed with their present qualities4 ,this view may still be said to represent the general or 8orthodox0 doctrine4 ,this view has also been called the ,linnan thesis1 because ,linn1 in the fifth edition of his ,genera ,plantarum1 adheres to it strictly3 8,species tot sunt1 quot diversas formas ab initio produxit ,infinitum ,ens1 quae deinde formae secundum generationis inditas leges produxere plures1 at sibi semper similes1 ut species nunc nobis non sint plures quam fuerunt ab initio40 ,which we may render3 8,there are as many different kind of species as the ,infinite ,being has created different forms in the beginning4 ,these forms have later engendered other beings according to the laws of inheritance1 always resembling them1 so that we have at the present time not any more species than there were from the beginning40 ,time was ripe1 however1 even then for a less rigid conception of nature1 more in accordance with our present views4 ,the first foundations of the theory of evolution in the biological sciences were laid by ,lamarck 7in #agid71 ,treviranus 7in #ahji71 ,goethe and ,oken 7in #ahbj74 ,but a reaction set in4 ,cuvier and his authority forced public opinion back to the ancient stand-point4 ,in his view the now extinct species of past geological epochs had been destroyed by natural revolutions1 and new species had again been generated by a new act of the ,creator4 ,within the last few decades1 however1 the general belief has rapidly been revolutionized1 and the theory of evolution1 especially since the immortal ,charles ,darwin came forth with his epoch-making researches1 now meets with universal acceptance4 ,according to this theory the species adapt themselves in the course of time to their surroundings1 and the changes may become so great that a new species may be considered to have originated from an old species4 ,the researches of ,de ,vries have1 within quite recent times1 further accentuated this view1 so that we now concede cases to be extant where new species spring forth from old ones under our very eyes4 ,this thesis has become known as the theory of mutation4 ,at the present time we accordingly imagine that living organisms1 such as we see around us1 have all descended from older organisms1 rather unlike them1 of which we still find traces and remnants in the geological strata which have been deposited during past ages4 ,from this stand-point all living organisms might possibly have originated from one single1 most simple organism4 ,how that was generated still remains to be explained4 ,the common view1 to which the ancients inclined1 is that the lower organisms need not necessarily have originated from seeds4 ,it was noticed that some low-type organisms1 larv1 etc41 took rise in putrid meat2 ,vergil describes this in his ,georgicas4 ,it was not until the seventeenth century that this belief was disproved by many experiments1 among others by those of ,swammerdam and ,leuwenhoek4 ,the thesis of the so-called 8,generatio spontanea0 once more blossomed into new life upon the discovery of the so-called infusoria1 the small animal organisms which seem to arise spontaneously in infusions and concoctions4 ,spallanzani1 however1 demonstrated in #aggg that when the infusions1 and the vessel containing them1 as well as the air above them1 were heated to a sufficiently high temperature to kill all the germs present1 the infusions would remain sterile1 and no living organisms could develop in them4 ,to this fact we owe our ordinary methods of making preserves4 ,it is true that objections were raised against this demonstration4 ,the air1 it was objected1 is so changed by heating that subsequent development of minute organisms is rendered impossible4 ,but this last objection was refuted by the chemists ,chevreul and ,pasteur1 as well as by the physicist ,tyndall in the sixties and seventies of the past century4 ,these scientists demonstrated that no organisms are produced in air which is freed from the smallest germs by some other means than heating--i4e41 by filtration through cotton-wool4 ,the researches of ,pasteur1 in particular1 and the methods of sterilization which are based upon them and which are applied every day in bacteriological laboratories1 have more and more forced the conviction upon us that a germ is indispensable for the origination of life4 ,and yet eminent scientists take up the pen again and again in order to demonstrate the possibility of the 8,generatio spontanea40 ,in this they do not rely upon the safe methods of natural science1 but they proceed on philosophical lines of argument4 ,life1 they suggest1 must once have had a beginning1 and we are hence forced to believe that spontaneous generation1 even if not realizable under actual conditions1 must have once occurred4 ,considerable interest was excited when the great ,english physiologist ,huxley believed he had discovered in the mud brought up from the very bottom of the sea an albuminoid substance which he called 8,bathybius ,haeckelii10 in honor of the zealous ,german ,darwinist ,haeckel4 ,in this bathybius 7deep-sea organism7 one fancied for a time that the primordial ooze1 which had originated from inorganic matter and from which all organisms might have been evolved1 and of which ,oken had been dreaming1 had been discovered4 ,but the more exact researches of the chemist ,buchanan demonstrated that the albuminoid substance in this primordial ooze consisted of flocks of gypsum precipitated by alcohol4 ,people then had recourse to the most fantastic speculations4 ,life1 it was argued1 might possibly have had its origin in the incandescent mass of the interior of the earth4 ,at high temperatures organic compounds of cyanogen and its derivatives might be formed which would be the carriers of life 7,pflger74 ,there is1 however1 little need of our entering into any of these speculations until they have been provided with an experimental basis4 ,almost every year the statement is repeated in biological literature that we have at last succeeded in producing life from dead matter4 ,among the most recent assertions of this kind1 the discovery claimed by ,butler-,burke has provoked much comment4 ,he asserted that he had succeeded1 with the aid of the marvellous substance radium1 in instilling life into lifeless matter--namely1 a solution of gelatine4 ,criticism has1 however1 relegated this statement1 like all similar ones1 to the realm of fairy tales4 ,we fully share the opinion which the great natural philosopher ,lord ,kelvin has expressed in the following words3 8,a very ancient speculation1 still clung to by many naturalists 7so much so that ,i have a choice of modern terms to quote in expressing it71 supposes that1 under meteorological conditions very different from the present1 dead matter may have run together or crystallized or fermented into #8germs of life1' or #8organic cells1' or #8protoplasm4' ,but science brings a vast mass of inductive evidence against this hypothesis of spontaneous generation4 ,dead matter cannot become living without coming under the influence of matter previously alive4 ,this seems to me as sure a teaching of science as the law of gravitation40 ,although the latter verdict may be a little dogmatic1 it yet demonstrates how strongly many scientists feel the necessity of finding another way of solving the problem4 ,the so-called theory of panspermia really shows a way4 ,according to this theory life-giving seeds are drifting about in space4 ,they encounter the planets1 and fill their surfaces with life as soon as the necessary conditions for the existence of organic beings are established4 ,this view was probably foreshadowed long ago4 ,definite suggestions in this direction we find in the writings of the ,frenchman ,sales-,guyon de ,montlivault 7#ahba71 who assumed that seeds from the moon had awakened the first life on the surface of the earth4 ,the ,german physician ,h4 ,e4 ,richter attempted to supplement the doctrine of ,darwin by combining the conception of panspermia with it4 ,flammarion's book on the plurality of inhabited worlds suggested to ,richter the idea that seeds had come from some other inhabited world to our earth4 ,he emphasizes the fact that carbon has been found in meteorites which move in orbits similar to those of the comets which wander about in space2 and in this carbon he sees the rests of organic life4 ,there is no proof at all for this latter opinion4 ,the carbon found in meteorites has never exhibited any trace of organic structure1 and we may well imagine the carbon--e4g41 that which appears to occur in the sun--to be of inorganic origin4 ,still more fantastic is his idea that organisms floating high in our atmosphere are caught by the attraction of meteorites flying past our planet1 and are in this way carried out into universal space and deposited upon other celestial bodies4 ,as the surface of meteorites becomes incandescent in their flight through the atmosphere1 any germs which they might possibly have caught would be destroyed2 and if1 in spite of that1 a meteorite should become the conveyor of live germs1 those germs would be burned in the atmosphere of the planet on which they descended4 ,in one point1 however1 we must agree with ,richter4 ,there is logic in his statement that 8,the infinite space is filled with1 or 7more correctly7 contains1 growing1 mature1 and dying celestial bodies4 ,by mature worlds we understand those which are capable of sustaining organic life4 ,we regard the existence of organic life in the universe as eternal4 ,life has always been there2 it has always propagated itself in the shape of living organisms1 from cells and from individuals composed of cells40 ,man used to speculate on the origin of matter1 but gave that up when experience taught him that matter is indestructible and can only be transformed4 ,for similar reasons we never inquire into the origin of the energy of motion4 ,and we may become accustomed to the idea that life is eternal1 and hence that it is useless to inquire into its origin4 ,the ideas of ,richter were taken up again in a popular lecture delivered in #ahgb by the famous botanist ,ferdinand ,cohn4 ,the best-known expression of opinion on the subject1 however1 is that of ,sir ,william ,thomson 7later ,lord ,kelvin7 in his presidential address to the ,british ,association at ,edinburgh in #ahga3 8,when two great masses come into collision in space1 it is certain that a large part of each is melted2 but it seems also quite certain that in many cases a large quantity of dbris must be shot forth in all directions1 much of which may have experienced no greater violence than individual pieces of rock experience in a landslip or in blasting by gunpowder4 ,should the time when this earth comes into collision with another body1 comparable in dimensions to itself1 be when it is still clothed as at present with vegetation1 many great and small fragments carrying seed and living plants and animals would undoubtedly be scattered through space4 ,hence1 and because we all confidently believe that there are at present1 and have been from time immemorial1 many worlds of life besides our own1 we must regard it as probable in the highest degree that there are countless seed-bearing meteoric stones moving about through space4 ,if at the present instant no life existed upon this earth1 one such stone falling upon it might1 by what we blindly call natural causes1 lead to its becoming covered with vegetation4 ,i am fully conscious of the many objections which may be urged against this hypothesis4 ,i will not tax your patience further by discussing any of them on the present occasion4 ,all ,i maintain is that ,i believe them to be all answerable40 ,unfortunately we cannot share ,lord ,kelvin's optimism regarding this point4 ,it is1 in the first instance1 questionable whether living beings would be able to survive the violent impact of the collision of two worlds4 ,we know1 further1 that the meteorite in its fall towards the earth becomes incandescent all over its surface1 and any seeds on it would therefore be deprived of their germinating power4 ,meteorites1 moreover1 show quite a different composition from that of the fragments from the surface of the earth or a similar planet4 ,plants develop almost exclusively in loose soil1 and a lump of earth falling through our atmosphere would1 no doubt1 be disintegrated into a shower of small particles by the resistance of the atmosphere4 ,each of these particles would by itself flash up like a shooting-star1 and could not reach the earth in any other shape than that of burned dust4 ,another difficulty is that such collisions1 which1 as we presume1 are responsible for the flashing-up of so-called new stars1 are rather rare phenomena1 so that little likelihood remains of small seeds being transported to our earth in this manner4 ,the question has1 however1 entered into a far more favorable stage since the effects of radiation have become understood4 ,bodies which1 according to the deductions of ,schwarzschild1 would undergo the strongest influence of solar radiation must have a diameter of #j4jjjaf mm41 supposing them to be spherical4 ,the first question is1 therefore3 are there any living seeds of such extraordinary minuteness8 ,the reply of the botanist is that the so-called permanent spores of many bacteria have a size of #j4jjjc or #j4jjjb mm41 and there are1 no doubt1 much smaller germs which our microscopes fail to disclose4 ,thus1 yellow-fever in man1 rabies in dogs1 the foot-and-mouth disease in cattle1 and the so-called mosaic disease--common to the tobacco plant in ,netherlandish ,india1 and also observed in other countries--are1 no doubt1 parasitical diseases2 but the respective parasites have not yet been discovered1 presumably because they are too minute to be visible under the microscope4#bj ,footnote #bj3 ,meanwhile a large number of organisms which are invisible under the ordinary microscope have been rendered visible by the aid of the ultra-microscope1 among others the presumable microbe of the foot-and-mouth disease4 ,it is1 therefore1 very probable that there are organisms so small that the radiation pressure of a sun would push them out into space1 where they might give rise to life on planets1 provided they met with favorable conditions for their development4 ,we will1 in the first instance1 make a rough calculation of what would happen if such an organism were detached from the earth and pushed out into space by the radiation pressure of our sun4 ,the organism would1 first of all1 have to cross the orbit of ,mars2 then the orbits of the smaller and of the outer planets2 and1 having passed the last station of our solar system1 the orbit of ,neptune1 it would drift farther into infinite space towards other solar systems4 ,it is not so difficult to estimate the time which the smallest particles would require for this journey4 ,let their specific gravity be that of water1 which will very fairly correspond to the facts4 ,the organisms would cross the orbit of ,mars after twenty days1 the ,jupiter orbit after eighty days1 and the orbit of ,neptune after fourteen months4 ,our nearest solar system1 ,alpha ,centauri1 would be reached in nine thousand years4 ,these calculations have been made under the supposition that the radiation pressure is four times as strong as gravitation1 which would be nearly correct according to the figures of ,schwarzschild4#ba ,footnote #ba3 ,the radiation pressure has here been assumed to be somewhat greater than on page #ajc1 because the spores are here regarded as opaque1 while the drops of hydrocarbons have been regarded as partially translucid to luminous rays4 ,these time intervals required for the organisms to reach the different planets of our solar system are not too long for the germs in question to preserve their germinating power4 ,the estimate is more unfavorable in the case of their transference from one planetary system to another1 which will require thousands of years4 ,but we shall see further on that the very low temperature of those parts of space 7about -#bbj ,c47 would suspend the extinction of the germinating power1 as it arrests all chemical reactions4 ,as regards the period during which the germinating power can be preserved at ordinary temperature1 we have been told that the so-called 8mummy wheat0 which had been found in ancient ,egyptian tombs was still capable of germination4 ,critics1 however1 have established that the respective statements of the ,arabs concerning the sources of that wheat are very doubtful4 ,the ,french scientist ,baudoin asserts that bacteria capable of germination were found in a ,roman tomb which had certainly remained untouched for eighteen hundred years2 but this statement is to be received with caution4 ,it is certain1 however1 that both seeds of some higher plants and spores of certain bacteria--e4g41 anthrax--do maintain their germinating power for several years 7say1 twenty71 and thus for periods which are much longer than those we have estimated as necessary for their transference to our planet4 ,on the road from the earth the germs would for about a month be exposed to the powerful light of the sun1 and it has been demonstrated that the most highly refrangible rays of the sun can kill bacteria and their spores in relatively short periods4 ,as a rule1 however1 these experiments have been conducted in such a manner that the spores could germinate on the moist surface on which they were deposited 7for instance1 in ,marshall ,ward's experiments74 ,that1 however1 does not at all conform to the conditions prevailing in planetary space4 ,for ,roux has shown that anthrax spores1 which are readily killed by light when the air has access1 remain alive when the air is excluded4 ,some spores do not suffer from insulation at all4 ,that applies1 for instance1 according to ,duclaux1 to ,thyrothrix scaber1 which occurs in milk and which may live for a full month under the intense light of the sun4 ,all the botanists that ,i have been able to consult are of the opinion that we can by no means assert with certainty that spores would be killed by the light rays in wandering through infinite space4 ,it may further be argued that the spores1 in their journey through universal space1 would be exposed during most of that period to an extreme cold which possibly they might not be able to endure4 ,when the spores have passed the orbit of ,neptune1 their temperature will have sunk to -#bbj1 and farther out it will sink still lower4 ,in recent years experiments have been made in the ,jenner ,institute1 in ,london1 with spores of bacteria which were kept for twenty hours at a temperature of -#beb in liquid hydrogen4 ,their germinating power was not destroyed thereby4 ,professor ,macfadyen has1 indeed1 gone still further4 ,he has demonstrated that micro-organisms may be kept in liquid air 7at -#bjj7 for six months without being deprived of their germinating power4 ,according to what ,i was told on the occasion of my last visit to ,london1 further experiments1 continued for still longer periods1 have only confirmed this observation4 ,there is nothing improbable in the idea that the germinating power should be preserved at lower temperatures for longer periods than at our ordinary temperatures4 ,the loss of germinating power is no doubt due to some chemical process1 and all chemical processes proceed at slower rates at lower temperatures than they do at higher4 ,the vital functions are intensified in the ratio of #a3 #b4e when the temperature is raised by #aj ,c4 7#ah ,f474 ,by the time that the spores reached the orbit of ,neptune and their temperature had been lowered to -#bbj1 their vital energy would1 according to this ratio1 react with one thousand millions less intensity than at #aj4 ,the germinating power of the spores would hence1 at -#bbj1 during the period of three million years1 not be diminished to any greater degree than during one day at #aj4 ,it is1 therefore1 not at all unreasonable to assert that the intense cold of space will act like a most effective preservative upon the seeds1 and that they will in consequence be able to endure much longer journeys than we could assume if we judged from their behavior at ordinary temperatures4 ,it is similar with the drying effect which may be so injurious to plant life4 ,in interplanetary space1 which is devoid of atmosphere1 absolute dryness prevails4 ,an investigation by ,b4 ,schrber demonstrates that the green alga ,pleurococcus vulgaris1 which is so common on the trunks of trees1 can be kept in absolute dryness 7over concentrated sulphuric acid in a desiccator7 for twenty weeks without being killed4 ,seeds and spores may last still longer in a dry atmosphere4 ,now1 the tension of water vapor decreases in nearly the same ratio as the speed of the reaction with lower temperatures4 ,the evaporation of water--i4e41 the drying effect--may hence1 at a temperature of -#bbj1 not proceed further in three million years than it will in one day at #aj4 ,we have thus several plausible reasons for concluding that spores which oppose an effective resistance to drying may well be carried from one planet to another and from one planetary system to another without sacrificing their vital energy4 ,the destructive effect of light is1 according to the experiments of ,roux1 no doubt due to the fact that the rays of light call forth an oxidation by the intermediation of the surrounding air4 ,this possibility is excluded in interplanetary space4 ,moreover1 the radiation of the sun is nine hundred times fainter in the orbit of ,neptune than in the orbit of the earth1 and half-way to the nearest fixed star1 ,alpha ,centauri1 twenty million times feebler4 ,light1 therefore1 will not do much harm to the spores during their transference4 ,if1 therefore1 spores of the most minute organisms could escape from the earth1 they might travel in all directions1 and the whole universe might1 so to say1 be sown with them4 ,but now comes the question3 how can they escape from the earth against the effect of gravitation8 ,corpuscles of such small weight would naturally be carried away by any aerial current4 ,a small rain-drop1 #a_/ej mm4 in diameter1 falls1 at ordinary air pressure1 about #d cm4 per second4 ,we can calculate from this observation that a bacteria spore #j4jjjaf mm4 in diameter would only fall #hc m4 in the course of a year4 ,it is obvious that particles of this minuteness would be swept away by every air current they met until they reached the most diluted air of the highest strata4 ,an air current of a velocity of #b m4 per second would take them to a height where the air pressure is only #j4jja mm4--i4e41 to a height of about #ajj km4 7#fj miles74 ,but the air currents can never push the particle outside of our atmosphere4 ,in order to raise the spores to still higher levels we must have recourse to other forces1 and we know that electrical forces can help us out of almost any difficulty4 ,at heights of #ajj km4 the phenomena of the radiating aurora take place4 ,we believe that the auror are produced by the discharge of large quantities of negatively charged dust coming from the sun4 ,if1 therefore1 the spore in question should take up negative electricity from the solar dust during an electric discharge1 it may be driven out into the sea of ether by the repulsive charges of the other particles4 ,we suppose1 now1 that the electrical charges--like matter--cannot be subdivided without limit4 ,we must finally come to a minimum charge1 and this charge has been calculated at about #c4e4aj-aj electrostatic units4 ,we can1 without difficulty1 calculate the intensity of an electric field capable of urging the charged spore of #j4jjjaf mm4 upward against the force of gravity4 ,the required field-strength is only #bjj volts per metre4 ,such fields are often observed on the surface of the earth with a clear sky1 and they are1 indeed1 almost normal4 ,the electric field of a region in which an auroral display takes place is probably much more intense1 and would1 without doubt1 be of sufficient intensity to urge the small electrically charged spores which convection currents had carried up to these strata1 farther out into space against the force of gravity4 ,it is thus probable that germs of the lowest organisms known to us are continually being carried away from the earth and the other planets upon which they exist4 ,as seeds in general1 so most of these spores1 thus carried away1 will no doubt meet death in the cold infinite space of the universe4 ,yet a small number of spores will fall on some other world1 and may there be able to spread life if the conditions be suitable4 ,in many cases conditions will not be suitable4 ,occasionally1 however1 the spores will fall on favorable soil4 ,it may take one million or several millions of years from the age at which a planet could possibly begin to sustain life to the time when the first seed falls upon it and germinates1 and when organic life is thus originated4 ,this period is of little significance in comparison with the time during which life will afterwards flourish on the planet4 ,the germs which in this way escape from the planets on which their ancestors had found abode1 may either wander unobstructed through space1 or they may1 as we have indicated1 reach outer planets1 or planets moving about other suns1 or they may meet with larger particles of dust rushing towards the sun4 ,we have spoken of the ,zodiacal ,light and that part of it which has been designated the counter-glow4 ,this latter glow is regularly seen in the tropics and occasionally in that portion of our heavens which is just opposite the sun4 ,astronomers ascribe the counter-glow to streams of fine dust which are drawn towards the sun 7compare page #adg74 ,let us assume that a seed of the diameter of #j4jjjaf mm4 strikes against a grain of dust which is a thousand times as large 7#j4jjaf mm4 diameter71 and attaches itself to its surface4 ,this spore will be carried by the grain of dust towards the sun2 it will cross the orbits of the inner planets1 and it may descend in their atmospheres4 ,those grains of dust do not1 by any means1 require very long spaces of time to pass from one planetary orbit to another4 ,if we assume that the spore starts with zero velocity near ,neptune 7in which case the seed might originate from the moon of ,neptune2 for ,neptune itself1 like ,uranus1 ,saturn1 and ,jupiter1 is not yet sufficiently cooled to sustain life71 the spore would reach the orbit of ,uranus in twenty-one years1 and of ,mercury in twenty-nine years4 ,with the same initial velocity such particles would be twelve years in passing between the orbits of ,uranus and ,saturn1 four years between ,saturn and ,jupiter1 two years between ,jupiter and ,mars1 eighty-four days between ,mars and the earth1 forty days between the earth and ,venus1 and twenty-eight days between ,venus and ,mercury4 ,we see from these time estimates that the germs1 together with the grains of dust to which they have attached themselves1 might move towards the sun with much smaller velocity 7from ten to twenty times smaller7 without our having to fear any loss of their germinating powers during the transit4 ,in other words1 if these seeds adhere to the particles1 ninety or ninety-five per cent4 of whose weight is balanced by the radiation pressure1 they may soon fall into the atmosphere of some inner planet with the moderate velocity of a few kilometres per second4 ,it is easy to calculate that if such a particle should1 in falling1 be arrested in its motion after the first second1 it would yet1 thanks to the strong heat radiation from it1 not be heated by more than #ajj ,cent4 7#bab ,f47 above the temperature of its surroundings4 ,such a temperature can be borne by the spores of bacteria without fatal effects for much more than one second4 ,after the particles1 together with the seed adhering to them1 have once been stopped1 they will slowly descend1 or will be carried down to the surface of the nearest planet by descending convection currents4 ,in this way life would be transferred from one point of a planetary system1 on which it had taken root1 to other locations in the same planetary system which favor the development of life4 ,the seeds not caught by such particles of dust may be taken over to other solar systems1 and finally be stopped by the radiation pressure of their suns4 ,they cannot penetrate any farther than to spots at which the radiation pressure is as strong as at their starting-points4 ,consequently1 germs from the earth1 which is five times as near the sun as ,jupiter1 could approach another sun within a fifth of the distance at which germs from ,jupiter would be stopped4 ,somewhere near the suns1 where the seeds are arrested by the radiation pressure to be turned back into space1 there will evidently be accumulations of these seeds4 ,the planets which circulate around their suns have therefore more chance of meeting them than if they were not in the vicinity of a sun4 ,the germs will have lost the great velocity with which they wandered from one solar system to another1 and they will not be heated so greatly in falling through the atmospheres of the planets which they meet4 ,the seeds which are turned back into space when coming near a sun will there perhaps meet with particles whose weight is somewhat greater than the repelling power of the radiation pressure4 ,they would1 therefore1 turn back to the suns4 ,like the germs1 and for similar reasons1 these particles would consequently be concentrated about the sun4 ,the small seeds have1 therefore1 a comparatively better chance of being arrested before their return to space by contact with such particles1 and of being carried to the planets near that sun4 ,in this manner life may have been transplanted for eternal ages from solar system to solar system and from planet to planet of the same system4 ,but as among the billions of grains of pollen which the wind carries away from a large tree--a fir-tree1 for instance--only one may on an average give birth to a new tree1 thus of the billions1 or perhaps trillions1 of germs which the radiation pressure drives out into space1 only one may really bring life to a foreign planet on which life had not yet arisen1 and become the originator of living beings on that planet4 ,finally1 we perceive that1 according to this version of the theory of panspermia1 all organic beings in the whole universe should be related to one another1 and should consist of cells which are built up of carbon1 hydrogen1 oxygen1 and nitrogen4 ,the imagined existence of living beings in other worlds in whose constitution carbon is supposed to be replaced by silicon or titanium must be relegated to the realm of improbability4 ,life on other inhabited planets has probably developed along lines which are closely related to those of our earth1 and this implies the conclusion that life must always recommence from its very lowest type1 just as every individual1 however highly developed it may be1 has by itself passed through all the stages of evolution from the single cell upward4 ,all these conclusions are in beautiful harmony with the general properties characteristic of life on our earth4 ,it cannot be denied that this interpretation of the theory of panspermia is distinguished by perfect consistency1 which is the most important criterion of the probability of a cosmogonical theory4 ,there is little probability1 though1 of our ever being able to demonstrate the correctness of this view by an examination of seeds falling down upon our earth4 ,for the number of germs which reach us from other worlds will be extremely limited--not more1 perhaps1 than a few within a year all over the earth's surface2 and those1 moreover1 will presumably strongly resemble the single-cell spores with which the winds play in our atmosphere4 ,it would be difficult1 if not impossible1 to prove the celestial origin of any such germs if they should be found contrary to our assumption4 ,,the ,,end