,transcriber ,note ,text emphasis denoted as ,italics and ,bold4 ,whole and fractional parts of numbers as #abc-d_/e and some fractions as #a_/ejth4 ,a ,,manual ,,of ,,photographic ,,chemistry4 ,a ,,manual ,,of ,,photographic ,,chemistry1 ,,including ,,the ,,practice ,,of ,,the ,,collodion ,,process4 ,by ,t4 ,,frederick ,,hardwich1 ,,lecturer ,,on ,,photography ,,in ,,king',s ,,college1 ,,london2 ,,late ,,demonstrator ,,of ,,chemistry ,,in ,,king',s ,,college4 ,fourth ,edition4 ,illustration ,,london3 ,,john ,,churchill1 ,,new ,,burlington ,,street4 ,,mdccclvii4 ,the ,author reserves to himself the right of translating this ,edition4 ,,printed ,,by ,,john ,,edward ,,taylor1 ,,little ,,queen ,,street1 ,,lincoln',s ,,inn ,,fields4 ,,preface ,,to ,,the ,,third ,,edition4 ,illustration ,it is a source of much1 gratification to the ,author to find himself called upon to prepare a ,third ,edition of his ,manual in less than fourteen months from the date of its first publication4 ,no greater proof could have been afforded of the rapid advance which the ,photographic ,art is now making in this country4 ,on once more entering upon the task of revision1 the ,writer has been led to reflect in what way the utility of the ,work may be promoted2 and from numerous inquiries he believes that this result will best be attained by carefully omitting everything which does not possess practical as well as scientific interest4 ,the majority of ,photographers look to the ,art to furnish them with amusement as well as instruction1 and they are deterred from entering upon a study which seems to involve a great amount of technical detail3 these remarks however are not intended to discourage a habit of perseverance and careful observation1 but simply to distinguish between the essential and the non-essential in the theory of the subject4 ,the present ,edition differs in many important particulars from those which have preceded it4 ,it has undergone a fresh arrangement throughout4 ,in some parts it is condensed1 in others enlarged4 ,the ,chapters on ,photographic ,printing are entirely re-written1 and include the whole of the ,author's investigations1 as published in the ,society's ,journal4 ,the minute directions given in this part of the ,work will show how much success in ,photography is thought to depend upon a careful attention to minor particulars4 ,another point which has been kept in view1 is to recommend1 as far as possible1 the employment of chemical agents which are used in medicine and vended by all druggists throughout the united kingdom4 ,it is often an advantage to the ,amateur to be able to purchase his materials near at hand2 and1 if the common impurities of the commercial articles are pointed out1 and directions given for their removal1 the ',london ,pharmacopia' will be found to include almost all the chemicals necessary for the practice of the ,art4 ,great additions have been made to the ,index of the present ,edition1 which is now so complete that a reference to it will at once point out the most important facts relating to each subject1 and the different parts of the ,work at which they are described4 ,in conclusion1 a hope is expressed that this ',manual of ,photographic ,chemistry' may be found to be a complete and trustworthy guide on every point connected with the theory and practice of the ,collodion process4 ,london1 ,june #bnd1 #ahef4 ,,preface ,,to ,,the ,,fourth ,,edition4 ,illustration ,the ,author has endeavoured to keep pace with the improvements which are daily being introduced in the science and art of ,photography4 ,in the present ,edition alterations have been made in the style and general arrangement of the work1 and additional matter has been inserted4 ,since the publication of the ,third ,edition1 a series of experiments have been made on the manufacture of ,collodion1 the results of which have thrown further light upon the conditions affecting the sensitiveness of the excited film1 and have enabled the writer to introduce an organic substance1 ,glycyrrhizine1 which will be found of service in making ,photographic copies of ,engravings and similar works of ,art4 ,dr4 ,norris1 of ,birmingham1 has within the last few months communicated a paper on dry ,collodion1 which places the theory of that subject upon a better footing than before4 ,the ,oxymel preservative process is now also thoroughly understood1 and may be considered certain4 ,in addition to the above1 the ,albuminized ,collodion of ,m4 ,taupenot1 which experience proves to be one of the best dry processes at present known1 is included in this ,edition4 ,king's ,college1 ,london1 ,april #fth1 #aheg4 ,,errata4 ,page #bd1 line #e1 for conditions read condition4 ,page #aae1 line #cb1 for ,iodide read ,iodine4 ,page #aid1 line #ae1 for p4 #hh read p4 #ahh4 ,transcriber ,note--,corrections have been applied ,,contents4 ,,part ,i4 ,,the ,,science ,,of ,,photography4 ,page ,introduction #a ,,chapter ,i4 ,historical ,sketch of ,photography #f ,,chapter ,,ii4 ,,the ,,salts ,,of ,,silver ,,employed ,,in ,,photography4 ,section ,i4--,the ,chemistry of the ,salts of ,silver4--,the preparation and properties of the ,nitrate of ,silver--of the ,chloride1 ,bromide1 and ,iodide of ,silver4--,the ,chemistry of the ,oxides of ,silver #ab ,section ,,ii4--,the ,photographic ,properties of the ,salts of ,silver4--,the action of ,light upon ,nitrate of ,silver--upon ,chloride1 ,bromide1 and ,iodide of ,silver4--,the blackening of ,chloride of ,silver explained4--,simple experiments with sensitive ,photographic ,paper #ah ,,chapter ,,iii4 ,,on ,,the ,,development ,,of ,,an ,,invisible ,,image4 ,simple experiments illustrating the process #be ,section ,i4--,chemistry of the ,substances1 employed as ,developers4--,development shown to be a process of reduction4--,the chemistry of the principal reducing agents1 ,gallic ,acid1 ,pyrogallic ,acid1 and the ,protosalts of ,iron #bf ,section ,,ii4--,the ,reduction of ,salts of ,silver by ,developing ,agents4--,reduction of ,oxide of ,silver--of ,nitrate and ,acetate of ,silver4--,varied appearance of ,metallic ,silver when finely divided4--,the reduction of the ,iodide and other ,salts of ,silver containing no ,oxygen #cj ,section ,,iii4--,hypothesis on the ,formation and ,development of the ,latent ,image4--,explanation of the terms under- and over-exposure4--,diagram of ,molecular change4--,moser's experiments on development4--,curious perversions of development #cd ,,chapter ,,iv4 ,,on ,,fixing the ,,photographic ,,image4 ,chemistry of the various substances which may be employed as ,fixing ,agents4--,ammonia1 ,alkaline ,chlorides1 ,bromides1 and ,iodides4--,hyposulphite of ,soda4--,cyanide of ,potassium #da ,,chapter ,v4 ,,on ,,the ,,nature ,,and ,,properties ,,of ,,light4 ,section ,i4--,the compound ,nature of ,light4--,its decomposition into elementary coloured rays4--,division of these rays into ,luminous1 ,heat-producing1 and ,chemical ,rays #df ,section ,,ii4--,the ,refraction of ,light4--,phenomena of simple refraction by parallel and inclined surfaces4--,refraction from curved surfaces4--,the various forms of ,lenses4--,the ,foci of ,lenses4--,formation of a ,luminous ,image by a ,lens #di ,section ,,iii4--,the ,photographic ,camera4--,its simplest form4--,the field of the ,camera4--,chromatic aberration4--,spherical aberration4--,the use of ,stops4--,the double1 or ,portrait combination of ,lenses4--,variation between the ,visual and ,chemical ,foci in ,lenses #ed ,section ,,iv4--,the ,photographic ,action of ,coloured ,light4--,diagram of ,chemical ,spectrum4--,illustrative experiments4--,superior sensibility of ,bromide of ,silver to coloured light4--,mode in which dark-coloured objects are ,photographed #fj ,section ,v4--,on ,binocular ,vision and the ,stereoscope4--,phenomena of ,binocular ,vision4--,theory of the ,stereoscope4--,wheatstone's reflecting ,stereoscope4--,brewster's ,stereoscope4--,rules for taking ,stereoscopic pictures #ff ,,chapter ,,vi4 ,,the ,,photographic ,,properties ,,of ,,iodide ,,of ,,silver ,,upon ,,collodion4 ,section ,i4--,collodion4--,pyroxyline--its varieties--its chemical composition4--,means of obtaining ,nitro-,sulphuric ,acid of the proper strength4--,solvents for ,pyroxyline4--,variation of physical properties in different samples of ,collodion4--,the changes which ,iodized ,collodion undergoes by keeping #ge ,section ,,ii4--,the ,chemistry of the ,nitrate ,bath4--,its solvent action on ,iodide of ,silver4--,acidity and ,alkalinity of the ,nitrate ,bath4--,formation of ,acetate of ,silver in the ,bath4--,the substances which decompose the ,bath4--,changes in the ,nitrate ,bath by use #hf ,section ,,iii4--,the ,conditions which influence the ,formation and ,development of the ,latent ,image4--,causes which increase or diminish the sensitiveness of the film to ,light4--,conditions which hasten or retard development #ia ,section ,,iv4--,on certain irregularities in the ,developing ,process4--,effect of particular states of the ,bath1 and of the ,collodion1 in producing clouding of the ,image1 and of acids in obviating it #ajc ,,chapter ,,vii4 ,,on ,,positive ,,and ,,negative ,,collodion ,,photographs4 ,definition of the terms ,positive and ,negative4--,the same ,photograph often capable of representing both varieties #ajf ,section ,i4--,on ,collodion ,positives4--,the ,collodion and ,nitrate ,bath best suited for ,positives4--,peculiarities of ,pyrogallic ,acid1 and the ,protosalts of ,iron employed to develope ,collodion ,positives4--,the colour of ,positives affected by the length of exposure to light4--,a ,process for whitening ,glass ,positives by means of ,bichloride of ,mercury #ajh ,section ,,ii4--,on ,collodion ,negatives4--,the ,collodion and ,nitrate ,bath best adapted for ,negatives4--,use of ,glycyrrhizine to increase the intensity4--,developing solutions for ,negatives4--,conversion of finished ,positives into ,negatives #aac ,,chapter ,,viii4 ,,on ,,the ,,theory ,,of ,,positive ,,printing4 ,section ,i4--,the ,preparation of the ,sensitive ,paper4--,its darkening by light4--,the conditions which affect its sensitiveness and the vigour of the ,image4--,the colour of the print influenced by the preparation of the paper #aba ,section ,,ii4--,the ,processes for ,fixing and ,toning the ,proof4--,conditions of a proper fixing4--,the ,salts of ,gold used as toning agents4--,the properties of the single fixing and toning ,bath1 with the conditions which affect its action #abh ,section ,,iii4--,the ,author's ,photographic ,researches4--,the chemical composition of the ,photographic ,image4--,the various agencies destructive to ,photographs4--,the action of damp air upon ,positive ,prints4--,the change in properties of the fixing ,bath by constant use #adj ,section ,,iv4--,the ,fading of ,photographic ,prints4--,the causes which produce it4--,the comparative permanency of ,prints4--,the mode of testing permanency #afj ,,chapter ,,ix4 ,,on ,,the ,,daguerreotype ,,and ,,talbotype ,,processes4 ,section ,i4--,the ,daguerreotype4--,the nature of the sensitive film4--,development of the ,latent ,image4--,the strengthening of the ,image by means of ,hyposulphite of ,gold #aga ,section ,,ii4--,the ,processes of ,talbot1 etc4--,the ,calotype4--,the ,waxed ,paper ,process of ,le ,grey4--,the ,albumen ,negative ,process4--,taupenot's ,collodio-,albumen ,process #agf ,,part ,,ii4 ,,practical ,,details ,,of ,,the ,,collodion ,,process4 ,,chapter ,i4 ,,preparation ,,of ,,collodion4 ,mode of preparing soluble ,pyroxyline by the mixed ,acids--by the ,nitre process4--,purification of the ,ether and ,alcohol4--,preparation of the iodizing compounds in a state of purity #ahe ,,chapter ,,ii4 ,,formul ,,for ,,solutions ,,required ,,for ,,collodion ,,photographs4 ,section ,i4--,formul for direct ,positive ,solutions4--,the ,collodion4--,the ,nitrate ,bath4--,developing fluids4--,fixing liquids4--,whitening solution #bja ,section ,,ii4--,formula for ,negative ,solutions4--,the ,collodion4--,the ,nitrate ,bath4--,developing fluid4--,fixing liquid #bjh ,,chapter ,,iii4 ,,manipulations ,,of ,,the ,,collodion ,,process4 ,section ,i4--,manipulations with moist ,collodion4--,cleaning the ,plates4--,coating with ,collodion4--,exciting4--,exposing4--,de veloping4 --,fixing #bac ,section ,,ii4--,simple directions for the use of ,photographic ,lenses4--,portrait ,lenses4--,view ,lenses4--,mode of finding the chemical ,focus #bbg ,section ,,iii4--,on copying ,engravings1 ,etchings1 ,diagrams1 etc4--,mode of intensifying the ,collodion #bca ,section ,,iv4--,rules for taking ,stereoscopic ,photographs4--,mr4 ,latimer ,clark's arrangement for working with a single ,camera #bcb ,section ,v4--,the ,photographic delineation of ,microscopic ,objects4--,arrangement of the apparatus4--,mode of finding the chemical ,focus4--,use of artificial light #bce ,,chapter ,,iv4 ,,the ,,details ,,of ,,photographic ,,printing4 ,section ,i4--,positive ,printing by the ordinary process4--,selection of the paper4--,preparation of ,albuminized paper--of plain paper--of ,ammonio-,nitrate ,paper4--,preparation of the fixing and toning ,bath4--,manipulatory details of ,photographic ,printing4--,mode of washing and mounting the ,proofs #bdj ,section ,,ii4--,positive ,printing by ,development4--,a process on ,chloride and ,citrate of ,silver4--,on ,iodide of ,silver4--,on ,bromide of ,silver #bei ,section ,,iii4--,the ,sel d',or ,process for toning ,positives4--,preparation of the toning ,bath4--,manipulatory details #bfg ,section ,,iv4--,on ,printing enlarged ,positives1 ,transparencies1 etc41 upon ,collodion #bgb ,,chapter ,v4 ,,classification ,,of ,,causes ,,of ,,failure ,,in ,,the ,,collodion ,,process4 ,section ,i4--,imperfections in ,negative and ,positive ,collodion ,photographs4--,fogging4--,transparent and opaque spots4--,markings of all kinds--under- and over-exposure1 etc4 #bgf ,section ,,ii4--,imperfections in ,paper ,positives #bhe ,,chapter ,,vi4 ,,landscape ,,photography ,,by ,,the ,,collodion ,,preservative ,,and ,,collodio-,,albumen ,,processes4 ,the ,honey ,keeping ,process4--,the ,oxymel ,process4--,photography on dry ,collodion4--,taupenot's ,collodio-,albumen ,process #bhh ,,part ,,iii4 ,,outlines ,,of ,,general ,,chemistry4 ,,chapter ,i4 ,,the ,,chemical ,,elements ,,and ,,their ,,combinations4 ,the more important ,elementary ,bodies1 with their ,symbols and ,atomic ,weights4--,the ,compounds formed by their union4--,the class of ,salts4--,illustrations of the nature of ,chemical ,affinity4--,chemical ,nomenclature4--,symbolic ,notation4--,the ,laws of ,combination4--,the ,atomic ,theory4--,the ,chemistry of ,organic ,bodies #cje ,,chapter ,,ii4 ,vocabulary of ,photographic ,chemicals #cbg ,,appendix4 ,quantitative testing of ,nitrate ,baths4--,recovery of ,silver from waste solutions4--,reduction of ,chloride of ,silver4--,mode of taking the ,specific ,gravity of liquids4--,on ,filtration4--,the use of ,test-papers4--,the removal of ,silver stains from the hands1 etc4--,dr4 ,ure's tables of the strength of ,sulphuric and ,nitric ,acid of different densities4--,table of ,weights and ,measures #cga ,a ,,manual ,,of ,,photographic ,,chemistry4 ,illustration ,,introduction4 ,in attempting to impart knowledge on any subject1 it is not sufficient that the writer should himself be acquainted with that which he professes to teach4 ,even supposing such to be the case1 yet much of the success of his effort must depend upon the manner in which the information is conveyed2 for as1 on the one hand1 a system of extreme brevity always fails of its object1 so1 on the other1 a mere compilation of facts imperfectly explained tends only to confuse the reader4 ,a middle course between these extremes is perhaps the best to adopt2 that is1 to make selection of certain fundamental points1 and to explain them with some minuteness1 leaving others of less importance to be dealt with in a more summary manner1 or to be altogether omitted4 ,but independently of observations of this kind1 which apply to educational instruction in general1 it may be remarked1 that there are sometimes difficulties of a more formidable description to be overcome4 ,for instance1 in treating of any science1 such as that of ,photography1 which may be said to be comparatively new and unexplored1 there is great danger of erroneously attributing effects to their wrong causes6 ,perhaps none but he who has himself worked in the laboratory can estimate this point in its proper light4 ,in an experiment where the quantities of material acted upon are infinitesimally small1 and the chemical changes involved of a most refined and subtle description1 it is soon discovered that the slightest variation in the usual conditions will suffice to alter the result4 ,nevertheless ,photography is truly a science1 governed by fixed laws2 and hence1 as our knowledge increases1 we may fairly hope that uncertainty will cease1 and the same precision at length be attained as that with which chemical operations are usually performed4 ,the intention of the author in writing this work1 is to impart a thorough knowledge of what may be termed the ,first ,principles of ,photography1 that the amateur may arm himself with a theoretical acquaintance with the subject before proceeding to the practice of it4 ,to assist this object1 care will be taken to avoid needless complexity in the formul1 and all ingredients will be omitted which are not proved to be of service4 ,the impurities of chemicals will be pointed out as far as possible1 and special directions given for their removal4 ,amongst the variety of ,photographic processes devised1 those only will be selected which are correct on theoretical grounds1 and are found in practice to succeed4 ,as the work is addressed to one supposed to be unacquainted both with ,chemistry and ,photography1 pains will be taken to avoid the employment of all technical terms of which an explanation has not previously been given4 ,a ,,sketch ,,of ,,the ,,main ,,divisions ,,to ,,be ,,adopted1 ,,with ,,the ,,principal ,,subject-,,matter ,,of ,,each4 ,the title given to the ,work is ,a ,manual of ,photographic ,chemistry1 and it is proposed to include in it a familiar explanation of the nature of the various chemical agents employed in the ,art of ,photography1 with the rationale of the manner in which they are thought to act4 ,the division adopted is threefold3-- ,part ,i4 enters minutely into the theory of ,photographic processes2 ,part ,,ii4 treats of the practice of ,photography upon ,collodion2 ,part ,,iii4 embraces a simple statement of the main laws of ,chemistry1 with the principal properties of the various substances1 elementary or compound1 which are employed by ,photographers4 ,part ,i41 or the ,science of ,photography1 includes a full description of the chemical action of ,light upon the ,salts of ,silver1 with its application to artistic purposes2 all mention of manipulatory details1 and of quantities of ingredients1 being1 as a rule1 omitted4 ,in this division of the ,work will be found nine ,chapters1 the contents of which are as follows3-- ,chapter ,i4 is a sketch of the history of ,photography1 intended to convey a general notion of the origin and progress of the ,art1 without dwelling on minute particulars4 ,chapter ,,ii4 describes the ,chemistry of the ,salts of ,silver employed by ,photographers2 their preparation and properties2 the phenomena of the action of ,light upon them1 with experiments illustrating it4 ,chapter ,,iii4 leads us on to the formation of an invisible image upon a sensitive surface1 with the development or bringing out to view of the same by means of chemical re-agents4 ,this point1 being of elementary importance1 is described carefully2--the reduction of metallic oxides1 the properties of the bodies employed to reduce1 and the hypotheses which have been entertained on the nature of the ,light's action1 are all minutely explained4 ,chapter ,,iv4 treats of the fixing of ,photographic impressions1 in order to render them indestructible by diffused light4 ,chapter ,v4 contains a sketch of the ,optics of ,photography--the decomposition of white ,light into its elementary rays1 the ,photographic properties of the different colours1 the refraction of ,light1 and construction of ,lenses4 ,in the last ,section of the same ,chapter will be found a short sketch of the history and use of the ,stereoscope4 ,chapter ,,vi4 embraces a more minute description of the sensitive ,photographic processes upon ,collodion4 ,in it is explained the chemistry of ,pyroxyline1 with its solution in ,alcoholized ,ether1 or ,collodion2 also the ,photographic properties of ,iodide of ,silver upon ,collodion1 with the causes which affect its sensitiveness to ,light1 and the action of the developing solutions in bringing out the image4 ,chapter ,,vii4 continues the same subject1 describing the classification of ,collodion ,photographs as ,positives and ,negatives1 with the distinctive peculiarities of each4 ,chapter ,,viii4 contains the theory of the production of ,positive ,photographs upon paper4 ,in this ,chapter will be found an explanation of the somewhat complex chemical changes involved in printing ,positives1 with the precautions which are required to ensure the permanency of the proofs4 ,chapter ,,ix4 is supplementary to the others1 and a brief notice of it will suffice4 ,it explains the theory of the ,photographic processes of ,daguerre and ,talbot2 especially noticing those points in which they may be contrasted with ,photography upon ,collodion1 but omitting all description of manipulatory details1 which if included would extend the ,work beyond its proposed limits4 ,the title of the second principal division of the ,work1 viz4 ,the practice of ,photography upon ,collodion1 explains itself4 ,attention however may be invited to the fifth ,chapter1 in which a classification is given of the principal imperfections in ,photographs1 with short directions for their removal2 and to ,chapter ,,vi41 which describes the preservation of the sensitiveness of ,collodion plates and the mode of operating upon films of ,albumenized ,collodion4 ,in ,part ,,iii4 will be found1 in addition to a statement of the laws of chemical combination1 etc41 a list of ,photographic chemicals1 alphabetically arranged1 including their preparation and properties as far as required for their employment in the ,art4 ,the reader will at once gather from this sketch of the contents of the volume before him1 that whilst the general theory of every ,photographic process is described1 with the preparation and properties of the chemicals employed1 minute directions in the minor points of manipulation are restricted to ,photography upon ,collodion1 that branch of the ,art being the one to which the time and attention of the author have been especially directed4 ,collodion is allowed by all to be the best vehicle for the sensitive ,silver ,salts which is at present known1 and successful results can be obtained with a very small expenditure of time and trouble1 if the solutions employed in the process are prepared in a state of purity4 ,,chapter ,i4 ,,historical ,,sketch ,,of ,,photography4 ,the ,art of ,photography1 which has now attained such perfection1 and has become so popular amongst all classes1 is one of comparatively recent introduction4 ,the word ,photography means literally writing by means of ,light2 and it includes all processes by which any kind of picture can be obtained by the chemical agency of ,light1 without reference to the nature of the sensitive surface upon which it acts4 ,the philosophers of antiquity1 although chemical changes due to the influence of ,light were continually passing before their eyes1 do not appear to have directed their attention to them4 ,some of the ,alchemists indeed noticed the fact that a substance which they termed ,horn ,silver1 which was probably a ,chloride of ,silver which had undergone fusion1 became blackened by exposure to ,light2 but their ideas on such subjects being of the most erroneous nature1 nothing resulted from the discovery4 ,the first philosophical examination of the decomposing action of ,light upon compounds containing ,silver was made by the illustrious ,scheele1 no longer than three-quarters of a century ago1 viz4 in #aggg4 ,it was also remarked by him that some of the coloured rays of ,light were peculiarly active in promoting the change4 ,earliest application of these facts to purposes of ,art4--,the first attempts to render the blackening of ,silver ,salts by ,light available for artistic purposes were made by ,wedgwood and ,davy about ,a4,d4 #ahjb4 ,a sheet of white paper or of white leather was saturated with a solution of ,nitrate of ,silver1 and the shadow of the figure intended to be copied projected upon it4 ,under these circumstances the part on which the shadow fell remained white1 whilst the surrounding exposed parts gradually darkened under the influence of the sun's rays4 ,unfortunately these and similar experiments1 which appeared at the outset to promise well1 were checked by the experimentalists being unable to discover any means of fixing the pictures1 so as to render them indestructible by diffused ,light4 ,the unchanged ,silver ,salt being permitted to remain in the white portions of the paper1 naturally caused the proofs to blacken in every part1 unless carefully preserved in the dark4 ,introduction of the ,camera ,obscura1 and other ,improvements in ,photography4--,the ,camera ,obscura1 or darkened chamber1 by means of which a luminous image of an object may be formed1 was invented by ,baptista ,porta1 of ,padua2 but the preparations employed by ,wedgwood were not sufficiently sensitive to be easily affected by the subdued light of that instrument4 ,in the year #ahad1 however1 twelve years subsequent to the publication of ,wedgwood's paper1 ,m4 ,nipce1 of ,chalons1 having directed his attention to the subject1 succeeded in perfecting a process in which the ,camera could be employed1 although the sensibility was still so low that an exposure of some hours was required to produce the effect4 ,in the process of ,m4 ,nipce1 which was termed ,heliography1 or sun-drawing1 the use of the ,silver ,salts was discarded1 and a resinous substance1 known as ,bitumen of ,juda1 substituted4 ,this resin was smeared on the surface of a metal plate1 and exposed to the luminous image4 ,the light in acting upon it so changed its properties1 that it became insoluble in certain essential oils4 ,hence1 on subsequent treatment with the oleaginous solvent1 the shadows dissolved away1 and the lights were represented by the unaltered resin remaining on the plate4 ,the ,discoveries of ,m4 ,daguerre4--,,mm4 ,nipce and ,daguerre appear at one time to have been associated as partners1 for the purpose of mutually prosecuting their researches2 but it was not until after the death of the former1 viz4 in #ahci1 that the process named the ,daguerreotype was given to the world4 ,daguerre was dissatisfied with the slowness of action of the ,bitumen sensitive surface1 and directed his attention mainly to the use of the ,salts of ,silver1 which are thus again brought before our notice4 ,even the earlier specimens of the ,daguerreotype1 although far inferior to those subsequently produced1 possessed a beauty which had not been attained by any ,photographs prior to that time4 ,the sensitive plates of ,daguerre were prepared by exposing a silvered tablet to the action of the vapour of ,iodine1 so as to form a layer of ,iodide of ,silver upon the surface4 ,by a short exposure in the ,camera an effect was produced1 not visible to the eye1 but appearing when the plate was subjected to the vapour of ,mercury4 ,this feature1 viz4 the production of a latent image upon ,iodide of ,silver1 with its subsequent development by a chemical reagent1 is one of the first importance4 ,its discovery at once reduced the time of taking a picture from hours to minutes1 and promoted the utility of the ,art4 ,daguerre also succeeded in fixing his proofs1 by removal of the unaltered ,iodide of ,silver from the shadows4 ,the processes employed however were imperfect1 and the matter was not set at rest until the publication of a paper by ,sir ,john ,herschel1 on the property possessed by ,hyposulphites of dissolving the ,salts of ,silver insoluble in water4 ,on a means of ,multiplying ,photographic ,impressions1 and other ,discoveries of ,mr4 ,fox ,talbot4--,the first communication made to the ,royal ,society by ,mr4 ,fox ,talbot1 in ,january1 #ahci1 included only the preparation of a sensitive paper for copying objects by application4 ,it was directed that the paper should be dipped first in solution of ,chloride of ,sodium1 and then in ,nitrate of ,silver4 ,in this way a white substance termed ,chloride of ,silver is formed1 more sensitive to light than the ,nitrate of ,silver originally employed by ,wedgwood and ,davy4 ,the object is laid in contact with the prepared paper1 and1 being exposed to light1 a copy is obtained1 which is ,negative1--id est1 with the light and shade reversed4 ,a second sheet of paper is then prepared1 and the first1 or ,negative impression1 laid upon it1 so as to allow the sun's light to pass through the transparent parts4 ,under these circumstances1 when the ,negative is raised1 a natural representation of the object is found below2 the tints having been again reversed by the second operation4 ,this production of a ,negative ,photograph1 from which any number of ,positive copies may be obtained1 is a cardinal point in ,mr4 ,talbot's invention1 and one of great importance4 ,the patent issued for the process named ,talbotype or ,calotype dates from ,february1 #ahda4 ,a sheet of paper is first coated with ,iodide of ,silver by soaking it alternately in ,iodide of ,potassium and ,nitrate of ,silver2 it is then washed with solution of ,gallic ,acid containing ,nitrate of ,silver 7sometimes termed ,gallo-,nitrate of ,silver71 by which the sensibility to light is greatly augmented4 ,an exposure in the ,camera of some seconds or minutes1 according to the brightness of the light1 impresses an invisible image1 which is brought out by treating the plate with a fresh portion of the mixture of ,gallic ,acid and ,nitrate of ,silver employed in exciting4 ,on the use of ,glass ,plates to retain ,sensitive ,films4--,the principal defects in the ,calotype process are attributable to the coarse and irregular structure of the fibre of paper1 even when manufactured with the greatest care1 and expressly for ,photographic purposes4 ,in consequence of this1 the same amount of exquisite definition and sharpness of outline as that resulting from the use of metal plates1 cannot be obtained4 ,we are indebted to ,sir ,john ,herschel for the first employment of glass plates to receive sensitive ,photographic films4 ,the ,iodide of ,silver may be retained upon the glass by means of a layer of ,albumen or white of egg1 as proposed by ,m4 ,nipce de ,saint-,victor1 nephew to the original discoverer of the same name4 ,a more important improvement still is the employment of ,collodion for a similar purpose4 ,collodion is an ethereal solution of a substance almost identical with ,gun-,cotton4 ,on evaporation it leaves a transparent layer1 resembling gold-beater's skin1 which adheres to the glass with some tenacity4 ,m4 ,le ,grey of ,paris originally suggested that this substance might possibly be rendered available in ,photography1 but our own countryman1 ,mr4 ,archer1 was the first to carry out the idea practically4 ,in a communication to ',the ,chemist' in the autumn of #ahea1 this gentleman gave a description of the ,collodion process much as it now stands2 at the same time proposing the substitution of ,pyro-gallic acid for the ,gallic acid previously employed in developing the image4 ,at that period no idea could have been entertained of the stimulus which this discovery would render to the progress of the ,art2 but experience has now abundantly demonstrated1 that1 as far as all qualities most desirable in a ,photographic process are concerned1 none at present known can excel1 or perhaps equal1 the ,collodion process4 ,,chapter ,,ii4 ,,the ,,salts ,,of ,,silver ,,employed ,,in ,,photography4 ,by the term ,salt of ,silver we understand that the compound in question contains ,silver1 but not in its elementary form2 the metal is in fact in a state of chemical union with other elements which disguise its physical properties1 so that the ,salt possesses none of the external characters of the ,silver from which it was produced4 ,silver is not the only metal which forms ,salts2 there are ,salts of ,lead1 ,copper1 ,iron1 etc4 ,sugar of ,lead is a familiar instance of a ,salt of ,lead4 ,it is a white crystalline body1 easily soluble in water1 the solution possessing an intensely sweet taste2 chemical tests prove that it contains ,lead1 although no suspicion of such a fact could be entertained from a consideration of its general properties4 ,common ,salt1 or ,chloride of ,sodium1 which is the type of the salts generally1 is constituted in a similar manner2 that is to say1 it contains a metallic substance1 the characters of which are masked1 and lie hid in the compound4 ,the contents of this ,chapter may be arranged in three ,sections3 the first describing the ,chemistry of the ,salts of ,silver2 the second1 the action of ,light upon them2 the third1 the preparation of a sensitive surface1 with experiments illustrating the formation of the ,photographic image4 ,,section ,i4 ,chemistry of the ,salts of ,silver4 ,the principal ,salts of ,silver employed in the ,photographic processes are four in number1 viz4 ,nitrate of ,silver1 ,chloride of ,silver1 ,iodide of ,silver1 and ,bromide of ,silver4 ,in addition to these1 it will be necessary to describe the ,oxides of ,silver4 ,,the ,,preparation ,,and ,,properties ,,of ,,the ,,nitrate ,,of ,,silver4 ,nitrate of ,silver is prepared by dissolving metallic ,silver in ,nitric ,acid4 ,nitric ,acid is a powerfully acid and corrosive substance1 containing two elementary bodies united in definite proportions4 ,these are ,nitrogen and ,oxygen2 the latter being present in greatest quantity4 ,nitric ,acid is a powerful solvent for the metallic bodies generally4 ,to illustrate its action in that particular1 as contrasted with other acids1 place pieces of silver-foil in two test-tubes1 the one containing dilute ,sulphuric1 the other dilute ,nitric ,acid2 on the application of heat a violent action soon commences in the latter1 but the former is unaffected4 ,in order to understand this1 it must be borne in mind that when a metallic substance dissolves in an acid1 the nature of the solution is different from that of an aqueous solution of salt or sugar4 ,if salt water be boiled down until the whole of the water has evaporated1 the salt is recovered with properties the same as at first2 but if a similar experiment be made with a solution of ,silver in ,nitric ,acid1 the result is different3 in that case metallic ,silver is not obtained on evaporation1 but ,silver combined with ,oxygen and ,nitric ,acid1 both of which are strongly retained1 being in fact in a state of chemical combination with the metal4 ,if we closely examine the effects produced by treating ,silver with ,nitric ,acid1 we find them to be of the following nature3--first1 a certain amount of ,oxygen is imparted to the metal1 so as to form an ,oxide1 which ,oxide dissolves in another portion of the ,nitric ,acid1 producing ,nitrate of the ,oxide1 or1 as it is shortly termed1 ,nitrate of ,silver4#a #a ,the preparation of ,nitrate of ,silver from the standard coin of the realm is described in ,part ,,iii41 ,art4 ,silver4 ,it is the instability of ,nitric ,acid therefore--its proneness to part with ,oxygen--which renders it superior to the ,sulphuric and to most acids in dissolving ,silver and various other substances1 both organic and inorganic4 ,properties of ,nitrate of ,silver4--,in preparing ,nitrate of ,silver1 when the metal has dissolved1 the solution is boiled down and set aside to crystallize4 ,the salt however as so obtained is still acid to test-paper1 and requires either re-crystallization1 or careful heating to about #cjj ,fahrenheit4 ,it is this retention of small quantities of ,nitric ,acid1 and sometimes probably of ,nitrous ,acid1 which renders much of the commercial ,nitrate of ,silver useless for ,photography1 until rendered neutral by fusion and a second crystallization4 ,pure ,nitrate of ,silver occurs in the form of white crystalline plates1 which are very heavy and dissolve readily in an equal weight of cold water4 ,the solubility is much lessened by the presence of free ,nitric ,acid1 and in the concentrated ,nitric ,acid the crystals are almost insoluble4 ,boiling ,alcohol takes up about one-fourth part of its weight of the crystallized ,nitrate1 but deposits nearly the whole on cooling4 ,nitrate of ,silver has an intensely bitter and nauseous taste2 acting as a caustic1 and corroding the skin by a prolonged application4 ,its aqueous solution does not redden blue litmus-paper4 ,heated in a crucible the salt melts1 and when poured into a mould and solidified1 forms the white lunar caustic of commerce4 ,at a still higher temperature it is decomposed1 and bubbles of ,oxygen ,gas are evolved3 the melted mass cooled and dissolved in water leaving behind a black powder1 and yielding a solution1 which is faintly alkaline to test-paper1 from the presence of minute quantities of ,nitrite or basic ,nitrite of ,silver4#b #b ,nitrite of ,silver differs from the ,nitrate in containing less ,oxygen1 and is formed from it by the abstraction of two atoms of that element2 it is described in the vocabulary1 ,part ,,iii4 ,,the ,,chemistry ,,of ,,the ,,chlorides ,,of ,,silver4 ,preparation of ,protochloride of ,silver4--,the ordinary white ,chloride of ,silver may be prepared in two ways1--by the direct action of ,chlorine upon metallic ,silver1 and by double decomposition between two salts4 ,if a plate of polished silver be exposed to a current of ,chlorine ,gas1#c it becomes after a short time coated on the surface with a superficial film of white powder4 ,this powder is ,chloride of ,silver1 containing the two elements ,chlorine and ,silver united in single equivalents4 #c ,for the properties of the element ,chlorine1 see the third division of the ,work4 ,preparation of ,chloride of ,silver by double decomposition4--,in order to illustrate this1 take a solution in water of ,chloride of ,sodium or common salt1 and mix it with a solution containing ,nitrate of ,silver2 immediately a dense1 curdy1 white precipitate falls1 which is the substance in question4 ,in this reaction the elements change places2 the ,chlorine leaves the ,sodium with which it was previously combined1 and crosses over to the ,silver2 the ,oxygen and ,nitric ,acid are released from the ,silver1 and unite with the ,sodium2 thus ,chloride of ,sodium plus ,nitrate of ,silver equals ,chloride of ,silver plus ,nitrate of ,soda4 ,this interchange of elements is termed by chemists double decomposition2 further illustrations of it1 with the conditions necessary to the proper establishment of the process1 are given in the first ,chapter of ,part ,,iii4 ,the essential requirements in two salts intended for the preparation of ,chloride of ,silver1 are simply that the first should contain ,chlorine1 the second ,silver1 and that both should be soluble in water2 hence the ,chloride of ,potassium or ,ammonium may be substituted for the ,chloride of ,sodium1 and the ,sulphate or ,acetate for the ,nitrate of ,silver4 ,in preparing ,chloride of ,silver by double decomposition1 the white clotty masses which first form must be washed repeatedly with water1 in order to free them from soluble ,nitrate of ,soda1 the other product of the change4 ,when this is done1 the salt is in a pure state1 and may be dried1 etc41 in the usual way4 ,properties of ,chloride of ,silver4--,chloride of ,silver differs in appearance from the ,nitrate of ,silver4 ,it is not usually crystalline1 but forms a soft white powder resembling common chalk or whiting4 ,it is tasteless and insoluble in water2 unaffected by boiling with the strongest ,nitric ,acid1 but sparingly dissolved by concentrated ,hydrochloric ,acid4 ,ammonia dissolves ,chloride of ,silver freely1 as do solutions of ,hyposulphite of ,soda and ,cyanide of ,potassium4 ,concentrated solutions of alkaline ,chlorides1 ,iodides1 and ,bromides are likewise solvents of ,chloride of ,silver1 but to a limited extent1 as will be more fully shown in ,chapter ,,iv41 when treating of the modes of fixing the ,photographic proofs4 ,dry ,chloride of ,silver carefully heated to redness fuses1 and concretes on cooling into a tough and semi-transparent substance1 which has been termed horn silver or luna cornea4 ,placed in contact with metallic ,zinc or ,iron acidified with dilute ,sulphuric ,acid1 ,chloride of ,silver is reduced to the metallic state1 the ,chlorine passing to the other metal under the decomposing influence of the galvanic current which is established4 ,preparation and ,properties of the ,subchloride of ,silver4--,if a plate of polished ,silver be dipped in solution of ,perchloride of ,iron1 or of ,bichloride of ,mercury1 a black stain is produced1 the ,iron or ,mercury ,salt losing a portion of ,chlorine1 which passes to the ,silver and converts it superficially into ,subchloride of ,silver4 ,this compound differs from the white ,chloride of ,silver in containing less ,chlorine2 the composition of the latter being represented by the formula ,ag,cl1 that of the former may perhaps be written as ,ag#b;,cl7874 ,subchloride of ,silver is interesting to the ,photographer as corresponding in properties and composition with the ordinary ,chloride of ,silver blackened by light4 ,it is a pulverulent substance of a bluish-black colour not easily affected by ,nitric ,acid but decomposed by fixing agents such as ,ammonia1 ,hyposulphite of ,soda1 or ,cyanide of ,potassium1 into ,chloride of ,silver which dissolves1 and insoluble metallic ,silver4 ,,the ,,chemistry ,,of ,,iodide ,,of ,,silver4 ,the properties of ,iodine are described in the third division of the ,work3 they are analogous to those of ,chlorine and ,bromine1 the ,silver ,salts formed by these elements bearing also a strong resemblance to each other4 ,preparation and ,properties of ,iodide of ,silver4--,iodide of ,silver may be formed in an analogous manner to the ,chloride1 viz4 by the direct action of the vapour of ,iodine upon metallic ,silver1 or by double decomposition1 between solutions of ,iodide of ,potassium and ,nitrate of ,silver4 ,when prepared by the latter mode it forms an impalpable powder1 the colour of which varies slightly with the manner of precipitation4 ,if the ,iodide of ,potassium be in excess1 the ,iodide of ,silver falls to the bottom of the vessel nearly white2 but with an excess of ,nitrate of ,silver it is of a straw-yellow tint4 ,this point may be noticed1 because the yellow salt is the one adapted for ,photographic use1 the other being insensible to the influence of light4 ,iodide of ,silver is tasteless and inodorous2 insoluble in water and in dilute ,nitric ,acid4 ,it is scarcely dissolved by ,ammonia1 which serves to distinguish it from the ,chloride of ,silver1 freely soluble in that liquid4 ,hyposulphite of ,soda and ,cyanide of ,potassium both dissolve ,iodide of ,silver2 it is also soluble in solutions of the alkaline ,bromides and ,iodides1 as will be further explained in ,chapter ,,iv4 ,iodide of ,silver is reduced by ,metallic ,zinc in the same manner as the ,chloride of ,silver1 forming soluble ,iodide of ,zinc and leaving a black powder4 ,,the ,,preparation ,,and ,,properties ,,of ,,bromide ,,of ,,silver4 ,this substance so closely resembles the corresponding salts containing ,chlorine and ,iodine1 that a short notice of it will suffice4 ,bromide of ,silver is prepared by exposing a silvered plate to the vapour of ,bromine1 or by adding solution of ,bromide of ,potassium to ,nitrate of ,silver4 ,it is an insoluble substance1 slightly yellow in colour1 and distinguished from ,iodide of ,silver by dissolving in strong ,ammonia and in ,chloride of ,ammonium4 ,it is freely soluble in ,hyposulphite of ,soda and in ,cyanide of ,potassium4 ,the properties of the element ,bromine are described in ,part ,,iii4 ,,chemistry ,,of ,,the ,,oxides ,,of ,,silver4 ,the ,protoxide of ,silver 7,ag ,o74--,if a little ,potash or ,ammonia be added to solution of ,nitrate of ,silver1 an olive-brown substance is formed1 which1 on standing1 collects at the bottom of the vessel4 ,this is ,oxide of ,silver1 displaced from its previous state of combination with ,nitric ,acid by the stronger oxide4 ,potash4 ,oxide of ,silver is soluble to a very minute extent in pure water1 the solution possessing an alkaline reaction to ,litmus2 it is easily dissolved by ,nitric or ,acetic ,acid1 forming a neutral ,nitrate or ,acetate2 also soluble in ,ammonia 7,ammonio-,nitrate of ,silver71 and in ,nitrate of ,ammonia1 ,hyposulphite of ,soda1 and ,cyanide of ,potassium4 ,long exposure to light converts it into a black substance1 which is probably a ,suboxide4 ,the ,suboxide of ,silver 7,ag#b,o87--,this substance was obtained by ,faraday on exposing a solution of the ,ammonio-,nitrate of ,silver to the action of the air4 ,it bears a relation to the ordinary brown ,protoxide of ,silver similar to that which the ,subchloride bears to ,protochloride of ,silver4 ,suboxide of ,silver is a black or grey powder1 which assumes the metallic lustre on rubbing1 and when treated with dilute ,acids is resolved into ,protoxide of ,silver which dissolves1 and metallic ,silver4 ,,section ,,ii4 ,on the ,photographic ,properties of the ,salts of ,silver4 ,in addition to the ,salts of ,silver described in the first ,section of this ,chapter there are many others well known to chemists1 as the ,acetate of ,silver1 the ,sulphate1 the ,citrate of ,silver1 etc4 ,some occur in crystals which are soluble in water1 whilst others are pulverulent and insoluble4 ,the ,salts of ,silver formed by colourless ,acids are white when first prepared1 and remain so if kept in a dark place2 but they possess the remarkable peculiarity of being darkened in colour by exposure to ,light4 ,action of ,light upon the ,nitrate of ,silver4--,the ,nitrate of ,silver is one of the most permanent of the ,silver salts4 ,it may be preserved unchanged in the crystalline form1 or in solution in distilled water1 for an indefinite length of time1 even when constantly exposed to the diffused light of day4 ,this is partly explained by the nature of the acid with which ,oxide of ,silver is associated in the ,salt2 ,nitric ,acid1 possessing strong oxidizing properties1 being opposed to the darkening influence of ,light upon the ,silver compounds4 ,nitrate of ,silver may1 however1 be rendered susceptible to the influence of ,light1 by adding to its solution organic matter1 vegetable or animal4 ,the phenomena produced in this case are well illustrated by dipping a pledget of cotton-wool1 or a sheet of white paper1 in solution of ,nitrate of ,silver1 and exposing it to the direct rays of the sun2 it slowly darkens1 until it becomes nearly black4 ,the stains upon the skin produced by handling ,nitrate of ,silver are caused in the same way1 and are seen most evidently when the part has been exposed to light4 ,the varieties of organic matter which especially facilitate the blackening of ,nitrate of ,silver are such as tend to absorb ,oxygen2 hence pure vegetable fibre1 free from ,chlorides1 such1 for instance1 as the ,swedish filtering-paper1 is not rendered very sensitive by being simply brushed with solution of the ,nitrate1 but a little grape sugar added soon determines the decomposition4 ,decomposition of ,chloride1 ,bromide1 and ,iodide of ,silver by ,light4--,pure moist ,chloride of ,silver#d changes slowly from white to violet on exposure to light4 ,bromide of ,silver becomes of a grey colour1 but is less affected than the ,chloride4 ,iodide of ,silver 7if free from excess of ,nitrate of ,silver7 does not alter in appearance by exposure even to the sun's rays1 but retains its yellow tint unchanged4 ,of these three compounds therefore ,chloride of ,silver is the most readily acted on by light1 and papers prepared with this salt will become far darker on exposure than others coated with ,bromide or ,iodide of ,silver4 #d ,the ,chloride here spoken of is the compound prepared by adding a soluble ,chloride to a solution of ,nitrate of ,silver3 the product of the direct action of ,chlorine upon metallic ,silver is sometimes insensitive to light4 ,there are certain conditions which accelerate the action of light upon the ,chloride of ,silver4 ,these are1 first1 an excess of ,nitrate of ,silver1 and second1 the presence of organic matter4 ,pure ,chloride of ,silver would be useless as a ,photographic agent1 but a ,chloride with excess of ,nitrate is very sensitive4 ,even ,iodide of ,silver1 ordinarily unaffected1 is blackened by light when moistened with a solution of the ,nitrate of ,silver4#e #e ,the reader will understand that the ,acetate1 ,sulphate1 or any other soluble ,salt of ,silver1 might be substituted for the ,nitrate in this experiment4 ,organic matter combined with ,chloride and ,nitrate of ,silver gives a still higher degree of sensibility1 and in this way the ,photographic papers are prepared4 ,the blackening of ,chloride of ,silver by ,light explained4--,this may be studied by suspending pure ,chloride of ,silver in distilled water1 and exposing it to the sun's rays for several days4 ,when the process of darkening has proceeded to some extent1 the supernatant liquid is found to contain free ,chlorine1 or1 in place of it4 ,hydrochloric ,acid 7,h ,cl71 the result of a subsequent action of the ,chlorine upon the water4 ,the luminous rays appear to loosen the affinity of the elements ,chlorine and ,silver for each other2 hence a portion of ,chlorine is separated1 and the white ,protochloride is converted into the violet ,subchloride of ,silver4 ,if an atom of ,nitrate of ,silver be present1 the liberated ,chlorine unites with it1 displacing ,nitric ,acid1 and forming again ,chloride of ,silver1 which is decomposed in its turn4 ,the excess of ,nitrate of ,silver thus exerts an accelerating influence upon the darkening of ,chloride of ,silver1 by rendering the chain of chemical affinities more complete1 and preventing an accumulation of ,chlorine in the liquid1 which would be a check to the continuance of the action4 ,action of ,light upon organic ,salts of ,silver4--,on adding diluted ,albumen1 or white of egg1 to solution of ,nitrate of ,silver1 a flocculent deposit forms which is a compound of the animal matter with ,protoxide of ,silver1 and is known as ,albuminate of ,silver4 ,this substance is at first quite white1 but on exposure to light it turns to a brick-red colour4 ,the change which takes place is one of deoxidation1 the ,protoxide of ,silver losing a portion of its ,oxygen1 and a ,suboxide of ,silver1 the product of the reduction1 remaining in union with the oxidized ,albumen4 ,the red compound may therefore be loosely designated as an ,albuminate of ,suboxide of ,silver4 ,gelatine does not precipitate ,nitrate of ,silver in the same manner as ,albumen3 but if a sheet of transparent ,gelatine be allowed to imbibe a solution of the ,nitrate1 it becomes of a clear ruby-red tint on exposure to light1 and a true chemical compound of ,gelatine1 or a product of its oxidation1 with a low ,oxide of ,silver1 is produced4 ,caseine1 the animal principle of milk1 is coagulated by ,nitrate of ,silver1 and the red substance formed on exposing the curds to light may be viewed as analogous in composition to the corresponding compounds with ,albumen and ,gelatine4 ,many other organic salts of ,silver are darkened by light4 ,the white ,citrate of ,protoxide of ,silver changes to a red substance1 reacting with chemical tests in the same manner as ,whler's ,citrate of ,suboxide of ,silver1 which he obtained by reducing the ordinary ,citrate in ,hydrogen ,gas4 ,glycyrrhizin1 the ,sugar of ,liquorice1 also forms a white compound with ,oxide of ,silver which becomes brown or red in the sun's rays4#f #f ,for further particulars on the action of light upon the ,salts of ,silver associated with organic matter1 see the ,author's paper on the composition of the photographic image1 in the eighth ,chapter4 ,,simple ,,experiments ,,illustrating ,,the ,,action ,,of ,,light ,,upon ,a ,,sensitive ,,layer ,,of ,,chloride ,,of ,,silver ,,on ,,paper4 ,in the performance of the most simple experiments on the decomposition of ,silver ,salts by ,light1 the student may employ ordinary test-tubes1 in which small quantities of the two liquids required for the double decomposition may be mixed together4 ,when however concentrated solutions are used in this way1 the insoluble ,silver ,salt falls in dense and clotted masses1 which1 exposed to the sun's rays1 quickly blacken on the exterior1 but the inside is protected1 and remains white4 ,it is of importance therefore in ,photography that the sensitive material should exist in the form of a surface1 in order that the various particles of which it is composed may each one individually be brought into relation with the disturbing force4 ,full directions for the preparation of sensitive ,photographic paper are given in the second division of this work4 ,the following is the theory of the process3--,a sheet of paper is treated with solution of ,chloride of ,sodium or ,ammonium1 and subsequently with ,nitrate of ,silver2 hence results a formation of ,chloride of ,silver in a fine state of division1 with an excess of ,nitrate of ,silver1 the ,silver bath having been purposely made stronger in proportion than the salting solution4 ,illustrative ,experiment ,no4 ,i4--,place a square of sensitive paper 7prepared according to the directions given in the ,second ,part of the work7 in the direct rays of the sun1 and observe the gradual process of darkening which takes place2 the surface passes through a variety of changes in colour until it becomes of a deep chocolate-brown4 ,if the ,light is tolerably intense1 the brown shades are probably reached in from three to five minutes2 but the sensibility of the paper1 and also the nature of the tints1 will vary much with the character of the organic matter present4 ,experiment ,no4 ,,ii4--,lay a device cut from black paper upon a sheet of sensitive paper1 and compress the two together by means of a sheet of glass4 ,after a proper length of exposure the figure will be exactly copied1 the tint however being reversed3 the black paper protecting the sensitive ,chloride beneath1 produces a white figure upon a dark ground4 ,experiment ,no4 ,,iii4--,repeat the last experiment1 substituting a piece of lace or gauze-wire for the paper device4 ,this is intended to show the minuteness with which objects can be copied1 since the smallest filament will be distinctly represented4 ,experiment ,no4 ,,iv4--,take an engraving in which the contrast of light and shade is tolerably well marked1 and having laid it closely in contact with the sensitive paper1 expose as before4 ,this experiment shows that the surface darkens in degrees proportionate to the intensity of the light1 so that the half shadows of the engraving are accurately maintained1 and a pleasing gradation of tone produced4 ,in the darkening of ,photographic papers1 the action of the light is quite superficial1 and although the black colour may be intense1 yet the amount of reduced ,silver which forms it is so small that it cannot conveniently be estimated by chemical reagents4 ,this is well shown by the results of an analysis performed by the ,author1 in which the total weight of ,silver obtained from a blackened sheet measuring nearly #bd by #ah inches amounted to less than half a grain4 ,it becomes therefore of great importance in preparing sensitive paper to attend to the condition of the surface layer of particles1 the action rarely extending to those beneath4 ,the use of ,albumen1 ,gelatine1 etc41 which will be explained in the eighth ,chapter1 has reference to this amongst other advantages1 and secures a better and more sharply defined print4 ,,chapter ,,iii4 ,,on ,,the ,,development ,,of ,,an ,,invisible ,,image ,,by ,,means ,,of ,a ,,reducing ,,agent4 ,it has been shown in the previous ,chapter that the majority of the ,salts of ,silver1 both organic and inorganic1 are darkened in colour on exposure to light1 and1 by the loss of ,oxygen1 ,chlorine1 etc41 become reduced to the condition of ,subsalts4 ,many of the same compounds are also susceptible of a change under the influence of light1 which is even more remarkable4 ,this change takes place after a comparatively short exposure1 and as it does not affect the appearance of the sensitive layer1 for some time it escaped notice3 but it was afterwards discovered that an impression1 before invisible1 might be brought out by treating the plate with certain chemical agents which are without effect on the original unchanged salt1 but quickly blacken it after exposure4 ,it is a remarkable fact that the ,silver compounds most readily affected by light alone1 are not the most sensitive to the reception of the invisible image4 ,thus1 of ,photographic papers prepared with ,chloride1 ,bromide1 or ,iodide of ,silver1 the former assume the deepest shade of colour under the influence of the sun's rays1 but if all be exposed momentarily1 and then removed1 the greatest amount of effect will be developed upon the ,iodide paper4 ,iodide of ,silver therefore is the salt commonly used when sensibility is an object1 but it should be noted that images nearly or quite latent can be impressed upon many other of the compounds of ,silver1 including those belonging to the animal and vegetable kingdoms4 ,experiments illustrating the ,formation of an ,invisible ,image4--,take a sheet of sensitive paper1 prepared with ,iodide of ,silver by the method given in the fourth ,chapter of ,part ,,ii41 and having divided it into two parts1 expose one of them to the luminous rays for a few seconds4 ,no visible decomposition takes place1 but on removing the pieces to a room dimly illuminated1 and brushing with a solution of ,gallic ,acid1 a manifest difference will be observed2 the one being unaffected1 whilst the other darkens gradually until it becomes black4 ,experiment ,,ii4--,a prepared sheet is shielded in certain parts by an opaque substance1 and then after the requisite exposure1 which is easily ascertained by a few trials1 treated with the ,gallic ,acid as before2 in this case the protected part remains white1 whilst the other darkens to a greater or less extent4 ,in the same way1 copies of leaves1 engravings1 etc4 may be made1 very correct in the shading and much resembling those produced by the prolonged action of light alone upon the ,chloride of ,silver4 ,the object of employing a substance like ,gallic ,acid to develope or bring out to view an invisible image1 in preference to forming the picture by the direct action of light1 unassisted by a developer1 is the economy of time thereby effected4 ,this is well shown in the results of some experiments conducted by ,m4 ,claudet in the ,daguerreotype process3 he found that with a sensitive layer of ,bromo-,iodide of ,silver1 an intensity of light three thousand times greater was required if the use of a developer was omitted1 and the exposure continued until the picture became visible upon the plate4 ,to increase the sensitiveness of ,photographic preparations is a point of great consequence2 and indeed1 when the ,camera is used1 from the low intensity of the luminous image formed in that instrument1 no other plan than the one above described would be practicable4 ,hence the advancement1 and indeed the very origin1 of the ,photographic ,art1 may be dated from the first discovery of a process for bringing out to view an invisible image by means of a reducing agent4 ,the present ,chapter is divided into three ,sections3--first1 the chemical properties of the substances usually employed as developers2--second1 their mode of action in reducing the ,salts of ,silver2--third1 hypotheses on the action of light in impressing a latent image4 ,,section ,i4 ,chemistry of the various ,substances employed as ,developers4 ,development is essentially a process of reduction1 or1 in other words1 of deoxidation4 ,if we take a certain metal1 we can1 by means of ,nitric ,acid1 impart ,oxygen to it1 so that it becomes first an ,oxide1 and afterwards1 by solution of the ,oxide in the excess of acid1 a salt4 ,when this salt is formed1 by a series of chemical operations the reverse of the former it may be deprived of all its ,oxygen1 and the metallic element again isolated4 ,the degree of facility with which oxidation as well as reduction is performed1 depends upon the affinity for ,oxygen which the particular metal under treatment possesses4 ,in this respect there is considerable difference1 as may be shown by a reference to the two well-known metals1 ,iron and ,gold4 ,how speedily does the first become tarnished and covered with rust1 whilst the other remains bright even in the fire6 ,it is indeed possible1 by a careful process1 to form ,oxide of ,gold2 but it retains its ,oxygen so loosely that the mere application of heat is sufficient to drive it off1 and leave the metal in a pure state4 ,silver1 ,gold1 and ,platinum all belong to the class of noble metals1 having the least affinity for ,oxygen3 hence their ,oxides are unstable1 and any body tending strongly to absorb ,oxygen will reduce them to the metallic state4 ,observe1 therefore1 that the substances employed by the ,photographer to assist the action of the light1 and to develope the picture1 act by removing ,oxygen4 ,the sensitive ,salt of ,silver is thus reduced1 more or less completely1 in the parts touched by light1 and an opaque deposit results which forms the image4#g #g ,these remarks do not apply to the vapour of ,mercury employed as a developing agent in the ,daguerreotype4 ,the chemistry of that process will be explained in a separate ,chapter4 ,the most important of the developers are as follows3-- ,gallic ,acid1 ,pyrogallic ,acid1 and the ,protosalts of ,iron4 ,,chemistry ,,of ,,gallic ,,and ,,pyrogallic ,,acids4 a4 ,of ,gallic ,acid4--,gallic ,acid is obtained from ,gall ,nuts1 which are peculiar excrescences formed upon the branches and shoots of the ,quercus infectoria by the puncture of a species of insect4 ,the best kind is imported from ,turkey1 and sold in commerce as ,aleppo ,galls4 ,gall ,nuts do not contain ,gallic ,acid ready formed1 but an analogous chemical principle termed ,tannic ,acid1 well known for its astringent properties and employment in the process of tanning raw hides4 ,gallic ,acid is produced by the decomposition and oxidation of ,tannic ,acid when powdered galls are exposed for a long time in a moist state to the action of the air4 ,by boiling the mass with water and filtering whilst hot1 the acid is extracted1 and crystallizes on cooling1 on account of its sparing solubility in cold water4 ,gallic ,acid occurs in the form of long silky needles1 soluble in #ajj parts of cold and #c of boiling water2 they are also readily soluble in ,alcohol1 but sparingly in ,ether4 ,the aqueous solution becomes mouldy on keeping1 to obviate which1 the addition of ,acetic ,acid or a drop or two of ,oil of ,cloves is recommended4 ,gallic ,acid is a feeble acid1 scarcely reddening litmus2 it forms salts with the alkaline and earthy bases1 such as ,potash1 ,lime1 etc41 but not with the oxides of the noble metals4 ,when added to ,oxide of ,silver the metallic element is separated and the ,oxygen absorbed4 b4 ,pyrogallic ,acid4--,the term pyro prefixed to ,gallic ,acid implies that the new substance is obtained by the action of heat upon that body4 ,at a temperature of about #daj ,fahr41 ,gallic ,acid is decomposed1 and a white sublimate forms1 which condenses in lamellar crystals2 this is ,pyrogallic ,acid4 ,pyrogallic ,acid is very soluble in cold water1 and in ,alcohol and ,ether2 the solution decomposes and becomes brown by exposure to the air4 ,it gives an indigo blue colour with ,protosulphate of ,iron1 which changes to dark green if any ,persulphate be present4 ,although termed an acid1 this substance is strictly neutral2 it does not redden litmus-paper1 and forms no salts4 ,the addition of ,potash or ,soda decomposes ,pyrogallic ,acid1 at the same time increasing the attraction for ,oxygen2 hence this mixture may conveniently be employed for absorbing the ,oxygen contained in atmospheric air4 ,the compounds of ,silver and ,gold are reduced by ,pyrogallic ,acid even more rapidly than by ,gallic ,acid1 the reducing agent absorbing the ,oxygen1 and becoming converted into ,carbonic ,acid and a brown matter insoluble in water4 ,commercial ,pyrogallic ,acid is often contaminated with empyreumatic oil1 and also with a black insoluble substance known as ,metagallic ,acid1 which is formed when the heat is raised above the proper temperature in the process of manufacture4 ,,chemistry ,,of ,,the ,,protosalts ,,of ,,iron4 ,the combinations of ,iron with ,oxygen are somewhat numerous4 ,there are two distinct ,oxides which form ,salts1 viz4 the ,protoxide of ,iron1 containing an atom of ,oxygen to one of metal2 and the ,peroxide1 with an atom and a half of ,oxygen to one of metal4 ,as half atoms however are not allowed in chemical language1 it is usual to say that the ,peroxide of ,iron contains three equivalents of ,oxygen to two of metallic ,iron4 ,expressed in symbols1 the composition is as follows3-- ,protoxide of ,iron1 ,fe ,o4 ,peroxide of ,iron1 ,fe#b,o#c4 ,the ,proto- and ,persalts of ,iron do not resemble each other in their physical and chemical properties4 ,the former are usually of an apple-green colour1 and the aqueous solutions almost colourless1 if not highly concentrated4 ,the latter1 on the other hand1 are dark1 and give a yellow or even blood-red solution4 ,the ,protosalts of ,iron are alone useful in ,photography2 but the following experiment will serve to illustrate the properties of both classes of salts3--,take a crystal of ,protosulphate of ,iron1 and1 having reduced it to powder1 pour a little ,nitric ,acid upon it in a test-tube4 ,on the application of heat1 abundance of fumes will be given off1 and a red solution obtained4 ,the ,nitric ,acid in this reaction imparts ,oxygen1 and converts the ,protosulphate entirely into a ,persulphate of ,iron4 ,it is this feature1 viz4 the tendency to absorb ,oxygen1 and to pass into the state of ,persalts1 which makes the ,protosalts of ,iron useful as developers4 ,there are two ,protosalts of ,iron commonly employed by ,photographers3 the ,protosulphate and the ,protonitrate of ,iron4 a4 ,protosulphate of ,iron4--,this salt1 often termed ,copperas or ,green ,vitriol1 is an abundant substance1 and used for a variety of purposes in the arts4 ,commercial ,sulphate of ,iron however1 being prepared on a large scale1 requires re-crystallization to render it sufficiently pure for ,photographic purposes4 ,pure ,sulphate of ,iron occurs in the form of large transparent1 prismatic crystals1 of a delicate green colour3 by exposure to the air they gradually absorb ,oxygen and become rusty on the surface4 ,solution of ,sulphate of ,iron1 colourless at first1 afterwards changes to a red tint1 and deposits a brown powder2 this powder is a basic ,persulphate of ,iron1 that is1 a ,persulphate containing an excess of the oxide or base4 ,by the addition of ,sulphuric or ,acetic ,acid to the solution1 the formation of a deposit is prevented1 the brown powder being soluble in acid liquids4 ,the ,crystals of ,sulphate of ,iron include a large quantity of water of crystallization1 a part of which they lose by exposure to dry air4 ,by a higher temperature1 the salt may be rendered perfectly anhydrous1 in which state it forms a white powder4 b4 ,protonitrate of ,iron4--,this salt is prepared by double decomposition between ,nitrate of ,baryta or of ,lead and ,protosulphate of ,iron4 ,it is an unstable substance and crystallizes with great difficulty2 its aqueous solution is pale green at first1 but very prone to decomposition1 even more so than the corresponding ,sulphate of ,iron4 ,,section ,,ii4 ,the ,reduction of ,salts of ,silver by ,developing ,agents4 ,the general theory of the reduction of metallic oxides having been explained1 it may be desirable to enter more minutely into the exact nature of the process as applied to the compounds of ,silver4 ,first1 the ,reduction of the ,oxide of ,silver will be taken1 as the most simple illustration2 then that of ,salts of ,silver formed by ,oxygen-acids2 and lastly1 of the ,chloride1 ,iodide1 and ,bromide of ,silver containing no ,oxygen4 ,reduction of ,oxide of ,silver4--,to illustrate this conveniently1 the ,oxide of ,silver should be in a state of solution2 water dissolves ,oxide of ,silver very sparingly1 but it is freely soluble in ,ammonia1 forming the liquid known as ,ammonio-,nitrate of ,silver4 ,if1 therefore1 a little of the ,ammonio-,nitrate of ,silver be placed in a test-tube1 and solution of ,sulphate of ,iron be added to it1 immediately it becomes discoloured1 and a deposit settles to the bottom4 ,this deposit is metallic ,silver1 produced by the reducing agent appropriating to itself the ,oxygen previously combined with the metal4 ,as metallic ,silver does not dissolve in ,ammonia1 the liquid becomes turbid1 and the metal subsides in the form of a bulky precipitate4 ,reduction of the ,oxyacid ,salts of ,silver4--,the term ,oxyacid includes those salts which contain the ,oxide of ,silver intimately combined with ,oxygen-acids2 as e4 g4 the ,nitrate of ,silver1 the ,sulphate1 the ,acetate of ,silver1 etc4 ,these salts1 soluble in water1 are reduced by developing agents in the same manner as ,oxide of ,silver1 but more slowly4 ,the presence of an acid united with the base is a hindrance to the process and tends to keep the oxide in solution1 especially when that acid is powerful in its affinities4 ,to illustrate the effect of the acid constituent of the salt in retarding reduction1 take two test-tubes1 the one containing ,ammonio-,nitrate1 and the other ordinary ,nitrate of ,silver--a single drop of solution of ,sulphate of ,iron added to each will indicate an evident difference in the rapidity of deposition4 ,the precipitate of metallic ,silver obtained by the action of reducing agents upon the ,nitrate1 varies much in colour and in general appearance4 ,if ,gallic or ,pyrogallic ,acid be employed1 it is a black powder2#h whilst the salts of ,iron1 and especially the same with free ,nitric ,acid added1 produce a sparkling precipitate1 resembling what is termed frosted silver4 ,grape ,sugar and many of the essential oils1 such as the ,oil of ,cloves1 etc41 separate the metal from ,ammonio-,nitrate of ,silver in the form of a brilliant mirror film1 and are often employed in silvering glass4 #h ,silver precipitated by ,gallic or ,pyrogallic ,acid does not appear to be free from organic matter1 and probably contains also a small proportion of ,oxygen4 ,in remarking upon these peculiarities in the molecular condition of precipitated ,silver1 it should be observed that the appearance of a metal whilst in mass is no indication of its colour when in the state of fine powder4 ,platinum and ,iron1 both bright metals1 and susceptible of a high polish1 are dull and intensely black when in a fine state of division2 ,gold is of a purple or yellowish brown2 ,mercury a dirty grey4 ,reduction of the ,hydracid ,salts of ,silver4--,by the term ,hydracid is meant ,salts of ,silver which contain no ,oxygen or ,oxygen-acids1 but simply elements like ,chlorine or ,iodine combined with ,silver4 ,these elements are characterized by forming acids with ,hydrogen1 which acids are hence called ,hydracids4 ,hydrochloric ,acid 7,h,cl7 is an example2 so also is ,hydriodic ,acid 7,,hi74 ,the reduction of the ,hydracid ,salts requires to be discussed separately1 because it is evidently different from that already described2 the reducing agent tending only to absorb ,oxygen1 which is not present in these salts4 ,the explanation is as follows3 ,when a ,chloride of a noble metal is reduced by a developer1 an atom of water1 composed of ,oxygen and ,hydrogen1 takes a part in the reaction4 ,the ,oxygen of the water passes to the developer1 the ,hydrogen to the ,chlorine4 ,to illustrate this1 take a solution of ,chloride of ,gold1 and add to it a little ,sulphate of ,iron4 ,a yellow deposit of metallic ,gold soon forms1 and the supernatant liquid is found1 by testing1 to be acid from free ,hydrochloric ,acid4 ,the following simple diagram1 in which however the number of the atoms concerned is omitted1 may assist the comprehension of the change4 ,illustration3 ,compound ,atom of ,compound ,atom ,atom of ,chloride of ,gold4 of ,water4 ,sulphate of ,iron4 ,the symbol ,au represents ,gold1 ,cl ,chlorine1 ,h ,hydrogen1 and ,o ,oxygen4 ,observe that the molecules ,h and ,o separate from each other and pass in opposite directions3 the latter unites with the ,sulphate of ,iron2 the former meets ,cl1 and produces ,hydrochloric ,acid 7,h,cl71 whilst the atom of ,gold is left alone4 ,hence there is no theoretical difficulty in supposing a reduction of ,iodide of ,silver by a developer1 if we associate with the ,iodide an atom of water to furnish the ,oxygen4 ,unless the sensitive plate however has been exposed to the light1 the reduction does not readily take place2 nor can it be produced under any circumstances1 with or without light1 when the whole of the free ,nitrate of ,silver has been washed away from the plate4 ,pure ,iodide of ,silver is therefore unaffected by a developer1 and the compound which blackens on the application of ,sulphate of ,iron or ,pyrogallic acid is an ,iodide with excess of ,nitrate of ,silver4 ,illustration3 ,compound ,atom of ,compound ,atom of ,atom of ,iodide of ,silver4 ,nitrate of ,silver4 ,sulphate of ,iron4 ,the mode in which a ,salt of ,silver1 such as the ,nitrate1 soluble in water1 may act in facilitating the reduction of ,iodide of ,silver1 is shown in the preceding diagram1 which corresponds closely with the last4 ,notice that the compound atom of ,nitrate of ,silver contains a molecule of ,oxygen for the developer1 one of ,silver 7,ag7 for the separated ,iodine1 and an atom of ,nitric ,acid 7,,no#e71 which is liberated1 and takes no further part in the change4 ,the chain of chemical affinities is more complete in this diagram than in the last1 where an atom of water only was present1 the affinity of ,iodine for ,silver being greater than that of ,iodine for ,hydrogen4 ,hence it is possible that an excess of ,nitrate of ,silver may1 by furnishing an elementary basis for which ,iodine has an attraction1 assist in drawing off that element1 so to speak1 from the original particle of ,iodide of ,silver touched by light4#i #i ,the reader must not suppose from the remarks which have been made in this ,section that images obtained by development consist invariably of pure metallic ,silver4 ,it can be shown that such is not the case1--that the process of reduction is in many cases suspended when a part only of the ,oxygen has been removed2 and hence results a subsalt similar to that produced by the direct action of light upon organic compounds of ,silver1 and differing in properties from metallic ,silver4 ,for further particulars see the ,author's ,photographic researches in the eighth ,chapter4 ,,section ,,iii4 ,the formation and development of the ,latent ,image4 ,it was shown in the second ,chapter that the continued action of white light upon certain of the ,salts of ,silver resulted in the separation of elements like ,chlorine and ,oxygen and the partial reduction of the compound4 ,we have also seen that bodies possessing affinity for ,oxygen1 such as ,sulphate of ,iron and ,pyrogallic ,acid1 tend to produce a similar effect2 acting in some cases with great energy and precipitating metallic ,silver in a pure state4 ,in forming an extemporaneous theory on the production of the latent image in the ,camera1 it would therefore be natural to suppose that the process consisted in setting up a reducing action upon the sensitive surface by means of light1 afterwards to be continued by the application of the developing solution4 ,this idea is to a certain extent correct1 but it requires some explanation4 ,the effects produced by the light and the developer are not so precisely similar that the one agency can always be substituted for the other3 an insufficient exposure in the ,camera cannot be remedied by prolonging the development of the image4 ,in the ,photographic processes on paper it is indeed found that a certain latitude may be allowed2 but1 as a rule1 it should be stated that a definite time is occupied in the formation of the invisible image1 which may not be shortened or extended beyond its proper limits with impunity4 ,there is a maximum point beyond which no advance is made2 hence if the plate be not then removed from the ,camera1 those portions of the image formed by the brightest lights are speedily overtaken by the half tones1 so that1 on developing1 an image appears without that contrast between lights and shadows which is essential to the artistic effect4 ,on the other hand1 in a case of insufficient exposure1 the feeble rays of light not having been allowed time to impress the plate1 the half shadows cannot be brought out on subsequent treatment with the developing agent4 ,a careful study of the phenomena involved in this part of the process cannot fail to show that the ray of ,light determines a molecular change of some kind in the particles of ,iodide of ,silver forming the sensitive surface4 ,this change is not of a nature to alter the composition or the chemical properties of the salt4 ,the ,iodine does not leave the surface1 or there would be a difference in the appearance of the film1 or in its solubility in ,hyposulphite of ,soda4 ,the following diagrams may perhaps be useful in mechanically illustrating what is meant by a molecular change4 ,fig4 #a represents a compound molecule of ,iodide of ,silver1 the component atoms of which are closely associated4 ,fig4 #b4 ,the same after the action of a disturbing force4 ,the simple molecules have not altogether separated1 but they are prepared to do so1 touching only at a single point4 ,illustration3 ,fig4 #a4 ,illustration3 ,fig4 #b4 ,now the effect produced on this combination by a developer is understood1 if we suppose that in the first case the affinity of the ,iodine for ,silver is too great to allow of its separation2 but in the second1 this affinity having been loosened1 the structure gives way1 and metallic ,silver is the result4 ,this hypothesis has the merit of simplicity1 and is not opposed to known facts2 it may therefore for the present be received4 ,the point however on which a doubt must rest is--whether the molecular disturbance produced by light upon ,iodide of ,silver leads to a reduction of that ,salt by the developer4 ,no image can be produced on the application of ,pyrogallic ,acid unless the particles of ,iodide are in contact with ,nitrate of ,silver2 and hence it may be the ,nitrate and not the ,iodide which is reduced--that is1 the impressed molecule of ,iodide may determine the decomposition of a contiguous particle of ,nitrate1 itself remaining unchanged4 ,this view is supported to some extent by ,moser's experiments1 shortly to be quoted2 and also by the fact that the delicate image first formed can be intensified by treating it with a mixture of the developing solution and ,nitrate of ,silver1 even after the ,iodide has been removed by a fixing agent4 ,the following experiment will serve to illustrate this4-- ,take a sensitive ,collodion plate1 and having impressed an invisible image upon it by a proper exposure in the ,camera1 remove it to the dark room1 and pour over it the solution of ,pyrogallic ,acid4 ,when the picture has fully appeared1 stop the action by washing the plate with water1 and remove the unaltered ,iodide of ,silver by ,cyanide of ,potassium4 ,an examination of the image at this stage will show that it is perfect in the details1 but pale and translucent4 ,the plate is then to be taken back again to the dark room and treated with fresh ,pyrogallic ,acid1 to which ,nitrate of ,silver has been added2 immediately the picture becomes much blacker1 and continues to darken1 even to complete opacity1 if the supply of ,nitrate be kept up4 ,now in this experiment it is evident that the additional deposit upon the image is produced from the ,nitrate of ,silver1 the whole of the ,iodide having been previously removed4 ,observe also1 that it forms only upon the image1 and not upon the transparent parts of the plate4 ,even if the ,iodide1 untouched by light1 be allowed to remain1 the same rule holds good2--the ,pyrogallic ,acid and ,nitrate of ,silver react upon each other and produce a metallic deposit2 this deposit however has no affinity for the unaltered ,iodide upon the part of the plate corresponding to the shadows of the picture1 but attaches itself in preference to the ,iodide already blackened by light4 ,this second stage of the development1 by which a feeble image may be strengthened and rendered more opaque1 is sometimes termed development by precipitation1 and should be correctly understood by the practical operator4 ,researches of ,m4 ,moser4--,the papers of ,m4 ,ludwig ,moser ',on the ,formation and ,development of ,invisible ,images1' published in #ahdb1 explain so clearly many remarkable phenomena of occasional occurrence in the ,collodion and paper processes1 that no apology need be offered for referring to them somewhat at length4 ,his first proposition may be stated thus3--,if a polished surface has been touched in particular parts by anybody1 it acquires the property of precipitating certain vapours on these spots differently to what it does on the other untouched parts4 ,to illustrate this1 take a thin plate of metal1 having characters excised2 warm it gently1 and lay it upon the surface of a clean mirror glass for a few minutes3 then remove1 allow to cool1 and breathe upon the glass1 when the outlines of the device will be distinctly seen4 ,a plate of polished ,silver may be substituted for the glass1 and in place of developing the image by the breath1 it may be brought out by ,mercurial vapour4 ,the second proposition of ,m4 ,moser is as follows3--,light acts on bodies1 and its influence may be tested by vapours that adhere to the substance4--,a plate of mirror glass is exposed in the ,camera to a bright and intense light2 it is then removed and breathed upon1 when an image before invisible will be developed1 the breath settling most strongly upon the parts where the light has acted4 ,a plate of polished ,silver may be used as before instead of glass1 the vapour of ,mercury or of water being employed to develope the image4 ,an iodized ,silver plate is still more sensitive to the influence of the light1 and receives a very sharp and perfect impression under the action of the ,mercury4 ,it seems therefore from these experiments and others not quoted1 that the surfaces of various bodies are capable of being modified by contact with each other1 or by contact with a ray of light1 in such a way as to impart an affinity for a vapour2 and further1 that many of the ,salts of ,silver are in the list of substances admitting of such modification4 ,but it is also evident that the same condition of surface which causes a vapour to settle in a peculiar manner also affects the behaviour of the ,silver ,salt when treated with a reducing agent4 ,thus1 if a clean glass plate be touched in certain spots by the warm finger1 the impression soon disappears1 but is again seen on breathing upon the glass2 and if this same plate be coated with a very delicate layer of ,iodized ,collodion and passed through the ,nitrate bath1 the solution of ,pyrogallic acid will commonly produce a well-defined outline of the figure even before the plate has been exposed to the light4 ,this experiment1 although it does not invariably succeed1 is nevertheless an instructive one1 and shows the necessity of cleaning the plates used in ,photography with care4 ,if there be any irregularity in the manner in which the breath settles upon the glass when it is breathed on1 a condition of surface exists at that point which will probably so modify the layer of ,iodide of ,silver1 that the action of the developing fluid will be in some way interfered with4 ,one more remarkable fact observed by ,m4 ,moser may be quoted4 ,he finds that the action of light upon the ,daguerreotype plate is of an alternating kind3 it first gives an affinity for ,mercury1 and then removes it4 ,if light acts on ,iodide of ,silver1 he says1 it imparts to it the power of condensing mercurial vapours2 but if it acts beyond a certain time1 it then diminishes this power and at length takes it away altogether4 ,this is precisely in accordance with phenomena observed also in the ,collodion process1 where the deposit of metallic ,silver is sometime less marked than usual if the plate has been exposed in the ,camera beyond the proper period of time4 ,a curious perversion of the developing process is occasionally met with1 in which on the application of the ,pyrogallic ,acid1 the deposit of ,silver takes place upon the shadows of the picture1 and not upon the lights2 hence on viewing the image by transmitted light1 the usual appearance is reversed4 ,this may perhaps be explained by an alternating action of the light as above suggested4 ,a phenomenon at first sight even more remarkable has occurred1 in which1 on developing the plate1 two images start out instead of one4 ,the secondary image in such a case is probably the remains of a previous impression which1 although apparently removed by washing1 had nevertheless modified the surface of the glass so as to affect the layer of ,iodide of ,silver2 and if the glass were breathed upon before again coating it with ,collodion1 there is every reason to suppose that the outlines of the accidental image would be seen4#aj #aj ,since writing the above1 the ,author has perused with pleasure a paper by ,mr4 ,grove on the production of latent images by electricity1 with a mode of fixing them4 ,in the experiments described1 a plate of glass1 electrized in certain portions only1 was breathed upon1 or exposed to the fumes of ,hydrofluoric ,acid4 ,in either case the vapour settled exclusively upon the non-electrical part of the glass1 thus developing a latent image4 ,when the plate was first submitted to electrization1 and then coated with ,iodide of ,silver upon ,collodion1 and exposed to light1--solution of ,pyrogallic ,acid produced a reduction of ,silver only upon the parts of the glass corresponding to those on which the breath settled in the previous experiment2 thus indicating that the electricity neutralized the effect of light upon the sensitive ,iodide of ,silver4 ,,chapter ,,iv4 ,,on ,,fixing ,,the ,,photographic ,,image4 ,a sensitive layer of ,chloride or ,iodide of ,silver on which an image has been formed1 either with or without the aid of a developing agent1 must pass through further treatment in order to render it indestructible by diffused light4 ,it is true that the image itself is sufficiently permanent1 and cannot be said1 in correct language1 to need fixing2 but the unchanged ,silver ,salt which surrounds it1 being still sensitive to light1 tends to be decomposed in its turn1 and so the picture is lost4 ,it is therefore necessary to remove this salt by applying some chemical agent capable of dissolving it4 ,the list of solvents of ,chloride and ,iodide of ,silver has been given in ,chapter ,,ii41 but some are better adapted for fixing than others4 ,in order that any body may be employed with success as a fixing agent1 it is required not only that it should dissolve unchanged ,chloride or ,iodide of ,silver1 but that it should produce no injurious effect upon the same salts reduced by light4 ,this solvent action upon the image1 as well as upon the parts which surround it1 is most liable to happen when the agency of light alone1 without a developer1 has been employed4 ,in that case the darkened surface1 not being reduced perfectly to the metallic state1 remains soluble to a certain extent in the fixing liquid4 ,,chemistry ,,of ,,the ,,various ,,fixing ,,agents4 ,the following will be mentioned3--,ammonia--,alkaline ,chlorides--,alkaline ,iodides--,alkaline ,hyposulphite--,alkaline ,cyanides4 ,,ammonia4 ,the properties of the alkaline liquid ,ammonia are given in ,part ,,iii4 ,ammonia dissolves ,chloride of ,silver readily1 but not ,iodide of ,silver3 hence its use is necessarily confined to the paper proofs upon ,chloride of ,silver4 ,even these however cannot advantageously be fixed in ,ammonia unless a deposit of ,gold has been previously produced upon the surface by a process of toning1 presently to be explained3 a peculiar and unpleasant red tint is always caused by ,ammonia acting upon the darkened material of a sun picture as it comes from the printing-frame3 but this is obviated by the employment of the ,gold4 ,,alkaline ,,chlorides1 ,,iodides1 ,,and ,,bromides4 ,the ,chlorides of ,potassium1 ,ammonium1 and ,sodium possess the property of dissolving a small portion of ,chloride of ,silver4 ,in the act of solution a double salt is formed2 that is1 a compound of ,chloride of ,sodium with ,chloride of ,silver1 which may be crystallized out by allowing the liquid to evaporate spontaneously4 ,the earlier ,photographers employed a saturated solution of common ,salt for fixing paper prints2 but the fixing action of the ,alkaline ,chlorides is slow and imperfect1 and their use may now be said to be obsolete4 ,the ,iodide and ,bromide of ,potassium have both been used as fixing agents4 ,they dissolve ,iodide of ,silver1 forming with it a double salt in the manner before described4 ,it is important to remark in the solution of the insoluble ,silver ,salts by ,alkaline ,chlorides1 ,iodides1 etc41 that the amount dissolved is not in proportion to the quantity of the solvent1 but to the degree of concentration of its aqueous solution4 ,this is not usual with solvents which act by entering into chemical combination with the substance dissolved4 ,commonly a given weight of the one salt dissolves a given weight of the other1 independent of the amount of water present4 ,the peculiarity in the case before us depends upon the fact that the double salt formed is decomposed by a large quantity of water4 ,hence it is a saturated solution of ,chloride of ,sodium which possesses the greatest power of fixing paper prints2 and with the ,bromide or ,iodide of ,potassium the same rule holds good--the stronger the solution the more ,iodide of ,silver will be taken up4 ,the addition of water produces milkiness and a deposit of the silver ,salt previously dissolved4 ,,alkaline ,,hyposulphites4 ,hyposulphurous ,acid is one of the ,oxides of ,sulphur4 ,it is1 as its name implies1 of an acid nature1 and takes its place upon the list immediately below ,sulphurous ,acid 71 under74 ,the ,hyposulphite of ,soda commonly employed by ,photographers is a neutral combination of ,hyposulphurous ,acid and the alkali ,soda4 ,it is selected as being more economical in preparation than any other ,hyposulphite adapted for fixing4 ,hyposulphite of ,soda occurs in the form of large translucent groups of crystals1 which include five atoms of water4 ,these crystals are soluble in water almost to any extent1 the solution being attended with the production of cold2 they have a nauseous and bitter taste4 ,in the solution of ,silver compounds by ,hyposulphite of ,soda a double decomposition always takes place2 thus3-- ,hyposulphite of ,soda ,chloride of ,silver ,hyposulphite of ,silver ,chloride of ,sodium4 ,the ,hyposulphite of ,silver with an excess of ,hyposulphite of ,soda forms a soluble double salt1 which may be crystallized out by evaporating the solution4 ,it possesses an intensely sweet taste1 and contains one atom of ,hyposulphite of ,silver1 chemically combined with two of ,hyposulphite of ,soda4 ,in addition to this there is a second double ,salt1 differing from the first in being very sparingly soluble in water4 ,it is formed by acting upon ,chloride of ,silver with a solution of ,hyposulphite of ,soda already saturated1 or nearly so1 with ,silver ,salts2 and contains single atoms of each constituent4 ,the fact that the ,silver contained in an ordinary fixing ,bath is present in the state of ,hyposulphite must be borne in mind1 because this salt is liable to undergo peculiar chemical changes1 as will be better shown in ,chapter ,,viii4 ,iodide of ,silver is dissolved by ,hyposulphite of ,soda more slowly than ,chloride of ,silver1 and the amount eventually taken up is less4 ,this is explained as follows3-- ,during the solution of ,iodide of ,silver1 ,iodide of ,sodium is formed1 and this alkaline ,iodide has a prejudicial effect upon the continuance of the process4 ,chloride of ,sodium has not the same action1 neither has ,bromide of ,sodium1 consequently the corresponding ,silver ,salts dissolve to a greater extent than the ,iodide4 ,,alkaline ,,cyanides4 ,the chemistry of ,cyanogen is sketched in ,part ,,iii4 ,the ,cyanide of ,potassium is the salt most frequently employed in fixing4 ,it occurs in commerce in the form of fused lumps of considerable size4 ,in this state it is usually contaminated with a large percentage of ,carbonate of ,potash1 amounting in some cases to more than half its weight4 ,by boiling in proof ,spirit the ,cyanide may be extracted and crystallized1 but this operation is scarcely required as far as its use in ,photography is concerned4 ,cyanide of ,potassium absorbs moisture on exposure to the air4 ,it is very soluble in water1 but the solution decomposes on keeping2 changing in colour and evolving the odour of ,prussic ,acid1 which is a ,cyanide of ,hydrogen4 ,cyanide of ,potassium is highly poisonous1 and must be used with caution4 ,solution of ,cyanide of ,potassium is a most energetic agent in dissolving the insoluble ,silver ,salts3 far more so1 in proportion to the quantity used1 than the ,hyposulphite of ,soda4 ,the ,salts are in all cases converted into ,cyanides1 and exist in the solution in the form of soluble double ,salts1 which1 unlike the double ,iodides1 are not affected by dilution with water4 ,cyanide of ,potassium is unadapted for fixing positive proofs upon ,chloride of ,silver2 and even when a developer has been used1 unless the solution is tolerably dilute1 it is apt to attack the image and dissolve it4 ,,chapter ,v4 ,,on ,,the ,,nature ,,and ,,properties ,,of ,,light4 ,the present ,chapter is devoted to a discussion of the more remarkable properties of ,light2 the object being to select certain prominent points1 and to state them as clearly as possible1 referring1 for information of a more complete kind1 to acknowledged works on the subject of ,optics4 ,the ,chapter will be divided into five ,sections3--first1 the compound nature of ,light2 second1 the laws of refraction of ,light2 third1 the construction of ,lenses and of the ,camera2 fourth1 the ,photographic action of coloured ,light2 fifth1 on ,binocular ,vision and the ,stereoscope4 ,,section ,i4 ,the ,compound ,nature of ,light4 ,the ideas entertained on the subject of ,light1 before the time of ,sir ,isaac ,newton1 were vague and unsatisfactory4 ,it was shown by that eminent philosopher1 that a ray of sunlight was not homogeneous1 as had been supposed1 but consisted of several rays of vivid colours1 united and intermingled4 ,this fact may be demonstrated by throwing a pencil of ,sunlight upon one angle of a prism1 and receiving the oblong image1 so formed1 upon a white screen4 ,the space illuminated and coloured by a pencil of rays analyzed in this way is called the ,solar ,spectrum4 ,the action of a prism in decomposing white light will be more fully explained in the next ,section4 ,at present we notice only that seven principal colours may be distinguished in the ,solar ,spectrum1 viz4 red1 orange1 yellow1 green1 blue1 indigo1 and violet4 ,sir ,david ,brewster has made observations which lead him to suppose that the primary colours are in reality but three in number1 viz4 red1 yellow1 and blue1 and that the others are compound1 being produced by two or more of these overlapping each other2 thus the red and yellow spaces intermingled constitute orange2 the yellow and blue spaces1 green4 ,illustration ,the composition of white light from the seven prismatic colours may be roughly proved by painting them on the face of a wheel1 and causing it to rotate rapidly2 this blends them together1 and a sort of greyish-white is the result4 ,the white is imperfect1 because the colours employed cannot possibly be obtained of the proper tints or laid on in the exact proportions4 ,the decomposition of light is effected in other ways besides that already given3--- ,first1 by reflection form the surfaces of coloured bodies4 ,all substances throw off rays of light1 which impinge upon the retina of the eye and produce the phenomena of vision4 ,colour is caused by a portion only1 and not the whole1 of the elementary rays1 being projected in this way4 ,surfaces termed white reflect all the rays2 coloured surfaces absorb some and reflect others3 thus red substances reflect only red rays1 yellow substances1 yellow rays1 etc1 the ray which is reflected in all cases deciding the colour of the substance4 ,secondly1 light may be decomposed by transmission through media which are transparent to certain rays1 but opaque to others4 ,ordinary transparent glass allows all the rays constituting white light to pass2 but by the addition of certain metallic oxides to it whilst in a state of fusion1 its properties are modified1 and it becomes coloured4 ,glass stained by ,oxide of ,cobalt is permeable only to blue rays4 ,oxide of ,silver imparts a pure yellow tint2 ,oxide of ,gold or ,suboxide of ,copper a ruby red1 etc4 ,,division ,,of ,,the ,,elementary ,,rays ,,of ,,white ,,light ,,into ,,luminous1 ,,heat-,,producing1 ,,and ,,chemical4 ,,rays4 ,the agency of ,light produces a variety of distinct effects upon the bodies which surround us4 ,these may be classed together as the properties of light4 ,they are of three kinds--the phenomena of colour and vision1 of heat1 and of chemical action4 ,by resolving white light into its constituent rays1 we find that these properties are associated each one with certain of the elementary colours4 ,the yellow is decidedly the most luminous ray4 ,on examining the ,solar ,spectrum1 it is seen that the brightest part is that occupied by the yellow1 and that the light diminishes rapidly on either side4 ,so again1 rooms glazed with yellow glass always appear abundantly illuminated1 whilst the effect of red or blue glass is dark and sombre4 ,the yellow colour therefore constitutes that portion of white light by which surrounding objects are rendered visible2 it is essentially the visual ray4 ,the heating properties of the sunlight reside principally in the red ray1 as is shown by the expansion of a mercurial thermometer placed in that part of the spectrum4 ,the chemical action of light corresponds more to the indigo and violet rays1 and is wanting1 as regards its influence upon ,iodide of ,silver1 both in the red and yellow4 ,strictly speaking however it cannot be localized in either of the coloured spaces1 as will be more fully shown in the ,fourth ,section of this ,chapter1 to which the reader is referred4 ,,section ,,ii4 ,the ,refraction of ,light4 ,a ray of light1 in its passage through any transparent medium1 travels in a straight line as long as the density of the medium continues unchanged4 ,but if the density varies1 becoming either greater or less1 then the ray is refracted1 or bent out of the course which it originally pursued4 ,the degree to which the refraction or bending takes place depends upon the nature of the new medium1 and in particular upon its density as compared with that of the medium which the ray had previously traversed4 ,hence ,water refracts light more powerfully than ,air1 and ,glass more so than ,water4 ,the following diagram illustrates the refraction of a ray of light4 ,illustration ,the dotted line is drawn perpendicularly to the surface1 and it is seen that the ray of light on entering is bent towards this line4 ,on emerging1 on the other hand1 it is bent to an equal extent away from the perpendicular1 so that it proceeds in a course parallel to1 but not coincident with1 its original direction4 ,if we suppose the new medium1 in place of being more dense than the old1 to be less dense1 then the conditions are exactly reversed1--the ray is bent away from the perpendicular on entering1 and towards it on leaving4 ,it must be observed that the laws of refraction apply only to rays of light which fall upon the medium at an angle3 if they enter perpendicularly--in the direction of the dotted lines in the last figure--they pass straight through without suffering refraction4 ,notice also1 that it is at the surfaces of bodies that the deflecting power acts4 ,the ray is bent on entering1 and bent again on leaving2 but whilst within the medium it continues in a straight line4 ,hence it is evident that by variously modifying the surfaces of refractive media the rays of light may be diverted almost at pleasure4 ,this will be rendered clear by a few simple diagrams4 ,in the figures given below1 and in the following page1 the dotted lines represent perpendiculars to the surface at the point where the ray falls1 and it is seen that the usual law of bending towards the perpendicular on entering1 and away from it on leaving the dense medium1 is in each case correctly observed4 ,illustration3 ,fig4 #a4 ,illustration3 ,fig4 #b4 ,fig4 #b1 termed a prism1 bends the ray permanently to one side2 fig4 #c1 consisting of two prisms placed base to base1 causes rays before parallel to meet in a point2 and conversely1 fig4 #d1 having prisms placed edge to edge1 diverts them further asunder4 ,illustration3 ,fig4 #c4 ,illustration3 ,fig4 #d4 ,the various forms of ,lenses4--,the phenomena of the refraction of light are seen in the case of curved surfaces in the same manner as with those which are plane4 ,glasses ground of a curvilinear form are termed ,lenses4 ,the following are examples4 ,illustration3 ,fig4 #a4 ,illustration3 ,fig4 #b4 ,illustration3 ,fig4 #c4 ,fig4 #a is a biconvex lens2 fig4 #b1 a biconcave lens2 and fig4 #c1 a meniscus lens4 ,as far as regards their refractive powers1 such figures may be represented1 nearly1 by others bound by straight lines1 and thus it becomes evident that a biconvex lens tends to condense rays of light to a point1 and a biconcave to scatter them4 ,a meniscus combines both actions1 but the rays are eventually bent together1 the convex curve of a meniscus lens being always greater than the concave4 ,the ,foci of ,lenses4--,it has been shown that convex lenses tend to condense rays of light and bring them together to a point4 ,this point is termed the focus of the ,lens4 ,the following laws as regards the focus may be laid down3-- ,that rays of light which are pursuing a parallel course at the time they enter the ,lens are brought to a focus at a point nearer to the ,lens than diverging rays4 ,the rays proceeding from very distant objects are parallel2 those from objects near at hand diverge4 ,the sun's rays are always parallel1 and the divergence of the others becomes greater as the distance from the ,lens is less4 ,the focus of a ,lens for parallel rays is termed the principal focus1 and is not subject to variation2 this is the point referred to when the focal length of a ,lens is spoken of4 ,when the rays are not parallel1 but diverge from a point1 that point is associated with the focus1 and the two are termed conjugate foci4 ,illustration ,in the above diagram ,a is the principal focus1 and ,b and ,c are conjugate foci4 ,any object placed at ,b has its focus at ,g1 and conversely when placed at ,c it is in focus at ,b4 ,therefore1 although the principal focus of a ,lens 7as determined by the degree of its convexity7 is always the same1 yet the focus for objects near at hand varies1 being longer as they are brought closer to the ,lens4 ,formation of a ,luminous ,image by a ,lens4--,as the rays of light proceeding from a point are brought to a focus by means of a ,lens1 so are they when they proceed from an object1 and in that case an image of the object is the result4 ,illustration ,the above figure illustrates this4 ,the size of the image varies with the distance of the arrow from the glass--being larger and formed at a point further from the ,lens as the object is brought nearer4 ,the refracting power of the ,lens also influences the result--lenses of short focal length1 i4 e4 more convex1 giving a smaller image4 ,in order that the course pursued by pencils of rays proceeding from an object may be easily traced1 the lines from the barb of the arrow in the last figure are dotted4 ,observe that the object is necessarily inverted1 and also that those rays which traverse the central point of the ,lens1 or the centre of the axis1 as it is termed1 are not bent away1 but pursue a course either coincident with1 or parallel to1 the original1 as in the case of refracting media with parallel surfaces4 ,,section ,,iii4 ,the ,photographic ,camera4 ,the ,photographic ,camera is in its essential nature an extremely simple instrument4 ,it consists merely of a dark chamber1 having an aperture in front in which a ,lens is inserted4 ,the accompanying figure shows the simplest form of ,camera4 ,illustration ,the body is represented as consisting of two portions which slide within each other2 but the same object of lengthening or shortening the focal distance may be attained by making the ,lens itself movable4 ,a luminous image of any object placed in front of the ,camera is formed by means of the ,lens1 and received upon a surface of ground glass at the back part of the instrument4 ,when the ,camera is required for use1 the object is focussed upon the ground glass1 which is then removed1 and a slide containing the sensitive layer inserted in its place4 ,the luminous image1 as formed upon the ground glass1 is termed the ,field of the ,camera2 it is spoken of as being flat or curved1 sharp or indistinct1 etc4 ,these and other peculiarities which depend upon the construction of the ,lens will now be explained4 ,chromatic ,aberration of ,lenses4--,the outside of a biconvex lens is strictly comparable with the sharp edge of a prism1 and therefore necessarily produces decomposition in the white light which passes through it4 ,the action of a prism in separating white light into its constituent rays may be simply explained2--all the coloured rays are refrangible1 but not to the same extent4 ,the indigo and violet are more so than the yellow and red1 and consequently they are separated from them1 and occupy a higher position in the ,spectrum4 7,see the diagram at p4 #dg47 ,a little reflection will show that in consequence of this unequal refrangibility of the coloured rays1 white light must invariably be decomposed on entering any dense medium4 ,this is indeed the case2 but if the surfaces of the medium are parallel to each other the effect is not seen1 because the rays recombine on their emergence1 being bent to the same extent in the opposite direction4 ,hence light is transmitted colourless through an ordinary pane of glass1 but yields the tints of the ,spectrum in its passage through a prism or a lens1 where the two surfaces are inclined to each other at an acute angle4 ,chromatic aberration is corrected by combining two lenses cut from varieties of glass which differ in their power of separating the coloured rays4 ,these are the dense flint-glass containing ,oxide of ,lead1 and the light crown-glass4 ,of the two lenses1 the one is biconvex1 and the other biconcave2 so that when fitted together they produce a compound ,achromatic lens of a meniscus form1 thus3-- ,illustration ,the first ,lens in this figure is the flint- and the second the crown-glass4 ,of the two the biconvex is the most powerful1 so as to overcome the other1 and produce a total of refraction to the required extent4 ,each of the ,lenses produces a spectrum of a different length2 and the effect of passing the rays through both1 is1 by overlapping the coloured spaces1 to unite the complementary tints1 and to form again white light4 ,spherical ,aberration of ,lenses4--,the field of a ,camera is not often equally sharp and distinct at every part4 ,if the centre be rendered clear and well defined1 the outside is misty2 whilst1 by slightly altering the position of the ground glass1 so as to define the outside portion sharply1 the centre is thrown out of focus4 ,opticians express this by saying that there is a want of proper flatness of field2 two causes may be mentioned as concurring to produce it4 ,the first is spherical aberration1 by which is meant the property possessed by ,lenses which are segments of spheres1 of refracting rays of light unequally at different parts of their surfaces4 ,the following diagram shows this3-- ,illustration ,observe that the dotted lines which fall upon the circumference of the ,lens are brought to a focus at a point nearer to the ,lens than those passing through the centre2 in other words1 the outside of the ,lens refracts light the most powerfully4 ,this causes a degree of confusion and indistinctness in the image1 from various rays crossing1 and interfering with1 each other4 ,spherical aberration may be avoided by increasing the convexity of the centre part of the ,lens1 so as to add to its refracting power at that particular point4 ,the surface is then no longer a segment of a sphere1 but of an ellipse1 and refracts light more equally4 ,the difficulty of grinding ,lenses to an elliptical form however is so great1 that the spherical ,lens is still used1 the aberration being corrected in other ways4 ,a second cause interfering with the distinctness of the outer portions of the image in the ,camera is the obliquity of some rays proceeding from the object2 in consequence of which the image has a curved form1 with the concavity inwards1 as may be seen by referring to the figure given at page #ec4 ,the following diagram is meant to explain curvature of the image4 ,the centre line running at right angles to the general direction of the ,lens is the axis2 an imaginary line1 on which the ,lens may be said to rotate as a wheel turns on its axle4 ,the lines ,a ,a represent rays of light falling parallel to the axis2 and the dotted lines1 others which have an oblique direction2 ,b and ,c show the points at which the two foci are formed4 ,observe that these points1 although equidistant from the centre of the ,lens1 do not fall in the same vertical plane1 and therefore they cannot both be received distinct upon the ground glass of the ,camera1 which would occupy the position of the perpendicular double line in the diagram4 ,hence it is that with most lenses1 when the centre of the field has been focussed1 the glass must be shifted forwards a little to define the outside sharply4 ,illustration ,the ,use of ,stops in ,lenses4--,curvature of the image and indistinctness of outline from spherical aberration are both remedied to a great extent by fixing in front of the ,lens a diaphragm having a small central aperture4 ,the diagram gives a sectional view of a ,lens with a stop attached2 the exact position it should occupy with reference to the ,lens is a point of importance1 and influences the flatness of the field4 ,illustration ,by using a diaphragm the quantity of light admitted into the ,camera is diminished in proportion to the size of the aperture4 ,the image is therefore less brilliant1 and a longer exposure of the sensitive plate is required4 ,in other respects however the result is improved2 the spherical aberration is lessened by cutting off the outside of the ,lens1 and a portion of the oblique rays being intercepted1 the focus of the remainder is lengthened out1 and the image is rendered flatter1 and improved in distinctness4 ,hence also1 when a small stop is affixed to a ,lens1 a variety of objects1 situated at different distances1 are all in focus at once2 whereas1 with the full aperture of the ,lens1 objects near at hand cannot be rendered distinct upon the ground glass at the same time with distant objects1 or vice vers4 ,the ,double or ,portrait ,combination of ,achromatic ,lenses4--,the brightness of illumination of an image formed by a ,lens is in proportion to the diameter of the ,lens1 that is1 to the size of the aperture by which the ,light is admitted4 ,the clearness or distinctness of outline however is independent of this1 being improved by using a stop1 which lessens the diameter4 ,the ,portrait combination of ,lenses is constructed to ensure rapidity of action by admitting a large volume of light4 ,the following diagram gives a sectional view4 ,illustration ,in this combination the front ,lens is an ,achromatic plano-convex1 with1 the convex side turned toward the object2 and the second1 which takes up the rays and refracts them further1 is a compound ,biconvex ,lens2 there are therefore in all four distinct glasses concerned in forming the image1 which may appear at first to be an unnecessarily complex arrangement4 ,it is found however that a good result cannot be secured by using a single ,lens1 when a stop is inadmissible4 ,by combining two glasses of different curves1 the aberrations of one correct those of the other to a great extent1 and the field is both flatter and more distinct than in the case of an ,achromatic ,meniscus employed without a diaphragm4 ,the manufacture of ,portrait ,lenses is a point of great difficulty1 the glasses requiring to be ground with extreme care1 in order to avoid distortion of the image3 hence the most rapid ,portrait ,lenses1 having large aperture and short focus1 are often useless unless purchased of a good maker4 ,the ,variation between the ,visual and ,actinic ,foci in ,lenses4--,the same causes which produce chromatic aberration in a ,lens1 tend also to separate the chemical from the visual focus4 ,the violet and indigo rays are more strongly bent in than the yellow1 and still more than the red2 consequently the focus for each of those colours is at a different point4 ,the following diagram shows this4 ,illustration ,v represents the focus of the violet ray1 ,y of the yellow1 and ,e of the red4 ,hence1 as the chemical action corresponds more to the violet1 the most marked actinic effect would be produced at ,v4 ,the luminous portion of the spectrum however is the yellow1 consequently the visual focus is at ,y4 ,photographers have long recognized this point2 and therefore1 with ordinary ,lenses1 not corrected for colour1 rules are laid down as to the exact distance which the sensitive plate should be shifted away from the visual focus in order to obtain the greatest amount of distinctness of outline in the image impressed by chemical action4 ,these rules do not apply to the ,achromatic ,lenses recently described4 ,the coloured rays being in that case bent together again and reunited1 the two foci also nearly correspond4 ,by a little further correction to a point higher in the ,spectrum1 they are made to do so perfectly4 ,,section ,,iv4 ,on the ,photographic ,action of ,coloured ,light4 ,it has already been mentioned in the ,first ,section of this ,chapter that certain of the elementary colours of white light1 viz4 the violet and indigo1 are peculiarly active in decomposing the ,photographic ,salts of ,silver2 but there are some points of importance relating to the same subject which require a further notice4 ,the term actinism 7,gr4 1 a ray or flash7 has been proposed as convenient to designate the property possessed by light of producing chemical change2 the rays to which the effect is especially due being known as actinic rays4 ,if the pure ,solar ,spectrum formed by prismatic analysis in the manner represented at page #dg be allowed to impinge upon a prepared sensitive surface of ,iodide of ,silver1 the latent image being subsequently developed by a reducing agent1 the effect produced will be something similar to that represented in the following diagram3-- ,illustration3 ,fig4 #a4 ,illustration3 ,fig4 #b4 ,fig4 #a shows the visible spectrum as it appears to the eye2 the brightest part being in the yellow space1 and the light gradually shading off until it ceases to be seen4 ,fig4 #b represents the chemical effect produced by throwing the ,spectrum upon ,iodide of ,silver4 ,observe that the darkening characteristic of chemical action is most evident in the upper spaces1 where the light is feeble1 and is altogether absent at the point corresponding to the bright yellow spot of the visible spectrum4 ,the actinic and luminous spectra are therefore totally distinct from each other1 and the word ,photography1 which signifies the process of taking pictures by light1 is in reality inaccurate4 ,to those who have not the opportunity of working with the ,solar ,spectrum1 the following experiments will be useful in illustrating the ,photographic value of coloured light4 ,experiment ,i4--,take a sheet of sensitive paper prepared with ,chloride of ,silver1 and lay upon it strips of blue1 yellow1 and red glass4 ,on exposure to the sun's rays for a few minutes1 the part beneath the blue glass darkens rapidly1 whilst that covered by the red and yellow glass is perfectly protected4 ,this result is the more striking from the extreme transparency of the yellow glass1 giving the idea that the ,chloride would certainly be blackened first at that point4 ,on the other hand1 the blue glass appears very dark1 and effectually conceals the tissue of the paper from view4 ,experiment ,,ii4--,select a vase of flowers of different shades of scarlet1 blue1 and yellow1 and make a ,photographic copy of them1 by development1 upon ,iodide of ,silver4 ,the blue tints will be found to act most violently upon the sensitive compound1 whilst the reds and yellows are scarcely visible2 were it not that it is difficult to procure in nature pure and homogeneous tints1 free from admixture with other colours1 they would make no impression whatever upon the plate4 ,in exemplifying further the importance of distinguishing between visual and actinic rays of light1 we may observe that if the two were in all respects the same4 ,photography must cease to exist as an ,art4 ,it would be impossible to make use of the more sensitive chemical preparations from the difficulties which would attend the previous preparation and subsequent development of the plates4 ,these operations are now conducted in what is termed a dark room2 but it is dark only in a ,photographic sense1 being illuminated by means of yellow light1 which1 whilst it enables the operator easily to watch the progress of the work1 produces no injurious effect upon the sensitive surfaces4 ,if the windows of the room were glazed with blue in place of yellow glass1 then it would be strictly a dark room1 but one altogether unfitted for the purpose intended4 ,another point connected with the same subject and worthy of note is--the extent to which the sensibility of the ,photographic compounds is influenced by atmospheric conditions not visibly interfering with the brightness of the light4 ,it is natural to suppose that those days on which the sun's rays are the most powerful would be the best for rapid impression1 but such is not by any means the case4 ,if the light is at all of a yellow cast1 however bright it may be1 its actinic power will be small4 ,it will also be often observed in working towards the evening1 that a sudden diminution of sensibility in the plates begins to be perceptible at a time when but little difference can be detected in the brilliancy of the light2 the setting sun has sunk behind a golden cloud1 and all chemical action is soon at an end4 ,in the same manner is explained the difficulty of obtaining ,photographs in the glowing light of tropical climates2 the superiority ,of the early months of spring over those of the midsummer2 of the morning sun to that of the afternoon1 etc4 ,april and ,may are usually considered the best months for rapid impression in this country2 but the light continues good until the end of ,july4 ,in ,august and ,september a longer exposure of the plates will be required4 ,,the ,,superior ,,sensibility ,,of ,,bromide ,,of ,,silver ,,to ,,coloured ,,light4 ,in copying the ,solar ,spectrum alternately upon a surface of ,iodide and ,bromide of ,silver1 we notice a difference in the ,photographic properties of these two salts4 ,the latter is affected more extensively1 to a point lower in the spectrum1 than the former4 ,in the case of the ,iodide of ,silver1 the action ceases in the ,blue space2 but with the ,bromide it reaches to the ,green4 ,this is shown in the following diagrams1 which are drawn from the observations of ,mr4 ,crookes 7',photographic ,journal1' vol4 i4 p4 #ajj73-- ,illustration3 ,fig4 #a4 ,fig4 #b4 ,fig4 #c4 ,fig4 #a represents the chemical spectrum on ,bromide of ,silver2 fig4 #b1 the same upon ,iodide of ,silver2 and fig4 #c1 the visible spectrum4 ,it might perhaps be supposed that the superior sensibility of the ,bromide of ,silver to green rays of light would render that salt useful to the ,photographer in copying landscape scenery2 and indeed it is the opinion of many that1 in the ,calotype paper process1 the dark colour of foliage is better rendered by a mixture of ,bromide and ,iodide of ,silver than by the latter salt alone4 ,this however cannot depend upon the greater sensibility of the ,bromide to coloured light1 as may easily be proved4-- ,the diagrams given above are shaded to represent nearly1 the relative intensity of the chemical action exerted by the rays at different points of the spectrum2 and on referring to them it will be seen that the maximum point of blackness is in the indigo and violet space1 the action being more feeble in the blue space lower down2 there are also highly refrangible rays extending upwards far beyond the visible colours1 and these invisible rays are actively concerned in the formation of the image4 ,it is evident therefore that the amount of effect produced by a pure green1 or even a light blue tint1 upon a surface of ,bromide of ,silver is very small as compared with that of an indigo or violet2 and hence1 as in copying natural objects radiations of all kinds are present at the same time1 the green tints have not time to act before the image is impressed by the more refrangible rays4 ,sir ,john ,herschel proposed to render coloured light more available in ,photography by separating the actinic rays of high refrangibility1 and working only with those which correspond to the blue and green spaces in the spectrum4 ,this may be done by placing in front of the ,camera a vertical glass trough containing a solution of ,sulphate of ,quinine4 ,professor ,stokes has shown that this liquid possesses curious properties4 ,in transmitting rays of light it modifies them so that they emerge of lower refrangibility1 and incapable of producing the same actinic effect4 ,sulphate of ,quinine is1 if we may use the term1 opaque to all actinic rays higher than the blue-coloured space4 ,the proposition of ,sir ,john ,herschel above referred to was therefore to employ a bath of ,sulphate of ,quinine1 and having eliminated the actinic rays of high refrangibility1 to work upon ,bromide of ,silver with those corresponding to the lower-coloured spaces4 ,in this way he conceived that a more natural effect might be obtained4 ,if ,photographic compounds should be discovered of greater sensibility than any we at present possess1 the use of the ,quinine bath will perhaps be adopted2 but at present we trust to the superior intensity of the invisible rays for the formation of the image1 and hence the employment of ,bromide of ,silver is less strongly indicated4 ,these remarks apply to ,photographs taken by sunlight4 ,mr4 ,crookes states that in working with artificial light1 such as gas or camphine1 the case is different4 ,actinic rays of high refrangibility are comparatively wanting in gas-light1 the great bulk of the ,photographic rays beings found to lie within the limits of the visible spectrum1 and consequently acting more energetically upon ,bromide than on ,iodide of ,silver4 ,explanation of the mode in which ,coloured ,objects impress the ,sensitive ,film4--,the fact of which we have been speaking1 viz4 that the natural colours are not always correctly represented in photography1 is often urged in depreciation of the art1--when lights1 are represented by shadows1 it is said1 how can a truthful picture be expected8 ,the insensitiveness of ,iodide of ,silver to the colours occupying the lower portion of the spectrum would indeed present an insuperable difficulty if the tints of ,nature were pure and homogeneous3 such however is not the case4 ,even the most sombre colours are accompanied by scattered rays of white light in quantity amply sufficient to affect the sensitive film4 ,this is especially seen when the coloured body possesses a good reflecting surface2 and hence some varieties of foliage1 as for instance the ,ivy1 with its smooth and polished leaf1 are more easily photographed than others4 ,so again with regard to drapery in the department of portraiture--it is necessary to attend not only to the colour1 but also to the material of which it is composed4 ,silks and satins are favourable1 as reflecting much light1 whilst velvets and coarse stuffs of all kinds1 if at all dark1 produce very little effect upon the sensitive film4 ,,section ,v4 ,on ,binocular ,vision and the ,stereoscope4 ,an object is said to be stereoscopic 7 solid1 and 1 ,i see7 when it stands out in relief1 and gives to the eye the impression of solidity4 ,this subject was first explained by ,professor ,wheatstone in a memoir on binocular vision1 published in the ',philosophical ,transactions' for #ahch2 in which he shows that solid bodies project different perspective figures upon each retina1 and that the illusion of solidity may be artificially produced by means of the ,stereoscope4 ,the phenomena of binocular vision may be simply sketched as follows3--,if a cube1 or a small box of an oblong form1 be placed at a short distance in front of the observer1 and viewed attentively with the right and left eye separately and in succession1 it will be found that the figure perceived in the two cases is different2 that each eye sees more of one side of the box1 and less of the other2 and that in neither instance is the effect exactly the same as that given by the two eyes employed conjointly4 ,a silver pencil-case1 or a pen-holder1 may be used to illustrate the same fact4 ,it should be held at about six or eight inches distant from the root of the nose1 and quite at right angles to the face1 so that the length of the pencil is concealed by the point4 ,then1 whilst it remains fixed in this position1 the left and right eye are to be alternately closed3 in each case a portion of the opposite side of the pencil will be rendered visible4 ,illustration3 ,fig4 #a4 ,fig4 #b4 ,the preceding diagrams exhibit the appearance of a bust as seen by each eye successively4 ,observe that the second figure1 which represents the impression received by the right eye1 is more of a full face than fig4 #a1 which1 being viewed from a point removed a little to the left1 partakes of the character of a profile4 ,the human eyes are placed about #b-a_/b inches1 or from that to #b-e_/h inches1 asunder2 hence it follows that1 the points of sight being separated1 a dissimilar image of a solid object is formed by each eye4 ,we do not however see two images1 but a single one1 which is stereoscopic4 ,in looking at a picture painted on a flat surface the case is different3 the eyes1 as before1 form two images1 but these images are in every respect similar2 consequently the impression of solidity is wanting4 ,a single picture1 therefore1 cannot be made to appear stereoscopic4 ,to convey the illusion two pictures must be employed1 the one being a right and the other a left perspective projection of the object4 ,the pictures must also be so arranged1 that each is presented to its own eye1 and that the two appear to proceed from the same spot4 ,the reflecting stereoscope1 employed to effect this1 forms luminous images of the binocular pictures1 and throws these images together1 so that1 on looking into the instrument1 only a single image is seen1 in a central position4 ,it should1 however1 be understood1 that no optical arrangement of any kind is indispensably required1 since it is quite possible1 with a little effort1 to combine the two images by the unaided organs of vision4 ,the following diagram will make this obvious3-- ,illustration ,the circles ,a and ,b represent two wafers1 which are stuck on paper at a distance of about three inches from each other4 ,they are then viewed by squinting strongly1 or turning the eyes inwards towards the nose1 until the right eye looks at the left wafer1 and the left eye at the right wafer4 ,each wafer will then appear to become double1 four images being seen1 the two central of which will gradually approach each other until they coalesce4 ,stereoscopic pictures1 properly arranged1 may be examined in the same manner2 and it will be found that the resultant solid image is formed midway1 at a point where two lines1 drawn across from the eyes to the pictures1 cut one another4 ,the experiment here mentioned is sometimes a painful one1 and cannot easily be made if the eyes are not of equal strength2 but it will serve to show that the essential principle resides in the binocular representation of the object1 and not in the instrument employed to view it4 ,in ,mr4 ,wheatstone's reflecting ,stereoscope mirrors are used4 ,the principle of the instrument is as follows3-- objects placed in front of a mirror have their reflected images apparently behind the mirror4 ,by arranging two mirrors at a certain inclination to each other1 the images of the double picture may be made to approach until they coalesce1 and the eye perceives a single one only4 ,the following diagram will explain this4 ,illustration ,the rays proceeding from the star on either side pass in the direction of the arrows1 being thrown off from the mirror 7represented by the thick black line7 and entering the eyes at ,r and ,l4 ,the reflected images appear behind the mirror1 uniting at the point ,a4 ,the reflecting ,stereoscope is adapted principally for viewing large pictures4 ,it is a very perfect instrument1 and admits of a variety of adjustments1 by which the apparent size and distance of the ,stereoscopic image may be varied almost at pleasure4 ,the lenticular ,stereoscope of ,sir ,david ,brewster is a more portable form of apparatus4 ,a sectional view is given in the diagram4 ,illustration ,the brass tubes to which the eyes of the observer are applied contain each a semi-lens1 formed by dividing a common lens through the centre and cutting each half into a circular form 7fig4 #a in the following page74 ,the half-lens viewed in section 7fig4 #b7 is therefore of a prismatic shape1 and when placed with its sharp edge as in the diagram above1 alters the direction of the rays of light proceeding from the picture1 bending them outwards or away from the centre1 so that in accordance with well-known optical laws they appear to come in the direction of the dotted lines in the diagram 7in the last page71 and the two images coalesce at their point of junction4 ,in the instrument as it is often sold1 one of the lenses is made movable1 and by turning it round with the finger and thumb it will be seen that the positions of the images may be shifted at pleasure4 ,fig4 #a4 ,illustration ,fig4 #b4 ,rules for taking ,binocular ,photographs4--,in viewing very distant objects with the eyes1 the images formed on the retin are not sufficiently dissimilar to produce a very ,stereoscopic effect2 hence it is often required1 in taking binocular pictures1 to separate the ,cameras more widely than the two eyes are separated1 in order to give a sufficient appearance of relief4 ,mr4 ,wheatstone's original directions were1 to allow about one foot of separation for each twenty-five feet of distance1 but considerable latitude may be permitted4 ,if the ,cameras be not placed far enough apart1 the dimensions of the stereoscopic image from before backwards will be too small1--statues looking like bas-reliefs1 and the circular trunks of trees appearing oval1 with the long diameter transverse4 ,on the other hand1 when the separation is too wide1 the reverse obtains1--objects for instance which are square1 assuming an oblong shape pointing towards the observer4 ,to understand the cause of this1 the following law in optics should be studied3--,the distance of objects is estimated by the extent to which the axes of the eyes must be converged to view them4 ,if we have to turn our eyes strongly inwards1 we judge the object to be near2 but if the eyes remain nearly parallel1 we suppose it to be distant4 ,illustration ,the above figures represent six-sided truncated pyramids1 each with its apex towards the observer1 the centres of the two smaller interior hexagons being more widely separated than those of the larger exterior ones4 ,by converging the eyes upon them so as to unite the central images in the manner represented in page #fh a greater amount of convergence will be required to bring together the two summits than the bases1 and hence the summits will appear the nearest to the eye2 that is to say1 the resultant central figure will acquire the additional dimension of height1 and appear as a solid cone1 standing perpendicularly upon its base3 further1 the more widely the summits are separated in relation to the bases1 the taller will the cone be1 although a greater effort will be required to coalesce the figures4 ,binocular ,photographs taken with too much separation of the ,cameras1 are distorted from a similar cause1--so strong a convergence being required to unite them that certain parts of the picture appear to approach near to the eye2 and the depth of the solid image is increased4 ,this effect is most observable when the picture embraces a variety of objects1 situated in different planes4 ,in the case of views which are quite distant1 no near objects being admitted1 the ,cameras may be placed with especial reference to them1 even as far as twelve feet apart1 without producing distortion4 ,it is sometimes observable1 in looking at ,stereoscopic pictures1 that they convey an erroneous impression of the real size and distance of the object4 ,for instance1 in using the large reflecting ,stereoscope1 if1 when the adjustments have been made and the images properly united1 the two pictures be moved slowly forward1 the eyes remaining fixed upon the mirrors1 the ,stereoscopic image will gradually change its character1 the various objects it embraces appearing to become diminished in size1 and approaching near to the observer3 whilst if the pictures be pushed backwards1 the image will enlarge and recede to a distance4 ,so1 again1 if an ordinary slide for the lenticular ,stereoscope be divided in the centre1 and1 looking into the instrument until the images coalesce1 the two halves be slowly separated from each other1 the solid picture will seem to become larger and to recede from the eye4 ,it is easy to understand the cause of this4 ,when the pictures in the reflecting ,stereoscope are moved forwards1 the convergence of the optic axes is increased3 the image therefore appears nearer1 in accordance with the last-mentioned law4 ,but to convey the impression of nearness is equivalent to an apparent diminution in size1 for we judge of the dimensions of a body very much in relation to its supposed distance4 ,of two figures1 for instance1 appearing of the same height1 one known to be a hundred yards off might be considered colossal1 whilst the other1 obviously near at hand1 would be viewed as a statuette4 ,these facts1 with others not mentioned1 are of great interest and importance1 but their further consideration does not fall within the bounds originally prescribed to us4 ,the practical details of ,stereoscopic ,photography have been arranged in a distinct ,section1 and will be found included in the ,second ,part of the ,work4#aa #aa ,for a more full and detailed explanation of the ,stereoscopic phenomena1 see an abstract of ,professor ,tyndall's lectures in the third volume of the ',photographic ,journal4' ,,chapter ,,vi4 ,,the ,,photographic ,,properties ,,of ,,iodide ,,of ,,silver ,,upon ,,collodion4 ,in the preceding part of this ,work the physical and chemical properties of ,chloride and ,iodide of ,silver have been described1 with the changes which they experience by the action of ,light4 ,nothing however has been said of the surface used to support the ,iodide of ,silver1 and to expose it in a finely divided state to the influence of the actinic radiations4 ,this omission will now be supplied1 and the use of ,collodion will engage our attention4 ,the sensibility of ,iodide of ,silver upon ,collodion is greatly superior to that of the same salt employed in conjunction with any other vehicle at present known4 ,hence the ,collodio-,iodide film will supersede the paper and ,albumen processes in all cases where objects liable to move are to be copied4 ,the causes of this superior sensitiveness1 as far as ascertained1 may be referred to the state of loose coagulation of a ,collodion film and other particulars presently to be noticed4 ,it must however be allowed that there are yet some points affecting the sensitiveness of ,iodide of ,silver1 both mechanical and chemical1 of the exact nature of which we are ignorant4 ,the present ,chapter may be divided into four ,sections3--the nature of ,collodion2 the chemistry of the ,nitrate ,bath2 the causes affecting the formation and development of the ,image upon ,collodion2 the various irregularities in the development of the ,image4 ,,section ,i4 ,collodion4 ,collodion 7so named from the ,greek word 1 to stick7 is a glutinous1 transparent fluid1 procured1 as generally said1 by dissolving ,gun-,cotton in ,ether4 ,it was originally used for surgical purposes only1 being smeared over wounds and raw surfaces1 to preserve them from contact with the air by the tough film which it leaves on evaporation4 ,photographers employ it to support a delicate film of ,iodide of ,silver upon the surface of a smooth glass plate4 ,two elements enter into the composition of ,collodion3 first1 the ,gun-,cotton2 second1 the fluids used to dissolve it4 ,each of these will be treated in succession4 ,,chemistry ,,of ,,pyroxyline4 ,gun-,cotton or ,pyroxyline is ,cotton or ,paper which has been altered in composition and properties by treatment with strong acids4 ,both ,cotton and ,paper are1 chemically1 the same4 ,they consist of fibres which are found on analysis to have a constant composition1 containing three elementary bodies1 ,carbon1 ,hydrogen1 and ,oxygen1 united together in fixed proportions4 ,to this combination the term ,lignine or ,cellulose#ab has been applied4 #ab ,lignine and ,cellulose are not precisely identical substances4 ,the latter is the material composing the cell-wall2 the former1 the contained matter in the cell4 ,cellulose is a definite chemical compound1 in the same sense as ,starch or ,sugar1 and consequently1 when treated with various reagents1 it exhibits properties peculiar to itself4 ,it is insoluble in most liquids1 such as ,water1 ,alcohol1 ,ether1 etc41 and also in dilute acids2 but when acted upon by ,nitric ,acid of a certain strength it liquefies and dissolves4 ,it has been already shown 7p4 #ab7 that when a body dissolves in ,nitric ,acid the solution is not usually of the same nature as an aqueous solution2 and so in this case--the ,nitric ,acid imparts ,oxygen first to the ,cotton1 and afterwards dissolves it4 ,preparation of ,pyroxyline4--,if1 instead of treating ,cotton with ,nitric ,acid1 a mixture of ,nitric and ,sulphuric ,acids in certain proportions be used1 the effect is peculiar4 ,the fibres contract slightly1 but undergo no other visible alteration4 ,hence we are at first disposed to think the mixed ,acids ineffectual4 ,this idea however is not correct1 since on making the experiment the properties of the cotton are found to be changed4 ,its weight has increased by more than one-half2 it has become soluble in various liquids1 such as ,acetic ,ether1 ,ether and ,alcohol1 etc41 and1 what is more remarkable1 it no longer burns in the air quietly1 but explodes on the application of flame with greater or less violence4 ,this change of properties clearly shows1 that although the fibrous structure of the material is unaffected1 it is no longer the same substance1 and consequently chemists have assigned it a different name1 viz4 ,pyroxyline4 ,to produce the peculiar change by which ,cotton is converted into ,pyroxyline1 both ,nitric and ,sulphuric ,acids are1 as a rule1 required2 but of the two the former is the most important4 ,on analyzing ,pyroxyline1 ,nitric ,acid1 or a body analogous to it1 is detected in considerable quantity1 but not ,sulphuric ,acid4 ,the latter ,acid1 in fact1 serves but a temporary purpose1 viz4 to prevent the ,nitric ,acid from dissolving the pyroxyline1 which it would be liable to do if employed alone4 ,the ,sulphuric ,acid prevents the solution by removing water from the ,nitric ,acid1 and so producing a higher degree of concentration2 ,pyroxyline1 although soluble in a dilute1 is not so in the strong ,acid1 and hence it is preserved4 ,the property possessed by ,oil of ,vitriol of removing water from other bodies1 is one with which it is well to be acquainted4 ,a simple experiment will serve to illustrate it4 ,let a small vessel of any kind be filled to about two-thirds with ,oil of ,vitriol1 and set aside for a few days2 at the end of that time1 and especially if the atmosphere be damp1 it will have absorbed sufficient moisture to cause it to flow over the edge4 ,now even the strongest reagents employed in chemistry contain1 almost invariably1 water in greater or lesser quantity4 ,pure ,anhydrous ,nitric ,acid is a white1 solid substance2 ,hydrochloric ,acid is a gas3 and the liquids sold under those names are merely solutions4 ,the effect then of mixing strong ,oil of ,vitriol with aqueous ,nitric ,acid is to remove water in proportion to the amount used1 and to produce a liquid containing ,nitric ,acid in a high state of concentration1 and ,sulphuric ,acid more or less diluted4 ,this liquid is the ,nitro-,sulphuric ,acid employed in the preparation of ,pyroxyline4 ,various forms of ,pyroxyline4--,very soon after the first announcement of the discovery of ,pyroxyline1 most animated discussions arose amongst chemists with regard to its solubility and general properties4 ,some spoke of a solution of ,gun-,cotton in ,ether2 whilst others denied its solubility in that menstruum2 a third class1 by following the process described1 obtained a substance which was not explosive1 and therefore could scarcely be termed ,gun-,cotton4 ,on further investigations some of these anomalies were cleared up1 and it was found that there were varieties of ,pyroxyline1 depending mainly upon the degree of strength of the ,nitro-,sulphuric ,acid employed in the preparation4 ,still the subject was obscure until the publication of researches by ,mr4 ,e4 ,a4 ,hadow4 ,these investigations1 conducted in the ,laboratory of ,king's ,college1 ,london1 were published in the ,journal of the ,chemical ,society4 ,constant reference will be made to them in the following remarks4 ,we notice--first1 the chemical constitution of ,pyroxyline2 secondly1 its varieties2 and thirdly1 the means adopted to procure a ,nitro-,sulphuric ,acid of the proper strength4 a4 ,constitution of ,pyroxyline4--,pyroxyline has been sometimes spoken of as a ,salt of ,nitric ,acid1 a ,nitrate of ,lignine4 ,this view however is erroneous1 since it can be shown that the substance present is not ,nitric ,acid1 although analogous to it4 ,it is the ,peroxide of ,nitrogen1 which is intermediate in composition between ,nitrous ,acid 7,,no#c7 and ,nitric ,acid 7,,no#e74 ,peroxide of ,nitrogen 7,,no#d7 is a gaseous body of a dark red colour2 it possesses no acid properties1 and is incapable of forming a class of salts4 ,in order to understand in what state this body is combined with cotton fibre to form ,pyroxyline1 it will be necessary to digress for a short time4 ,law of ,substitution4--,by the careful study of the action of ,chlorine1 and of ,nitric ,acid1 upon various organic substances1 a remarkable series of compounds has been discovered1 containing a portion of ,chlorine or of ,peroxide of ,nitrogen in the place of ,hydrogen4 ,the peculiarity of these substances is1 that they strongly resemble the originals in their physical1 and often in their chemical properties4 ,it might have been supposed that agents of such active chemical affinities as ,chlorine and ,oxide of ,nitrogen would1 by their mere presence in a body1 produce a marked effect2 yet it is not so in the case before us4 ,the primitive type or constitution of the substance modified remains the same1 even the crystalline form being often unaffected4 ,it seems as if the body by which the ,hydrogen had been displaced had stepped in quietly and taken up its position in the framework of the whole without disturbance4 ,many compounds of this kind are known2 they are termed by chemists substitution compounds4 ,the law invariably observed is1 that the substitution takes place in equal atoms3 a single atom of ,chlorine1 for instance1 displaces one of ,hydrogen2 two of ,chlorine displace two of ,hydrogen1 and so on1 until1 in some cases1 the whole of the latter element is separated4 ,in illustration of these remarks1 take the following instances3--,acetic ,acid contains ,carbon1 ,hydrogen1 and ,oxygen2 by the action of ,chlorine the ,hydrogen may be removed in the form of ,hydrochloric ,acid1 and an equal number of atoms of ,chlorine be substituted4 ,in this way a new compound is formed1 termed ,chloracetic ,acid1 resembling in many important particulars the ,acetic ,acid itself4 ,notice particularly that the peculiar properties characteristic of ,chlorine are completely masked in the substitution body1 and no indication of its presence is obtained by the usual tests6 ,a soluble ,chloride gives with ,nitrate of ,silver a white precipitate of ,chloride of ,silver1 unaffected by ,acids1 but the ,chloracetic ,acid does not2 hence it is plain that the ,chlorine exists in a peculiar and ,ultimate state of combination different from what is usual4 ,the substance we have been previously considering1 viz4 ,pyroxyline1 affords another illustration of the ,law of ,substitution4 ,omitting1 for the sake of simplicity1 the number of atoms concerned in the change1 the action of concentrated ,nitric ,acid upon ligneous fibre may be thus explained3-- ,carbon ,cotton or ,hydrogen ,nitric ,acid ,hydrogen ,oxygen equals ,carbon ,pyroxyline or ,hydrogen ,water ,peroxide ,nitrogen ,oxygen ,or in symbols3-- ,,ch#aa,o ,,no#e ,c 7,hn-#a,,no#d7 ,o ,,ho ,by a reference to the formula1 it is seen that the fifth atom of ,oxygen contained in the ,nitric ,acid takes one of ,hydrogen1 and forms an atom of ,water2 the ,,no#d then steps in1 to fill the gap which the atom of ,hydrogen has left4 ,all this is done with so little disturbance that even the fibrous structure of the cotton remains as before4 b4 ,chemical ,composition of the varieties of ,pyroxyline4--,mr4 ,hadow has succeeded in establishing four different substitution compounds1 which1 as no distinctive nomenclature has been at present proposed1 may be termed compounds ,a1 ,b1 ,c1 and ,d4 ,compound ,a is the most explosive ,gun-,cotton1 and contains the largest amount of ,peroxide of ,nitrogen4 ,it dissolves only in ,acetic ,ether1 and is left on evaporation as a white powder4 ,it is produced by the strongest ,nitro-,sulphuric ,acid which can be made4 ,compounds ,b and ,c1 either separate or in a state of mixture1 form the soluble material employed by the ,photographer4 ,they both dissolve in ,acetic ,ether1 and also in a mixture of ,ether and ,alcohol4 ,the latter1 viz4 ,c1 also dissolves in glacial ,acetic ,acid4 ,they are produced by a ,nitro-,sulphuric ,acid slightly weaker than that used for ,a1 and contain a smaller amount of ,peroxide of ,nitrogen4 ,compound ,d resembles what has been termed ,xyloidine1 that is1 the substance produced by acting with ,nitric ,acid upon ,starch4 ,it contains less ,peroxide of ,nitrogen than the others1 and dissolves in ,ether and ,alcohol1 and also in ,acetic ,acid4 ,the ethereal solution leaves1 on evaporation1 an opaque film1 which is highly combustible1 but not explosive4 ,by bearing in mind the properties of these compounds1 many of the anomalies complained of in the manufacture of ,gun-,cotton disappear4 ,if the ,nitro-,sulphuric ,acid employed is too strong1 the product will be insoluble in ,ether2 whilst if it is too weak1 the fibres are gelatinized by the ,acid and partly dissolved4 c4 ,means adopted to procure a ,nitro-,sulphuric ,acid of the requisite strength for preparing ,pyroxyline4--,this is a point of more difficulty than would at first appear4 ,it is easy to determine an exact formula for the mixture1 but not so easy to hit upon the proper proportions of the acids required to produce that formula2 and a very slight departure from them altogether modifies the result4 ,the main difficulty lies in the uncertain strength of commercial ,nitric ,acid4 ,oil of ,vitriol is more to be depended upon1 and has a tolerably uniform ,sp4 ,gr4 of #ahcf2ac but ,nitric ,acid is constantly liable to variation2 hence it becomes necessary to make a preliminary determination of its real strength1 which is done either by taking the specific gravity and referring to tables1 or1 better still1 by a direct analysis4 ,as each atom of ,sulphuric ,acid removes only a given quantity of water1 it follows that the weaker the ,nitric ,acid1 the larger the amount of ,sulphuric which will be required to bring it up to the proper degree of concentration4 #ac ,the later experience of the writer induces him to believe1 that the specific gravity of ,oil of ,vitriol cannot always be taken as an indication of its real strength2 which is best ascertained by analysis4 ,to avoid the trouble necessarily attendant upon these preliminary operations1 many prefer to use1 in place of ,nitric ,acid itself1 one of the salts formed by the combination of ,nitric ,acid with an alkaline base4 ,the composition of these salts1 provided they are pure and nicely crystallized1 can be depended on4 ,nitrate of ,potash1 or ,saltpetre1 contains a single atom of ,nitric ,acid united with one of ,potash4 ,it is an anhydrous salt1 that is1 it has no water of crystallization4 ,when strong ,sulphuric ,acid is poured upon ,nitrate of ,potash in a state of fine powder1 in virtue of its superior chemical affinities it appropriates to itself the ,alkali and liberates the ,nitric ,acid4 ,if care be taken to add a sufficient excess of the ,sulphuric ,acid1 a solution is obtained containing ,sulphate of ,potash dissolved in ,sulphuric ,acid1 and free ,nitric ,acid4 ,the presence of the ,sulphate of ,potash 7or1 more strictly speaking1 of the ,bi-,sulphate7 does not in any way interfere with the result1 and the effect is the same as if the mixed acids themselves had been used4 ,the reaction may be thus represented3-- ,nitrate of ,potash plus ,sulphuric ,acid in excess ,bisulphate ,potash plus ,nitro-,sulphuric ,acid4 ,,chemistry ,,of ,,the ,,solution ,,of ,,pyroxyline ,,in ,,ether ,,and ,,alcohol1 ,,or ,,collodion4 ,the substitution compounds ,b and ,c1 already alluded to as forming the ,soluble ,cotton of ,photographers1 are both abundantly soluble in ,acetic ,ether4 ,this liquid however is not adapted for the purpose required1 inasmuch as on evaporation it leaves the ,pyroxyline in the form of a white powder1 and not as a transparent layer4 ,the rectified ,ether of commerce has been found to answer better than any other liquid as a solvent for ,pyroxyline4 ,if the sp4 gravity be about #gej1 it contains invariably a small proportion of ,alcohol1 which appears to be necessary2 the solution not taking place with absolutely pure ,ether4 ,the ,pyroxyline1 if properly prepared1 begins almost immediately to gelatinize by the action of the ,ether1 and is soon completely dissolved4 ,in this state it forms a slimy solution1 which1 when poured out on a glass plate1 dries up into a horny transparent layer4 ,in preparing ,collodion for ,photographic purposes1 we find that its physical properties are liable to considerable variation4 ,sometimes it appears very thin and fluid1 flowing on the glass almost like water1 whilst at others it is thick and glutinous4 ,the causes of these differences will now engage our attention4 ,they may be divided into two classes3 first1 those relating to the ,pyroxyline2 second1 to the solvents employed4 a4 ,variation of ,properties in different ,samples of soluble ,pyroxyline4--,the substitution compounds ,a1 ,b1 ,c1 and ,d differ1 as already shown1 in the percentage amount of ,peroxide of ,nitrogen present1 and the former are more explosive and insoluble than the latter4 ,but it often happens in preparing ,pyroxyline1 that two portions of ,nitro-,sulphuric ,acid taken from the same bottle yield products which vary in properties1 although they are necessarily the same in composition4 ,taking extremes in illustration1 we notice two principal modifications of soluble ,pyroxyline4 ,the first1 when treated with the mixture of ,ether and ,alcohol1 sinks down to a gummy or gelatinous mass1 which gradually dissolves on agitation ,the solution is very fluid in proportion to the number of grains used1 and when poured out spreads into a beautifully smooth and glassy surface1 which is quite structureless1 even when highly magnified4 ,the film adheres tightly to the glass1 and when the finger is drawn across it1 separates in short fragments1 and broken pieces4 ,the second variety produces a ,collodion which is thick and glutinous1 flowing over the glass in a slimy manner1 and soon setting into numerous small waves and cellular spaces4 ,the film lies loose upon the glass1 is apt to contract on drying1 and may be pushed off by the finger in the form of a connected skin4 ,this subject is not thoroughly understood1 but it is known that the temperature of the ,nitro-,sulphuric ,acid at the time of immersing the ,cotton influences the result4 ,the soluble variety is produced by hot acids2 the second1 or glutinous1 by the same acids employed cold1 or only slightly warm4 ,the best temperature appears to be from #acj to #aee ,fahrenheit2 if it rises much beyond that point1 the acids act upon and dissolve the ,cotton4 b4 ,the physical properties of ,collodion affected by the proportions and purity of the ,solvents4--,pyroxyline of the varieties termed ,b and ,c dissolves freely in a mixture of ,ether and ,alcohol2 but the characters of the resulting solution vary with the relative proportions of the two solvents4 ,when the ,ether is in large excess1 the film is inclined to be strong and tough1 so that it can often be raised by one corner and lifted completely off the plate without tearing4 ,it is also very contractile1 so that a portion of the ,collodion poured on the hand draws together and puckers the skin as it dries4 ,if spread upon a glass plate in the usual way1 the same property of contractility causes it to retract and separate from the sides of the glass4 ,these properties1 produced by ,ether in large proportion1 disappear entirely on the addition of more ,alcohol4 ,the transparent layer is now soft and easily torn1 possessing but little coherency4 ,it adheres to the surface of the glass more firmly1 and exhibits no tendency to contract and separate from the sides4 ,from these remarks it will be gathered that an excess of ,ether1 and a low temperature in preparing the ,pyroxyline1 both favour the production of a contractile ,collodion2 whilst on the other hand an abundance of ,alcohol1 and a hot ,nitro-,sulphuric ,acid1 tend to produce a short and non-contractile ,collodion4 ,the physical properties of ,collodion are affected by another cause1 viz4 by the strength and purity of the solvents1 or1 in other words1 their freedom from dilution with water4 ,if a few drops of water be purposely added to a sample of ,collodion1 the effect is seen to be to precipitate the ,pyroxyline in flakes to the bottom of the bottle4 ,there are many substances known in chemistry which are soluble in spirituous liquids1 but behave in the same manner as ,pyroxyline in this respect4 ,the manner in which water gains entrance into the ,photographic ,collodion is usually by the employment of ,alcohol or ,spirit of ,wine which has not been highly rectified4 ,in that case the ,collodion is thicker1 and flows less readily than if the ,alcohol were stronger4 ,sometimes the texture of the film left upon evaporation is injured2 it is no longer homogeneous and transparent1 but semi-opaque1 reticulated1 or honeycombed1 and so rotten that a stream of water projected upon the plate washes it away4 ,these effects are to be attributed not to the ,alcohol1 but to the water introduced with it2 and the remedy will be to procure a stronger spirit1 or1 if that cannot be done1 to increase the amount of ,ether4 ,collodion prepared with a large proportion of ,ether1 and water1 but a small quantity of ,alcohol1 is often very fluid and structureless at first1 adhering to the glass with some tenacity and having a short texture2 but it tends to become rotten when used to coat many plates successively1 the water on account of its lesser volatility accumulating in injurious quantity in the last portions4 ,,the ,,coloration ,,of ,,iodized ,,collodion ,,explained4 ,collodion iodized with the ,iodides of ,potassium1 ,ammonium1 or ,zinc1 soon assumes a yellow tint1 which in the course of a few days or weeks1 according to the temperature of the atmosphere1 deepens to a full brown4 ,this gradual coloration1 due to a development of ,iodine1 is caused partly by the ,ether and partly by the ,pyroxyline4 ,ether may1 with proper precautions1 be preserved for a long time in a pure state1 but on exposure to the joint action of air and light it undergoes a slow process of oxidation1 attended with formation of ,acetic ,acid and a peculiar principle resembling in properties ozone1 or ,oxygen in an allotropic and active condition4 ,iodide of ,potassium or ,ammonium is decomposed by ,ether in this state4 ,acetate of the ,alkali1 and ,hydriodic ,acid 7,,hi71 being first produced4 ,the ozonized substance then removes ,hydrogen from the latter compound1 and liberates ,iodine1 which dissolves and tinges the liquid yellow4 ,a simple solution of an ,alkaline ,iodide in ,alcohol and ,ether does not1 however1 become so quickly coloured as ,iodized ,collodion2 and hence it is evident that the presence of the ,pyroxyline produces an effect4 ,it may be shown that ,alkaline ,iodides slowly decompose ,pyroxyline1 and that a portion of ,peroxide of ,nitrogen is set free3 this body1 containing loosely combined oxygen1 tends powerfully to eliminate ,iodine1 as may be seen by adding a few drops of the yellow commercial ,nitrous acid to a solution of ,iodide of ,potassium4 ,the stability of the particular ,iodide used in ,iodizing ,collodion1 influences mainly the rate of coloration1 though elevation of temperature and exposure to light are not without effect4 ,iodide of ,ammonium is the least stable1 and ,iodide of ,cadmium the most so2 ,iodide of ,potassium being intermediate4 ,collodion iodized with pure ,iodide of ,cadmium usually remains nearly colourless to the last drop1 if kept in a cool and dark place4 ,as the presence of free ,iodine in ,collodion affects its photographic properties1 it may sometimes be necessary to remove it4 ,this is done by inserting a strip of ,silver-foil2 which decolorizes the liquid1 by forming ,iodide of ,silver1 soluble in the excess of ,alkaline ,iodide 7p4 #db74 ,metallic ,cadmium1 and metallic ,zinc1 have the same effect4 ,when ,methylated ,spirits are employed in the manufacture of ,collodion1 the ,iodine first liberated is afterwards either partially or entirely reabsorbed1 the liquid acquiring at the same time an acid reaction to test-paper4 ,,section ,,ii4 ,the ,chemistry of the ,nitrate ,bath4 ,the solution of ,nitrate of ,silver in which the plate coated with iodized ,collodion is dipped1 to form the layer of ,iodide of ,silver1 is known technically as the ,nitrate ,bath4 ,the chemistry of ,nitrate of ,silver has been explained at page #ac1 but there are some points relating to the properties of its aqueous solution which require a further notice4 ,solubility of ,iodide of ,silver in the ,nitrate ,bath4--,aqueous solution of ,nitrate of ,silver may be mentioned in the list of solvents of ,iodide of ,silver4 ,the proportion dissolved is in all cases small1 but it increases with the strength of the solution4 ,if no attention were paid to this point1 and the precaution of previously saturating the ,nitrate ,bath with ,iodide of ,silver neglected1 the film would be dissolved when left too long in the liquid4 ,this solvent power of ,nitrate of ,silver on the ,iodide is well shown by taking the excited ,collodion plate out of the ,bath1 and allowing it to dry spontaneously4 ,the layer of ,nitrate on the surface1 becoming concentrated by evaporation1 eats away the film1 so as to produce a transparent1 spotted appearance4 ,in the solution of ,iodide of ,silver by ,nitrate of ,silver a double salt is formed1 which corresponds in properties to the double ,iodide of ,potassium and ,silver in being decomposed by the addition of water4 ,consequently1 in order to saturate a ,bath with ,iodide of ,silver it is only necessary to dissolve the total weight of ,nitrate of ,silver in a small bulk of water1 and to add to it a few grains of an ,iodide2 perfect solution takes place1 and on subsequent dilution with the full amount of water1 the excess of ,iodide of ,silver is precipitated in the form of a milky deposit4 ,acid condition of ,nitrate of ,silver4--,a solution of pure ,nitrate of ,silver is neutral to blue litmus-paper1 but that prepared from the commercial ,nitrate has usually an acid reaction2 the crystals having been imperfectly drained from the acid mother-liquor in which they were formed4 ,hence1 in making a new ,bath it is often advisable not only to saturate it with ,iodide of ,silver1 but to neutralize the free ,nitric acid it contains4 ,there is also a peculiar condition of ,nitrate of ,silver crystallized from a solution of the metal in ,nitric ,acid1 which renders it quite unfit for photographic purposes 7see p4 #aja74 ,it is thought to depend upon the presence of an oxide of ,nitrogen1 possibly of ,nitrous ,acid1 and the remedy is to dry the crystals very strongly1 or1 better still1 to fuse them at a moderate heat3 mere neutralization with ,carbonate of ,soda does not suffice4 ,in melting ,nitrate of ,silver great care should be taken not to raise the heat so high as to decompose the salt1 or a basic ,nitrite will be formed1 which affects the properties of the solution 7p4 #ac73 fused ,nitrate of ,silver ought1 when cold1 to be quite white1 and to dissolve perfectly in water without leaving any residue4 ,the only objection to the employment of ,nitrate of ,silver in this form is the facility with which it may be adulterated with ,nitrates of ,potash and ,soda1 the presence of which would lessen the available strength of the ,bath4 ,the ,nitrate ,bath1 although perfectly neutral when first prepared1 may become acid by continued use1 if ,collodion containing much free ,iodine be constantly employed4 ,in that case a portion of ,nitric ,acid is liberated1 thus3-- ,nitrate of ,silver ,iodine ,iodide of ,silver ,nitric ,acid ,oxygen4 ,when ,collodion is iodized entirely with alkaline ,iodides1 it liberates ,iodine by keeping2 and hence the occasional use of ,ammonia may be required to remove acidity from the ,bath4 ,but since the introduction of the ,iodide of ,cadmium1 which preserves the ,collodion nearly or quite colourless1 the necessity for neutralizing ,nitric ,acid in the ,bath has ceased4 ,alkaline condition of the ,bath4--,by alkalinity of the ,bath is meant a condition in which the blue tint is rapidly restored to reddened litmus-paper4 ,this indicates that an ,oxide of some kind is present in solution1 which1 by combining with the acid in the reddened paper1 neutralizes it and removes the red colour4 ,if a small portion of caustic ,potash or ,ammonia be added to a strong solution of ,nitrate of ,silver1 it produces a brown precipitate1 which is ,oxide of ,silver4 ,ammonia ,nitrate of ,silver ,oxide of ,silver ,nitrate ,ammonia4 ,the solution however1 from which the precipitate has separated1 is not left in a neutral state1 but possesses a faint alkaline reaction4 ,oxide of ,silver and ,carbonate of ,silver are also abundantly soluble in water containing ,nitrate of ,ammonia2 which salt is continually accumulating in the ,bath when compounds of ,ammonium are used for iodizing4 ,an alkaline ,bath is perhaps of all conditions the one most fatal to success in photography4 ,it leads to that universal darkening of the film on applying the developer to which the name of fogging has been given4 ,hence care must be used in adding to the ,bath substances which tend to make it alkaline4 ,collodion containing free ,ammonia1 often sold in the shops1 gradually does so4 ,the use of ,potash1 ,carbonate of ,soda1 ,chalk1 or ,marble1 to remove free ,nitric ,acid from the ,bath1 has the same effect2 and hence1 when they are employed1 a trace of ,acetic acid must afterwards be added4 ,the mode of testing a bath for alkalinity is as follows3-- a strip of porous blue litmus-paper is taken and held to the mouth of a bottle of glacial ,acetic acid until it becomes reddened2 it is then placed in the liquid to be examined and left for ten minutes or a quarter of an hour4 ,if ,oxide of ,silver be present in solution1 the original blue colour of the paper will slowly but gradually be restored4 ,occasional formation of ,acetate of ,silver in the ,nitrate ,bath4--,in preparing a new ,bath1 if the crystals of ,nitrate of ,silver are acid1 it is usual to add an alkali in small quantity4 ,this removes the ,nitric ,acid1 but leaves the solution faintly alkaline4 ,acetic ,acid is then dropped in1 which1 by combining with the ,oxide of ,silver1 forms ,acetate of ,silver4 ,acetate of ,silver is not formed by the simple addition of ,acetic ,acid to the ,bath1 because its production under such circumstances would imply the liberation of ,nitric ,acid2 but if an alkali be present to neutralize the ,nitric ,acid1 then the double decomposition takes place1 thus-- ,acetate of ,ammonia ,nitrate of ,silver ,acetate of ,silver ,nitrate of ,ammonia4 ,acetate of ,silver is a white flaky salt1 sparingly soluble in water4 ,it dissolves in the ,bath only in small proportion1 but yet sufficiently to affect the ,photographic properties of the film 7see p4 #aaa and #aag74 ,the observance of the following simple rules will obviate its production in injurious quantity3--,first1 when it is required to remove free ,nitric ,acid from a bath not containing ,acetic ,acid a solution of ,potash or ,carbonate of ,soda may be dropped in freely2 but the liquid must be filtered before adding any ,acetic ,acid1 otherwise the brown deposit of ,oxide of ,silver will be taken up by the ,acetic ,acid1 and the ,bath will be charged with ,acetate of ,silver4 ,secondly1 in dealing with a ,bath containing both ,nitric and ,acetic ,acids1 employ an alkali much diluted 7,liquor ,ammoni with #aj parts of ,water71 and add a single drop at a time1 coating and trying a plate between each addition2 the ,nitric ,acid will neutralize itself before the ,acetic1 and with care there will be no formation of ,acetate of ,silver in quantity4 ,substances which decompose the ,nitrate ,bath4--,most of the common metals1 having superior affinity for ,oxygen1 separate the ,silver from a solution of the ,nitrate2 hence the ,bath must be kept in glass1 porcelain1 or gutta-percha1 and contact with ,iron1 ,copper1 ,mercury1 etc41 must be avoided1 or the liquid will be discoloured1 and a black deposit of metallic ,silver precipitated4 ,all developing agents1 such as ,gallic and ,pyrogallic ,acids1 the ,protosalts of ,iron1 etc41 blacken the ,nitrate ,bath1 and render it useless by reducing metallic ,silver4 ,chlorides1 ,iodides1 and ,bromides produce a deposit in the ,bath2 but the solution1 although weakened1 may again be used after passing through a filter4 ,hyposulphites1 ,cyanides1 and all fixing agents decompose ,nitrate of ,silver4 ,organic matters1 generally1 reduce ,nitrate of ,silver1 either with or without the aid of light4 ,grape ,sugar1 ,albumen1 ,serum of ,milk containing caseine1 etc41 blacken the ,bath1 even in the dark4 ,alcohol and ,ether act more slowly1 and produce no injurious effect unless the liquid is constantly exposed to light4 ,these facts indicate that the ,nitrate ,bath containing volatile organic matters must be preserved in a dark place2 also that it should be kept exclusively for sensitizing the ,collodion plates1 and not used in floating papers intended for the printing process4 ,changes in the ,nitrate ,bath by use4--,the solution of ,nitrate of ,silver employed in exciting the ,collodion film gradually decreases in strength1 but not so quickly as the ,bath used in sensitizing papers for printing4 ,if the amount of ,nitrate be allowed to fall as low as twenty grains to the ounce of water1 the decomposition will be imperfect1 and the film will be pale and blue1 even with a highly iodized ,collodion4 ,a gradual accumulation of ,ether and ,alcohol also takes place in the ,bath after long use1 in consequence of which the developing solutions flow less readily upon the ,collodionized plates1 and oily stains are apt to be produced4 ,diminished sensitiveness of the ,iodide film is sometimes traced to impurities in the ,bath1 when it is very old1 and has been much used4 ,these are probably of an organic nature and may often be partially removed by agitation with kaolin1 or animal charcoal4 ,the latter however is objectionable1 being usually contaminated with ,carbonate of ,lime1 which makes the ,bath alkaline2 or 7in the case of purified animal charcoal7 with traces of ,hydrochloric ,acid1 which liberate ,nitric ,acid in the ,bath4 ,even the kaolin may as a preliminary precaution be washed with dilute ,acetic ,acid to remove ,carbonate of ,lime if any should be present4 ,,section ,,iii4 ,the ,conditions which affect the ,formation and ,development of the ,latent ,image in the ,collodion process4 ,it will be necessary to preface the observations contained in this ,section by defining two terms which are frequently confounded with each other1 but are in reality of distinct meaning4 ,these terms are ,sensitiveness and ,intensity4 ,by ,sensitiveness is meant a facility of receiving impression from very feeble rays of light1 or of receiving it quickly from brighter rays4 ,intensity1 on the other hand1 relates to the appearance of the finished ,photograph1 independently of the time taken to produce it1--to the degree of opacity of the image1 and the extent to which it obstructs transmitted light4 ,it will be seen as we proceed that the conditions necessary to obtain extreme sensitiveness of the ,iodide film are different from1 and often opposed to1 those which give the maximum intensity of image4 ,,causes ,,which ,,influence ,,the ,,sensitiveness ,,of ,,iodide ,,of ,,silver ,,on ,,collodion4 ,some of the most important are as follows3-- a4 ,the presence of free ,nitrate of ,silver4--,when the sensitive film is removed from the ,nitrate ,bath1 the ,iodide of ,silver is left in contact with excess of ,nitrate of ,silver4 ,the presence of this compound is not essential to the action of the light1 since1 if it be removed by washing in distilled water1 the image may still be impressed4 ,in such a case however the effect is produced slowly1 and a longer exposure in the ,camera is required4 ,the sensitiveness of the ,iodide film does not increase uniformly with the amount of the excess of ,nitrate of ,silver1 as measured by the strength of the ,bath4 ,it is found that no advantage in this respect can be gained by using a proportion of ,nitrate of ,silver greater than #cj or #ce grains to the ounce of water1 although solutions of three times this strength have been sometimes employed4 ,it has been asserted that a chemically pure ,iodide of ,silver1 which is unaffected in colour by the direct action of light1 is also incapable of receiving the invisible image in the ,camera2 and that the sensitiveness of a washed ,collodion film is due to a minute quantity of ,nitrate of ,silver still remaining4 ,iodide of ,silver in the state in which it is thrown down on diluting with water a strong solution of the salt known as the double ,iodide of ,potassium and ,silver1--and which must1 from the mode of its preparation1 be free from ,nitrate of ,silver1--is quite insensitive2 but this form of ,iodide differs from the other in colour1 and not only so1 but is likely to contain an excess of ,iodide of ,potassium4 ,the application of a solution of ,nitrate of ,silver to this compound at once renders it sensitive to light4 b4 ,free acids in the ,nitrate ,bath4--,strong oxidizing agents1 such as ,nitric ,acid1 greatly diminish the sensibility of the film1 and hence the importance of removing the free acid often met with in commercial samples of the ,nitrate of ,silver4 ,the effect of even a single drop of strong ,nitric ,acid in an eight-ounce ,nitrate ,bath will be appreciable2 and when the proportion is increased to one drop per ounce1 it will be difficult to obtain a rapid impression4 ,acetic ,acid has far less effect upon the sensitiveness than ,nitric ,acid1 and being found useful during the development of the image is commonly employed2 but when great rapidity is desired1 it should be added cautiously1 and in a proportion very much less than that in the solution known as the ,aceto-,nitrate of ,silver1 which contains about one drop of the glacial acid to each grain of ,nitrate of ,silver4 c4 ,addition of certain organic matters4--,it has long been remarked that the use of bodies like ,albumen1 ,gelatine1 ,caseine1 etc41 which combine with ,oxides of ,silver1 retard the action of light upon ,iodide of ,silver2 and the recent observations of the ,author enable him to confirm this statement4 ,it is probable that one cause1 amongst others1 of the great sensibility of the ,collodion film is due to the fact that ,pyroxyline is a substance peculiarly indifferent to the ,salts of ,silver1 exhibiting no tendency to reduce them to the metallic state2 and it is proved by experiment that the addition of ,grape ,sugar1 or of the resinous body1 ,glycyrrhizine1 which resembles ,albumen in causing a white precipitate in strong solution of ,nitrate of ,silver1 renders necessary a longer exposure in the ,camera4 ,alkaline ,citrates have a still more marked effect1 as also have ,tartrates1 ,oxalates1 etc4 d4 ,impurities in the soluble ,iodides4--,commercial ,iodide of ,potassium often contains ,iodate of ,potash1 which is found to have a retarding effect upon the action of light2 also ,carbonate of ,potash1 which1 in ,collodion1 produces ,iodoform1#ad and in the paper processes1 where ,aceto-,nitrate is used for sensitizing1 forms ,acetate of ,silver4 ,iodoform has a marked influence in diminishing the sensitiveness of ,iodide of ,silver2 ,acetate of ,silver may perhaps increase it a little by securing the absence of free ,nitric ,acid 7p4 #aag74 ,iodide of ,potassium prepared by the process in which ,sulphuretted ,hydrogen and ,alcohol are used1 and having a smell of ,garlic1 contains probably ,xanthate of ,potash1 and is nearly useless for ,photography4 #ad ,see the ,vocabulary1 ,part ,,iii41 ,art4 ,iodoform4 ,commercial ,iodide of ,cadmium is a purer salt than the ,iodide of ,potassium1 and may be advantageously substituted for it2 but it possesses the property of coagulating ,albumen1 and hence cannot be employed in conjunction with that substance4 e4 ,presence of free ,iodine4--,both in the waxed paper and the ,collodion processes1 the solutions often contain a small quantity of free ,iodine4 ,this ,iodine1 in contact with the ,nitrate of ,silver of the ,bath1 produces a mixed ,iodide and ,iodate of ,silver1 and liberates ,nitric ,acid4 ,it thus retards the sensitiveness of the film in proportion to the quantity of ,iodine present4 ,collodion of a full yellow colour is perceptibly less sensitive than the same rendered colourless2 and when enough ,iodine has been liberated to give a red or brown tint1 double the original exposure will probably be required4 ,if brown ,collodion be much used1 the ,nitrate ,bath may by degrees become sufficiently contaminated with free ,nitric ,acid to interfere with the sensitiveness of the film2 but if colourless or lemon-yellow tinted ,collodion be employed1 this evil need not be anticipated4 ,certain substances may be added to coloured ,collodion which possess the property of counteracting the retarding influence of the free ,iodine1 such1 for instance1 as the ,oils of ,cloves1 ,cinnamon1 etc42 they probably act in virtue of their affinity for ,oxygen1 by preventing the formation of ,iodate of ,silver4 ,in colourless ,collodion they produce little or no effect1 neither do they remove the insensitiveness of the film when dependent upon a too acid condition of the ,nitrate ,bath4 f4 ,addition of ,bromide or ,chloride to ,collodion4--,in the ,daguerreotype a very exalted state of sensibility is obtained by exposing the silvered plate first to the vapour of ,iodine1 and afterwards to that of ,bromine or ,chlorine2 but this rule does not apply to the ,collodion process1 which differs essentially in principle4 ,soluble ,bromides added to ,collodion lessen its sensibility to an appreciable extent1 as also do ,chlorides4 ,this rule however may perhaps be liable to an exception when artificial light is used1 which contains a greater proportion of the rays of small refrangibility1 known to act more powerfully upon the ,bromide than upon the ,iodide of ,silver 7p4 #ff74 g4 ,density of the sensitive film4--,when the proportion of soluble ,iodide in the iodizing solution is too great1 the film is very dense1 and the ,iodide of ,silver is apt to burst out upon the surface1 and fall away in loose flakes into the ,bath4 ,this condition1 which is highly unfavourable to sensitiveness1 is very common in ,collodion1 and constitutes what is termed over-iodizing4 ,the ,iodide1 in fact1 is formed in such a case too much upon the surface1 and consequently1 when the fixing agent is applied1 the image not being retained by the film1 is washed off and lost4 ,on the other hand1 the sensibility of the film is not lessened by reducing the amount of ,iodide in ,collodion to a minimum1 if all the solutions are neutral2 but the pale blue films formed by a dilute ,collodion1 and which almost rival the ,daguerreotype itself in delicacy1 are nearly useless in practice2 for if free ,iodine or other bodies of a retarding nature are present in any quantity1 either in the ,collodion or in the ,bath1 they almost destroy the action of a weak light1 producing a far more injurious effect than if the film were more yellow and opaque4 h4 ,impurities in ,ether and ,alcohol4--,pure ,ether should be neutral to test-paper1 but the commercial samples of this article have usually either an acid or an alkaline reaction4 ,the frequent occurrence of a peculiar oxidizing principle in ,ether has also been pointed out 7p4 #he74 ,each of these three conditions is injurious to sensitiveness2 the first and last by liberating ,iodine when alkaline ,iodides are used2 and the second1 by producing ,iodoform under the same circumstances4 ,in this case the ,collodion remains colourless1 but gives inferior results4 ,the ,author has also observed that ,ether which has been redistilled from the residues of ,collodion may contain a volatile principle 7probably a compound ,ether87 which produces a retarding effect upon the action of light4 ,commercial ,spirit of ,wine is not always uniform in composition1 as sufficiently evidenced by the test of smell4 ,it may contain fusel oil or other volatile substances1 which become milky on dilution with water1 and are believed to injure the quality of the spirit for ,photographic use4 i4 ,relative proportions of ,ether and ,alcohol in ,collodion4--,it was shown at p4 #hd that the addition of ,alcohol to ,collodion lessens the contractility of the film1 and renders it soft and gelatinous4 ,this condition is favourable to the formation of the invisible image in the ,camera1 the play of affinities being promoted by the loose manner in which the particles of ,iodide are held together4 ,it is therefore usual to add to ,collodion as much ,alcohol as it will bear without becoming glutinous1 or leaving the glass2 the exact quantity required varying with the strength of the spirit or its freedom from dilution with water4 k4 ,decomposition in the ,collodion4--,collodion iodized with the metallic ,iodides generally1 excepting the ,iodide of ,cadmium1 becomes brown and loses sensitiveness in the course of a few days or weeks4 ,if the free ,iodine1 the cause of the brown colour1 be removed1 the greater part1 but not the whole1 of the sensitiveness is regained4 ,the experiments of the ,author1 and of others1 have proved that a solution of ,pyroxyline in contact with an unstable iodide1 slowly undergoes decomposition1 the result of which is that ,iodine is set free1 and an equivalent quantity of the base remains in union with certain organic elements of the ,collodion4 ,decomposition also gradually ensues when iodized ,collodion is placed in contact with reducing agents1 such as ,proto-iodide of ,iron1 ,gallic ,acid1 ,grape ,sugar1 ,glycyrrhizine1 etc41 so that these combinations do not retain a constant sensibility for any length of time4 ,even plain ,collodion uniodized cannot be preserved many months without a small but perceptible amount of change4 l4 ,decomposition in the ,nitrate ,bath4--,a ,collodion ,nitrate ,bath which has been much used1 often gives a less sensitive film than when newly made4 ,it is known also that many organic substances which reduce ,nitrate of ,silver1 if added to the ,bath1 produce a state which is favourable to sensitiveness whilst the decomposition is taking place1 but is eventually unfavourable2 hence the solution will be injured by adding either ,gallic or ,pyrogallic ,acid1 and by organic matters generally if exposed to light4 ,recapitulation4--,the conditions most favourable to extreme sensitiveness of the ,iodide of ,silver on ,collodion may be condensed as follows3--perfect neutrality of the solutions employed2 a soft1 gelatinous state of the film2 absence of ,chlorides and other salts which precipitate ,nitrate of ,silver2 an undecomposed ,collodion1 containing no organic matter of that kind which is precipitated by basic ,acetate of ,lead1 and combines with oxides of ,silver4 ,,the ,,conditions ,,which ,,affect ,,the ,,development ,,of ,,the ,,latent ,,image4 ,the general theory of the development of a latent image by means of a reducing agent1 having been simply explained in the third ,chapter1 may now be more fully examined in its application to the ,iodide of ,silver on ,collodion4 a4 ,the presence of free ,nitrate of ,silver essential to the development4--,this subject has already been mentioned 7p4 #cf74 ,a sensitive ,collodion plate1 carefully washed in distilled water1 is still capable of receiving the radiant impression in the ,camera1 but it does not admit of development until it has been redipped in the ,bath1 or treated with a reducing agent to which ,nitrate of ,silver has been added3 and if the proportion of free ,nitrate of ,silver on a ,collodion film be too small1 the image will be feeble or altogether imperfect in parts1 with patches of green or blue1 due to deficient reduction4 b4 ,comparative strength of deducing ,agents4--,no increase of power in a developer will suffice to bring out a perfect image on an under-exposed plate1 or upon a film containing too little ,nitrate of ,silver4 ,but there is considerable difference in the length of time which the various developers require to act4 ,gallic ,acid is the most feeble1 and ,pyrogallic ,acid the strongest1 producing at least four times more effect than an equal weight of the crystallized ,protosulphate of ,iron1 and twenty times more than the ,protonitrate of ,iron4 c4 ,the effect of free ,acid upon the development4--,acids tend to retard the reduction of the image as well as to diminish the sensibility of the film to light4 ,nitric ,acid especially does so1 from its powerful oxidizing and solvent properties4 ,the effect of ,nitric ,acid is particularly seen when the film of ,iodide of ,silver is very blue and transparent1 and the quantity of ,nitrate of ,silver retained upon its surface small4 ,under such circumstances the proper development of the image may be suspended1 and spangles of metallic ,silver separate4 ,this indicates that the quantity of the acid should be diminished1 or the strength of the ,nitrate ,bath and of the reducing agent be increased1 as a counterpoise to the retarding action of acid upon the development4 ,acetic ,acid also moderates the rapidity of development1 but it has not that tendency altogether to suspend it possessed by ,nitric ,acid4 ,it is therefore usefully employed1 to enable the operator to cover the plate evenly with liquid before the development commences1 and to preserve the white parts of the impression from any accidental deposit of metallic ,silver due to irregular action of the reducing agent4 ,on comparing the retarding effects of free acid upon the light's action1 and upon the development1 we see that the former is the most marked1--that a small quantity of ,nitric ,acid produces a more decided influence upon the impression of the image in the ,camera than upon the bringing out of that image by means of a developer4 d4 ,accelerating effect of certain organic matters4--,organic bodies1 like ,albumen1 ,gelatine1 ,glycyrrhizine1 etc41 which combine chemically with oxides of ,silver1 and were shown in the last ,section to lessen the sensitiveness of the ,iodide film1--facilitate the development of the image1 producing often a dense deposit of a brown or black colour by transmitted light4 ,in the same way1 viz4 by a retention of organic matter1 may partly be explained the fact1 that the image developed by ,pyrogallic ,acid1 although proved by the application of tests to contain no more than an equal quantity of ,silver1 possesses greater opacity by transmitted light1 than that resulting from the use of protosalts of ,iron3 and in the case of the ,collodion itself the same rule applied--if it be pure1 it is liable to give a less vigorous impression than when by long keeping a partial decomposition has taken place1 and products have been formed which combine with reduced ,oxide of ,silver more easily than the unaltered ,pyroxyline4 e4 ,molecular conditions affecting ,intensity4--,the physical structure of the ,collodion film is thought to exert an influence upon the mode in which the reduced ,silver is thrown down during the development4 ,a short and almost powdery state1 such as ,collodion iodized with the alkaline iodides acquires by keeping1 is considered favourable1 and a glutinous1 coherent structure unfavourable1 to density4 ,this is certainly the case when the film is allowed to dry before development1 as in the process with desiccated ,collodion and1 to some extent1 in the ,oxymel preservative process4 ,the mode of conducting the development also affects the density2 a rapid action tending to produce an image of which the particles are finely divided and offer a considerable resistance to the passage of light1 whilst a slow and prolonged development often leaves a metallic and almost crystalline deposit1 comparatively translucent and feeble4 ,the writer has observed1 that with certain samples of ,collodion the image is much enfeebled by keeping the plate for a considerable time1--a quarter of an hour or longer1--after sensitizing1 but before development4 ,this effect is not the result of the ,nitrate of ,silver having partially drained away1 since a second dip in the ,nitrate ,bath immediately before applying the ,pyrogallic ,acid1 does not remedy it4 ,an alteration of molecular structure may therefore be the correct explanation1 and if so1 a contractile ,collodion would suffer more than one possessing less coherency4 ,the actinic power of the light at the time of taking the picture1 influences the appearance of the developed image2 the most vigorous impressions being produced by a strong light acting for a short time4 ,on a dull dark day1 or in copying badly lighted interiors1 the photograph will often lack bloom and richness1 and be blue and inky by transmitted light4 f4 ,development of images upon ,bromide and ,chloride of ,silver4--,of the three principal ,salts of ,silver1 the ,iodide is the most sensitive to light1 but the ,bromide and ,chloride1 under some conditions1 are more easily developed and give a darker image4 ,in the ,collodion process the difference is principally seen when organic bodies1 like ,grape ,sugar1 ,glycyrrhizine1 etc41 are introduced in order to increase the intensity2 a far more decided effect being produced by adding both ,glycyrrhizine and a portion of ,bromide or ,chloride1 than by using the ,glycyrrhizine alone4#ae #ae ,see the ,author's ,paper on the chemical composition of the photographic image1 in the eighth ,chapter4 g4 ,the intensity of the image affected by the length of exposure4--,this point has been briefly alluded to in the third ,chapter4 ,if the exposure in the ,camera be prolonged beyond the proper time the development takes place rapidly but without any intensity1 the picture being pale and translucent4 ,the effects produced by over-action of the light are particularly seen when the ,nitrate ,bath contains ,nitrite of ,silver1 or ,acetate of ,silver2 the image being frequently in such a case dark by reflected light1 and red by transmitted light1--more nearly resembling in fact a photographic print1 developed on paper prepared with ,chloride of ,silver4 ,when ,collodion plates are coated with honey without previously removing the free ,nitrate of ,silver1 a slow reducing action is set up1 which may give rise to the characteristic appearance above referred to1 after development4 ,other organic substances1 such as biliary matter1 etc41 will act in the same way4 h4 ,certain conditions of the ,bath which affect development4--,attention may be called to a peculiar state of the ,nitrate ,bath1 in which the ,collodion image developes unusually slowly1 and has a dull grey metallic appearance1 with an absence of intensity in the parts most acted on by the light4 ,this condition1 which occurs only when using a newly mixed solution1 is thought by the ,author to depend upon the presence of an ,oxide of ,nitrogen retained by the ,nitrate of ,silver4 ,it is removed partially by neutralizing the ,bath with an alkali1 more perfectly so by adding an excess of alkali followed by ,acetic ,acid2 but most completely by carefully fusing the ,nitrate of ,silver before dissolving it4 ,commercial ,nitrate of ,silver has sometimes a fragrant smell1 similar to that produced by pouring strong ,nitric ,acid upon ,alcohol4 ,when such is the case1 it contains organic matter1 and produces a ,bath which yields red and misty pictures4 ,nitrate of ,silver which has been sufficiently strongly fused to decompose the ,salt1 and produce a portion of the basic ,nitrite of ,silver exhibits great peculiarity of development1 the image coming out instantaneously and with great force4 ,this condition is exactly the reverse of that produced by the presence of acids1 in which the development is slow and gradual4 ,in summing up the different conditions of the ,nitrate ,bath which affect the development of the image1 as many as four might be mentioned1 each of which gives a more rapid reduction than the one which precedes it4 ,these are--the acid ,nitrate ,bath1 the neutral ,bath1 the ,bath of strongly fused ,nitrate of ,silver1 and the ,bath containing ,ammoniacal ,nitrate of ,silver1 which is quite unmanageable1 and produces an instantaneous and universal blackening of the film on the application of the developer4 ,greater intensity of image is commonly obtained in a ,nitrate ,bath which has been a long time in use1 than in a newly mixed solution3 this may be due to minute quantities of organic matter dissolved out of the ,collodion film1 which1 having an affinity for ,oxygen1 partially reduce the ,nitrate of ,silver2 and also to the accumulation of ,alcohol and ,ether in an old ,bath producing a short and friable structure of the film4 i4 ,effect of ,temperature on ,development4--,reduction of the oxides of noble metals proceeds more rapidly in proportion as the temperature rises4 ,in cold weather it will be found that the development of the image is slower than usual1 and that greater strength of the reducing agent and more free ,nitrate of ,silver is required to produce the effect4 ,on the other hand1 if the heat of the atmosphere be excessive1 the tendency to rapid reduction will be greatly increased1 the solutions decomposing each other almost immediately on mixing4 ,in this case the remedy will be to use ,acetic ,acid freely both in the ,bath and in the developer1 at the same time lessening the quantity of ,pyrogallic ,acid1 and omitting the ,nitrate of ,silver which is sometimes added towards the end of the development4 ,also in the case of films which are to be kept for a long time in a sensitive condition by means of honey1 etc41 the modifying influence of temperature must be observed1 and the quantity of free ,nitrate of ,silver left upon the film be reduced to a minimum if the thermometer stands higher than usual4 ,,section ,,iv4 ,on certain ,irregularities in the ,developing ,process4 ,the characteristics of the proper development of a latent image are--that the action of the reducing agent should cause a blackening of the ,iodide in the parts touched by light1 but produce no effect upon those which have remained in shadow4 ,in operating both on ,collodion and paper however there is a liability to failure in this respect2 the film beginning1 after the application of the developer1 to change in colour to a greater or less extent over the whole surface4 ,there are two main causes which produce this state of things3--the first being due to an irregularity in the action of the light2 the second to a faulty condition of the chemicals employed4 ,if from a defect in the construction of the instrument1 or from other causes which will be pointed out more particularly in the ,second ,part of this work1 diffused white light gains entrance into the ,camera1 it produces indistinctness of the image by affecting the ,iodide more or less universally4 ,the luminous image of the ,camera not being perfectly pure1 mere over-exposure of the sensitive plate will usually have the same effect4 ,in such a case1 when the developer is poured on1 a faint image first appears1 and is followed by a general cloudiness4 ,the clearness of the developed ,collodion picture is much influenced by the condition of all the solutions employed1 but particularly so by that of the ,nitrate ,bath4 ,if this liquid be in the state termed alkaline 7p4 #hh71 it will be impossible to obtain a good picture2 and even when neutral1 care and avoidance of all disturbing causes will be required to prevent a deposition of ,silver upon the shadows of the image3 especially so when ,nitrite of ,silver or ,acetate of ,silver are present1 both of these salts being more easily reduced than the ,nitrate of ,silver4 ,the use of ,acid is the principal resource in obviating cloudiness of the image4 ,acids lessen the facility of reduction of the ,salts of ,silver by developing agents 7p4 #ih71 and hence when they are present the metal is deposited more slowly1 and only on the parts where the action of the light has so modified the particles of ,iodide as to favour the decomposition3 whereas if acids be absent or present in insufficient quantity1 the equilibrium of the mixture of ,nitrate of ,silver and reducing agent which constitutes the developer is so unstable1 that any rough point or sharp edge is likely to become a centre from which the chemical action1 once started1 radiates to all parts of the plate4 ,various acids have been employed1 such as ,acetic acid1 ,citric acid1 ,tartaric acid1 etc4 ,nitric acid is the most effectual of all1 but is seldom used1 because1 although the image can often be developed with great clearness when the ,bath contains a small quantity of ,nitric acid1 yet such a condition is not favourable to intensity2 on the other hand1 films which are prone to irregular reduction1 such as those prepared in a chemically neutral bath or a bath containing ,acetate or ,nitrite of ,silver1 are likely to give the greatest vigour of impression4 ,hence1 when this quality is desired1 the use of ,nitric ,acid will be adopted cautiously4 ,the state of the ,collodion must be attended to as well as that of the bath2 it should be either acid or neutral1 not alkaline4 ,colourless ,collodion may be used successfully as a rule1 but sometimes a little free ,iodine is advantageously added4 ,care should be taken in introducing organic substances1 many of which dissolve out into the bath1 and spoil it for giving clear pictures4 ,glycyrrhizine1 however1 which is recommended to produce intensity of ,negatives1 has no effect of that kind1 and may be employed with safety4 ,the condition of the developing agent is a point of importance in producing clear and distinct pictures4 ,the ,acetic acid1 which is advised in the formul1 cannot be omitted or even lessened in quantity without danger4 ,this is particularly the case in hot weather or under any other condition which favours reduction1 such as neutrality of the bath1 etc42 at all times1 in fact1 when the solutions of ,pyrogallic acid and ,nitrate of ,silver decompose each other with unusual rapidity4 ,in addition to the points now mentioned1 viz4 the state of the ,bath1 of the ,collodion1 and of the developer1 the reader should also study the remarks made in the ,third ,section of ,chapter ,,iii4 on the effect of surface conditions in modifying the deposition of vapour and of metallic ,silver3 he will then in all probability experience but little difficulty in dealing with those numerous irregularities in the action of the developing fluid1 which often prove the greatest hindrance to the successful practice of the ,collodion process4 ,,chapter ,,vii4 ,,on ,,positive ,,and ,,negative ,,collodion ,,photographs4 ,the terms ,positive and ,negative occur so frequently in all works upon the subject of ,photography1 that it will be impossible for the student to make progress without thoroughly understanding their meaning4 ,a ,positive may be defined to be a ,photograph which gives a natural representation of an object1 as it appears to the eye4 ,a ,negative ,photograph1 on the other hand1 has the lights and shadows reversed1 so that the appearance of the object is changed or negatived4 ,in ,photographs taken upon ,chloride of ,silver1 either in the ,camera or by superposition1 the effect must necessarily be ,negative2 the ,chloride being darkened by luminous rays1 the lights are represented by shadows4 ,the following simple diagrams will make this obvious4 ,illustration3 ,fig4 #a4 ,fig4 #b4 ,fig4 #c4 ,fig4 #a is an opaque image drawn upon a transparent ground2 fig4 #b represents the effect produced by placing it in contact with a layer of sensitive ,chloride and exposing to light2 and fig4 #c is the result of copying this negative again on ,chloride of ,silver4 ,fig4 #c therefore is a ,positive copy of ,fig4 #a1 obtained by means of a ,negative4 ,by the first operation the tints are reversed2 by the second1 being reversed again1 they are made to correspond to the original4 ,the possession of a ,negative therefore enables us to obtain ,positive copies of the object1 indefinite in number and all precisely similar in appearance4 ,this capability of multiplying impressions is of the utmost importance1 and has rendered the production of good ,negative ,photographs of greater consequence than any other branch of the ,art4 ,the same ,photograph may often be made to show either as a ,positive or as a ,negative4 ,for instance1 supposing a piece of silver-leaf to be cut into the shape of a cross and pasted on a square of glass1 the appearance presented by it would vary under different circumstances4 ,illustration3 ,fig4 #a4 ,fig4 #b4 ,fig4 #a represents it placed on a layer of black velvet2 fig4 #b as held up to the light4 ,if we term it ,positive in the first case1 i4 e4 by reflected light1 then it is ,negative in the second1 that is1 by transmitted light4 ,the explanation is obvious4 ,therefore to carry our original definition of ,positives and ,negatives a little further1 we may say1 that the former are usually viewed by reflected1 and the latter by transmitted1 light4 ,all ,photographs however cannot be made to represent both ,positives and ,negatives4 ,in order to possess this capability1 it is necessary that a part of the image should be transparent1 and the other opaque but with a bright surface4 ,these conditions are fulfilled when the ,iodide of ,silver upon ,collodion is employed1 in conjunction with a developing agent4 ,every ,collodion picture is to a certain extent both ,negative and ,positive1 and hence the processes for obtaining both varieties of ,photographs are substantially the same4 ,although however the general characters of a ,positive and a ,negative are similar1 there are some points of difference4 ,a surface which appears perfectly opaque when looked down upon1 becomes somewhat translucent on being held up to the light2 hence1 to give the same effect1 the deposit of metal in a ,negative must be proportionally thicker than in a positive2 otherwise the minor details of the image1 will be invisible1 from not obstructing the light sufficiently4 ,with these preliminary remarks1 we are prepared to investigate more closely the rationale of the processes for obtaining ,collodion ,positives and ,negatives4 ,all that refers to paper ,positives upon ,chloride of ,silver will be treated in a subsequent ,chapter4 ,,section ,i4 ,on ,collodion ,positives4 ,collodion ,positives are sometimes termed direct1 because obtained by a single operation4 ,the ,chloride of ,silver1 acted upon by light alone1 is not adapted to yield direct ,positives1 the reduced surface being dark and incapable of representing the lights of a picture4 ,hence a developing agent is necessarily employed1 and the ,iodide of ,silver substituted for the ,chloride1 as being a more sensitive preparation4 ,collodion ,positives are closely allied in their nature to ,daguerreotypes4 ,the difference between the two consists principally in the surface used to sustain the sensitive layer1 and the nature of the substance by which the invisible image is developed4 ,in a ,collodion ,positive the lights are formed by a bright surface of reduced ,silver1 and the shadows by a black background showing through the transparent portions of the plate4 ,two main points are to be attended to in the production of these ,photographs4 ,first1 to obtain an image distinct in every part1 but of comparatively small intensity4--,if the deposit of reduced metal be too thick1 the dark background is not seen to a sufficient extent1 and the picture in consequence is deficient in shadow4 ,secondly1 to whiten the surface of the reduced metal as much as possible1 in order to produce a sufficient contrast of light and shade4 ,iodide of ,silver developed in the usual way presents a dull yellow appearance which is sombre and unpleasing4 ,the ,collodion and ,nitrate ,bath for ,positives4--,good ,positives may be obtained by diluting down a sample of ,collodion with ,ether and ,alcohol until it gives a pale bluish film in the ,bath4 ,the proportion of ,iodide of ,silver being in that case small1 the action of the high lights is less violent1 and the shadows are allowed more time to impress themselves4 ,the dilution lessens the amount of ,pyroxyline in the ,collodion at the same time with the ,iodide1 which is an advantage1 the slight and transparent films always giving more sharpness and definition in the picture4 ,the employment of a very thin film for ,positives is not however always a successful process4 ,the particles of the ,iodide of ,silver being closely in contact with the glass1 unusual care is required in cleaning the plates in order to avoid stains2 and the amount of free ,nitrate of ,silver retained upon the surface of the film being small1 circular patches of imperfect development are liable to occur1 unless the reducing agent be scattered evenly and perfectly over the surface4 ,also if free ,iodine or organic substances which have a retarding effect on the action of light are present to a considerable extent1 the ,collodion will not work well with a small proportion of ,iodide4 ,the ,author found in experimenting on this subject that with perfectly pure ,collodion and a neutral ,bath most vigorous impressions were produced when the density of the film had been so far reduced by dilution that scarcely anything could be seen upon the glass2 but with ,collodion strongly tinted with ,iodine1 or with a ,bath containing ,nitric ,acid1 it was necessary to stop the dilution at a certain point or the film became absolutely insensitive to feeble radiations of light1 and the shadows could not be brought out by any amount of exposure4 ,in this case1 by adding more ,iodide a better effect was obtained4 ,a thicker ,collodion may be used for positives if a little free ,iodine be added1 for the purpose of diminishing intensity and keeping the shadows clear during the development4 ,this process is easier to practice than the last1 but does not always give the same perfect definition4 ,no organic substance of the class to which ,glycyrrhizine and ,sugars belong should be added to ,collodion which is to be used for ,positives4 ,by so doing the image would be rendered intense1 and the high lights liable to solarization1 id est1 a dark appearance by reflected light4 ,the ,nitrate ,bath4--,if the materials are pure1 the ,nitrate ,bath may advantageously be diluted down at the same time with the ,collodion1 when ,positives are to be taken2 but the employment of a very weak ,nitrate ,bath 7such as one of #bj grains to the ounce71 although highly useful in obviating excess of development1 has some disadvantages2 it becomes necessary to exclude free ,nitric ,acid1 and to avoid the employment of a ,collodion too highly tinted with ,iodine4 ,on the other hand1 with a strong ,nitrate ,bath1 and a tolerably dense film of ,iodide of ,silver1 a better result is often secured by the use of ,nitric ,acid4 ,the sensitiveness of the plates is impaired1 but at the same time the intensity is diminished1 and the picture shows well upon the surface of the glass4 ,a new ,bath is better for taking ,positives than one which has been a long time in use4 ,the latter often causes haziness and irregular markings on the film during the action of the developer4 ,this is due partly to the accumulation of ,alcohol and ,ether in the ,bath1 which causes the solution of ,sulphate of ,iron to flow in an oily manner2 and partly to a reduction of the ,nitrate of ,silver by organic matter4 ,the presence of ,acetate of ,silver is objectionable in a ,positive ,nitrate ,bath as producing solarization and intensity of image2 hence those precautions which obviate its formation must be adopted 7p4 #hi74 ,if fused ,nitrate of ,silver be used for the ,positive ,nitrate ,bath1 it is very important that the fusion should not be carried too far1 or the solution would contain a basic ,nitrite of ,silver1 and yield an intense1 solarized1 and misty image4 ,the ,developers for ,collodion ,positives4--,pyrogallic ,acid when used with ,acetic ,acid1 as is usual for negative pictures1 produces a surface which is dull and yellow4 ,this may be obviated by substituting ,nitric ,acid in small quantity for the ,acetic4 ,the surface produced by ,pyrogallic ,acid with ,nitric ,acid is lustreless1 but very white1 if the solution be used of the proper strength4 ,on attempting to increase the amount of ,nitric ,acid the deposit becomes metallic1 and the half-tones of the picture are injured2 ,pyrogallic ,acid1 although an active developer1 does not allow of the addition of mineral acid to the same extent as the ,salts of ,iron4 ,it requires also1 when combined with ,nitric ,acid1 a fair proportion of ,nitrate of ,silver on the film1 or the development will be imperfect in parts of the plate4 ,sulphate of ,iron4--,the ,protosalts of ,iron were first employed in ,photography by ,mr4 ,hunt4 ,the ,sulphate is a most energetic developer1 and often brings out a picture when others would fail4 ,to produce by means of it a dead white tint with absence of metallic lustre1 it may be used in conjunction with ,acetic ,acid1 and in a somewhat concentrated condition1 so as to develope the picture quickly4 ,the addition of ,nitric ,acid to ,sulphate of ,iron modifies the development1 making it more slow and gradual1 and producing a bright sparkling surface of reduced ,silver4 ,too much of this acid however must not be used1 or the action will be irregular4 ,the ,nitrate ,bath also must be tolerably concentrated1 in order to compensate for the retarding effect of ,nitric ,acid upon the development4 ,the blue and transparent films of ,iodide of ,silver1 formed in a very dilute ,nitrate ,bath1 are not well adapted for ,positives to be developed in this way4 ,they are injured by the acid1 and the development of the image becomes imperfect4 ,protonitrate of ,iron4--,this salt1 first used by ,dr4 ,diamond1 is remarkable as giving a surface of brilliant metallic lustre without any addition of free acid4 ,theoretically1 it may be considered as closely corresponding to the ,sulphate of ,iron with ,nitric ,acid added4 ,there are however slight practical differences between them1 which are perhaps in favour of the ,protonitrate4 ,the reducing powers of ,protoxide of ,iron appear to be in inverse ratio to the strength of the acid with which it is associated in its salts2 hence the ,nitrate is by far the most feeble developer of the ,protosalts of ,iron4 ,the rules already given for the use of ,sulphate of ,iron acidified with ,nitric ,acid1 apply also to the ,nitrate of ,iron2 the proportion of free ,nitrate of ,silver must be large1 and the film of ,iodide of ,silver not too transparent4 ,in developing direct ,positives either by ,pyrogallic ,acid or the ,salts of ,iron1 the colour of the image will be found liable to some variation2 the character of the light1 whether bright or feeble1 and the length of exposure in the ,camera1 affecting the result4 ,a ,process for whitening the ,positive ,image by means of ,bichloride of ,mercury4--,in place of brightening the ,positive image by modifying the developer1 it was proposed some time since by ,mr4 ,archer to effect the same object by the use of the salt known as ,corrosive ,sublimate1 or ,bichloride of ,mercury4 ,the image is first developed in the usual way1 fixed1 and washed4 ,it is then treated with the solution of ,bichloride1 the effect of which is to produce almost immediately an interesting series of changes in colour4 ,the surface first darkens considerably1 until it becomes of an ash-grey1 approaching to black2 shortly it begins to get lighter1 and assumes a pure white tint1 or a white slightly inclining to blue4 ,it is then seen1 on examination1 that the whole substance of the deposit is entirely converted into this white powder4 ,the rationale of the reaction of ,bichloride of ,mercury appears to be1 that the ,chlorine of the mercurial salt divides itself between the ,mercury and the ,silver1 a portion of it passing to the latter metal and converting it into a ,protochloride4 ,the white powder is therefore probably a compound salt1 as is further evidenced by the effects produced on treating it with various reagents4 ,,section ,,ii4 ,on ,collodion ,negatives4 ,as in the case of a direct ,positive we require an image which is feeble though distinct1 so1 on the other hand1 for a negative1 it is necessary to obtain one of considerable intensity4 ,in the ,chapter immediately following the present1 it will be shown that in using glass ,negatives to produce ,positive copies upon ,chloride of ,silver paper1 a good result cannot be secured unless the ,negative is sufficiently dark to obstruct light strongly4 ,the ,collodion and ,nitrate ,bath for ,negatives4--,a ,collodion containing a very small portion of ,iodide and yielding a blue transparent film in the ,bath is not well adapted for taking ,negatives4 ,pale opalescent films often give too little intensity in the high lights1 and1 unless the ,nitrate ,bath be acid1 do not admit of being exposed in the ,camera for the proper length of time without cloudiness and indistinctness of image being produced under the action of the developer4 ,the effect known as solarization of negatives1 i4 e4 a red and translucent appearance of the highest lights1 is also more liable to occur when operating with a very pale film4 ,on the other hand1 if the layer of ,iodide be too yellow and creamy1 the half-tones of the image will often be imperfectly developed1 so that a middle point between these extremes is the best4 ,a pure and newly prepared ,collodion1 although highly sensitive to light1 does not always give1 with one application of the developer1 a sufficiently vigorous image to serve as a negative matrix2 and this particularly in the most brightly illuminated parts1 such as the sky in a landscape photograph1 or the white borders of an engraving4 ,but on keeping the ,collodion for some weeks or months it becomes yellow1 if iodized with the alkaline iodides1 and a decomposition takes place in it1 as before shown 7p4 #ig71 which lessens the rapidity of action1 but adds to the intensity of the negative4 ,grape ,sugar may be employed for the purpose of giving intensity to newly mixed ,collodion3 also ,glycyrrhizine1 which is a resinous body extracted from the root of ,liquorice2 but as both substances have an effect in lessening the sensitiveness and keeping qualities of the fluid1 they should be used cautiously4 ,in taking portraits in the open air1 on bright days1 and with a ,bath which has been mixed for a considerable time1 it will rarely be found that the intensity will be deficient2 and especially so if the developer be applied a second time to the film with a few drops of solution of ,nitrate of ,silver added4 ,in landscape ,photography however1 or in copying engravings1 where extreme sensitiveness is not an object1 the ,glycyrrhizine may sometimes be added with advantage in order to obtain perfect opacity of the blacks4 ,when the use of this substance is resorted to1 the mode of iodizing the ,collodion appears to be of importance1 the increase of intensity being greater with the ,iodide of ,cadmium than with the ,iodides of the ,alkalies2 the latter probably exercising a decomposing action4 ,an addition of a ,bromide or a ,chloride to the ,collodion in small quantity has also a marked effect in adding to the intensity when ,glycyrrhizine is used with alkaline ,iodides 7p4 #aja74 ,substances which produce intensity of the ,collodion image have often1 if added in too large quantity1 a tendency to lower the half-tone1 and prevent the darker parts of the picture from being sufficiently brought out4 ,the print from the ,negative is then pale and white1 or chalky as it is termed1 in the high lights4 ,collodion in this condition is often preferred by the beginner1 from the facility with which the ,negatives are obtained1 but it does not give the finest results4 ,an excess of ,glycyrrhizine in ,collodion has also the effect of interfering with the precipitation of the ,iodide of ,silver1 producing a blue and smoky film which is nearly useless for ,negatives4 ,a judicious employment of free ,iodine in ,collodion which has been previously intensified with ,glycyrrhizine1 has a remarkable effect in improving the gradation of tone4 ,the excessive opacity of the high lights is diminished1 and hence the operator is enabled by a longer exposure of the sensitive plate to bring out the shadows and minor details of the image with great distinctness4 ,collodion prepared in this manner is too slow to be used for portraits1 excepting in a strong light1 but often gives an image with great roundness and stereoscopic effect4 ,the ,iodine and the liquorice sugar employed conjointly1 tend also to preserve the clearness of the plates under the influence of the developer1 and to give sharpness to the lines and dots of engravings1 etc41 which1 with a new and sensitive ,collodion1 are often imperfectly rendered4 ,these advantages will be appreciated by the operator who has failed from working with a too feeble ,collodion2 but it must be borne in mind1 that all substances acting as intensifiers have a bad effect when the state of the film is not such as to call for their employment4 ,the ,proto-iodide of ,iron has been recommended as an addition to ,negative ,collodion4 ,in the ,nitrate ,bath it forms1 in addition to ,iodide of ,silver1 ,protonitrate of ,iron1 an unstable substance and a developer4 ,the use of ,iodide of ,iron gives great sensibility1 but it is difficult to preserve it pure and unchanged4 ,it also decomposes the ,collodion in the course of a few hours1 becoming itself peroxidized1 and producing an insensitive condition of film4 ,in addition to this1 the negatives taken by the aid of ,iodide of ,iron are commonly of an inferior kind1 the reduction being too marked in the high lights2 so that its employment is of doubtful utility4 ,the ,nitrate ,bath4--,this should be prepared from ,nitrate of ,silver which has been melted at a moderate heat 7see pp4 #ac and #aja74 ,if this point be neglected1 the best ,collodion will sometimes fail in producing an intense negative4 ,acetic ,acid must be added in minute quantity1 to preserve the solution from a too ready reduction by the ,alcohol and ,ether of the ,collodion4 ,also1 unless the ,nitrate of ,silver be quite pure and free from organic matter 7p4 #ajd71 clear pictures will not be obtained without the use of ,acid4 ,acetate of ,silver has often been advised as an addition to the ,negative ,nitrate ,bath4 ,it is produced by dropping into the solution an alkali1 such as ,ammonia1 followed by ,acetic ,acid in excess4 ,the ,negatives are rendered blacker and more vigorous by this proceeding1 but especially so when the ,bath is contaminated with ,nitric ,acid2 which neutralizes itself at the expense of the ,acetate of ,silver1 thus3-- ,acetate of ,silver ,nitric ,acid ,nitrate of ,silver ,acetic ,acid4 ,as a rule1 it will be better to avoid adding ,acetate of ,silver to the ,bath1 since with1 pure melted ,nitrate of ,silver no ,nitric ,acid can be present1 and perfect intensity is easily obtained4 ,when the ,bath is saturated with ,acetate of ,silver1 it is in a more reducible state1 and hence unless the glass plates are very perfectly cleaned1 black lines and markings1 the results of irregular action1 will be produced on the application of the developer to the film 7p4 #ajd74 ,solarization1 or reddening by over-exposure1 is also promoted by the presence of ,acetate of ,silver4 ,developing solutions for ,negatives4--,the ,protosalts of ,iron are not usually employed in developing ,negative impressions4 ,they are liable to yield a violet-coloured image1 which cannot easily be rendered more intense by continuing the action4 ,gallic ,acid is too feeble for developing ,collodion pictures4 ,pyrogallic ,acid is much superior1 and may be used of any strength1 according to the effect desired4 ,when the light is bad1 the temperature low1 and the ,negative developes slowly and appears blue and inky by transmitted light1 the proportion of the reducing agent should be increased4 ,but with an intense ,collodion1 on a clear summer's day1 the finest gradation is obtained with a weak solution1 which does not begin to act until the plate has been evenly covered4 ,a strong developer might in such1 a case produce too much opacity in the highest lights1 and would probably occasion stains of irregular reduction4 ,modes of strengthening a finished impression which is too feeble to be used as a ,negative4--,the ordinary plan of pushing the development cannot be applied with advantage after the picture has been washed and dried4 ,in that case1 if it is found to be too feeble to print well1 its intensity may be increased by one of the following methods4-- ,it must be premised however1 that the same degree of excellence is not to be expected in a ,negative ,photograph which has been improperly developed in the first instance and more especially if the exposure to light was too short4 ,any instantaneous ,positive may be rendered sufficiently intense for a ,negative1 but in that case the shadows are almost invariably imperfect4 #a4 ,treatment of the image with ,sulphuretted ,hydrogen or ,hydrosulphate of ,ammonia4--,the object is to convert the metallic ,silver into ,sulphuret of ,silver1 and if this could be done it would be of service4 ,the mere application of an ,alkaline ,sulphuret has however but little effect upon the image1 excepting to darken its surface and destroy the ,positive appearance by reflected light2 the structure of the metallic deposit being too dense to admit of the ,sulphur reaching its interior4 ,professor ,donny 7',photographic ,journal1' vol4 i47 proposes to obviate this by first converting the image into the white ,salt of ,mercury and ,silver by the application of ,bichloride of ,mercury1 and afterwards treating it with solution of ,sulphuretted ,hydrogen or ,hydrosulphate of ,ammonia4 ,negatives produced in this way are of a brown-yellow colour by transmitted light1 and opaque to chemical rays to an extent which would not1 priori1 have been anticipated4 #b4 ,,mm4 ,barreswil and ,davanne's process4--,the image is converted into ,iodide of ,silver by treating it with a saturated solution of ,iodine in water4 ,it is then washed--to remove the excess of ,iodine1--exposed to the light1 and a portion of the ordinary developing solution1 mixed with ,nitrate of ,silver1 poured over it4 ,the changes which ensue are precisely the same as those already described2 the whole object of the process being to bring the metallic surface back again into the condition of ,iodide of ,silver modified by light1 that the developing action may be commenced afresh1 and more ,silver deposited from the ,nitrate in the usual way4 #c4 ,the process with ,bichloride of ,mercury and ,ammonia4--,the image is first converted into the usual white double ,salt of ,mercury and ,silver by the application of a solution of the ,corrosive ,sublimate4 ,it is then treated with ,ammonia1 the effect of which is to blacken it intensely4 ,probably the alkali acts by converting ,chloride of ,mercury into the black ,oxide of ,mercury4 ,in place of ,ammonia1 a dilute solution of ,hyposulphite of ,soda or ,cyanide of ,potassium may be used1 with very similar results4 ,,chapter ,,viii4 ,,on ,,the ,,theory ,,of ,,positive ,,printing4 ,the subject of ,collodion ,negatives having been explained in the previous ,chapter1 we proceed to show how they may be made to yield an indefinite number of copies with the lights and shadows correct as in nature4 ,such copies are termed ,positives1 or sometimes ,positive prints1 to distinguish them from direct ,positives upon ,collodion4 ,there are two distinct modes of obtaining photographic prints2--first by development1 or1 as it is termed1 by the ,negative process1 in which a layer of ,iodide or ,chloride of ,silver is employed1 and the invisible image developed by ,gallic ,acid2 and second1 by the direct action of light upon a surface of ,chloride of ,silver1 no developer being used4 ,these processes1 involving chemical changes of great delicacy1 require a careful explanation4 ,the action of light upon ,chloride of ,silver was described in ,chapter ,,ii4 ,it was shown that a gradual process of darkening took place1 the compound being reduced to the condition of a coloured subsalt2 also1 that the rapidity and perfection of the change were increased by the presence of excess of ,nitrate of ,silver1 and of organic matters1 such as ,gelatine1 ,albumen1 etc4 ,we have now to suppose that a sensitive paper has been prepared in this way1 and that a ,negative having been laid in contact with it1 the combination has been exposed to the agency of light for a sufficient length of time4 ,upon removing the glass1 a ,positive representation of the object will be found below1 of great beauty and detail4 ,now if this image were in its nature fixed and permanent1 or if there were means of making it so1 without injury to the tint1 the production of ,paper ,positives would certainly be a simple department of the ,photographic ,art2 for it will be found that with almost any ,negative1 and with sensitive paper however prepared1 the picture will look tolerably well on its first removal from the printing-frame4 ,immersion in the bath of ,hyposulphite of ,soda however1 which is essentially necessary in order to fix the picture1 produces an unfavourable effect upon the tint2 decomposing the violet-coloured ,subchloride of ,silver1 and leaving behind a red substance which appears to be united to the fibre of the paper1 and1 when tested1 reacts in the manner of a ,suboxide of ,silver4 ,other chemical operations are therefore required to remove the objectionable red colour of the print1 and hence the consideration of the subject is naturally divided into two parts2 first1 the means by which the paper is rendered sensitive1 and the image impressed upon it2--and secondly1 the subsequent fixing and toning1 as it may be termed1 of the proof4 ,the present ,chapter will also include1 in two additional ,sections1 a condensed account of the most important facts relating to the properties and the mode of preservation of photographic prints4 ,,section ,i4 ,the ,preparation of the ,sensitive ,paper4 ,in this ,section the general theory of the preparation of ,positive paper1 in so far as it affects the tone and intensity of the print1 will be described2 the reader being referred to the second division of the ,work for the formul required4 ,the ,preparation of the ,sensitive ,paper4--,the conditions which are required for producing a sharp and well defined print are--that an even layer of ,chloride of ,silver should exist upon the very surface of the paper1 and that the particles of this ,chloride should be in contact with a sufficient excess of ,nitrate of ,silver4 ,these points have been already referred to at an early part of the ,work 7p4 #ai74 ,the material used for sizing the paper is of importance4 ,english papers are usually sized with ,gelatine1 which is a photographic agent1 and acts chemically in forming the image4 ,foreign papers on the other hand being sized with starch only1 require an addition of ,gelatine1 ,caseine1 or ,albumen1 to retain the ,salt at the surface of the paper1 and to assist in producing the picture3 if otherwise1 the print will be flat and mealy1 as it is termed4 ,albumen especially produces a beautifully smooth surface1 and is advantageously employed in printing small portraits and stereoscopic subjects4 ,the uniform surface distribution of the ,chloride of ,silver is sometimes interfered with by a faulty structure of the paper1 causing it to absorb liquids unevenly1 and in consequence the pictures1 when removed from the printing frame1 appear spotted4 ,another cause producing the same effect1 is the employment of too weak a solution of ,nitrate of ,silver1 or the removal of the sheet from the ,nitrate bath before the ,chloride of ,ammonium has been perfectly decomposed2 it is thus rendered unequally sensitive at different portions of the surface1 and the prints have the characteristic marbled appearance above referred to4 ,a sufficient excess of ,nitrate of ,silver being essential1 it is important to bear in mind1 that the quantity of this salt eventually remaining in the paper1 is much influenced by the manner in which the solution is applied4 ,if it be laid on by floating1 then the proportion of ,nitrate to that of ,chloride of ,sodium should be about as #c to #a 7the atomic weights are nearly as #e to #b72 but if the plan of brushing or spreading with a glass rod be adopted1 #g to #a or #h to #a will not be too much4 ,the ,darkening of the ,sensitive ,paper by ,light4--,the operator should be familiar with the changes of colour which indicate the progress of the reduction of the sensitive layer4 ,much in this respect depends upon the kind of organic matter used1 but there is always a regular sequence of tints2 in the case of a paper prepared simply with ,chloride of ,ammonium and ,nitrate of ,silver1 it is as follows3 pale violet1 violet-blue1 slate-blue1 bronze or copper-colour4 ,when the bronzed stage is reached1 there is no further change4 ,on immersion in the fixing bath of ,hyposulphite1 the violet tones due to ,subchloride of ,silver are destroyed1 and the print assumes a red or brown colour1 which is deepest and most intense in the parts where the light has acted longest4 ,hence we see1 that1 to produce a good print1 it is essential that the ,negative should possess considerable intensity in the dark parts4 ,pale and feeble ,negatives yield proofs which are wanting in vigour1 and have a flat and indistinct appearance4 ,the combination cannot be exposed to light for a sufficient length of time to bring about the requisite degree of reduction of the ,chloride of ,silver2 and hence the deepest shadows of the resulting ,positive are not sufficiently dark1 and there is a want of contrast which is fatal to the effect4 ,a good ,negative should be so opaque as to preserve the lights of the printed image beneath clear1 until the darkest shades are about to pass into the bronze or coppery condition4 ,if the amount of intensity be less than this1 the finest effect cannot be obtained4 ,,conditions ,,affecting ,,the ,,sensitiveness ,,of ,,the ,,paper ,,and ,,the ,,intensity ,,of ,,the ,,image4 ,some of the principal of these are as follows3-- a4 ,the ,strength of the ,salting ,bath4--,the sensibility of the paper is regulated up to a certain point by the amount of salt#af used in the preparation4 ,the quantity of alkaline ,chloride determines the amount of ,chloride of ,silver2 and with a proper excess of ,nitrate of ,silver1 papers are to a certain point more sensitive in proportion as they contain more of the ,chloride4 #af ,the difference in the atomic weights of the various soluble ,chlorides used in salting must be borne in mind4 ,ten grains of ,chloride of ,ammonium contain as much ,chlorine as eleven of ,chloride of ,sodium1 or twenty-two grains of ,chloride of ,barium4 7,see the ,vocabulary1 ,part ,,iii47 ,highly sensitized papers darken rapidly1 and pass very completely into the bronze stage4 ,those containing less ,chloride darken more slowly1 and do not become bronzed with the same intensity of light4 ,a ,photographic print1 formed upon paper highly salted and sensitized1 is usually vigorous1 with great contrast of light and shade2 particularly so when the printing is conducted in a strong light4 ,hence it will be an advantage1 with a feeble ,negative1 and in dull weather1 to double the ordinary quantity of ,salt1 whereas in the case of an intense ,negative1 and with direct sunlight1 the deep shadows will be too much bronzed unless the quantity of ,chloride and ,nitrate of ,silver in the paper be kept low4 ,in proportion as ,photographic papers are highly salted and sensitized1 they become more prone to change colour spontaneously in the dark4 b4 ,proportion of ,nitrate of ,silver4--,the compound on which a positive print is formed is a ,chloride1 or an organic ,salt of ,silver1 with an excess of ,nitrate of ,silver4 ,nothing is gained by increasing the proportion of ,chloride of ,sodium1 unless at the same time an addition be made to the quantity of free ,nitrate in the sensitizing ,bath4 ,a surface of ,chloride of ,silver with a bare excess of ,nitrate1 darkens on exposure1 but it does not reach the bronzed stage2 the action appearing to stop at a certain point4 ,on placing the print in ,hyposulphite of ,soda1 it becomes very red and pale1 and when tinted1 looks cold and slaty1 without depth or intensity4 c4 ,the sensitiveness and intensity affected by substituting the ,oxide of ,silver for the ,nitrate4--,many operators employ a solution of ,oxide of ,silver in ,ammonia#ag or ,nitrate of ,ammonia1 in preparing ,chloride of ,silver paper4 ,by doing so1 a great increase of sensitiveness1 and also of intensity of image1 is obtained4 ,this will be understood if we remember that the action of light in producing the print is of a reducing nature4 ,hence the substitution of ,oxide for ,nitrate of ,silver facilitates the decomposition2 just as ,ammonio-,nitrate of ,silver is more readily reduced by ,gallic or ,pyrogallic ,acid than the simple ,nitrate 7see p4 #ca74 #ag ,the chemistry of ,ammonio-,nitrate of ,silver is explained in the ,vocabulary1 ,part ,,iii4 ,ammonio-,nitrate paper has the disadvantage of soon discolouring when kept2 but it is very serviceable in printing during the winter months4 ,the proportion of ,chloride in the salting ,bath may1 if desired1 be considerably reduced2 the intensity of action being greatly exalted by the use of the ,oxide of ,silver4 d4 ,employment of organic matters4--,those recommended in this work are--,albumen1 ,gelatine1 and ,iceland ,moss4 ,albumen adds much to the sensibility of the paper1 and gives very fine surface definition4 ,a less amount of ,chloride is required than in the case of plain paper simply salted1 the glutinous character of ,albuminous liquids causing more of the fluid to be retained upon the surface of the paper1 and the animal matter assisting the reduction4 ,by varying the proportion of salt1 both feeble and intense ,negatives may be printed successfully upon albuminized paper4 ,no process gives better results1 either as regards sensitiveness1 or in faithfully rendering all the finer details of the ,negative1 than the process with ,albumen4 ,iceland ,moss1 when boiled in water1 yields a mucilaginous liquid which is conveniently employed as a vehicle for ,chloride of ,silver2 it increases the sensitiveness of the paper and gives additional power of bronzing1 by assisting to reduce the free ,nitrate of ,silver4 ,many other organic matters1 tending to absorb oxygen1 would act in the same way4 ,gelatine is used in positive printing2 it is analogous to ,albumen in composition1 and1 like it1 forms a red compound with ,suboxide of ,silver4 ,it is serviceable in keeping the print at the surface of the paper1 but does not alter the sensibility or the general appearance of the finished picture so greatly as ,albumen4 e4 ,impurities in ,nitrate of ,silver4--,nitrate of ,silver used for ,photographic printing should be free from even a trace of ,protonitrate of ,mercury1 since it is known that the precipitation of ,chloride of ,mercury prevents the darkening of ,chloride of ,silver by light4 ,the peculiar condition of ,nitrate of ,silver spoken of at page #aja1 in which it is thought to contain ,oxides of ,nitrogen1 is likely to interfere with ,photographic printing4 ,this is probably the explanation of a faulty state of the ,nitrate solution1 in which it yields red and feeble positives1 and does not darken in colour in exciting albuminized paper4 ,the remedy will be1 to fuse the ,nitrate of ,silver at a moderate heat before dissolving it4 ,,the ,,colour ,,of ,,the ,,image ,,influenced ,,by ,,the ,,preparation ,,of ,,the ,,sensitive ,,paper4 ,this subject should be studied by those who desire to print with taste4 ,by introducing a few simple modifications into the mode of preparing the sensitive paper1 almost any variety of tint may be obtained4 ,the tendency of the toning process1 to which the print is afterwards to be submitted1 is to darken the colour1 and1 if gold be used1 to give a shade of blue4 ,hence1 if the ,positive be printed of a red tone1 it will change in the gold ,bath to a purple2 whereas if left1 after exposure to light and fixing1 of a dark brown or sepia tint1 it passes by toning into a pure black4 ,the ,positive should look warm and bright on its removal from the printing frame2 but the tint which remains after immersion in ,hyposulphite of ,soda is the proper colour of the simply fixed print4 ,the following points may be mentioned as affecting the colour and general appearance of the picture4 a4 ,the proportions of ,salt and ,nitrate of ,silver4--,highly salted and sensitized papers give a darker image than those which1 containing a small proportion of ,chloride of ,silver1 are less sensitive to light4 ,hence in printing upon paper weakly sensitized1 in order to bring out the finer details of a highly intense negative1 we find the image unusually red after fixing1 and of a brown or mulberry colour when toned4 ,the above remarks apply also in some degree to the strength of the ,nitrate ,bath1 and especially so when no organic matter excepting ,gelatine is employed1--in such a case the image will be darker after fixing1 if the proportion of free ,nitrate of ,silver be large4 b4 ,effect of ,oxide of ,silver on the colour4--,prints formed upon ,ammonio-,nitrate papers highly salted are of a sepia colour after fixing1 and usually of a pure black or a purple-black when toned4 ,with the increased facility of reduction by light afforded by use of ,oxide of ,silver1 there is also less redness in the print4 ,but if the quantity of salt used in preparing the paper be reduced to a minimum 7one grain to the ounce or less71 for the sake of economy or to improve the half-tone1 then the usual red colour returns1 and the ,positive is brown or purple after toning1 in place of black4 ,thus by employing a solution of ,oxide of ,silver1 the operator is enabled1 without the addition of organic matter1 to print ,positives of a pleasing variety of tint1 combined with a peculiar softness and delicacy1 which cannot easily be obtained with the simple ,nitrate of ,silver4 c4 ,the colour affected by organic matter4--,albumen is coagulated by ,nitrate of ,silver1 and forms a permanent gloss upon the paper4 ,the sensitive albuminized paper darkens in the sun to a chocolate-brown colour1 which becomes very red on immersion in the ,hyposulphite4 ,the finished prints are clear and transparent2 usually of a brown tone1 or with a shade of purple when the gold ,bath is newly made and active2 pure blacks are not easily obtained4 ,iceland ,moss affects the colour of the proof to a certain extent1 but less than ,albumen2 the finished prints are nearly black if the paper is highly salted4 ,the ,gelatinous sizing used for the ,english papers1 and obtained by boiling hides in water1 and hardening the product by an admixture of ,alum1 has a reddening influence upon reduced ,silver salts1 analogous to that of ,albumen1 or of ,caseine1 the characteristic animal principle of milk4 ,positives printed upon ,english paper1 commonly assume some shade of brown more or less removed from black2 the darker tones being more readily obtained upon the foreign papers4 ,citrates and ,tartrates have a marked effect upon the colour of prints4 ,paper prepared with ,citrate1 in addition to ,chloride of ,silver1 darkens to a fine purple colour which changes to brick-red in the fixing ,bath4 ,the ,positives1 when toned1 are usually of a violet-purple or of a bistre tint1 with a general aspect of warmth and transparency4 ,,section ,,ii4 ,the ,processes for ,fixing and ,toning the ,proof4 ,this part of the operation is one to which great attention should be paid1 in order to secure bright and lasting colours3 it involves more of delicate chemical change than perhaps any other department of the ,art4 ,the first point requiring explanation is the process of fixing2 to which 7p4 #da7 brief reference has already been made4 ,the methods adopted to improve the tint of the finished picture will then be described4 ,,conditions ,,of ,a ,,proper ,,fixing ,,of ,,the ,,proof4 ,this subject is not always understood by operators1 and consequently they have no certain guide as to how long the prints should remain in the fixing ,bath4 ,the time occupied in fixing will of course vary with the strength of the solution employed2 but there are simple rules which may be usefully followed4 ,in the act of dissolving the unaltered ,chloride of ,silver in the proof1 the fixing solution of ,hyposulphite of ,soda converts it into ,hyposulphite of ,silver 7p4 #dc71 which is soluble in an excess of ,hyposulphite of ,soda4 ,but if there be an insufficient excess1--that is1 if the ,bath be too weak1 or the print removed from it too speedily1--then the ,hyposulphite of ,silver is not perfectly dissolved1 and begins by degrees to decompose1 producing a brown deposit in the tissue of the paper4 ,this deposit1 which has the appearance of yellow spots and patches1 is not usually seen upon the surface of the print1 but becomes very evident when it is held up to the light1 or if it be split in half1 which can be readily done by gluing it between two flat surfaces of deal1 and then forcing them asunder4 ,the reaction of ,hyposulphite of ,soda with ,nitrate of ,silver4--,in order to understand more fully how decomposition of ,hyposulphite of ,silver may affect the process of fixing1 the peculiar properties of this salt should be studied4 ,with this view ,nitrate of ,silver and ,hyposulphite of ,soda may be mixed in equivalent proportions1 viz4 about twenty-one grains of the former salt to sixteen grains of the latter1 first dissolving each in separate vessels in half an ounce of distilled water4 ,these solutions are to be added to each other and well agitated2 immediately a dense deposit forms1 which is ,hyposulphite of ,silver4 ,at this point a curious series of changes commences4 ,the precipitate1 at first white and curdy1 soon alters in colour3 it becomes canary-yellow1 then of a rich orange-yellow1 afterwards liver-colour1 and finally black4 ,the rationale of these changes is explained to a certain extent by studying the composition of the ,hyposulphite of ,silver4 ,the formula for this substance is as follows3-- ,ag,o ,s#b,o#b4 ,but ,ag,o ,s#b,o#b plainly equals ,ag,s1 or ,sulphuret of ,silver1 and ,,so#c1 or ,sulphuric ,acid4 ,the acid reaction assumed by the supernatant liquid is due therefore to ,sulphuric ,acid1 and the black substance formed is ,sulphuret of ,silver4 ,the yellow and orange-yellow compounds are earlier stages of the decomposition1 but their exact nature is uncertain4 ,the instability of ,hyposulphite of ,silver is principally seen when it is in an isolated state3 the presence of an excess of ,hyposulphite of ,soda renders it more permanent1 by forming a double salt1 as already described4 ,in fixing ,photographic prints1 this brown deposit of ,sulphuret of ,silver is very liable to form in the ,bath and upon the picture2 particularly so when the temperature is high4 ,to obviate it1 observe the following directions3--,it is especially in the reaction between ,nitrate of ,silver and ,hyposulphite of ,soda that the blackening is seen2 the ,chloride and other insoluble ,salts of ,silver being dissolved1 even to saturation1 without any decomposition of the ,hyposulphite formed4 ,hence if the print be washed in water to remove the soluble ,nitrate1 a very much weaker fixing ,bath than usual may be employed4 ,but if the proofs are taken at once from the printing frame and immersed in a dilute ,bath of ,hyposulphite 7one part of the salt to six or eight of water71 a shade of brown may often be observed to pass over the surface of the print1 and a large deposit of ,sulphuret of ,silver soon forms as the result of the decomposition4 ,on the other hand1 with a strong ,hyposulphite ,bath there is little or no discoloration1 and the black deposit is absent4 ,the print must also be left for a sufficient time in the fixing bath1 or some appearance of brown patches1#ah visible by transmitted light1 may occur4 ,each atom of ,nitrate of ,silver requires three atoms of ,hyposulphite of ,soda to form the sweet and soluble double salt1 and hence1 if the action be not continued sufficiently long1 another compound will be formed almost tasteless and insoluble 7p4 #dd74 ,even immersion in a new ,bath of ,hyposulphite of ,soda does not fix the print when once the yellow stage of decomposition has been established4 ,this yellow salt is insoluble in ,hyposulphite of ,soda1 and consequently remains in the paper4 #ah ,the writer has noticed that when sensitive paper is kept for some time before being used for printing1 these yellow patches of imperfect fixation are very liable to occur4 ,the ,nitrate of ,silver appears gradually to enter into combination with the organic matter of the size of the paper1 and cannot then be so easily extracted by the fixing bath4 ,in fixing prints by ,ammonia the ,author has found that the same rule may be applied as in the case of ,hyposulphite of ,soda1 viz4 that if the process be not properly performed1 the white parts of the print will appear spotted when held up to the light1 from a portion of insoluble ,silver ,salt remaining in the paper4 ,prints imperfectly fixed by ,ammonia are also usually brown and discoloured upon the surface of the paper4 ,more exact directions as to the strength of the fixing bath and the time occupied in the process1 will be given in the ,second ,part of the ,work2 at present it may be noticed only that ,albuminized paper1 from the horny nature of its surface-coating1 requires a longer treatment with the ,hyposulphite than the plain paper4 ,,the ,,salts ,,of ,,gold ,,as ,,toning ,,agents ,,for ,,photographic ,,prints4 ,the ,salts of ,gold have been successfully applied to the improvement of the tones obtained by simply fixing the proof in ,hyposulphite of ,soda4 ,the following are the principal modes followed3-- ,m4 ,le ,grey's ,process4--,the print1 having been exposed to light until it becomes very much darker than it is intended to remain1 is washed in water to remove the excess of ,nitrate of ,silver4 ,it is then immersed in a dilute solution of ,chloride of ,gold1 acidified by ,hydrochloric ,acid4 ,the effect is to reduce the intensity considerably1 and at the same time to change the dark shades to a violet or bluish tint4 ,after a second washing with water1 the proof is placed in plain ,hyposulphite of ,soda1 which fixes it and alters the tone to a pure black or a blue-black1 according to the manner of preparing the paper and the time of exposure to light4 ,the rationale of the process appears to be as follows3-- the ,chlorine1 previously combined with ,gold1 passes to the reduced ,silver ,salt2 it bleaches the lightest shades1 by converting them again into white ,protochloride of ,silver1 and gives to the others a violet tint more or less intense according to the reduction4 ,at the same time metallic ,gold is deposited1 the effect of which is not visible at this stage1 since the same violet tint is perceived when a solution of ,chlorine is substituted for ,chloride of ,gold4 ,the ,hyposulphite of ,soda subsequently employed1 decomposes the violet ,subchloride of ,silver1 and leaves the surface of a black tint1 due to the ,gold and the reduced ,silver ,salt4 ,m4 ,le ,grey's process is objectionable on account of the excessive over-printing required4 ,this however is to a great extent obviated by a modification of the process in which an alkaline instead of an acid solution of the ,chloride is employed2 one grain of ,chloride of ,gold is dissolved in about six ounces of water1 to which are added twenty to thirty grains of the common ,carbonate of ,soda4 ,the alkali moderates the violence of the action1 so that the print washed with water and immersed in the ,gold ,bath1 is less reduced in intensity1 and does not acquire the same inky blueness4 ,on subsequent fixing in the ,hyposulphite1 the tint changes from violet to a dark chocolate-brown1 which is permanent4 ,the ,tetrathionate and ,hyposulphite of ,gold employed in toning4--,after the discovery of ,le ,grey's mode1 it was proposed1 as an improvement1 to add ,chloride of ,gold to the fixing solution1 so as to obviate the necessity of using two ,baths4 ,the print1 in that case1 although darkened considerably1 is less reduced in intensity1 and the same amount of over-printing is not required4 ,the chemical changes which ensue are different from before3 they may be described as follows3-- ,chloride of ,gold1 added to ,hyposulphite of ,soda1 is converted into ,hyposulphite of ,gold1 ,tetrathionate of ,gold1 and 7if the ,chloride of ,gold be free from excess of acid7 a red compound1 containing more of the metal than1 either of the others1 but the exact nature of which is uncertain4 ,each of these three ,gold ,salts possesses the property of darkening the print1 but not to the same extent4 ,the activity is less as the stability of the salt is greater1 and hence the red compound1 which is so highly unstable that it cannot be preserved many hours without decomposing and precipitating metallic ,gold1 is far more active than the ,hyposulphite of ,gold1 which1 when associated with an excess of ,hyposulphite of ,soda1 is comparatively permanent4 ,when rapidity of colouring is an object it will therefore be advisable to add ,chloride of ,gold to the fixing ,bath of ,hyposulphite rather than an equivalent quantity of ,sel d'or2 and by dropping a little ,ammonia into the ,chloride of ,gold so as to precipitate fulminating gold#ai 7a compound which dissolves in ,hyposulphite of ,soda with considerable formation of the unstable red salt71 the activity of the ,bath will be promoted4 #ai ,read the observations on the ,explosive ,properties of ,fulminating ,gold in the ,vocabulary1 ,part ,,iii4 ,the ,author explains the action of these ,salts of ,gold upon the ,positive print as follows3--they are unstable1 and contain an excess of ,sulphur loosely combined2 hence1 when placed in contact with the image1 which has an affinity for ,sulphur1 the existing compound is broken up1 and ,sulphuret of ,silver1 ,sulphuric ,acid1 and metallic ,gold are the results4 ,that a minute proportion of ,sulphuret of ,silver is formed seems certain2 but the change must be superficial1 as the stability of the print is very little lessened when the process is properly performed4 ,sel ,or employed as a toning agent4--,this process1 which was communicated to the ',photographic ,journal' by ,mr4 ,sutton of ,jersey1 has been found serviceable4 ,the prints are first washed in water1 to which is added a little ,chloride of ,sodium1 to decompose the free ,nitrate of ,silver4 ,they are then immersed in a dilute solution of ,sel d'or1 or double ,hyposulphite of ,gold and ,soda1 which quickly changes the tint from red to purple without destroying any of the details or lighter shades4 ,lastly1 the ,hyposulphite of ,soda is employed to fix the print in the usual way4 ,this process differs theoretically from the last in some important particulars4 ,the toning solution is applied to the print before fixing1 which experience proves to have an important influence upon the result1 it having been found that when the print is previously acted upon by ,hyposulphite of ,soda1 the rapidity of deposition of the ,gold is interfered with2--thus1 a dilute solution of ,sel d'or colours a print rapidly1 but if to this same liquid a few crystals of ,hyposulphite of ,soda be added1 the picture becomes red and may be kept in the ,bath for comparatively a long time without acquiring the purple tones4 ,as ,hyposulphite of ,soda in excess lessens the action of the ,sel d'or1 so on the other hand the addition of an acid increases it4 ,the acid does not precipitate ,sulphur1 as might be expected from a knowledge of the reaction of ,hyposulphite with acid bodies 7p4 #acg71 but it favours the reduction of metallic ,gold4 ,hence it is usual to add a little ,hydrochloric ,acid to the toning solution of ,sel d'or1 to increase the rapidity and perfection of the colouring process4 ,,the ,,conditions ,,which ,,affect ,,the ,,action ,,of ,,the ,,fixing ,,and ,,toning ,,bath ,,of ,,gold ,,and ,,hyposulphite ,,of ,,soda4 ,although the process of toning ,positives by ,sel d'or is very certain in its results and gives good tints1 yet1 as involving a somewhat greater expenditure of time and trouble1 it is not at present universally adopted4 ,the ordinary plan of fixing and toning in one bath has been proved to yield permanent prints if the proper precautions are observed1 but it is quite necessary1 in order to ensure success1 that the conditions by which its action is modified should be understood4 ,the more important of these are as follows3-- a4 ,the ,,age of the ,bath4--,when ,chloride of ,gold is added to ,hyposulphite of ,soda1 several unstable salts are produced1 which decompose by keeping4 ,hence the solution is very active during the first few days after mixing2 but at the expiration of some weeks or months1 if not used1 it becomes almost inert1 a reddish deposit of ,gold first forming1 and eventually a mixture of black ,sulphuret of ,silver and ,sulphur1 the former of which often adheres to the sides of the bottle in dense shining lamin4 ,when the ,bath is constantly kept in use there is a loss of ,gold1 which1 although it is less perceived than it otherwise would be1 from the fact that sulphuretting principles are formed 7see next page7 capable of replacing the ,gold as toning agents--yet makes the ,bath work more slowly1 and hence over-printing is required4 b4 ,presence of free ,nitrate of ,silver upon the surface of the proof4--,this produces an accelerating effect1 as may be shown by soaking the print in salt and water1 to convert the ,nitrate into ,chloride of ,silver2 the action then takes place more slowly4 ,the free ,nitrate of ,silver increases the instability of the ,gold salts2 but if present in too great an excess1 it is apt to cause a decomposition of ,hyposulphite of ,silver1 and consequent yellowness in the white parts of the proof4 ,it is therefore particularly recommended to wash the print in water before immersing it in the fixing and toning ,bath4 c4 ,temperature of the solution4--,in cold weather1 the thermometer standing at #cb to #dj1 the ,bath works more slowly than usual2 whereas in the height of summer1 and especially in hot climates1 it occasionally becomes quite unmanageable4 ,the best temperature for operating successfully appears to be about #fj to #fe ,fahrenheit2 if higher than this the solutions must be employed more dilute4 d4 ,addition of ,iodide of ,silver4--,some operators associate ,iodide with ,chloride in the preparation of sensitive paper for printing4 ,another source of the same salts is the admixture of a portion of the fixing ,bath used for ,negatives with the ,positive toning solution4 ,the presence of ,iodides in the fixing and toning ,bath is injurious3 when in large excess1 they dissolve the image1 or produce yellow patches of ,iodide of ,silver on the lights2 in smaller quantity1 the deposition of the ,gold is hindered1 and the action proceeds more slowly4 ,bromides and ,chlorides have not the same effect4 e4 ,mode of preparing the paper4--,the rapidity of toning varies with causes independent of the ,bath3 thus1 plain paper prints are toned more quickly than prints upon albuminized paper1 and the use of ,english paper sized with ,gelatine retards the action4 ,foreign papers rendered sensitive with ,ammonio-,nitrate tone the most quickly4 ,on certain states of the fixing and toning ,bath which are injurious to the proofs4--,the object of using the ,hyposulphite ,bath is to fix the proof and to tone it by means of ,gold4 ,but it is a fact familiar to the photographic chemist1 that ,positives can also be toned by a sulphuretting action1 and that the colours so obtained are not very different from those which follow the employment of ,gold4#bj ,now the ,hyposulphite of ,soda is a substance which can be very readily made to yield up ,sulphur to any bodies which possess an affinity for that element1 and as the reduced ,silver compound in the print has such an affinity1 there is always a tendency to absorption of ,sulphur when the proofs are immersed in the ,bath4 ,consequently in many cases a sulphur toning-process is set up1 and as the picture is improved by it in appearance1 losing its brick-red colour and assuming a purple shade1 it was at first adopted by ,photographers4 ,experience however has shown that colours brightened in this way are less permanent than others1 and are liable to fade unless kept perfectly dry4 ,hence the process will be discarded by all careful operators1 and the object will be to avoid sulphuration as far as possible4 ,this can be done to a great extent1 and1 when the ,bath is properly managed1 the prints will be toned almost entirely by ,gold1 and will1 with care1 be permanent4 #bj ,for a more detailed account of the toning process by ,sulphur1 see the ,third ,section of this ,chapter1 page #ade4 ,the instability of sulphuretted prints is shown in the fourth ,section4 ,some of the conditions which facilitate a sulphuretting action upon the proof are as follows3-- a4 ,the addition of an ,acid to the ,bath4--,it was at one time common to add a few drops of ,acetic ,acid to the fixing ,bath of ,hyposulphite of ,soda1 immediately before immersing the proofs4 ,the ,bath then assumes an opalescent appearance in the course of a few minutes1 and1 when this milkiness is perceptible1 the print begins to tone rapidly and becomes nearly black4 ,the chemical changes produced in a ,hyposulphite ,bath by addition of acid1 may be explained thus3--,the acid first displaces the feeble ,hyposulphurous acid from its combination with ,soda4 ,acetic ,acid ,hyposulphite ,soda4 ,acetate ,soda ,hyposulphurous ,acid4 ,then the ,hyposulphurous ,acid1 not being a stable substance when isolated1 begins spontaneously to decompose1 and splits up into ,sulphurous ,acid--which remains dissolved in the liquid1 communicating the characteristic odour of burning ,sulphur--and ,sulphur1 which separates in a finely divided state and forms a milky deposit4#ba #ba ,from the ,vocabulary1 ,part ,,iii41 it will be seen that commercial ,chloride of ,gold usually contains free ,hydrochloric ,acid2 hence a considerable deposit of ,sulphur takes place on adding it to the ,hyposulphite solution1 and the liquid must not be used immediately4 ,observe therefore that free acids of all kinds must be excluded from the fixing ,bath1 or1 if inadvertently added1 the liquid must be set aside for some hours until the ,hyposulphurous ,acid has decomposed1 and1 the ,sulphur having settled to the bottom1 the ,bath has regained its original neutral condition4#bb #bb ,the chemical reader will understand the decomposition of free ,hyposulphurous ,acid by the following equation3--,s#b,o#b ,,so#b and ,s4 b4 ,decomposition of the ,bath by constant use4--,it has long been known that a solution of ,hyposulphite of ,soda undergoes a peculiar change in properties when much used in fixing4 ,when first prepared it leaves the image of a red tone1 the characteristic colour of the reduced ,silver ,salt1 but soon acquires the property of darkening this red colour by a subsequent communication of ,sulphur4 ,hence a simple fixing ,bath becomes at last an active toning bath1 without any addition of ,gold4 ,this change of properties will be found more fully explained in the abstract of the ,author's researches given in the next ,section 7p4 #aef74 ,at present we remark only that it is due principally to a reaction between ,nitrate of ,silver and ,hyposulphite of ,soda1 attended with decomposition of ,hyposulphite of ,silver 7p4 #acj72 and hence1 if the prints are washed in water before immersion in the ,bath1 the solution will be less quickly liable to change4 ,many operators state that the toning ,bath having at first been prepared with ,chloride of ,gold1 no further addition of this substance will be required4 ,this no doubt is correct1 but in such case the proofs will at last be toned by ,sulphur more than by ,gold1 and will not possess the same stability2 the ,bath will also1 after long use1 be found to acquire a distinct acid reaction to test-paper1 the acidity being due to a peculiar principle generated by decomposing ,hyposulphite of ,silver1 and which is shown to have an injurious action upon the print 7p4 #aeh74 ,to avoid this the solution should be kept neutral to test-paper by means of a drop of ,ammonia1 if required2 and when it begins to be exhausted1 and does not tone 7quickly7 a print from which the free ,nitrate of ,silver has been removed by washing1 a fresh quantity of ,chloride of ,gold should be added4 c4 ,tetrathionate in the ,hyposulphite ,bath4--,the ,author has shown that the ,tetrathionates1 which are analogous to the ,hyposulphites1 have an active sulphuretting action upon ,positive prints 7see the papers in the next ,section74 ,very fine colours can be obtained in this way2 but toning by ,sulphur having been proved to be wrong in principle1 the formul given in the first two editions of this ,work have been omitted4#bc #bc ,the preparation of a toning bath by ,tetrathionate1 without ,gold1 is described in the next ,section1 but it is not recommended for practical use4 ,the bodies which produce ,tetrathionate when added to a solution of ,hyposulphite of ,soda1 and hence are inadmissible in the toning process1 are as follows3--,free ,iodine1 ,perchloride of ,iron1 ,chloride of ,copper1 ,acids of all kinds 7in the latter case the acid first produces ,sulphurous ,acid1 and the ,sulphurous ,acid1 if present in any quantity1 by reacting upon ,hyposulphite of ,soda1 forms ,tetrathionate and ,trithionate of ,soda74 ,chloride of ,gold also produces a mixed ,tetrathionate of ,gold and ,soda when added to the fixing ,bath 7p4 #acc72 but as the quantity of ,chloride used is small1 the prints are far less sulphuretted than in the case of toning ,baths prepared by ,tetrathionate without ,gold4 ,,section ,,iii4 ,the ,author's ,researches in ,photographic ,printing4 ,having been long engaged in conducting experiments upon the composition and properties of the reduced material forming the ,photographic image1 and especially with a view of determining the exact conditions under which the picture may be considered permanent1 the ,author has thought it advisable to give the results of these researches in the form of an abstract of the original papers read at the meetings of the ,photographic ,society4 ,a previous perusal of these papers will put the reader in possession of the principal facts upon which are founded the precautions advised in the next ,section for the preservation of ,photographic prints4 ,in order to keep the ,work as nearly as possible within its original limits1 and also for the purpose of distinguishing the present ,section from the others1 as one referring principally to scientific details1 the type has been reduced to the size of that used in the ,appendix4 ,,on ,,the ,,chemical ,,composition ,,of ,,the ,,photographic ,,image4 ,the determination of the chemical nature of the ,photographic image in its various forms is a point of much importance1 both as indicating the conditions required for the preservation of works of art of that class1 and also as a guide to the experimenter in selecting bodies likely to have an effect as chemical agents in ,photography4 ,it has been stated by some who have given attention to the subject1 that the image is formed in all cases of pure metallic ,silver1 and that any observable variations in its colour and properties1 are due to a difference in the molecular arrangement of the particles4 ,but this hypothesis1 although involving much that is correct1 yet does not contain the whole truth1 for it is evident that the chemical properties of the ,photographic image often bear no resemblance to those of a metal4 ,one ,photograph may also differ essentially from another1 so that we are led to infer the existence of two varieties1 the first of which is less of a metallic nature than the second4 ,in investigating the subject1 the principal point appeared to be to examine the action of light upon ,chloride of ,silver1 and afterwards to associate the ,chloride with organic matter in order to imitate the conditions under which ,photographs are obtained4 ,the following is an epitome of the conclusions arrived at3-- ,action of ,light upon ,chloride of ,silver4--,the process is accompanied by a separation of ,chlorine1 but its product is not a mere mixture of ,chloride of ,silver and ,metallic ,silver2 if it were so1 we cannot suppose that the darkening would take place beneath the surface of ,nitric ,acid1 which it is found to do4 ,a definite ,subchloride of ,silver seems to be formed1 the most important property of which is its decomposition by fixing agents1 such as ,ammonia1 and ,hyposulphite of ,soda1 both of which destroy the violet colour1 dissolving out ,protochloride of ,silver1 and leaving a small quantity of a grey residue of metallic ,silver4 ,inasmuch therefore as all ,photographic pictures require fixing1 we may conclude that if they could be produced upon pure and isolated ,chloride of ,silver 7which however is not the case71 they would consist solely of metallic ,silver4 ,decomposition of organic ,salts of ,silver by ,light4--,compounds of ,oxide of ,silver with organic bodies1 are as a rule darkened by exposure to light1 but the process does not always consist in a simple reduction to the metallic state4 ,this assertion is proved by the employment of the following tests4 a4 ,mercury4--,little or no amalgamation takes place on triturating the darkened salt with this metal4 b4 ,ammonia and fixing agents4--,these usually produce only a limited amount of action4 ,thus1 the ,albuminate of ,protoxide of ,silver is perfectly soluble in ,ammonia2 but after having been reddened by exposure to light1 it is little or not at all affected4 c4 ,potash4--,animal matters coagulated by ,nitrate of ,silver1 and reduced by the sun's rays1 are dissolved by boiling ,potash1 the solution being clear and of a blood-red colour4 ,metallic ,silver1 it is presumed1 if present1 would remain insoluble4 d4 ,boiling ,water4--,gelatine treated with ,nitrate of ,silver and exposed to light1 loses its characteristic property of dissolving in hot water4 ,this experiment is conclusive4 ,the above facts justify us in supposing the existence of combinations of organic matter with a low ,oxide of ,silver2 and analysis indicates further that the relative proportion of each constituent in these compounds may vary4 ,for instance1 when ,citrate of ,silver is reduced by light1 and acted on with ,ammonia1 a black powder remains1 which was found to contain as much as #ie per cent1 real ,silver2 but ,albuminate of ,silver treated in the same way yields on analysis less of metallic ,silver1 and more volatile and carbonaceous matter4 ,the use of ,ammonio-,nitrate of ,silver in preparing the salt tends also to increase the relative quantity of metal left in the compound after reduction and fixing4 ,the length of time during which the light has acted1 has also a modifying effect of the same kind1--the product of reduction by a powerful light being more nearly in the state of metal1 and containing less both of ,oxygen and organic matter4 ,action of ,light upon ,chloride of ,silver associated with organic matter4--,photographs formed on ,chloride of ,silver alone1 would1 after fixing1 consist of metallic ,silver1 but such a process could not be carried out in practice4 ,the addition of organic matter is absolutely necessary in order to increase the sensitiveness1 and to prevent the image from being dissolved in the ,bath of ,hyposulphite of ,soda4 ,the blue ,subchloride of ,silver is decomposed by fixing1 a very scanty proportion of grey metallic ,silver remaining insoluble2 but the red compound of ,suboxide of ,silver with organic matter is almost unaffected by ,hyposulphite of ,soda1 or ,ammonia4 ,the increase of sensitiveness and intensity produced by the use of organic matter is accompanied also by a change in the composition of the picture2 the image losing the metallic character which it possesses when formed on pure ,chloride of ,silver1 and resembling in every respect the product of the action of light upon organic ,salts of ,silver4 ,there are certain characteristic tests which may usefully be employed in distinguishing the metallic image from what may be termed the organic or non-metallic image4 ,one of these tests is ,cyanide of ,potassium4 ,an image formed upon pure ,chloride of ,silver1 although pale and feeble1 may1 after fixing1 be immersed ill dilute solution of ,cyanide of ,potassium without injury4 ,but a photograph on ,chloride of ,silver supported by an organic basis1 is much acted upon by ,cyanide of ,potassium1 quickly losing its finer details4 ,a second test is the ,hydrosulphate of ,ammonia4 ,if no organic matter be employed1 the image becomes darker and more intense by treatment with a soluble ,sulphuret2 whilst the non-metallic image1 formed on an organic surface1 is quickly bleached and faded4 ,the action of ,sulphur upon the image is indeed a mode of determining the real quantity of ,silver present4 ,when existing in a very finely divided layer1 ,sulphuret of ,silver often appears yellow2 but in a thicker layer it is black4 ,hence the colour of the ,photograph1 after treatment with ,sulphuretted ,hydrogen1 is an indication of the proportion of metal present1 and the reason of the organic image becoming so perfectly faded is because it contains a minimum of ,silver in relation to the intensity4 ,we see1 therefore1 that the addition of organic matter to ,chloride of ,silver does not so much increase the actual quantity of ,silver reduced by light1 as it adds to its opacity by associating other elements with the ,silver1 and altogether modifying the composition of the image4 ,the employment of oxidizing agents shows also that in an ordinary ,photographic process by the direct action of light1 other elements besides ,silver assist in forming the image3 the pictures being found to be easily susceptible of oxidation1 whereas the metallic image formed on pure ,chloride of ,silver resists oxidation4 ,composition of developed images4--,by exposing sensitive layers of the ,iodide1 the ,bromide1 and the ,chloride of ,silver to the light for a short time only1 and subsequently developing with ,gallic ,acid1 ,pyrogallic ,acid1 and the protosalts of ,iron1 a variety of images may be obtained1 which differ from each other materially in every important particular1 and a comparison of which assists the determination of the disputed point4 ,the appearance and properties of the developed ,photograph are found to vary with the existence of the following conditions4 #ast4 ,the surface used to sustain the sensitive layer4--,there is a peculiarity in the image formed on ,collodion4 ,collodion contains ,pyroxyline1 a substance which behaves towards the salts of ,silver in a manner different from that of most organic bodies1 exhibiting no tendency to assist their reduction by light4 ,hence ,chloride of ,silver on ,collodion darkens far more slowly than the same salt upon ,albumen1 and the image1 after fixing1 is feeble and metallic4 ,iodide of ,silver on ,collodion1 exposed and developed1 gives usually a more metallic image1 with less intensity1 than ,iodide of ,silver upon ,albumen1 or on paper sized with ,gelatine4 ,by adding to the ,collodion a body which has an affinity for low oxides of ,silver1 such for instance as ,glycyrrhizine1 the opacity of the developed image is increased4 #bnd4 ,the nature of the sensitive salt4--,when ,iodide of ,silver is used to receive the latent impression1 the image after development1 although lacking intensity of colour by reflected light1 is more nearly in the condition of metallic ,silver than if ,bromide or ,chloride of ,silver be substituted2 and of the three salts1 the ,chloride gives the most intensity1 with the least quantity of metallic ,silver4 ,this rule applies especially when organic matters1 ,gelatine1 ,glycyrrhizine1 etc41 are present4 #crd4 ,the developing agent employed4--,an organic developing agent like ,pyrogallic ,acid may be expected to produce a ,collodion image more intense1 but less metallic1 than an inorganic developer1 such as the ,protosulphate of ,iron4 #dth4 ,the length of time during which the light has acted4--,over-action of the light favours the production of an image which is dark by reflection and brown or red by transmission1 corresponding in these particulars to what may be termed the non-metallic image containing an oxide of ,silver4 #eth4 ,the stage of the development4--,the red image first formed on the application of the developer to a gelatinized or albuminized surface of ,iodide of ,silver is less metallic1 and more easily injured by destructive tests1 than the black image1 which is the result of prolonging the action4 ,developed photographs which are of a bright red colour after fixing1 correspond in properties to images obtained by the direct action of light on paper prepared with ,chloride of ,silver1 more nearly than to ,collodion1 or even to fully developed ,talbotype ,negatives4 ,to conclude the ,paper1 the following may be offered in the way of recapitulation3--,an image consisting of metallic silver1 as a rule1 reflects white light1 and shows as a positive when laid on black velvet2 but a non-metallic organic image is dark1 and represents the shadows of a picture4 ,collodion positives developed with protosalts of ,iron are nearly or quite metallic4 ,photographs on ,albumen or ,gelatine less so than those on ,collodion4 ,developed ,photographs contain more ,silver than others1 if the development has been prolonged4 ,the half shadows of the image in a ,positive ,print are especially liable to suffer under injurious conditions1 since they contain the ,silver in a less perfect state of reduction4#bd #bd ,the ,author omits1 in this place1 all mention of molecular conditions affecting intensity1 inasmuch as at the present time nothing positive has been determined with regard to them4 ,it is however known that in the use of the protosalts of ,iron as developing agents1 the appearance of the image is much influenced by the rapidity with which the reduction is effected--the particles of ,silver being larger and more metallic when the development is conducted slowly4 ,the process of electro-plating and other chemical operations of a similar kind prove that the physical properties of metals precipitated from solutions of their salts1 vary greatly with the degree of fineness and arrangement of their particles4 ,,on ,,the ,,various ,,agencies ,,destructive ,,to ,,photographic ,,prints4 ,action of ,sulphuretting ,compounds upon ,positive ,prints4--,it was first noticed by ,mr4 ,t4 ,a4 ,malone1 that the most intense ,photograph might be destroyed by acting upon it with solution of ,sulphuretted ,hydrogen or a soluble ,sulphuret1 for a sufficient length of time4 ,the changes produced by a sulphuretting compound acting upon the red image of a simply fixed print are these3--the colour is first darkened1 and a degree of brilliancy imparted to it2 this is the effect termed toning4 ,then the warm tint by degrees alters to a colder shade1 the intensity of the whole image is lessened1 and the half-tones turn yellow4 ,lastly1 the full shadows pass also from black to yellow1 and the print fades4 ,now in this peculiar reaction we notice the following points of interest4 ,if at that particular stage at which the print has reached its maximum of blackness1 it be raised partially out of the liquid and allowed to project into the air1 the part so treated becomes yellow before that which remains immersed4 ,again1 if a print toned by ,sulphur be placed in a pan of water to wash1 after the lapse of several hours it is apt to assume a faded appearance in the half-tones4 ,the full shadows1 in which the reduced ,silver salt is thicker and more abundant1 retain their black colour for a longer time1 but if the action of the sulphuretting ,bath be continued1 every portion of the print becomes yellow4 ,these facts prove that ,oxygen has an influence in accelerating the destructive action of the ,sulphur compounds upon ,positive prints2 and this idea is borne out by the results of further experiments1 for it is found that moist ,sulphuretted ,hydrogen has little or no effect in darkening the colour when every trace of air is excluded4 ,when prints are washed in water they are exposed to the influence of the dissolved air which water always contains1 and hence the change from black to yellow is produced4#be #be ,further remarks upon the action of damp air upon ,positives toned by ,sulphur are given at p4 #aec4 ,there are some substances which facilitate the yellow degeneration of ,positives toned by ,sulphur1 a knowledge of which will be useful3 they are--#ast1 powerful oxidizers1 such as ,chlorine1 ,permanganate of ,potash1 and ,chromic ,acid2 these1 even when highly diluted1 act with great rapidity3 #bnd1 bodies which dissolve ,oxide of ,silver1 as soluble ,cyanides1 ,hyposulphites1 ,ammonia2 also acids of various kinds1 and hence the frequency of yellow finger impressions upon old sulphuretted prints1 which are probably caused by a trace of organic 7,lactic87 ,acid left by contact with the warm hand4 ,it was at one time supposed that the ,photograph in the stage at which it appears blackened by ,sulphur1 consisted of ,sulphuret of ,silver1 and that this black ,sulphuret became yellow by absorption of ,oxygen and conversion into ,sulphate4 ,,mm4 ,davanne and ,girard1 who examined the subject1 thought that there might be two isomeric forms of ,sulphuret of ,silver1 a black and a yellow form2 the former of which passing gradually into the latter produced the fading of the impression4 ,but neither of these views are correct2 for it is proved by careful experiment1 that the ,sulphuret of ,silver is a highly stable compound1 not prone to oxidize1 and1 further1 that the change of colour from black to yellow has no reference to a modification of this salt4 ,the truth appears to be that the image whilst in the black stage contains other elements besides ,sulphur and ,silver1 but when it has become yellow by the continued action of the sulphuretting compound1 it is then a true ,sulphuret4 ,comparative permanence of ,photographs under the action of ,sulphur4--,developed ,positives1 as a rule1 stand better than those printed by direct exposure to light2 but much depends upon the nature of the negative process followed2 and hence no general statement can be made which will not be liable to many exceptions4 ,the mode of conducting the development must not be overlooked4 ,the prints1 which become very red in the ,hyposulphite fixing ,bath from the action of the developer having been stopped at too early a period1 are often sulphuretted and destroyed even more readily than a vigorous sun-print obtained by direct exposure to light4 ,a point of even greater importance is the nature of the sensitive surface which receives the latent image4 ,it is the print developed upon ,iodide of ,silver which especially resists sulphuration4 ,in that case1 not only is the preliminary toning effect of the ,sulphur more slow than usual1 but the impression cannot be made to fade by any continuance of the action4 ,it loses much of its brilliancy1 and is reduced in intensity1 but it is not so completely destroyed as to be useless4 ,the reason of this1 as shown in the last paper1 depends upon the fact that the ,talbotype proofs contain the largest amount of ,silver in the image4 ,the employment of ,gold in toning does not render an ordinary sun-print as permanent as a ,positive developed upon ,iodide of ,silver4 ,the deep shadows of the picture are protected by the ,gold1 but the lighter shades not so perfectly4 ,hence after the ,sulphur has acted1 in place of the universal yellow and faded aspect presented by the simple untoned print1 the ,positive fully toned by ,gold has black shadows with yellow half-tones4 ,therefore1 whilst recommending the use of ,gold as a toning agent1 it does not seem advisable to lay too much stress upon it as a preservative from the destructive action of ,sulphur4 ,exposure of ,positive ,prints to a ,sulphuretting ,atmosphere4--,in testing the action of a solution of ,sulphuretted ,hydrogen upon paper ,positives1 it did not appear that the conditions under which the prints were placed bore a sufficiently close resemblance to the case of ,positives exposed to an atmosphere contaminated with minute traces of the gas2 and this more particularly because it is known that dry ,sulphuretted ,hydrogen has comparatively little effect upon ,photographic ,prints4 ,the experiments were therefore repeated in a somewhat different form4 ,a number of ,positives 7about three dozen7 printed in various ways1 were suspended in a glass case1 measuring #b-a_/b feet by #ba inches1 and containing #g-a_/b cubic feet of air2 into which was introduced1 occasionally1 a few bubbles of ,sulphuretted ,hydrogen1 just sufficient to keep the air of the chamber smelling perceptibly of the gas4 ,a polished ,daguerreotype plate was hung up in the centre1 to serve as a guide to the progress of the sulphuretting action4 ,by the second day the metal plate had acquired a faint yellow hue1 not easily seen except in certain positions2 but the ,positives were unaffected4 ,at the expiration of three days the majority of the pictures exhibited no signs of change1 but a few untoned prints of a pale red colour1 some of which had been printed by development1 and others by direct exposure to light1 had perceptibly darkened4 ,after the eighth day1 the action1 appearing to progress more slowly than at first1 was stopped1 and the prints removed4 ,the general results obtained were as follows3-- ,the ,daguerreotype plate had become strongly tarnished with a film of ,sulphuret of ,silver1 which appeared yellowish-brown in some parts and steel-blue in others4 ,the ,positives were1 as a rule1 toned to a slightly colder shade1 but many of them had scarcely changed4 ,no obvious difference was observed between prints developed on paper prepared with ,chloride of ,silver1 and others printed by direct exposure to light2 but in all cases the prints obtained by those methods which give a very red image after fixing1 were the first to show the change of colour due to sulphuration1 the proofs submitted to the test having all been previously toned with ,gold4 ,effect of ,oxidizing ,agents upon ,positive ,prints4--,it appeared of importance to ascertain to what extent ,photographic ,prints are susceptible of oxidation2 on account of the atmospheric influences to which they are necessarily exposed4 ,in experimenting upon this subject the following results have been obtained4 ,powerful oxidizers destroy ,positive ,prints rapidly2 the action usually commencing at the corners and edges of the paper1 or at any isolated point1 such as a metallic speck or particle of extraneous matter1 which can serve as a centre of chemical action4 ,this same fact is often noticed in the fading of ,positives by long keeping1 and therefore since other destructive actions 7with the exception of that of ,chlorine7 do not appear to follow the same rule1 it is an argument in addition to others which can be adduced1 that ,photographic ,prints are frequently destroyed by oxidation4 ,air which has been ,ozonized by ,phosphorus1 and in which blue litmus-paper becomes reddened1 quickly bleaches the ,positive image4 ,oxygen gas1 obtained by voltaic decomposition of acidified water and which should contain ,ozone1 did not appear to have an equal amount of effect1 the action being comparatively slight1 or altogether wanting4 ,peroxide of ,hydrogen obtained in solution1 and in conjunction with ,acetate of ,baryta1 by adding ,peroxide of ,barium to dilute ,acetic ,acid1#bf bleaches darkened ,positive paper2 but the effect is slow1 and does not take place to a very perceptible extent if the liquid be kept alkaline to test-paper4 #bf ,hydrochloric ,acid1 which is usually recommended in place of ,acetic ,acid1 cannot be employed in this experiment2 it seems to cause a liberation of free ,chlorine1 which bleaches the print instantly4 ,nitric ,acid applied in a concentrated form acts immediately upon the darkened surface1 bleaching every part of the print with the exception of the bronzed shadows1 which usually retain a slight residual colour4 ,a solution of ,chromic ,acid is still more active4 ,this liquid may usefully be applied to distinguish prints toned by ,sulphur from others toned by ,gold2 the presence of metallic ,gold protecting the shadows of the picture in some measure from the action of the acid4 ,the solution should be prepared as follows3-- ,bichromate of ,potash #f grains4 ,strong ,sulphuric ,acid #d minims4 ,water #ab ounces4 ,a solution of ,permanganate of ,potash is an energetic destroyer of paper positives2 and1 as it is a neutral substance1 may conveniently be employed in testing the relative capability of withstanding oxidation possessed by different ,photographic ,prints4 ,the solution should be dilute1 of a pale pink hue1 and the ,positives must be moved occasionally1 as the first effect is to decolorize a great portion of the liquid1 the ,permanganate oxidizing the size and organic tissue of the paper4 ,after an immersion of twenty minutes to half an hour1 varying with the degree of dilution1 the half-tones of the picture begin to die out1 and the full shadows become darker in colour2 the bronzed portions of the print withstand the action longer1 but at length the whole is changed to a yellow image much resembling in appearance the ,photograph faded by ,sulphur4 ,comparative permanence of ,photographs treated with ,permanganate of ,potash4--,developed prints prepared by a ,negative process withstand the action better than others4 ,but to this rule there are exceptions2 much depending upon the time of exposure to light1 and the extent to which the development is carried4 ,those prints which1 being exposed for a short time1 and afterwards strongly developed1 become dark in colour and vigorous in outline1 are more permanent than others which having been over-exposed and under-developed1 lose their dark colour and become red and comparatively faint in the ,hyposulphite fixing ,bath4 ,positives developed upon a surface of ,chloride of ,silver on plain paper do not resist the oxidizing action so perfectly as those on ,iodide of ,silver4 ,prints developed upon paper prepared with ,serum of ,milk containing ,caseine stand better than those on plain paper4 ,of prints obtained by the ordinary process of direct exposure to light1 those on plain paper are the first to fade1 the oxidizing action being most seen upon the half-tones4 ,the use of ,albumen gives a great advantage4 ,developed prints on ,albumen stand far better than the same upon plain paper2 and even the ,albuminized sun prints are less injured by the ,permanganate than the best of the ,negative prints prepared without ,albumen4 ,caseine has the same effect1 but to a less extent2 and as ,serum of ,milk almost invariably contains uncoagulated ,caseine1 its efficacy is thus explained4 ,the manner of toning the print is a point of importance2 previous sulphuration in an old ,hyposulphite ,bath always facilitating the oxidizing action4 ,action of ,chlorine upon ,positive ,prints4--,aqueous solution of ,chlorine destroys the ,photographic image1 changing it first to a violet tint 7probably ,subchloride71 and subsequently obliterating it by conversion into white ,chloride of ,silver4 ,the impression1 although invisible1 remains in the paper1 and may be developed in the form of yellow or brown ,sulphuret of ,silver by the action of ,sulphuretted ,hydrogen4 ,it also becomes visible on exposure to light1 and assumes considerable intensity if the paper be previously brushed with free ,nitrate of ,silver4 ,sulphate of ,iron produces no effect upon the invisible image of ,chloride of ,silver2 but ,gallic or ,pyrogallic ,acid1 rendered alkaline by ,potash1 converts it into a black deposit4 ,the ,action of ,chlorine water usually commences at the edges and corners of the print1 in the same manner as that of oxidizing agents4 ,the proofs upon ,albumen are the least readily injured1 and next1 those developed on ,iodide of ,silver4 ,hydrochloric ,acid4--,the liquid acid of sp4 gr4 #aaf1 even when free from ,chlorine1 acts immediately upon the half-tones of a positive print1 and destroys the full shadows in the course of a few hours2 a slight residual colour however usually remains in the darkest parts4 ,the prints developed on ,iodide of ,silver are the most permanent4 ,sulphuric1 ,acetic ,acids1 etc4--,acids of all kinds appear to exert an injurious influence upon ,positive prints1 and especially so upon the half-tones of the image1 the effect varying with the strength of the acid and the degree of dilution with water4 ,even a vegetable acid like ,acetic gradually darkens the colour and destroys partially or entirely the faint outlines of the picture4 ,bichloride of ,mercury4--,the most important particulars relating to the action of this test upon ,photographs are well known4 ,the image is ultimately converted into a white powder1 and hence1 in the case of a ,positive print1 it becomes invisible2 immersion in ,ammonia or ,hyposulphite of ,soda however restores it in a form often resembling in tint the original impression4 ,a point worthy of note is the protective effect of a deposit of ,gold1 which is very marked1 the proof1 after toning1 resisting the action of the ,bichloride for comparatively a long time4 ,ammonia4--,the effect of ,ammonia upon a print is rather to redden the image than to destroy it2 the half-tones become pale and faint1 but they do not disappear4 ,toning with ,gold enables the proof to resist the action of the strongest solution of ,ammonia1 and hence ,ammonia may safely be employed as a fixing agent after the use of the ,sel d'or ,bath4 ,hyposulphite of ,soda4--,a concentrated solution of ,hyposulphite of ,soda exercises a gradual solvent action upon the image of ,photographic ,prints1 at the same time tending to communicate ,sulphur and to darken the colour of the impression4 ,a faint yellow outline of ,sulphuret of ,silver usually remains after the solution of the image is completed4 ,developed prints of all kinds1 but in particular the ,talbotype proofs upon ,iodide of ,silver1 are less readily dissolved by ,hyposulphite of ,soda than those obtained by the direct action of light4 ,there is also a slight difference between plain and ,albuminized prints1 which is in favour of the former1 the albuminized paper always losing somewhat more by immersion in the ,hyposulphite ,bath than plain ,chloride paper sensitized by ,nitrate of ,silver4 ,cyanide of ,potassium4--,the solvent action of ,cyanide of ,potassium is most energetic upon ,photographs formed on paper4 ,these images1 whether developed or not1 do not withstand the test so well as the impressions on ,collodion4 ,albuminized proofs are also somewhat more easily affected than prints on simple ,chloride paper sensitized with ,nitrate or ,ammonio-,nitrate of ,silver4 ,heat1 moist and dry4--,long-continued boiling in distilled water has a reddening action upon ,positive ,prints4 ,the image becomes at length pale and faint1 resembling a print treated with ,ammonia before toning4 ,a deposit of ,gold upon the image lessens1 but does not altogether neutralize1 the effect of the hot water4 ,if the boiling be long continued1 the violet-purple tone often imparted by the ,gold invariably gives place to a chocolate-brown1 which appears to be the most permanent colour4 ,prints developed by ,gallic ,acid upon paper prepared with ,serum of ,milk or with a ,citrate1 suffer as much as others obtained by direct action of light4 ,ammonio-,nitrate prints on highly salted paper1 which become nearly black when toned with ,gold1 retain their original appearance the most perfectly2 a slight diminution of brightness being the only observable difference after long boiling in water4 ,albumen proofs1 and prints on ,english papers1 or foreign papers prepared with ,serum of ,milk1 ,citrates1 ,tartrates1 or any of those bodies which redden the reduced ,salt1 are1 as a rule1 rendered lighter in colour1 and pass from purple to brown when boiled in water4 ,dry heat has an opposite effect to that of hot water1 usually darkening the colour of the image4 ,on exposing a plain paper print simply fixed1 and thoroughly freed from ,hyposulphite of ,soda by washing1 to a current of heated air1 it changes gradually from red to dark brown1 in which state it continues until the temperature rises to the point at which the paper begins to char1 when it resumes its original red tone1 becoming at the same time faint and indistinct4 ,the ,products of ,combustion of ,coal-gas a cause of ,fading4--,coal-gas contains ,sulphur compounds1 which in combustion are oxidized into ,sulphurous and ,sulphuric ,acids2 other substances of a deleterious nature may also be present4 ,a plate of polished silver suspended in a glass tube1 through which was directed the current of heated air rising from a small gas jet1 became tarnished with a white film in the course of twenty-four hours4 ,positive prints exposed to the same1 absorbed moisture and faded2 the action resembling that of oxidation1 in being preceded by a general darkening in colour4 ,of four prints exposed1 an ,iodide-developed print was the least injured1 and next1 a print upon ,albuminized paper4 ,,on ,,the ,,action ,,of ,,damp ,,air ,,upon ,,positive ,,prints4 ,in order to ascertain this point1 more than six dozen ,positives1 printed on every variety of paper1 were mounted in new and perfectly clean stoppered glass bottles1 at the bottom of each of which was placed a little distilled water1 to keep the contained air always moist4 ,they were removed at the expiration of three months1 having been kept during that time1 some in the dark1 and others exposed to the light4 ,as the prints were prepared by various methods1 toned in different ways1 and mounted with or without substances likely to exercise a deleterious action1 this series of experiments will possess considerable value in determining some of the intrinsic causes of fading of ,positives4#bg #bg ,for a more detailed account of the experiments1 see the original paper in the ',photographic ,journal1' vol4 iii4 ,the general results obtained were as follows3--,positives which had been simply fixed in ,hyposulphite of ,soda remained quite uninjured4 ,whether developed by ,gallic ,acid on either of the three ,salts of ,silver usually employed1 or printed by direct action of light1 the result was the same4 ,hence we may infer that the darkened material which forms the image of ,photographic ,prints does not readily oxidize in a damp atmosphere4 ,toned ,positives were found in many cases to be less permanent than ,positives simply fixed4 ,this was especially the case when the toning had been effected by ,sulphur2 all the sulphuretted prints1 fixed in solution of ,hyposulphite which had been long used1 became yellow in the half-tones when exposed to moisture4 ,positives fixed and toned in ,hyposulphite containing ,gold were variously affected2 some prepared when the solution was in an active state being unchanged1 others losing a little half-tone1 and others1 again1 fading badly4 ,these latter were prepared in a ,bath which had lost ,gold and acquired sulphuretting properties2 and it was noticed that they were more injured by the action of boiling water than those ,positives which proved to be permanent under the influence of the moisture4 ,toning by means of ,chloride of ,gold appeared to be highly satisfactory1 but the number of prints operated upon was small4 ,the ,sel d'or process also did not injure the integrity of the image1 no commencing yellowness or bleaching of half-tones being visible after exposure to the moist air4 ,this series of experiments confirmed the statement made in a former paper1 that some tints obtained in ,positive printing are more permanent than others4 ,violet tones produced by ,sulphur invariably passed into a dull brown by the action of the moist air2 and even when ,gold was employed in toning1 these same purple colours were usually reddened4 ,this was especially the case when ,english papers were used1 or foreign papers re-sized with ,serum of ,milk containing ,caseine4 ,the chocolate-brown tints which best stand the action of boiling water1 and in particular those upon ,ammonio-,nitrate paper1 were least affected by the damp air2 and indeed it was evident that the two agents1 viz4 moist air and hot water1 acted alike in tending to redden the print1 although the latter did so in the most marked manner4 ,it seemed also1 from the results of these experiments1 to be a point of great importance that the size should be removed from the print in order to render it indestructible by damp air4 ,this was evidently seen in two cases where ,positives1 toned in an old ,hyposulphite and ,gold ,bath1 were divided into halves1 one of which was treated with a strong solution of ,ammonia4 ,the result was that the halves in which the size was allowed to remain1 faded1 whilst the others were comparatively uninjured4 ,the ,albumen proofs especially suffered when the size was left in the paper1 a destructive mouldiness forming1 and fading the picture4 ,the use of boiling water obviated this1 and the prints so treated remained clean and bright4 ,a partial decomposition of ,albumen however occurred in some cases even when hot water was used1 the gloss disappearing from the paper in isolated patches4 ,with ,caseine substituted for ,albumen there was also a loss of half-tone2 thus seeming to indicate that both these animal principles1 although stable under ordinary conditions1 will1 even when coagulated by ,nitrate of ,silver1 decompose if kept long in a moist state4 ,the use of improper substances for mounting proved to be another determining cause of fading by oxidation4 ,those bodies which combine with ,oxide of ,silver1 are likely upon theoretical grounds to destroy the half-tones of the image2 and it was found1 that if the picture were left in contact with ,alum1 ,acetic ,acid1 etc41 or with the substances which generate an acid by fermentation1 such as paste or starch1 it invariably faded4 ,the supposed accelerating influence of ,light upon the fading of ,positives was not confirmed by these experiments1 as far as they extended4 ,many of the bottles containing the ,photographs were placed outside the window of a house with a southern aspect during the whole of the three months with the exception of two or three weeks1 but no difference whatever could be detected between ,positives so treated and others kept in total darkness4 ,it will be proper however that this part of the investigation should be repeated1 allowing a longer time4 ,an examination of the various modes employed for coating ,positives1 in order to exclude the atmosphere1 showed that many of them were not fitted to fulfil the purpose intended4 ,waxed prints faded quite as much when exposed to moisture as others not waxed4 ,white wax is a substance often adulterated1 and ,oil of ,turpentine has been shown to contain a body resembling ,ozone in properties1 and possessing the power of bleaching a dilute solution of ,sulphate of ,indigo4 ,spirit varnish applied to the surface of the picture after re-sizing with ,gelatine was plainly superior to white wax1 but nevertheless it did not obviate the fading effect of the moisture upon an unstable ,positive which had been toned by sulphuration4 ,its protective influence is therefore limited4 ,,on ,,the ,,change ,,in ,,composition ,,which ,,hyposulphite ,,of ,,soda ,,experiences ,,by ,,use ,,in ,,fixing ,,paper ,,proofs4#bh #bh ,these observations are condensed and re-arranged from the papers published by the ,author in the ',photographic ,journal' for ,september and ,october1 #ahed4 ,it was remarked by ,photographers at an early period that the properties of the ,fixing ,bath of ,hyposulphite of ,soda became altered by constant use2 that it gradually acquired the power of darkening the colour of the ,positive image4 ,this change was at first referred to the accumulation of ,salts of ,silver in the ,bath1 and hence directions were given to dissolve a portion of blackened ,chloride of ,silver in the ,hyposulphite in preparing a new solution4 ,careful experiments performed by the ,author convinced him that an error had been entertained2 since it was found that the simple solution of ,chloride of ,silver in ,hyposulphite of ,soda had no power of yielding the black tones4 ,but it afterwards appeared that if the fixing ,bath1 containing dissolved ,silver ,salts1 were set aside for a few weeks1 a decomposition occurred in it1 evidenced by the formation of a black deposit of ,sulphuret of ,silver2 and then it became active in toning the proofs4 ,the presence of this deposit of ,sulphuret of ,silver indicated that a portion of ,hyposulphite of ,silver had spontaneously decomposed1 and1 knowing the products which are generated by the spontaneous decomposition of this salt1 a clue to the difficulty was afforded4 ,one atom of ,hyposulphite of ,silver includes the elements of one of ,sulphuret of ,silver and one of ,sulphuric ,acid4 ,sulphuric ,acid in contact with ,hyposulphite of ,soda produces ,sulphurous ,acid by a process of displacement2 and ,plessy has shown that ,sulphurous ,acid reacts upon an excess of ,hyposulphite of ,soda1 forming two of that interesting series of ,sulphur compounds designated by ,berzelius the ,polythionic ,acids4 ,it appeared therefore probable1 upon theoretical grounds1 that the ,penta-1 ,tetra-1 and ,trithionates might produce some effect in the ,hyposulphite fixing ,bath4 ,upon making the trial these expectations were verified2 and it was found that ,tetrathionate of ,soda added to ,hyposulphite of ,soda yielded a fixing and toning ,bath quite equal in activity to that produced by means of ,chloride of ,gold4 ,it may be useful to review for an instant the composition of the ,polythionic series of acids2 it is thus represented3-- ,sulphur4 ,oxygen4 ,formul4 ,dithionic or ,hyposulphuric ,acid #b atoms #e atoms ,s#b,o#e ,trithionic ,acid #c #e ,s#c,o#e ,tetrathionic ,acid #d #e ,s#d,o#e ,pentathionic ,acid #e #e ,s#e,o#e ,the amount of ,oxygen in all is the same1 that of the other element increases progressively2 hence it is at once evident that the highest member of the series might by losing ,sulphur descend gradually until it reached the condition of the lowest4 ,this transition is not only theoretically possible1 but there is an actual tendency to it1 all the acids being unstable with the exception of the ,hyposulphuric4 ,the ,alkaline ,salts of these acids are more unstable than the acids themselves2 a solution of ,tetrathionate of ,soda becomes milky in the course of a few days from deposition of ,sulphur1 and1 if tested1 is then found to contain ,trithionate and eventually ,dithionate of ,soda4 ,the cause of the change in properties of the fixing ,bath being thus clearly traced to a decomposition of ,hyposulphite of ,silver1 and a consequent generation of unstable principles capable of imparting ,sulphur to the immersed proofs1 it seemed desirable to continue the experiments4-- ,there is a peculiar acid condition commonly assumed by old fixing ,baths1 which could not be satisfactorily explained1 since it was known that acids do not exist long in a free state in solution of ,hyposulphite of ,soda1 but tend to neutralize themselves by displacing ,hyposulphurous ,acid spontaneously decomposable into ,sulphurous ,acid and ,sulphur4 ,this point is set at rest by the discovery of a peculiar reaction which takes place between certain salts of the ,polythionic acids and ,hyposulphite of ,soda4 ,a solution of ,tetrathionate of ,soda may be preserved for many hours unchanged2 but if a few crystals of ,hyposulphite of ,soda be dropped in1 it begins very shortly to deposit ,sulphur1 and continues to do so for several days4 ,at the same time the liquid acquires an acid reaction to test-paper1 and produces effervescence on the addition of ,carbonate of ,lime4 ,it is evident that a ,sulphur acid exists which has not hitherto been described1 and that this acid is formed as one of the products of the decomposition of the ,hyposulphite of ,silver contained in the fixing ,bath4 ,the subject is an important one to ,photographers1 because it is found that ,hyposulphite ,baths which have acquired the acid reaction1 although toning quickly1 yield ,positives which fade on keeping4 ,the acid may perhaps combine with the reduced ,silver ,salt1 which1 if the image be allowed to contain ,suboxide of ,silver1 is theoretically probable4 ,the experiments were next directed towards ascertaining more carefully the effect of the acid fixing ,bath upon the ,positive proofs4 ,tetrathionate of ,soda added to solution of ,hyposulphite of ,soda produces1 at the expiration of twelve hours1 a liquid which1 when filtered from the deposited ,sulphur1 reddens blue litmus-paper slowly4 ,positive prints immersed in the ,bath pass from red to black1 dissolving in the half-tones1 and becoming yellow and faded if the action be too long continued4 ,on adding ,carbonate of ,soda in quantity sufficient to remove the acid reaction1 the power of toning is much diminished1 but dark colours can still be obtained by continuing the action4 ,the solvent effect upon the half-tones1 evidently caused in great measure by the acid1 is lessened2 whilst the tendency to yellowness in the white parts of the proof1 almost disappears4 ,these effects are more particularly manifested when the prints are immersed in the ,bath immediately on their removal from the printing frame2 and it is found almost impossible to preserve the whites of the impression clear1 in the acid ,bath1 unless the ,nitrate of ,silver has been washed away4 ,solution of half-tones and yellowness in the lights1 both a source of annoyance to the operator1 are thus traced in great measure to an acid condition of the fixing and toning ,bath2 and the remedy is obvious4 ,the ,author's experiments upon the ,tetrathionates and their reaction with ,hyposulphite of ,soda likewise elicited the important fact that alkalies decompose the unstable sulphuretted principle4 ,if the ,bath be treated with ,potash or ,carbonate of ,soda1 an alkaline ,sulphuret appears to be gradually formed1 which precipitates ,sulphuret of ,silver1 and in the course of a few days the liquid returns to its original condition and ceases to act as a toning agent upon the proof4 ,the same effect takes place to a great extent when the solution is set aside for several weeks or months2 a process of spontaneous change going forward1 which issues in a deposition of ,sulphur and ,sulphuret of ,silver1 and a partial loss of sulphuretting properties in the liquid4 ,it may be interesting to the scientific investigator to describe the mode of preparing a fixing and toning ,bath1 illustrating the above remarks3-- ,take of ,nitrate of ,silver #c drachms4 ,hyposulphite of ,soda #d ounces4 ,water #h ounces4 ,dissolve the ,nitrate of ,silver in #b ounces of the water1 then from the total quantity of ,hyposulphite of ,soda1 weigh out ,hyposulphite of ,soda #b drachms2 dissolve this likewise in #b ounces of water1 and the remainder of the ,hyposulphite in the other #d ounces4 ,then1 having the three solutions in separate vessels1 pour the ,nitrate of ,silver at once into the #b-ounce solution of ,hyposulphite1 agitating the precipitated ,hyposulphite of ,silver rapidly4 ,in a short time it will begin to decompose1 passing from white to canary-yellow1 and then to orange-yellow4 ,when the orange-yellow begins to verge towards brown1 add the #d-ounce concentrated solution of ,hyposulphite1 which will at once complete the decomposition1 a part of the precipitate dissolving and the remainder becoming perfectly black4 ,after filtering out the black ,sulphuret of ,silver1 the solution is ready for use4 ,a ,bath prepared by this formula is not usually very active1 but it shows clearly the process by which an ordinary fixing ,bath may be converted into a toning ,bath by the immersion of positives having free ,nitrate of ,silver upon the surface4 ,the following formula is more economical and gives a better result1 but it cannot be used for ,ammonio-,nitrate prints2 the addition of an alkali precipitating ,sulphuret of ,iron4 ,strong solution of ,perchloride of ,iron #f fluid drachms4 ,hyposulphite of ,soda #d ounces4 ,water #h ounces4 ,nitrate of ,silver #cj grains4 ,dissolve the ,hyposulphite of ,soda in seven ounces of the water1 the ,nitrate of ,silver in the remaining one ounce2 then pour the ,perchloride of ,iron into the solution of ,hyposulphite1 by degrees1 stirring all the time4 ,the addition of the ,iron ,salt strikes a fine purple colour1 but this soon disappears4 ,when the liquid has become again colourless1 which it does in a few minutes1 add the ,nitrate of ,silver1 stirring briskly4 ,perfect solution will take place without any formation of black ,sulphuret4 ,a toning ,bath prepared with ,chloride of ,iron will be ready for use twelve hours after mixing1 but it will be more active at the expiration of a week4 ,the solution is acid to test-paper1 and milky from a deposit of ,sulphur1 which must be filtered out4 ,the ,perchloride of ,iron should be prepared by boiling ,peroxide of ,iron with ,hydrochloric ,acid1 in preference to dissolving ,iron wire in ,aqua-,regia4 ,the addition of the ,nitrate of ,silver is made in order to produce a portion of ,hyposulphite of ,silver in the bath2 the presence of a ,silver ,salt having been found to modify the tint of the ,positives1 and to prevent their quickly turning yellow4 ,,section ,,iv4 ,on the ,fading of ,photographic ,prints4 ,for many years subsequent to the discovery of the process of ,photographic ,printing by ,mr4 ,fox ,talbot1 it was not generally known that pictures so produced were easily susceptible of injury from various causes1 and in particular from traces of the fixing-agent remaining in the paper4 ,hence1 due care not being taken in the proper cleansing and preservation of the proofs1 the majority of them faded4 ,this matter became at last one of such importance that the ,council of the ,photographic ,society decided upon forming a ,committee for the purpose of examining the subject4 ,the ,author was honoured by being placed upon this ,committee1 and the researches of which an abstract has been given in the previous ,section1 were undertaken at the request of the ,society4 ,the present ,section is intended to explain practically and in a concise manner the causes of the fading of ,photographic ,prints1 and the precautions which should be taken to ensure their permanency4 ,the chemistry of the subject having been fully explained in the last ,section1 it will suffice to refer the reader to its pages for more detailed information4 ,historical evidence of the permanence of ,photographs4--,it is a point of interest to collect information as to the existence of old ,photographs which have remained many years unchanged4 ,there are numerous instances of ,positives printed more than ten years ago1 which have not perceptibly altered up to the present time4 ,these prints are mostly on plain paper1 ,albumen not having come into use at so early a date4 ,the general impression of practical operators however is1 that fading has occurred less frequently since the introduction of ,albuminized paper4 ,positives printed by development on paper prepared by ,talbot's method seem as a rule to have stood remarkably well1 and instances of ,talbotype ,negatives having faded are rare4 ,of the prints which have proved to be permanent1 some are red or brown in colour1 but many1 being of a dark or purple shade1 have evidently been toned1 although not with ,gold1 the use of which was unknown to the earlier ,photographers4 ,it is plain from data thus collected1 that ,photographs do not necessarily fade by time2 and the fact that in one and the same portfolio are constantly seen prints which appear permanent1 and others in an advanced state of change1 cannot but lead to the inference that the main causes of deterioration are intrinsic1 depending upon some injurious matters left in the paper2 which is confirmed by experiment4 ,causes of fading4--,the ,author believes that the fading of ,photographic ,prints may almost invariably be referred to one or other of the following conditions3-- a4 ,imperfect washing4--,this is perhaps the most important of all1 and the most frequent4 ,when ,hyposulphite of ,soda is allowed to remain in the paper1 even in minute quantity1 it gradually decomposes1 with liberation of ,sulphur1 and destroys the print in the same way and quite as effectually as a solution of ,sulphuretted ,hydrogen or an alkaline sulphuret4 ,imperfect washing may be suspected1 if the ,photograph1 within a few months from the date of its preparation1 begins to get darker in colour3 the half-tints1 which are the first to show the action1 afterwards passing into the yellow stage1 whilst the dark shadows remain black or brown for a longer time4 ,the proper mode of washing ,photographs is sometimes misunderstood4 ,the length of time during which the print lies in the water is a point of less importance1 than that the water should be continually changed4 ,when a number of ,positives are placed together in a pan1 and a tap turned upon them1 the circulation of fluid does not necessarily extend to the bottom4 ,this is proved by the addition of a little colouring matter1 which shows that the stream flows actively above1 but at the lower part of the vessel1 and between the prints1 there is a stationary layer of water which is of little use in washing out the ,hyposulphite4 ,care should therefore be taken that the pictures are kept as far as possible separate from each other1 and when running water cannot be had1 that they are frequently moved and turned over1 fresh water being constantly added4 ,when this is done1 and especially if the size be removed from the paper in the manner presently to be advised1 four or five hours washing will be sufficient4 ,it is a mistake to allow the pictures to remain in the water for several days2 which produces no good effect1 and may tend to encourage a putrefactive fermentation1 or the formation of a white deposit upon the image when the water contains ,carbonate of ,lime4 b4 ,acid matters left in the ,paper4--,upon examining collections of old ,photographs1 it is not uncommon to find prints which are stated to have remained unaltered for a long time after their first production1 but in the course of time to have lost their brilliancy1 and become pale and indistinct4 ,this kind of fading often commences at the corners and edges of the paper1 and works inwards towards the centre4 ,the ,author's experiments have shown that it is principally caused by a slow process of oxidation4 ,the ,photographic ,image does not appear readily susceptible of oxidation unless it be previously darkened by the action of ,sulphur1 or placed in contact with acids or bodies which act as solvents of ,oxide of ,silver 7p4 #adf74 ,the materials often used in sizing papers1 such as ,alum and ,resin1 being of an acid nature1 are directly injurious to the image2 and the removal of the size1 which may easily be effected by means of a dilute alkali or an alkaline carbonate1 without injury to the tint1 has the additional advantage of carrying out the last traces of ,hyposulphite of ,soda1 and also the germs of fungi1 which if allowed to remain would vegetate and produce a destructive mouldiness on exposure to damp 7,chap4 ,,iii4 ,part ,,ii474 ,the fact that acids facilitate oxidation of the image is likewise a hint that ,photographic ,prints should not be handled too frequently1 or touched with the finger more than is necessary2 the warm hand may leave behind a trace of acid#bi which would tend in time to produce a yellow mark4 #bi ,the writer has seen blue litmus-paper immediately reddened by being laid upon the arm of a person suffering from acute ,rheumatism4 ,this acid is probably ,lactic ,acid6 c4 ,moisture as a cause of fading4--,although4 ,photographs properly printed are not readily injured by damp air 7p4 #aec71 yet as there are impurities of various kinds constantly floating in the atmosphere1 a state of comparative dryness may be said to be essential to the preservation of all ,photographs4 ,in collecting evidence upon the subject1 wet and damp are frequently alleged as having been causes of fading--the prints were hung against a damp wall during frosty weather1 in a room without a fire3 or the rain had been allowed to penetrate the frame6 ,no pictures will long survive such treatment1 and ,photographs1 like engravings and water-colour paintings1 require common care to be exercised in their preservation4 d4 ,the modes of ,mounting the ,proof4--,this subject has been alluded to in the abstract of the ,author's papers at p4 #aee4 ,all cements which are of an acid nature1 or which are liable to become sour by acetous fermentation1 should be avoided4 ,flour paste is especially injurious1 and many cases of fading have been traced to this cause4 ,the addition of ,bichloride of ,mercury1 which is often made to prevent the paste from becoming mouldy1 would still more unfit it for ,photographic use 7p4 #aea74 ,starch is not much preferable4 ,no substance appears better than ,gelatine1 which does not readily decompose1 and shows no tendency to absorb atmospheric moisture4 ,the deliquescent nature of many bodies is a point which should be borne in mind in mounting ,photographs1 and hence the use of a salt like ,carbonate of ,potash1 which the writer has known to be added to paste to prevent the formation of acid1 would be unadvisable4 e4 ,the effect of ,imperfect ,fixation4--,the earlier ,photographers did not always succeed in properly fixing their prints1 since old ,photographs are often found thickly studded with spots and blotches in the tissue of the paper4 ,these prints however are not invariably faded upon the surface1 and hence it cannot be said that imperfect fixation will certainly end in the total destruction of the picture4 ,still a notice of the subject may properly be introduced in this place1 and the attention of the reader be once more drawn to the importance of washing the print in water on removing it from the printing frame2 a decomposition invariably occurring when paper ,positives saturated with free ,nitrate of ,silver are plunged in a dilute solution of ,hyposulphite of ,soda1 containing an insufficient quantity of the salt to dissolve away the ,hyposulphite of ,silver before it begins to undergo spontaneous change4 f4 ,exposure to an impure ,atmosphere as a cause of ,fading4--,the five causes of fading which precede1 have mostly reference to an intrinsically faulty condition of the print4 ,this1 the sixth1 explains the mode in which a ,photograph carefully prepared may yet suffer injury from deleterious matters often present in the atmosphere4 ,the air of large cities1 and particularly that emanating from sewers and drains1 contains ,sulphuretted ,hydrogen1 and hence articles of silver-plate become tarnished unless placed beneath glass4 ,the injury which a print sustains by exposure to air contaminated with ,sulphuretted ,hydrogen1 is less than the tarnish produced upon the bright surface of a silver plate 7see p4 #adh72 but it is recommended as a precautionary measure1 that ,photographic ,pictures be protected by glass or kept in a portfolio1 and that they be not exposed too freely to the air4 ,the products of the combustion of coal-gas are probably more likely than the cause last named1 to be a source of injury to ,photographs suspended without any covering4 ,the sulphur compounds in gas burn into ,sulphurous and ,sulphuric ,acids1 the latter of which1 in combination with ,ammonia1 produces the sparkling crystals often observed upon the shop windows4 ,the question as to the manner in which the ,photographic ,image may best be protected from these extraneous causes of fading has been mooted1 and many plans of coating prints with some impervious material have been devised4 ,if the pictures are to be glazed or kept in a portfolio1 this of itself will be sufficient1 but in other cases it may perhaps be useful to apply a layer of spirit or gutta-percha varnish4 ,the use of wax1 resin1 and such bodies is likely1 by introducing impurities1 to act injuriously rather than otherwise4 g4 ,decomposition of ,pyroxyline a source of ,injury to ,collodion ,photographs4--,collodion ,positives and ,negatives are usually esteemed permanent2 but some have been exhibited which1 having been put away in a damp place1 gradually became pale and indistinct4 ,the change commences at rough edges and isolated points1 leaving the centre1 as a rule1 the last affected4 ,on examination1 numerous cracks are often visible1 thus seeming to indicate that the ,collodion film has undergone decomposition4 ,the result of this would be the liberation of corrosive ,oxides of ,nitrogen1 which destroy the image4 ,substitution compounds containing ,peroxide of ,nitrogen are known to be liable to spontaneous change4 ,the bitter resin produced by acting upon white sugar with ,nitro-,sulphuric ,acid1 if not kept perfectly dry1 will sometimes evolve enough gas to destroy the cork of the bottle in which it is kept2 the solution of the resin has then a strong acid reaction1 and rapidly fades an ordinary ,positive ,print4 ,these facts are interesting1 and indicate that ,collodion ,pictures1 containing in themselves the elements of their destruction1 should be protected from moisture by a coating of varnish4 ,comparative ,permanence of ,photographic ,prints4--,there is every reason to think that the ,photographic ,image1 however formed1 is permanent1 if certain injurious conditions are avoided2--in other words1 that prints do not necessarily fade1 in the same manner as fugitive colours1 by a simple exposure to light and air4 ,but supposing a case1 which is the common one1 of injurious influences which cannot altogether be removed1 it may be useful to inquire what mode of printing gives the greatest amount of stability4 ,positives produced by a short exposure to light and subsequent development with ,gallic ,acid1 may be expected to be more permanent than ordinary sun-prints2 not that there is any reason to suppose the chemical composition of a developed image to be peculiar1 but that the use of the ,gallic ,acid enables us to increase the intensity of the red picture first formed1 and to add to its stability by precipitating fresh ,silver upon it4 ,this point has not always been attended to4 ,it has been recommended to remove the print from the developing solution whilst in the red and early stage of development1 and to produce the dark tones subsequently by means of gold2 but this plan1 although giving very good results as regards colour and gradation of tone1 appears to lessen the advantage which would otherwise accrue from the adoption of a ,negative process1 and to leave the picture1 as regards permanency1 much in the condition of an ordinary print obtained by direct action of light4 ,the original ,talbotype process1 in which the latent image is formed upon ,iodide of ,silver1 produces1 next to ,collodion1 the most stable image2 but the difficulty of obtaining bright and warm tints on ,iodide of ,silver1 will stand in the way of its adoption4 ,the toning of ,paper ,positives is the part of the process which is likely to injure their stability2 inasmuch as the finest results cannot easily be obtained without incurring sulphuration1 and the action of ,sulphur1 if carried to any extent1 has been shown to be detrimental4 ,the point to be kept in view1 is to alter the original structure of the image as little as possible in toning2 and it is best to use ,gold in preference to ,sulphur as the colouring agent4 ,on theoretical grounds1 toning by an alkaline solution of ,chloride of ,gold 7p4 #acb71 and fixing by ,ammonia1 is the best process2 but the employment of ,sel d'or1 which gives a more agreeable colour and has not been found practically to injure the image1 will be generally preferred4 ,in using a single fixing and toning ,bath the same object of working by ,gold rather than by ,sulphur may be best attained by maintaining the activity of the ,bath by constant additions of ,chloride of ,gold4 ,the prints which are least stable are such as have been toned in acid ,hyposulphite ,baths1 without ,gold2 and the difficulty of preserving such pictures from becoming yellow in the half-tones is very great4 ,possibly a portion of the ,sulphuretted ,acid may unite with the ,suboxide of ,silver and cannot be removed by washing 7see p4 #aeh72 but even if this be not the case1 it is certain that no ordinary amount of care will obviate the occasional occurrence of fading1 unless the ,hyposulphite ,bath be kept neutral to test-paper4 ,and all those plans of toning in which ,acetic or ,hydrochloric ,acid is mixed with ,hyposulphite of ,soda1 and the ,positive immersed whilst the liquid is in a milky state from precipitation of ,sulphur1 ought studiously to be avoided4 ,it will be well also to avoid pushing the action of the fixing and toning ,bath to its utmost limits1 since practice and theory both teach us that the ,positives which have been long in the ,hyposulphite1 and consequently show a tendency to yellowness in the light parts1 are most liable to lose their half-tones on keeping4 ,photographic ,prints are found often to darken slightly in the course of years2 and therefore by suspending the toning action at an earlier stage a margin is left for what some have termed an improvement by time4 ,the use of ,albuminized in preference to plain paper gives an advantage in protecting the image from oxidation2 but if constantly exposed to moisture1 a putrefactive decomposition of the animal matter may occur4 ,the proper colour of the ,albumen image being a pale red1 the black tones should not be sought for on that variety of paper3 their production1 if ,hyposulphite of ,soda were used in toning1 would probably imply an amount of ,sulphuration which would more than counterbalance any advantage otherwise derivable from the ,albumen4 ,permanent ,positives of a black colour may easily be obtained by sensitizing plain paper1 free from animal matters1 with ,oxide of ,silver in place of ,nitrate4 ,the simply fixed image being in that case of a sepia tint1 requires a less amount of toning to change it to black4 ,an impression was at one time prevalent that ,ammonio-,nitrate prints were unstable2 but so far from such being the case1 they are proved to withstand the action of all destructive tests better than pictures prepared upon the same kind of paper sensitized with plain ,nitrate of ,silver4 ,mode of testing the permanence of ,positives4--,the tests for ,hyposulphite of ,soda are not sufficiently delicate to indicate with certainty when the process of washing has been properly performed4 ,the quantity of that salt left in the paper is usually so small and so much mixed up with organic matter1 that the application of ,protonitrate of ,mercury or of ,nitrate of ,silver to the liquid which drains from the corner of the print1 would probably mislead the operator4 ,a dilute solution of ,permanganate of ,potash1 prepared by dissolving from half a grain or two grains of the salt1 according to its purity1 in one gallon of distilled water1 affords a convenient mode of testing ,positives as regards their power of resisting oxidation2 and to an experienced eye it will prove the presence or absence of ,hyposulphite of ,soda1 the smallest trace of which is sufficient to remove the pink colour of the ,permanganate4 ,the most available and simple plan of testing permanence is to enclose the pictures in a stoppered glass bottle with a small quantity of water4 ,if they retain their half-tones after a course of three months of this treatment1 and do not become mouldy1 the mode of printing followed is satisfactory4 ,boiling water will also be found useful in distinguishing the unstable colours produced by ,sulphur from those following the judicious employment of ,gold2 in all cases the image will at first be reddened by the hot water1 but if toned without ,sulphur it will1 as a rule1 recover much of its dark colour on drying4 ,the characteristic appearance of prints which have been ,i much sulphuretted in the toning ,bath1 and are very liable to fade1 should be known4 ,a yellow colour in the lights is a bad sign2 and if the half-tones are at all faint and indistinct1 with an aspect of commencing yellowness1 it is almost certain that the ,positive will not last for any considerable length of time4 ,,chapter ,,ix4 ,,on ,,the ,,theory ,,of ,,the ,,daguerreotype ,,and ,,talbotype ,,processes1 ,,etc4 ,,section ,i4 ,the ,daguerreotype4 ,it was not the original intention of the ,author to include a description of the ,daguerreotype ,process within the limits of the present ,work4 ,the ,daguerreotype is a branch of the ,photographic ,art so distinct from the others1 that1 in manipulatory details1 it bears very little analogy to them2 a slight sketch of the theory of the process may not however be unacceptable4 ,all necessary remarks will fall under three heads3--,the preparation of the ,daguerreotype film2--the means by which the latent image is developed2--and the strengthening of the image by ,hyposulphite of ,gold4 ,the ,preparation of the ,daguerreotype ,film4--,the sensitive film of the ,daguerreotypist is in many respects different from that of the ,calotype or ,collodiotype4 ,the latter may be termed wet processes1 in contradistinction to the former1 where aqueous solutions are not employed4 ,the ,daguerreotype film is a pure and isolated ,iodide of ,silver1 formed by the direct action of ,iodine upon the metal4 ,hence it lacks one element of sensitiveness possessed by the others1 viz4 the presence of soluble ,nitrate of ,silver in contact with the particles of ,iodide of ,silver4 ,it is important to remember that the ,iodide of ,silver prepared by acting with vapour of ,iodine upon metallic ,silver1 is different in its ,photographic action from the yellow salt obtained by double decomposition between ,iodide of ,potassium and ,nitrate of ,silver4 ,a ,daguerreotype film1 when exposed to a bright light1 first darkens to an ash-grey colour and then becomes nearly white2 the solubility in ,hyposulphite of ,soda being at the same time lessened4 ,a ,collodion film1 on the other hand1 if the excess of ,nitrate of ,silver be washed off1 although it is still capable of receiving the radiant impression in the ,camera1 does not alter either in colour or in solubility1 by exposure even to the sun's rays4 ,details of the process for preparing a ,daguerreotype ,plate4--,a copper plate of moderate thickness is coated upon the surface with a layer of pure ,silver1 either by the electrotype or in any other convenient manner4 ,it is then polished with great care1 until the surface assumes a brilliant metallic lustre4 ,this preliminary operation of polishing is one of great practical importance1 and the troublesome details attending it constitute one of the main difficulties to be overcome4 ,after the polishing is complete1 the plate is ready to receive the sensitive coating4 ,this part of the process is conducted in a peculiar manner4 ,a simple piece of cardboard or a thin sheet of wood1 previously soaked in solution of ,iodine1 evolves enough of the vapour to attack the silver plate2 which being placed immediately above1 and allowed to remain for a short time1 acquires a pale violet hue1 due to the formation of an excessively delicate layer of ,iodide of ,silver4 ,by prolonging the action of the ,iodine the violet tint disappears and a variety of prismatic colours are produced1 much in the same way as when light is decomposed by thin plates of mica or the surface of mother-of-pearl4 ,from violet the plate becomes of a straw-yellow1 then rose-colour1 and afterwards steel-grey4 ,by continuing the exposure1 the same sequence of tints is repeated2 the steel-grey disappears1 and the yellow and rose-colours recur4 ,the deposit of ,iodide of ,silver gradually increases in thickness during these changes2 but to the end it remains excessively thin and delicate4 ,in this respect it contrasts strongly with the dense and creamy layer often employed in the ,collodion process1 and shows that a large proportion of the ,iodide of ,silver must in such a case be superfluous1 as far as any influence produced by the light is concerned4 ,an inspection of a sensitive ,daguerreotype plate reveals the microscopic nature of the actinic changes involved in the ,photographic ,art1 and teaches a useful lesson4 ,increase of sensibility obtained by combining the joint action of ,bromine and ,iodine4--,the original process of ,daguerre was conducted with the vapour of ,iodine only2 but in the year #ahdj it was discovered by ,mr4 ,john ,goddard that the sensibility of the plate was greatly promoted by exposing it to the vapours of ,iodine and ,bromine in succession1--the proper time for each being regulated by the tints assumed4 ,the composition of this ,bromo-,iodide of ,silver1 so called1 is uncertain1 and has not been proved to bear any analogy to that of the mixed salt obtained by decomposing a solution of ,iodide and ,bromide of ,potassium with ,nitrate of ,silver4 ,observe also that the ,bromo-,iodide of ,silver is more sensitive than the simple ,iodide only token the vapour of ,mercury is employed as a developer4 ,m4 ,claudet proves that if the image be formed by the direct action of light alone 7see page #agd71 the usual condition is reversed1 and that the use of ,bromine under such circumstances retards the effect4 ,the ,development and ,properties of the ,image4--,the latent image of the ,daguerreotype is developed in a manner different from that of the humid processes generally1--viz4 by the action of ,mercurial vapour4 ,mercury1 or ,quicksilver1 is a metallic fluid which boils at #ffb ,fahrenheit4 ,we are not however to suppose that the iodized plate is subjected to the vapour of ,mercury at a temperature at all approaching to #ffb4 ,the cup containing the ,quicksilver is previously heated by means of a spirit-lamp to about #adj1 a temperature easily borne by the hand1 in most cases1 without inconvenience4 ,the amount of ,mercurial vapour evolved at #adj is very small1 but it is sufficient for the purpose1 and after continuing the action for a short time the image is perfectly developed4 ,there are few questions which have given rise to greater discussion amongst chemists than the nature of the ,daguerreotype image4 ,unfortunately1 the quantity of material to be operated on is so small1 that it becomes almost impossible to ascertain its composition by direct analysis4 ,some suppose it to consist of ,mercury alone4 ,others have thought that the ,mercury is in combination with metallic ,silver4 ,the presence of the former metal is certain1 since ,m4 ,claudet shows that1 by the application of a strong heat1 it can actually be volatilized from the image in sufficient quantity to develope a second impression immediately superimposed4 ,it is a remarkable fact that an image more or less resembling that developed by ,mercury can be obtained by the prolonged action of light alone upon the iodized plate4 ,the substance so formed is a white powder1 insoluble in solution of ,hyposulphite of ,soda2 amorphous to the eye1 but presenting the appearance of minute reflecting crystals when highly magnified4 ,its composition is uncertain4 ,for all practical purposes the production of the ,daguerreotype image by light alone is useless1 on account of the length of time required to effect it4 ,this was alluded to in the third ,chapter1 where it was shown that in the case of the ,bromo-,iodide of ,silver an intensity of light #cjjj times greater is required1 if the use of the ,mercurial vapour be omitted4 ,m4 ,ed4 ,becquerel's discovery of the continuing action of rays of yellow light4--,pure homogeneous yellow light has no action upon the ,daguerreotype plate2 but if the iodized surface be first exposed to white light for a sufficient time to impress a latent image1 and then afterwards to the yellow light1 the action already commenced is continued1 and even to the extent of forming the peculiar white deposit1 insoluble in ,hyposulphite of ,soda1 already alluded to4 ,yellow light may therefore in this sense be spoken of as a developing agent1 since it produces the same effect as the ,mercurial vapour in bringing out to view the latent image4 ,a singular anomaly however requires notice1 viz4 that if the plate be prepared with the mixed vapours of ,bromine and ,iodine1 in place of ,iodine alone1 then the yellow light cannot be made to develope the image4 ,in fact1 the same coloured ray which continues the action of white light upon a surface of ,iodide of ,silver1 actually destroys it1 and restores the particles to their original condition1 with a surface of ,bromo-,iodide of ,silver4 ,these facts1 although not of great practical importance1 are interesting in illustration of the delicate and complex nature of the chemical changes produced by light4 ,the ,strengthening of the ,daguerreotype ,image by means of ,hyposulphite of ,gold4--,the use of the ,hyposulphite of ,gold to whiten the ,daguerreotype image1 and render it more lasting and indestructible1 was introduced by ,m4 ,fizeau1 subsequent to the original discovery of the process4 ,after removal of the unaltered ,iodide of ,silver by means of ,hyposulphite of ,soda1 the plate is placed upon a levelling stand and covered with a solution of ,hyposulphite of ,gold1 containing about one part of the salt dissolved in #ejj parts of water4 ,the flame of a spirit-lamp is then applied until the liquid begins to boil4 ,shortly a change is seen to take place in the appearance of the image2 it becomes whiter than before1 and acquires great force4 ,this fact seems to prove conclusively that metallic ,mercury enters into its composition1 since a surface of ,silver--such1 for instance1 as that of the ,collodion image--is darkened by ,hyposulphite of ,gold4 ,the difference in the action of the gilding solution upon the image and the pure ,silver surrounding it illustrates the same fact4 ,this ,silver1 which appears of a dark colour1 and forms the shadows of the image1 is rendered still darker2 a very delicate crust of metallic ,gold gradually forming upon it1 whereas with the image the whitening effect is immediate and striking4 ,,section ,,ii4 ,theory of the ,talbotype and ,albumen ,processes4 ,the ,talbotype or ,calotype4--,this process1 as practiced by many at the present time1 is almost identical with that originally described by ,mr4 ,fox ,talbot4 ,the object is to obtain an even and finely divided layer of ,iodide of ,silver upon the surface of a sheet of paper2 the particles of the ,iodide being left in contact with an excess of ,nitrate of ,silver1 and usually with a small proportion of ,gallic ,acid1 to heighten1 still further1 the sensibility to light4 ,the ,english papers sized with ,gelatine are commonly used for the ,calotype process2 they retain the film more perfectly at the surface1 and the ,gelatine in all probability assists in forming the image4 ,with a foreign starch-paper1 unless it be re-sized with some organic substance1 the solutions sink in too deeply1 and the picture is wanting in clearness and definition4 ,there are two modes of iodizing and sensitizing the sheets3 first1 by floating alternately upon ,iodide of ,potassium and ,nitrate of ,silver1 in the same manner as in the preparation of papers for ,positive ,printing2 and second1 by what is termed the single wash1 which is thought by many to give superior results as regards sensitiveness and intensity of image4 ,to iodize by this mode1 the yellow ,iodide of ,silver1 prepared by mixing solutions of ,iodide of ,potassium and ,nitrate of ,silver1 is dissolved in a strong solution of ,iodide of ,potassium2 the sheets are floated for an instant upon this liquid and dried2 they are then removed to a dish of water1 by the action of which the ,iodide of ,silver is precipitated upon the surface of the paper in a finely divided state4 ,the properties of a solution of ,iodide of ,silver in ,iodide of ,potassium1 or of the double ,iodide of ,potassium and ,silver1 are described at page #dc1 a reference to which will show that the double salt is decomposed by a large quantity of water1 with precipitation of the ,iodide of ,silver1 this substance being insoluble in a dilute solution of ,iodide of ,potassium1 although soluble in a strong solution4 ,paper coated with ,iodide of ,silver by this mode1 after proper washing in water to remove soluble salts 7which if allowed to remain would attract damp71 will keep good for a long time4 ,the layer of ,iodide appears of a pale primrose colour1 and is perfectly insensitive to light4 ,even exposure to the sun's rays produces no change1 thus indicating that an excess of ,nitrate of ,silver is essential to the visible darkening of ,iodide of ,silver by light4 ,the paper is also insensitive to the reception of an invisible image1 differing in this respect from the washed ,collodion plate1 which receives an impression in the ,camera1 although apparently freed from ,nitrate of ,silver4 ,to render ,calotype paper sensitive to light1 it is brushed with a solution of ,nitrate of ,silver containing both ,acetic and ,gallic ,acids1 termed ,aceto-,nitrate and ,gallo-,nitrate solution4 ,the ,gallic ,acid lessens the keeping qualities of the paper1 but increases the sensitiveness4 ,the ,acetic ,acid prevents the paper from blackening all over during the development1 and preserves the clearness of the white parts2 its employment is indispensable4 ,the paper is commonly excited upon the morning of the day upon which it is intended to be used2 and the longer it is kept1 the less active and certain it becomes4 ,an exposure of five to eight minutes in the ,camera is the average time with an ordinary view lens4 ,the picture is developed with a saturated solution of ,gallic ,acid1 to which a portion of ,aceto-,nitrate of ,silver is added to heighten the intensity4 ,both ,sulphate of ,iron1 and ,pyrogallic ,acid have also been used1 but they are unnecessarily strong1 the invisible image being more easily developed upon paper than upon ,collodion 7see page #adc74 ,after fixing the ,negative by removing the unaltered ,iodide of ,silver with ,hyposulphite of ,soda1 it is well washed and dried4 ,white wax is then melted in with a hot iron1 so as to render the paper transparent1 and to facilitate the after-process of printing4 ,the ,calotype cannot be compared with the ,collodion process for sensitiveness and delicacy of detail1 but it possesses advantages for tourists and those who do not wish to be encumbered with large glass plates4 ,the principal difficulty appears to be in obtaining a uniformly good paper1 many samples giving a speckled appearance in the black parts of the ,negative4 ,the ,waxed ,paper process of ,le ,grey4--,this is a useful modification of the ,talbotype introduced by ,m4 ,le ,grey4 ,the paper is waxed before iodizing1 by which1 without involving any additional operation1 a very fine surface layer of ,iodide of ,silver can be obtained4 ,the ,waxed ,paper ,process is well adapted for tourists1 from its extreme simplicity and the length of time which the film may be kept in a sensitive condition4 ,both ,english and foreign papers are employed3 but the former take the wax with difficulty4 ,mr4 ,crookes1 who has devoted his attention to this process1 gives clear directions for waxing paper2 it is essential that pure white wax should be obtained direct from the bleachers1 since the flat cakes sold in the shops are commonly adulterated4 ,the temperature must also be carefully kept below that point at which decomposition of the wax takes place2 the use of too hot an iron being a common source of failure 7see ',photographic ,journal1' vol4 ii4 p4 #bca74 ,the sheets of paper1 having been properly waxed1 are soaked for two hours in a solution containing ,iodide and ,bromide of ,potassium1 with1 enough free ,iodine to tinge the liquid of a port-wine colour4 ,the greasy nature of wax impedes the entry of liquids1 and hence a long immersion is required4 ,the iodizing formul of the ,french ,photographers have been encumbered by the addition of a variety of substances which appear to introduce complications without giving proportional advantage1 and ,mr4 ,townshend has done the art a service by proving that the ,iodide and ,bromide of ,potassium1 with free ,iodine1 are sufficient4 ,this latter ingredient was first used by ,mr4 ,crookes2 it seems to add to the clearness and sharpness of the ,negatives2 and as the papers are coloured by the ,iodine1 air-bubbles cannot escape detection4 ,the process of exciting with ,nitrate of ,silver is also rendered more certain by the employment of free ,iodine1 the action of the ,bath being continued until the purple colour gives place to the characteristic yellow tint of the ,iodide of ,silver4 ,waxed ,paper is rendered sensitive by immersion in a ,bath of ,nitrate of ,silver containing ,acetic ,acid2 the quantity of which latter ingredient should be increased when the papers are to be long kept4 ,as the excess of ,nitrate is subsequently removed1 the solution may be used weaker than in the ,calotype or ,collodion process4 ,after exciting1 the papers are washed with water to reduce the amount of free ,nitrate of ,silver to a minimum4 ,this lessens the sensitiveness1 but greatly increases the keeping qualities1 and the paper will often remain good for ten days or longer4 ,it is a very important point1 in operating with ,waxed ,paper1 to keep the developing dishes clean4 ,the development is conducted by immersion in a ,bath of ,gallic ,acid containing ,acetic ,acid and ,nitrate of ,silver2 and being retarded by the superficial coating of wax1 there is always a tendency to an irregular reduction of ,silver upon the white portions of the ,negative4 ,when the developer becomes brown and discoloured1 this is almost sure to happen2 and it is well known to chemists that the length of time during which ,gallic ,acid and ,nitrate of ,silver may remain mixed without decomposing1 is much lessened by using vessels which are dirty from having been before employed for a similar purpose4 ,the black deposit of ,silver exercises a catalytic 71 decomposition by contact7 action upon the freshly-mixed portion1 and hastens its discoloration4 ,the ,waxed ,paper process is exceedingly simple and inexpensive1--very suitable for tourists1 as requiring but little experience1 and a minimum of apparatus4 ,it is however slow and tedious in all its stages1 the sensitive papers frequently taking an exposure of twenty minutes in the ,camera1 and the development extending over an hour or an hour and a half4 ,several ,negatives however may be developed at the same time2 and as the removal of the free ,nitrate of ,silver gives the process a great advantage during hot weather1 it will in all probability continue to be extensively followed4 ,the prints which have been sent to the ,exhibition of the ,photographic ,society1 show that waxed paper in the hands of a skilful operator may be made to delineate architectural subjects with great fidelity1 and also to give the details of foliage and landscape ,photography with distinctness4 ,the ,albumen process upon ,glass4--,the process with ,albumen originated in a desire to obtain a more even surface layer of ,iodide of ,silver than the coarse structure of the tissue of paper will allow4 ,it is conducted with simple ,albumen1 or white of eggs1 diluted with a convenient quantity of water4 ,in this glutinous liquid ,iodide of ,potassium is dissolved2 and the solution1 having been thoroughly shaken1 is set aside1 the upper portion being drawn off for use1 in the same manner as in the preparation of ,albuminized paper for printing4 ,the glasses are coated with the ,iodized ,albumen1 and are then placed horizontally in a box to dry4 ,this part of the process is considered the most troublesome1 the moist ,albumen easily attracting particles of dust1 and being apt to blister and separate from the glass4 ,if an even layer of the dried and ,iodized material can be obtained1 the chief difficulty of the process has been overcome4 ,the plates are rendered sensitive by immersion in a ,bath of ,nitrate of ,silver with ,acetic ,acid added1 and are then washed in water and dried4 ,they may be kept for a long time in an excited state4 ,the exposure in the ,camera must be unusually long2 the free ,nitrate of ,silver having been removed by washing1 and the ,albumen exercising a direct retarding influence upon the sensitiveness of ,iodide of ,silver4 ,the development is conducted in the ordinary way by a mixture of ,gallic ,acid and ,nitrate of ,silver1 with ,acetic ,acid added to preserve the clearness of the lights4 ,it usually requires one hour or more1 but may be accelerated by the gentle application of heat4 ,albumen pictures are remarkable for elaborate distinctness in the shadows and minor details1 and are admirably adapted for viewing in the ,stereoscope2 but they do not often possess the peculiar and characteristic softness of the ,photograph upon ,collodion4 ,the process is well adapted for hot climates1 being very little prone to the cloudiness and irregular reduction of ,silver which are often complained of with moist ,collodion under such circumstances4 ,m4 ,taupenot's ,collodio-,albumen process4--,this is a recent discovery which seems to involve a new principle in the ,art1 and gives promise of great utility4 ,one of the greatest objections to the ,albumen process has been its want of sensitiveness2 but ,m4 ,taupenot found that this was obviated to a great extent by pouring the ,albumen upon a plate previously coated with ,iodide of ,silver4 ,in this way two layers of that sensitive salt are formed1 and the sensibility of the surface layer1 which alone receives the image1 is promoted by its resting upon a substratum of ,iodide rather than upon the inert surface of the glass4 ,in this view1 if the theory be correct1 the lower particle of ,iodide of ,silver promotes the molecular disturbance of the upper1 itself remaining unchanged4 ,other experimenters1 pursuing the subject further1 have asserted that a successful result may be obtained by coating the plate with plain ,collodion and subsequently with ,iodized ,albumen4 ,if this observation should prove correct1 the process will be simplified and its utility increased4 ,in the sixth ,chapter of ,part ,,ii4 the practical details of the ,collodio-,albumen process will be described4 ,,end ,,of ,,part ,i4 ,,part ,,ii4 ,,practical ,,details ,,of ,,the ,,collodion ,,process4 ,,practical ,,details ,,of ,,the ,,collodion ,,process4 ,illustration ,,chapter ,i4 ,,preparation ,,of ,,collodion4 ,this includes--the soluble ,paper2--the ,alcohol and ,ether2--and the iodizing compounds4 ,the formul for ,negative and ,positive ,collodion1 and for the ,nitrate ,bath and developing fluids1 are given in the second ,chapter4 ,,the ,,soluble ,,paper4 ,pyroxyline may be prepared either from cotton wool or from ,swedish ,filtering-paper4 ,most operators prefer the latter1 from its giving a product of constant solubility1 and yielding a fluid solution4#cj ,the ,cotton ,wool however is better adapted for use with the ,sulphuric ,acid and ,nitre1 since the ,paper1 from its closeness of texture1 requires a longer immersion in the mixture4 #cj ,swedish filtering-paper may be procured at the operative chemists'1 at about five shillings the quire4 ,each half-sheet has the water-mark ,j4 ,h4 ,munktell4 ,preparation of a ,nitro-,sulphuric ,acid of the proper strength4--,there are two modes of preparing the ,nitro-,sulphuric ,acid3 first1 by mixing the acids2 second1 by the ,oil of ,vitriol and ,nitre ,process4 ,the former is the best in cases where large quantities of the material are operated on1 but the amateur is recommended to begin by trying the ,nitre ,process 7p4 #aij7 as the most simple4 ,,preparation ,,of ,,nitro-,,sulphuric ,,acid ,,by ,,the ,,mixed ,,acids4 ,the operator may proceed in either of two ways2 first1 by taking the strength of each sample of acid1 and mixing according to fixed rule2 second1 by a more ready plan1 which may be used when the exact strength of the acids is not known4 ,each of these will be described in succession4 a4 ,directions for mixing according to fixed rule4--,this process is given from ,mr4 ,hadow's original paper in the ',quarterly ,journal of the ,chemical ,society4' ,it is certain in its results if the strength of both acids be accurately determined4 ,a very perfect process for taking the strength of ,nitric ,acid is by means of powdered ,marble or ,carbonate of ,lime1 as described in various works on practical ,chemistry4 ,sulphuric ,acid may be estimated by precipitating with ,nitrate of ,baryta1 and weighing the insoluble ,sulphate with the proper precautions4 ,the specific gravity is not a criterion of strength to be perfectly relied on1 but if it be adopted as a test1 the following points must be attended to4 #ast4 ,that the temperature of the acid be at or near #fj ,fahrenheit2 the density of ,sulphuric ,acid especially is1 from its small specific heat1 greatly influenced a change of temperature4 #bnd4 ,the sample of ,nitric ,acid must be free from ,peroxide of ,nitrogen1 or only slightly coloured by it4 ,this substance1 when present1 increases the specific gravity of the acid without adding to its available properties4 ,a yellow sample of ,nitric ,acid will therefore be somewhat weaker than is indicated by the specific gravity4 #crd4 ,the ,oil of ,vitriol should yield no solid residue on evaporation4 ,sulphate of ,lead and ,bisulphate of ,potash are often found in the commercial acid1 and add much to its density4 ,oil of ,vitriol containing ,sulphate of ,lead becomes milky on dilution4 ,the formula for a definite ,nitro-,sulphuric ,acid1 of the proper strength for making the soluble ,pyroxyline1 may be stated thus3-- ,,ho ,,no#e1 #b 7,,ho ,,so#c7 #c-a_/b ,,ho or ,atoms4 ,atomic weight4 ,nitric ,acid #a #ed ,sulphuric ,acid #b #hj ,water #f-a_/b #eh --- #aib ,having found the percentage of real acid which is present1#ca the following calculation will give the relative weights of the ingredients required to produce the formula3-- ,let a percentage of real ,nitric ,acid1 b ,sulphuric ,acid1 then #edjj_/;a quantity of ,nitric ,acid1 #hjjj_/;b ,sulphuric ,acid1 #aib - #edjj_/;a - #hjjj_/;b ,water4 #ca ,tables are given in the ,appendix for calculation by specific gravity2 but direct analysis of the acids is the most certain4 ,observe that the numbers in the calculation correspond to the atomic weights recently given2 and that the amount of water is derived from the total atomic weight1 viz4 #aib1 minus the sum of the weights of both acids4 ,hence if the samples of acid employed are too weak for the purpose1 the formula for the water gives a negative quantity4 ,the weight of mixed acids produced by the formula is #aib grains1 which would measure somewhere about two fluid drachms4 ,ten times this quantity forms a convenient bulk of liquid1 in which about #ej or #fj grains of ,paper may be immersed4 ,in weighing corrosive liquids1 such as ,sulphuric and ,nitric ,acid1 a small glass may be counterbalanced in the scale-pan1 and the acid poured in carefully4 ,if too much is added1 the excess can be removed by a glass rod1 or by the pipette commonly employed for such a purpose4 ,the following example of a calculation similar to the above may be given3-- #ajj parts of the ,oil of ,vitriol #gffe real acid4 ,nitric ,acid #fed real acid4 therefore #hjjj_/gffe #ajdc grains of ,oil of ,vitriol4 #edjj_/fed #hbe ,nitric ,acid #aib - #ajdc - #hbe #eb ,water4 ,multiplying these weights ten times1 we have ,oil of ,vitriol #ajdc grains4 ,nitric ,acid #hbe ,water #eb ---- ,total weight of the #aibj grains4 ,nitro-,sulphuric ,acid ,having prepared the acid mixture of a definite strength by the above formula1 the paper must be immersed according to directions given at page #aia4 b4 ,process for mixing ,nitro-,sulphuric ,acid1 the strength of the two acids not having been previously determined4--,take a strong sample of ,nitric ,acid 7the yellow ,nitrous acid1 so called1 succeeds well71 and mix it with ,oil of ,vitriol as follows3-- ,sulphuric ,acid #aj fluid drachms1 ,nitric ,acid #aj ,now immerse a thermometer and note the temperature2#cb it should be from #acj ,fahr4 to #aej4 ,if it sinks below #abj1 place the mixture in a capsule1 and float upon boiling water for a few minutes4 #cb ,in the preparation of soluble cotton1 and indeed in all ,photographic manipulations1 a thermometer is almost indispensable4 ,instruments of sufficient delicacy for common purposes are sold in ,hatton ,garden and elsewhere1 at a low price4 ,the bulb should be uncovered1 to admit of being dipped in acids1 etc41 without injury to the scale4 ,a preliminary experiment with a small tuft of ,cotton ,wool 7cotton shows it better than paper7 will then indicate the actual strength of the ,nitro-,sulphuric ,acid4 ,stir the tuft in the mixture for five minutes4 ,remove with a glass rod1 and wash with water for a short time1 until no acid taste can be perceived4 ,if the ,wool becomes matted1 and gelatinizes slightly on its first immersion in the acid1 or if1 in the subsequent washing1 the fibres appear to adhere and to be disintegrated by the action of the water1 the ,nitro-,sulphuric ,acid is too weak4 ,in that case add to the acid mixture4 ,oil of ,vitriol1 #c drachms4 ,if the cotton was actually dissolved in the first trial1 an addition of half of a fluid ounce of ,oil of ,vitriol may be required4 ,supposing the cotton not to be gelatinized and to wash well1 then wring it out very dry1 pull out the fibres1 and treat it in a test-tube with rectified ,ether1#cc to which a few drops of ,alcohol have been added4 ,if it be insoluble1 dry it by a gentle heat and apply a flame3 a brisk explosion indicates that the ,nitro-,sulphuric ,acid employed is too strong4 ,in that case1 add to the twenty drachms of mixed acids1 one drachm of water1 and test again1 repeating the process until a soluble product is obtained4 #cc ,observe that the ,ether be pure2 if it contains too much water and ,alcohol1 it will not dissolve the ,pyroxyline1 or will yield an opalescent solution4 ,there is a third condition of ,pyroxyline1 different from either of the above1 which may be puzzling3--the fibres of the ,cotton mat together very slightly or not at all on immersion1 and the washing proceeds tolerably well2 the compound formed is scarcely explosive1 and dissolves imperfectly in ,ether1 leaving little nodules or hard lumps4 ,the ethereal solution yields1 on evaporation1 a film which is opaque instead of transparent4 ,in this case 7presuming the ,ether to be good7 the acid mixture is slightly too weak1 or the temperature is too low1 being probably about #ij1 instead of #acj to #adj 7874 ,when the acid mixture has been brought to the proper strength by a few preliminary trials1 proceed according to the directions given at the next page4 ,,preparation ,,of ,,nitro-,,sulphuric ,,acid ,,by ,,oil ,,of ,,vitriol ,,and ,,nitre4 ,this process is recommended1 in preference to the other1 to the amateur who is unable to obtain ,nitric ,acid of convenient strength4 ,the common ,oil of ,vitriol sold in the shops is often very good for ,photographic purposes2 but it is best1 if possible1 to take the specific gravity1 when any doubt exists of its genuineness4 ,at a temperature of #eh to #fj1 specific gravity #ahcc is the usual strength1 and if it falls below this1 it should be rejected4 7,see ,part ,,iii4 for ',impurities of ,commercial ,sulphuric ,acid4'7 ,the ,nitre must be the purest sample which can be obtained4 ,commercial ,nitre often contains a large quantity of ,chloride of ,potassium1 detected on dissolving the ,nitre in distilled water1 and adding a drop or two of solution of ,nitrate of ,silver4 ,if a milkiness and subsequent curdy deposit is formed4 ,chlorides are present4 ,these ,chlorides are injurious2 after the ,oil of ,vitriol is added1 they destroy a portion of ,nitric ,acid by converting it into brown fumes of ,peroxide of ,nitrogen1 and so alter the strength of the solution4 ,nitrate of ,potash is an anhydrous salt1--it contains simply ,nitric ,acid and ,potash1 without any water of crystallization2 still1 in many cases1 a little water is retained mechanically between the interstices of the crystals1 and therefore it is better to dry it before use4 ,this may be done by laying it in a state of fine powder upon blotting-paper1 close to a fire1 or upon a heated metallic plate4 ,the sample must also be reduced to a fine powder before adding the ,oil of ,vitriol2 otherwise portions of the salt escape decomposition4 ,these preliminaries having been properly observed1 weigh out ,pure ,nitre1 powdered and dried1 #fjj grains4 ,this quantity is equivalent to #a-a_/d ounce ,troy or ,apothecaries' weight2--and to #a-a_/d ounce ,avoirdupois weight plus #ed grains4 ,place this in a teacup or any other convenient vessel1 and pour upon it4 ,water #a-a_/b fluid drachms mixed with ,oil of ,vitriol #ab ,stir well with a glass rod for two or three minutes1 until all effervescence has ceased1 and an even1 pasty mixture1 free from lumps1 is obtained4 ,during the whole process1 abundance of dense fumes of ,nitric ,acid will be given off1 which must be allowed to escape up the flue or into the open air4 ,a modification of the formula4--,the above formula will invariably succeed with a good sample of acid and pure ,nitre4 ,when tried however with ,oil of ,vitriol rather weaker than ordinary1 and commercial ,nitre1 it may fail1 the cotton being gelatinized and dissolved4 ,when such is the case1 the addition of water must be omitted or the quantity reduced from one drachm and a half to half a drachm4 ,,general ,,directions ,,for ,,immersing1 ,,washing1 ,,and ,,drying ,,the ,,pyroxyline4 ,the mixture of ,sulphuric ,acid and ,nitre requires to be used immediately after its preparation1 as it solidifies into a stiff mass on cooling2 but the mixed acids may be kept for any length of time in a stoppered bottle4 ,when ,cotton is used1 the fibres should be well pulled out1 and small tufts added one by one to the acid mixture1 stirring with a glass rod in order to keep up a constant change of particles4 ,the ,paper is cut into squares or strips1 which are introduced singly4 ,in either case the quantity must not be too great1 or some portions will be imperfectly acted upon2 about #bj grains to each fluid ounce of the mixture will be sufficient4 ,the time of immersion required varies from ten minutes with ,cotton1 to twenty minutes or even half an hour with the ,paper4 ,when an unusually large proportion of ,sulphuric ,acid is used1 as in the case of a weak sample of ,nitric ,acid1 the ,cotton should be removed at the expiration of six or seven minutes1 as there is a tendency to partial solution of the ,pyroxyline in the acid mixture under those circumstances4 ,it is an advantage in some cases to prepare the material at a high temperature1 but unless the proportions of the ,acids are strictly according to ,mr4 ,hadow's formula1 solution of the ,cotton may take place if the thermometer indicates more than #adj4 ,after the action is complete1 the ,nitro-,sulphuric ,acid is left weaker than before1 from addition of various atoms of water necessarily formed during the change4 ,hence1 if the same portion be used more than once1 an addition of ,sulphuric ,acid will be required4 ,directions for ,washing4--,in removing the ,pyroxyline from the ,nitro-,sulphuric ,acid1 press out as much of the liquid as possible1 and wash it rapidly in a large quantity of cold water1 using a glass rod to preserve the fingers from injury4 ,if it were simply thrown into a small quantity of water and allowed to remain1 the rise in temperature and weakening of the acid mixture might do mischief4 ,the washing should be continued for at least a quarter of an hour1 or longer in the case of ,paper1 as it is essential to get rid of every trace of acid4 ,when the ,nitre plan has been adopted1 a portion of the ,bisulphate of ,potash formed adheres to the fibres1 and if not carefully washed out1 an opalescent appearance is seen in the ,collodion1 resulting from the insolubility of this salt in the ethereal mixture4 ,if no acid taste can be perceived1 and a piece of blue litmus-paper remains in contact with the fibres for five minutes without changing in colour1 the product is thoroughly washed4 ,it is however a safe plan to place the ,pyroxyline in running water and allow it to remain for several hours4 ,lastly1 wring it out in a cloth1 pull out the fibres1 and dry slowly1 by a moderate heat4 ,after drying1 it may be kept for any length of time in a stoppered bottle4 ,,recapitulation ,,of ,,the ,,general ,,characters ,,of ,,pyroxyline ,,prepared ,,in ,,nitro-,,sulphuric ,,acid ,,of ,,various ,,degrees ,,of ,,concentration4 ,the acid mixture too strong4--,the appearance of the cotton is not much altered on its first immersion in the mixture4 ,it washes well1 without any disintegration4 ,on drying1 it is found to be strong in texture1 and produces a peculiar crackling sensation between the fingers1 like starch4 ,it explodes on the application of flame1 without leaving any ash4 ,it is insoluble in the mixture of ,ether and ,alcohol1 but dissolves if treated with ,acetic ,ether4 ,the acid mixture of the proper strength4--,no agglutination of the fibres of the cotton on immersion1 and the product washes well2 soluble in the ethereal mixture1 and yields a transparent film on evaporation4 ,the acid mixture too weak4--,the fibres of the cotton agglutinate1 and the ,pyroxyline is washed with difficulty4 ,on drying1 the texture is found to be short and rotten4 ,it does not explode on being heated1 but either burns quietly with a flame1 leaving behind a black ash--in which case it consists simply of unaltered cotton1--or is only slightly combustible1 and not explosive4 ,it dissolves more or less perfectly in glacial ,acetic ,acid4 ,when treated with the ethereal mixture1 it is acted on partially1 leaving behind lumps of unchanged cotton2 the solution does not form an even transparent layer on evaporation1 but becomes opaque and cloudy as it dries4 ,this opacity however may be seen to a small extent with any sample of ,pyroxyline1 if the solvents contain too much water4 ,in using ,swedish ,paper in place of ,cotton1 the ,pyroxyline formed in too weak a ,nitro-,sulphuric ,acid is usually insoluble in ,ether and ,alcohol1 and burns slowly like unchanged paper4 ,by studying these characters1 and at the same time bearing in mind that a drachm and a half of water in the quantities of acid given in the formula 7p4 #ahh7 will suffice to cause the difference1 the operator will overcome all difficulties4 ,,purification ,,of ,,the ,,solvents ,,required ,,for ,,collodion4 ,the purity of the ,ether employed is a matter of as much importance in the manufacture of a good ,collodion as that of any other ingredient2 this point must be attended to in order to secure a good result4 ,there are four kinds of ,ether sold by manufacturing chemists2 first1 ordinary rectified ,sulphuric ,ether1 containing a certain percentage of ,alcohol and of water2 specific gravity about #gej4 ,second1 the washed ,ether1 which is the same agitated with an equal bulk of water1 to remove the ,alcohol3 by this proceeding the specific gravity of the fluid is reduced considerably4 ,third1 ,ether both washed and re-rectified from a caustic alkali1 so as to contain neither ,alcohol nor water2 in this case the specific gravity should not be higher than #gbj4 ,fourth1 ,methylated ,ether1 manufactured at a lower price than the others4 ,rectified ,ether of #gej is not to be depended on1 inasmuch as the specific gravity is often made up by adding water instead of alcohol4 ,methylated ,ether should be used only when economy is an object1 as it is prone to acidity and less certain in its properties4 ,some of the qualities which render ,ether unfit for ,photographic purposes1 are as follows3--a peculiar and disagreeable smell1 either of some essential oil1 or of ,acetic ,ether2 an acid reaction to test-paper2 a property of turning alcoholic solution of ,iodide of ,potassium brown with unusual rapidity2 an alkaline reaction to test-paper2 a high specific gravity1 from superabundance of ,alcohol and water4 ,the ,ether which has been both washed and redistilled is always the most uniform in composition1 and especially so if the second distillation be conducted from ,quicklime1 ,carbonate of ,potash1 or ,caustic ,potash4 ,these ,alkaline substances retain the impurities1 which are often of an acid nature1 and leave the ,ether in a fit state for use4 ,the redistillation of ,ether is a simple process3 in dealing with this fluid however the greatest caution must be exercised1 on account of its inflammable nature4 ,even in pouring ,ether from one bottle into another1 if a light of any kind be near1 the vapour is apt to take fire2 and severe injuries have been occasioned from this cause4 ,purification of ,ether by redistillation from a caustic or carbonated alkali4--,take ordinary rectified ,sulphuric ,ether1 and agitate it with an equal bulk of water to wash out the ,alcohol2 stand for a few minutes until the contents of the bottle separate into two distinct strata1 the lower of which--id est1 the watery stratum--is to be drawn off and rejected4 ,then introduce ,caustic ,potash1 finely powdered1 in the proportion of about one ounce to a pint of the washed ,ether2 shake the bottle again many times1 in order that the water--a small portion of which is still present in solution in the ,ether--may be thoroughly absorbed4 ,afterwards set aside for twenty-four hours 7not longer1 or the ,potash may begin to decompose the ,ether71 when it will probably be observed that the liquid has become yellow1 and that a flocculent deposit has formed in small quantity4 ,transfer to a retort of moderate capacity1 supported in a saucepan of warm water1 and properly connected with a condenser4 ,on applying a gentle heat1 the ,ether distils over quietly1 and condenses with very little loss2 care must be taken that none of the alkaline liquid contained in the body of the retort finds its way1 by projection or otherwise1 into the neck1 so as to run down and contaminate the distilled fluid4 ,a more economical plan of purifying ,ether is1 without previous washing with water1 to agitate with ,carbonate of ,potash or with ,quicklime1 and redistil at a moderate temperature4 ,in order to preserve ,ether from decomposition1 it must be kept in stoppered bottles1 nearly full1 and in a dark place4 ,the stoppers should be tied over with bladder and luted1 or a considerable amount of evaporation will take place1 unless the neck of the bottle has been ground with unusual care4 ,after the lapse of some months1 probably a certain amount of decomposition1 evidenced by the liberation of ,iodine on adding ,iodide of ,potassium1 will be found to have taken place4 ,this however is small in amount1 and not of a character to injure the fluid4 ,rectification of ,spirits of ,wine from ,carbonate of ,potash4--,the object of this operation is to remove a portion of water from the spirit1 and so to increase its strength4 ,alcohol thus purified may be added to ,collodion almost to any extent1 without producing glutinosity and rottenness of film4 ,the salt termed ,carbonate of ,potash is a deliquescent salt1--that is1 it has a great attraction for water2 consequently when ,spirits of ,wine are agitated with ,carbonate of ,potash1 a portion of water is removed1 the salt dissolving in it and forming a dense liquid1 which refuses to mix with the ,alcohol1 and sinks to the bottom4 ,at the expiration of two or three days1 if the bottle has been shaken frequently1 the action is complete1 and the lower stratum of fluid may be drawn off and rejected4 ,pure ,carbonate of ,potash is an expensive salt1 and a commoner variety may be taken4 ,it should be well dried on a heated metal plate1 and powdered1 before use4 ,the quantity may be about two ounces to a pint of spirit2 or more1 if an unusually concentrated ,alcohol is required4 ,after the distillation is complete1 a fluid is obtained containing about #ij per cent1 of absolute ,alcohol1 the remaining #aj per cent1 being water4 ,the specific gravity at #fj ,fahrenheit should be from #hae to #hbe2 commercial ,spirit of ,wine being #hcf to #hdj4 ,,preparation ,,of ,,the ,,iodizing ,,compounds ,,in ,a ,,state ,,of ,,purity4 ,these are the ,iodides of ,potassium1 ,ammonium1 and ,cadmium4 ,the properties of each are more fully described in ,part ,,iii4 a4 ,the ,iodide of ,potassium4--,iodide of ,potassium1 as sold in the shops1 is often contaminated with various impurities4 ,the first and most remarkable is ,carbonate of ,potash4 ,when a sample of ,iodide of ,potassium contains much ,carbonate of ,potash1 it forms small and imperfect crystals1 which are strongly alkaline to test-paper1 and become moist on exposure to the air1 from the deliquescent nature of the ,alkaline ,carbonate4 ,sulphate of ,potash is also a common impurity2 it may be detected by ,chloride of ,barium4 ,a third impurity of ,iodide of ,potassium is ,chloride of ,potassium2 it is detected as follows3--,precipitate the salt by an equal weight of ,nitrate of ,silver1 and treat the yellow mass with solution of ,ammonia2 if any ,chloride of ,silver is present1 it dissolves in the ,ammonia1 and1 after filtration1 is precipitated in white curds by the addition of an excess of pure ,nitric ,acid4 ,if the ,nitric ,acid employed is not pure1 but contains traces of free ,chlorine1 the ,iodide of ,silver must be well washed with distilled water before treating it with ,ammonia1 or the excess of free ,nitrate of ,silver dissolving in the ,ammonia would1 on neutralizing1 produce ,chloride of ,silver1 and so cause an error4 ,iodate of ,potash is a fourth impurity often found in ,iodide of ,potassium3 to detect it1 add a drop of dilute ,sulphuric ,acid1 or a crystal of ,citric ,acid1 to the solution of the ,iodide2 when1 if much ,iodate be present1 the liquid will become yellow from liberation of free ,iodine4 ,the rationale of this reaction is as follows3--,the ,sulphuric ,acid unites with the base of the salt1 and liberates ,hydriodic ,acid 7,,hi71 a colourless compound2 but if ,iodic ,acid 7,,io#e7 be also present1 it decomposes the ,hydriodic ,acid first formed1 oxidizing the ,hydrogen into ,water 7,,ho71 and setting free the ,iodine4 ,the immediate production of a yellow colour on adding a weak acid to aqueous solution of ,iodide of ,potassium1 is therefore a proof of the presence of an ,iodate4 ,as ,iodate of ,potash renders ,collodion insensitive1 this point should be attended to4 ,iodide of ,potassium may be rendered very pure by recrystallizing from ,spirit1 or by dissolving in strong ,alcohol of sp4 gr4 #hbc1 in which ,sulphate1 ,carbonate1 and ,iodate of ,potash are insoluble4 ,the proportion of ,iodide of ,potassium contained in saturated ,alcoholic solutions varies with the strength of the spirit 7vide ,part ,,iii41 article ,iodide of ,potassium74 ,solution of ,chloride of ,barium is commonly used to detect impurities in ,iodide of ,potassium2 it forms a white precipitate if ,carbonate1 ,iodate1 or ,sulphate be present4 ,in the two former cases the precipitate dissolves on the addition of pure dilute ,nitric ,acid1 but in the latter it is insoluble4 ,the commercial ,iodide however is rarely so pure as to remain quite clear on the addition of ,chloride of ,barium4 b4 ,the ,iodide of ,ammonium4--,this salt may be prepared by adding ,carbonate of ,ammonia to ,iodide of ,iron1 but more easily by the following process3--,a strong solution of ,hydrosulphate of ,ammonia is first made1 by passing ,sulphuretted ,hydrogen gas into ,liquor ,ammoni4 ,to this liquid1 ,iodine is added until the whole of the ,sulphuret of ,ammonium has been converted into ,iodide4 ,when this point is reached1 the solution at once colours brown from solution of free ,iodine4 ,on the first addition of the ,iodine1 an escape of ,sulphuretted ,hydrogen gas and a dense deposit of ,sulphur take place4 ,after the decomposition of the ,hydrosulphate of ,ammonia is complete1 a portion of ,hydriodic ,acid--formed by the mutual reaction of ,sulphuretted ,hydrogen and ,iodine--attacks any ,carbonate of ,ammonia which may be present1 and causes an effervescence4 ,the effervescence being over1 the liquid is still acid to test-paper1 from excess of ,hydriodic ,acid2 it is to be cautiously neutralized with ,ammonia1 and evaporated by the heat of a water-bath to the crystallizing point4 ,the crystals should be thoroughly dried over a dish of ,sulphuric ,acid1 and then sealed in tubes2 by this means it will be preserved colourless4 ,iodide of ,ammonium is very soluble in ,alcohol1 but it is not advisable to keep it in solution1 from the rapidity with which it decomposes and becomes brown4 ,the most common impurity of commercial ,iodide of ,ammonium is ,sulphate of ,ammonia2 it is detected by its sparing solubility in ,alcohol4 ,carbonate of ,ammonia is also frequently present to a large extent1 in which case an alkaline ,collodion and eventually an alkaline ,nitrate ,bath will be produced4 e4 ,iodide of ,cadmium4--,this salt is formed by heating filings of metallic ,cadmium with ,iodine1 or by mixing the two together with addition of water4 ,iodide of ,cadmium is very soluble both in ,alcohol and ,water2 the solution yielding on evaporation large six-sided tables of a pearly lustre1 which are permanent in the air4 ,the commercial ,iodide is sometimes contaminated with ,iodide of ,zinc2 the crystals being imperfectly formed and slowly liberating ,iodine when dissolved in ,ether and ,alcohol4 ,pure ,iodide of ,cadmium remains nearly or quite colourless in ,collodion1 if the fluid be kept in a cool and dark place4 ,,chapter ,,ii4 ,,formul ,,for ,,solutions ,,required ,,in ,,the ,,collodion ,,process4 ,section ,i4--,solutions for direct ,positives4 ,section ,,ii4--,solutions for ,negative ,photographs4 ,,section ,i4 ,formul for ,solutions for direct ,positives4 ,the solutions are taken in the following order2--,the ,collodion4--,the ,nitrate ,bath4--,developing fluids4--,fixing liquids4--,whitening solution4 ,,the ,,collodion4 ,formula ,no4 #a4 ,purified ,ether1 sp4 gr4 #gbj #e fluid drachms4 ,purified ,alcohol1 sp4 gr4 #hbe #c ,pyroxyline #c to #e grains4 ,pure ,iodide of ,cadmium or ,ammonium #d grains4 ,formula ,no4 #b4 ,rectified ,ether1 sp4 gr4 #gej #f fluid drachms4 ,spirits of ,wine1 sp4 gr4 #hcf #b ,pyroxyline #b to #d grains4 ,iodide of ,potassium or ,ammonium #c to #d ,if the operator wishes to prepare a stock of the plain ,collodion1 and to iodize as required1 the last formula will stand thus3-- ,rectified ,ether1 #gej #c fluid ounces4 ,alcohol of #hcf #b fluid drachms4 ,pyroxyline #h to #ad grains4 ,dissolve the ,pyroxyline1 and let the fluid stand for forty-eight hours to subside1 then draw off clear1 with a siphon4 ,to each fluid ounce of this plain ,collodion add about two fluid drachms of the following iodizing mixture3-- ,alcohol1 sp4 gr4 #hcf #a fluid ounce4 ,iodide of ,potassium #af grains4 ,of the two formul above given1 the first is considered the best1 but the second may be substituted for it when highly rectified spirits cannot be obtained4 ,iodide of ,ammonium chemically pure is perhaps superior to any other ,iodide for preparing a portrait ,collodion1 but ,iodide of ,cadmium1 with addition of free ,iodine1 possesses better keeping properties1 and gives very good results4 ,a mixture of the two ,iodides may also be used advantageously1 or ,iodide of ,potassium may be combined with ,iodide of ,cadmium3 this preparation has been much recommended1 but the ,collodion will be liable to produce a spotted film unless the salts are quite pure4 ,the exact quantity of ,pyroxyline will vary with the temperature at which the preparation was made4 ,the ,collodion should flow smoothly on the glass and remain free from crapy lines on setting4 ,when ,iodide of ,cadmium is used1 the tendency to glutinosity will be a little greater than usual1 which must be obviated by the directions given at page #hc4 ,the film1 after dipping in the ,bath1 should appear opalescent and not too yellow and creamy4 ,pale-blue films yield very good ,positives1 but with more liability to failure than thicker films 7p4 #aji74 ,if the ,positives are not perfectly clear and transparent in the shadows1 dissolve #e grains of ,iodine in an ounce of ,spirits of ,wine 7not methylated71 and add a few drops until the ,collodion assumes a golden-yellow colour4 ,in hot weather advantage will be gained by somewhat increasing the quantity of ,alcohol in ,collodion2 the evaporation of the solvents being retarded1 and the film rendered less liable to become dry before development4 ,anhydrous ,alcohol of ,sp4 ,gr4 #gif1 may be mixed with pure ,ether of #gae1 even to the extent of equal parts2 but this is the extreme limit1 and with the strongest spirit ordinarily obtainable1 the ,collodion will often become somewhat glutinous if the proportions 7by measure7 of #e parts of ,ether to #c of ,alcohol be exceeded4 ,collodion prepared by ,formula ,no4 #a1 and iodized with ,iodide of ,cadmium1 may be kept for weeks or months without much loss of sensitiveness2 but when ,alkaline ,iodides are employed as in the second ,formula1 ,iodine is liberated1 and the fluid becomes at last brown and insensitive4 ,,the ,,nitrate ,,bath4 ,nitrate of ,silver #cj grains4 ,nitric ,acid #a_/bj minim1 or ,acetic ,acid 7glacial7 #a_/f minim4 ,alcohol #ae minims4 ,distilled water #a fluid ounce4 ,nitrate of ,silver which has been melted1 in order to expel ,oxides of ,nitrogen1 is always the most certain in its action3 but the heat must not be raised too high or the salt will be contaminated with ,nitrite of ,silver4 ,in the ,vocabulary 7see ,part ,,iii47 directions are given for the preparation and purification of ,nitrate of ,silver2 also for the testing of distilled water1 and the best substitutes when it cannot be obtained4 ,the ,bath must be saturated with ,iodide of ,silver1 and ,nitric ,acid neutralized if it be present4 ,nitrate of ,silver however which has undergone fusion is free from ,nitric ,acid4 ,weigh out the total quantity of crystals of ,nitrate required for the ,bath1 and dissolve in about two parts of water4 ,then take a quarter of a grain of ,iodide of ,potassium to each #ajj grains of ,nitrate1 dissolve in half a drachm of water1 and add to the strong solution2 a yellow deposit of ,iodide of ,silver first forms1 but on stirring is completely re-dissolved4 ,when the liquid is clear1 test for free ,nitric ,acid by dropping in a piece of blue litmus-paper4 ,if at the expiration of two minutes the paper appears reddened1 ,nitric ,acid is present1 to neutralize which1 add solution of ,potash or ,carbonate of ,soda 7not ,ammonia7 until a distinct turbidity1 remaining after agitation1 is produced 7an excess does no harm74 ,then dilute down the concentrated solution with the remaining portion of the water1 stirring all the time1 and filter out the milky deposit4 ,if the liquid does not at first run clear1 it will probably do so on passing it again through the same filter4 ,lastly1 add the ,acetic ,acid 7previously tested for impurities1 see ,part ,,iii47 and the ,alcohol to the filtered liquid4 ,as the bulk of the ,bath becomes lessened by use1 fill it up with a solution containing #dj grains of ,nitrate to the ounce1 which will be found sufficient to maintain the strength nearly at the original point4 ,the common practice of occasionally dropping ,ammonia or ,potash into the solution1 to remove ,nitric ,acid liberated by free ,iodine in the ,collodion1 is not recommended 7see p4 #hi74 ,when the ,bath becomes old1 and yields ,positives which are highly intense or stained1 and slightly foggy1 with a deficiency of half-tone1 it will be advisable to precipitate it with a ,chloride and prepare a new one4 ,,the ,,developing ,,fluids4 ,either of the three following formul may be used1 according to the taste of the operator3-- ,,formula ,no4 #a4 ,sulphate of ,iron1 recrystallized #ab to #bj grains4 ,acetic ,acid 7glacial7 #bj minims4 ,alcohol #aj minims4 ,water #a fluid ounce4 ,,formula ,no4 #b4 ,pyrogallic ,acid #b grains4 ,nitric ,acid #a drop4 ,water #a fluid ounce4 ,,formula ,no4 #c4 ,solution of ,protonitrate of ,iron #a fluid ounce4 ,alcohol #bj minims4 ,in all these formul1 if distilled water is not at hand1 read the directions in the ,vocabulary1 ,part ,,iii41 ,article ,water1 for the best substitute4 ,remarks upon these ,formul4--,formula ,no4 #a is the most simple1 since the solution can be used as a ,bath1 the same portion being employed many times successively4 ,if it acts too rapidly1 lessen the proportion of ,sulphate of ,iron4 ,an addition of ,nitric ,acid1 half a minim to the ounce1 makes the image whiter and more metallic2 but if too much is used1 the development proceeds irregularly1 and spangles of ,silver are formed4 ,the ,alcohol and ,acetic ,acid render the development uniform by causing the solution of ,protosulphate to combine more readily with the film4 ,the latter also has an effect in whitening the image and increasing its brightness4 ,solution of ,sulphate of ,iron becomes red on keeping1 from a gradual formation of persalt4 ,when it is too weak1 add more of the ,protosulphate4 ,the muddy deposit which settles to the bottom of the ,bath is metallic ,silver1 reduced from the soluble ,nitrate upon the plates4 ,some operators add pure ,nitrate of ,potash to this developing solution1 to form a small portion of ,protonitrate of ,iron4 ,it is said to improve the colour slightly4 ,the proportions are #aj grains of ,nitrate of ,potash to about #ad or #ae grains of ,protosulphate of ,iron4 ,formula ,no4 #b4--;,in this formula1 if the colour of the image is not sufficiently white1 try the effect of increasing the amount of ,nitric ,acid slightly4 ,on the other hand1 if the development is imperfect in parts1 and patches of a green colour are seen1 use three grains of ,pyrogallic ,acid to the ounce1 with less ,nitric ,acid4 ,a few drops of ,nitrate of ,silver solution added to the ,pyrogallic1 immediately before use1 will augment the energy of development when blue and green spots occur4 ,formula ,no4 #c1 or ,protonitrate of ,iron1 does not require any addition of ,acid2 but it will be advisable1 in some cases1 to add to it a few drops of ,nitrate of ,silver immediately before developing4 ,it gives a bright metallic image1 resembling that obtained by adding ,nitric ,acid to ,protosulphate of ,iron4 ,the following process is commonly followed for preparing ,protonitrate of ,iron3-- ,take of ,nitrate of ,baryta #cjj grains2--powder and dissolve by the aid of heat in three ounces of water4 ,then throw in by degrees1 with constant stirring1 crystallized ,sulphate of ,iron1 powdered1 #cbj grains4 ,continue to stir for about five or ten minutes4 ,allow to cool1 and filter from the white deposit1 which is the insoluble ,sulphate of ,baryta4 ,in place of ,nitrate of ,baryta1 the ,nitrate of ,lead may be used 7,sulphate of ,lead being an insoluble salt71 but the quantity required will be different4 ,the atomic weights of ,nitrate of ,baryta and ,nitrate of ,lead are as #aca to #aff2 consequently #cjj grains of the former are equivalent to #chj grains of the latter4 ,,the ,,fixing ,,solution4 ,cyanide of ,potassium #b to #ab grains4 ,common ,water #a fluid ounce4 ,cyanide of ,potassium is usually preferred to ,hyposulphite of ,soda for fixing direct ,positives2 it is less liable to injure the purity of the white colour4 ,the percentage of ,carbonate of ,potash in commercial ,cyanide of ,potassium is so variable that no exact directions can be given for the formula4 ,it is best however to use it rather dilute--of such a strength that the plate is cleared gradually in from half a minute to a minute4 ,the solution of ,cyanide of ,potassium decomposes slowly on keeping1 but it will usually retain its solvent power for several weeks4 ,in order to escape inconvenience from the pungent odour evolved by this salt1 many employ a vertical ,bath to hold the solution2 but in that case the plates must be carefully washed before fixing1 as the ,iron salts hasten the decomposition of the ,cyanide4 ,,the ,,whitening ,,solution4 ,bichloride of ,mercury #cj grains4 ,distilled ,water #a fluid ounce4 ,by a gentle application of heat the corrosive sublimate dissolves and forms a solution as nearly as possible saturated at common temperatures4 ,the addition of a portion of ,muriatic ,acid enables the water to take up a larger quantity of ,bichloride2 but this concentrated solution1 at the same time that it whitens more quickly than the other1 is apt to act unequally upon different parts of the image4 ,before applying the ,bichloride1 the image is to be fixed and the plate well washed4 ,either the ,protosulphate of ,iron or the ,pyrogallic ,acid with ,acetic 7p4 #bbc7 may be used for the development2 but the whitening process is more rapid and uniform in the latter case4 ,,section ,,ii4 ,formul1 etc41 for ,negative ,solutions4#cd #cd ,the same ,collodion and ,nitrate ,bath may be used both for ,positives and ,negatives if required2 but there are a few minor points of difference which are included in the following remarks4 ,,the ,,collodion4 ,,formula ,no4 #a4 ,purified ,ether1 sp4 gr4 #gbj #e fluid drachms4 ,purified ,alcohol1 sp4 gr4 #hbe #c fluid drachms4 ,soluble ,pyroxyline #d to #h grains4 ,pure ,iodide of ,cadmium or ,ammonium #d to #e grains4 ,,formula ,no4 #b4 ,rectified ,ether1 sp4 gr4 #gej #f fluid drachms4 ,alcohol1 sp4 gr4 #hcf #b fluid drachms4 ,soluble ,pyroxyline #d to #h grains4 ,iodide of ,potassium or ,ammonium #d grains4 ,when the ,collodion and ,iodizing mixture are kept separate1 the second formula will stand thus3-- ,rectified ,ether #gej #c fluid ounces4 ,alcohol of #hcf #b fluid drachms4 ,pyroxyline #ae to #cj grains4 ,to each fluid ounce of this plain ,collodion add #b fluid drachms of the following ,iodizing solution3-- ,alcohol1 sp4 gr4 #hcf #a fluid ounce4 ,iodide of ,potassium #bj grains4 ,when the temperature of the ,nitro-,sulphuric ,acid used in making the ,pyroxyline is high 7#adj to #aee71 it often happens that the ,collodion is too fluid with #d grains of soluble paper to the ounce1 and forms a blue transparent film of ,iodide on dipping the plate in the ,bath4 ,in that case1 increase the quantity of ,pyroxyline from #d grains to #f1 or even to #h grains to each ounce4 ,if the ,collodion is glutinous1 and produces a wavy surface1 with less than #d grains of ,pyroxyline to the ounce1 it is probable that the ,alcohol is too weak1 or that the soluble ,cotton is badly made4 ,if flakes of ,iodide of ,silver are seen loose upon the surface of the film1 and falling away into the ,bath1 the ,collodion is over-iodized1 and it will be impossible to obtain a good picture4 ,after the ,collodion has been employed to coat a number of plates1 the relative proportions of ,alcohol and ,ether contained in it become changed1 from the superior volatility of the latter fluid3 when it ceases to flow readily1 and gives a more dense film than usual1 thin it down by the addition of a little rectified ,ether4 ,in dissolving the ,pyroxyline1 any fibrous or flocculent matter which resists the action of the ,ether1 must be allowed to subside1 the clear portion being decanted for use4 ,the ,iodide of ,potassium is to be finely powdered1 and digested with the spirit until dissolved2 it is better not to apply any heat4 ,both ,iodide of ,ammonium and ,iodide of ,cadmium dissolve almost immediately1 if the salts are pure4 ,the ,collodion must be kept in a cool and dark place4 ,when prepared with ,iodide of ,ammonium or ,potassium it becomes at length high coloured and insensitive4 ,the free ,iodine may then be removed by a strip of pure zinc or silver-foil4 ,when sensitiveness is not an object1 many prefer working with an old1 coloured ,collodion1 finding that it gives more intensity4 ,it has been shown at page #ig that a peculiar change takes place in ,collodion after iodizing1 by which the intensity of the image is increased4 ,directions for using ,glycyrrhizine in ,collodion4--,the action of this material has been described at page #aad4 ,the ,collodion should be iodized with the ,iodide of ,cadmium only1 or with a mixture of the ,iodides and ,bromides of the alkalies4 ,the condition which calls for the employment of ,glycyrrhizine is that often found in a newly made and rather glutinous ,collodion1 viz4 sensitiveness of film1 with good half-tones1 but insufficient intensity in the high lights4 ,dissolve the ,glycyrrhizine in ,alcohol 7not ,methylated7 in the proportion of #e grains to the ounce3 this solution may perhaps keep unchanged for three or four months4 ,to each ounce of the ,collodion add from one to four drops1 and expose in the ,camera a few seconds longer than before4 ,the effect of the ,glycyrrhizine upon the ,collodion may not be fully produced immediately2 if so1 the fluid must be set aside for twenty-four hours4 ,use of ,nitro-glucose in ,collodion4--,nitro-glucose is a substance analogous to ,pyroxyline1 but more unstable4 ,when added to ,collodion iodized with the alkaline ,iodides1 it slowly decomposes1 liberates ,iodine1 lessens the sensitiveness to a certain extent1 and confers intensity4 ,like ,glycyrrhizine1 it may be used to remedy feebleness of the image1 and to give opacity to the blacks4 ,prepare the ,nitro-glucose by the directions given in the ,vocabulary1 ,part ,,iii4 ,dissolve twenty grains in an ounce of pure spirit1 and agitate with powdered chalk to remove free acid4 ,add from five to eight drops to each ounce of ,collodion4 ,in a few days1 more or less1 according to temperature1 the ,collodion will deepen in colour1 and will be found on trial to produce a more vigorous picture4 ,collodion for hot ,climates4--,in this case the ,iodide of ,ammonium should be avoided1 as unstable and prone to change colour4 ,iodide of ,cadmium may be substituted1 which has been shown to remain quite colourless when dissolved in ,alcohol and ,ether4 ,collodion iodized with the ,iodide of ,potassium will usually keep for about six weeks or two months2 but no certain rule can be given1 much depending upon the condition of the ,ether and the heat of the weather4 ,plain ,collodion may retain its properties unimpaired for five or six months1 sometimes much longer2 but there is a tendency to a formation of the acid principle 7p4 #he72 and hence1 on the addition of an alkaline ,iodide to old ,collodion1 the coloration is commonly very rapid4 ,the structure of the transparent film may also be injured by keeping plain ,collodion for too long a time4 ,photographers who wish to operate with ,collodion in hot climates will find it advantageous to carry with them the prepared ,pyroxyline and the spirituous solvents1 observing that the bottles are carefully luted1 and that a bubble of air is left in the neck of each1 to allow for the necessary expansion1 which might otherwise burst the glass or force out the stopper4 ,,the ,,nitrate ,,bath4 ,this solution may be prepared by the same formula as that given for direct ,positives at page #bjc1 acidifying the solution with ,acetic ,acid in preference to ,nitric ,acid4 ,,the ,,developing ,,solution4 ,pyrogallic ,acid #a grain4 ,acetic ,acid 7glacial7 #aj to #bj minims1 or ,beaufoy's ,acetic ,acid fort4 #a fluid drachm4 ,alcohol #aj minims4 ,distilled ,water #a fluid ounce4 ,in place of ,distilled ,water1 pure ,rain-,water may be used 7see ,part ,,iii41 ,art4 ,water74 ,the quantity of ,acetic ,acid required will vary with the strength of the ,acid and the temperature of the atmosphere4 ,an excess enables the manipulator to cover the plate more easily before the action begins1 but when the picture is taken in a dull light1 is apt to give a bluish1 inky hue to the image4 ,in cold weather1 use less of the ,acetic and twice the quantity of ,pyrogallic ,acid4 ,with ,collodion prepared from ,spirits nearly anhydrous1 and iodized with ,iodide of ,cadmium1 the full quantity of ,acetic ,acid will be required1 as there is sometimes a little difficulty in making the developer flow up to the edge of the film4 ,if the image cannot be rendered sufficiently black1 two or three minims of the ,nitrate ,bath solution may be added to each drachm towards the end of the development4 ,if the solution be kept for some time after its first preparation1 it becomes brown and discoloured4 ,in this state it will still develope the image1 but is less likely to give a clear and vigorous picture4 ,a solution of ,pyrogallic ,acid in ,acetic ,acid will keep for many weeks1 and may be diluted down when required for use4 ,the following is a good formula3-- ,pyrogallic acid #ab grains4 ,beaufoy's ,acetic acid #a fluid ounce4 ,to one drachm add seven drachms of water4 ,,the ,,fixing ,,liquid4 ,cyanide of ,potassium #b to #ab to #bj grains4 ,water #a fluid ounce4 or1 ,hyposulphite of ,soda #a_/b ounce4 ,water #a fluid ounce4 ,for remarks on the ,cyanide of ,potassium ,fixing ,bath1 see the last ,section1 page #bjg4 ,,chapter ,,iii4 ,,manipulations ,,of ,,the ,,collodion ,,process4 ,these may be classed under five heads3--,cleaning the ,plates4--,coating with ,iodide of ,silver4--,exposure in the ,camera4--,developing the image4--,fixing the image4--,in addition to this1 the present ,chapter will include in separate ,sections directions for the choice and management of lenses1 for copying engravings1 manuscripts1 etc41 and for taking stereoscopic and microscopic photographs4 ,,cleaning ,,the ,,glass ,,plates4 ,care should be taken in selecting glass for use in ,photography4 ,the ordinary window-glass is inferior1 having scratches upon the surface1 each of which may cause an irregular action of the developing fluid2 and the squares are seldom flat1 so that they are apt to be broken in compression during the printing process4 ,the patent plate answers better than any other description of glass2 but if it cannot be procured1 the flatted crown glass may be substituted4 ,before washing the glasses1 each square should be roughened on the edges by means of a file or a sheet of emery-paper2 or more simply1 by drawing the edges of two plates across each other4 ,if this precaution be omitted1 the fingers are liable to injury1 and the ,collodion film may contract and separate from the sides4 ,in cleaning glasses1 it is not sufficient1 as a rule1 to wash them simply with water2 other liquids are required to remove grease1 if present4 ,a cream of ,tripoli powder and ,spirits of ,wine1 with a little ,ammonia added1 is commonly employed4 ,a tuft of cotton is dipped in this mixture1 and the glasses are well rubbed with it for a few minutes4 ,they are then rinsed in plain water and wiped dry with a cloth4 ,the cloths used for cleaning glasses should be kept expressly for that purpose2 they are best made of a material sold as fine diaper1 and very free from flocculi and loosely-adhering fibres4 ,they are not to be washed in soap and water1 but always in pure water or in water containing a little ,carbonate of ,soda4 ,after wiping the glass carefully1 complete the process by polishing with an old silk handkerchief1 avoiding contact with the skin of the hand4 ,some object to silk1 as tending to render the glass electrical1 and so to attract particles of dust1 but in practice no inconvenience will be experienced from this source4 ,before deciding that the glass is clean1 hold it in an angular position and breathe upon it4 ,the importance of attending to this simple rule will be at once seen by referring to the remarks made at page #ci4 ,in the ,honey preservative and ,collodio-,albumen processes it is especially needful that the glasses should be thoroughly cleaned1 on account of the tendency which the film has to become loosened or to blister during the development and washings4 ,caustic ,potash1 sold by the druggists under the name of ,liquor ,potass1 is very efficacious1 or in place of it1 a warm solution of washing ,soda 7,carbonate of ,soda74 ,liquor ,potass1 being a caustic and alkaline liquid1 softens the skin and dissolves it2 it must therefore be diluted with about four parts of water and applied to the glass by means of a cylindrical roll of flannel4 ,after wetting both sides thoroughly1 allow the glass to stand for a time until several have been treated in the same way2 then wash with water and rub dry in a cloth4 ,the use of an alkaline solution is usually sufficient to clean the glass1 but some plates are dotted on the surface with small white specks1 not removable by ,potash4 ,these specks may consist of hard particles of ,carbonate of ,lime1 and when such is the case they dissolve readily in a dilute acid1--,oil of ,vitriol1 with about four parts of water added1 or dilute ,nitric ,acid4 ,the objection to the use of ,nitric ,acid is1 that if allowed to come in contact with the dress1 it produces stains which cannot be removed unless immediately treated with an alkali4 ,a drop of ,ammonia should be applied to the spot before it becomes yellow and faded4 ,when ,positives are to be taken1 it is advisable to use additional care in preparing the glass1 and especially so with pale transparent films and neutral1 ,nitrate ,bath4 ,after a glass has been once coated with ,collodion1 it is not necessary in cleaning it a second time to use anything but pure water2 but if the film has been allowed to harden and become dry1 possibly dilute ,oil of ,vitriol or ,cyanide of ,potassium may be required to remove stains4 ,when glasses have been repeatedly used in photography they often become at length so dull and stained1 that it is better to reject them4 ,,coating ,,the ,,plate ,,with ,,the ,,collodio-,,iodide ,,of ,,silver4 ,this part of the process1 with that which follows1 must be conducted in a room from which chemical rays of light are excluded4 ,it is inferred therefore that the operator has provided himself with an apartment of that kind4 ,the most simple plan of preparing the room is to nail a treble thickness of yellow calico completely over the window1 or a part of it1 the remainder being darkened4 ,to this a single thickness of a waterproof material made by coating linen with gutta-percha may be added as a further security against the entrance of white light1 the smallest pencil of which admitted into the room would cause fogging4 ,it is often convenient to illuminate by means of a candle screened by yellow glass4 ,a dark orange yellow1 approaching to brown1 is more impervious to chemical rays than a lighter canary-yellow4 ,lamps suitable for the purpose are sold by the manufacturers of apparatus and chemicals4 ,before coating the plate with ,collodion1 see that the fluid is perfectly clear and transparent1 and that all particles have settled to the bottom2 also that the neck of the bottle is free from hard and dry crusts1 which1 if allowed to remain1 would partially dissolve and produce stri upon the film4 ,in taking small portraits and stereoscopic subjects1 these points are of especial importance1 and every picture will be spoiled if they are not attended to4 ,a useful piece of apparatus for clearing ,collodion is that represented in the following woodcut4 ,illustration ,the ,collodion1 having been iodized some hours previously1 is allowed to settle down and become clear in this bottle2 then by gently blowing at the point of the shorter tube1 the small glass siphon is filled1 and the fluid drawn off more closely than could be done by simply pouring from one bottle to another4 ,when the ,collodion is properly cleared from sediment1 the operator takes a glass plate1 previously cleaned1 and wipes it gently with a silk handkerchief1 in order to remove any particles of dust which may have subsequently collected4 ,if it be a plate of moderate size1 it may be held by the corners in a horizontal position1 between the forefinger and thumb of the left hand4 ,the ,collodion is to be poured on steadily until a circular pool is formed1 extending nearly to the edges of the glass4 ,illustration ,by a slight inclination of the plate the fluid is made to flow towards the corner marked #a1 in the above diagram1 until it nearly touches the thumb by which the glass is held3 from corner #a it is passed to corner #b1 held by the forefinger2 from #b to #c1 and lastly1 the excess poured back into the bottle from the corner marked ,no4 #d4 ,it is then to be held vertically over the bottle for a moment1 until it nearly ceases to drip1 and then1 by raising the thumb a little1 the direction of the plate is changed1 so as to cause the diagonal lines to coalesce and produce a smooth surface4 ,the operation of coating a plate with ,collodion must not be done hurriedly1 and nothing is required to ensure success but steadiness of hand and a sufficiency of the fluid poured in the first instance upon the plate4 ,in coating larger plates1 the pneumatic holder1 which fixes itself by suction1 will be found the most simple and useful4 ,the ,proper ,time for immersing the ,film in the ,bath4--,after exposing a layer of ,collodion to the air for a short time1 the greater part of the ,ether evaporates1 and leaves the ,pyroxyline in a state in which it is neither wet nor dry1 but receives the impression of the finger without adhering to it4 ,photographers term this setting1 and when it takes place it is a sign that the time has come for submitting it to the action of the ,bath4 ,if the film be lowered into the ,nitrate before it has set1 the effect is the same as that produced by adding ,water to ,collodion4 ,the ,pyroxyline is precipitated in part1 and consequently there are cracks1 and the developer will not always run up to the edge of the film4 ,on the other hand1 if it be allowed to become too dry1 the ,iodide of ,silver does not form perfectly1 and the film1 on being washed and brought out to the light1 exhibits a peculiar iridescent appearance1 and is paler in some parts than in others4 ,no rule can be given as to the exact time which ought to elapse3 it varies with the temperature of the atmosphere1 and with the proportions of ,ether and of ,pyroxyline2 thin ,collodion containing but little ,alcohol requiring to be immersed more speedily4 ,twenty seconds in the common way1 or ten seconds in hot weather1 will be found an average time4 ,when the plate is ready1 rest it upon the glass dipper1 ,collodion side uppermost1 and lower it into the solution by a slow and steady movement3 if any pause be made1 a horizontal line corresponding to the surface of the liquid will be formed4 ,then place the cover upon the vertical trough#ce and darken the room1 if this has not already been done4 ,as the presence of white light does no injury to the plate previous to its immersion in the ,bath1 it is not necessary to exclude it during the time of coating with ,collodion4 #ce ,troughs made of gutta-percha1 glass1 or porcelain are commonly used2 the latter are the best1 being quite opaque and not liable to cracks or leaking4 ,when the plate has remained in the solution about twenty seconds1 lift it partially out two or three times1 in order to wash away the ,ether from the surface4 ,an immersion of one minute to a minute and a half will usually be sufficient2 or two minutes in cold weather1 and with ,collodion containing but little ,alcohol4 ,continue to move the plate until the liquid flows off in a uniform sheet1 when the decomposition may be considered to be sufficiently perfect4 ,the principal impediment in this part of the process lies in the difficulty with which ,ether and ,water mix together1 which causes the ,collodion surface on its first immersion to appear oily and covered with streaks4 ,by gentle motion the ,ether is washed away1 and a smooth and homogeneous layer obtained4 ,the plate is next removed from the dipper1 and held vertically in the hand for a few seconds upon blotting paper1 to drain off as much as possible of the solution of ,nitrate of ,silver4#cf ,it is then wiped on the back with filtering-paper1 placed in a clean and dry slide1 and is ready for the ,camera4 #cf ,this blotting-paper must be frequently changed1 or stains will be produced at the lower edge of the plate during the development4 ,the amateur is strongly recommended not to proceed to take pictures in the ,camera until by a little practice he has succeeded in producing a perfect film which is uniform in every part and will bear inspection when washed and brought out to the light4 ,it should1 if properly prepared1 present the following appearance3--,smooth and uniform1 both by reflected and transmitted light2 free from wavy lines or markings such as would be caused by a glutinous ,pyroxyline1 and from opaque dots due to small particles of dust or ,iodide of ,silver in suspension in the ,collodion4 ,the evidences of a too rapid immersion in the ,bath are sought for on the side of the plate from which the ,collodion was poured off4 ,this part remains wet longer than the other1 and always suffers the most2 horizontal cracks or marks resembling vegetation are seen1 each of which would cause an irregular action of the developing fluid4 ,on the other hand1 the upper part of the plate must be examined for the pale colour characteristic of a film which had become too dry before immersion1 since the ,collodion is thinner at that point than at any other4 ,,exposure ,,of ,,the ,,plate ,,in ,,the ,,camera4 ,after the plate has been rendered sensitive1 it should be exposed and developed with all convenient despatch2 the intensity of the ,negatives being1 with some ,collodion1 materially lessened by neglecting this point 7see p4 #ajj74 ,ascertain that the joints of the ,camera are tight in every part--that the sensitive plate1 when placed in the slide1 falls precisely in the same plane as that occupied by the ground glass--and that the chemical and visual foci of the ,lens accurately correspond4#cg #cg ,see the ,second ,section of this ,chapter4 ,supposing the case of a portrait1 next proceed to arrange the sitter as nearly as possible in a vertical position1 that every part may be equidistant from the lens4 ,then1 an imaginary line being drawn from the head to the knee1 point the ,camera slightly downwards1 so that it may stand at right angles to the line4 ,if this point be neglected the figure will be liable to be distorted in a manner presently to be shown 7p4 #bbh74 ,in order to succeed well with portraits1 the sitter should be illuminated by an even1 diffused light falling horizontally4 ,a vertical light causes a deep shadow on the eyes and makes the hair appear grey3 it must therefore be cut off by a curtain of blue or white calico suspended over the head4 ,the direct rays of the sun are generally to be avoided1 as causing too great a contrast of light and shade4 ,this is a point on which the operator must exercise his judgment4 ,with a feeble ,collodion1 a better ,negative picture may often be obtained by placing the sitter quite in the open air1 but when the ,collodion and ,bath are in the condition for giving great intensity of image1 the gradation of tone will be inferior unless the light be prevented from falling too strongly upon the face and hands4 ,in focussing the object1 cover the head and the back part of the ,camera with a black cloth1 and shift the ,lens gently until the greatest possible amount of distinctness is obtained4 ,then insert the sensitive plate1 and having raised the door of the slide1 cover all with a black cloth during the exposure1 as a security against white light finding entrance at any part excepting through the ,lens4 ,with regard to the proper time for the exposure1 so much depends upon the brightness of the light and the nature of the ,collodion1 that it must be left almost entirely to experience4 ,the following general rules however may be of use3-- ,in a tolerably bright day in the spring or summer months1 and with a newly-mixed ,collodion1 allow four seconds for a ,positive ,portrait1 and eight seconds for a ,negative4 ,with a double-combination ,lens of large aperture and short focus1 perhaps three seconds1 and six seconds1 or even less1 may be sufficient4 ,in the dull winter months1 in the smoky atmosphere of large cities1 or when using an old ,collodion brown from free ,iodine1 multiply these numbers three or four times1 which will be an approximation to the exposure required4 ,it is by the appearance presented under the influence of the developer1 which will immediately be described1 that the operator ascertains the proper time for exposure to light4 ,,the ,,development ,,of ,,the ,,image4 ,the details of developing the latent image differ so much in the case of ,positive and ,negative pictures1 that it is better to describe the two separately4 ,the development of direct ,positives4--,with ,sulphate of ,iron as a developer1 it is most simple to develope the image by immersion4 ,the solution may conveniently be poured into a vertical trough1 such as that used for exciting1 and the plate immersed by means of a glass dipper in the usual way4 ,unless the weather be cold1 the image makes its appearance in three or four seconds1 and the film is then immediately washed with clear water4 ,whilst in the ,bath1 the plate is kept in gentle motion1 and the operator must not expect to see the image very distinctly1 except the high lights2 the shadows1 being faint1 are partially concealed by the unaltered ,iodide1 but they come out during the fixing4 ,the action of the ,sulphate of ,iron is stopped at an early period1 or an excess of development will be incurred4 ,the ,bath may be used repeatedly4 ,in using ,pyrogallic ,acid or ,nitrate of ,iron to develope glass ,positives1 the plate may be placed upon a levelling-stand1 or held in the hand1 or by the pneumatic holder1 and the solution poured on quickly at one corner2 by blowing gently or inclining the hand1 as the case may be1 it is scattered evenly over the film before the development commences4 ,if any difficulty is experienced in covering a plate evenly with a strong developer before the action commences1 it may be overcome by using a shallow cell formed by cementing two or three thicknesses of window-glass on a piece of patent plate to the depth of a quarter of an inch4 ,the size of the cell should be only slightly larger than the plate intended to be developed1 that the waste of fluid may be as little as possible4 ,the cell is held in the left hand1 and the plate being placed in it1 a sufficient quantity of the developer is poured on at one corner4 ,by a slight inclination1 the fluid is caused to flow in a uniform sheet over the surface of the film1 backwards and forwards4 ,the image starts out quickly1 and the developer is then at once poured off1 and the film washed as before4 ,it is very important in developing ,positives to use a sufficient quantity of the solution to cover the plate easily2 otherwise oily stains and marks are formed1 from the developer not combining properly with the surface of the film4 ,for a plate five inches by four1 three or four drachms will be required1 and so in proportion for larger sizes4 ,the appearance of the ,positive image after developing1 as a guide to the proper time of exposure4--,when the plate has been developed1 it is washed1 fixed1 and laid upon a dark ground1 such as a piece of black velvet1 for inspection4 ,in the case of a portrait1 if the features have an unnaturally black and gloomy appearance1 the dark portions of the drapery1 etc41 being invisible1 the picture has been under-exposed4 ,on the other hand1 in an over-exposed plate1 the face is usually pale and white1 and the drapery misty and indistinct4 ,much however in this respect depends upon the dress of the sitter 7see p4 #ff71 and the manner in which the light is thrown2 if the upper part of the figure is shaded too much1 the face may perhaps be the last to be seen4 ,the operator should accustom himself to expend pains in the preliminary focussing upon the ground glass1 and to ascertain at that time that every part of the object is equally illuminated4 ,for this reason1 pictures taken in a room are seldom successful2 the light falls entirely upon one side1 and hence the shadows are dark and indistinct4 ,the development of ,negative ,pictures4--,this process differs in most respects from that of ,positives4 ,in the latter case1 there is a tendency to over-develope the image2 but in the former1 to stop the action at too early a period2 hence it is common to find ,negative ,pictures which are insufficiently developed1 and too pale to print well4 ,in developing ,negatives1 many operators place the plate upon a levelling-stand1 and distribute the fluid by blowing gently upon the surface2 others prefer holding it in the hand and pouring the fluid on and off from a glass measure4 ,the quantity of developer required will be less than that used for ,positives1 inasmuch as1 if the ,acetic ,acid be present in sufficient excess1 it is easy to cover the plate before the action begins4 ,some ,collodion however1 especially the glutinous kind1 seems to repel the developer and prevent it from running up to the edge of the plate4 ,when this is the case1 or when oiliness and stains are produced1 from the ,bath being old and containing ,ether1 ,alcohol must be added to the solution of ,pyrogallic ,acid4 ,with ordinary ,negative ,collodion1 an addition of ,nitrate of ,silver to the developer will often be required2 but the ,pyrogallic ,acid is to be used alone until the image has reached its maximum of intensity1 which it will do in a minute or so1 according to the temperature of the developing room4 ,the plate may then be examined leisurely by placing it in front of1 and at some distance from1 a sheet of white paper4 ,if it is not sufficiently black1 add about four drops of the ,nitrate ,bath to each drachm of developer1 stir well with a glass rod1 and continue the action until the requisite amount of intensity is obtained4 ,when there is any disposition in the plate to fog towards the end of the development1 it may be obviated by fixing with ,cyanide of ,potassium 7not ,hyposulphite71 and then1 after a careful washing1 intensifying with ,pyrogallic ,acid and ,nitrate of ,silver in the usual way4 ,the glass which contains the mixture of ,pyrogallic ,acid and ,nitrate of ,silver must be washed out after each plate1 as the black deposit hastens the discoloration of the fresh solution 7p4 #agi74 ,appearance of the ,negative image during and after the reducing process1 as a guide to the exposure to light4--,an under-exposed plate developes slowly4 ,by continuing the action of the ,pyrogallic ,acid1 the high lights become very black1 but the shadows are invisible1 nothing but the yellow ,iodide being seen on those portions of the plate4 ,after treatment with the ,cyanide1 the picture shows well as a ,positive1 but by transmitted light all the minor details are invisible2 the image is black and white1 without any half-tone4 ,an over-exposed ,negative developes rapidly at first1 but soon begins to blacken slightly at every part of the plate4 ,after the fixing is completed1 nothing can often be seen by reflected light but a uniform grey surface of metallic ,silver1 without any appearance 7or1 at most1 an indistinct one7 of an image4 ,by transmitted light the plate may appear of a red or brown colour1 and the image is faint and dull4 ,the clear parts of the ,negative being obscured by the fogging1 and the half-shadows having acted so long as nearly to overtake the lights1 there is a want of proper contrast2 hence the over-exposed plate is the exact converse of the under-exposed1 where the contrast between lights and shadows is too well marked1 from the absence of intermediate tints4 ,a ,negative which has received the proper amount of exposure1 usually possesses the following characters after the development is completed3--,the image is partially but not fully seen by reflected light4 ,in the case of a portrait1 any dark portions of drapery show well as a ,positive1 but the features of the sitter are scarcely to be discerned4 ,the plate has a general aspect as of fogging about to commence1 but not actually established4 ,by transmitted light the figure is bright1 and appears to stand out from the glass3 the dark shadows are clear1 without any misty deposit of metallic ,silver2 the high lights black almost to complete opacity4 ,the colour of the image however varies much with the state of the ,bath and ,collodion and with the brightness of the light4 ,the remarks already made under the head of ,positives1 apply equally well to ,negatives2 that is1 it will be difficult to secure gradation of tone1 unless the object be equally illuminated1 without any strong contrast of light and shade4 ,hence the direct rays of the sun are1 as a rule1 to be avoided1 and curtains1 etc41 employed when practicable4 ,,fixing ,,and ,,varnishing ,,the ,,image4 ,after the development is completed1 and the plate has been carefully washed by a stream of water1 it may be brought out to the light and treated with the ,hyposulphite or ,cyanide1 until the unaltered ,iodide is entirely cleared off4 ,some use a ,bath for the ,cyanide2 but it is doubtful whether much saving is effected by doing so4 ,the plate is again to be carefully washed after the fixing2 and especially if ,hyposulphite of ,soda be used4 ,three or four minutes in running water will not be too long1 or the glass may be left in a dish of water for an hour or two4 ,if such precautions are neglected1 crystals form on drying1 and the image is injured4 ,collodion pictures should be protected by a coat of varnish1 both ,negatives and ,positives having been known to fade when exposed to damp air without any covering 7see p4 #aff74 ,to prepare transparent varnish4 ,amber may be dissolved in ,chloroform according to ,dr4 ,diamond's formula2--about #hj grains of amber-beads or pipe-stems should be digested with one ounce of the ,chloroform1 and the clear portion separated by filtration4 ,it may be poured on the plate in the same manner as ,collodion1 and dries up speedily into a hard and transparent layer4 ,the ,spirit ,varnish ordinarily sold for ,negatives requires the aid of heat to prevent the gum from chilling as it dries2 the plate is first warmed gently and the varnish poured on and off in the usual way2 it is then1 whilst still dripping1 held to the fire until the ,spirit has evaporated4 ,a few trials will render the operation easy to perform4 ,white ,lac dissolved in strong ,alcohol or in ,benzole has also been recommended for clear varnish4 ,direct ,positives are to be varnished1 first with a layer of transparent varnish1 and then with black japan4 ,suggett's patent jet is sometimes employed1 but it has a disagreeable smell1 and is apt to crack on drying4 ,the best black japan used by coachmakers is more elastic and less liable to crack4 ,asphalt 7#d oz47 dissolved in mineral ,naphtha 7#aj oz471 with the addition of #cj grains of ,caoutchouc dissolved in half an ounce of the same menstruum1 is also said to stand well4 ,a third formula contains black sealing-wax dissolved in ,alcohol4 ,in either case it will be best to apply first a layer of clear varnish to the film1 and afterwards the black varnish1 which should combine with the other without dissolving it4 ,positives whitened with ,bichloride of ,mercury are injured by varnishing2 they must therefore be backed up with black velvet1 or ,japan laid upon the opposite side of the glass4 ,many prefer taking the picture upon coloured glass1 using only a layer of clear varnish2 but in this case the ,collodion side being left uppermost1 the image is necessarily reversed4 ,,section ,,ii4 ,directions for the use of ,photographic ,lenses4 ,those who are comparatively unacquainted with the science of optics require simple rules to guide them in the choice of a photographic lens1 and in the proper mode of using it4 ,two kinds of ,achromatic lenses are sold1 the ,portrait lens and the ,view lens2 the former of which is constructed to admit a large volume of light1 for the purpose of copying living objects1 etc4 ,a convenient-sized ,camera for small portraits is the half-plate with a lens of about #b-a_/d inches diameter1 and giving a tolerably flat field on a surface of #e inches by #d4 ,much however in this respect will depend upon the quality of the glass and also upon its focal length2 a short focus lens taking a picture more quickly1 but giving a smaller image1 and a field which is misty towards the edge4 ,there is also a great tendency to distortion of the image in portrait lenses of large aperture and short focus1 such as those employed for operating in a dull light4 ,the whole plate portrait lens may be expected to cover #f-a_/b by #d-c_/d inches1 and has a diameter of about #c-a_/d inches4 ,it will take larger pictures than the last1 but not necessarily in a shorter time2 since1 although the aperture for admitting the light is larger1 the focal length is proportionately greater and the light less condensed4 ,the quarter-plate portrait lens of #a-a_/d inch diameter is useful for stereoscopic subjects and small portraits2 which are usually more sharply defined when taken with a small lens4 ,the distance at which the ,camera is to be placed from the sitter in taking a portrait1 will depend upon the focal length of the lens4 ,the effect of bringing the ,camera nearer is to add to the size of the image1 but at the same time to increase the chance of distortion2 hence with every lens of full aperture1 there is a practical limit to the size of picture which can be taken4 ,when it is required to obtain a large image with a small lens1 a stop with a central aperture 7which may be readily made of a piece of circular cardboard blackened with ,indian ink7 must be placed in front of the lens4 ,this will diminish the amount of light1 but will render the picture more distinct towards the edge1 and bring a variety of objects at different distances into focus at the same time4 ,with a stop attached1 the lens may also be brought nearer to the object without distorting4 ,with regard to this subject of the distortion often produced by lenses1 observe particularly1 that with the portrait combination of full aperture1 and especially when the powers of the glass are rather strained by its being advanced too near to the sitter1--all objects near to the lens will be magnified1 and those more removed will appear diminished2 hence1 as the position of the sitter is never quite vertical1 the ,camera must be inclined a little downwards1 or the hands and feet will be enlarged1 the figure in fact becoming pyramidal with the base below2 whereas on the other hand1 if the inclination of the ,camera be too great1 the head and forehead will be enlarged1 and the figure becomes a pyramid with the base above4 ,when groups are taken1 arrange the objects as near as possible equidistant from the lens1 and use a stop if practicable4 ,long-focus lenses are the best for this purpose1 allowing the ,photograph to be taken further off1 and giving a greater variety of objects in focus at the same time4 ,portrait lenses may often be advantageously substituted for ,view lenses in copying objects of still life which are badly lighted4 ,the aperture of the lens being large1 a ,negative can be obtained with an amount of light which would not suffice if a small stop were used4 ,on the other hand1 if the light be unusually bright1 the lens of full aperture is always the most likely1 from its extent of reflecting surface1 to produce a misty and indistinct image4 ,hence the object should be well backed up with some neutral colour1 or1 if that cannot be done1 a pasteboard funnel1 projecting about a foot and a half1 may be fastened in front of the lens1 in order to exclude rays of light not immediately concerned in the formation of the image4 ,if the lens were turned towards distant objects brightly illuminated1 and a portion of sky included1 there would probably be diffused light1 and consequent fogging of the plate on the application of the developer4 ,this effect will also invariably follow if the sun's rays be allowed to fall directly upon the glass4 ,directions for finding the ,plane at which the ,sharpest ,image can be obtained4--,non-,achromatic ,lenses are understood by all to require correction for the chemical focus2 but it is usually said of the compound glasses1 that their two foci correspond4 ,the amateur is recommended1 in order to avoid disappointment1 to test the accuracy of this statement1 and also to see that his ,camera is constructed with care4 ,to do this1 proceed as follows3-- ,first ascertain that the prepared sensitive plate falls precisely in the plane occupied by the ground glass4 ,suspend a newspaper or a small engraving at the distance of about three feet from the ,camera1 and focus the letters occupying the centre of the field2 then insert the slide1 with a square of ground glass substituted for the ordinary plate 7the rough surface of the glass looking inwards71 and observe if the letters are still distinct4 ,in place of the ground glass1 a transparent plate with a square of silver-paper which has been oiled or wetted1 may be used1 but the former is preferable4 ,if the result of this trial seems to show that the ,camera is good1 proceed to test the correctness of the ,lens4-- ,take a ,positive ,photograph with the full aperture of the portrait ,lens1 the central letters of the newspaper being carefully focussed as before4 ,then examine at what part of the plate the greatest amount of distinctness of outline is to be found4 ,it will sometimes happen1 that whereas the exact centre was focussed visually1 the letters on a spot midway between the centre and edge are the sharpest in the ,photograph4 ,in that case the chemical focus is longer than the other1 and by a distance equivalent to1 but in the opposite direction of1 the space which the ground glass has to be moved1 in order to define those particular letters sharply to the eye4 ,when the chemical focus is the shorter of the two1 the letters in the ,photograph are indistinct at every portion of the plate2 the experiment must therefore be repeated1 the lens being shifted an eighth of an inch or less4 ,indeed it will be proper to take many ,photographs at minute variations of focal distance before the capabilities of the lens will be fully shown4 ,the object of finding the point at which the sharpest image is obtained will also be assisted by placing several small figures in different planes and focussing those in the centre4 ,this being done1 if the more distant figures come out distinctly in the ,photograph1 the chemical focus is longer than the ,visual1 or vice vers when the nearest ones are most sharply defined4 ,the ,single ,achromatic ,lens4--,a useful lens for landscape ,photography is one of about #c inches diameter and #ae inches focal length1 which may be expected to cover a field of #aj inches by #h4 ,with the lens1 stops are supplied of various diameters1 the largest of which will be useful in dull weather2 the smaller when the field is required to be rendered sharp to the very edge4 ,the stop is arranged at a certain distance in front of the lens1 and must not be moved4 ,if it were brought close up to the glass1 the field would not be so flat2 the effect being then the same as that of a stop placed in front of a ,portrait ,lens1 viz4 simply to cut off the outside portion of the glass4#ch #ch ,see this subject explained in ',photographic ,journal1' vol4 ii4 p4 #acc4 ,in taking ,photographs of architectural and other subjects with vertical outlines1 it is very important to have the ,camera placed perfectly horizontal2 since1 if it be inclined either upwards or downwards1 the perpendiculars will be destroyed and the object will appear of a pyramidal form1 falling inwards or outwards1 as before shown4 ,it is convenient to rule the ground focussing glass with a number of parallel lines in both directions1 which enables the operator at once to see that the position of the instrument is correct4 ,,section ,,iii4 ,mode of copying ,engravings1 ,etchings1 etc4 ,the engraving to be ,photographed should be removed from its frame 7the glass causing irregular reflection7 and suspended vertically and in a reversed position1 in a good diffused light4 ,a black cloth may be placed behind the picture with advantage if any surface likely to reflect light be presented to the lens4 ,the ,camera must be fixed immovably1 so as not to vibrate in the least degree when the cap of the lens is taken off4 ,it should be pointed at right angles to the picture1 and the focus determined in the ordinary way4 ,either a portrait or a single lens may be used1 with a diaphragm sufficiently small to render the image distinct up to the edge4 ,it is not desirable to employ too thin a ,collodion1 since perfect opacity of the darkest parts of the ,negative is essential4 ,an old ,collodion containing free ,iodine is better than a contractile ,collodion1 as giving a more intense and clear image4 ,pure ,collodion iodized with ,iodide of ,cadmium1 if found wanting in intensity1 may be at once rendered fit for use in copying engravings by adding ,glycyrrhizine 7p4 #bji71 until the dark parts of the negative become very opaque1 and subsequently softening the excessive hardness1 if necessary1 by dropping alcoholic solution of ,iodine into the ,collodion until it reaches a straw-yellow tint4 ,a second formula useful in iodizing ,collodion for a similar purpose is as follows4 ,iodide of ,potassium #d grains4 ,bromide of ,potassium #a grain4 ,this1 with addition of ,glycyrrhizine1 will give a very black image4 ,etchings1 diagrams1 and drawings with pencil or ink1 without much middle-tint1 if on thin paper1 are easily copied without the aid of the ,camera1 by simply laying the sketch upon a sheet of ,negative ,paper1 exposing for a brief time to the light1 and developing with ,gallic ,acid4 ,this yields a ,negative which is employed for printing ,positives in the usual way4 ,full directions on this subject will be found in the ,second ,section of the following ,chapter4 ,a more simple plan1 and one which will succeed when great delicacy is not required1 consists in laying the sketch upon a sheet of ,positive printing paper 7a highly salted paper will be the best1 as giving most intensity7 and exposing to the light until a copy is obtained4 ,all the details are faithfully rendered in this way1 but it is sometimes difficult to obtain a ,negative sufficiently black to yield a vigorous print4 ,,section ,,iv4 ,rules for taking ,stereoscopic ,photographs4 ,binocular pictures of a large size1 for the reflecting ,stereoscope1 may be taken with an ordinary ,view lens of about #ae inches focus4 ,the ground glass of the ,camera having been ruled with cross lines in the manner described at page #bca1 the position of some prominent object is marked upon one of the lines with a pencil1 and the first view is taken4 ,the stand is then moved laterally to the proper distance1 and the ,camera adjusted to its second position by shifting it until the marked object occupies the same place as before4 ,the distance between the two positions should be about one foot when the foreground of the picture is twenty-five feet from the instrument1 or four feet when it is at thirty or forty yards4 ,but1 as before shown at page #ga1 this rule is not to be followed implicitly1 much depending upon the character of the picture and the effect desired4 ,photographs for the lenticular ,stereoscope are taken with small lenses of about #d-a_/b inches focus4 ,for portraits1 a ,camera may advantageously be fitted with two double-combination lenses1 of #a-c_/d inches diameter1 exactly equal in focal length and in rapidity of action4 ,the caps are removed simultaneously1 and the pictures impressed at the same instant4 ,the centres of the lenses may be separated by three inches when the ,camera is placed at about six feet from the sitter1 or four inches when the distance is increased to eight feet4 ,pictures taken with a binocular ,camera of this kind1 require to be mounted in a reversed position to that which they occupy on the glass3 for since the image of the ,camera is inverted1 when it is turned round and made erect1 the right-hand picture will necessarily stand on the left side1 and vice vers4 ,mr4 ,latimer ,clark has devised an arrangement for taking stereoscopic pictures with a single ,camera1 which is exceedingly ingenious4 ,its most important feature is a contrivance for rapidly moving the ,camera in a lateral direction without disturbing the position of the image upon the ground glass4 ,this will be understood by a reference to the following woodcut4 ,illustration ,a strongly-framed ,camera-stand carries a flat table1 about #bj inches wide by #af1 furnished with the usual adjustments4 ,upon this are laid two flat bars of wood in the direction of the object1 and parallel1 and about the width of the ,camera asunder4 ,they are #ah inches in length2 their front ends carry stout pins1 which descend into the table and form centres upon which they turn4 ,their opposite ends also carry similar pins1 but these are directed upwards1 and fit into two corresponding holes in the tail-board of the ,camera4 ,now when the ,camera is placed upon these pins1 and moved to and fro laterally1 the whole system exactly resembles the common parallel ruler4 ,the two bars form the guides1 and the ,camera1 although capable of free lateral motion1 always maintains a parallel position4 ,in this condition of things it is only suited to take stereoscopic pictures of an object at an infinite distance2 but to make it move in an arc1 converging on an object at any nearer distance1 it is only necessary to make the two guide-bars approximate at their nearer end so as to converge slightly towards the object2 and by a few trials some degree of convergence will be readily found at which the image will remain as it were fixed on the focussing glass while the ,camera is moved to and fro4 ,to admit of this adjustment1 one of the pins descends through a ,slot in the table and carries a clamping-screw1 by means of which it is readily fixed in any required position4 ,in order however to render the motion of the ,camera smoother1 it is advisable not to place it directly upon the two guides1 but to interpose two thin slips of wood1 lying across them at right angles1 beneath the front and back of the ,camera respectively 7and which may be fixed to the ,camera if preferred71 and to dust the surfaces with powdered soap-stone or ,french chalk4 ,in addition to this arrangement for moving the ,camera laterally1 the slide for holding the sensitive plates must be modified from the common form4 ,it is oblong in shape1 and being about ten or eleven inches long1 requires some little adaptation to fit it to the end of an ordinary ,camera4 ,the glasses are cut to about #f-c_/d inches by #c-a_/d2 and when coated with ,iodide of ,silver1 the two images are impressed side by side1 the plate being shifted laterally about #b-a_/b inches1 at the same time and in the same direction as the ,camera itself4 ,the operation of taking a portrait is thus performed4 ,the focus having been adjusted for both positions1 and the ,camera and the slide both drawn to the left-hand1 the door is raised and the plate exposed2 the ,camera and the slide are then shifted to the right-hand1 and the plate in its new position having been again exposed1 the door is closed and the operation completed4#ci #ci ,see ',photographic ,journal1' vol4 i4 page #ei4 ,pictures taken with this instrument do not require to be reversed in mounting1 the left picture being purposely formed on the right-hand side of the glass4 ,,section ,v4 ,on the ,photographic delineation of ,microscopic objects4 ,many specimens of ,micro-photography which have been exhibited are exceedingly elaborate and beautiful2 and their production is not difficult to one thoroughly acquainted with the use of the ,microscope and with the manipulations of the ,collodion ,process4 ,it is important however to possess a good apparatus1 and to have it properly arranged4 ,the object-glass of the ordinary compound ,microscope is the only part actually required in ,photography1 but it is useful to retain the body for the sake of the adjustments1 and the mirrors used in the illumination4 ,the eye-piece however1 which simply magnifies the image formed by the object-glass1 is not necessary1 since the same effect of enlargement may be obtained by lengthening out the dark chamber1 and throwing the image further off4 ,arrangement of the ,apparatus4--,the ,microscope is placed with its body in a horizontal position1 and the eye-piece being removed1 a tube of paper1 properly blackened in the interior1 or lined with black velvet1 is inserted into the instrument1 to prevent irregular reflection of light from the sides4 ,a dark chamber of about two feet in length1 having at one end an aperture for the insertion of the eye-piece end of the body1 and at the other a groove for carrying the slide containing the sensitive plate1 is then attached2 care being taken to stop all crevices likely to admit diffused light4 ,an ordinary ,camera may be employed as the dark chamber1 the lens being removed1 and the body lengthened out if required by a conical tube of gutta-percha1 made to fasten into the flange of the lens in front4 ,the whole apparatus should be placed exactly in a straight line1 that the ground glass used in focussing may fall at right angles to the axis of the ,microscope4 ,the length of the chamber1 measuring from the object-glass1 may be from two to three feet1 according to the size of image required2 but if extended beyond this1 the pencil of light transmitted by the object-glass is diffused over too large a surface1 and a faint and unsatisfactory picture is the result4 ,the object should be illuminated by sunlight if it can be obtained1 but a bright diffused daylight will succeed with low-power glasses1 and especially when ,positives are taken4 ,employ the concave mirror for reflecting the light on the object in the latter case2 but in the former the plane mirror is the best1 except with powers exceeding a quarter of an inch1 and of large angular aperture4 ,the image upon the ground glass should appear bright and distinct1 and the field of a circular form and evenly illuminated2 when this is the case1 all is ready for inserting the sensitive plate4 ,the time of exposure must be varied according to the intensity of the light1 the sensibility of the ,collodion1 and the degree of magnifying power2 a few seconds to a minute will be about the extremes2 but minute directions are not required1 as the operator1 if a good ,photographer1 will easily ascertain the proper time for exposing 7see page #bbd74 ,at this point a difficulty will probably occur from the plane of the chemical focus not corresponding1 as a rule1 with that of the visual focus4 ,this arises from the fact that the object-glasses of ,microscopes are over-corrected for colour1 in order to compensate for a little chromatic aberration in the eye-piece4 ,the violet rays1 in consequence of the over-correction1 are projected beyond the yellow1 and hence the focus of chemical action is further from the glass than the visible image4 ,the allowance may be made by shifting the sensitive plate1 or1 what amounts to the same thing1 by removing the object-glass a little away from the object with the fine adjustment screw2 the latter is the most convenient4 ,the exact distance must be determined by careful experiment for each glass2 but it is greatest with the low powers1 and decreases as they ascend4 ,mr4 ,shadbolt gives the following as a guide3--,an inch and a half objective of ,smith and ,beck's make required to be shifted #a_/ejth of an inch1 or two turns of their fine adjustment2 a #b_/crds of an inch1 #a_/bjjth of an inch1 or half a turn2 and a #d_/ajths of an inch1 #a_/ajjjth of an inch1 or about two divisions of the adjustment4 ,with the #a_/dth and higher powers1 the difference between the foci was so small as to be practically unimportant4 ,there is also reason to think that the kind of light employed has an influence upon the separation of the foci4 ,mr4 ,delves finds that with sunlight the difference between them is very small even with the low powers1 and inappreciable with the higher2 whereas in using diffused daylight which has undergone a previous reflection from white clouds1 it is considerable4 ,the object-glasses of the same maker1 and particularly those of different makers1 also vary much2 so that it will be necessary to test each glass separately1 and to register the allowance which is required4 ,having found the chemical focus1 the principal difficulty has been overcome1 and the remaining steps are the same in every respect as for ordinary ,collodion ,photographs4 ,to those who cannot devote their time to ,photography during the day1 ,mr4 ,shadbolt's observations on the use of artificial light may be of service4 ,he employs ,camphine1 which gives a whiter flame than gas1 or a moderator lamp2 placing the source of light in the focus of a plano-convex lens of #b-a_/b to #c inches diameter 7the flat side towards the lamp71 and condensing the parallel rays so obtained on the object1 by a second lens of about #a-a_/b-;inch diameter and #c-;inch focus4 ,this mode of illumination1 being feeble in chemical rays1 is best adapted for object-glasses of low power4 ,the exposure required to produce a ,negative impression with the one-inch glass may be from three to five minutes4 ,as the sensitive plate would be liable to become dry during that time1 it is recommended to coat it with some preservative solution by the modes described in the sixth ,chapter4 ,mr4 ,crookes having lately shown that the ,bromide of ,silver is more sensitive than the ,iodide to artificial light1 a mixture of the two salts may conveniently be used 7see pp4 #ff and #bcb74 ,the development may be conducted in the same manner as that for preserved sensitive plates2 fixing with ,cyanide of ,potassium before the development is fully complete1 if any tendency to fogging is observed 7see page #bbd74 ,the ,rev4 ,w4 ,towler ,kingsley has communicated a process by which very beautiful ,microscopic ,photographs have been obtained4 ,he illuminates 7in the absence of sunlight7 with the brilliant light produced by throwing a jet of mixed ,oxygen and ,hydrogen gases upon a small cone of ,lime or ,magnesia4 ,particular stress is laid upon the object-glass of the ,microscope being a good one for the purpose2 and indeed all who have given attention to the subject are agreed upon this point--that there is a considerable difference in the ,photographic value of objectives1 and this independent of the angular aperture of the glass4 ,,chapter ,,iv4 ,,the ,,practical ,,details ,,of ,,photographic ,,printing4 ,this ,chapter is divided as follows3-- ,section ,i4--,the ordinary direct process of positive printing4 ,section ,,ii4--,positive printing by development4 ,section ,,iii4--,the mode of toning ,positives by ,sel d'or4 ,section ,,iv4--,on printing enlarged or reduced ,positives1 transparencies1 etc4 ,,section ,i4 ,positive ,printing by the direct action of ,light4 ,this includes--the preparation of sensitive paper1--of fixing and toning ,baths1--and the manipulatory details of the process4 ,selection of ,paper for ,photographic ,printing4--,the ordinary varieties of paper sold in commerce are not well adapted for the production of ,positive prints4 ,papers are manufactured purposely which are more smooth and uniform in texture4 ,many samples of even the finest paper are however defective1 and hence each sheet should be examined separately by holding it against the light1 and if spots or irregularities of texture are seen1 it should be rejected4 ,these spots usually consist of small particles of brass or iron1 which1 when the paper is rendered sensitive1 decompose the ,nitrate of ,silver and leave a circular mark very noticeable after fixing4 ,the foreign papers1 ,french and ,german1 are different from the ,english4 ,they are porous and sized with starch1 the ,english being sized with gelatinous animal matter4 ,in all cases there is a difference in smoothness between the two sides of the paper1 which may be detected by holding each sheet in such a manner that the light strikes it at an angle2 the wrong side is that on which dark wavy bands1 of an inch to an inch and a half in breadth1 are seen1 caused by the strips of felt on which the paper was dried4 ,with most qualities of paper no difficulty whatever will be experienced in detecting the broad and regular bands above referred to2 but when they cannot be seen1 the wrong side of the sheet may be known by wire markings crossing each other1 or if the paper be wetted at the corner1 one side may appear evidently smoother than the other4 ,,preparation ,,of ,,sensitive ,,paper4 ,there are three principal varieties of sensitive paper in common use1 viz4 the ,albuminized1 the plain1 and the ,ammonio-,nitrate paper4 ,formula ,i4 ,preparation of ,albuminized ,paper4--,this includes the salting and albuminizing1 and the sensitizing with ,nitrate of ,silver4 ,the ,salting and ,albuminizing4--,take of ,chloride of ,ammonium1 or ,pure ,chloride of ,sodium #bjj grains4 ,water #aj fluid ounces4 ,albumen #aj fluid ounces4 ,if distilled water cannot be procured1 rain water or even common spring water#dj will answer the purpose4 ,to obtain the ,albumen1 use new-laid eggs1 and be careful that in opening the shell the yolk is not broken2 each egg will yield about one fluid ounce of ,albumen4 #dj ,if the water contained much ,sulphate of ,lime1 it is likely that the sensitiveness of the paper would be impaired 7874 ,when the ingredients are mixed1 take a bundle of quills or a fork1 and beat the whole into a perfect froth4 ,as the froth forms1 it is to be skimmed off and placed in a flat dish to subside4 ,the success of the operation depends entirely upon the manner in which this part of the process is conducted2--if the ,albumen be not thoroughly beaten1 flakes of animal membrane will be left in the liquid1 and will cause streaks upon the paper4 ,when the froth has partially subsided1 transfer it to a tall and narrow jar1 and allow to stand for several hours1 that the membranous shreds may settle to the bottom4 ,then pour off the upper clear portion1 which is fit for use4 ,albuminous liquids are too glutinous to run well through a paper filter1 and are better cleared by subsidence4 ,a more simple plan than the above1 and one equally efficacious1 is to fill a bottle to about three parts with the salted mixture of ,albumen and water1 and to shake it well for ten minutes or a quarter of an hour until it loses its glutinosity and can be poured out smoothly from the neck of the bottle4 ,it is then to be transferred to an open jar1 and allowed to settle as before4 ,the solution prepared by the above directions will contain exactly ten grains of salt to the ounce1 dissolved in an equal bulk of ,albumen and water4 ,some operators employ the ,albumen alone without an addition of water2 but this commonly gives a highly varnished appearance1 which is thought by most to be objectionable4 ,much however will depend upon the kind of paper which is employed1 certain varieties taking more gloss than others2 ,papier ,rive1 for instance1 often requires the ,albumen to be nearly or quite undiluted4 ,the principal difficulty in ,albuminizing paper1 is to avoid the occurrence of streaky lines1 which1 when the paper is rendered sensitive1 bronze strongly under the influence of the light4 ,to avoid them1 use the eggs quite fresh1 and lower the paper on to the liquid by one steady movement2 if a pause be made1 a line will probably be formed4 ,some papers are not readily wetted by the ,albumen1 and when such is the case1 a few drops of spirituous solution of bile1 or a fragment of the prepared ,ox-,gall sold by the artists' colour-men1 will be found a useful adjunct4 ,care must be taken however not to add an excess1 or the ,albumen will be rendered too fluid1 and will sink into the paper1 leaving no gloss4 ,in salting and albuminizing ,photographic paper by the formula above given1 it is found that each quarter-sheet1 measuring eleven inches by nine inches1 removes one fluid drachm and a half from the bath1 equivalent to about one grain and three-quarters of salt 7including droppings74 ,in salting plain paper1 each quarter-sheet takes up only one drachm2 so that the glutinous nature of the ,albumen causes a third part more of salt to be retained by the paper4 ,english papers are not good for albuminizing2 they do not take the ,albumen properly1 and curl up when laid upon the liquid3 the process of toning the prints is also slow and tedious4 ,the thin negative paper of ,canson1 the ,papier ,rive1 and ,papier ,saxe1 have succeeded with the writer better than ,canson's ,positive paper1 which is often recommended2 they have a finer texture1 and give more smoothness of grain4 ,to apply the ,albumen1 pour a portion of the solution into a flat dish to the depth of half an inch4 ,then1 having previously cut the paper to the proper size1 take a sheet by the two corners1 bend it into a curved form1 convexity downwards1 and lay it upon the ,albumen1 the centre part first touching the liquid1 and the corners being lowered gradually4 ,in this way all bubbles of air wall be pushed forwards and excluded4 ,one side only of the paper is wetted3 the other remains dry4 ,allow the sheet to rest upon the solution for one minute and a half1 and then raise it off1 and pin it up by two corners4 ,if any circular spots1 free from ,albumen1 are seen1 caused by bubbles of air1 replace the sheet for the same length of time as at first4 ,the paper must not be allowed to remain upon the salting ,bath much longer than the time specified1 because the solution of ,albumen being alkaline 7as is shown by the strong smell of ,ammonia evolved on the addition of the ,chloride of ,ammonium7 tends to remove the size from the paper and to sink in too deeply2 thus losing its surface gloss4 ,albuminized paper will keep a long time in a dry place4 ,some have recommended to press it with a heated iron1 in order to coagulate the layer of ,albumen upon the surface2 but this precaution is unnecessary1 since the coagulation is perfectly effected by the ,nitrate of ,silver used in the sensitizing2 and it is doubtful whether a layer of dry ,albumen would admit of coagulation by the simple application of a heated iron4 ,to render the paper sensitive4--,this operation must be conducted by the light of a candle1 or by yellow light4 ,take of ,fused ,nitrate of ,silver #fj grains4 ,glacial ,acetic ,acid #a_/c minim4 ,distilled ,water #a ounce4 ,prepare a sufficient quantity of this solution1 and lay the sheet upon it in the same manner as before4 ,three minutes' contact will be sufficient with the thin ,negative paper1 but if the ,canson ,positive paper be used1 four or five minutes must be allowed for the decomposition4 ,the papers are raised from the solution by a pair of bone forceps or common tweezers tipped with sealing-wax2 or a pin may be used to lift up the corner1 which is then taken by the finger and thumb and allowed to drain a little before again putting in the pin1 otherwise a white mark will be produced upon the paper1 from decomposition of the ,nitrate of ,silver4 ,when the sheet is hung up1 a small strip of blotting-paper suspended from the lower edge of the paper will serve to drain off the last drop of liquid4 ,a ,bath prepared by the above formula is stronger than is really necessary4 ,forty grains of ,nitrate to the ounce of water is abundantly sufficient if the sample be pure2 but it must be borne in mind that the strength of the ,bath diminishes rapidly by use1 and hence1 when the prints begin to be wanting in vigour1 with pale shadows and perhaps a spotted appearance1 an addition of ,nitrate of ,silver must be made4 ,fused ,nitrate of ,silver is recommended in preference to the crystallized ,nitrate1 on account of the latter being occasionally contaminated with an impurity alluded to at page #aja4 ,this when present will be likely to redden the pictures and to interfere with the rapidity of bronzing4 ,the solution of ,nitrate of ,silver becomes after a time discoloured by the ,albumen1 but may be used for sensitizing until it is nearly black4 ,the colour can be removed by ,animal ,charcoal1#da but a better plan is to use the kaolin1 or pure white china clay4 ,this substance often contains ,carbonate of ,lime1 and effervesces with acids3 it must in such a case be purified by washing in vinegar1 or the ,bath will become alkaline1 and dissolve off the ,albumen4 ,it has been stated that an addition of ,alcohol to the ,nitrate ,bath prevents it discolouring with ,albumen4 #da ,common ,animal ,charcoal contains ,carbonate and ,phosphate of ,lime1 the former of which renders the ,nitrate of ,silver alkaline2 purified ,animal ,charcoal is usually acid from ,hydrochloric ,acid4 ,sensitive albuminized paper will usually keep for several days1 if protected from the light1 but afterwards turns yellow from partial decomposition4 ,formula ,,ii4 ,preparation of plain paper4--,take of ,chloride of ,ammonium or ,sodium #afj grains4 ,purified ,gelatine #bj grains4 ,iceland ,moss#db #fj grains4 ,water #bj ounces4 #db ,iceland ,moss is recommended because the writer finds that ,positives so printed stand the action of destructive tests better than prints on plain paper1 and equal to prints upon ,ammonio-,nitrate paper4 ,pour boiling water upon the ,moss and ,gelatine and stir until the latter is dissolved1 then cover the vessel and set aside until cold2 add the salt1 and strain4 ,use ,papier ,saxe or ,towgood's paper1#dc floated upon the salting ,bath in the same manner as directed for ,albumen at p4 #bdc4 #dc ,the writer does not recommend the ,positive paper of ,de ,canson1 having noticed that prints upon that paper do not withstand the action of sulphuretting agents so well as others 7874 ,render sensitive by floating for two or three minutes upon a solution of ,nitrate of ,silver1 #dj grains to the ounce4 ,thirty grains to the ounce1 or less1 will be sufficient if the sample be pure2 but in that case occasional additions of fresh ,nitrate of ,silver must be made1 as the ,bath loses strength4 ,a second ,formula for plain paper4--,take of ,chloride of ,ammonium #bjj grains4 ,citrate of ,soda#dd #bjj ,gelatine #bj ,water #bj fluid ounces4 #dd ,this salt may be obtained at the operative chemists2 or it may be prepared extemporaneously by neutralizing #aab grains of pure ,citric ,acid1 free from ,tartaric ,acid1 with #acc grains of the dried ,bicarbonate or ,sesquicarbonate of ,soda1 used for effervescing draughts4 ,if ,towgood's or any ,english paper be used1 the ,citric ,acid1 ,carbonate of ,soda1 and ,gelatine may be omitted4 ,with a foreign paper the ,citrate tends to give a purple tone to the ,positive1 when toned by ,sel d'or1 but the gold toning ,bath must be in active order1 or the prints will be too red4 ,the ,citric ,acid also should not be in excess over the alkaline ,carbonate4 ,render sensitive by floating for three minutes upon a ,nitrate ,bath of sixty grains to the ounce of water4 ,formula ,,iii4 ,ammonio-,nitrate ,paper4--,this is always prepared without ,albumen1 which is dissolved by ,ammonio-,nitrate of ,silver4 ,take of ,chloride of ,ammonium #ajj grains4 ,citrate of ,soda #bjj ,gelatine #bj ,water #bj fluid ounces4 ,dissolve the ,gelatine by the aid of heat2 add the other ingredients1 and filter4 ,the solution cannot be kept longer than two or three weeks without becoming mouldy4 ,the ,saxony paper1 or ,towgood's ,english paper1 may be employed2 the ,gelatine and ,citrate being retained or omitted1 according to the taste of the operator and the mode of toning which is adopted4 ,render sensitive by a solution of ,ammonio-,nitrate of ,silver1 #fj grains to the ounce of water1 which is prepared as follows3-- ,dissolve the ,nitrate of ,silver in one-half of the total quantity of water4 ,then take a pure solution of ,ammonia and drop it in carefully1 stirring meanwhile with a glass rod4 ,a brown precipitate of ,oxide of ,silver first forms1 but on the addition of more ,ammonia it is re-dissolved4#de ,when the liquid appears to be clearing up1 add the ,ammonia very cautiously1 so as not to incur an excess4 ,in order still further to secure the absence of free ,ammonia1 it is usual to direct1 that when the liquid becomes perfectly clear1 a drop or two of solution of ,nitrate of ,silver should be added until a slight turbidity is again produced4 ,lastly1 dilute with water to the proper bulk4 ,if the crystals of ,nitrate of ,silver employed contain a large excess of free ,nitric ,acid1 no precipitate will be formed on the first addition of ,ammonia4 ,the free ,nitric ,acid1 producing ,nitrate of ,ammonia with the alkali1 keeps the ,oxide of ,silver in solution4 ,this cause of error however is not likely to happen frequently1 since the amount of ,nitrate of ,ammonia required to prevent all precipitation would be considerable4 ,from the same reason1 viz4 the presence of ,nitrate of ,ammonia1 it is often useless to attempt to convert an old ,nitrate ,bath already used for sensitizing1 into ,ammonio-,nitrate4 #de ,if the excess of ,ammonia does not readily dissolve it1 probably the ,nitrate of ,silver is impure4 ,ammonio-,nitrate of ,silver should be kept in a dark place1 being more prone to reduction than the ,nitrate of ,silver4 ,sensitizing paper with ,ammonio-,nitrate4--,it is not usual to float the paper when1 the ,ammonio-,nitrate of ,silver is used4 ,if a bath of this liquid were employed1 it would not only become quickly discoloured by the action of organic matter dissolved out of the papers1 but would soon contain abundance of free ,ammonia 7see the ,vocabulary1 ,part ,,iii41 art4 ,ammonio-,nitrate72 and an excess of ,ammonia in the liquid produces an injurious effect by dissolving away the sensitive ,chloride of ,silver4 ,the ,ammonio-,nitrate is therefore applied with a glass rod1 or by brushing1 and in neither case is any of the liquid which has once touched the paper allowed to return into the bottle4 ,brushes are manufactured purposely for applying ,silver solutions1 but the hair is soon destroyed unless the brush be kept scrupulously clean4 ,lay the salted sheet upon blotting-paper1 and wet it thoroughly by drawing the brush first lengthways and then across4 ,allow it to remain flat for a minute or so1 in order that a sufficient quantity of the solution may be absorbed 7you will see when it is evenly wet by looking along the surface71 and then pin up by the corner in the usual way4 ,if1 on drying1 white lines appear at the points last touched by the brush1 it is probable that the ,ammonio-,nitrate contains free ,ammonia4 ,the employment of a glass rod is a very simple and economical mode of applying ,silver solutions4 ,procure a flat piece of board somewhat smaller than the sheet to be operated on1 and having turned over the edges of the paper1 secure them with a pin4 ,next bring the board near to the corner of the table1 and laying the glass rod along the edge of the paper1 allow the fluid to drop into the groove so formed2 then carry the rod directly across the sheet1 when an even wave of fluid will be spread over the surface4 ,a pipette made of glass tubing1 when dipped into the bottle and the upper end closed with the finger1 will withdraw as much of the ,ammonio-,nitrate as is required2 and if a scratch be made upon the tube at a point corresponding to #cj or #dj minims1 it will be found sufficient for a quarter sheet of the ,papier ,saxe4 ,ammonio-,nitrate paper1 however prepared1 cannot be kept many hours without becoming brown and discoloured4 ,use of a solution of ,oxide of ,silver in ,nitrate of ,ammonia4--,the great objection to the use of ,ammonio-,nitrate of ,silver is the decomposition which it sometimes experiences by keeping1 metallic ,silver separating and ,ammonia being set free4 ,to obviate this liberation of ,ammonia1 the ,author employs ,nitrate of ,ammonia as the solvent for the ,oxide of ,silver4 ,the solution is prepared as follows3--,dissolve #fj grains of ,nitrate of ,silver in half an ounce of water1 and drop in ,ammonia until the precipitated ,oxide of ,silver is exactly re-dissolved4 ,then divide this solution of ,ammonio-,nitrate of ,silver into two equal parts1 to one of which add ,nitric ,acid cautiously1 until a piece of immersed litmus-paper is reddened by an excess of the acid2 then mix the two together1 fill up to one ounce with water1 and filter from the milky deposit of ,chloride or ,carbonate of ,silver1 if any be formed4 ,this solution of ,oxide of ,silver in ,nitrate of ,ammonia appears to possess all the advantages of the ,ammonio-,nitrate without the inconvenience of liberating so much free ,ammonia upon the surface of the sensitive sheets4 ,hints in selecting from the above ,formul4--,albuminized paper is the most simple and generally useful2 it is well fitted for small portraits and stereoscopic ,photographs4 ,the ,ammonio-,nitrate ,process requires more experience1 but gives excellent results when black tones are required3 it may be used for larger portraits1 engravings1 etc4 ,plain paper rendered sensitive by floating upon a ,bath of ,nitrate of ,silver is easier of manipulation than the ,ammonio-,nitrate1 and will be found to be better adapted for toning by the ,sel d'or ,bath 7p4 #bfg7 than the ,albuminized ,paper4 ,,preparation ,,of ,,the ,,fixing ,,and ,,toning ,,bath4 ,take of ,chloride of ,gold #d grains4 ,nitrate of ,silver #af grains4 ,hyposulphite of ,soda#df #d ounces4 ,water #h fluid ounces4 #df ,the common kind of ,hyposulphite of ,soda occurring in yellow and discoloured masses1 is too impure for use in ,photography1 and requires re-crystallization4 ,dissolve the ,hyposulphite of ,soda in four ounces of the water1 the ,chloride of ,gold in three ounces1 the ,nitrate of ,silver in the remaining ounce2 then pour the diluted ,chloride by degrees into the ,hyposulphite1 stirring with a glass rod2 and afterwards the ,nitrate of ,silver in the same way4 ,this order of mixing the solutions is to be strictly observed3 if it were reversed1 the ,hyposulphite of ,soda being added to the ,chloride of ,gold1 the result would be the reduction of ,metallic ,gold2 ,hyposulphite of ,gold1 which is formed1 being an unstable substance1 and not capable of existing in contact with unaltered ,chloride of ,gold4 ,if however it be dissolved by ,hyposulphite of ,soda immediately on its formation1 it is rendered more permanent1 by conversion into a double salt of ,soda and ,gold4 ,in place of ,nitrate of ,silver1 recommended in the formula1 ,chloride of ,silver may be used1 but not ,iodide of ,silver1 as the formation of ,iodide of ,sodium would be objectionable 7p4 #acf74 ,for the same reason it is better not to add any part of the ,hyposulphite ,bath used for fixing ,negatives1 to the ,positive colouring solution4 ,this toning ,bath is not to be employed immediately after mixing1 but should be set aside until a portion of ,sulphur 7produced by free ,hydrochloric ,acid1 and ,tetrathionate of ,soda reacting upon the ,hyposulphite7 has subsided4 ,it will be very active at the expiration of a few days or a week2 but upon keeping for a longer time1 loses much of its efficacy by a process of spontaneous change4 ,the immersion of prints also lessens the quantity of ,gold2 and hence1 when the ,bath begins to work slowly1 more of the ,chloride must be added1 the ,sulphur being allowed to deposit as before4 ,filtration through blotting-paper will not be required4 ,the writer finds that after a certain time1 when the ,bath has been long used1 and organic matters1 ,albumen1 etc41 have accumulated in it1 it is better1 and more economical1 to throw away what remains1 and to prepare a new solution4 ,the addition of ,chloride of ,gold to an old ,bath will not always make it work as quickly as one recently mixed4 ,,the ,,manipulatory ,,details ,,of ,,photographic ,,printing4 ,these include--the exposure to light1 or printing properly so called2 the fixing and toning2 and the washing1 drying1 and mounting of the proof4 ,the ,exposure to ,light4--,for this purpose reversing frames are sold1 which admit of being opened at the back1 in order to examine the progress of the darkening by light1 without producing any disturbance of position4 ,simple squares of glass however succeed equally well1 when a little experience has been acquired4 ,they may be held together by the wooden clips sold at the ,american warehouses at one shilling per dozen4 ,the lower plate should be covered with black cloth or velvet4 ,supposing the frame to be employed1 the shutter at the back is removed1 and the ,negative laid flat upon the glass1 ,collodion side uppermost4 ,a sheet of sensitive paper is then placed upon the ,negative1 sensitive side downwards1 and the whole tightly compressed by replacing and bolting down the shutter4 ,this operation may be conducted in the dark room2 but unless the light be strong1 such a precaution will not be required4 ,the time of exposure to light varies much with the density of the ,negative and the power of the actinic rays1 as influenced by the season of the year and other obvious considerations4 ,as a general rule1 the best ,negatives print slowly2 whereas ,negatives which have been under-exposed and under-developed print more quickly4 ,in the early spring or summer1 when the light is powerful1 probably about ten to fifteen minutes will be required2 but from three-quarters of an hour to an hour and a half may be allowed in the winter months1 even in the direct rays of the sun4 ,it is always easy to judge of the length of time which will be sufficient1 by exposing a small slip of the sensitive paper1 unshielded1 to the sun's rays1 and observing how long it takes to reach the coppery stage of reduction4 ,whatever that time may be1 nearly the same will be occupied in the printing1 if the ,negative be a good one4 ,when the darkening of the paper appears to have proceeded to a considerable extent1 the frame is to be taken in and the picture examined4 ,if squares of plate glass are used to keep the ,negative and sensitive paper in contact1 some difficulty may be experienced at first in returning it precisely to its former position after the examination is complete1 but this will easily be overcome by practice4 ,the finger and thumb should be fixed on the lower corners or edge1 and the plate raised evenly and quickly4 ,if the exposure to light has been sufficiently long1 the print appears slightly darker than it is intended to remain4 ,the toning ,bath dissolves away the lighter shades1 and reduces the intensity1 for which allowance is made in the exposure to light4 ,a little experience soon teaches what is the proper point2 but much will depend upon the state of the toning ,bath2 and albuminized paper will require to be printed somewhat more deeply than plain paper4 ,if1 on removal from the printing-frame1 a peculiar spotted appearance is seen1 produced by unequal darkening of the ,chloride of ,silver1 either the ,nitrate ,bath is too weak1 the sheet removed from its surface too speedily1 or the paper is of inferior quality4 ,on the other hand1 if the general aspect of the print is a rich chocolate-brown in the case of ,albumen1 a dark slate-blue with ,ammonio-,nitrate ,paper1 or a reddish purple with paper prepared with ,chloride and ,citrate of ,silver1 probably the subsequent parts of the process will proceed well4 ,if1 in the exposure to light1 the shadows of the proof become very decidedly coppery before the lights are sufficiently printed1 the ,negative is in fault4 ,ammonio-,nitrate paper highly salted is particularly liable to this fault of excess of reduction1 and especially so if the light be powerful2 hence it is best1 in the summer months1 not to print by the direct rays of the ,sun4 ,this point is important also1 because the excessive heat of the ,sun's rays often cracks the glasses by unequal expansion1 and glues the ,negative firmly down to the sensitive paper4 ,an exception however may be made in the case of ,negatives of great intensity2 which are printed most successfully upon1 a weakly sensitized paper 7p4 #abd7 exposed to the full rays of the ,sun2 a feeble light not fully penetrating the dark parts4 ,the fixing and toning of the proof4--,no injury results from postponing this part of the process for many hours1 provided the print be kept in a dark place4 ,the mode often followed is to immerse the ,positive in the ,hyposulphite ,bath in the state in which it comes from the printing-frame2 moving it about in the liquid in order to displace air-bubbles1 which1 if allowed to remain1 produce spots4 ,but the ,author1 for reasons given in the first part of the ,work 7pp4 #abi and #afe71 recommends that the print should first be washed in common water until the soluble ,nitrate of ,silver has been removed4#dg ,this is known to be the case when the liquid flows away clear2 the first milkiness being caused by the soluble ,carbonates and ,chlorides in the water precipitating the ,nitrate of ,silver4 ,greater security is thus afforded that the print will be toned in a really permanent manner1 since after removing the ,nitrate of ,silver from the proof1 the ,bath does not work quickly unless the supply of ,gold be well maintained4 #dg ,this water must be free from ,hyposulphite of ,soda1 or the print will become discoloured4 ,immediately on coming in contact with the ,hyposulphite of ,soda in the fixing and toning ,bath1 the chocolate brown or violet tint of the ,positive disappears1 and leaves the image of a red tone4 ,albumen proofs become brick-red2 ,ammonio-,nitrate a sepia or brown-black4 ,if the colour is unusually pale at this stage1 probably the ,silver ,bath is too weak1 or the quantity of ,chloride of ,ammonium or ,sodium insufficient4 ,after the print has been thoroughly reddened1 the toning action begins1 and must be continued until the desired effect is obtained4 ,this may happen in from ten minutes to a quarter of an hour1 if the solution is in good working order and the thermometer at #fj2 but much depends upon the temperature1 and the activity of the ,bath4 ,english papers1 and especially the same prepared with ,albumen1 tone more slowly than foreign papers plain salted4 ,the brown and purple tints are an earlier stage of coloration than the black tones1 and therefore the latter require more time4 ,it must be borne in mind however that prolonged immersion in the ,bath is favourable to sulphuration and yellowness2 tending also to render the image unstable and liable to fade in the half-tones4 ,this fading may not be seen decidedly whilst the print is in the ,bath1 but will show itself in the after-processes of washing and drying4 ,the ultimate colour of the ,print will vary much with the density of the ,negative and the character of the subject2 copies of line engravings1 having but little half-tone1 are easily obtained of a dark shade resembling the original impression4 ,some advise that on removal from the toning ,bath the ,print should be soaked in new ,hyposulphite for ten minutes1 to complete the fixation2 but this precaution is not required with a ,bath of the strength given in the formula4 ,an analysis of an old ,bath which had been extensively used1 indicated only ten grains of ,hyposulphite of ,silver to the ounce1 so that it was far from saturated4 ,the occasional addition of fresh crystals of ,hyposulphite of ,soda to keep up the strength of the ,bath1 is useful1 the exact quantity added not being material4 ,the washing1 drying1 and mounting of the ,positive ,proofs4--,it is essential to wash out every trace of ,hyposulphite of ,soda from the ,print if it is to be preserved from fading1 and to do this properly requires considerable care4 ,always wash with running water when it can be obtained1 and choose a large shallow vessel exposing a considerable surface in preference to one of lesser diameter4 ,a constant dribbling of water must be maintained for four or five hours1 and the prints should not lie together too closely1 or the water does not find its way between them1 7see the remarks at p4 #afb74 ,when running water cannot be obtained1 proceed as follows3--first wash the ,prints gently1 to remove the greater part of the ,hyposulphite solution4 ,then transfer them to a large shallow pan1 in which may be placed as many ,prints as it will conveniently hold4 ,leave them in for about a quarter of an hour1 with occasional movement1 and then pour off the water quite dry4 ,this point is important1 viz4 to drain off the last portion of liquid completely before adding fresh water4 ,repeat the process of changing at least five or six times1 or more1 according to the bulk of water1 number of ,prints1 and degree of attention paid to them4 ,lastly1 proceed to remove the size from the ,print by immersion in boiling water4#dh ,this process will give some idea of the permanency of the tints1 since1 if they become dull and red1 and do not darken on drying1 the ,print is probably toned without ,gold4 ,ammonio-,nitrate and plain paper ,prints prepared on foreign papers by the modes described in this ,work1 may be expected to stand the test of boiling water2 ,albumen ,prints and ,positives on ,english paper are a little reddened1 although not to an objectionable degree4 #dh ,the ,print must be well washed in cold water1 to remove the ,hyposulphite1 before using the hot water2 or the half-tones will be liable to be darkened1 or changed to incipient yellowness1 by sulphuration4 ,this point is important as regards the permanency4 ,the size may also be effectually removed from the ,print by the common ,carbonate of ,soda used in washing1 although the former process is recommended as the most secure4 ,dissolve about a handful of the ,soda in a pint of water1 and when the milky deposit1 if any occurs1 has subsided1 immerse the washed ,positives for twenty minutes or half an hour4 ,the ,soda renders the paper quite porous1 but produces no alteration of tint4 ,if the process be properly performed1 ink will run in attempting to write upon the back of the finished picture4 ,after removal from the ,soda ,bath a second washing will be required1 but the time of the first washing may be proportionally shortened4 ,here a difficulty will occur with many kinds of water2 the ,carbonate of ,soda precipitating ,carbonate of ,lime1 in the form of a white powder which obscures the picture4 ,to obviate this1 use rain water until the greater part of the alkaline salt has been removed1 and do not allow a stationary layer of liquid to rest too long upon the ,print4 ,the ,new ,river water supplied to many parts of ,london1 being comparatively soft1 answers perfectly1 and produces no white deposit1 if the proofs are moved about occasionally4 ,when the ,prints have been thoroughly washed1 blot them off between sheets of porous paper and hang up to dry4 ,some press them with a hot iron1 which darkens the colour slightly1 but does so in an injurious manner when ,hyposulphite of ,soda is left in the paper4 ,albumen proofs when dry are sufficiently bright without further treatment2 but in the case of plain paper1 salted simply1 the effect is improved by laying the ,print face downwards upon a square of plate-glass and rubbing the back with an agate burnisher1 sold at the artists' colour-men's4 ,this hardens the grain of the paper and brings out the details of the picture4 ,hot-pressing has a similar effect and is often employed4 ,mount the proofs with a solution of ,gelatine in hot water1 freshly made2 the best ,scotch glue answers well4 ,gum water1 prepared from the finest commercial gum1 and free from acidity1 may also be used1 but it should be made very thick1 that it may not sink into the paper1 nor produce an unpleasant cockling up of the cardboard1 which is caused by the damp and expanded print contracting as it dries4 ,caoutchouc dissolved in mineral ,naphtha to the consistence of thick glue or gold-beaters' size1 is employed by many for mounting ,photographic ,prints2 it may be obtained at the varnish shops1 and is sold in tin boxes4 ,the mode of using it is as follows3--with a broad brush made of stiff bristles1 apply the cement to the back of the picture2 then take a strip of glass with a straight edge1 and by drawing it across the paper1 scrape off as much as possible of the excess4 ,the print will then be found to adhere very readily to the cardboard1 without causing expansion or cockling2 and any portion of the cement which oozes out during the pressing may1 when dry1 be removed with a penknife without leaving a stain4 ,,remarks ,,upon ,,the ,,want ,,of ,,correspondence ,,between ,,the ,,formula ,,of ,,different ,,operators4 ,the formul for ,positive printing given in the works on practical ,photography exhibit great variety2 and it has been proposed to attempt to reduce them to more uniform proportions4 ,this cannot however easily be done1 both on account of the difference in the structure and preparation of the various ,photographic papers1 and also because the mode of applying the solutions is not always the same4 ,take as an illustration the following process1 which has long been recommended for its simplicity1 and which is in every respect a good one3--,dissolve #dj grains of ,chloride of ,ammonium in #bj ounces of ,distilled ,water1 and immerse about a dozen sheets of ,towgood's ,positive paper1 removing air-bubbles with a camels'-hair brush4 ,when the last sheet has been placed in the liquid1 turn the batch over and take them out one by one1 so that each sheet1 remaining in the liquid at least ten minutes1 may be thoroughly saturated4 ,when dry1 excite by brushing with a #dj or #fj-;grain solution of ,ammonio-,nitrate of ,silver in the usual way4 ,now this formula contains less than one-fifth of the amount of salt often employed1 and if a thick foreign paper sized with starch1 such as ,canson's ,positive1 were floated upon such a salting ,bath1 it would be difficult to obtain a good picture4 ,by immersing however a paper sized with ,gelatine like the one recommended1 a much larger quantity of salt is retained upon the surface1 and the film is sufficiently sensitive4 ,there are three modes of applying solutions1 viz4 by brushing1 floating1 and immersion4 ,the quantity of solution left on the paper varies with each1 and consequently each requires a different formula4 ,immersion in a strong salting ,bath tends to give a coarse picture wanting in definition2 whereas the plan of brushing a weak salting solution1 produces a paper deficient in sensitiveness1 and yielding a pale red image without proper depth of shadow4 ,but independent of these differences1 the chemical nature of the size employed also influences the toning of the ,print4 ,for instance1 in the process above given1 if the ,positives1 after having been fully toned in the ,gold ,bath1 and washed in cold water1 be treated with boiling water1 the tint immediately changes to a dull red2 but on blotting off between sheets of bibulous paper and pressing with a hot iron1 the dark tones are restored4 ,this destruction of the tint by boiling water1 and its restoration by dry heat1 is due in great part to the animal substance employed in sizing the paper2 and it will be found that prints upon a foreign paper1 such as the ,saxony ,positive1 salted with a ten-grain solution and sensitized with ,ammonio-,nitrate1 do not lose their tones in hot water and are not much darkened by ironing4 ,the peculiarity of the sizing of the ,english ,photographic papers must therefore be borne in mind1 and allowance made for the additional sensitiveness and alteration of colour which it produces4 ,when a formula is given1 the paper which is recommended for that particular formula should alone be used4 ,,section ,,ii4 ,positive ,printing by ,development4 ,negative printing processes will be found useful during the dull winter months1 and at other times when the light is feeble1 or when it is required to produce a large number of impressions from a ,negative in a short space of time4 ,the plan of development also enables the operator to obtain ,positives of greater stability than those yielded by the direct action of light4 ,three processes may be described1 the first of which gives ,positives of an agreeable colour1 but the second1 on ,iodide of ,silver1 the greatest permanency under unfavourable conditions4 ,,negative ,,printing ,,processes ,,upon ,,chloride ,,of ,,silver4 ,positives may be obtained by exposing paper prepared with ,chloride of ,silver to the action of light until a faint image is perceptible1 and subsequently developing by ,gallic ,acid2 but in this process it is difficult to obtain sufficient contrast of light and shade2 the impression1 if sufficiently exposed and not too much developed1 being feeble1 with a want of intensity in the dark parts4 ,by associating with the ,chloride an organic salt of ,silver1 such as the ,citrate1 this difficulty may be overcome1 and the shadows be brought out with great depth and distinctness4 ,the papers are salted with a mixed ,chloride and ,citrate as in the formula for the ,ammonio-,nitrate ,process4#di ,they are then rendered sensitive upon a ,bath of ,nitrate of ,silver containing either ,citric or ,acetic ,acid1 which are used in ,negative processes to preserve the clearness of the white parts under the influence of the developer4 #di ,the formula at p4 #bdf may be modified with advantage3 use double the quantity of ,gelatine1 and half the amount of ,citrate and ,chloride4 ,the ,bath of ,aceto-,nitrate is prepared as follows3-- ,nitrate of ,silver #cj grains4 ,glacial ,acetic ,acid #cj minims4 ,water #a fluid ounce4 ,float the papers 7,papier ,saxe or ,papier ,rive7 upon the ,bath for three minutes1 and suspend them to dry in a room from which actinic rays are perfectly excluded4 ,the exposure to light1--which is conducted in the ordinary printing frame1 the ,negative and sensitive paper being laid in contact in the usual way1--will seldom be longer than three or four minutes1 even upon a dull day4 ,it may be regulated by the colour assumed by the projecting margin of the paper2 but it is quite possible to tell by the appearance of the image when it has received a sufficient amount of exposure3--the whole of the picture should be seen1 excepting the lightest shades1 and it will be found that very few details can be brought out in the development which were altogether invisible before the ,gallic ,acid was applied4 ,the developing solution is prepared as follows3-- ,gallic ,acid #b grains4 ,water #a fluid ounce4 ,in very cold weather it may be necessary to employ a saturated solution of ,gallic ,acid1 containing about four grains to the ounce2 whereas in warm weather the image will develope too quickly1 and ,acetic ,acid must be added 7see the remarks at the end of the process1 p4 #bff74 ,to facilitate the solution of the ,gallic ,acid1 stand the bottle in a warm place near the fire4 ,a lump of ,camphor floated in the liquid1 or a drop of ,oil of ,cloves added1 will to a great extent prevent it from becoming mouldy by keeping2 but if once mould has formed1 the bottle must be well cleansed with ,nitric ,acid1 or the decomposition of the fresh ,gallic ,acid will be hastened4 ,pour the solution of ,gallic ,acid into a flat dish1 and immerse the ,prints two or three at a time1 moving them about1 and using a glass rod to remove air-bubbles4 ,the development is rapid1 and will be completed in three or four minutes4 ,if the ,print developes slowly1 becomes very dark in colour by continuing the action of the ,gallic ,acid1 but shows no half-tones1 it has not been exposed sufficiently long to the light4 ,an over-exposed proof1 on the other hand1 developes with unusual rapidity1 and it is necessary to remove it speedily from the ,bath in order to preserve the clearness of the white parts2 when taken out to the light1 it appears pale and red1 with no depth of shadow4 ,the extent to which the development should be carried depends upon the kind of ,print desired4 ,by pushing the action of the ,gallic ,acid1 a dark picture not much altered by the fixing ,bath will be produced4 ,but a better result as regards colour and gradation of tone will be obtained by removing the ,print from the developing solution whilst in the light red stage1 and toning it subsequently by means of ,gold2 in which case it will correspond both in appearance and properties to a ,positive obtained by the direct action of light 7see the remarks at page #afg74 ,when it is intended to follow the latter plan1 the action of the developer must be stopped at a point when the proof appears lighter than it is to remain2 since the ,sel d'or ,bath adds a little to the intensity1 and the image becomes somewhat more vigorous on drying4 ,wash the ,prints in cold water in order to extract all the ,gallic ,acid4 ,then tone with ,sel d'or in the manner described in the next ,section1 and fix in the usual way4 ,the whites will with care be kept pure2 or with only a faint yellow tinge1 which is not objectionable4 ,upon comparing the developed ,prints with others obtained by the direct action of light upon the same sensitive paper1 it is evident that the advantage is slightly on the side of the latter2 but the difference is so small that it would be overlooked in printing large subjects1 for which the ,negative ,process is more especially adapted4 ,the colour of both kinds of ,positives is the same1 or perhaps a shade darker in the developed proofs1 which are usually of a violet-purple tone1 but sometimes of a dark chocolate-brown4 ,a developing process with ,serum of ,milk4--,the use of whey as a vehicle for ,chloride of ,silver has something the same effect as that produced by adding a ,citrate4 ,this may be traced to the presence of the ,milk ,sugar and of a portion of uncoagulated ,caseine left in the ,serum4 ,the only difficulty in the process is to coagulate the milk in such a way as to separate the greater part but not the whole of the ,caseine4 ,milk which has become sour1 or to which an acid has been added1 is not considered so good for the purpose as that which has been treated with rennet2 and even when rennet is used it must be of the best quality or its action will be imperfect4 ,the serum must filter clear through1 blotting-paper2 but it should not run very rapidly1 or in all probability the whole of the ,caseine has been separated1 and the fluid contains little besides sugar4 ,the whey which is left after cheese-making1 commonly answers the purpose1 if clarified by beating it up with the white of an egg and subsequently boiling and filtering4 ,globules of oil must be separated as far as possible1 or they will produce a greasiness of the paper4#ej #ej ,see the ,vocabulary1 ,part ,,iii11 ,art4 ,milk1 for further particulars4 ,salt the prepared ,serum with ,chloride of ,sodium or ,ammonium2 in quantity about eight or ten grains to each fluid ounce1 and render sensitive upon the same ,bath as that recommended for the ,citrate ,process4 ,a ,,negative ,,printing ,,process ,,upon ,,iodide ,,of ,,silver4 ,iodide of ,silver is more sensitive to the reception of the invisible image than the other compounds of that metal2 and hence it is usefully employed in printing enlarged ,positives from small ,negatives1 by means of the ,camera4 ,the great stability of the proofs upon ,iodide of ,silver will also be a recommendation of this process when unusual permanency is required4 ,take of ,iodide of ,potassium #afj grains4 ,water #bj fluid ounces4 ,the best paper to use will be either ,turner's ,calotype1 or ,whatman's or ,hollingworth's ,negative2 the foreign papers do not succeed with the above formula 7p4 #beh74 ,float the paper on the iodizing ,bath until it ceases to curl up and lies flat upon the liquid3 then pin up to dry in the usual way4 ,render sensitive upon a ,bath of ,aceto-,nitrate of ,silver containing #cj grains of ,nitrate of ,silver with #cj minims of ,glacial ,acetic ,acid to each ounce of water4 ,when the sheet is quite dry1 place it in contact with the ,negative in a pressure frame1 and expose to a feeble light4 ,about #cj seconds will be an average time upon a dull winter's day1 on which it would be impossible to print at all in the ordinary way4 ,on removing the ,negative nothing whatever is seen upon the paper1 the image being strictly invisible in this process unless the exposure has been carried too far4 ,develope by immersion in a saturated solution of ,gallic ,acid1 prepared in the manner described at page #bfa4 ,the image appears slowly1 and the process may last from #ae minutes to half an hour4 ,if the exposure has been correctly timed1 the ,gallic ,acid appears at length almost to cease acting2 but when the proof has been over-exposed1 the development goes on uninterruptedly1 and the image becomes too dark1 partaking more of the character of a ,negative than a ,positive4 ,the usual rule1 that under-exposed proofs develope slowly but show no half-tones1 and that the over-exposed develope with unusual rapidity1 is also observed in the process with ,iodide of ,silver4 ,after the picture is fully brought out1 wash in cold1 and subsequently in warm water1 to remove the ,gallic ,acid1 which1 if allowed to remain1 would discolour the ,hyposulphite ,bath4 ,then fix the ,print in a solution of ,hyposulphite of ,soda1 one part to two of water1 continuing the action until the yellow colour of the ,iodide disappears4 ,the fixing ,bath ought not to produce much change in the tint4 ,if the ,positive loses its dark colour on immersion in the ,hyposulphite1 and becomes pale and red1 it has been insufficiently developed4 ,the theory of this part of the process should be understood3--,it is particularly the second stage of the development of a ,photograph 7see p4 #add7 on which the fixing ,bath produces no effect2 and therefore a considerable change of colour in the ,hyposulphite indicates that too little ,silver has been deposited1 and the remedy will be to push the development1 adding a little ,aceto-,nitrate to the ,gallic ,acid if the strength of the ,bath be found insufficient to yield dark tones4 ,the colour of ,positives developed upon ,iodide of ,silver is not agreeable1 and they become blue and inky when toned with gold4 ,by fixing the proof in ,hyposulphite of ,soda which has been long used and has acquired sulphuretting properties1 the tint is much improved2 but the permanency of the ,print under unfavourable conditions is lessened by adopting that mode of toning4 ,a ,,negative ,,printing ,,process ,,upon ,,bromide ,,of ,,silver4 ,by substituting the ,bromide for the ,iodide of ,silver in the above process1 the proportions and details of manipulation being in other respects the same1 a more agreeable colour is obtained4 ,paper prepared with ,bromide of ,silver is less sensitive than the ,iodide1 but an exposure of one minute 7in the printing frame7 will usually be sufficient even on a dull day4 ,the image is nearly latent1 but sometimes a very faint outline of the darkest shadows can be seen4 ,the proportion of ,bromide used is likely to influence this point2 the sensitiveness being diminished1 but the image showing more of the details before development1 when the quantity of the ,silver ,salt is reduced to a minimum4 ,either ,english or ,french papers may be used1 but in the latter case the ,bromide should be dissolved in ,serum of ,milk 7p4 #bfb71 or it will be difficult to obtain a good surface picture4 ,the proportion of ,bromide may be five grains to the ounce of ,serum4 ,these proofs1 even when simply fixed in plain ,hyposulphite of ,soda1 are superior in colour to the ,positives printed by the last formula upon ,iodide of ,silver2 and the permanency is very great if the development be sufficiently pushed4 ,the use of the ,serum of ,milk gives an advantage in resisting the oxidizing influences to which ,positives are liable to be exposed 7p4 #aej74 ,,general ,,remarks ,,on ,,negative ,,printing4 ,printing by development should not be attempted until the manipulation of the ordinary process by direct exposure to light has been acquired4 ,perfect cleanliness is essential4 ,the salting or iodizing solution and the ,aceto-,nitrate ,bath must be filtered clear1 as the effect of small suspended particles in producing spots is more seen when the image is brought out by a developer4 ,it will be necessary to be far more careful in excluding white light than in the ordinary process2 and when ,iodide of ,silver is used1 all the precautions required in the case of ,collodion ,negatives must be taken4 ,observe particularly that the dishes are kept clean1 or the ,gallo-,nitrate of ,silver will be rapidly discoloured 7read the remarks at page #agi74 ,stereoscopic ,negatives and small portraits are not successfully printed by development2 since it is difficult to obtain the most elaborate definition1 and there is a slight tendency to yellowness in the white parts4 ,positives may be developed upon ,albumen paper1 but the ,gallic ,acid is apt to discolour the lights4 ,in printing by development upon ,chloride of ,silver1 the theory of the subject must be particularly studied4 ,when the weather is cold and the light bad1 the development of the image proceeds slowly1 the ,gallic ,acid ,bath remains clear1 and good half-tones are obtained2 but under opposite conditions1 the developer may become turbid and the shadows be lost by excessive deposit of ,silver4 ,this over-development will be remedied by printing the ,negative in a more feeble light 7near to the open window of a room71 and by adding ,acetic ,acid to the developer1 about #e or #aj minims to the ounce1 so as to bring out the image more slowly4 ,the intensity of action is thus lessened1 and if the picture be not under-exposed1 the half-tones will be good4 ,observe also when preparing papers with ,citrate1 that if too much ,carbonate of ,soda be added in neutralizing the ,citric ,acid1 ,carbonate of ,silver will be deposited in the paper1 the effect of which is to remove by degrees the acidity of the ,nitrate ,bath1 and to produce over-development and excessive sensibility to light4 ,the colour of the proofs when taken from the ,gallic ,acid should be light red2 the gradation of tone not being usually so perfect when the development is carried into the second or black stage4 ,it is not recommended to prepare too large a stock of the salted papers1 as they will probably be liable to mouldiness and decomposition unless kept perfectly dry4 ,,section ,,iii4 ,the ,sel d'or ,process for toning ,positives4 ,this process is somewhat more troublesome than the plan of fixing and toning in one solution1 but possesses advantages which will presently be enumerated4 ,the description may be divided into the preparation of the toning ,bath1 and the manipulatory details4 ,,the ,,preparation ,,of ,,the ,,toning ,,bath4 ,take of ,chloride of ,gold #a grain4 ,pure ,hyposulphite of ,soda #c grains4 ,hydrochloric ,acid #d minims4 ,water1 distilled or common #d fluid ounces4 ,dissolve the ,gold and ,hyposulphite of ,soda each in two ounces of the water2 then mix quickly by pouring the former solution into the latter1 and add the ,hydrochloric ,acid4 ,if the ,chloride of ,gold be neutral1 the liquid will have a red tinge1 but if acid1 then the solution may be colourless4 ,the commercial ,chloride of ,gold1 containing usually much free ,hydrochloric ,acid1 will not require any addition of that substance4 7,see the ,vocabulary1 ,part ,,iii47 ,in place of making an extemporaneous ,hyposulphite of ,gold by mixing the ,chloride with ,hyposulphite of ,soda1 the ,crystallized ,sel d'or may be used1 adding about half a grain to the ounce of water1 acidified as before2 but the objection to the employment of this salt is its expense1 and also the difficulty of obtaining it in a pure form2 some samples containing less than five per cent1 of ,gold4 ,it will be found very convenient to keep the two solutions on hand ready for mixing1 viz4 the ,chloride of ,gold dissolved in water in the proportion of a grain to the drachm1 and the ,hyposulphite of ,soda1 three grains to the drachm4 ,when required for use1 measure out a fluid drachm of each1 dilute with water to two ounces1 and mix4 ,it is possible that the three-grain solution of ,hyposulphite of ,soda may by long keeping become decomposed1 with precipitation of ,sulphur4 ,the effect of this would be to produce a turbidity and deposit of ,gold on mixing the ingredients for the ,bath1 the ,chloride of ,gold being in excess over the ,hyposulphite of ,soda 7see p4 #bej74 ,the ,bath of ,sel d'or is always most active when recently mixed1 but it will keep good for some days if contact with free ,nitrate of ,silver be avoided4 ,the addition of this substance produces a red deposit in the ,bath1 containing ,gold1 and the solution then becomes useless4 ,,details ,,of ,,manipulation4 ,the paper may be prepared by either of the formul given in the first ,section of this ,chapter1 according to the tint desired4 ,the pure black tones are obtained most easily with the ,ammonio-,nitrate paper1 and the purple tints1 without gloss1 on paper prepared with plain ,chloride and ,citrate of ,soda4 ,the printing is not carried quite to the usual intensity1 as the half-tones are very little dissolved in this process4 ,on being taken from the frame1 the prints are washed thoroughly in common water until it ceases to become milky2 that is1 until the greater part of the ,nitrate of ,silver has been removed4 ,the washing must be conducted in a dark place1 but it is not necessary to hasten it2 the proofs may be thrown into a pan of water covered with a cloth1 and allowed to remain until required for tinting4 ,a trace of free ,nitrate of ,silver usually escapes the washing2 this would cause a yellow deposit on the ,print1 and also in the toning ,bath4 ,it must therefore be removed1 either by adding a little common salt to the water during the last washings1 or by means of a dilute solution of ,ammonia4 ,for plain paper ,prints the former plan will be found the least troublesome2 but with ,albumen proofs#ea the ,ammonia is required1 in order to dissolve away a portion of the ,albuminate of ,silver which has escaped the action of light1 before submitting the print to the gold2 otherwise the dark tones would nearly disappear in the fixing ,bath1 the ,hyposulphite carrying away the ,gold with this superficial layer of silver salt4 #ea ,the amateur is recommended not to use ,albuminized paper in this process until he has become accustomed to the manipulations2 the plain paper prints being toned with more ease and certainty4 ,to prepare the ,ammonia ,bath1 take of ,liquor ,ammoni #a drachm4 ,common ,water #a pint4 ,the exact quantity is not material2 if the liquid smells faintly of ,ammonia1 it will be sufficient4 ,place the washed ,prints in this ,bath1 two or three at a time1 and allow them to remain until the purple tint gives place to a red tone4 ,the action must be watched1 because if the ,ammonia ,bath be strong1 the proof becomes unusually pale and red1 and when this is the case a little brilliancy is lost in the after-tinting4 ,as the ,print is comparatively insensitive to light when the excess of ,nitrate has been washed away1 it is not necessary to darken the room2 but a bright light proceeding from an open door or window should be avoided4 ,after using the salt or the ,ammonia1 soak the ,prints again for a minute or so in common water4 ,then place them in the toning ,bath of ,gold and acid2 do not put in too many at once1 and move them about occasionally1 to prevent spots of imperfect action at the point where the sheets touch each other4 ,the foreign papers1 plain salted1 colour rapidly in two or three minutes4 ,english papers require five to ten minutes2 ,albuminized1 ten minutes to a quarter of an hour4 ,the tendency of the ,gold ,bath is to give a blue tone to the image2 hence proofs which are light red after using the salt or ,ammonia1 become1 first red-purple1 and then violet-purple in the ,sel d'or4 ,albumen ,prints assume some shade of brown1 or of purple if not too strongly ,albuminized4 ,ammonio-,nitrate papers highly salted1 and prepared without ,citrate1 become first dark purple1 and then blue and inky2 the ,citrate is intended to obviate this inky tint4 ,when the darkest tones are reached1 the ,bath produces no further effect1 but eventually 7more especially if the solution be not shielded from light 87 there is a little decomposition1 producing a cream-coloured deposit upon the lights4 ,the toning being completed1 the ,prints are again washed for an instant in water1 to remove the excess of gold solution4 ,this washing must not be continued longer than two or three minutes1 or there will be danger of yellowness of the whites2 this however ought not to happen with proper precautions4 ,lastly1 the proofs are fixed in a solution of ,hyposulphite of ,soda1 one part to four of water2 which may be used many times successively4 ,this ,bath alters the tone very little if the deposit of ,gold be well fixed on the ,print2 but the writer has often observed in the case of ,albumen paper and paper prepared with ,citrate 7,formula ,,ii47 that if removed too quickly from the ,sel d'or1 the purple tones change by immersion in the ,hyposulphite to a chocolate-brown4 ,ammonio-,nitrate ,prints are less liable to alter in this way4 ,in order that the fixing may be properly performed1 the time of immersion should not be less than ten minutes with a porous paper1 plain salted2 or fifteen minutes in the case of an ,english or albuminized paper4 ,ammonia may be used for fixing plain paper ,prints2 about one part of the ,liquor ,ammoni1 to four of water4 ,ten minutes' immersion will usually be sufficient1 and the tone is very little affected4 ,this process is a good one1 but the pungent smell of the ,ammonia is an objection1 and the ,bath discolours by use4 ,some care too is required in order to ensure a proper fixing of the prints 7see the remarks at page #aca74 ,for directions to wash and mount the proofs1 see page #bee4 ,it will sometimes happen in the ,sel d'or process1 from the toning ,bath having but little solvent action on the light shades1 that the ,prints1 after being washed and dried1 appear too dark2 this may be remedied by laying them for a few minutes in a very dilute solution of ,chloride of ,gold 7five or six drops of the yellow solution of the ,chloride to a few ounces of water7 and washing for an additional quarter of an hour4 ,or an over-printed ,positive may be saved by toning it with ,chloride of ,gold instead of ,sel d'or4 ,in that case1 after proper removal of the free ,nitrate of ,silver1 a few drops of a lemon-yellow solution of ,chloride of ,gold 7with a fragment of ,carbonate of ,soda added to remove acidity1 p4 #acb71 should be poured over the ,print1 which is to be subsequently fixed in the usual way4 ,advantages of toning by ,sel d'or4--,this process will be found especially useful by those who print large ,positives4 ,the solutions may be mixed in a few minutes1 and1 being very dilute1 are economical4 ,it is not even necessary to employ a ,bath for toning1 but if the ,sel d'or solution be prepared of about twice or three times the strength given in the formula1 it will be sufficient to pour a few drachms upon the surface of the print4 ,as the ,gold solution is always used soon after mixing1 a uniform and permanent tint can be obtained2 whereas the single fixing and toning ,bath of ,gold and ,hyposulphite loses much of its efficacy by keeping1 and over-printing of the proof is required in proportion as the ,bath becomes older4 ,,section ,,iv4 ,on a mode of ,printing enlarged and reduced ,positives1 ,transparencies1 etc41 from ,collodion ,negatives4 ,to explain the manner in which a ,photograph may be enlarged or reduced in the process of printing1 it will be necessary to refer to the remarks made at page #eb1 on the conjugate foci of lenses4 ,if a ,collodion ,negative be placed at a certain distance in front of a ,camera1 and 7by using a tube of black cloth7 the light be admitted into the dark chamber only through the ,negative1 a reduced image will be formed upon the ground glass2 but if the ,negative be advanced nearer1 the image will increase in size1 until it becomes first equal to1 and then larger than1 the original ,negative2 the focus becoming more and more distant from the lens1 or receding1 as the ,negative is brought nearer4 ,again1 if a ,negative portrait be placed in the ,camera slide1 and the instrument being carried into a dark room1 a hole be cut in the window-shutter so as to admit light through the ,negative1 the luminous rays1 after refraction by the lens1 will form an image of the exact size of life upon a white screen placed in the position originally occupied by the sitter4 ,these two planes1 in fact1 that of the object and of the image1 are strictly conjugate foci1 and1 as regards the result1 it is immaterial from which of the two1 anterior or posterior1 the rays of light proceed4 ,therefore in order to obtain a reduced or enlarged copy of a ,negative1 it is necessary only to form an image of the size required1 and to project the image upon a sensitive surface either of ,collodion or paper4 ,a good arrangement for this purpose may be made by taking an ordinary ,portrait ,camera1 and prolonging it in front by a deal box blackened inside and with a double body1 to' admit of being lengthened out as required2 or1 more simply1 by adding a framework of wood covered in with black cloth4 ,a groove in front carries the ,negative1 or receives the slide containing the sensitive layer1 as the case may be4 ,in reducing ,photographs1 the ,negative is placed in front of the lens1 in the position ordinarily occupied by the object2 but in making an enlarged copy1 it must be fixed behind the lens1 or1 which is equivalent1 the lens must be turned round1 so that the rays of light transmitted by the ,negative enter the back glass of the combination1 and pass out at the front4 ,this point should be attended to in order to avoid indistinctness of image from spherical aberration4 ,a ,portrait combination of lenses of #b-a_/b or #c-a_/d inches diameter is the best form to use1 and the actinic and luminous foci should accurately correspond1 as any difference between them would be increased by enlarging4 ,a stop of an inch or an inch and a half aperture placed between the lenses obviates to some extent the loss of sharp outline usually following enlargement of the image4 ,the light may be admitted through the ,negative by pointing the ,camera towards the sky2 or direct sunlight may be used1 thrown upon the ,negative by a plane reflector4 ,a common swing looking-glass1 if clear and free from specks1 does very well2 it should be so placed that the centre on which it turns is on a level with the axis of the lens4 ,the best ,negatives for printing enlarged ,positives are those which are distinct and clear2 and it is important to use a small ,negative1 which strains the lens less and gives a better result than one of larger size4 ,in printing by a #b-a_/d lens for instance1 prepare the ,negative upon a plate about two inches square1 and afterwards enlarge it four diameters4 ,paper containing ,chloride of ,silver is not sufficiently sensitive to receive the image1 and the ,print should be formed upon ,collodion1 or on iodized paper developed by ,gallic ,acid 7see p4 #bfc74 ,the exposure required will vary not only with the intensity of the light and the sensibility of the surface used1 but also with the degree of reduction or enlargement of the image4 ,in printing upon ,collodion the resulting picture is ,positive by transmitted light2 it should be backed up with white varnish1 and then becomes ,positive by reflected light4 ,the tone of the blacks is improved by treating the plate first with ,bichloride of ,mercury1 and then with ,ammonia1 in the manner described at pages #aac and #bjg4 ,mr4 ,wenham1 who has written a paper on the mode of obtaining ,positives of the life size1 operates in the following way3--he places the ,camera1 with the slide containing the ,negative1 in a dark room1 and reflects the sunlight in through a hole in the shutter1 so as to pass first through the ,negative and then through the lens2 the image is received upon iodized paper1 and developed by ,gallic ,acid1 in the mode described in the second ,section of this ,chapter 7p4 #bfc74 ,on printing ,collodion transparencies for the ,stereoscope4--,this may be done by using the ,camera to form an image of the ,negative in the mode described in the last page2 but more simply by the following process3--,coat the glass1 upon which the ,print is to be formed1 with ,collodio-,iodide of ,silver in the usual way2 then lay it upon a piece of black cloth1 ,collodion side uppermost1 and place two strips of paper of about the thickness of cardboard and one-fourth of an inch broad1 along the two opposite edges1 to prevent the ,negative being soiled by contact with the film4 ,both glasses must be perfectly flat1 and even then it may happen that the ,negative is unavoidably wetted2 if so1 wash it immediately with water1 and if it be properly varnished1 no harm will result4 ,a little ingenuity will suggest a simple framework of wood1 on which the ,negative and sensitive plate are retained1 separated only by the thickness of a sheet of paper2 and the use of this will be better than holding the combination in the hand4 ,the printing is conducted by the light of gas1 or of a camphine or moderator lamp2 diffused daylight would be too powerful4 ,the employment of a concave reflector1 which may be purchased for a few shillings1 ensures parallelism of rays1 and is a great improvement4 ,the lamp is placed in the focus of the mirror1 which may at once be ascertained by moving it backwards and forwards until an evenly illuminated circle is thrown upon a white screen held in front4 ,this in fact is one of the disadvantages of printing by a naked flame--that the light falls most powerfully upon the central part1 and less so upon the edges1 of the ,negative4 ,the picture must be exposed for a longer or shorter time 7about ten seconds will be an average7 according to its behaviour during development 7see p1 #bbd72 this process1 as well as the fixing1 is conducted in the same manner as for ,collodion pictures generally4 ,some adopt the plan of whitening by ,corrosive ,sublimate1 and again blackening by dilute ,ammonia1 as an improvement to the colour of the dark shadows 7see p4 #aac74 ,if this mode of printing upon ,collodion be conducted with care1 the ,negative being separated from the film by the smallest interval only1 the loss of distinctness in outline will scarcely be perceived4 ,stereoscopic transparencies may also be printed by the dry ,collodion process described in ,chapter ,,vi41 or by the ,collodio-,albumen process4 ,mr4 ,llewellyn recommends the employment of a solution of ,oxymel1 so dilute that the plate becomes nearly dry1 and may be laid in contact with the ,negative without fear of injury 7see the footnote at page #cjb74 ,,chapter ,v4 ,,classification ,,of ,,causes ,,of ,,failure ,,in ,,the ,,collodion ,,process4 ,section ,i4--,imperfections in ,collodion ,photographs4 ,section ,,ii4--,imperfections in ,paper ,positives4 ,,section ,i4 ,imperfections in ,negative and ,positive ,collodion ,photographs4 ,the following may be mentioned3--fogging--spots-- markings1 etc4 ,,causes ,,of ,,fogging ,,of ,,collodion ,,plates4 #a4 ,over-exposure of the ,plate4--,this is likely to happen when using the full aperture of a double combination lens for distant objects brightly illuminated1 the ,collodion being highly sensitive4 ,also from the film being very blue and transparent1 with too little ,iodide of ,silver 7p4 #aad74 #b4 ,diffused ,light4--a4 ,in the developing room4 ,this is a frequent cause of fogging1 and especially so when the common yellow calico is employed1 which is apt to fade4 ,use a treble thickness1 or procure the waterproof material1 in which the pores are stopped with gutta-percha4--b4 ,in the ,camera4 ,the slide may not fit accurately1 or the door does not shut close4 ,throw a black cloth over the ,camera during the exposure of the plate4--c4 ,from direct rays of the sun or the light of the sky falling upon the lens4 ,with the full aperture of a double combination ,lens1 a portion of sky included in the field 7as for instance to form the background of a portrait7 is apt to cause fogging4 ,the portrait will probably be more brilliant if a funnel-shaped canvas bag1 or a curtain with an oblong aperture admitting only the rays proceeding from the sitter1 be placed in front of the ,camera4 #c4 ,alkalinity of the ,bath4--,this condition1 explained at page #hh1 may be due to one of the following causes3--a4 ,the use of ,nitrate of ,silver which has been too strongly fused 7p4 #ac74--;b4 ,constant employment of a ,collodion containing free ,ammonia or ,carbonate of ,ammonia 7p4 #hi74--;c4 ,addition of ,potash1 ,ammonia1 or ,carbonate of ,soda to the ,nitrate ,bath1 in order to remove free ,nitric ,acid 7p4 #hi74--;d4 ,use of rain-water or hard water for making the ,nitrate ,bath 7rain-water usually contains traces of ,ammonia2 hard water often abounds with ,carbonate of ,lime74 ,in either case the alkalinity may easily be removed by the addition of ,acetic ,acid1 one drop to four ounces of the solution4 ,the proper mode of testing for alkalinity is described at p4 #hi4 #d4 ,decomposition of the ,nitrate ,bath4--a4 ,by constant exposure to light 7the injurious effects of this will be mostly seen when ,positives are taken74--b4 ,by organic matter3 this is sometimes present in ,nitrate of ,silver which has been prepared from the residues of old ,baths2 or it may be introduced by floating papers for the printing process upon the ,bath1 or by dissolving the crystals of ,nitrate of ,silver in putrid rain-water1 or in impure distilled water collected from the condensed water of steam-boilers and contaminated with oily matter4--c4 ,decomposition of the ,bath by contact with metallic iron or copper1 or with a fixing agent1 or a developing agent 7p4 #ij74 #e4 ,faults of the developing solution4--a4 ,brown and decomposed solution of ,pyrogallic ,acid2 this may sometimes be used with impunity1 but it tends1 as a rule1 to facilitate irregular reduction of ,silver4--h4 ,impure ,acetic ,acid having a smell of ,garlic and which probably contains ,sulphur in organic combination4--c4 ,omission of the ,acetic ,acid in the developer3 this will produce a universal blackness4 #f4 ,sundry other causes of fogging4--a4 ,vapour of ,ammonia or ,hydrosulphate of ,ammonia1 or the products of the combustion of coal-gas1 escaping into the developing room4--b4 ,development of the image by immersion in solution of ,sulphate of ,iron3 this is a safe plan when the films are formed in an acid ,nitrate ,bath2 but with pale films formed in a chemically neutral ,bath it is better to pour the fluid over the plate1 and not to use the same portion twice4--c4 ,redipping the plate in the ,bath before development3 this is apt to give a foggy picture when using an old ,bath1 and is not recommended4 ,systematic plan of proceeding to detect the cause of the fogging4--,if the amateur has had but little experience in the ,collodion process1 and is using ,collodion of moderate sensitiveness and a new ,bath1 the probability is that the fogging is caused by over-exposure4 ,having obviated this1 proceed to test the ,bath2 if it is made from pure materials1 and does not restore the blue colour of a piece of litmus-paper previously reddened by holding it over the mouth of a glacial ,acetic ,acid bottle1 it may be considered in working order4 ,next prepare a sensitive plate1 and after draining it for two or three minutes in a dark place1 pour on the developer3 wash1 fix1 and bring out to the light2 if any mistiness is perceptible1 either the developing room is not sufficiently dark1 or the ,bath was prepared with a bad sample of ,nitrate of ,silver1 or with impure ,alcohol1 or impure water4 ,on the other hand1 if the plate remains absolutely clear under these circumstances1 the cause of error may be in the ,camera2--therefore prepare another sensitive film1 place it in the ,camera1 and proceed exactly as if taking a picture1 with the exception of not removing the brass cap of the lens3 allow to remain for two or three minutes1 and then remove and develope as usual4 ,if no indication of the cause of the fogging is obtained in either of these ways1 there is every reason to suppose that it is due to diffused ,light gaining entrance through the lens4 ,this cause of error may often be detected by looking into the ,camera from the front1 when an irregular reflection will be seen upon the glass4 ,,spots ,,upon ,,collodion ,,plates4 ,spots are of two kinds3 spots of opacity1 which appear black by transmitted light1 and white by reflected light2 and spots of transparency1 the reverse of the others1 being white when seen upon ,negatives1 and black on ,positives4 ,opaque ,spots are referable to an excess of development at the point where the spot is seen2 they may be caused by-- #a4 ,the use of ,collodion holding small particles in suspension4--,each particle becomes a centre of chemical action1 and produces a speck1 or a speck with a tail to it4 ,the ,collodion should be placed aside to settle for several hours1 after which the upper portion may be poured off4 #b4 ,turbidity of the ,nitrate solution4--a4 ,from flakes of ,iodide of ,silver having fallen away into the solution1 by use of an over-iodized ,collodion4--b4 ,from a deposit formed by degrees upon the sides of the gutta-percha trough4--c4 ,from the inside of the trough being dusty at the time of pouring in the solution4 ,in order to obviate these inconveniences1 it is well to make at least half as much again of the ,nitrate solution as is necessary1 and to keep it in a stock-bottle1 from which the upper part may be poured off when required4 ,the frequent filtration of ,silver ,baths is unadvisable1 since the paper employed may be contaminated with impurities4 #c4 ,dust upon the surface of the glass at the time of pouring on the ,collodion4--,perfectly clean glasses1 if set aside for a few minutes1 acquire small particles of dust2 each plate should therefore be gently wiped with a silk handkerchief immediately before being used4 #d4 ,faults of the ,slide4--,sometimes a small hole exists1 which admits a pencil of light1 and produces a spot1 known by its being always in the same part of the plate2 occasionally the door works too tightly so that small particles of wood1 etc41 are scraped off1 and projected against the plate when it is raised4 ,or perhaps the operator1 after the exposure is finished1 shuts down the door with a jerk1 and so causes a splash in the liquid which has drained down and accumulated in the groove below2 this cause1 although not a common one1 may sometimes occur4 #e4 ,insoluble particles in the ,pyrogallic ,acid4--,the solution of ,pyrogallic ,acid will not usually require filtering1 but if specks of ,metagallic ,acid are present1 the developer should be passed through blotting paper before use4 ,spots of ,transparency may generally be traced to some cause which renders the ,iodide of ,silver insensible to light at particular points1 so that on the application of the developer no reduction takes place4 #a4 ,concentration of the ,nitrate of ,silver on the surface of the film by evaporation4--,when the film becomes too dry after removal from the ,bath1 the solvent power of the ,nitrate increases so much that it eats away the ,iodide and produces spots4 #b4 ,small particles of undissolved ,iodide of ,potassium in the ,collodion4--,these are likely to occur when ,anhydrous ,ether and ,alcohol are employed4 ,they produce transparent specks at every part of the plate4 ,allow the ,collodion to settle1 or add a drop of water1 which will dissolve the ,iodide4 #c4 ,alcohol or ,ether containing too much water4--,this causes a reticulated appearance of the film1 which is rotten and full of holes4 #d4 ,use of glasses improperly cleaned4--,this cause is perhaps the most frequent of all1 when the film of ,pyroxyline is very thin and the ,bath neutral4 ,after glasses have been long used it is often difficult to clean them so thoroughly that the breath lies smoothly2 but the use of ,potash gives the best chance4 ,,markings ,,of ,,various ,,kinds ,,on ,,collodion ,,plates4 #a4 ,a reticulated appearance on the film after developing4--,when this is universal1 it often depends upon the employment of ,collodion containing water4 ,or1 if not due to this cause1 the plate may have been immersed too quickly in the ,bath1 and the soluble ,pyroxyline partially precipitated4 #b4 ,oily spots or lines4--a4 ,from raising the plate out of the ,nitrate ,bath before it has been immersed sufficiently long to have become thoroughly wetted4--b4 ,removal of the plate from the ,bath before the ,ether upon the surface has been washed away4--c4 ,redipping the plate in the ,nitrate ,bath after exposure to light1 and pouring on the developer immediately2 if a few minutes be not allowed to drain off the excess of ,nitrate1 the ,pyrogallic ,acid will not amalgamate readily with the surface of the film4--d4 ,from the ,nitrate ,bath being covered with an oily scum1 which is carried down by the plate4 ,draw a slip of blotting-paper gently along the surface of the liquid before using it4 #c4 ,straight lines traversing the film horizontally4--,from a check having been made in immersing the plate in the ,bath4 #d4 ,curved lines of over-development4--,by employing the developer too concentrated2 or by not pouring it on sufficiently quickly to cover the surface before the action begins2 or by using too little ,acetic ,acid1 and omitting the ,alcohol4 ,the addition of ,alcohol to the developer will not be required as a rule when the ,bath is newly made2 but when much ,ether has accumulated in it1 the developer has a tendency to run into oily lines1 unless containing ,alcohol4 #e4 ,stains from too small a quantity of fluid having been employed to develope the image4--,in this case1 the whole plate not being thoroughly covered during the development1 the action does not always proceed with regularity4 #f4 ,irregular stri4--,from fragments of dried ,collodion accumulating in the neck of the bottle1 and being washed on the film2 to avoid this1 the finger should be passed gently round the inside of the neck before use4 #g4 ,markings like those represented in the woodcut4--,they are caused by using an inferior sample of ,pyroxyline made from too hot acids1 and are most seen when using an old ,bath4 ,illustration #h4 ,stains on the upper part of the plate1 from using a dirty slide4--,to avoid these1 place1 if necessary1 strips of blotting-paper between the supports and the glass4 #i4 ,wavy marks at the lower parts of the plate4--a4 ,if the ,collodion is becoming thick and glutinous from constant use1 dilute it with a little ,ether containing an eighth part of ,alcohol4--b4 ,from reversing the direction of the plate after its removal from the ,bath1 so that the ,nitrate of ,silver flows back again over the surface and causes a stain on the application of ,pyrogallic ,acid4--c4 ,impurities on the woodwork of the frame ascending the film by capillary attraction4 ,this is a frequent source of stains4 #aj4 ,marks from the developer not running up to the edge of the film 7p4 #bab74 ,remedy this as far as possible by allowing the ,collodion to set a little more firmly before dipping the plate in the ,bath4 ,,imperfections ,,in ,,collodion ,,negatives4 #a4 ,a want of ,intensity4--a4 ,from the development not having been sufficiently pushed 7p4 #bbd74--;b4 ,from the ,collodion film being too blue and transparent for ,negatives4--c4 ,the ,collodion newly made from pure materials 7p4 #aad74--;d4 ,the plate kept too long between exciting and development 7p4 #ajj74--;e4 ,the ,bath newly prepared from commercial crystallized ,nitrate of ,silver 7p4 #aja74--;f4 ,the light too feeble1 as on very dark wintry days1 or in copying interiors1 etc4 #b4 ,inferior half-tones1 with great intensity of the high ,lights4--a4 ,from the plate being insufficiently exposed4--b4 ,the ,collodion of inferior quality1 either too strongly tinted with ,iodine or made from impure materials4--c4 ,the ,nitrate ,bath old and partially decomposed4--d4 ,the light reflected too strongly from the object4 ,when the light is unusually bright1 a feeble ,collodion and a newly mixed ,nitrate ,bath will be found to give better definition in the high lights than an intense ,collodion1 which may produce chalky ,negatives4 #c4 ,the image pale and misty4--,the plate is over-exposed 7if so1 the image will probably be a reddish-brown colour by transmitted light71 or there is diffused light in the ,camera or developing room4 ,the presence of ,bromides or ,chlorides in the ,collodion may occasionally produce the same effect4 #d4 ,the high lights of the image are solarized4--,a change of colour to a light brown or red tint by transmitted light1 with a dark shade by reflected light1 is favoured by over-exposure of the plate1 by organic decomposition of the ,collodion1 and by ,acetate of ,silver and other organic bodies in the ,bath4 #e4 ,the image dissolves off on applying the ,cyanide of ,potassium4--,the ,collodion is probably over-iodized4 ,the same thing may also happen in the ,honey preservative process1 when the plates have been long kept and the indurated layer of syrup not properly removed before applying the developer4 #f4 ,the developer does not run up to the edge of the film4--,this is likely to occur when using ,collodion nearly anhydrous2 and particularly so with a new ,bath not containing much ,alcohol4 ,the film will be less repellent1 if a longer time be allowed before dipping in the ,bath4 #g4 ,the film does not stick to the glass4--,clean the plates very carefully1 and make the ,collodion a little thinner if required4 ,allow a longer time before dipping in the ,bath4 ,a very effectual plan is to roughen the surface of the plates1 about an eighth of an inch round the edges4 ,,imperfections ,,in ,,collodion ,,positives4 ,the principal difficulty in the production of ,negatives is to ascertain the right time of exposure to light and the proper point to which to carry the development of the image4 ,a minor amount of fogging1 stains1 etc41 is of less consequence1 and will scarcely be noticed in the printing4 ,with direct ,positives however the case is different4 ,the beauty of these pictures depends entirely upon their being clean and brilliant1 without fogging1 specks1 or imperfections of any kind4 ,on the other hand1 the exposure and development of ,positives is comparatively simple and easily ascertained4 #a4 ,the shadows dark and heavy4--,the plate has not received sufficient exposure in the ,camera2--or the film being very transparent and the ,silver solution weak1 ,nitric ,acid is present in the ,bath1 or the ,collodion is brown from free ,iodine2 in the latter case make the ,collodion a little thicker1 and develope with ,sulphate of ,iron in preference to ,pyrogallic ,acid4 #b4 ,the shadows good1 but the lights overdone4--,the developing fluid may have been kept on too long2 or the object is not properly illuminated 7p4 #bbj72 or the ,collodion is not adapted for ,positives4 #c4 ,the high lights pale and flat1 the shadows misty4--,the plate is over-exposed4 ,indistinctness of outline caused by over-exposure is distinguished from that produced by fogging by holding the plate up to the light2 in the former case the image shows as a ,negative4 ,if the ,collodion is colourless1 clearer shadows will probably be obtained by dropping in ,tincture of ,iodine until a yellow colour is produced4 #d4 ,the picture developes slowly2 spangles of metallic ,silver are formed4--,too much ,nitric ,acid is present in proportion to the strength of the ,bath1 to the amount of ,iodide in the film1 and to the quantity of ,protosalt of ,iron in the developer 7p4 #aab74 #e4 ,circular spots of a black colour after hacking up with the varnish4--,these are often caused by lifting the plate too quickly out of the ,bath2 or by pouring on the developer at one spot1 so as to wash away the ,nitrate of ,silver2 or by the use of glasses imperfectly cleaned4 #f4 ,the image becomes metallic on drying4--,if ,sulphate of ,iron is employed1 the solution is too weak1 or free ,nitric ,acid has been added in excess4 ,if ,pyrogallic ,acid is used to develope1 the proportion of ,nitric ,acid is too great4 #g4 ,a green or blue tint in certain parts of the image4--,this is caused by the deposit of ,silver being too scanty1 which may happen from over-action of the light1 or from the film of ,pyroxyline being very thin2--if the ,collodion is diluted down beyond a certain point1 the same quantity of free ,nitrate of ,silver is not retained upon the surface of the film4 ,add a few drops of the ,bath to the developer before pouring it on the plate4 #h4 ,vertical lines1 and mistiness1 on the image4--,if the ,bath has been much used1 add to it a third part of a simple solution of ,nitrate of ,silver in water1 without any ,alcohol or ,iodide4 ,also prepare the developer with addition of ,alcohol1 to make it flow more readily 7p4 #baa74 ,,section ,,ii4 ,imperfections in ,paper ,positives4 #a4 ,the ,print marbled and spotty4--,the quality of the paper is often inferior1 which causes it to imbibe liquids unevenly at different points2 or the amount of ,silver in the ,nitrate ,bath is insufficient4 ,in this case the spots are often absent at the lower and most depending part of the sheet1 where the excess of liquid drains off4 #b4 ,the ,print clean on the surface1 but spotted when held up to the light4--,in this case the spots are probably due to imperfect fixation 7see p4 #abi74 #c4 ,the ,print becomes pale in the ,hyposulphite ,bath1 and has a cold and faded appearance when finished4--,the ,chloride of ,silver in the paper may have been in excess with regard to the free ,nitrate of ,silver2 which is especially likely if no bronzing could be obtained by prolonged action of the light1 or if a weak solution of ,nitrate of ,silver was laid on with a brush1 or by a glass rod4 ,prints formed on paper which has been kept too long after sensitizing present the same appearance1 the free ,nitrate of ,silver having entered into combination with the organic matter4 #d4 ,yellowness of the light parts of the proof4--,the following causes are likely to produce yellowness3--acidity of the fixing and toning ,bath 7p4 #aci71--;its action continued for too long a time1--the first washings of the proof not performed quickly1--the toning ,bath laid aside until it had become decomposed and nearly useless1--the paper kept for several days after sensitizing4 ,a creamy yellowness is also common in ,prints toned by ,sel d'or1 when the ,hydrochloric ,acid has been omitted from the formula2 the proof exposed to light during the toning and fixing process2 or too long a time allowed to elapse between the toning and fixing4 ,it is also more frequently met with on albuminized paper4 #e4 ,intense bronzing of the deep shadows4--,in this case the ,negative is in fault2 remedy the evil as far as possible by printing on paper containing but little salt4 #f4 ,the definition of the ,print imperfect1 the ,negative being a good one4--,much will depend upon the quality of the paper4 ,towgood's ,positive gives good definition4 ,the use of ,albumen will be a great advantage4 ,citrate of ,soda 7p4 #bdf7 will also improve the definition on plain paper4 #g4 ,markings of a yellow tint in the dark portions of the ,positive4--,these are common on ,prints toned without ,gold2 care should be taken not to handle the paper too much1 either before or after sensitizing2 to wash the prints in a clean vessel2 and not to lay them down whilst wet on a wooden table or in contact with anything likely to communicate impurities4 #h4 ,small specks and spots of different hinds4--,these1 when not corresponding to similar marks upon the ,negative1 are usually due to metallic specks in the paper2 or to insoluble particles floating in the bath4 #i4 ,markings of the brush in ,ammonio-,nitrate pictures4--,in this case there is probably an excess of ,ammonia1 which dissolves the ,chloride of ,silver4 ,add a little fresh ,nitrate of ,silver1 or use the ,oxide of ,silver dissolved in ,nitrate of ,ammonia 7p4 #bdi74 #aj4 ,marbled stains on the surface of the ,print4--,draw a strip of blotting-paper gently over the surface of the ,nitrate ,bath before sensitizing the paper2 and see that the sheet does not touch the bottom of the dish4 #aa4 ,streaks on ,albuminized paper4--,apply the ,albumen more rapidly and evenly to the paper4 ,if this does not succeed1 add a little ,ox-,gall 7p4 #bdc74 #ab4 ,removal of the ,albumen from the paper during sensitizing4--,the ,nitrate ,bath is probably alkaline 7see page #hi74 ,,chapter ,,vi4 ,,landscape ,,photography ,,on ,,preserved ,,collodion ,,and ,,collodio-,,albumen4 ,the ,collodion process may be applied with success to landscape ,photography2 but as the plates become dry and lose their sensitiveness shortly after their removal from the ,bath1 the operator will require to provide himself with a yellow tent or some portable vehicle in which the operations of sensitizing and developing can be conducted4 ,as it is a point of great importance in the ,collodion process that the plate should receive exactly the right amount of exposure in the ,camera1--a few seconds more or less sufficing to affect the character of the picture1--many will submit to much trouble and inconvenience in order to have the apparatus complete upon the spot at which the view is taken4 ,the object of the ,collodion ,preservative ,processes is to maintain the sensitiveness of the film for a certain length of time after it has been excited in the ,bath4 ,there is some difficulty in doing this1 because if the plate be allowed to dry spontaneously1 the solution of free ,nitrate of ,silver upon the surface1 becoming concentrated by evaporation1 eats away the ,iodide of ,silver1 and produces transparent spots4 ,some operators have attempted to use a second plate of glass in such a way as to enclose the sensitive film with an intervening stratum of liquid4 ,the difficulty however of separating the glasses again without tearing the film1 is considerable4 ,in the process of ,messrs4 ,spiller and ,crookes1 the property possessed by certain saline substances of remaining for a long time in a moist condition was turned to account4 ,such salts are termed deliquescent1 and many of them have so great an attraction for water that they absorb it eagerly from the air3 the solution having been formed1 the water cannot entirely be driven off except by the application of a considerable heat4 ,more recently1 ,honey has been employed by ,mr4 ,shadbolt4#eb ,this substance can scarcely be termed deliquescent1 but it possesses1 like other uncrystallizable sugars1 the property of remaining moist and sticky for a long time4 ,honey is1 according to the ,author's views1 superior to inorganic deliquescent salts as a preservative agent1 from its possessing an affinity for ,oxides of ,silver1 and thus acting chemically in communicating organic intensity to the image4--,collodion plates when kept long in a moist and sensitive state often give a pale and blue image1 even although the ,nitrate of ,silver be left upon the film2 and neither ,nitrate of ,magnesia nor ,glycerine appears capable of supplying the deficient element1 both being nearly or quite indifferent to the ,salts of ,silver4 #eb ,a claim has lately been advanced by ,mr4 ,maxwell ,lyte to be considered as the discoverer of the ,honey ,process4 ,this gentleman appears to have worked simultaneously with ,mr4 ,shadbolt1 and to have anticipated him in publishing2 but the object of ,mr4 ,lyte's process was rather to increase the sensibility of the plates than to confer upon them keeping qualities4 ,,the ,,honey ,,and ,,oxymel ,,keeping ,,processes4 ,when the weather is cool1 ,collodion plates may be preserved with tolerable certainty for a few hours1 by simply applying ,honey to them in the state in which they are taken from the ,nitrate of ,silver ,bath4 ,the best pure ,virgin ,honey should be obtained by dripping it immediately from the comb4 ,this point is of importance1 since if the sample of honey be of inferior quality1 or adulterated1 the process may not succeed4 ,the quantity of water to be added will vary with the consistence of the honey1 from about an equal bulk to two parts3 it should be sufficient to make the preservative solution pass slowly through filtering-paper4 ,after the plate is removed from the ,nitrate ,bath1 it is to be drained and wiped on the back in the usual way4 ,the ,honey is then poured along the edge in such a manner as to form a broad wave which forces the ,nitrate of ,silver solution before it and covers the film4 ,next drain the plate into a measure and pour on a second portion of ,honey as before4 ,this second dose may be used again for the first application to the succeeding plate4 ,lastly1 stand the glass on blotting-paper in a dark place for about a quarter of an hour or twenty minutes1 and wipe the lower edge before putting it into the plate box4 ,the exposure required will probably be about four or five times as long as that for new and sensitive ,collodion1 or twice as long as the exposure required for old and brown ,collodion4 ,before applying the developer1 immerse the plate in a ,bath of rain-water for five minutes1 moving it about occasionally to soften the honey4 ,this will probably be sufficient for plates which have not been kept longer than four hours1 and beyond that time the process is not considered certain1 since the ,honey exercises a slow reducing action upon the ,nitrate of ,silver4 ,the solution of ,pyrogallic ,acid may be used of the ordinary strength1 with a full dose of ,acetic ,acid4 ,only a faint image comes out at first1 but on pouring over the plate a fresh portion of the developer with two or three drops of the ,nitrate ,bath added to each fluid drachm1 it may be intensified to any extent4 ,fix with ,hyposulphite of ,soda1 and wash in the usual way4 ,when the process fails1 from heat of the weather or other causes1 the image will probably be feeble and red by transmitted light1 and the shadows defective and misty4 ,this is especially likely to happen when the ,nitrate ,bath is very old and contains much ,acetate of ,silver2 or when the same portion of ,honey is used more than once1 and has undergone partial decomposition by the action of the ,nitrate of ,silver4 ,the use of pure ,honey1 free from mouldiness and fermentation1 will1 in cool weather1 almost certainly ensure success4 ,a modification of the process when the plates are to be kept over four hours4--,in this case the whole1 or the greater part of the ,nitrate of ,silver must be removed before applying the preservative agent4 ,wash the sensitive plate in water in the manner described for the ,oxymel process in the next page4 ,then apply the syrup as before1 using it as thick as possible4 ,honeyed plates1 free from ,nitrate of ,silver1 may commonly be kept for five or six days2 often much longer4 ,dr4 ,mansell1 who has employed this process with great success1 speaks of temperature as a point to be attended to4 ,in hot weather the same length of keeping properties will not be attained4 ,use of ,oxymel for preserving ,collodion plates4--,the principal difficulty in the employment of ,honey in ,photography1 is its disposition to ferment1 or to become mouldy4 ,fermentation occurs most readily in a dilute solution1 and will be obviated by using the syrup as thick and free from water as possible4 ,mr4 ,llewellyn employs ,oxymel1 which is a mixture of ,honey and ,vinegar1 as a preservative agent4 ,this substance will keep even in dilute solution for a long time without decomposition2 and1 being very readily removed from the plates1 does not interfere with the development of the image4 ,the preparation of ,oxymel is described in the ,vocabulary1 ,part ,,iii42 it must be diluted with three or four parts of water1 and filtered4 ,certain facts to which attention has been lately drawn by ,dr4 ,norris and ,mr4 ,barnes in working with dry ,collodion1 may be advantageously borne in mind when using ,oxymel2 the preservative solution of which is employed in so dilute a state that the process resembles to a great extent a dry ,collodion process4 ,the observations above referred to relate to the quality of the ,collodion best adapted for the purpose1 and will be found at page #bih1 to which the reader is referred4 ,the manipulation of the ,oxymel process is very simple4 ,two flat gutta-percha dishes are provided1 the one containing common water and the other diluted and filtered ,oxymel4 ,the ,collodion plate1 on its removal from the ,bath1 is placed in the first dish1 which is gently tilted up and down1 to wash away the free ,nitrate of ,silver4 ,in a few seconds1 when the liquid is rendered milky1 it is poured away1 and fresh water being introduced1 the process is repeated until the oily lines disappear1 and the surface of the film becomes smooth and glassy4 ,the plate is then1 after a slight draining1 removed to the second tray1 and the ,oxymel waved backwards and forwards for about half a minute1 after which the glass is lifted out and placed vertically on blotting-paper1 which must be renewed when it becomes wet and saturated4 ,the plates may be used any time within a fortnight from the date of their preparation1 and it is not necessary to develope immediately after the exposure4 ,the sensitiveness will be considerably less than that of fresh ,collodion3 from two to five minutes may be allowed with a ,stereoscopic view lens having a quarter-inch diaphragm4 ,before developing1 the film should be gently washed for a few seconds with common water4 ,solution of ,pyrogallic ,acid1 of the ordinary strength1 but previously mixed with a portion of the ,nitrate ,bath solution1 one or two drops to each drachm1 may then be poured on in the ordinary way4 ,use less ,nitrate of ,silver and more ,acetic ,acid in hot weather4 ,when discoloration of the developer occurs1 mix a fresh portion and proceed as before4 ,,precautions ,,to ,,be ,,observed ,,in ,,keeping ,,processes4 ,the plates must be roughened at the edges1 and also upon the surface1 to make the film adhere4 ,it is advisable to use a tolerably thick ,collodion1 giving a yellow film2 the pale opalescent films being more easily affected by markings on the glass1 and not retaining so much of syrup or ,nitrate of ,silver upon the surface4 ,the room in which the plates are prepared must be carefully guarded from scattered pencils of white light2 the films are exposed to injury from this cause during the whole of the time occupied in applying the preservative syrup2 and hence anything short of absolute chemical darkness will be likely to cause fogging2 especially so when free ,nitrate of ,silver is left upon the film4 ,the water used for washing away the free ,nitrate of ,silver before applying the preserving liquid1 need not be distilled4 ,common hard water containing ,carbonates and ,chlorides1 and producing milkiness with ,nitrate of ,silver1 will often suffice4 ,the water of the ,new ,river and of the ,river ,thames1 with which many parts of ,london are supplied1 may certainly be used2 but in the case of a very hard water1 containing much ,sulphate of ,lime1 it might perhaps be advisable to substitute clean rain-water1 free from brown organic discoloration4 ,the preservative ,oxymel must be carefully filtered1 and kept covered1 in order to protect it from dust4 ,it will also be necessary occasionally1 before using it1 to run it through a piece of white cambric1 to stop back suspended particles1 which1 if allowed to remain1 would be a source of spots4 ,if it becomes mouldy1 or discoloured by ,silver1 or ferments and evolves gas1 throw it away4 ,after the syrup is applied and the plates are drained1 stow them in a grooved box perfectly protected from light2 or place them in slides1 which must be kept scrupulously clean1 since any trace of impurity would be likely to produce a stain when the plate was left a long time in the slide4 ,if the preserved plates are kept in a cupboard or box1 see that no volatile matter1 such as ,ammonia1 coal-gas1 etc41 can find entrance4 ,in changing the plates after the exposure in the ,camera1 use a large bag made of several thicknesses of black calico1 with a square of yellow calico let in at the top2 an elastic band securing it round the waist4 ,,the ,,collodio-,,albumen ,,process4 ,this process1 the theory of which has been briefly explained at page #aha1 is more sensitive than the one last described1 and has the additional advantage of giving dry plates1 which do not attract dust1 and are less liable to injury4 ,the details of manipulation are complex1 but this inconvenience is not so much felt when preparing a large number of plates4 ,cleaning the ,glasses4--,success will greatly depend upon the mode in which this part of the process is performed4 ,the layer of ,albumen which is applied to the ,collodion film tends to swell and to raise the latter in blisters2 the most effectual mode of obviating which will be to clean the glass so that the film adheres with unusual tenacity4 ,the ,liquor ,potass of the ,druggists1 diluted with three or four parts of water1 and rubbed on the glass by a roll of flannel 7page #bad71 is very effectual4 ,a mixture of ,tripoli-water and ,nitric ,acid may however1 if desired1 be substituted3-- ,tripoli #a drachm4 ,nitric ,acid #cj minims4 ,water #a ounce4 ,lay the glass flat on a cloth1 and rub the surface carefully with a tuft of cotton-wool dipped in the ,tripoli2 then1 before the cream dries1 wipe it off with a second tuft1 and polish with a third4 ,lastly1 breathe upon the glass1 and having ascertained that it is chemically clean1 apply the ,collodion4 ,coating with ,collodion4--,choose a rather thin ,collodion which adheres tightly to the glass4 ,a preparation which has been kept a long time after iodizing will usually answer the purpose very well1 and1 as a rule1 a non-contractile1 structureless ,collodion is better than one which is glutinous and wavy4 ,the degree of sensibility of the ,collodion is not thought to have much influence upon the result4 ,coating the ,plate4--,apply the ,collodion in the usual manner1 and allow it full time to set perfectly1 before dipping in the ,bath1 in order to favour its adherence to the glass4 ,with ,collodion prepared from anhydrous spirits1 about half a minute may be given in cool weather4 ,the ,nitrate ,bath4--,take of ,fused ,nitrate of ,silver #dj grains4 ,glacial ,acetic ,acid #cj minims4 ,alcohol #bj minims4 ,water #a fluid ounce4 ,saturate with ,iodide of ,silver as described at page #bjd1 and filter4 ,an immersion of one minute will be sufficient2 after which1 give the plate an up-and-down movement1 and wash it in plain water1 in the manner advised for the ,oxymel preservative process1 at page #bib4 ,then stand it on blotting-paper1 to drain for a minute or two1 wipe the back of the glass1 and pour on the ,albumen4 ,this ,bath may become discoloured after a time2 continue to use it until it is of a dark sherry-colour1 and then treat it with ,kaolin1 in the manner and with the precautions advised at pages #ia and #bde4 ,the ,iodized ,albumen4--,procure eggs1 fresh laid1 or not more than two or three days old4 ,separate the whites in the same way as for ,albuminized paper 7p4 #bda71 and mix by the following ,formula3-- ,albumen #i fluid ounces4 ,water #c fluid ounces4 ,liquor ,ammoni #b fluid drachms4 ,iodide of ,potassium #dh grains4 ,bromide of ,potassium #ab grains4 ,the ,iodide and ,bromide should be free from ,carbonate of ,potash1 which is said to cause pin-holes in the ,negatives4 ,to ensure the absence of this salt1 dissolve the total quantity of both ,iodide and ,bromide in the three ounces of water advised in the formula2 then1 previously to adding the ,ammonia and ,albumen1 introduce an excessively minute particle of ,iodine1 enough barely to colour the liquid4 ,the ,iodine decomposes the ,carbonate of ,potash1 but it must not be used in excess1 since free ,iodine possesses the property of coagulating ,albumen4 ,iodide of ,cadmium also coagulates ,albumen1 so that the ,iodides of ,potassium and ,ammonium are the best4 ,having mixed the ingredients in the order above given1 introduce them into a bottle1 and shake it violently until they have thoroughly amalgamated4 ,then transfer to a tall narrow jar2 allow to settle for twenty-four hours1 and draw off the upper clear portion for use4 ,particulars of this part of the process have already been given under the head of ,albuminized ,paper1 to which the reader is referred 7p4 #bda74 ,the ammoniacal solution of ,albumen may be kept for some time in a stoppered bottle without much decomposition4 ,if mucous threads form in it1 filter through fine linen4 ,mode of applying the ,albumen4--,cover the moist film with the ,albumen in the same way as advised for ,collodion 7p4 #baf71 pouring on at once a sufficient quantity to cause it to spread in an even and undivided sheet2 otherwise a veined appearance may be produced1 which will show in the development4 ,return the excess of ,albumen into the bottle1 and pour it once again upon the plate3 the film will remain clear and transparent1 if the whole of the ,nitrate of ,silver has been properly washed away from the ,collodion4 ,lastly1 stand the plate nearly vertically on blotting-paper to dry4 ,this will occupy five or six hours2 but the process may be hastened by artificial heat4 ,after the ,albumen solution has been used to coat a number of plates successively1 it becomes diluted with water2 the result of which is1 that unequal intensity of image is produced at the upper and lower edge of the film4 ,the iodized ,albumen plates are at this stage of the process nearly or quite insensitive to light1 and may be preserved unchanged for many weeks4 ,sensitizing the ,albumen film4--,when the plate has become thoroughly dry1 it is again introduced into the ,bath of ,aceto-,nitrate of ,silver1 and allowed to remain for one minute3 then washed with water in the same manner as before1 but with even greater care1 in order to obviate clouding in the development4 ,if blisters should form on drying1 it will be found useful to hasten the process by holding the plates to the fire--or a hot iron may be placed in the centre of a covered box and the glasses reared up round the sides4 ,they will thus dry quickly1 and there will not be time for the ,albumen to swell much by imbibition4 ,exposure in the ,camera4--,this may be performed at any period within a few weeks from the date of preparation of the plates4 ,for a landscape view with a small ,stereoscopic single lens1 allow about three minutes in the winter1 or one minute and a half in the summer4 ,development of the image4--,this can be deferred as long as fourteen days after the exposure1 with successful results4 ,pour water over the plate until the film is thoroughly wetted2 then cover it with a solution of ,pyrogallic ,acid containing one grain of the acid to the ounce of water1 and twenty minims of ,glacial ,acetic ,acid4 ,two drops of a neutral solution of ,nitrate of ,silver made with forty grains of ,nitrate to the ounce of water must be previously added to each fluid drachm of the ,pyrogallic4 ,the development1 in the case of a landscape view taken with sunlight1 commences almost immediately1 and may be completed in about ten minutes1 but the time occupied in developing will vary greatly with the length of exposure1 the quantity of ,nitrate of ,silver1 and the nature of the subject copied--a badly lighted interior1 for instance1 often taking an hour or longer to appear in all its details4 ,if the developer should discolour before the proper intensity has been obtained1 pour it off and mix a fresh quantity4 ,fixing the image4--,hyposulphite of ,soda 7one ounce to four of water7 will be found preferable to the ,cyanide of ,potassium1 as the latter has a solvent effect upon the ,albumen4 ,an unusually long time will be required1 as the fixing agent must penetrate the ,albumen1 to reach the ,collodion beneath4 ,careful washing in water for five or ten minutes removes the excess of ,hyposulphite1 and the plate may then be varnished in the usual way4 ,,the ,,dry ,,collodion ,,process4 ,the earlier attempts to employ sensitive ,collodion plates in a desiccated condition were unsuccessful4 ,the film of ,pyroxyline shrinks on drying1 and becomes almost impervious to moisture3 hence1 the developing solution not penetrating properly1 density cannot easily be obtained4 ,we are indebted to ,dr4 ,hill ,norris1 of ,birmingham1 for establishing the theory of the subject upon a more correct basis4 ,he has pointed out the importance of distinguishing two different conditions of the ,collodion surface1#ec viz4 the contractile1 common in newly-mixed ,collodion1--and the short or powdery1 in ,collodion which has been iodized with the alkaline ,iodides1 and kept until much ,iodine has been set free4 ,the latter is the most suitable condition for the dry process2 and the practical mode of distinguishing between them is by sensitizing a plate and passing the finger across it2 if it can be easily pushed away in a firm and connected skin1 it will be unfit for the purpose required4 ,in order still further to preserve the film in a condition permeable by the developer1 it is recommended to coat it whilst moist with a solution of ,gelatine4 #ec ,see these states of the film more fully described at page #hc4 ,the dry ,collodion process1 although less sensitive1 is more simple than that on ,collodio-,albumen1 and possesses many of its advantages2 but it is less universally applicable1 since it depends entirely for success upon the peculiar state of the ,collodion1 resembling in this respect the ,oxymel process already described4 ,mode of preparing the plates4--,the glasses are coated with the ,collodion in the usual way4 ,blistering during development being liable to happen in this process as in the last1 every care must be taken to make the films adhere with the greatest possible tenacity1 both by cleaning the glasses with extra care 7see p4 #bid71 and also by allowing the ,collodion to set firmly before dipping in the ,bath4 ,the plate may be held from twenty to thirty seconds previous to immersion1 or even longer1 provided the film1 when lifted out of the ,bath1 appears of uniform thickness throughout 7see page #bah74 ,the sensitizing having been completed1 wash the plates with plain water1 exactly in the same way as for ,oxymel 7p4 #bib74 ,if ,nitrate of ,silver be left1 clouding will take place in the process of development4 ,after washing1 drain for a few seconds1 and immerse in the solution of ,gelatine4 ,to prepare this ,bath1 take of ,nelson's patent ,gelatine #abh grains4 ,distilled water #ad ounces4 ,alcohol #b-a_/b ounces4 ,put the ,gelatine in the cold water1 and allow it a quarter of an hour to soften and swell2 it will then readily dissolve on applying a gentle heat4 ,this may be done in a glazed saucepan or a pipkin of earthenware1 taking care not to scorch the bottom part by too strong a heat4 ,next clarify the solution by adding to it1 whilst barely warm1 a tea-spoonful of white of egg 7previously beaten up with a silver fork71 and afterwards heating nearly to the boiling point4 ,the ,alcohol must now be added1 to facilitate the coagulation of the ,albumen4 ,when this takes place and the liquid becomes clear1 filter through a clean piece of cambric folded three or four times4 ,if a hot water filtering apparatus can be obtained1 the solution may be made to pass through paper2 but as it tends to gelatinize on cooling1 the ordinary mode of filtration commonly fails4 ,the quantity of ,alcohol in the above formula is greater than is usually recommended1 allowance having been made for a partial evaporation of the spirit4 ,the filtered liquid may be poured into a flat porcelain dish1 or a vertical trough1 but in either case it will be necessary to stand the vessel in warm water1 in order to prevent gelatinization4 ,the ,collodion plate1 thoroughly washed1 is to be immersed in this solution and moved up and down for two or three minutes4 ,it is then removed1 drained on blotting-paper1 and dried4 ,the use of artificial heat in drying will be found a great advantage2 it prevents the gelatine from settling unequally upon the plate4 ,those who possess an apparatus made purposely for drying plates by hot air1 will experience no difficulty1 but an ordinary deal trunk may be made to answer1 with a little management4 ,cover the bottom of the box with blotting-paper1 and having heated one or two flat irons1 place them in the centre3 then range the glasses side by side1 with the coated surface looking inwards2 in a quarter of an hour1 or from that to twenty minutes1 the desiccation will be complete4 ,if the ,collodion plates are prepared in a room containing a fire1 they may be reared up side by side at a distance of two or three feet1 and in that way may be safely dried without fear of injury1 provided white light be excluded4 ,when dry they can be stowed away in a box2 all the precautions given at page #bic being observed4 ,the sensibility remains good for many days1 possibly for weeks or months in cold weather4 ,exposure in the ,camera4--,allow from four to eight times the exposure of the most sensitive moist ,collodion4 ,on a clear summer's day1 a sun-lit view may require one minute or a minute and a half1 with a short focus ,stereoscopic lens1 having a diaphragm of a quarter of an inch diameter4 ,the average time however with the same lens would be about twice as much1 viz4 three minutes4 ,development of the ,image4--,make a saturated solution of ,gallic ,acid in water by the directions given at page #bfa4 ,then dissolve forty grains of pure ,nitrate of ,silver in one ounce of distilled water4 ,pour into a flat porcelain dish a sufficient quantity of the ,gallic ,acid solution to flood the plate readily4 ,then measure it1 and to each fluid ounce add ten minims of the solution of ,silver1 or five minims in hot weather4 ,it is important that no discoloration should occur on mixing these liquids together1 to obviate which1 observe the following precautions3--,clean the porcelain vessel very carefully with ,nitric ,acid or ,cyanide before use4 ,employ a pure solution of ,nitrate of ,silver2 and mix it with the ,gallic ,acid1 in preference to adding the ,gallic ,acid to the ,silver solution 7read the remarks at p4 #agi74 ,the picture may be expected to appear in five or ten minutes1 and in one hour1 or from that to four hours 7p4 #bih71 the development will be complete4 ,it will not be necessary to keep the plates in motion1 but simply to lay them side by side in the solution of the ,gallic ,acid4 ,if in spite of all precautions the developer begins to blacken before the intensity has reached the proper point1 it must be poured off and a fresh mixture prepared4 ,this however will not often happen4 ,lastly1 when a full amount of opacity has been obtained1 wash the plate with water1 and fix it in a solution of ,hyposulphite of ,soda1 or dilute solution of ,cyanide of ,potassium4 ,failures in the process4--,stains in the development may arise from using dirty dishes1 or glasses which have been left in ,gallo-,nitrate of ,silver and improperly cleaned4 ,it must be borne in mind that these impurities are not visible to the eye1 although they produce the effect of discolouring the developer4 ,a thorough cleansing with strong ,nitric ,acid or ,potash will prove a remedy4 ,blisters1 unless of large size1 may often be disregarded1 as they disappear on drying4 ,general cloudiness may depend upon the film having been imperfectly washed4 ,irregular reduction at certain parts may be due to the ,gelatine setting before the plate has become dry1 or to stains produced by the finger applied to the upper edge of the plate4#ed #ed ,since the above was written1 ,mr4 ,maxwell ,lyte has communicated to the ',photographic ,journal' 7vol4 iii47 a dry process in which a modified ,gelatine is used4 ,the change is produced by boiling a solution of gelatine with dilute ,sulphuric ,acid1 which is afterwards neutralized and removed by means of chalk4 ,the result is to destroy the gelatinizing property of the animal substance2 the solution retains its fluidity on cooling1 and the necessity of employing artificial heat in drying the plates is avoided4 ,,part ,,iii4 ,,outlines or ,,general ,,chemistry4 ,,outlines or ,,general ,,chemistry4 ,illustration ,,chapter ,i4 ,,the ,,chemical ,,elements ,,and ,,their ,,combinations4 ,the limits of the present ,work allow only of a simple sketch of the subjects which it is proposed to treat in this ,chapter4 ,our attention therefore must be confined to an explanation of certain points which are alluded to in the ,first ,part of the ,work1 and without a proper understanding of which it will be impossible for the reader to make progress4 ,the following division may be adopted3--,the more important ,elementary ,bodies1 with their symbols and atomic weights2 the ,compounds formed by their union2 the class of ,salts2 illustrations of the nature of ,chemical ,affinity2 ,chemical ,nomenclature2 ,symbolic ,notation2 the laws of ,combination2 the ,atomic ,theory2 the ,chemistry of ,organic ,bodies4 ,,the ,,chemical ,,elements1 ,,with ,,their ,,symbols ,,and ,,atomic ,,weights4 ,the class of elementary bodies embraces all those substances which cannot1 in the present state of our knowledge1 be resolved into simpler forms of matter4 ,the chemical elements are divided into metallic and non-metallic1 according to the possession of certain general characters4 ,the following are some of the principal non-metallic elements1 with the symbols employed to designate them1 and their atomic weights3#ee-- ,symbol4 ,atomic ,wt4 ,gases4 ,oxygen ,o #h ,hydrogen ,h #a ,nitrogen ,n #ad ,chlorine ,cl #cf ,solids4 ,iodine ,i #abf ,carbon ,c #f ,sulphur ,s #af ,phosphorus ,p #cb ,liquid4 ,bromine ,br #gh ,unknown4 ,fluorine ,f #ai ,the metallic elements are more numerous4 ,the following list includes only those which are commonly known3-- ,symbol4 ,atomic ,wt4 ,metals of the ,potassium ,k #dj ,alkalies4 ,sodium ,na #bd ,metals of the ,barium ,ba #fi ,alkaline ,calcium ,ca #bj ,earths ,magnesium ,mg #ab ,metals ,iron ,fe #bh ,proper4 ,zinc ,zn #cb ,cadmium ,cd #ef ,copper ,cu #cb ,lead ,pb #ajd ,tin ,sn #ei ,arsenic ,as #ge ,antimony ,sb #abi ,nobel ,mercury ,hg #bjb ,metals4 ,silver ,ag #ajh ,gold ,au #aig ,platinum ,pt #ii #ee ,the atomic weights1 with the exception of that of ,gold1 are taken from the last edition of ,brande's ',manual of ,chemistry4' ,,on ,,the ,,binary ,,compounds ,,of ,,the ,,elements4 ,many of the elementary bodies exhibit a strong tendency to combine with each other1 and to form compounds1 which differ in properties from either of their constituent elements4 ,this attraction1 which is termed ,chemical ,affinity1 is exerted principally between bodies which are opposed to each other in their general characters4 ,thus1 taking for example the elements ,chlorine and ,iodine--they are analogous in their reactions1 and therefore there is but little attraction between them1 whereas either of the two combines eagerly with ,silver1 which is an element of a different class4 ,so1 again4 ,sulphur unites with the metals1 but two metallic elements are comparatively indifferent to each other4 ,oxygen is by far the most important in the list of chemical elements4 ,it combines with all the others1 with the single exception1 perhaps1 of ,fluorine4 ,the attraction1 or chemical affinity1 however1 which is exerted1 varies much1 in different cases4 ,the metals1 as a class1 are easily oxidized2 whilst many of the non-metallic elements1 such as ,chlorine1 ,iodine1 ,bromine1 etc41 exhibit but little affinity for ,oxygen4 ,nitrogen is also a peculiarly negative element1 showing little or no tendency to unite with the others4 ,classification of binary compounds containing ,oxygen4--,when one simple element unites with another1 the product is termed a binary compound4 ,there are three distinct classes of binary compounds of ,oxygen3--,neutral ,oxides1 basic ,oxides1 and acid ,oxides4 ,neutral and basic ,oxides4--,take as examples--the ,oxide of ,hydrogen1 or ,water1 a neutral ,oxide2 the ,oxide of ,potassium1 or ,potash1 a basic ,oxide4 ,water is termed a neutral oxide1 because its affinities are low1 and it is comparatively indifferent to other bodies4 ,potash and ,oxide of ,silver are examples of basic oxides2 but there is a great difference between the two in chemical energy1 the former belonging to a superior class of bases1 viz4 the alkaline4 ,by studying the properties of an alkali 7such as ,potash or ,soda7 which are familiar to all1 we gain a correct notion of the whole class of basic oxides4 ,an alkali is a substance readily soluble in water1 and yielding a solution which has a slimy feel from its solvent action upon the skin4 ,it immediately restores the blue colour of reddened litmus1 and changes the blue infusion of cabbage to green4 ,lastly1 it is neutralized and loses all its characteristic properties upon the addition of an acid4 ,the weaker bases are1 as a rule1 sparingly or not at all soluble in water1 neither have they the same caustic and solvent action upon the skin2 but they restore the colour of reddened litmus1 and neutralize acids in the same manner as the more powerful bases1 or alkalies4 ,the ,acid ,oxides4--,this class1 taking the stronger acids as the type1 may be described as follows3--very soluble in water1 the solution possessing an intensely sour taste1 and a corroding rather than a solvent action upon the skin2 changes the blue colour of litmus and other vegetable substances to red1 and neutralizes the alkalies and basic oxides generally4 ,observe however that these properties are possessed in very various degrees by different acids4 ,prussic ,acid and ,carbonic ,acid1 for instance1 are not sour to the taste1 and being feeble in their reactions1 redden litmus scarcely or not at all4 ,all acids however1 without any exception1 tend to combine with bases and to neutralize themselves2 so that this may be said to be the most characteristic property of the class4 ,chemical composition of ,acid and ,basic ,oxides contrasted4--,it is a law commonly observed1 although with many exceptions1 that bases are formed by the union of ,oxygen with metals2 and acids1 by ,oxygen uniting with non-metallic elements4 ,thus1 ,sulphuric ,acid is a compound of ,sulphur and ,oxygen2 ,nitric ,acid1 of ,nitrogen and ,oxygen4 ,but the alkali1 ,potash1 is an oxide of the metal ,potassium2 and the oxides of ,iron1 ,silver1 ,zinc1 etc4 are bases1 and not acids4 ,again1 the composition of acids and bases is different in another respect2 the former invariably contain more ,oxygen in proportion to the other element than the latter4 ,taking the same examples as before1 the two classes may be represented thus3-- ,acids ,oil of ,vitriol1 ,sulphur #a atom1 ,oxygen #c atoms4 ,aqua-fortis1 ,nitrogen #a ,oxygen #e ,bases ,oxide of ,silver1 ,silver #a atom4 ,oxygen #a atom4 ,oxide of ,iron1 ,iron #a ,oxygen #a ,the class of ,hydrogen ,acids4--,oxygen is so essentially the element which forms the acidifying principle of acids1 that its very name is derived from that fact 71 acid1 and 1 to generate74 ,still there are exceptions to this rule1 and in some acids ,hydrogen appears to play the same part2 the ,hydracids1 as they are termed1 are formed principally by ,hydrogen uniting with elements like ,chlorine1 ,bromine1 ,iodine1 ,fluorine1 etc4 ,thus1 ,muriatic or ,hydrochloric ,acid contains ,chlorine and ,hydrogen2 ,hydriodic ,acid contains ,iodine and ,hydrogen4 ,observe1 however1 that the position held by the ,hydrogen in these compounds1 is different from that of the ,oxygen in the ,oxyacids1 as regards the number of atoms usually present2 thus-- ,aqua-fortis ,nitrogen #a atom1 ,oxygen #e atoms1 ,muriatic ,acid ,chlorine #a ,hydrogen #a atom2 so that the composition of the ,hydracids is analogous to the basic oxides1 in containing a single atom of each constituent4 ,,the ,,ternary ,,compounds ,,of ,,the ,,elements4 ,as the various elementary substances unite with each other to form ,binary ,compounds1 so these binary compounds again unite and form ,ternary ,compounds4 ,compound bodies however do not1 as a rule1 unite with simple elements4 ,in illustration1 take the action of ,nitric ,acid upon ,silver1 described at page #ab4 ,no effect is produced upon the metal until ,oxygen is imparted2 then the ,oxide of ,silver so formed dissolves in the ,nitric ,acid4 ,in other words1 it is necessary that a binary compound should be first formed1 before the solution can take place4 ,the mutual attraction or chemical affinity exhibited by compound bodies is1 as in the case of elements1 most strongly marked when the two substances are opposed to each other in their general properties4 ,thus1 acids do not unite with other acids1 but they combine instantly with alkalies2 the two mutually neutralizing each other and forming a salt4 ,salts therefore are ternary compounds produced by the union of acids and bases2 common ,salt1 formed by neutralizing ,muriatic ,acid with ,soda1 being taken as the type of the whole class4 ,general characters of the ,salts4--,an aqueous solution of ,chloride of ,sodium1 or common ,salt1 possesses those characters which are usually termed saline2 it is neither sour nor corrosive1 but1 on the other hand1 has a cooling agreeable taste4 ,it produces no effect upon litmus and other vegetable colours1 and is wanting in those energetic reactions which are characteristic of both acids and alkalies2 hence1 although formed by the union of two binary compounds1 it differs essentially in properties from both4 ,all salts however do not correspond to this description of the properties of ,chloride of ,sodium4 ,the ,carbonate of ,potash1 for instance1 is an acrid and alkaline salt1 and the ,nitrate of ,iron reddens litmus-paper4 ,a perfectly neutral salt is formed when a strong acid unites with an energetic base2 but if1 of the two constituents1 one is more powerful than the other1 the properties of that one are often seen in the resulting salt4 ,thus the ,carbonate of ,potash is alkaline to test-paper1 because the ,carbonic ,acid is feeble in its reactions2 but if ,nitric ,acid and ,potash are brought together1 then a ,nitrate of ,potash is produced1 which is neutral in every sense of the term4 ,the ,chloride of ,sodium and salts of a similar kind are freely soluble in water1 but all salts are not so4 ,some dissolve only sparingly1 and others not at all4 ,the ,chloride and ,iodide of ,silver are examples of the latter class2 they are not bitter and caustic like the ,nitrate of ,silver1 but are perfectly tasteless from being insoluble in the fluids of the mouth4 ,it is seen therefore from these examples1 and many others which might be adduced1 that the popular notion of a saline body is far from being correct1 and that1 in the language of strict definition1 any substance is a salt which is produced by the union of an acid with an alkali1 independent of the properties it may possess4 ,thus1 ,cyanide of ,potassium is a true salt1 although highly poisonous2 ,nitrate of ,silver is a salt2 the green ,sulphate of ,iron is a salt2 so also is ,chalk or ,carbonate of ,lime1 which has neither taste1 colour1 nor smell4 ,on the ,hydracid class of ,salts4--,the distinction between ,oxyacids and ,hydracids has already been pointed out 7p4 #cji71 the latter having been shown to consist of ,hydrogen united with elements analogous in their reactions to ,chlorine1 ,iodine1 ,bromine1 etc4 ,in a salt formed by an ,oxygen ,acid1 both the basic and acid elements appear4 ,thus the common ,nitre1 which is a ,nitrate of ,potash1 is found by analysis to contain ,oxide of ,potassium as a base1 in a state of combination with ,nitric ,acid4 ,but if a salt be formed by neutralizing an alkali with a ,hydrogen ,acid1 the product in that case does not contain all the elements4 ,this is seen from the following example3-- ,hydrochloric ,acid ,soda ,chloride of ,sodium ,water2 or1 stated more at length1-- 7,chlorine ,hydrogen7 7,oxygen ,sodium7 7,chlorine ,sodium7 7,oxygen ,hydrogen74 ,observe that the ,hydrogen and ,oxygen1 being present in the correct proportions1 unite to form ,water1 which is an ,oxide of ,hydrogen4 ,this water passes off when the solution is evaporated1 and leaves the dry crystals of salt4 ,on the other hand1 with the ,oxyacid ,salts1 the elementary ,hydrogen being absent1 no water is formed1 and the ,oxygen remains4 ,it must therefore be borne in mind that salts like the ,chlorides1 ,bromides1 ,iodides1 etc4 contain only two elements2 but that in the ,oxyacid ,salts1 such as ,sulphates1 ,nitrates1 ,acetates1 three are present4 ,thus1 ,nitrate of ,silver consists of ,nitrogen1 ,oxygen1 and ,silver1 but ,chloride of ,silver contains simply ,chlorine and metallic ,silver united1 without ,oxygen4 ,the ,hydracid salts however1 when decomposed1 yield products similar to the ,oxyacid salts4 ,for instance1 if ,iodide of ,potassium be dissolved in water1 and dilute ,sulphuric ,acid added1 this acid1 being powerful in its chemical affinities1 tends to appropriate to itself the alkali2 but it does not remove ,potassium and liberate ,iodine1 but takes the ,oxide of ,potassium and sets free ,hydriodic ,acid1 ,in other words1 as an atom of water is produced during the formation of a ,hydracid ,salt1 so is an atom destroyed and made to yield up its elements in the decomposition of a ,hydracid ,salt4 ,the reaction of dilute ,sulphuric ,acid upon ,iodide of ,potassium may be stated thus3-- ,sulphuric ,acid plus 7,iodine ,potassium7 plus 7,hydrogen ,oxygen7 equals 7,sulphuric ,acid1 ,oxygen ,potassium7 or ,sulphate of ,potash1 and 7,hydrogen ,iodine7 or ,hydriodic ,acid4 ,,the ,,nature ,,of ,,chemical ,,affinity ,,further ,,illustrated4 ,illustration from the ,non-metallic ,elements4--,if a stream of ,chlorine gas be passed into a solution containing the same salt as before mentioned1 viz4 the ,iodide of ,potassium1 the result is to liberate a certain portion of ,iodine1 which dissolves in the liquid1 and tinges it of a brown colour4 ,the element ,chlorine1 possessing a degree of chemical energy superior to that of ,iodine1 prevails over it1 and removes the ,potassium with which the ,iodine was previously combined4 ,chlorine ,iodide of ,potassium ,iodine ,chloride of ,potassium4 ,the same ,law illustrated by the ,metals4--,a strip of ,iron dipped in solution of ,nitrate of ,silver becomes immediately coated with metallic ,silver2 but a piece of ,silver-foil may be left for any length of time in ,sulphate of ,iron without undergoing change3 the difference depends upon the fact1 that metallic ,iron has a greater attraction for ,oxygen than ,silver1 and hence it displaces it from its solution4 ,iron ,nitrate of ,silver ,silver ,nitrate of ,iron4 ,illustrations amongst ,binary ,compounds4--,if a few drops of solution of ,potash be added to solution of ,nitrate of ,silver1 a brown deposit is formed1 which is the ,oxide of ,silver1 sparingly soluble in water4 ,that is to say1 as a stronger metal displaces metallic ,silver1 so does an oxide of the same metal displace ,oxide of ,silver4 ,therefore bases like the alkalies1 alkaline earths1 etc4 cannot exist in a free state in solutions of the salts of weaker bases1--a liquid containing ,nitrate of ,silver could not also contain free ,potash or ,ammonia4 ,in the list given at page #cjf1 the metallic elements are arranged principally in the order of their chemical affinities2 those of ,potassium1 ,sodium1 ,barium1 etc4 being the most marked4 ,as the alkalies displace the weaker bases from their combination with acids1 so the strong acids displace weak acids from their combination with bases4 ,thus1 as ,oxide of ,potassium ,acetate of ,silver ,oxide of ,silver ,acetate of ,potash2 ,so ,nitric ,acid ,acetate of ,silver ,acetic ,acid ,nitrate of ,silver4 ,in the list of acids4 ,sulphuric ,acid is usually placed first as being the strongest1 and ,carbonic ,acid1 which is a gaseous substance1 last4 ,the vegetable acids1 such as ,acetic1 ,tartaric1 etc41 are intermediate1 being weaker than the mineral acids1 but stronger than ,carbonic1 or ,hydrocyanic ,acid4 ,the order of decompositions affected by the insolubility or the volatility of the products which may be formed4--,it might be inferred from remarks already made1 that on mixing saline solutions1 a gradual interchange of elements would take place1 until the strongest acids were associated with the strongest bases1 and vice vers4 ,there are many causes however which interfere to prevent this2 one of which is volatility4--- ,the violent effervescence which takes place on treating a ,carbonate of any kind with an acid is due to the gaseous nature of ,carbonic ,acid and its escape in that form1 which greatly facilitates the decomposition4 ,insolubility is also a cause which exercises a great influence on the result which will follow in mixing solutions4 ,if the formation of an insoluble substance is possible by any interchange of elements1 it will take place4 ,a solution of ,chloride of ,sodium added to ,nitrate of ,silver invariably produces ,chloride of ,silver2 the insolubility of ,chloride of ,silver being the cause which determines its formation4 ,so again1 ,sulphate of ,lead and ,protonitrate of ,iron are produced by mixing ,nitrate of ,lead with ,sulphate of ,iron2 but if ,nitrate of ,potash be substituted for ,nitrate of ,lead1 the result is uncertain1 because there are no elements present which can1 by interchanging1 form an insoluble salt2 ,sulphate of ,potash1 although sparingly soluble in water1 not being insoluble1 like the ,sulphate of ,lead or the ,sulphate of ,baryta4 ,,on ,,chemical ,,nomenclature4 ,the nomenclature of the chemical elements is mostly independent of any rule2 but an attempt has been made to obviate this in the case of those of later discovery4 ,thus the names of the newly-found metals usually end in um1 as ,potassium1 ,sodium1 ,barium1 ,calcium1 etc42 and those elements which possess analogous characters have corresponding terminations assigned to them1 as ,chlorine1 ,bromine1 ,iodine1 ,fluorine1 etc4 ,nomenclature of ,binary ,compounds4--,these are often named by attaching the termination ide to the more important element of the two2 as1 the ,oxide of ,hydrogen1 or ,water2 the ,chloride of ,silver2 the ,sulphide of ,silver4 ,binary compounds of ,sulphur however are sometimes termed ,sulphurets1 as the ,sulphuret or the ,sulphide of ,silver indifferently4 ,when the same body combines with ,oxygen1 or the corresponding element1 in more than one proportion1 the prefix proto is applied to that containing the least ,oxygen2 sesqui to that with once and a half as much as the proto2 bi or bin to that with twice as much2 and per to the one containing the most ,oxygen of all4 ,as examples1 take the following3--,the ,protoxide of ,iron2 the ,sesquioxide of ,iron3 the ,protochloride of ,mercury2 the ,bichloride of ,mercury4 ,in these examples the ,sesquioxide of ,iron is also a ,peroxide1 because no higher simple oxide is known1 and the ,bichloride of ,mercury is a ,perchloride for a similar reason4 ,when an inferior compound is discovered1 it is often termed sub2 as the ,suboxide of ,silver1 the ,subchloride of ,silver4 ,these bodies contain the least known quantity of ,oxygen and ,chlorine respectively1 and are hence entitled to the prefix proto2 but being of minor importance1 they are excepted from the general rule4 ,the combinations of metallic elements with each other are termed alloys2 or if containing ,mercury1 amalgams4 ,nomenclature of binary ,compounds possessing acid properties4--,these are named on a different principle4 ,the termination ic is applied to one element4 ,thus1 taking as an illustration the liquid known as ,oil of ,vitriol1 it is truly an ,oxide of ,sulphur1 but as it possesses strong acid properties it is termed ,sulphuric ,acid4 ,so ,nitric ,acid is an ,oxide of ,nitrogen2 ,carbonic ,acid is an ,oxide of ,carbon1 etc4 ,when there are two oxides of the same element1 both possessing acid properties1 the most important has the termination ic1 and the other ous2 as ,sulphuric ,acid1 ,sulphurous ,acid2 ,nitric ,acid1 ,nitrous ,acid4 ,nomenclature of the ,hydracids4--,the ,hydrogen ,acids are distinguished from ,oxyacids by retaining the names of both constituents1 the termination ic being annexed as usual4 ,thus1 ,hydrochloric ,acid1 or the ,chloride of ,hydrogen2 ,hydriodic ,acid1 or the ,iodide of ,hydrogen4 ,further illustrations of the nomenclature of ,binary ,compounds4--,the ,oxides of ,nitrogen1 and also of ,sulphur1 afford an interesting illustration of the principles of nomenclature4 ,the former are as follows3-- ,nitrogen4 ,oxygen4 ,protoxide of ,nitrogen #a atom4 #a atom4 ,binoxide of ,nitrogen #a #b ,nitrous ,acid #a #c ,peroxide of ,nitrogen #a #d ,nitric ,acid #a #e ,observe1 that two only out of the five possess acid properties1 the others being simple oxides4 ,nitric ,acid is1 strictly speaking1 the ,peroxide1 but as it belongs to the class of acids1 that term naturally falls to the compound below4 ,the binary compounds of ,sulphur with ,oxygen all possess acid properties2 they may be represented 7in part7 as follows3-- ,sulphur4 ,oxygen4 ,hyposulphurous ,acid #b atoms4 #b atoms4 ,sulphurous ,acid #a #b ,hyposulphuric ,acid #b #e ,sulphuric ,acid #a #c ,in this case the ,sulphuric and ,sulphurous ,acids had become familiarly known before the others1 intermediate in composition1 were discovered4 ,hence1 to avoid the confusion which would result from changing the nomenclature1 the new bodies are termed ,hyposulphuric and ,hyposulphurous 7from 1 under74 ,nomenclature of ,salts4--,salts are named according to the acid they contain2 the termination ic being changed into ate1 and ous into ite4 ,thus1 ,sulphuric ,acid forms ,sulphates2 ,nitric ,acid1 ,nitrates2 but ,sulphurous ,acid forms ,sulphites1 and ,nitrous ,acid1 ,nitrites4 ,in naming a salt1 the base is always placed after the acid1 the term oxide being omitted2 thus4 ,nitrate of ,oxide of ,silver is more shortly known as ,nitrate of ,silver1 the presence of ,oxygen being understood4 ,when there are two oxides of the same base1 both of which are salifiable1--in naming the salts1 the term proto is prefixed to the acid of the salt formed by the lowest1 and per to that of the higher oxide2 as1 the ,protosulphate of ,iron1 or ,sulphate of the ,protoxide2 the ,persulphate of ,iron1 or ,sulphate of the ,peroxide4 ,many salts contain more than one atom of acid to each atom of base4 ,in that case1 the usual prefixes expressive of quantity are adopted3 thus1 the ,bisulphate of ,potash contains twice as much ,sulphuric ,acid as the neutral ,sulphate1 etc4 ,on the other hand1 there are salts in which the base is in excess with regard to the acid1 and which are usually known as basic salts2 thus1 the red powder which deposits from solution of ,sulphate of ,iron1 is a basic ,persulphate of ,iron1 or a ,sulphate of the ,peroxide of ,iron with more than the normal proportion of oxide4 ,nomenclature of the ,hydracid ,salts4--,the composition of these salts being different from those formed by ,oxygen ,acids1 the nomenclature varies also4 ,thus1 in neutralizing ,hydrochloric ,acid with ,soda1 the product formed is not known as ,hydrochlorate of ,soda1 but as ,chloride of ,sodium2 this salt1 and others of a similar constitution1 being binary1 and not ternary1 compounds4 ,the salt produced by ,hydrochloric ,acid and ,ammonia however is often called ,muriate or ,hydrochlorate of ,ammonia1 although more strictly it should be the ,chloride of ,ammonium4 ,,on ,,symbolic ,,notation4 ,the list of symbols employed to represent the various elementary bodies is given at page #cjf4--;,commonly the initial letter of the ,latin name is used1 a second or smaller letter being added when two elements correspond in their initials3 thus ,c stands for ,carbon1 ,cl for ,chlorine1 ,cd for ,cadmium1 and ,cu for ,copper4 ,the chemical symbol however does not simply represent a particular element2 it denotes also a definite weight1 or equivalent proportion1 of that element4 ,this will be explained more fully in the succeeding pages1 when speaking of the ,laws of ,combination4 ,formul of ,compounds4--,in the nomenclature of compounds it is usual to place the ,oxygen or analogous element first in the case of binary compounds1 and the acid before the base in the ternary compounds1 or salts2 but in representing them symbolically this order is reversed3 thus1 ,oxide of ,silver is written ,ag,o1 and never as ,o,ag2 ,nitrate of ,silver as ,ag,o ,,no#e1 not ,,no#e;,ag,o4 ,the juxtaposition of symbols expresses combination2 thus1 ,fe,o is a compound of one proportion of ,iron with one of ,oxygen1 or the ,protoxide of ,iron1 ,if more than one equivalent be present1 small figures are placed below the symbols3 thus1 ,fe#b,o#c represents two equivalents of ,iron united with three of ,oxygen1 or the ,peroxide of ,iron2 ,,so#c1 one equivalent of ,sulphur with three of ,oxygen1 or ,sulphuric ,acid4 ,larger figures placed before and in the same line with the symbols1 affect the whole compound which the symbols express3 thus1 #b ,,so#c means two equivalents of ,sulphuric ,acid2 #c ,,no#e1 three equivalents of ,nitric ,acid4 ,the interposition of a comma prevents the influence of the large figure from extending further4 ,thus1 the double ,hyposulphite of ,soda and ,silver is represented as follows3-- #b ,na,o ,s#b,o#b1 ,ag,o ,s#b,o#b1 or two equivalents of ,hyposulphite of ,soda with one of ,hyposulphite of ,silver2 the large figure referring only to the first half of the formula4 ,sometimes brackets1 etc4 are employed1 in order to render a complicated formula more plain4 ,for example1 the formula for the double ,hyposulphite of ,gold and ,soda1 or ,sel d'or1 may be written thus2-- #c 7,na,o ,s#b,o#b7 ,au,o ,s#b,o#b #d ,,ho4 ,in this formula1 the plus sign 77 denotes that the four atoms of water which follow1 are less intimately united with the framework of the salt than the other constituents4 ,the use of a plus sign is commonly adopted in representing salts which contain water of crystallization4 ,thus1 the formula for the crystallized ,protosulphate of ,iron is written as follows3-- ,fe,o ,,so#c #g ,,ho4 ,these atoms of water are driven off by the application of heat1 leaving a white substance1 which is the ,anhydrous salt1 and would be written simply as ,fe,o ,,so#c4 ,the plus sign however is often employed in token of simple addition1 no combination of any kind being intended4 ,thus the decomposition which follows on mixing ,chloride of ,sodium with ,nitrate of ,silver may be written as follows3-- ,na,cl ,ag,o ,,no#e ,ag,cl ,na,o ,,no#e2 that is1-- ,chloride of ,sodium added to ,nitrate of ,silver4 ,chloride of ,silver and ,nitrate of ,soda4 ,,on ,,equivalent ,,proportions4 ,when elementary or compound bodies enter into chemical union with each other1 they do not combine in indefinite proportions1 as in the case of a mixture of two liquids1 or the solution of a saline body in water4 ,on the other hand1 a certain definite weight of the one unites with an equally definite weight of the other2 and if an excess of either be present1 it remains free and uncombined4 ,thus1 if we take a single grain of the element ,hydrogen--to convert that grain into ,water there will be required exactly #h grains of ,oxygen2 and if a larger quantity than this were added1 as for instance ten grains1 then two grains would be over and above4 ,so1 to form ,hydrochloric ,acid1 #a grain of ,hydrogen takes #cf grains of ,chlorine3--for the ,hydriodic ,acid1 #a grain of ,hydrogen unites with #abf grains of ,iodine4 ,again1 if separate portions of metallic ,silver1 of #ajh grains each1 are weighed out1--in order to convert them into ,oxide1 ,chloride1 and ,iodide of ,silver respectively1 there would be required ,oxygen #h grains4 ,chlorine #cf ,iodine #abf ,therefore it appears that #h grains of ,oxygen are equivalent to #cf grains of ,chlorine and to #abf grains of ,iodine1 seeing that these quantities all play the same part in combining2 and so it is with regard to the other elements1--to every one of them a figure can be assigned which represents the number of parts by weight in which that element unites with others4 ,these figures are the equivalents or combining proportions1 and they are denoted by the symbol of the element4 ,a symbol does not stand as a simple representative of an element1 but as a representative of one equivalent of an element4 ,thus ,o indicates #h parts by weight of ,oxygen2 ,cl one equivalent1 or #cf parts by weight1 of ,chlorine2 and so with the rest4 ,observe however that these figures1 termed equivalents1 do not refer to the actual number of parts by weight1 but only to the ratio which exists between them3 if ,oxygen is #h1 then ,chlorine is #cf2 but if we term ,oxygen #ajj1 as some have proposed1 then ,chlorine would be #ddbfe4 ,in the scale of equivalents now usually adopted1 ,hydrogen1 as being the lowest of all1 is taken as unity1 and the others are related to it4 ,equivalents of ,compounds4--,the law of equivalent proportions applies to compounds as well as to simple bodies1 the combining proportion of a compound being always the sum of the equivalents of its constituents4 ,thus ,sulphur is #af1 and ,oxygen #h1 therefore ,sulphuric ,acid1 or ,,so#c1 equals #dj4 ,the equivalent of ,nitrogen is #ad1 that of ,nitric ,acid1 or ,,no#e1 is #ed4 ,the same rule applies with regard to salts4 ,take for instance the ,nitrate of ,silver3 it contains ,equivalent4 ,nitrogen #ad #f ,oxygen #dh ,silver #ajh --- ,total of equivalents1 or equivalent #agj of the ,nitrate of ,silver ,practical application of the ,laws of ,combination 4--,the utility of being acquainted with the law of combining proportions is obvious when their nature is understood4 ,as bodies both unite with and replace each other in equivalents1 a simple calculation shows at once how much of each element or compound will be required in a given reaction4 ,thus1 supposing it be desired to convert #ajj grains of ,nitrate of ,silver into ,chloride of ,silver1 the weight of ,chloride of ,sodium which will be necessary is deduced thus3--one equivalent1 or #agj parts1 of ,nitrate of ,silver1 is decomposed by an equivalent1 or #fj parts1 of ,chloride of ,sodium4 ,therefore as #agj 3 #fj 33 #ajj 3 #ceb2 that is1 #ceb grains of ,salt will precipitate1 in the state of ,chloride1 the whole of the ,silver contained in #ajj grains of ,nitrate4 ,so again1 in order to form the ,iodide of ,silver1 the proportions in which the two salts should be mixed is thus shown4 ,the equivalent of ,iodide of ,potassium is #aff1 and that of ,nitrate of ,silver is #agj4 ,these numbers so nearly correspond1 that it is common to direct that equal weights of the two salts should be taken4 ,one more illustration will suffice4 ,supposing it be required to form #bj grains of ,iodide of ,silver--how much ,iodide of ,potassium and ,nitrate of ,silver must be used8 ,one equivalent1 or #aff parts1 of ,iodide of ,potassium1 will yield an equivalent1 or #bcd parts1 of ,iodide of ,silver2 therefore as #bcd 3 #aff 33 #bj 3 #adb4 ,hence1 if #adb grains of the ,iodide of ,potassium be dissolved in water1 and an equivalent quantity1 viz4 #ade grains1 of the ,nitrate of ,silver added1 the yellow precipitate1 when washed and dried1 will weigh precisely #bj grains4 ,,on ,,the ,,atomic ,,theory4 ,the atomic theory1 originally proposed by ,dalton1 so much facilitates the comprehension of chemical reactions generally1 that it may be useful to give a short sketch of it4 ,it is supposed that all matter is made up of an infinite number of minute atoms1 which are elementary1 and do not admit of further division4 ,each of these atoms possesses an actual weight1 although inappreciable by our present methods of investigation4 ,simple atoms1 by uniting with each other1 form compound atoms2 and when these compounds are broken up1 the elementary constituent atoms are not destroyed1 but separate from each other1 in possession of all their original properties4 ,in representing the simple atomic structure of bodies1 circles may be used1 as in the following diagram4 ,illustration3 ,fig4 #a4 ,fig4 #b4 ,fig4 #c4 ,fig4 #a is a compound atom of ,sulphuric ,acid1 consisting of an atom of ,sulphur united intimately with three of ,oxygen2 fig4 #b is an atom of ,peroxide of ,nitrogen1 ,,no#d2 and fig4 #c1 an atom of ,nitric ,acid1 composed of ,nitrogen #a atom4 ,oxygen #e atoms1 or in symbols ,,no#e4 ,the term atomic weight substituted for equivalent proportion4--,if we suppose that the simple atoms of different kinds of matter differ in weight1 and that this difference is expressed by their equivalent numbers1 the whole laws of combination follow by the simplest reasoning4 ,it is easy to understand that an atom of one element1 or compound1 would displace1 or be substituted for1 a single atom of another2 therefore1 taking as the illustration the decomposition of ,iodide of ,potassium by ,chlorine1--the weight of the latter element required to liberate #abf grains of ,iodine is #cf grains1 because the weights of the atoms of those two elementary bodies are as #cf to #abf4 ,so again1 in the reaction between ,chloride of ,sodium and ,nitrate of ,silver1 a compound atom of the former1 represented by the weight #fj1 reacts upon a compound atom of the latter1 which equals #agj4 ,therefore in place of the term equivalent or combining proportion1 it is more usual to employ that of atomic weight4 ,thus the atomic weight of ,oxygen is #h1 represented by the symbol ,o2 that of ,sulphur is #af2 hence the atomic weight of the compound atom of ,sulphuric ,acid1 or ,,so#c1 is necessarily equal to the combined weights of the four simple atoms2 id est1 #af #bd #dj4 ,,on ,,the ,,chemistry ,,of ,,organic ,,substances4 ,by organic substances are meant those which have possessed life1 with definite organs and tissues1 in contra-distinction to the various forms of dead inorganic matter1 in which no structural organization of that kind is found4 ,the term organic however is also applied to substances which are obtained by chemical processes from the vegetable and animal kingdoms1 although they cannot themselves be said to be living bodies2 thus ,acetic ,acid1 procured by the distillation of woody fibre1 and ,alcohol1 by fermentation from sugar1 are strictly organic substances4 ,the class of organic bodies embraces a great variety of products2 which1 like inorganic ,oxides1 may be divided into neutral1 acid1 and basic4 ,the organic acids are numerous1 including ,acetic ,acid1 ,tartaric1 ,citric1 and a variety of others4 ,the neutral substances cannot easily be assimilated to any class of inorganic compounds2 as examples1 take ,starch1 ,sugar1 ,lignine1 etc4 ,the bases are also a large class4 ,they are mostly rare substances1 not familiarly known3 ,morphia1 obtained from ,opium2 ,quinia1 from ,quinine2 ,nicotine1 from ,tobacco1 are illustrations4 ,composition of organic and inorganic bodies contrasted4--,there are more than fifty elementary substances found in the inorganic kingdom1 but only four1 commonly speaking1 in the organic3 these four are ,carbon1 ,hydrogen1 ,nitrogen1 and ,oxygen4 ,some organic bodies1--oil of turpentine1 naphtha1 etc41 contain only ,carbon and ,hydrogen2 many others1 such as sugar1 gum1 alcohol1 fats1 vegetable acids--,carbon1 ,hydrogen1 and ,oxygen4 ,the ,nitrogenous bodies1 so called1 containing ,nitrogen in addition to the other elements1 are principally substances derived from animal and vegetable tissues1 such as ,albumen1 ,caseine1 ,gelatine1 etc42 ,sulphur and ,phosphorus are also present in many of the ,nitrogenous bodies1 but only to a small extent4 ,organic substances1 although simple as regards the number of elements involved in their formation1 are often highly complex in the arrangement of the atoms2 this may be illustrated by the following formul3-- ,starch ,c#bd;,h#bj,o#bj ,lignine ,c#bd;,h#bj,o#bj ,cane ,sugar ,c#bd;,h#bb,o#bb ,grape ,sugar ,c#bd;,h#bh,o#bh ,inorganic bodies1 as already shown1 unite in pairs1--two elements join to form a binary compound2 two binary compounds produce a salt2 two salts associated together form a double salt4 ,with organic bodies however the arrangement is different1--the elementary atoms are all grouped equally in one compound atom1 which is highly complex in structure1 and cannot be split up into binary products4 ,observe also1 as characteristic of ,organic ,chemistry1 the apparent similarity in composition between bodies which differ widely in properties4 ,as examples take ,lignine1 or cotton fibre1 and ,starch1--each of which contains the three elements united as ,c#bd;,h#bj,o#bj4 ,mode of distinguishing between ,organic and ,inorganic matter4--,a simple means of doing this is as follows3-- place the suspected substance upon a piece of ,platinum-foil1 and heat it to redness with a spirit-lamp3 if it first blackens1 and then burns completely away1 it is probably of organic origin4 ,this test depends upon the fact1 that the constituent elements of organic bodies are all either themselves volatile1 or capable of forming volatile combinations with ,oxygen4 ,inorganic substances1 on the other hand1 are often unaffected by heat1 or1 if volatile1 are dissipated without previous charring4 ,the action of heat upon organic matter may further be illustrated by the combustion of coal or wood in an ordinary furnace2--first1 an escape of ,carbon and ,hydrogen1 united in the form of volatile gaseous matter1 takes place1 leaving behind a black cinder1 which consists of ,carbon and inorganic matter combined2 afterwards this ,carbon burns away into ,carbonic ,acid1 and a grey ash is left which is composed of inorganic salts1 and is indestructible by heat4 ,,chapter ,,ii4 ,,vocabulary ,,of ,,photographic ,,chemicals4 ,,acetic ,,acid4 ,symbol1 ,c#d;,h#c,o#c ,,ho4 ,atomic weight1 #fj4 ,acetic ,acid is a product of the oxidation of ,alcohol4 ,spirituous liquids1 when perfectly pure1 are not affected by exposure to air2 but if a portion of yeast1 or ,nitrogenous organic matter of any kind1 be added1 it soon acts as a ferment1 and causes the spirit to unite with oxygen derived from the atmosphere1 and so to become sour from formation of ,acetic ,acid1 or vinegar4 ,acetic ,acid is also produced on a large scale by heating wood in close vessels3 a substance distils over which is ,acetic ,acid contaminated with empyreumatic and tarry matter2 it is termed ,pyroligneous ,acid1 and is much used in commerce4 ,the most concentrated ,acetic ,acid may be obtained by neutralizing common vinegar with ,carbonate of ,soda1 and crystallizing out the ,acetate of ,soda so formed2 this ,acetate of ,soda is then distilled with ,sulphuric ,acid1 which removes the ,soda and liberates ,acetic ,acid3 the ,acetic ,acid being volatile1 distils over1 and may be condensed4 ,properties of ,acetic ,acid4--,the strongest acid contains only a single atom of water2 it is sold under the name of ,glacial ,acetic ,acid1 so called from its property of solidifying at a moderately low temperature4 ,at about #ej the crystals melt1 and form a limpid liquid of pungent odour and a density nearly corresponding to that of water2 the specific gravity of ,acetic ,acid however is no test of its real strength1 which can only be estimated by analysis4 ,the commercial ,glacial ,acetic ,acid is often diluted with water1 which may be suspected if it does not solidify during the cold winter months4 ,sulphurous and ,hydrochloric ,acids are also common impurities4 ,they are injurious in ,photographic ,processes1 from their property of precipitating ,nitrate of ,silver4 ,to detect them proceed as follows3--dissolve a small crystal of ,nitrate of ,silver in a few drops of water1 and add to it about half a drachm of the ,glacial ,acid2 the mixture should remain quite clear even when exposed to the light4 ,hydrochloric and ,sulphurous ,acid produce a white deposit of ,chloride or ,sulphite of ,silver2 and if ,aldehyde or volatile tarry matter be present in the ,acetic ,acid1 the mixture with ,nitrate of ,silver1 although clear at first1 becomes discoloured by the action of light4 ,glacial ,acetic ,acid sometimes has a smell of garlic4 ,in this state it probably contains an organic ,sulphur ,acid1 and is unfit for use4 ,many employ a cheaper form of ,acetic ,acid1 sold by druggists as ,beaufoy's acid2 it should be of the strength of the ,acetic ,acid fortiss4 of the ,london ,pharmacopia1 containing #cj per cent1 real acid4 ,it will be advisable to test it for ,sulphuric ,acid 7see ,sulphuric ,acid71 and other impurities1 before use4 ,,acetate ,,of ,,silver4 ,see ,silver1 ,acetate of4 ,,albumen4 ,albumen is an organic principle found both in the animal and vegetable kingdom4 ,its properties are best studied in the white of egg1 which is a very pure form of ,albumen4 ,albumen is capable of existing in two states2 in one of which it is soluble1 in the other insoluble1 in water4 ,the aqueous solution of the soluble variety gives a slightly alkaline reaction to test-paper2 it is somewhat thick and glutinous1 but becomes more fluid on the addition of a small quantity of an alkali1 such as ,potash or ,ammonia4 ,soluble ,albumen may be converted into the insoluble form in the following ways3-- #a4 ,by the application of heat4--,a moderately strong solution of ,albumen becomes opalescent and coagulates on being heated to about #aej ,fahrenheit1 but a temperature of #bab is required if the liquid is very dilute4 ,a layer of dried ,albumen cannot easily be coagulated by the mere application of heat4 #b4 ,by addition of strong acids4--,nitric ,acid coagulates ,albumen perfectly without the aid of heat4 ,acetic ,acid however acts differently1 appearing to enter into combination with the ,albumen1 and forming a compound soluble in warm water acidified by ,acetic ,acid4 #c4 ,by the action of metallic salts4--,many of the salts of the metals coagulate ,albumen completely4 ,nitrate of ,silver does so2 also the ,bichloride of ,mercury4 ,ammoniacal ,oxide of ,silver however does not coagulate ,albumen4 ,the white precipitate formed on mixing ,albumen with ,nitrate of ,silver is a chemical compound of the animal matter with ,protoxide of ,silver4 ,this substance1 which has been termed ,albuminate of ,silver1 is soluble in ,ammonia and ,hyposulphite of ,soda2 but after exposure to light1 or heating in a current of ,hydrogen gas1 it assumes a brick-red colour1 being probably reduced to the condition of an organic compound of a ,suboxide of ,silver4 ,it is then almost insoluble in ,ammonia1 but enough dissolves to tinge the liquid wine-red4 ,the red coloration of solution of ,nitrate of ,silver employed in sensitizing the ,albuminized photographic paper is probably produced by the same compound1 although1 often referred to the presence of ,sulphuret of ,silver4 ,albumen also combines with ,lime and ,baryta4 ,when ,chloride of ,barium is used with ,albumen1 a white precipitate of this kind usually forms4 ,chemical composition of ,albumen4--,albumen belongs to the ,nitrogenous class of organic substances 7see page #cbe74 ,it also contains small quantities of ,sulphur and ,phosphorus4 ,,alcohol4 ,symbol1 ,c#d;,h#f,o#b4 ,atomic weight1 #df4 ,alcohol is obtained by the careful distillation of any spirituous or fermented liquor4 ,if wine or beer be placed in a retort1 and heat applied1 the ,alcohol1 being more volatile than water1 rises first1 and is condensed in an appropriate receiver2 a portion of the vapour of water however passes over with the ,alcohol1 and dilutes it to a certain extent1 forming what is termed ,spirits of ,wine4 ,much of this water may be removed by redistillation from ,carbonate of ,potash1 in the manner described at page #aif of this work2 but in order to render the ,alcohol thoroughly anhydrous1 it is necessary to employ ,quicklime1 which possesses a still greater attraction for water4 ,an equal weight of this powdered lime is mixed with strong ,alcohol of #hbc1 and the two are distilled together4 ,properties of ,alcohol4--,pure anhydrous ,alcohol is a limpid liquid1 of an agreeable odour and pungent taste2 sp4 gr4 at #fj1 #gid4 ,it absorbs vapour of water1 and becomes diluted by exposure to damp air2 boils at #agc ,fahr4 ,it has never been frozen4 ,alcohol distilled from ,carbonate of ,potash has a specific gravity of #hae to #hbc1 and contains #ij to #ic per cent1 of real spirit4 ,the specific gravity of ordinary rectified ,spirits of ,wine is usually about #hdj1 and it contains #hj to #hc per cent1 of absolute ,alcohol4 ,,ammonia4 ,symbol1 ,,nh#c or ,,nh#d,o4 ,atomic weight1 #ag4 ,the liquid known by this name is an aqueous solution of the volatile gas ,ammonia4 ,ammoniacal gas contains one atom of ,nitrogen combined with three of ,hydrogen3 these two elementary bodies exhibit no affinity for each other1 but they can be made to unite under certain circumstances1 and the result is ,ammonia4 ,properties of ,ammonia4--,ammoniacal gas is soluble in water to a large extent2 the solution possessing those properties which are termed alkaline 7see page #cjh74 ,ammonia however differs from the other alkalies in one important particular--it is volatile3 hence the original colour of turmeric-paper affected by ,ammonia is restored on the application of heat4 ,solution of ,ammonia absorbs ,carbonic ,acid rapidly from the air1 and is converted into ,carbonate of ,ammonia2 it should therefore be preserved in stoppered bottles4 ,besides ,carbonate1 commercial ,ammonia often contains ,chloride of ,ammonium1 recognized by the white precipitate given by ,nitrate of ,silver after acidifying with pure ,nitric ,acid4 ,the strength of commercial ,ammonia varies greatly2 that sold for pharmaceutical purposes under the name of ,liquor ,ammoni1 contains about #aj per cent1 of real ,ammonia4 ,the sp4 gr4 of aqueous ,ammonia diminishes with the proportion of ,ammonia present1 the ,liquor ,ammoni being usually about #icf4 ,ammonia1 although forming a large class of salts1 appears at first sight to contrast strongly in composition with the alkalies proper1 such as ,potash and ,soda4 ,mineral bases generally are protoxides of metals1 as already shown at page #cjh1 but ,ammonia consists simply of ,nitrogen and ,hydrogen united without ,oxygen4 ,the following remarks may perhaps tend somewhat to elucidate the difficulty3-- ,theory of ,ammonium4--,this theory supposes the existence of a substance possessing the properties of a metal1 but differing from metallic bodies generally in being compound in structure3 the formula assigned to it is ,,nh#d1 one atom of ,nitrogen united with four of ,hydrogen4 ,this hypothetical metal is termed ,ammonium2 and ,ammonia1 associated with an atom of water1 may be viewed as its ,oxide1 for ,,nh#c ,,ho plainly equals ,,nh#d,o4 ,thus1 as ,potash is the ,oxide of ,potassium1 so ,ammonia is the ,oxide of ,ammonium4 ,the composition of the salts of ,ammonia is on this view assimilated to those of the alkalies proper4 ,thus1 ,sulphate of ,ammonia is a ,sulphate of the ,oxide of ,ammonium2 ,muriate or ,hydrochlorate of ,ammonia is a ,chloride of ,ammonium1 etc4 ,,ammonio-,,nitrate ,,of ,,silver4 ,see ,silver1 ,ammonio-,nitrate of4 ,,aqua-,,regia4 ,see ,nitro-,hydrochloric ,acid4 ,,baryta1 ,,nitrate ,,of4 ,see ,nitrate of ,baryta4 ,,bichloride ,,of ,,mercury4 ,see ,mercury1 ,bichloride of4 ,,bromine4 ,symbol1 ,br4 ,atomic weight1 #gh4 ,this elementary substance is obtained from the uncrystallizable residuum of sea-water1 termed bittern4 ,it exists in the water in very minute proportion1 combined with ,magnesium in the form of a soluble ,bromide of ,magnesium4 ,properties4--,bromine is a deep reddish-brown liquid of a disagreeable odour1 and fuming strongly at common temperatures2 sparingly soluble in water 7#a part in #bc1 ,lwig71 but more abundantly so in ,alcohol1 and especially in ,ether4 ,it is very heavy1 having a specific gravity of #cj4 ,bromine is closely analogous to ,chlorine and ,iodine in its chemical properties4 ,it stands on the list intermediately between the two2 its affinities being stronger than those of ,iodine1 but weaker than ,chlorine 7see ,chlorine74 ,it forms a large class of salts1 of which the ,bromides of ,potassium1 ,cadmium1 and ,silver are the most familiar to ,photographers4 ,,bromide ,,of ,,potassium4 ,symbol1 ,k,br4 ,atomic weight1 #aah4 ,bromide of ,potassium is prepared by adding ,bromine to ,caustic ,potash1 and heating the product1 which is a mixture of ,bromide of ,potassium and ,bromate of ,potash1 to redness1 in order to drive off the ,oxygen from the latter salt4 ,it crystallizes in anhydrous cubes1 like the ,chloride and ,iodide of ,potassium2 it is easily soluble in water1 but more sparingly so in ,alcohol2 it yields red fumes of ,bromine when acted upon by ,sulphuric ,acid4 ,,bromide ,,of ,,silver4 ,see ,silver1 ,bromide of4 ,,carbonate ,,of ,,soda4 ,symbol1 ,na,o ,,co#b #aj ,aq4 ,this salt was formerly obtained from the ashes of seaweeds1 but is now more economically manufactured on a large scale from common salt4 ,the ,chloride of ,sodium is first converted into ,sulphate of ,soda1 and afterwards the ,sulphate into ,carbonate of ,soda4 ,properties4--,the perfect crystals contain ten atoms of water1 which are driven off by the application of heat1 leaving a white powder--the anhydrous ,carbonate4 ,common ,washing ,soda is a neutral ,carbonate1 contaminated to a certain extent with ,chloride of ,sodium and ,sulphate of ,soda4 ,the ,carbonate used for effervescing draughts is either a ,bicarbonate with #a atom of water1 or a ,sesquicarbonate1 containing about #dj per cent1 of real alkali2 it is therefore nearly double as strong as the washing ,carbonate1 which contains about #bb per cent1 of ,soda4 ,carbonate of ,soda is soluble in twice its weight of water at #fj1 the solution being strongly alkaline4 ,,carbonate ,,of ,,potash4 ,see ,potash1 ,carbonate of4 ,,caseine4 ,see ,milk4 ,,charcoal1 ,,animal4 ,animal ,charcoal is obtained by heating animal substances1 such as bones1 dried blood1 horns1 etc41 to redness1 in close vessels1 until all volatile empyreumatic matters have been driven off1 and a residue of ,carbon remains4 ,when prepared from bones it contains a large quantity of inorganic matter in the shape of ,carbonate and ,phosphate of ,lime1 the former of which produces alkalinity in reacting upon ,nitrate of ,silver 7see p4 #hi74 ,animal ,charcoal is freed from these earthy salts by repeated digestion in ,hydrochloric ,acid2 but unless very carefully washed it is apt to retain an acid reaction1 and so to liberate free ,nitric ,acid when added to solution of ,nitrate of ,silver4 ,properties4--,animal ,charcoal1 when pure1 consists1 solely of ,carbon1 and burns away in the air without leaving any residue3 it is remarkable for its property of decolorizing solutions2 the organic colouring substance being separated1 but not actually destroyed1 as it is by ,chlorine employed as a bleaching agent4 ,this power of absorbing colouring matter is not possessed in an equal degree by all varieties of ,charcoal1 but is in great measure peculiar to those derived from the animal kingdom4 ,,china ,,clay1 ,,or ,,kaolin4 ,this is prepared1 by careful levigation1 from mouldering granite and other disintegrated felspathic rocks4 ,it consists of the ,silicate of ,alumina1--that is1 of ,silicic ,acid or ,flint1 which is an ,oxide of ,silicon1 united with the base ,alumina 7,oxide of ,aluminum74 ,kaolin is perfectly insoluble in water and acids1 and produces no decomposition in solution of ,nitrate of ,silver4 ,it is employed by ,photographers to decolorize solutions of ,nitrate of ,silver which have become brown from the action of ,albumen or other organic matters4 ,commercial ,kaolin may contain chalk1 in which state it produces alkalinity in solution of ,nitrate of ,silver4 ,the impurity1 detected by its effervescence with acids1 is removed by washing the ,kaolin in diluted vinegar and subsequently in water4 ,,chlorine4 ,symbol1 ,cl4 ,atomic weight1 #cf4 ,chlorine is a chemical element found abundantly in nature1 combined with metallic ,sodium in the form of ,chloride of ,sodium1 or ,sea-salt4 ,preparation4--,by distilling common ,salt with ,sulphuric ,acid1 ,sulphate of ,soda and ,hydrochloric ,acid are formed4 ,hydrochloric ,acid contains ,chlorine combined with ,hydrogen2 by the action of nascent ,oxygen 7see ,oxygen71 the ,hydrogen may be removed in the form of water1 and the ,chlorine left alone4 ,properties4--,chlorine is a greenish-yellow gas1 of a pungent and suffocating odour2 soluble to a considerable extent in water1 the solution possessing the odour and colour of the gas4 ,it is nearly #b-a_/b times as heavy as a corresponding bulk of atmospheric air4 ,chemical properties4--,chlorine belongs to a small natural group of elements which contains also ,bromine1 ,iodine1 and ,fluorine4 ,they are characterized by having a strong affinity for ,hydrogen1 and also for the metals2 but are comparatively indifferent to ,oxygen4 ,many metallic substances actually undergo combustion when projected into an atmosphere of ,chlorine1 the union between the two taking place with extreme violence4 ,the characteristic bleaching properties of ,chlorine gas are explained in the same manner3--,hydrogen is removed from the organic substance1 and in that way the structure is broken up and the colour destroyed4 ,chlorine is more powerful in its affinities than either ,bromine or ,iodine4 ,the salts formed by these three elements are closely analogous in composition and often in properties4 ,those of the ,alkalies1 ,alkaline ,earths1 and many of the ,metals1 are soluble in water2 but the ,silver salts are insoluble2 the ,lead salts sparingly so4 ,the combinations of ,chlorine1 ,bromine1 ,iodine1 and ,fluorine1 with ,hydrogen1 are acids1 and neutralize ,alkalies in the usual1 manner1 with formation of ,alkaline ,chloride and water 7see page #caa74 ,the test by which the presence of ,chlorine is detected1 either free or in combination with bases1 is ,nitrate of ,silver2 it gives a white curdy precipitate of ,chloride of ,silver1 insoluble in ,nitric ,acid1 but soluble in ,ammonia4 ,the solution of ,nitrate of ,silver employed as the test must not contain ,iodide of ,silver1 as this compound is precipitated by dilution4 ,,chloride ,,of ,,ammonium4 ,symbol1 ,,nh#d;,cl4 ,atomic weight1 #ed4 ,this salt1 also known as ,muriate or ,hydrochlorate of ,ammonia1 occurs in commerce in the form of colourless and translucent masses1 which are procured by sublimation1 the dry salt being volatile when strongly heated4 ,it dissolves in an equal weight of boiling1 or in three parts of cold water4 ,it contains more ,chlorine in proportion to the weight used than ,chloride of ,sodium1 the atomic weights of the two being as #ed to #fj4 ,,chloride ,,of ,,barium4 ,symbol1 ,ba,cl #b ,,ho4 ,atomic weight1 #abc4 ,barium is a metallic element very closely allied to ,calcium1 the elementary basis of ,lime4 ,the ,chloride of ,barium is commonly employed as a test for ,sulphuric ,acid1 with which it forms an insoluble precipitate of ,sulphate of ,baryta4 ,it is also said to affect the colour of the ,photographic image when used in preparing ,positive paper1 which may possibly be due to a chemical combination of ,baryta with ,albumen2 but it must be remembered that this ,chloride1 from its high atomic weight1 contains less ,chlorine than the alkaline ,chlorides 7see page #abd74 ,properties of ,chloride of ,barium4--,chloride of ,barium occurs in the form of white crystals1 soluble in about two parts of water1 at common temperature4 ,these crystals contain two atoms of water of crystallization1 which are expelled at #bab1 leaving the anhydrous ,chloride4 ,,chloride ,,of ,,gold4 ,see ,gold1 ,chloride of4 ,,chloride ,,of ,,sodium4 ,symbol1 ,na,cl4 ,atomic weight1 #fj4 ,common ,salt exists abundantly in nature1 both in the form of solid rock-salt and dissolved in the waters of the ocean4 ,properties of the pure ,salt4--,fusible without decomposition at low redness1 but sublimes at higher temperatures2 the melted salt concretes into a hard white mass on cooling4 ,nearly insoluble in absolute alcohol1 but dissolves in minute quantity in rectified spirit4 ,soluble in three parts of water1 both hot and cold4 ,crystallizes in cubes1 which are anhydrous4 ,impurities of ,common ,salt4--,table ,salt often contains large quantities of the ,chlorides of ,magnesium and ,calcium1 which1 being deliquescent1 produce a dampness by absorption of atmospheric moisture3 ,sulphate of ,soda is also commonly present4 ,the salt may be purified by repeated re-crystallization1 but it is more simple to prepare the pure compound directly1 by neutralizing ,hydrochloric ,acid with ,carbonate of ,soda4 ,,chloride ,,of ,,silver4 ,see ,silver1 ,chloride of4 ,,citric ,,acid4 ,this acid is found abundantly in lemon-juice and in lime-juice4 ,it occurs in commerce in the form of large crystals1 which are soluble in less than their own weight of water at #fj4 ,commercial ,citric ,acid is sometimes mixed with ,tartaric ,acid4 ,the adulteration may be discovered by making a concentrated solution of the acid and adding ,acetate of ,potash2 crystals of ,bitartrate of ,potash will separate if ,tartaric ,acid be present4 ,citric ,acid is tribasic4 ,it forms with ,silver a white insoluble salt1 containing #c atoms of ,oxide of ,silver to #a atom of ,citric ,acid4 ,when the ,citrate of ,silver is heated in a current of ,hydrogen gas1 a part of the acid is liberated and the salt is reduced to a ,citrate of ,suboxide of ,silver2 which is of a red colour4 ,the action of white light in reddening ,citrate of ,silver is shown by the ,author to be of a similar nature4 ,,cyanide ,,of ,,potassium4 ,symbol1 ,,kc#b,n1 or ,k,cy4 ,atomic weight1 #ff4 ,this salt is a compound of ,cyanogen gas with the metal ,potassium4 ,cyanogen is not an elementary body1 like ,chlorine or ,iodine1 but consists of ,carbon and ,nitrogen united in a peculiar manner4 ,although a compound substance1 it reacts in the manner of an element1 and is therefore 7like ,ammonium1 previously described7 an exception to the usual laws of chemistry4 ,many other bodies of a similar character are known4 ,properties of ,cyanide of ,potassium4--,these have been sufficiently described at page #dd1 to which the reader is referred4 ,,ether4 ,symbol1 ,c#d;,h#e,o4 ,atomic weight1 #cg4 ,ether is obtained by distilling a mixture of ,sulphuric ,acid and ,alcohol4 ,if the formula of ,alcohol 7,c#d;,h#f,o#b7 be compared with that of ,ether1 it will be seen to differ from it in the possession of an additional atom of ,hydrogen and of ,oxygen3 in the reaction the ,sulphuric ,acid removes these elements in the form of water1 and by so doing converts one atom of ,alcohol into an atom of ,ether4 ,the term ,sulphuric applied to the commercial ,ether has reference only to the manner of its formation4 ,properties of ,ether4--,the properties of ,ether have been described to some extent at pages #he and #aie4 ,the following particulars however may be added4 ,it is neither acid nor alkaline to test-paper4 ,specific gravity1 at #fj1 about #gbj4 ,boils at #ih ,fahrenheit4 ,the vapour is exceedingly dense1 and may be seen passing off from the liquid and falling to the ground3 hence the danger of pouring ,ether from one bottle to another if a flame be near at hand4 ,ether does not mix with water in all proportions2 if the two are shaken together1 after a short time the former rises and floats upon the surface4 ,in this way a mixture of ,ether and ,alcohol may be purified to some extent1 as in the common process of washing ,ether4 ,the water employed however always retains a certain portion of ,ether 7about a tenth part of its bulk71 and acquires a strong ethereal odour2 washed ,ether also contains water in small quantity4 ,bromine and ,iodine are both soluble in ,ether1 and gradually react upon and decompose it4 ,the strong alkalies1 such as ,potash and ,soda1 also decompose ,ether slightly after a time1 but not immediately4 ,exposed to air and light4 ,ether is oxidized and acquires a peculiar odour 7page #he74 ,ether dissolves fatty and resinous substances readily1 but inorganic salts are mostly insoluble in this fluid4 ,hence it is that ,iodide of ,potassium and other substances dissolved in ,alcohol are precipitated to a certain extent by the addition of ,ether4 ,,fluoride ,,of ,,potassium4 ,symbol1 ,,kf4 ,atomic weight1 #ei4 ,preparation4--,fluoride of ,potassium is formed by saturating ,hydrofluoric ,acid with ,potash1 and evaporating to dryness in a platinum vessel4 ,hydrofluoric ,acid contains ,fluorine combined with ,hydrogen2 it is a powerfully acid and corrosive liquid1 formed by decomposing ,fluor ,spar1 which is a ,fluoride of ,calcium1 with strong ,sulphuric ,acid2 the action which takes place being precisely analogous to that involved in the preparation of ,hydrochloric ,acid4 ,properties4--,a deliquescent salt1 occurring in small and imperfect crystals4 ,very soluble in water3 the solution acting upon glass in the same manner as ,hydrofluoric ,acid4 ,,formic ,,acid4 ,symbol1 ,c#b;,,ho#c4 ,atomic weight1 #cg4 ,this substance was originally discovered in the red ant 7,formica rufa71 but it is prepared on a large scale by distilling ,starch with ,binoxide of ,manganese and ,sulphuric ,acid4 ,properties4--,the strength of commercial ,formic ,acid is uncertain1 but it is always more or less dilute4 ,the strongest acid1 as obtained by distilling ,formiate of ,soda with ,sulphuric ,acid1 is a fuming liquid with a pungent odour1 and containing only one atom of water4 ,it inflames the skin in the same manner as the sting of the ant4 ,formic ,acid reduces the ,oxides of ,gold1 ,silver1 and ,mercury to the metallic state1 and is itself oxidized into ,carbonic ,acid4 ,the alkaline formiates also possess the same properties4 ,,gallic ,,acid4 ,symbol1 ,c#g;,h#c,o#e ,h#c,o4 ,atomic weight1 #id4 ,the chemistry of ,gallic ,acid is sufficiently described at page #bg1 to which the reader is referred4 ,,gelatine4 ,symbol1 ,c#ac;,h#aj,o#e,n#b4 ,atomic weight1 #aef4 ,this is an organic substance somewhat analogous to ,albumen1 but differing from it in properties4 ,it is obtained by subjecting bones1 hoofs1 horns1 calves' feet1 etc41 to the action of boiling water4 ,the jelly formed on cooling is termed size1 or1 when dried and cut into slices1 glue4 ,gelatine1 as it is sold in the shops1 is a pure form of ,glue4 ,isinglass is gelatine prepared1 chiefly in ,russia1 from the air-bladders of certain species of sturgeon4 ,properties of ,gelatine4--,gelatine softens and swells up in cold water1 but does not dissolve until heated3 the hot solution1 on cooling1 forms a tremulous jelly4 ,one ounce of cold water will retain about three grains of ,isinglass without gelatinizing2 but much depends upon the temperature1 a few degrees greatly affecting the result4 ,when long boiled in water1 and especially in presence of an acid1 such as the ,sulphuric1 ,gelatine undergoes a peculiar modification1 and the ,solution loses either partially or entirely its property of solidifying to a jelly4 ,,glycerine4 ,fatty bodies are resolved by treatment with an alkali into an ,acid--which combines with the alkali1 forming a soap1--and ,glycerine1 remaining in solution4 ,pure ,glycerine1 as obtained by ,price's patent process of distillation1 is a viscid liquid of sp4 gr4 about #abc2 miscible in all proportions with water and ,alcohol4 ,it is peculiarly a neutral substance1 exhibiting no tendency to combine with acids or bases4 ,it has little or no action upon ,nitrate of ,silver in the dark1 and reduces it very slowly even when exposed to light4 ,,glycyrrhizine4 ,glycyrrhizine1 obtained from the fresh root of ,liquorice1 is a substance intermediate in properties between a sugar and a resin4 ,sparingly soluble in water but very soluble in ,alcohol4 ,it precipitates strong solution of ,nitrate of ,silver white1 but the deposit becomes reddened by exposure to light4 ,its preparation is described in the larger works on organic chemistry4 ,,gold1 ,,chloride ,,of4 ,symbol1 ,au,cl#c4 ,atomic weight1 #cjc4 ,this salt is formed by dissolving pure metallic ,gold in ,nitro-hydrochloric ,acid1 and evaporating at a gentle heat4 ,the solution affords deliquescent crystals of a deep orange colour4 ,chloride of ,gold1 in a state fit for ,photographic use1 may easily be obtained by the following process3--,place a half-sovereign in any convenient vessel1 and pour on it half a drachm of ,nitric ,acid mixed with two and a half drachms of ,hydrochloric ,acid and three drachms of water2 digest by a gentle heat1 but do not boil the acid1 or much of the ,chlorine will be driven off in the form of gas4 ,at the expiration of a few hours add fresh ,aqua-,regia in quantity the same as at first1 which will probably complete the solution1 but if not1 repeat the process a third time4 ,lastly1 neutralize the liquid by adding ,carbonate of ,soda until all effervescence ceases1 and a green precipitate forms2 this is ,carbonate of ,copper1 which must be allowed several hours to separate thoroughly4 ,the ,chloride of ,gold is thus freed from ,copper and ,silver1 with which the metallic ,gold is alloyed in the standard coin of the realm4 ,the solution so prepared will be alkaline1 and consequently prone to a reduction of metallic ,gold3 a slight extra quantity of ,hydrochloric acid should therefore be added1 sufficient to redden a piece of immersed litmus-paper4 ,the weight of a half-sovereign is about #fa grains1 of which #ef grains are pure ,gold4 ,this is equivalent to #hf grains of ,chloride of ,gold1 which will be the quantity contained in the solution4 ,the following process for preparing ,chloride of ,gold is more perfect than the last3--,dissolve the ,gold coin in ,aqua-,regia as before2 then boil with excess of ,hydrochloric ,acid1 to destroy the ,nitric ,acid1--dilute largely with distilled water1 and add a filtered aqueous solution of common ,sulphate of ,iron 7#f parts to #a of ,gold72 collect the precipitated ,gold1 which is now free from copper2 redissolve in ,aqua-,regia1 and evaporate to dryness on a water bath4 ,avoid using ,ammonia to neutralize ,chloride of ,gold1 as it would occasion a deposit of ,fulminating ,gold1 the properties of which are described in the next page4 ,properties of ,chloride of ,gold4--,as sold in commerce it usually contains excess of ,hydrochloric ,acid1 and is then of a bright yellow colour2 but when neutral and somewhat concentrated1 it is dark red 7,leo ruber of the alchemists74 ,it gives no precipitate with ,carbonate of ,soda unless heat be applied2 the free ,hydrochloric ,acid present forms1 with the alkali4 ,chloride of ,sodium1 which unites with the ,chloride of ,gold1 and produces a double salt1 ,chloride of ,gold and ,sodium1 soluble in water4 ,chloride of ,gold is decomposed with precipitation of metallic ,gold by ,charcoal1 ,sulphurous ,acid1 and many of the vegetable acids2 also by ,protosulphate and ,protonitrate of ,iron4 ,it tinges the cuticle of an indelible purple tint4 ,it is soluble in ,alcohol and in ,ether4 ,,gold1 ,,fulminating4 ,this is a yellowish-brown substance1 precipitated on adding ,ammonia to a strong solution of ,chloride of ,gold4 ,it may be dried carefully at #bab1 but explodes violently on being heated suddenly to about #bij4 ,friction also causes it to explode when dry2 but the moist powder may be rubbed or handled without danger4 ,it is decomposed by ,sulphuretted ,hydrogen4 ,fulminating ,gold is probably an ,aurate of ,ammonia1 containing #b atoms of ,ammonia to #a atom of ,peroxide of ,gold4 ,,gold1 ,,hyposulphite ,,of4 ,symbol1 ,au,o ,s#b,o#b4 ,atomic weight1 #bec4 ,hyposulphite of ,gold is produced by the reaction of ,chloride of ,gold upon ,hyposulphite of ,soda 7see page #acc74 ,the salt sold in commerce as ,sel d'or is a double ,hyposulphite of ,gold and ,soda1 containing one atom of the former salt to three of the latter1 with four atoms of water of crystallization4 ,it is formed by adding one part of ,chloride of ,gold1 in solution1 to three parts of ,hyposulphite of ,soda1 and precipitating the resulting salt by ,alcohol3 the ,chloride of ,gold must be added to the ,hyposulphite of ,soda1 and not the ,soda salt to the ,gold 7see page #bej74 ,properties4--,hyposulphite of ,gold is unstable and cannot exist in an isolated state1 quickly passing into ,sulphur1 ,sulphuric ,acid1 and metallic ,gold4 ,when combined with excess of ,hyposulphite of ,soda in the form of ,sel d'or1 it is more permanent4 ,sel d'or occurs crystallized in fine needles1 which are very soluble in water4 ,the commercial article is often impure1 containing little else than ,hyposulphite of ,soda1 with a trace of ,gold4 ,it may be analyzed by adding a few drops of strong ,nitric ,acid 7free from ,chlorine71 diluting with water1 and afterwards collecting and igniting the yellow powder1 which is metallic ,gold4 ,,grape ,,sugar4 ,symbol1 ,c#bd;,h#bh,o#bh4 ,atomic weight1 #cif4 ,this modification of ,sugar1 often termed ,granular ,sugar1 or ,glucose1 exists abundantly in the juice of grapes and in many other varieties of fruit4 ,it forms the saccharine concretion found in honey1 raisins1 dried figs1 etc4 ,it may be produced artificially by the action of fermenting principles and of dilute mineral acids1 upon ,starch4 ,properties4--,grape ,sugar crystallizes slowly and with difficulty from a concentrated aqueous solution1 in small hemispherical nodules1 which are hard1 and feel gritty between the teeth4 ,it is much less sweet to the taste than ,cane ,sugar1 and not so soluble in water 7#a part dissolves in #a-a_/b of cold water74 ,grape ,sugar tends to absorb ,oxygen1 and hence it possesses the property of decomposing the salts of the noble metals1 and reducing them by degrees to the metallic state1 even without the aid of light4 ,cane ,sugar does not possess these properties to an equal extent1 and hence it is readily distinguished from the other variety4 ,the product of the action of ,grape ,sugar upon ,nitrate of ,silver appears to be a very low form of ,oxide of ,silver combined with organic matter4 ,,honey4 ,this substance contains two distinct kinds of ,sugar1 ,grape ,sugar1 and an uncrystallizable substance analogous to1 or identical with1 the ,treacle found associated with common ,sugar in the cane-juice4 ,the agreeable taste of ,honey probably depends upon the latter1 but its reducing power on metallic oxides is due to the former4 ,pure ,grape ,sugar can readily be obtained from inspissated ,honey1 by treating it with ,alcohol1 which dissolves out the syrup1 but leaves the crystalline portion4 ,much of the commercial article is adulterated1 and1 for ,photographic use1 the ,virgin ,honey should be obtained direct from the comb4 ,,hydrochloric ,,acid4 ,symbol1 ,h,cl4 ,atomic weight1 #cg4 ,hydrochloric ,acid is a volatile gas1 which may be liberated from most of the salts termed ,chlorides by the action of ,sulphuric ,acid4 ,the acid1 by its superior affinities1 removes the base2 thus1-- ,na,cl ,,ho ,,so#c ,na,o ,,so#c ,h,cl4 ,properties4--,abundantly soluble in water1 forming the liquid ,hydrochloric or ,muriatic ,acid of commerce4 ,the most concentrated solution of ,hydrochloric ,acid has a sp4 gr4 #ab1 and contains about #dj per cent1 of gas2 that commonly sold is somewhat weaker1 sp4 gr4 #aad #bh per cent1 real acid4 ,pure ,hydrochloric ,acid is colourless1 and fumes in the air4 ,the yellow colour of the commercial acid depends upon the presence of traces of ,perchloride of ,iron1 or of organic matter2 commercial ,muriatic ,acid also often contains a portion of free ,chlorine and of ,sulphuric ,acid4 ,,hydriodic ,,acid4 ,symbol1 ,,hi4 ,atomic weight1 #abg4 ,this is a gaseous compound of ,hydrogen and ,iodine1 corresponding in composition to the ,hydrochloric ,acid4 ,it cannot however1 from its instability1 be obtained in the same manner1 since1 on distilling an ,iodide with ,sulphuric ,acid1 the ,hydriodic ,acid first formed is subsequently decomposed into ,iodine and ,hydrogen4 ,an aqueous solution of ,hydriodic ,acid is easily prepared by adding ,iodine to water containing ,sulphuretted ,hydrogen gas2 a decomposition takes place1 and ,sulphur is set free3 thus1 ,,hs ,i ,,hi s4 ,properties4--,hydriodic ,acid is very soluble in water1 yielding a strongly acid liquid4 ,the solution1 colourless at first1 soon becomes brown from decomposition1 and liberation of free ,iodine4 ,it may be restored to its original condition by adding solution of ,sulphuretted ,hydrogen4 ,,hydrosulphuric ,,acid4 ,symbol1 ,,hs4 ,atomic weight1 #ag4 ,this substance1 also known as ,sulphuretted ,hydrogen1 is a gaseous compound of ,sulphur and ,hydrogen1 analogous in composition to the ,hydrochloric and ,hydriodic ,acid4 ,it is usually prepared by the action of dilute ,sulphuric ,acid upon ,sulphuret of ,iron1 as described at page #cgc2 the decomposition being similar to that involved in the preparation of the ,hydrogen acids generally3-- ,fe,s ,,ho ,,so#c ,fe,o ,,so#c ,,hs4 ,properties4--,cold water absorbs three times its bulk of ,hydrosulphuric ,acid1 and acquires the peculiar putrid odour and poisonous qualities of the gas4 ,the solution is faintly acid to test-paper1 and becomes opalescent on keeping1 from gradual separation of ,sulphur4 ,it is decomposed by ,nitric ,acid1 and also by ,chlorine and ,iodine4 ,it precipitates ,silver from its solutions in the form of black ,sulphuret of ,silver2 also ,copper1 ,mercury1 ,lead1 etc42 but ,iron and other metals of that class are not affected1 if the liquid contains free acid4 ,hydrosulphuric ,acid is constantly employed in the chemical laboratory for these and other purposes4 ,,hydrosulphate ,,of ,,ammonia4 ,symbol1 ,,nh#d,s ,,hs4 ,atomic weight1 #ea4 ,the liquid known by this name1 and formed on passing ,sulphuretted ,hydrogen gas into ,ammonia1 is a double ,sulphuret of ,hydrogen and ,ammonium4 ,in the preparation1 the passage of the gas is to be continued until the solution gives no precipitate with ,sulphate of ,magnesia1 and smells strongly of ,hydrosulphuric ,acid4 ,properties4--,colourless at first1 but afterwards changes to yellow1 from liberation and subsequent solution of ,sulphur4 ,becomes milky on the addition of any acid4 ,precipitates1 in the form of ,sulphuret1 all the metals which are affected by ,sulphuretted ,hydrogen1 and1 in addition1 those of the class to which ,iron1 ,zinc1 and ,manganese belong4 ,hydrosulphate of ,ammonia is employed in ,photography to darken the ,negative image1 and also in the preparation of ,iodide of ,ammonium1 the separation of ,silver from ,hyposulphite solutions1 etc4 ,,hyposulphite ,,of ,,soda4 ,symbol1 ,na,o ,s#b,o#b #e ,,ho4 ,atomic weight1 #abe4 ,the chemistry of ,hyposulphurous ,acid and the ,hyposulphite of ,soda has been sufficiently described at pages #dc1 #abi1 and #acg of the present ,work4 ,the crystallized salt includes five atoms of water of crystallization4 ,,hyposulphite ,,of ,,gold4 ,see ,gold1 ,hyposulphite of4 ,,hyposulphite ,,of ,,silver4 ,see ,silver1 ,hyposulphite of4 ,,iceland ,,moss4 ,cetraria ,islandica4--,a species of ,lichen found in ,iceland and the mountainous parts of ,europe2 when boiled in water1 it first swells up1 and then yields a substance which gelatinizes on cooling4 ,it contains ,lichen ,starch1 a bitter principle soluble in ,alcohol1 termed ,cetrarine1 and common ,starch2 traces of ,gallic ,acid and ,bitartrate of ,potash are also present4 ,,iodine4 ,symbol1 ,i4 ,atomic weight1 #abf4 ,iodine is chiefly prepared at ,glasgow1 from kelp1 which is the fused ash obtained on burning seaweeds4 ,the waters of the ocean contain minute quantities of the ,iodides of ,sodium and ,magnesium1 which are separated and stored up by the growing tissues of the marine plant4 ,in the preparation1 the mother-liquor of kelp is evaporated to dryness and distilled with ,sulphuric ,acid2 the ,hydriodic ,acid first liberated is decomposed by the high temperature1 and fumes of ,iodine condense in the form of opaque crystals4 ,properties4--,iodine has a bluish-black colour and metallic lustre2 it stains the skin yellow1 and has a pungent smell1 like diluted ,chlorine4 ,it is extremely volatile when moist1 boils at #cej1 and produces dense violet-coloured fumes1 which condense in brilliant plates4 ,specific gravity #didf4 ,iodine is very sparingly soluble in water1 #a part requiring #gjjj parts for perfect solution2 even this minute quantity however tinges the liquid of a brown colour4 ,alcohol and ,ether dissolve it more abundantly1 forming dark-brown solutions4 ,iodine also dissolves freely in solutions of the alkaline ,iodides1 such as the ,iodide of ,potassium1 of ,sodium1 and of ,ammonium4 ,chemical ,properties4--,iodine belongs to the ,chlorine group of elements1 characterized by forming acids with ,hydrogen1 and combining extensively with the metals 7see ,chlorine74 ,they are however comparatively indifferent to ,oxygen1 and also to each other4 ,the ,iodides of the alkalies and alkaline earths are soluble in water2 also those of ,iron1 ,zinc1 ,cadmium1 etc4 ,the ,iodides of ,lead1 ,silver1 and ,mercury are nearly or quite insoluble4 ,iodine possesses the property of forming a compound of a deep blue colour with ,starch4 ,in using this as a test1 it is necessary first to liberate the ,iodine 7if in combination7 by means of ,chlorine1 or ,nitric ,acids saturated with ,peroxide of ,nitrogen4 ,the presence of ,alcohol or ,ether interferes to a certain extent with the result4 ,,iodide ,,of ,,ammonium4 ,symbol1 ,,nh#d;,i4 ,atomic weight1 #add4 ,the preparation and properties of this salt are described at page #aih1 to which the reader is referred4 ,,iodide ,,of ,,cadmium4 ,symbol1 ,cd,i4 ,atomic weight1 #ahb4 ,see page #aii1 for the preparation and properties of this salt4 ,,iodide ,,of ,,iron4 ,symbol1 ,fe,i4 ,atomic weight1 #aed4 ,iodide of ,iron is prepared by digesting an excess of ,iron filings with solution of ,iodine in ,alcohol4 ,it is very soluble in water and ,alcohol1 but the solution rapidly absorbs ,oxygen and deposits ,peroxide of ,iron2 hence the importance of preserving it in contact with metallic ,iron1 with which the separated ,iodine may recombine4 ,by very careful evaporation1 hydrated crystals of ,proto-iodide may be obtained1 but the composition of the solid salt usually sold under that name cannot be depended on4 ,the ,periodide of ,iron1 corresponding to the ,perchloride1 has not been examined1 and it is doubtful if any such compound exists4 ,,iodide ,,of ,,potassium4 ,symbol1 ,,ki4 ,atomic weight1 #aff4 ,this salt is usually formed by dissolving ,iodine in solution of ,potash until it begins to acquire a brown colour2 a mixture of ,iodide of ,potassium and ,iodate of ,potash 7,,ko ,,io#e7 is thus formed2 but by evaporation and heating to redness1 the latter salt parts with its ,oxygen1 and is converted into ,iodide of ,potassium4 ,properties4--,it forms cubic and prismatic crystals1 which should be hard1 and very slightly or not at all deliquescent4 ,soluble in less than an equal weight of water at #f,o2 it is also soluble in ,alcohol1 but not in ,ether4 ,the proportion of ,iodide of ,potassium contained in a saturated alcoholic solution1 varies with the strength of the spirit3--with common ,spirits of ,wine1 sp4 gr4 #hcf1 it would be about #h grains to the drachm2 with ,alcohol rectified from ,carbonate of ,potash1 sp4 gr4 #hbc1 #d or #e grains2 with absolute ,alcohol1 #a to #b grains4 ,the solution of ,iodide of ,potassium is instantly coloured brown by free ,chlorine2 also very rapidly by ,peroxide of ,nitrogen 7page #hf72 ordinary acids however act less quickly1 ,hydriodic ,acid being first formed1 and subsequently decomposing spontaneously4 ,the impurities of commercial ,iodide of ,potassium1 with the means to be adopted for their removal1 are fully given at page #aig4 ,,iodide ,,of ,,silver4 ,see ,silver1 ,iodide of4 ,,iodoform4 ,the composition of this substance is analogous to that of ,chloroform1 ,iodine being substituted for ,chlorine4 ,it is obtained on boiling together ,iodine1 ,carbonate of ,potash1 and ,alcohol4 ,iodoform occurs in yellow nacrous crystals1 which have a saffron-like odour4 ,it is insoluble in water1 but soluble in spirit4 ,,iron1 ,,protosulphate ,,of4 ,symbol1 ,fe,o ,,so#c #g ,,ho4 ,atomic weight1 #aci4 ,the properties of this salt1 and of the two salifiable ,oxides of ,iron1 are described at page #bi4 ,it dissolves in rather more than an equal weight of cold water1 or in less of boiling water4 ,aqueous solution of ,sulphate of ,iron absorbs the ,binoxide of ,nitrogen1 acquiring a deep olive-brown colour3 as this gaseous ,binoxide is itself a reducing agent1 the liquid so formed has been proposed as a more energetic developer than the ,sulphate of ,iron alone 7874 ,,iron1 ,,protonitrate ,,of4 ,symbol1 ,fe,o ,,no#c #g ,,ho4 ,atomic weight1 #aec4 ,this salt1 by careful evaporation in vacuo over ,sulphuric ,acid1 forms transparent crystals1 of a light green colour1 and containing #g atoms of water1 like the ,protosulphate4 ,it is exceedingly unstable1 and soon becomes red from decomposition1 unless preserved from contact with air4 ,the preparation of solution of ,protonitrate of ,iron for developing ,collodion ,positives1 is given at page #bjf4 ,,iron1 ,,perchloride ,,of4 ,symbol1 ,fe#b;,cl#c4 ,atomic weight1 #afd4 ,there are two ,chlorides of ,iron1 corresponding in composition to the ,protoxide and the ,sesquioxide respectively4 ,the ,protochloride is very soluble in water1 forming a green solution1 which precipitates a dirty white ,protoxide on the addition of an alkali4 ,the ,perchloride1 on the other hand1 is dark brown1 and gives a foxy-red precipitate with alkalies4 ,properties4--,perchloride of ,iron may be obtained in the solid form by heating ,iron wire in excess of ,chlorine2 it condenses in the shape of brilliant and iridescent brown crystals1 which are volatile1 and dissolve in water1 the solution being acid to test-paper4 ,it is also soluble in ,alcohol1 forming the ,tinctura ,ferri ,sesquichloridi of the ,pharmacopia4 ,commercial ,perchloride of ,iron ordinarily contains an excess of ,hydrochloric ,acid4 ,,litmus4 ,litmus is a vegetable substance prepared from various lichens1 which are principally collected on rocks adjoining the sea4 ,the colouring matter is extracted by a peculiar process1 and afterwards made up into a paste with chalk1 plaster of ,paris1 etc4 ,litmus occurs in commerce in the form of small cubes of a fine violet colour4 ,in using it for the preparation of test-papers1 it is digested in hot water1 and sheets of porous paper are soaked in the blue liquid so formed4 ,the red papers are prepared at first in the same manner1 but afterwards placed in water which has been rendered faintly acid with ,sulphuric or ,hydrochloric ,acid4 ,,mercury1 ,,bichloride ,,of4 ,symbol1 ,hg,cl#b4 ,atomic weight1 #bgd4 ,this salt1 also called ,corrosive ,sublimate1 and sometimes ,chloride of ,mercury 7the atomic weight of ,mercury being halved71 may be formed by heating ,mercury in excess of ,chlorine1 or more economically1 by subliming a mixture of ,persulphate of ,mercury and ,chloride of ,sodium4 ,properties4--,a very corrosive and poisonous salt1 usually sold in semi-transparent1 crystalline masses1 or in the state of powder4 ,soluble in #af parts of cold1 and in #c of hot water2 more abundantly so in ,alcohol1 and also in ,ether4 ,the solubility in water may be increased by the addition of free ,hydrochloric ,acid1 or of ,chloride of ,ammonium4 ,the ,protochloride of ,mercury is an insoluble white powder1 commonly known under the name of ,calomel4 ,,methylic ,,alcohol4 ,this liquid1 known also by the names of wood naphtha and pyroxylic spirit1 is one of the volatile products of the destructive distillation of wood4 ,it is very volatile and limpid1 with a pungent odour4 ,by a recent excise regulation1 ordinary ,spirit mixed with ten per cent1 of wood naphtha is sold free of duty1 under the name of ,methylated ,spirit4 ,,milk4 ,the ,milk of herbivorous animals contains three principal constituents--,fatty matter1 ,caseine1 and ,sugar2 in addition to these1 small quantities of the ,chloride of ,potassium1 and of ,phosphates of ,lime and ,magnesia1 are present4 ,the fatty matter is contained in small cells1 and forms the greater part of the cream which rises to the surface of the milk on standing2 hence shimmed milk is to be preferred for ,photographic use4 ,the second constituent1 ,caseine1 is an organic principle somewhat analogous to ,albumen in composition and properties4 ,its aqueous solution however does not1 like ,albumen1 coagulate on boiling1 unless an acid be present1 which probably removes a small portion of alkali with which the ,caseine was previously combined4 ,the substance termed rennet1 which is the dried stomach of the calf1 possesses the property of coagulating ,caseine1 but the exact mode of its action is unknown4 ,sherry-wine is also commonly employed to curdle ,milk2 but brandy and other spirituous liquids1 when free from acid and astringent matter1 have no effect4 ,in all these cases a portion of the ,caseine usually remains in a soluble form in the whey2 but when the ,milk is coagulated by the addition of acids1 the quantity so left is very small1 and hence the use of the rennet is to be preferred1 since the presence of ,caseine facilitates the reduction of the sensitive ,silver salts4 ,caseine combines with ,oxide of ,silver in the same manner as ,albumen1 forming a white coagulum1 which becomes brick-red on exposure to light4 ,sugar of ,milk1 the third principal constituent1 differs from both cane and grape sugar2 it may be obtained by evaporating whey until crystallization begins to take place4 ,it is hard and gritty1 and only slightly sweet2 slowly soluble1 without forming a syrup1 in about two and a half parts of boiling1 and six of cold water4 ,it does not ferment and form ,alcohol on the addition of yeast1 like grape sugar1 but by the action of decomposing animal matter is converted into ,lactic ,acid4 ,when skimmed ,milk is exposed to the air for some hours1 it gradually becomes sour1 from ,lactic ,acid formed in this way2 and if then heated to ebullition1 the ,caseine coagulates very perfectly4 ,,nitric ,,acid4 ,symbol1 ,,no#e4 ,atomic weight1 #ed4 ,nitric ,acid1 or ,aqua-fortis1 is prepared by adding ,sulphuric ,acid to ,nitrate of ,potash1 and distilling the mixture in a retort4 ,sulphate of ,potash and free ,nitric ,acid are formed1 the latter of which1 being volatile1 distils over in combination with one atom of water previously united with the ,sulphuric ,acid4 ,properties4--,anhydrous ,nitric ,acid is a solid substance1 white and crystalline1 but it cannot be prepared except by an expensive and complicated process4 ,the concentrated liquid ,nitric ,acid contains #a atom of water1 and has a sp4 gr4 of about #ae2 if perfectly pure1 it is colourless1 but usually it has a slight yellow tint1 from partial decomposition into ,peroxide of ,nitrogen3 it fumes strongly in the air4 ,the strength of commercial ,nitric ,acid is subject to much variation4 ,an acid of sp4 gr4 #adb1 containing about #d atoms of water1 is commonly met with4 ,if the specific gravity is much lower than this 7less than #acf71 it will scarcely be adapted for the preparation of ,pyroxyline4 ,the yellow ,nitrous ,acid1 so called1 is a strong ,nitric ,acid partially saturated with the brown vapours of ,peroxide of ,nitrogen2 it has a high specific gravity1 but this is somewhat deceptive1 being caused in part by the presence of the ,peroxide4 ,on mixing with ,sulphuric ,acid1 the colour disappears1 a compound being formed which has been termed a ,sulphate of ,nitrous ,acid4 ,in the ,appendix a ,table is given which exhibits the quantity of real anhydrous ,nitric ,acid contained in samples of different densities4 ,chemical ,properties4--,nitric ,acid is a powerful oxidizing agent 7see page #ac72 it dissolves all the common metals1 with the exception of ,gold and ,platinum4 ,animal substances1 such as the cuticle1 nails1 etc41 are tinged of a permanent yellow colour1 and deeply corroded by a prolonged application4 ,nitric ,acid forms a numerous class of salts1 all of which are soluble in water4 ,hence its presence cannot be determined by any precipitating reagent1 in the same manner as that of ,hydrochloric and ,sulphuric ,acid4 ,impurities of ,commercial ,nitric ,acid4--,these are principally ,chlorine and ,sulphuric ,acid2 also ,peroxide of ,nitrogen1 which tinges the acid yellow1 as already described4 ,chlorine is detected by diluting the acid with an equal bulk of distilled water1 and adding a few drops of ,nitrate of ,silver1--a milkiness1 which1 is ,chloride of ,silver in suspension1 indicates the presence of ,chlorine4 ,in testing for ,sulphuric ,acid1 dilute the ,nitric ,acid as before1 and drop in a single drop of solution of ,chloride of ,barium2 if ,sulphuric ,acid be present1 an insoluble precipitate of ,sulphate of ,baryta will be formed4 ,,nitrous ,,acid4 ,see ,silver1 ,nitrite of4 ,,nitrate ,,of ,,potash4 ,symbol1 ,,ko ,,no#e4 ,atomic weight1 #ajb4 ,this salt1 also termed ,nitre1 or ,saltpetre1 is an abundant natural product1 found effloresced upon the soil in certain parts of the ,east ,indies4 ,it is also produced artificially in what are called ,nitre-beds4 ,the properties of ,nitrate of ,potash are described as far as necessary at page #aij4 ,,nitrate ,,of ,,baryta4 ,symbol1 ,ba,o ,,no#e4 ,atomic weight1 #aca4 ,nitrate of ,baryta forms octahedral crystals1 which are anhydrous4 ,it is considerably less soluble than the ,chloride of ,barium1 requiring #ab parts of cold and #d of boiling water for solution4 ,it may be substituted for the ,nitrate of ,lead in the preparation of ,protonitrate of ,iron4 ,,nitrate ,,of ,,lead4 ,symbol1 ,pb,o ,,no#e4 ,atomic weight1 #aff4 ,nitrate of ,lead is obtained by dissolving the metal1 or the ,oxide of ,lead1 in excess of ,nitric ,acid1 diluted with #b parts of water4 ,it crystallizes on evaporation in white anhydrous tetrahedra and octahedra1 which are hard1 and decrepitate on being heated2 they are soluble in #h parts of water at #fj4 ,nitrate of ,lead forms with ,sulphuric ,acid1 or soluble ,sulphates1 a white precipitate1 which is the insoluble ,sulphate of ,lead4 ,the ,iodide of ,lead is also very sparingly soluble in water4 ,,nitrate ,,of ,,silver1 ,see ,silver1 ,nitrate or4 ,,nitro-,,glucose4 ,when #c fluid ounces of cold ,nitro-,sulphuric ,acid1 consisting of #b ounces of ,oil of ,vitriol and #a ounce of highly concentrated ,nitric ,acid1 are mixed with #a ounce of finely powdered ,cane ,sugar1 there is formed at first a thin1 transparent1 pasty mass4 ,if it is stirred with a glass rod for a few minutes without interruption1 the paste coagulates as it were1 and separates from the liquid as a thick tenacious mass1 aggregating into lumps1 which can easily be removed from the acid mixture4 ,this substance has a very acid and intensely bitter taste4 ,kneaded in warm water until the latter no longer reddens litmus-paper1 it acquires a silver colour and a beautiful silky lustre4 ,it may be used in ,photography to confer intensity upon newly mixed ,collodion2 but is inferior to ,glycyrrhizine employed for the same purpose4 ,,nitro-,,hydrochloric ,,acid4 ,symbol1 ,,no#d ,cl4 ,this liquid is the ,aqua-,regia of the old alchemists4 ,it is produced by mixing ,nitric and ,hydrochloric ,acids3 the ,oxygen contained in the former combines with the ,hydrogen of the latter1 forming water and liberating ,chlorine1 thus3-- ,,no#e ,h,cl ,,no#d ,,ho ,cl4 ,the presence of free ,chlorine confers on the mixture the power of dissolving ,gold and ,platinum1 which neither of the two acids possesses separately4 ,in preparing ,aqua-,regia it is usual to mix one part1 by measure1 of ,nitric ,acid with four of ,hydrochloric ,acid1 and to dilute with an equal bulk of water4 ,the application of a gentle heat assists the solution of the metal2 but if the temperature rises to the boiling point1 a violent effervescence and escape of ,chlorine takes place4 ,,nitro-,,sulphuric ,,acid4 ,for the chemistry of this acid liquid1 see page #gg4 ,,oxygen4 ,symbol1 ,o4 ,atomic weight1 #h4 ,oxygen gas may be obtained by heating ,nitrate of ,potash to redness1 but in this case it is contaminated with a portion of ,nitrogen4 ,the salt termed ,chlorate of ,potash 7the composition of which is closely analogous to that of the ,nitrate1 ,chlorine being substituted for ,nitrogen7 yields abundance of pure ,oxygen gas on the application of heat1 leaving behind ,chloride of ,potassium4 ,chemical ,properties4--,oxygen combines eagerly with many of the chemical elements1 forming ,oxides4 ,this chemical affinity however is not well seen when the elementary body is exposed to the action of ,oxygen in the gaseous form4 ,it is the nascent ,oxygen which acts most powerfully as an oxidizer4 ,by nascent ,oxygen is meant ,oxygen on the point of separation from other elementary atoms with which it was previously associated2 it may then be considered to be in the liquid form1 and hence it comes more perfectly into contact with the particles of the body to be oxidized4 ,illustrations of the superior chemical energy of nascent ,oxygen are numerous1 but none perhaps are more striking than the mild and gradual oxidizing influence exerted by atmospheric air1 as compared with the violent action of ,nitric ,acid and bodies of that class which contain ,oxygen loosely combined4 ,,oxymel4 ,this syrup of ,honey and ,vinegar is prepared as follows4 ,take of ,honey #a pound4 ,acid1 ,acetic1 fortiss4 7,beaufoy's ,acid7 #aa drachms4 ,water #ac drachms4 ,stand the pot containing the ,honey in boiling water until a scum rises to the surface1 which is to be removed two or three times4 ,then add the ,acetic ,acid and water1 and skim once more if required4 ,allow to cool1 and it will be fit for use4 ,,potash4 ,symbol1 ,,ko ,,ho4 ,atomic weight1 #eg4 ,potash is obtained by separating the ,carbonic ,acid from ,carbonate of ,potash by means of ,caustic ,lime4 ,lime is a more feeble base than ,potash1 but the ,carbonate of ,lime1 being insoluble in water1 is at once formed on adding ,milk of ,lime to a solution of ,carbonate of ,potash 7see page #cad74 ,properties4--,usually met with in the form of solid lumps1 or in cylindrical sticks1 which are formed by melting the ,potash and running it into a mould4 ,it always contains one atom of water1 which cannot be driven off by the application of heat4 ,potash is soluble almost to any extent in water1 much heat being evolved4 ,the solution is powerfully alkaline 7p4 #cjh71 and acts rapidly upon the skin2 it dissolves fatty and resinous bodies1 converting them into soaps4 ,solution of ,potash absorbs ,carbonic ,acid quickly from the air1 and should therefore be preserved in stoppered bottles2 the glass stoppers must be wiped occasionally1 in order to prevent them from becoming immovably fixed by the solvent action of the ,potash upon the ,silica of the glass4 ,the ,liquor ,potass of the ,london ,pharmacopia has a sp4 gr4 of #ajfc1 and contains about #e per cent1 of real ,potash4 ,it is usually contaminated with ,carbonate of ,potash1 which causes it to effervesce on the addition of acids2 also1 to a less extent1 with ,sulphate of ,potash1 ,chloride of ,potassium1 ,silica1 etc4 ,,potash1 ,,carbonate ,,of4 ,symbol1 ,,ko ,,co#b4 ,atomic weight1 #gj4 ,the impure ,carbonate of ,potash1 termed ,pearlash1 is obtained from the ashes of wood and vegetable matter1 in the same manner as ,carbonate of ,soda is prepared from the ashes of seaweeds4 ,salts of ,potash and of ,soda appear essential to vegetation1 and are absorbed and approximated by the living tissues of the plant4 ,they exist in the vegetable structure1 combined with organic acids in the form of salts1 like the ,oxalate1 ,tartrate1 etc41 which1 when burned are converted into ,carbonates4 ,properties4--,the ,pearlash of commerce contains large and variable quantities of ,chloride of ,potassium1 ,sulphate of ,potash1 etc4 ,a purer ,carbonate is sold1 which is free from ,sulphates1 and with only a trace of ,chlorides4 ,carbonate of ,potash is a strongly alkaline salt1 deliquescent1 and soluble in twice its weight of cold water2 insoluble in ,alcohol1 and employed to deprive it of water 7see page #aif74 ,,pyrogallic ,,acid4 ,symbol1 ,c#h;,h#d,o#d 7,stenhouse74 ,atomic weight1 #hd4 ,the chemistry of ,pyrogallic ,acid has been described at page #bh4 ,,sel ,d',,or4 ,see ,gold1 ,hyposulphite of4 ,,silver4 ,symbol1 ,ag4 ,atomic weight1 #ajh4 ,this metal1 the ,luna or ,diana of the alchemists1 is found native in ,peru and ,mexico2 it occurs also in the form of ,sulphuret of ,silver4 ,when pure it has a sp4 gr4 of #aje1 and is very malleable and ductile2 melts at a bright red heat4 ,silver does not oxidize in the air1 but when exposed to an impure atmosphere containing traces of ,sulphuretted ,hydrogen1 it is slowly tarnished from formation of ,sulphuret of ,silver4 ,it dissolves in ,sulphuric ,acid1 but the best solvent is ,nitric ,acid4 ,the standard coin of the realm is an alloy of ,silver and ,copper1 containing about one-eleventh of the latter metal4 ,to prepare pure ,nitrate of ,silver from it1 dissolve in ,nitric ,acid and evaporate until crystals are obtained4 ,then wash the crystals with a little dilute ,nitric ,acid1 redissolve them in water1 and crystallize by evaporation a second time4 ,lastly1 fuse the product at a moderate heat1 in order to expel the last traces of ,nitric and ,nitrous ,acids4 ,,silver1 ,,ammonio-,,nitrate ,,of4 ,crystallized ,nitrate of ,silver absorbs ,ammoniacal gas rapidly1 with production of heat sufficient to fuse the resulting compound1 which is white1 and consists of #ajj parts of the ,nitrate #bie of ,ammonia4 ,the compound however which ,photographers employ under the name of ,ammonio-,nitrate of ,silver may be viewed more simply as a solution of the ,oxide of ,silver in ,ammonia1 without reference to the ,nitrate of ,ammonia necessarily produced in the reaction4 ,very strong ,ammonia1 in acting upon ,oxide of ,silver1 converts it into a black powder1 termed ,fulminating ,silver1 which possesses the most dangerous explosive properties4 ,its composition is uncertain4 ,in preparing ,ammonio-,nitrate of ,silver by the common process1 the ,oxide first precipitated occasionally leaves a little black powder behind1 on re-solution2 this does not appear however1 according to the observations of the ,author1 to be ,fulminating ,silver4 ,in sensitizing salted paper by the ,ammonio-,nitrate of ,silver1 free ,ammonia is necessarily formed4 ,thus-- ,chloride of ,ammonium ,oxide of ,silver in ,ammonia ,chloride of ,silver ,ammonia ,water4 ,,silver1 ,,oxide ,,of4 ,symbol1 ,ag,o4 ,atomic weight1 #aaf4 ,this compound has already been described in ,part ,i41 page #ag4 ,,silver1 ,,chloride ,,of4 ,symbol1 ,ag,cl4 ,atomic weight1 #add4 ,the preparation and properties of ,chloride of ,silver are given in ,part ,i4 page #ad4 ,,silver1 ,,bromide ,,of4 ,symbol1 ,ag,br4 ,atomic weight1 #ahf4 ,see ,part ,i4 page #ag4 ,,silver1 ,,citrate ,,of4 ,see ,citric ,acid4 ,,silver1 ,,iodide ,,of4 ,symbol1 ,ag,i4 ,atomic weight1 #bcd4 ,see ,part ,i4 page #af4 ,,silver1 ,,fluoride ,,of4 ,symbol1 ,ag,f4 ,atomic weight1 #abg4 ,this compound differs from those last described in being soluble in water4 ,the dry salt fuses on being heated1 and is reduced by a higher temperature1 or by exposure to light4 ,,silver1 ,,sulphuret ,,of4 ,symbol1 ,ag,s4 ,atomic weight1 #abd4 ,this compound is formed by the action of ,sulphur upon metallic ,silver1 or of ,sulphuretted ,hydrogen or ,hydrosulphate of ,ammonia upon the ,silver salts2 the decomposition of ,hyposulphite of ,silver also furnishes the black ,sulphuret4 ,sulphuret of ,silver is insoluble in water1 and nearly so in those substances which dissolve the ,chloride1 ,bromide1 and ,iodide1 such as ,ammonia1 ,hyposulphites1 ,cyanides1 etc42 but it dissolves in ,nitric ,acid1 being converted into soluble ,sulphate and ,nitrate of ,silver4 7,for a further account of the properties of the ,sulphuret of ,silver1 see page #adf47 ,,silver1 ,,nitrate ,,of4 ,symbol1 ,ag,o ,,no#e4 ,atomic weight1 #agj4 ,the preparation and properties of this salt have been explained at pages #ab and #cfb4 ,,silver1 ,,nitrite ,,of4 ,symbol1 ,ag,o ,,no#c4 ,atomic weight1 #aed4 ,nitrite of ,silver is a compound of ,nitrous ,acid1 or ,,no#c1 with ,oxide of ,silver4 ,it is formed by heating ,nitrate of ,silver1 so as to drive off a portion of its ,oxygen1 or more conveniently1 by mixing ,nitrate of ,silver and ,nitrite of ,potash in equal parts1 fusing strongly1 and dissolving in a small quantity of boiling water3 on cooling1 the ,nitrite crystallizes out1 and may be purified by pressing in blotting-paper4 ,mr4 ,hadow describes an economical method of preparing ,nitrite of ,silver in quantity1 viz4 by heating #a part of ,starch in #h of ,nitric ,acid of #abe specific gravity1 and conducting the evolved gases into a solution of pure ,carbonate of ,soda until effervescence has ceased4 ,the ,nitrite of ,soda thus formed is afterwards added to ,nitrate of ,silver in the usual way4 ,properties4--,nitrite of ,silver is soluble in #abj parts of cold water2 easily soluble in boiling water1 and crystallizes1 on cooling1 in long slender needles4 ,it has a certain degree of affinity for ,oxygen1 and tends to pass into the condition of ,nitrate of ,silver2 but it is probable that its ,photographic properties depend more upon a decomposition of the salt and liberation of ,nitrous ,acid4 ,properties of ,nitrous ,acid4--,this substance possesses very feeble acid properties1 its salts being decomposed even by ,acetic ,acid4 ,it is an unstable body1 and splits up1 in contact with water1 into ,binoxide of ,nitrogen and ,nitric ,acid4 ,the ,peroxide of ,nitrogen1 ,,no#d1 is also decomposed by water1 and yields the same products4 ,,silver1 ,,acetate ,,of4 ,symbol1 ,ag,o 7,c#d;,h#c,o#c74 ,atomic weight1 #afg4 ,this is a difficultly soluble salt1 deposited in lamellar crystals when an ,acetate is added to a strong solution of ,nitrate of ,silver4 ,if ,acetic ,acid be used in place of an ,acetate1 the ,acetate of ,silver does not fall so readily1 since the ,nitric ,acid which would then be liberated impedes the decomposition4 ,its properties have been sufficiently described at page #hi4 ,,silver1 ,,hyposulphite ,,of4 ,symbol1 ,ag,o ,s#b,o#b4 ,atomic weight1 #afd4 ,this salt is fully described in ,part ,i4 page #abi4 ,for the properties of the soluble double salt of ,hyposulphite of ,silver and ,hyposulphite of ,soda1 see page #dc4 ,,sugar ,,of ,,milk4 ,see ,milk4 ,,sulphuretted ,,hydrogen4 ,see ,hydrosulphuric ,acid4 ,,sulphuric ,,acid4 ,symbol1 ,,so#c4 ,atomic weight1 #dj4 ,sulphuric ,acid may be formed by oxidizing ,sulphur with boiling ,nitric ,acid2 but this plan would be too expensive to be adopted on a large scale4 ,the commercial process for the manufacture of ,sulphuric ,acid is exceedingly ingenious and beautiful1 but it involves reactions which are too complicated to admit of a superficial explanation4 ,the ,sulphur is first burnt into gaseous ,sulphurous ,acid 7,,so#b71 and then by the agency of ,binoxide of ,nitrogen gas1 an additional atom of ,oxygen is imparted from the atmosphere1 so as to convert the ,,so#b into ,,so#c1 or ,sulphuric ,acid4 ,properties4--,anhydrous ,sulphuric ,acid is a white crystalline solid4 ,the strongest liquid acid always contains one atom of water1 which is closely associated with it1 and cannot be driven off by the application of heat4 ,this mono-hydrated ,sulphuric ,acid1 represented by the formula ,,ho ,,so#c1 is a dense fluid1 having a specific gravity of about #ahde2 boils at #fbj1 and distils without decomposition4 ,it is not volatile at common temperatures1 and therefore does not fume in the same manner as ,nitric or ,hydrochloric ,acid4 ,the concentrated acid maybe cooled down even to zero without solidifying2 but a weaker compound1 containing twice the quantity of water1 and termed glacial ,sulphuric ,acid1 crystallizes at #dj ,fahr4 ,sulphuric ,acid is intensely acid and caustic1 but it does not destroy the skin or dissolve metals so readily as ,nitric ,acid4 ,it has an energetic attraction for water1 and when the two are mixed1 condensation ensues1 and much heat is evolved2 four parts of acid and one of water produce a temperature equal to that of boiling water4 ,mixed with aqueous ,nitric ,acid1 it forms the compound know a as ,nitro-,sulphuric ,acid4 ,sulphuric ,acid possesses intense chemical powers1 and displaces the greater number of ordinary acids from their salts4 ,it chars organic substances1 by removing the elements of water1 and converts ,alcohol into ,ether in a similar manner4 ,the strength of a given sample of ,sulphuric ,acid may be calculated1 nearly1 from its specific gravity1 and a ,table is given by ,dr4 ,ure for that purpose4 7,see ,appendix47 ,impurities of ,commercial ,sulphuric ,acid4--,the liquid acid sold as ,oil of ,vitriol is tolerably constant in composition1 and seems to be as well adapted for ,photographic use as the pure ,sulphuric ,acid1 which is far more expensive4 ,the specific gravity should be about #ahcf at #fj4 ,if a drop1 evaporated upon ,platinum-foil1 gives a fixed residue1 probably ,bisulphate of ,potash is present4 ,a milkiness1 on dilution1 indicates ,sulphate of ,lead 7see page #ahf74 ,test for ,sulphuric ,acid4--,if the presence of ,sulphuric ,acid1 or a soluble ,sulphate1 be suspected in any liquid1 it is tested for by adding a few drops of dilute solution of ,chloride of ,barium1 or ,nitrate of ,baryta4 ,a white precipitate1 insoluble in ,nitric ,acid1 indicates ,sulphuric ,acid4 ,if the liquid to be tested is very acid1 from ,nitric or ,hydrochloric ,acid1 it must be largely diluted before testing1 or a crystalline precipitate will form1 caused by the sparing solubility of the ,chloride of ,barium itself in acid solutions4 ,,sulphurous ,,acid4 ,symbol1 ,,so#b4 ,atomic weight1 #cb4 ,this is a gaseous compound1 formed by burning ,sulphur in atmospheric air or ,oxygen gas3 also by heating ,oil of ,vitriol in contact with metallic ,copper1 or with ,charcoal4 ,when an acid of any kind is added to ,hyposulphite of ,soda1 ,sulphurous ,acid is formed as a product of the decomposition of ,hyposulphurous ,acid1 but it afterwards disappears from the liquid by a secondary reaction1 resulting in the production of ,trithionate and ,tetrathionate of ,soda4 ,properties4--,sulphurous ,acid possesses a peculiar and suffocating odour1 familiar to all in the fumes of burning ,sulphur4 ,it is a feeble acid1 and escapes with effervescence1 like ,carbonic ,acid1 when its salts are treated with ,oil of ,vitriol4 ,it is soluble in water4 ,,tetrathionic ,,acid4 ,symbol1 ,s#d,o#e4 ,atomic weight1 #ajd4 ,the chemistry of the ,polythionic ,acids and their salts will be found described in the ,first ,part of this ,work1 page #aeg4 ,,water4 ,symbol1 ,,ho4 ,atomic weight1 #i4 ,water is an ,oxide of ,hydrogen1 containing single atoms of each of the gases4 ,distilled water is water which has been vaporized and again condensed2 by this means it is freed from earthy and saline impurities1 which1 not being volatile1 are left in the body of the retort4 ,pure distilled water leaves no residue on evaporation1 and should remain perfectly clear on the addition of ,nitrate of ,silver1 even when exposed to the light2 it should also be neutral to test-paper4 ,the condensed water of steam-boilers sold as distilled water is apt to be contaminated with oily and empyreumatic matter1 which discolours ,nitrate of ,silver1 and is therefore injurious4 ,rain-water1 having undergone a natural process of distillation1 is free from inorganic salts1 but it usually contains a minute portion of ,ammonia1 which gives it an alkaline reaction to test-paper4 ,it is very good for ,photographic purposes if collected in clean vessels1 but when taken from a common rain-water tank should always be examined1 and if much organic matter be present1 tingeing it of a brown colour and imparting an unpleasant smell1 it must be rejected4 ,spring or ,river water1 commonly known as hard water1 usually contains ,sulphate of ,lime1 and ,carbonate of ,lime dissolved in ,carbonic ,acid2 also ,chloride of ,sodium in greater or less quantity4 ,on boiling the water1 the ,carbonic ,acid gas is evolved1 and the greater part of the ,carbonate of ,lime 7if any is present7 deposits1 forming an earthy incrustation on the boiler4 ,in testing water for ,sulphates and ,chlorides1 acidify a portion with a few drops of pure ,nitric ,acid1 free from ,chlorine 7if this is not at hand1 use pure ,acetic ,acid72 then divide it into two parts1 and add to the first a dilute solution of ,chloride of ,barium1 and to the second1 ,nitrate of ,silver1--a milkiness indicates the presence of ,sulphates in the first case or of ,chlorides in the second4 ,the ,photographic ,nitrate ,bath cannot be used as a test1 since the ,iodide of ,silver it contains is precipitated on dilution1 giving a milkiness which might be mistaken for ,chloride of ,silver4 ,common hard water can often be used for making a ,nitrate ,bath when nothing better is at hand4 ,the ,chlorides it contains are precipitated by the ,nitrate of ,silver1 leaving soluble ,nitrates in solution1 which are not injurious4 ,the ,carbonate of ,lime1 if any is present1 neutralizes free ,nitric ,acid1 rendering the ,bath alkaline in the same manner as ,carbonate of ,soda4 7,see page #hi47 ,sulphate of ,lime1 usually present in well water1 is said to exercise a retarding action upon the sensitive ,silver ,salts1 but on this point the writer is unable to give certain information4 ,hard water is not often sufficiently pure for the developing fluids4 ,the ,chloride of ,sodium it contains decomposes the ,nitrate of ,silver upon the film1 and the image cannot be brought out perfectly4 ,the ,new ,river water1 however1 supplied to many parts of ,london1 is almost free from ,chlorides1 and answers very well4 ,in other cases a few drops of ,nitrate of ,silver solution may be added1 to separate the ,chlorine1 taking care not to use a large excess4 ,,appendix4 ,,quantitative ,,testing ,,of ,,solutions ,,of ,,nitrate ,,of ,,silver4 ,the amount of ,nitrate of ,silver contained in solutions of that salt may be estimated with sufficient delicacy for ordinary ,photographic operations by the following simple process4 ,take the pure crystallized ,chloride of ,sodium1 and either dry it strongly or fuse it at a moderate heat1 in order to drive off any water which may be retained between the interstices of the crystals2 then dissolve in distilled water1 in the proportion of #h-a_/b grains to #f fluid ounces4 ,in this way1 a standard solution of salt is formed1 each drachm of which 7containing slightly more than one-sixth of a grain of salt7 will precipitate exactly half a grain of ,nitrate of ,silver4 ,in order to use it1 measure out accurately one drachm of the ,bath in a minim measure and place it in a two-ounce stoppered phial1 taking care to rinse out the measure with a drachm of distilled water1 which is to be added to the former2 then pour in the salt solution1 in the proportion of a drachm for every #d grains of ,nitrate known to be present in an ounce of the ,bath which is to be tested2 shake the contents of the bottle briskly1 until the white curds have perfectly separated1 and the supernatant liquid is clear and colourless2 then add fresh portions of the standard solution1 by #cj minims at a time1 with constant shaking4 ,when the last addition causes no milkiness1 read off the total number of drachms employed 7the last half-drachm being subtracted71 and multiply that number by #d for the weight in grains of the ,nitrate of ,silver present in an ounce of the ,bath4 ,in this manner the strength of the ,bath is indicated within two grains to the ounce1 or even to a single grain if the last additions of standard salt-solution be made in portions of #ae1 instead of #cj minims4 ,supposing the ,bath to be tested is thought to contain about #ce grains of ,nitrate to the ounce1 it will be convenient to begin by adding to the measured drachm1 #g drachms of the standard solution2 afterwards1 as the milkiness and precipitation become less marked1 the process must be carried on more cautiously1 and the bottle shaken violently for several minutes1 in order to obtain a clear solution4 ,a few drops of ,nitric ,acid added to the ,nitrate of ,silver facilitate the deposition of the ,chloride2 but care must be taken that the sample of ,nitric ,acid employed is pure and free from ,chlorine1 the presence of which would cause an error4 ,,recovery ,,of ,,silver ,,from ,,waste ,,solutions1--,,from ,,the ,,black ,,deposit ,,of ,,hypo-,,baths1 ,,etc4 ,the manner of separating metallic ,silver from waste solutions varies according to the presence or absence of alkaline ,hyposulphites and ,cyanides4 a4 ,separation of metallic ,silver from old ,nitrate ,baths4--,the ,silver contained in solutions of the ,nitrate1 ,acetate1 etc4 may easily be precipitated by suspending a strip of sheet ,copper in the liquid2 the action is completed in two or three days1 the whole of the ,nitric ,acid and ,oxygen passing to the ,copper1 and forming a blue solution of the ,nitrate of ,copper4 ,the metallic ,silver however1 separated in this manner1 always contains a portion of ,copper1 and gives a blue solution when dissolved in ,nitric ,acid4 ,a better process is to commence by precipitating the ,silver entirely in the form of ,chloride of ,silver1 by adding common ,salt until no further milkiness can be produced4 ,if the liquid is well stirred1 the ,chloride of ,silver sinks to the bottom1 and may be washed by repeatedly filling the vessel with common water1 and pouring off the upper clear portion when the clots have again settled down4 ,the ,chloride of ,silver thus formed may afterwards be reduced to metallic ,silver by a process which will presently be described 7p4 #cgd74 b4 ,separation of ,silver from solutions containing alkaline ,hyposulphites1 ,cyanides1 or ,iodides4--,in this case the ,silver cannot be precipitated by adding ,chloride of ,sodium1 since the ,chloride of ,silver is soluble in such liquids4 ,it is necessary therefore to use the ,sulphuretted ,hydrogen1 or the ,hydrosulphate of ,ammonia1 and to separate the ,silver in the form of ,sulphuret4 ,sulphuretted ,hydrogen gas is readily prepared1 by fitting a cork and flexible tubing to the neck of a pint bottle1 and having introduced ,sulphuret of ,iron 7sold by operative chemists for the purpose71 about as much as will stand in the palm of the hand1 pouring upon it #a-a_/b fluid ounce of ,oil of ,vitriol diluted with #aj ounces of water4 ,the gas is generated gradually without the application of heat1 and must be allowed to bubble up through the liquid from which the ,silver is to be separated4 ,the smell of ,sulphuretted ,hydrogen being offensive1 and highly poisonous if inhaled in a concentrated form1 the operation must be carried on in the open air1 or in a place where the fumes may escape without doing injury4 ,when the liquid begins to acquire a strong and persistent odour of ,sulphuretted ,hydrogen1 the precipitation of ,sulphuret is completed4 ,the black mass must then be collected upon a filter1 and washed by pouring water over it1 until the liquid which runs through gives little or no precipitate with a drop of ,nitrate of ,silver4 ,the ,silver may also be separated in the form of ,sulphuret from old ,hypo-,baths1 by adding ,oil of ,vitriol in quantity sufficient to decompose the ,hyposulphite of ,soda2 and burning off the free ,sulphur from the brown deposit4 ,conversion of ,sulphuret of ,silver into metallic ,silver4--,the black ,sulphuret of ,silver may be reduced to the state of metal by roasting and subsequent fusion with ,carbonate of ,soda2 but it is more convenient1 in operating on a small scale1 to proceed in the following manner3--first convert the ,sulphuret into ,nitrate of ,silver1 by boiling with ,nitric ,acid diluted with two parts of water2 when all evolution of red fumes has ceased1 the liquid may be diluted1 allowed to cool1 and filtered from the insoluble portion1 which consists principally of ,sulphur1 but also contains a mixture of ,chloride and ,sulphuret of ,silver1 unless the ,nitric ,acid employed was free from ,chlorine2 this precipitate may be heated1 in order to volatilize the ,sulphur1 and then digested with ,hyposulphite of ,soda1 or added to the ,hypo-,bath4 ,the solution of ,nitrate of ,silver obtained by dissolving ,sulphuret of ,silver1 is always strongly acid with ,nitric ,acid1 and also contains ,sulphate of ,silver4 ,it may be crystallized by evaporation2 but unless the quantity of material operated on is large1 it will be better to precipitate the ,silver in the form of ,chloride1 by adding common ,salt1 as already recommended4 ,,reduction ,,of ,,chloride ,,of ,,silver ,,to ,,the ,,metallic ,,state4 ,the ,chloride of ,silver is first to be carefully washed1 by filling up the vessel which contains it1 many times with water1 and pouring off the liquid1 or drawing it off close with a siphon4 ,it may then be dried at a gentle heat1 and fused with twice its weight of dry ,carbonate of ,potash1 or better still1 with a mixture of the ,carbonates of ,potash and ,soda4 ,the process for reducing ,chloride of ,silver in the moist way1 by metallic ,zinc and ,sulphuric ,acid1 is more economical and less troublesome than that just given2 it is conducted as follows3--,the ,chloride1 after having been well washed as before1 is placed in a large flat dish1 and a bar of metallic ,zinc laid in contact with it4 ,a small quantity of ,oil of ,vitriol1 diluted with four parts of water1 is then added1 until a slight effervescence of ,hydrogen gas is seen to take place4 ,the vessel is set aside for two or three days1 and is not to be disturbed1 either by stirring or by moving the bar4 ,the reduction begins with the ,chloride immediately in contact with the ,zinc1 and radiates in all directions4 ,when the whole mass has become of a grey colour1 the bar is to be carefully removed and the adhering ,silver washed off with a stream of water2 the ,zinc usually presents a honeycombed appearance1 with irregularities upon the surface1 which however are not metallic ,silver2--they consist only of ,zinc or of ,oxide of ,zinc4 ,in order to ensure the purity of the ,silver1 a fresh addition of ,sulphuric ,acid must be made1 after the ,zinc bar has been removed1 and the digestion continued for several hours1 in order to dissolve any fragments of metallic ,zinc which may have been inadvertently detached4 ,the grey powder must be repeatedly washed1 first with ,sulphuric ,acid and water 7this is necessary to dissolve a portion of an insoluble ,salt of ,zinc1 probably an oxychloride7 and then with water alone1 until the liquid runs away neutral1 and gives no precipitate with ,carbonate of ,soda2 it may then be fused into a button1 to burn off organic matter if present1 and subsequently converted into ,nitrate of ,silver by boiling with ,nitric ,acid diluted with two parts of water4 ,in reducing ,chloride of ,silver precipitated from old ,nitrate ,baths containing ,iodide of ,silver1 the grey metallic powder is sometimes contaminated with unreduced ,iodide of ,silver1 which dissolves in the solution of ,nitrate of ,silver formed on treating the mass with ,nitric ,acid4 ,to avoid this1 wash the purified ,silver with solution of ,hyposulphite of ,soda1 and then again with water4 ,,mode ,,of ,,taking ,,the ,,specific ,,gravity ,,of ,,liquids4 ,instruments are sold1 termed ,hydrometers1 which indicate specific gravity by the extent to which a glass bulb containing air1 and properly balanced1 rises or sinks1 in the liquid2 but a more exact process1 and one equally simple1 is by the use of the specific gravity bottle4 ,these bottles are made to contain exactly #ajjj grains of distilled water1 and with each is sold a brass weight1 which counterbalances it when filled with pure water4 ,in taking the specific gravity of a liquid1 fill the bottle quite full and insert the stopper1 which being pierced through by a fine capillary tube allows the excess to escape4 ,then1 having wiped the bottle quite dry1 place it in the scale-pan1 and ascertain the number of grains required to produce equilibrium2 this number added to1 or subtracted from1 unity 7the assumed specific gravity of water71 will give the density of the liquid4 ,thus1 to take examples1 supposing the bottle filled with rectified ,ether to require #bej grains to enable it to counterbalance the brass weight1--then #a minus #bej1 or #gej1 is the specific gravity2 but in the case of ,oil of ,vitriol the bottle1 when full1 will be heavier than the counterpoise by perhaps #hcf grains2 therefore #a plus #hcf1 id est #ahcf1 is the density of the sample examined4 ,sometimes the bottle is made to hold only #ejj grains of distilled water1 in place of #ajjj2 in this case the number of grains to be added or subtracted must be multiplied by #b4 ,in taking specific gravities1 observe that the temperature be within a few degrees of #fj ,fahrenheit 7if higher or lower1 immerse the bottle in warm or cold water72 and wash out the bottle thoroughly with water each time after use4 ,,on ,,filtration ,,and ,,washing ,,precipitates4 ,in preparing filters1 cut the paper into squares of a sufficient size1 and fold each square neatly upon itself1 first into a half-square1 and then again1 at right angles1 into a quarter-square2--round off the corners with a pair of scissors1 and open out the filter into a conical form1 when it will be found to drop exactly into the funnel1 and to be uniformly supported throughout4 ,before pouring in the liquid1 always moisten the filter with distilled water1 in order to expand the fibres2 if this precaution be neglected1 the pores are apt to become choked in filtering liquids which contain finely divided matter in suspension4 ,the solution to be filtered may be poured gently down a glass rod1 held in the left hand 7a silver spoon may be used1 in case of necessity1 for ,nitrate ,baths1 and all liquids not containing ,nitric or ,hydrochloric ,acid71 and directed against the side of the funnel1 near to the upper part4 ,if it does not immediately run clear1 it will usually do so on returning it into the filter and allowing it to pass through a second time4 ,mode of ,washing ,precipitates4--,collect the precipitate upon a filter and drain off as much of the mother-liquor as possible2 then pour in distilled water by small portions at a time1 allowing each to percolate through the deposit before adding a fresh quantity4 ,when the water passes through perfectly pure1 the washing is complete2 in testing it1 a single drop may be laid upon a strip of glass and allowed to evaporate spontaneously in a warm place1 or the proper chemical reagents may be applied1 and the washing continued until no impurity can be detected4 ,thus1 for example1 in washing the ,sulphuret of ,silver precipitated from a ,hypo-,bath by means of ,hydrosulphate of ,ammonia1 the process will be completed when the water which runs through causes no deposit with a drop of ,nitrate of ,silver solution4 ,,on ,,the ,,use ,,of ,,test-,,papers4 ,the nature of the colouring matter which is employed in the preparation of litmus-paper has already been described at page #cec4 ,in testing for the alkalies and basic oxides generally1 the blue litmus-paper which has been reddened by an acid may be used1 or1 in place of it1 the turmeric-paper4 ,turmeric is a yellow vegetable substance which possesses the property of becoming brown when treated with an alkali2 it is however less sensitive than the reddened litmus1 and is scarcely affected by the weaker bases1 such as ,oxide of ,silver4 ,in using test-papers1 observe the following precautions3--they should be kept in a dark place1 and protected from the action of the air1 or they soon become purple from ,carbonic ,acid1 always present in the atmosphere in small quantity4 ,by immersion in water containing about one drop of ,liquor ,potass or ,ammoni1 or a grain of ,carbonate of ,soda to four ounces1 the blue colour is restored4 ,as the quantities which are tested for in ,photography are often infinitesimally small1 it is essential that the litmus-paper should be in good condition2 and test-papers prepared with porous paper will be found to show the colour better than those upon glazed or strongly-sized paper4 ,the mode of employing the paper is as follows3--,place a small strip in the liquid to be examined3 if it becomes at once bright red1 a strong acid is present2 but if it changes slowly to a wine-red tint1 a weak acid1 such as ,acetic or ,carbonic1 is indicated4 ,in the case of the ,photographic ,nitrate ,bath faintly acidified with ,acetic ,acid1 a purple colour only may be expected1 and a decided red colour would suggest the presence of ,nitric ,acid4 ,in the ,hypo fixing and toning ,bath which has acquired acidity1 the litmus-paper will perhaps redden in about three or four minutes4 ,blue litmus-papers may be changed to the red papers used for alkalies by soaking in water acidified with ,sulphuric ,acid1 one drop to half a pint2 or by holding for an instant near the mouth of a bottle containing ,glacial ,acetic ,acid4 ,in examining a ,nitrate ,bath for alkalinity by means of the reddened litmus-paper1 at least five or ten minutes should be allowed for the action1 since the change of colour from red to blue takes place very slowly4 ,,removal ,,of ,,silver ,,stains ,,from ,,the ,,hands1 ,,linen1 ,,etc4 ,the black stains upon the hands caused by ,nitrate of ,silver1 may readily be removed by moistening them and rubbing with a lump of ,cyanide of ,potassium4 ,as this salt however is highly poisonous1 many may prefer the following plan3--,wet the spot with a saturated solution of ,iodide of ,potassium1 and afterwards with ,nitric ,acid 7the strong ,nitric ,acid acts upon the skin and turns it yellow1 it must therefore be diluted with two parts of water before use72 then wash with solution of ,hyposulphite of ,soda4 ,stains upon white linen may be easily removed by brushing them with a solution of ,iodine in ,iodide of ,potassium1 and afterwards washing with water and soaking in ,hyposulphite of ,soda1 or ,cyanide of ,potassium1 until the yellow ,iodide of ,silver is dissolved out2 the ,bichloride of ,mercury 7neutral solution7 also answers well in many cases1 changing the dark spot to white 7p4 #aea74 ,a ,,table ,,showing ,,the ,,quantity ,,of ,,anhydrous ,,acid ,,in ,,dilute ,,sulphuric ,,acid ,,of ,,different ,,specific ,,gravities4 7,,ure47 ---------------------------------------- ----------------------- ,real ,acid ,real ,acid ,real ,acid ,specific in #ajj ,specific in #ajj ,specific in #ajj ,gravity4 parts of the,gravity4 parts of the,gravity4 parts of the ,liquid4 ,liquid4 ,liquid4 ---------------------------------------- ----------------------- #ahdhe #haed #ahaae #gcci #agabj #febc #ahdge #hjgb #ahjdc #gbeg #afiic #fddb #ahdfj #giij #agifb #gage #afhgj #fcfj #ahdci #giji #aghgj #gjid #afgej #fbgh #ahdaj #ghbh #agggd #gjab #affcj #faig #ahcgf #ggdf #agfgc #fica #afebj #faae #ahccf #gffe #agegj #fhdi #afdae #fjcd #ahbij #gehc #agdfe #fgfh #afcba #eieb #ahbcc #gejb #agcfj #ffhf #afbjd #ehga #ahagi #gdbj #agbde #ffje #afjij #eghi ---------------------------------------- ----------------------- ,a ,,table ,,showing ,,the ,,quantity ,,of ,,anhydrous ,,acid ,,in ,,the ,,liquid ,,nitric ,,acid ,,of ,,different ,,specific ,,gravities4 7,,ure47 ---------------------------------------- ----------------------- ,real ,acid ,real ,acid ,real ,acid ,specific in #ajj ,specific in #ajj ,specific in #ajj ,gravity4 parts of the,gravity4 parts of the,gravity4 parts of the ,liquid4 ,liquid4 ,liquid4 ---------------------------------------- ----------------------- #aejjj #gigjj #adfdj #ficci #adadg #ehigh #adihj #ghijc #adfjj #fhedb #adajg #ehaha #adifj #ghajf #adegj #fggde #adjfe #egchd #adidj #ggcji #adecj #ffidh #adjbc #efehg #adiaj #gfeab #adejj #ffaee #acigh #eegij #adhhj #gegae #addfj #feced #acide #ediic #adhej #gdiah #addbd #fdeeg #achhb #edaif #adhbj #gdaba #adche #fcgfj #achcc #eccii #adgij #gccbd #adcdf #fbifc #acghc #ebfjb #adgfj #gbebg #adcjf #fbaff #acgcb #eahje #adgcj #gagcj #adbfi #facfi #acfha #eajfh #adgjj #gjicc #adbbh #fjegb #acfcj #ejbaa #adfgj #gjacf #adahi #eigge #acegi #didad ---------------------------------------- ----------------------- ,,weights ,,and ,,measures4 ,troy1 or ,apothecaries' ,weight4 #a ,pound #ab ,ounces4 #a ,ounce #h ,drachms4 #a ,drachm #c ,scruples4 #a ,scruple #bj ,grains4 7#a ,ounce ,troy #dhj ,grains1 or #a ,ounce ,avoirdupois plus #dbe grains47 ,avoirdupois ,weight4 #a ,pound #af ,ounces4 #a ,ounce #af ,drachms4 #a ,drachm #bgcdc grains4 7#a ,ounce ,avoirdupois #dcge grains47 7#a ,pound ,avoirdupois #gjjj ,grains1 or #a ,pound ,troy plus #b-a_/b ,troy ,ounces plus #dj grains47 ,imperial ,measure4 #a ,gallon #h ,pints4 #a ,pint #bj ,ounces4 #a ,ounce #h ,drachms4 #a ,drachm #fj ,minims4 7,a ,wine ,pint of water measures #af ,ounces1 and weighs a ,pound47 ,an ,imperial ,gallon of water weighs #aj ,pounds ,avoirdupois1 or #gj1jjj ,grains4 ,an ,imperial ,pint of water weighs #a-a_/d ,pound ,avoirdupois4 ,a fluid ,ounce of water weighs #a ,ounce ,avoirdupois1 or #dcge ,grains4 ,a ,drachm of water weighs #edg ,grains4 ,french ,measures of ,weight4 #a ,kilogramme #ajjj ,grammes something less than #b-a_/d ,pounds ,avoirdupois4 #a ,gramme #aj ,decigrammes--#ajj ,centigrammes #ajjj ,milligrammes #aedcc ,english ,grains4 ,a ,gramme of water measures #ag ,english ,minims1 nearly4 #ajjj ,grammes of water measure #ce-a_/d ,english fluid ,ounces4 ,french ,measures of ,volume4 #a ,litre #ac ,decilitres #ajj ,centilitres #ajjj ,millilitres #ce-a_/d ,english fluid ,ounces4 #a ,litre #a ,cubic ,decimetre #ajjj ,cubic ,centimetres4 #a ,cubic ,centimetre #ag ,english ,minims4 ,a ,litre of water weighs a ,kilogramme1 or something less than #b-a_/d ,pounds ,avoirdupois4 ,a ,cubic ,centimetre of water weighs a ,gramme4 ,,index4#ef #ef ,the preparation and properties of the ,chemicals used in ,photography will be found in the ,alphabetical ,list commencing at page #cbg4 ,aberration1 chromatic1 #ed2 spherical1 #ef4 ,accelerating agents1 their mode of action in ,collodion explained1 #ie4 ,acetate of ,silver1 its preparation and formula1 #cfe2 its formation in ,nitrate ,bath explained1 #hi2 ensures absence of free ,nitric ,acid1 #aaf2 tends slightly to favour fogging and spots1 #ajd2 contra-indicated for glass ,positives1 #aaa4 ,acetic ,acid1 properties and mode of testing purity of1 #cbg2 useful in preventing fogging1 #ajd2 in rendering the development slow and even1 #ii2 does not coagulate ,albumen1 #cbi2 a good commercial form of acid1 #bab2 ,acetic ,acid essential in ,calotype1 waxed paper1 and ,albumen processes1 #agg2 also in printing paper ,positives by development1 #bfj4 ,aceto-,nitrate of ,silver1 term explained1 #agg4 ,achromatic ,lenses1 their construction explained1 #ee2 the visual and chemical foci often coincident in1 #fj4 ,acids1 nature of1 #cjh4 ,actinism1 explained1 #fa2 importance of distinguishing ,actinic from visual rays1 #fb2 mode of finding ,actinic focus1 #bbi4 ,affinity1 chemical1 #cab4 ,albumen1 its chemistry1 #cbh2 forms a compound with oxide of ,silver1 #bj2 used in ,positive printing to produce a fine surface layer1 #abb2 to increase sensitiveness1 #abe2 affects the colour of the prints1 #abg2 protects the image from oxidation1 #aej2 putrifies when exposed to moisture1 #aee2 discolours the ,nitrate ,bath1 #bde4 ,albumen negative process1 its invention1 #aj2 theory of1 #ahj2 ,collodio-,albumen process of ,m4 ,taupenot1 #bid4 ,albuminized paper1 formula for1 #bda2 slow in fixing1 #aca2 not well adapted for toning by ,sel d'or1 #bfi2 good for stereoscopic subjects and small portraits1 #bdi2 cannot be sensitized with ,ammonio-,nitrate of ,silver1 #bdf4 ,alcohol1 its chemistry1 #ccj2 sometimes too dilute for making ,collodion1 #hd2 mode of rectifying1 #aif2 must not contain impurities1 #if2 effects of adding to ,collodion1 #hd1 #if2 to developer1 #bje4 ,alkalies1 nature of1 #cjh4 ,alkalinity of ,nitrate ,bath1 explained1 #hh2 the evils it produces1 #ajd2 how to test for it1 #cgg2 how to remove it1 #bgg4 ,amber varnish1 #bbf4 ,ammonia1 preparation and properties1 #cca2 its use in fixing1 #db2 ,mr4 ,shadbolt's formula for1 #bga2 its action upon ,chloride of ,gold1 #cdc2 effect of concentrated ,ammonia upon ,oxide of ,silver1 #cfb4 ,ammonio-,nitrate of ,silver1 its chemistry1 #bfb2 used in ,positive printing to increase sensitiveness1 #abe2 to give black tones1 #abg2 cannot be used with ,albumen1 #bdf2 increases permanency of print1 #afi2 old ,nitrate ,baths not easily convertible into ,ammonio-,nitrate1 #bdh2 mode of preparing1 #bdg2 best applied to the paper by brush or rod1 #bdh2 ,oxide of ,silver in ,nitrate of ,ammonia1 a useful substitute for it1 #bdi4 ,ammonio-,nitrate paper1 formula for1 #bdf2 a more simple formula1 but less sensitive than the last1 #beh4 ,atomic theory explained1 #cbb4 ,bath for fixing and toning ,positives4 ,see ,fixing and toning ,bath4 ,bichloride of ,mercury1 whitening action on glass ,positives explained1 #aac2 solution for1 #bjg2 used to intensify ,negatives1 #aah2 bleaches paper prints1 #aea2 should not be added to paste used in mounting prints1 #afd2 removes ,silver stains1 #cgg4 ,binocular vision1 phenomena of1 explained1 #ff4 ,blackening ,negatives1 #cg1 #aag4 ,black tones1 mode of obtaining1 in paper ,positives1 #afh1 #bdf4 ,bromide of ,silver1 its preparation and properties1 #ag2 its superior sensibility to coloured light1 #fc2 less acted on by white light than ,chloride1 #ai2 less sensitive to invisible image than ,iodide1 #be2 employment in ,collodion1 #aja2 found useful in ,photographing by artificial light1 #ff2 diagram of chemical spectrum on1 #fd4 ,bromo-,iodide of ,silver1 #agc4 ,brushes1 mode of applying ,silver solutions by1 #bdh4 ,calotype process1 theory of1 #agf4 ,camera1 its first invention1 #g2 theory of its construction1 #ed2 mode of testing accuracy of1 #bbi2 cause of the image being inverted1 #ec2 the term flatness of field explained1 #ed2 best position of the ,camera for portraits1 #bbj2 for architectural subjects1 #bca2 a funnel-shaped tube placed in front of the lens1 #bbi2 stereoscopic ,camera1 #bcd2 microscope ,camera1 #bcf4 ,causes of failure in ,collodion process1 #bgf4 ,chemical affinity1 illustrations of1 #cab4 ,chemical elements1 #cjf4 ,chemical focus1 directions for finding1 #bbi2 shorter than visual in non achromatic lenses1 #fj2 longer than visual in microscopic objectives1 #bcg2 varies slightly with the nature of the light1 #bch4 ,chemical spectrum1 #fa4 ,chemicals1 ,photographic1 ,vocabulary of1 #cbg4 ,chloride of ,silver1 its preparation and properties1 #ad2 more sensitive to white light than ,bromide or ,iodide1 #ai2 less sensitive to invisible image1 #bd2 its blackening by light explained1 #bj1 #ada2 accelerated by excess of ,nitrate1 #ai2 by organic matter1 #bj1 #adb2 experiments illustrating darkening of papers prepared with1 #ba2 simple explanation of the mode of preparing sensitive papers with1 #bb2 agents which dissolve it1 #db2 mode of reducing it to metallic state4 #cgd4 ,chloride of ,gold1 its preparation and properties1 #cdb2 action of ,ammonia upon it1 #cdc2 use of an alkaline solution of1 for toning1 #acb1 #bga2 compounds formed on adding it to ,hyposulphite of ,soda1 #acc2 mode of preparing the fixing and toning ,bath with1 #bej2 the ,sel d'or ,bath with1 #bfg4 ,chromatic aberration1 #ed4 ,citric ,acid1 forms a red compound with ,suboxide of ,silver1 #ba1 #cch2 used in printing to give purple tones1 #abh2 formula for preparing paper with1 #bdf4 ,cleaning glass plates1 theory of1 #ci2 details of1 #bac4 ,collodion1 its discovery1 #aj2 chemistry of ,pyroxyline1 #ge2 physical effect of ,ether and ,alcohol in1 #hc2 of water in1 #he2 glutinosity of1 #hc2 coloration of iodized1 explained1 #he2 sensitiveness and intensity affected by the change1 #ig1 #ii2 details of manufacture of ,collodion1 #ahe2 ,positive ,collodion1 theory of1 #ajh2 formula for1 #bja2 ,negative ,collodion1 theory of1 #aac2 formula for1 #bjh2 ,collodion for copying engravings1 #bca2 for keeping processes1 #bih2 for hot climates1 #baj2 for working by artificial light1 #bch2 to remove the brown colour from ,collodion1 #hf4 ,collodion film1 the proper time for immersing it in the ,bath1 #bai2 a thin film often good for direct ,positives1 #aji2 a thicker film for ,negatives1 #aac2 cause of the film falling away from the glass1 #hc1 #bic2 spots and markings on1 #bha2 conditions which affect its sensitiveness to light1 #ib2 causes influencing its behaviour with the developer1 #ih2 mode of preserving sensitiveness of film1 #bhi4 ,collodio-,albumen process1 theory of1 #aha2 practical details of1 #bid4 ,colours1 their nature explained1 #dg2 their chemical action on sensitive film1 #fd2 their photographic action assisted by reflection of white light1 #ff4 ,combination1 laws of1 #cjg4 ,conjugate foci1 explained1 #eb1 #bgb4 ,crookes1 ,mr41 remarks upon chemical spectrum1 #fc2 upon waxed paper process1 #ahj2 preservative process for ,collodion films1 #bhi4 ,curvature of luminous image formed by lens1 explained1 #ec4 ,cyanide of ,potassium1 its fixing action explained1 #dd2 preparation of solution of1 #bjg2 used to remove stains1 #cgg4 ,daguerreotype1 its invention1 #h2 theory of the process1 #aga4 ,development of invisible image1 explanation of1 #cd-dj2 second1 or intensifying stage explained1 #cg2 details of developing glass ,positives and ,negatives1 #bba-bbc2 development of paper ,positives1 #bei2 conditions which increase or diminish rapidity of development1 #ih2 irregularities of development1 #ajc4 ,developers1 their preparation and properties1 #bf2 comparative strength of1 #ih2 theory of1 for ,positives1 #aaa2 for ,negatives1 #aag2 formul for ,positive developers1 #bje2 for ,negative1 #baa4 ,diagrams1 mode of copying1 #bcb4 ,diaphragms for lenses4 ,see ,stops4 ,double decomposition1 illustrated1 #ad2 explained1 #cad4 ,dry ,collodion process1 #bih4 ,elementary bodies1 table of1 #cjf2 combination of1 #cjg4 ,engravings1 mode of copying1 #bca2 often yield dark-coloured prints1 #bee4 ,equivalent proportions1 #cbj4 ,ether1 properties of1 #cci2 purification of1 for ,photography1 #aie2 must be kept in a dark place1 #aif2 should not be distilled from residues of old ,collodion1 #if4 ,experiments1 illustrating action of ,light upon ,chloride of ,silver1 #ba2 illustrating formation and development of invisible images1 #be2 illustrating photographic action of coloured light1 #fb4 ,exposure in the ,camera1 rules for ,positives1 #bba2 for ,negatives1 #bbe2 for preserved ,collodion plates1 #bib2 for microscopic photographs1 #bch2 effects of under and over-exposing1 #ce2 exposure required in ,calotype process1 #agg2 in waxed paper1 #ahj2 in ,albumen negative process1 #aha2 in dry ,collodion process1 #cja2 in ,taupenot's process1 #big4 ,fading of ,positives1 explained at length1 #afj2 ,author's researches on1 #aec4 ,film1 sensitive4 ,see ,collodion film4 ,filters1 mode of cutting1 #cgf4 ,fixing1 theory of1 #da2 of paper prints explained1 #abh2 solution for fixing glass ,positives and ,negatives1 #bab2 manipulatory details of fixing1 #bbe2 fixing paper ,positives with ,ammonia1 #bga4 ,fixing and toning ,bath1 its preparation1 #bej2 conditions which favour or retard its action1 #ace2 certain states of the ,bath injurious to the proofs1 #acf2 importance of keeping it in an active condition1 #afh2 must not be employed immediately after mixing1 #bea2 must not be allowed to become acid by constant use1 #afh2 theory of the gradual change of properties it undergoes1 #aef4 ,foci1 actinic and luminous1 #fj2 actinic1 mode of finding1 #bbi2 variation between them in microscopic objectives1 #bcg4 ,focussing the object1 #bbj4 ,fogging1 theory of1 #ajc2 mode of detecting causes of1 #bgf4 ,formul for solutions required in ,collodion process1 #bja2 for papers used in ,positive printing1 #bda2 want of correspondence between1 #beg4 ,gallic ,acid1 its preparation and properties1 #bg2 used in paper processes1 #agh2 becomes mouldy by keeping1 #bfa4 formula for developing paper ,positives with1 #bfa4 ,gallo-,nitrate of ,silver1 #agg2 discolours rapidly when developing dishes are not clean1 #agi4 ,gelatine1 its properties1 #cda2 forms a compound with an ,oxide of ,silver1 #ba2 employed in dry ,collodion process1 #bii2 modified form of1 #cjb2 affects the colour in printing processes1 #abh2 used in ,positive printing to form an even surface layer1 #abf2 as a cement to mount ,photographs1 #beg4 ,glass plates1 rules for cleaning1 #ci2 details of cleaning1 #bac2 mode of coating with ,collodion1 #bae2 with ,albumen1 #ahj4 ,glutinous ,collodion1 explained1 #hc4 ,glycyrrhizine1 its nature1 #cdb2 its action in ,collodion1 #aad2 formula for solution of1 #bji4 ,gold1 ,chloride of4 ,see ,chloride of ,gold4 ,gold salts1 their use in ,photographic printing explained1 #aca2 in the ,daguerreotype process1 #age4 ,gradation of tone1 in ,collodion ,photographs1 affected by the density of the film1 #aji1 #aac2 by use of ,glycyrrhizine1 #aae4 ,gradation of tone1 in paper ,positives1 conditions affecting it in prints obtained by direct exposure1 #abc2 in ,positives printed by development1 #bff4 ,hadow1 ,mr41 researches on ,collodion1 #gg2 formula for making ,pyroxyline1 #ahg4 ,heliography1 invented by ,m4 ,nipce1 #g4 ,historical sketch of ,photography1 #f4 ,honey keeping process1 #bhi4 ,hunt1 ,mr41 introduces ,protosalts of ,iron in developing1 #aaa4 ,hypo ,bath4 ,see ,fixing and ,toning ,bath4 ,hyposulphite of ,silver1 its peculiar changes in colour1 #abi2 the sweet compound which it forms with ,hyposulphite of ,soda1 #dd4 ,hyposulphite of ,soda1 preparation and properties1 #dc2 theory of its fixing action1 #dc2 blackens ,nitrate of ,silver1 #abi2 causes a milkiness with acids1 #acg2 its decomposition by constant use in fixing1 #ach2 the salts it forms with ,chloride of ,gold1 #acc2 its conversion into sulphuretting ,tetrathionate by ,iodine and ,perchloride of ,iron1 #aci2 test for presence of1 #afi4 ,iceland moss1 its use in ,positive printing1 #abh2 formula for preparing paper with1 #bde4 ,imperfections in ,collodion ,negatives1 #bhb2 in ,positives1 #bhd2 in paper ,positives1 #bhe4 ,intensity1 explanation of term1 #ib2 mode of increasing in ,negatives1 #ii1 #aad2 effect of ,acetate of ,silver upon1 #aaf2 of ,nitrite of ,silver upon1 #ajb2 mode of diminishing1 in glass ,positives1 #aji1 #aaj2 conditions affecting intensity in paper ,positives1 #abc2 in developed paper ,positives1 #bff4 ,invisible images1 theory of formation of1 #cd2 development of1 #cf2 experiments illustrating1 #be4 ,iodate1 how formed in ,collodion film1 #id2 produces insensitiveness1 #aih4 ,iodide of ,ammonium1 preparation of1 #aih2 not fitted for iodizing ,collodion required to be kept long1 #baj4 ,iodide of ,iron1 an accelerator to ,collodion1 #aaf4 ,iodide of ,potassium and ,silver1 properties of1 #db2 mode of iodizing ,calotype papers by1 #agg4 ,iodide of ,potassium1 tests of purity of1 #aig2 extent of solubility in ,alcohol1 #cea2 dissolves ,iodide of ,silver1 #db4 ,iodide of ,silver1 its preparation and properties1 #af2 unaffected by direct action of light1 #ai2 highly sensitive to invisible image1 #bd2 hypothesis of formation of latent image on1 #cd2 possibility of its reduction by ,pyrogallic ,acid shown1 #cc2 excess of ,nitrate of ,silver essential to its blackening by developer1 #cf2 diagrams of chemical spectrum on1 #fa1 #fd2 fixing agents for1 #db2 its solubility in the ,nitrate ,bath1 #hf2 retards the action of ,hypo fixing and toning ,bath1 #acf2 superior permanency of developed prints on1 #afg2 details of ,negative printing process on1 #bfc4 ,iodine1 in ,collodion1 diminishes sensitiveness1 #id2 forms ,nitric ,acid and ,iodate in the ,bath1 #id2 often useful in ,positive ,collodion1 #aaj2 in ,negative ,collodion1 if fogging occurs1 #aje2 mode of removing from ,collodion1 #hf4 ,iodized ,collodion4 ,see ,collodion4 ,kaolin1 properties of1 #cce2 used to decolorize ,nitrate ,baths1 #ia2 importance of purifying it before use1 #bde4 ,landscape ,photography1 with preserved ,collodion plates1 #bhh4 ,latent image4 ,see ,invisible image4 ,laws of substitution explained1 #gh4 ,le ,grey1 ,m11 his toning process with ,chloride of ,gold1 #acb2 his waxed paper ,negative process1 #agh4 ,lenses1 various forms of1 #ea2 foci of1 #eb2 formation of images by1 #ec2 use of stops1 #eh2 portrait1 #ei2 chemical foci of1 #fj2 chromatic aberration of1 #ed2 spherical aberration of1 #ef2 simple directions for using lenses1 #bbg2 for finding chemical focus1 #bbi4 ,light2 its action upon ,silver ,salts1 #ai2 experiments illustrating1 #ba2 formation of invisible images by1 #bd2 its alternating action upon ,daguerreotype plate1 #ci2 its compound nature1 #df2 photographic action of coloured light1 #fj2 refraction of light1 #di4 ,llewellyn1 ,mr41 his ,oxymel process1 #bia4 ,manipulations of ,collodion process1 #bac2 of ,photographic printing1 #bea2 of toning by ,sel d'or1 #bfg4 ,manuscripts1 mode of copying1 #bca4 ,markings on ,collodion ,pictures1 #bha4 ,measures and ,weights1 #cgi4 ,microscopic ,photography1 #bce4 ,moser1 ,m4 ,ludwig1 his researches on the development of invisible images1 #cg4 ,mounting ,positive ,prints1 substances which should be avoided in1 #aee1 #afd2 details of1 #beg4 ,negative processes for printing ,positives1 #bei1 #bfc4 ,negatives1 definition of1 #ajf2 ,collodion ,negatives1 theory of production of1 #aac2 ,calotype1 #agf2 waxed paper1 #agh2 ,albumen1 #ahj2 ,collodio-,albumen 7,taupenot71 #aha2 mode of developing ,collodion ,negatives1 #cg1 #aag1 #bbc2 of converting ,positives into1 #aag2 formula for solutions for ,negatives1 #bjh2 the ,collodion best adapted for ,negatives1 #aad2 spots and markings upon ,negatives1 #bhb2 decomposition of ,pyroxyline a cause of fading of1 #aff4 ,nitrate of ,silver1 preparation and properties of1 #ab2 preparation of from standard coin of realm1 #cfb2 often contains free ,nitric ,acid1 #ac2 when very strongly fused1 contains ,nitrite 7see ,nitrite of ,silver71 #ad2 not acted on by light1 #ah2 its reduction by ,pyrogallic ,acid explained1 #ca2 the melted ,nitrate more certain in its action1 #ac1 #aja2 its presence essential in developing the image1 #cf1 #ih2 increases sensitiveness of ,collodion plate1 #ib2 dissolves ,iodide of ,silver1 #hf2 discoloured by ,albumen1 #bde1 #cbi2 forms a compound with ,honey1 #bhi2 with various other organic bodies1 #ba2 very little acted on by ,glycerine1 #cdb2 mode of recovering the ,silver from1 #cgb4 ,nitrate ,bath1 mode of preparing for glass ,positives1 #aaj1 #bjc2 for ,collodion ,negatives1 #aaf1 #baa2 its power of dissolving ,iodide of ,silver1 its occasional acidity and alkalinity explained1 #hf2 the mode in which ,acetate of ,silver may be formed in it1 #hi2 a list of the substances by which it is decomposed1 #ij2 changes by use1 #ia2 effect of these changes on sensitiveness1 #ig2 on intensity1 #ajb2 care required to prevent it from yielding foggy pictures1 #ajd2 a caution against the too frequent addition of alkali1 #bjd2 quantitative testing of the ,bath1 #cga4 ,nitrite of ,silver1 adds to rapidity of development1 #ajb2 tends slightly to produce fogging1 #ajd2 solarizes the high lights1 #aaa4 ,nitric ,acid1 its preparation and properties1 #cee2 its oxidizing powers1 #ab2 impairs sensitiveness of ,collodion film1 #ic2 lessens rapidity of development1 #ih2 tends to prevent fogging1 #ajd2 sometimes usefully employed for glass ,positives1 #aaj2 contra-indicated for ,negatives1 #aaf2 its accumulation in the ,nitrate ,bath explained1 #id2 mode of removing it1 #ij2 cannot exist in contact with ,acetate of ,silver1 #aaf2 produces stains on cloth1 #bae2 mode of determining the strength of ,commercial ,nitric ,acid1 #ahf2 table of strength of ,nitric ,acid of different densities1 #cgh4 ,nitro-,sulphuric ,acid1 explained1 #gg2 process for making by mixed acids1 #ahf2 by ,oil of ,vitriol and ,nitre1 #aij2 should not be used cold1 #hc4 ,nomenclature1 chemical1 #cae4 ,norris1 ,dr41 his dry ,collodion process1 #bih4 ,notation1 chemical1 #cah4 ,organic bodies1 chemistry of1 #cbd4 ,oxide of ,silver1 preparation and properties1 #ag2 dissolves in the ,nitrate ,bath1 rendering it alkaline1 #hh2 properties of its solution in ,ammonia1 #cfb2 preparation of ditto1 #bdg2 its solution in ,nitrate of ,ammonia used in ,photography1 #bdi4 ,oxymel1 keeping process1 #bia2 preparation of ,oxymel1 #cfj4 ,paper1 ,photographic1 selection of1 #bdj2 peculiarity of ,english papers1 #bda4 ,paper1 sensitive1 for printing4 ,see ,sensitive ,paper4 ,perchloride of ,iron1 preparation of toning ,bath with1 #afj4 ,permanence of ,positives1 mode of testing1 #afi4 ,photographic image1 chemical composition of1 #adj2 action of destructive tests on1 #ade4 ,photographic properties of ,salts of ,silver1 #ah2 of ,iodide of ,silver upon ,collodion1 #gd4 ,photographic researches by the ,author1 #adj4 ,photography1 historical sketch of1 #f2 the term explained1 #fa4 ,portrait lenses1 theory of their construction1 #ei2 rules for their use1 #bbg2 mode of finding chemical focus1 #bbi4 ,portraits1 drapery for1 #ff2 directions for taking1 #bbj2 the position of the ,camera1 and other points of importance1 #bbh2 the time of exposure1 #bba4 ,positive printing1 on ,albuminized paper1 formul for1 #bda2 on plain paper1 formul for1 #bde2 on ,ammonio-,nitrate paper1 formula for1 #bdf1 #beh2 by development1 formul for1 #bei2 manipulatory details of printing1 fixing1 toning1 washing1 and mounting1 #bea2 process of toning by ,sel d'or1 theory of1 #acd2 practice of1 #bfg2 reasons for the want of correspondence between different formul1 #beg2 use of ,chloride of ,gold in toning1 #acb1 #bga2 theory of the preparation of the sensitive paper for ,positives1 #abb2 theory of the process of fixing1 #abi2 of toning by ,gold1 #acb2 the ,author's researches1 #adj2 rationale of the printing process1 #abj2 composition of the image1 #adj2 fading of ,positive prints1 #afj2 destructive action of ,sulphur on1 #ade2 of oxidizing agents on1 #adh2 of ,chlorine1 acids1 boiling water1 etc41 on1 #aea2 of combustion of coal-gas on1 #aec2 effect of damp air on1 #aec2 theory of mode of washing ,positives1 #afb2 comparative permanency of prints1 #aff2 mode of testing permanency1 #afi4 ,positives1 definition of1 #ajf2 ,collodion ,positives1 theory of production of1 #ajh2 formul for solutions for1 #bja2 development of1 #aaa1 #bba2 ,collodion and ,nitrate ,bath best adapted for1 #aji2 mode of whitening by ,bichloride of ,mercury1 #aab2 solution for whitening1 #bjg2 mode of backing up1 #bbf2 spots and markings on1 #bhd2 mode of printing ,positives on ,collodion1 #bgb4 ,positives1 enlarged1 mode of printing1 #bgb4 ,practice of ,collodion process1 #ahc4 ,preservative processes for ,collodion plates1 #bhi4 ,printing1 ,photographic1 theory of1 #abj2 practical details of1 #bdj4 ,prism1 refraction of light by1 #ea2 diagram of formation of spectrum by1 #dg2 explained1 #ed4 ,prismatic spectrum1 #dg1 #fa4 ,protonitrate of ,iron1 preparation of1 #bjf2 a feeble developer when free from excess of ,sulphate of ,iron1 #ih2 theory of its mode of action1 and rules for its use1 #aab2 cannot be prepared in quantity by adding ,nitrate of ,potash to ,sulphate of ,iron1 #cad2 sometimes requires the addition of ,nitrate of ,silver1 #bjf4 ,protosulphate of ,iron1 its preparation and properties1 #bi2 its characteristics as a developer for ,collodion ,positives1 #aaa2 not well adapted for developing ,collodion ,negatives1 #aag2 preparation of the solution for ,positives1 #bje2 mode of applying it to the plate1 #bba2 to remove iron stains on glass1 #bae4 ,pyrogallic ,acid1 its preparation and properties1 #bh2 solution for developing glass ,positives1 #bje2 for ,negatives1 #baa2 cannot be used without ,acetic ,acid1 #aje2 less adapted for developing paper pictures1 #agh2 requires addition of ,nitric ,acid when used for ,positives1 #aaa2 superior to ,sulphate of ,iron for developing ,negatives1 #aag1 #add2 mode of obviating the brown discoloration of developing solutions1 #bab4 ,pyroxyline1 its nature and properties1 #ge2 preparation of1 by ,mr4 ,hadow's formula1 #ahf2 by a rule-of-thumb mixture of the acids1 #ahh2 by the ,oil of ,vitriol and ,nitre process1 #aij2 details of immersing1 washing1 and drying1 #aia2 the glutinous variety produced by cold acids1 #hc2 recapitulation of the effects of varying the strength of the acid mixture1 #aic2 spontaneous decomposition of ,pyroxyline1 #aff4 ,reduction of metallic oxides by developers1 theory of1 #bf2 of ,silver salts by developers1 theory of1 #cj2 practical details of reducing ,silver compounds to metallic state1 #cgb4 ,salts nature of1 #caj4 ,salts of ,silver1 their preparation and properties1 #ab2 their ,photographic action1 #ah2 theory of their reduction by developer1 #cj2 directions for obtaining metal from1 #cgb4 ,sel d'or1 toning process by1 its theory1 #acd2 its practical details1 #bfg2 its advantages1 #bga2 gives permanent prints1 #afg4 ,sensitiveness1 term explained1 #ib2 conditions favourable to1 #ig4 ,sensitiveness of ,collodion film1 causes influencing1 #ib2 superior sensitiveness partially explained1 #gd2 preservation of sensitive film1 #bhh4 ,sensitive paper1 theory of preparation of1 #bb1 #abb2 its darkening by light described1 #abc2 preparation of ,albuminized paper1 #bda2 of plain paper1 #bde2 of ,ammonio-,nitrate paper1 #bdf1 #beh2 of paper for ,negative processes1 #bei2 causes which affect the sensitiveness of ,positive paper1 #abc2 which alter the colour of the image1 #abf2 spots and markings on1 #bhe2 a large excess of ,nitrate of ,silver essential1 #abd2 the paper should not be kept too long1 #acj1 #bhf4 ,serum of ,milk1 preparation of1 #bfb1 #cee2 used in ,negative printing process1 #bfb4 ,shadbolt1 ,mr41 his ,honey keeping process1 #bhi2 employs artificial light in ,micro-,photography1 #bcg4 ,silver1 properties of1 #cfb2 estimation of1 in ,nitrate ,baths1 #cga2 recovery from waste solutions1 #cgb2 reduction from ,chloride1 #cgd2 stains1 removal of1 #cgg4 ,size1 mode of removing1 from paper ,positives1 #bee4 ,solar spectrum1 #dg1 #fa4 ,soluble paper4 ,see ,pyroxyline4 ,specific gravity of liquids1 mode of finding1 #cge4 ,spherical aberration1 #ef4 ,spirits of ,wine1 preparation and properties1 #ccj2 not always sufficiently strong for ,collodion1 #hd2 mode of rectifying1 #aif2 sometimes contaminated with1 fusel oil1 #if4 ,spots on ,collodion plates1 #bgi2 on paper ,positives1 #bhe2 on prints obtained by development1 #bff2 on preserved ,collodion plates1 #bic4 ,stains1 ,silver1 removal of1 #cgg4 ,stereoscope1 invention of1 #fg2 theory of1 #fh2 ,wheatstone's1 #fi2 ,brewster's1 #gj4 ,stereoscopic ,photographs1 rules for taking1 #ga2 practical details of1 #bcb4 ,stops1 theory of use of1 #eg2 simple mode of making1 #bbh2 position of the stop often important1 #bcj4 ,strength of acids1 tables of1 #cgh4 ,subchloride of ,silver1 its preparation and properties1 #ae2 decomposed by fixing agents1 #ada4 ,suboxide of ,silver1 its properties1 #ah2 forms compounds with organic matters4 ,citric ,acid1 ,albumen1 etc41 #ba4 ,substitution1 laws of1 explained1 #gh4 ,sulphate of ,iron4 ,see ,protosulphate of ,iron4 ,sulphate of ,quinine1 absorption of chemical rays by1 #fe4 ,sulphuric ,acid1 table of strength of1 #cgh4 ,sutton1 ,mr41 theory of ,sel d'or toning process1 #acd2 practical details of1 #bfg2 ,negative printing process1 #bfb2 preparation of ,serum of ,milk for1 #cee4 ,symbols1 use of1 #cah4 ,syruped ,collodion film1 #bhi4 ,talbot1 ,mr41 his discoveries1 #i2 theory of ,calotype process1 #agf4 ,taupenot1 ,m41 his ,collodio-,albumen process1 #aha2 practical details of1 #bid4 ,temperature1 its effect upon development of ,collodion film1 #ajb2 upon fogging1 #aje2 upon keeping ,collodion1 #baj2 upon action of fixing ,bath for paper ,positives1 #acj2 upon ,hypo toning ,bath1 #acf4 ,test-papers1 use of1 #cgf4 ,toning ,bath for ,positives1 with ,sel d'or1 #acd1 #bfg2 with ,hyposulphite and ,gold1 see ,fixing and ,toning ,bath2 with ,chloride of ,iron and ,hyposulphite1 #afj4 ,toning of ,positives1 term defined1 #aba2 may injure the stability of the proof1 #aed2 points to be kept in view to avoid fading1 #afg2 manipulatory details of1 #bec2 by ,sel d'or1 #bfg4 ,transparencies1 mode of printing1 #bgc4 ,varnishes for ,collodion ,photographs1 #bbf4 ,view ,lenses1 directions for using1 #bcj4 ,vocabulary of ,photographic chemicals1 #cbg4 ,washing ,positive prints1 rules for1 #afb2 details of1 #bee4 ,waxed paper process1 theory of1 #agh4 ,weights and ,measures1 table of1 #cgi4 ,,printed ,,by ,,john ,,edward ,,taylor1 ,,little ,,queen ,,street1 ,,lincoln',s ,,inn ,,fields4 99 99 99 99 99 ,transcriber ,note ,minor typos have been corrected4 ,images moved to prevent splitting paragraphs4