PHOTOGRAPHIC CHEMisTKY.] UNDULATORY FORCES. LIGHT PHOTOGRAPHY. 



145 



metal silver. This is never used in the state we observe 

 it in our silver coins, because it is therein alloyed with 

 copper, to give it sufficient hardness to withstand the 

 wear and tear of daily use. 



Silver is employed both in the metallic state and in 

 the form of nitrate of silver: the former is used in 

 plates for the Daguerreotype process, or as plating or 

 covering a sheet of copper ; and the salt is universally 

 used in the calotype, the collodion, albumen, and other 

 processes of photography generally. 



The salt, nitrate of silver, is easily formed, by putting 

 a piece of pure silver in a little nitric acid diluted with 

 water. A violent action takes place ; the metal is dis- 

 solved ; and by evaporating the solution, crystals of the 

 salt are easily obtained. Tt, however, is far better that 

 the operator should purchase the salt at a photographic 

 chemist's or instrument-maker ; as such prepare it more 

 especially for photographic purposes, and thus avoid a 

 variety of impurities which are ordinarily found in the 

 nitrate of silver of commerce. It may perhaps be as 

 well for us to remark, that all photographic chemicals 

 may be procured by post from any of the leading towns 

 in Great Britain and Ireland. This may be valuable in- 

 formation to those resident at a distance from such 

 places. In our own journeys, we have met with several 

 "towns" so called, wherein we could not purchase a 

 chemical of any kind ; and while writing this, we have 

 in our mind's eye two places in Scotland, containing 

 each over two thousand inhabitants, wherein neither 

 medical man or chemist and druggist could be met 

 with, or could their professed wares be obtained at any 

 price a fact which unutterably astonished us, consider- 

 ing the prevalence of "medicinal taste" amongst us as a 

 people. 



On adding a salt containing chlorine, iodine, and 

 bromine, to nitrate of silver, other substances are formed, 

 being respectively the chloride, iodide, and bromide of 

 that metal. The chloride of silver is a white powder, 

 turning to a dark mulberry tint on being exposed to 

 the action of light. The iodide of silver is of a yellow 

 colour, as is also the bromide ; each being produced on 

 adding eithoi the iodide or bromide of potassium to 

 nitrate of silver. These salts are formed when a plate, 

 covered with iodised and bromised collodion, is dipped 

 into a bath of nitrate of silver a process into which we 

 shall extensively enter when we consider and describe 

 the usual method of taking likenesses, <fcc., by means of 

 collodion. 



In chlorine, iodine, bromine, and their salts, or salt- 

 like combinations, we have described the chief active 

 agents employed in photography ; and in silver we have 

 the substance acted on by them. The resulting products 

 of their mutual action are the media which the photo- 

 grapher employs directly to obtain his result ; but these 

 have to be placed or used in combination with others. 

 His chemicals are as the colours of the painter ; whilst 

 his collodion, <tc., take the place of the painter's 

 canvas. 



Collodion, like its basis, gun-cotton, is of very recent 

 discovery; but it has done more to extend and render 

 popular the pursuit of photography, than any other in- 

 vention which has been connected with the art ; and the 

 process deriving its name from the substance, at once 

 affords beautiful and satisfactory results. Whilst we 

 urge on o\ir readers the wisdom of purchasing, rather 

 than of attempting to make their collodion, we shall give 

 a general direction as to its manufacture, so that its 

 composition, (tc. , maybe fully understood ; and to this 

 end we suggest an experiment by which gun-cotton may 

 easily be made. 



Experiment 8. Put some perfectly clean jewellers' 

 cotton, or cotton wool, into a glass jar, or a common 

 t mnlil'T. Mix in a porcelain vessel equal parts of strong 

 sulphuric and the strongest nitric acid; and, after 

 stirring these together, allow the mixture to cool. We 

 name a porcelain instead of a glass vessel, because of the 

 great heat produced, which would almost certainly 

 destroy glass. The mixture should also be made in the 

 open air, or in a place where a draught would carry off 



VOL. I. 



the suffocating fumes which are given off. When the 

 acids have cooled, pour them on to the cotton wool, and 

 press them together repeatedly for about five minutes 

 by means of a glass rod, so that every part may be well 

 moistened with the acids. It then should be removed 

 by means of the glass rod, and washed under a tap of 

 running water, until every vestige of acid is removed. 

 This can be judged of by the taste of tht water as it 

 runs away. The cotton may then be pressed between a 

 clean cloth ; and afterwards being opened out, so as to 

 expose plenty of surface to the air, it may thus be dried 

 either in the sun or in a warm room. Artificial heat 

 should only be used with great care, because an explo- 

 sion might ensue if too great heat be applied. Blotting- 

 paper, sawdust, Ac. , may be treated in a similar manner, 

 and they will undergo identical chemical changes. 



If the cotton thus prepared be ignited, it will be found 

 to explode suddenly like gunpowder, and not in the 

 slow fashion that takes place when common cotton wool 

 is placed in a flame. Indeed, the substance has been 

 frequently employed instead of gunpowder for blasting 

 purposes. It, however, undergoes too rapid a combus- 

 tion to be of use in rifles, cannon, <tc. ; and is a dangerous 

 agent if so employed, generally causing the bursting 

 of the piece. We have lately been informed, by a 

 friend, that in the summer of the past year (1860), it was 

 successfully adopted by the Austrian service to a con- 

 siderable extent. 



Having thus obtained the gun-cotton, or, as it is 

 chemically termed, "pyroxyliue" the next step is to 

 dissolve it, and so to form collodion. This is done by 

 adding a few grains of the cotton to a mixture of six 

 parts of ether with three parts of alcohol, highly recti- 

 fied ; or the following proportions may be taken : 

 Gun-cotton . . 10 grains. 



Ether . . . half an ounce. 

 Rectified alcohol . quarter of an ounce. 



These should be added together in a glass-stoppered 

 bottle, and, after a short time, the greater portion of the 

 gun-cotton will dissolve, and this solution will be col- 

 lodion. 



If a little of this solution is dropped on to a glass 

 plate, the spirit will soon evaporate, and a fine film will 

 form on its surface. It is this film that is employed to 

 hold the chemical agents which are subsequently placed 

 on its surface, and by whose changes, under the action 

 of light, the photographer effects his beautiful results. 



The collodion thus prepared, requires, however, other 

 additions to fit it for use ; and these consist of substances 

 which, by their action on the silver solution in which 

 they are afterwards placed, render the coating sensitive. 

 Fur this purpose, iodide of potassium, iodide of ammo- 

 nium, or that of cadmium, may be added to the collodion. 

 The iodising of the collodion is a question on which 

 almost every operator differs ; and it is, to a large extent, 

 an empirical subject. We shall, therefore, state gene- 

 rally, that the addition of a few grains of either of the 

 three salts, iodide of potassium, ammonium, or cadmium, 

 to the quantity of collodion manufactured as we have 

 just directed, may be employed by the student. We 

 hKi! however, only mention this that he may under- 

 stand the constitution of iodised collodion ; and, as 

 before, advise its purchase as ready prepared. The 

 uses and modifications of this liquid we shall fully con- 

 sider hereafter. 



We have thus dealt with most of the substances em- 

 ployed for the ordinary processes so far as preparing the 

 plate for the <-amera is concerned. We now proceed to 

 speak of others which are employed for developing and 

 fixing the pictures on the removal of the plate from the 

 camera after its exposure to light, referring chiefly 

 to those of the collodion process. Omitting alcohol and 

 ether, we shall cliiefly refer to those which are most in 

 demand ; and shall thus avoid distracting the attention 

 of the operator by too extended a description. 



The following acids and salts are variously employed 

 for developing solutions; namely: acids acetic, gallic, 

 pyrogallic, and nitric ; salts protosulphate, and proto- 

 nit rate of iron. Acetic acid is the basis of the vinegar 



