ON THE PHOTOGRAPHIC IMAGE. Ill 



conferred on the pictures. Sulphide of ammonium converts the fixed image on 

 paper into, first, an intensely black compound, and subsequently, by its con- 

 tinued action, into a dull yellowish, scarcely visible stain. The latter, there can 

 be little doubt, is sulphide of silver. It seems highly probable that the inter- 

 mediate step in the process is the production of a subsulphide, and that it is at 

 that stage that the progress of sulphurizing is arrested in a successfully-toned 

 picture. This explanation would be quite in harmony with the conditions 

 under which the toning is performed. 



The results, then, at which we conceive that photographic chemistry may 

 be said to have now arrived, in respect to the direct processes involving the 

 use of silver-salts, may be thus stated. 



The materials employed perform various functions : — 



1st. One of these is that of supporting the picture, as a mechanical material 

 or basis for holding the chemical bodies. Of the substances so employed 

 the tissue of paper is one. Pyroxyline (the product of a substitution effected 

 in the elements of the cellulose) is spread on glass to afford another. The 

 latter appears to be inert. The former, on the other hand, seems to aid in 

 the reduction, and possibly in some cases to remain in union with the reduced 

 result. 



2ndly. The silver-salts employed, whereof the chloride — for which may 

 be substituted other salts, as the tribasic phosphate, the tartrate, the citrate, 

 and many others, though each with a specific effect — appears to act by im- 

 parting sensitiveness. The nitrate, on the other hand, is present in excess to 

 keep up a constant succession of sensitive material, and so to give vigour and 

 intensity to the image. 



Srdly. Gelatine as a size, or albumen as a glaze, and various other sub- 

 stitutes for these (though but little linked together by any chemical analogy 

 amongst themselves), cooperate by conferring rich tints and deep tones, while 

 they at once impart to the image formed on them an immunity from the 

 destroying action of the fixing process, and form a mechanical surface more 

 or less impenetrable, which prevents the other sensitive compounds from 

 sinking into the paper. 



Each of these substances can, provided nitrate of silver be present, be 

 employed to produce an image. Thus, the chloride rapidly produces a faint 

 picture ; the " gelatino-nitrate " slowly yields an intense one ; together they 

 produce the required result. Whether that result is a cumulative one, the 

 sum of the separate results, or a conjoint one produced by a combination of 

 the chloride with the gelatine compound, it were difficult to say. 



The image is, however, a mixed one, for treatment of it with dilute nitric 

 acid leaves the slaty violet subchloride of silver. It seems therefore to be a 

 mixture of subchloride with a gelatinous, and perhaps also a cellulose-com- 

 pound of suboxide of silver. 



The next great division of our subject which we have to enter upon is that 

 of photographs produced by development. 



Fortunately, in dealing with the images thus formed, we are able to dis- 

 sever the results from the magic influence that calls them into being. We 

 need only show that certain conditions are necessary for the impress of the 

 invisible image ; we are not called on to explain the character of the impress 

 itself. Without attempting to explain what goes on in the camera obscura, 

 we may determine the conditions for a favourable action in it, and interpret 

 the results of that action after development ; though even here, from the 

 great delicacy of the processes employed, the task is a most difficult one. 



With regard then, first, to the preparatory portion of these processes in- 

 volving the production of the sensitive surface. This consists, in the pro- 



