114 REPORT 1859. 



ill others these images widely diverge from them. Thus, we seldom find in 

 them those purple and violet tones which seem to characterize the subchlo- 

 ride of silver before fixing. On the other hand, we observe two classes of 

 developed images : — the one is of a dull metallic appearance, of a slaty grey 

 character by transmitted light, and in but a feeble degree opake ; the other 

 varies in colour, exhibiting brown or red hues, and sometimes even presenting 

 perfect opacity to transmitted light, closely similar to the picture formed by 

 direct processes. But, on testing these two varieties of image by the method 

 of conversion into suiphide of silver before described, it is found that the dull 

 translucent metallic image teems with silver, and becomes very opake in the 

 form of sulphide, while the more richly coloured and dense-seeming image 

 loses opacity under the sulphurizing action, and exhibits at last a subdued tone 

 of colour that brings it more on a par with the sulphuretted metallic image. 

 Clearly then, here, density, and the qualities which give photographic value 

 to an image, do not depend on the amount of metal that goes to form it, so 

 much as on the chemical, and even perhaps mechanical state, in which that 

 silver is present in it. 



The several causes which determine the deposit of the images in these 

 several states appear to be these : — 



1. The materials forming the sensitive film. — Pyroxyline, in chemical 

 purity, has little tendency to form the darker image. Albumen and the hete- 

 rogeneous substances (including decomposed collodions), which we have had 

 to yoke in the same class with it, have this tendency. 



In general (speaking of the ordinary moist process) the tendency to pro- 

 duce the darker image is found to be in something like an inverse ratio, 

 cceteris paribus, with the sensitiveness. 



The use of the bromide of silver with the iodide imparts to a collodion 

 film a tendency to deposit the grey metallic image, at the same time that a 

 more powerful reducing agent is needed to develope it. It is a remarkable 

 fact, bearing upon this singular property of bromide, that no compounds ana- 

 logous to that formed by A. Kremer with the iodide have yet been formed 

 with it. In the case of albumen, this influence of bromide is not felt; for with 

 albumen, bromide of silver is held to increase the opacity of the image. 



2. The nature of the developing agent. — The substances used to develope 

 the latent image, besides the free nitrate of silver invariably necessary, 

 embrace also without exception one ingredient, the character and the pur- 

 pose of which is to reduce the salts of silver. In some cases organic bodies 

 are employed for this purpose, in others the reducing agent is inorganic. 

 Now, whether the grey or metallic form of image is completely reduced 

 silver, and the more opake forms are an argentous compound (mixed or not 

 with metallic silver), or whether all the forms of image are silver in different 

 mechanical states of deposition, is a very important inquiry, and one on 

 which the facts of the development and the nature of the developing agent 

 may throw some light. 



But no one who is intimate with the complex and perplexing details of this 

 step in the photographic process will expect the chemist to come in and 

 remove the difficulty by the use of a few formula?. All we can hope to do 

 is to point to a few sure results of experience, and indicate any explanation 

 which may be suggested by facts from the laboratory analogous to these. 



Jt is known, then, that to produce a " positive" picture in the camera, the 

 developing agent should be sulphate of iron, acidified in some cases even by 

 nitric acid. The result is the crystalline white deposit of metallic silver. 

 Protonitrate of iron is used with a similar result. So likewise in the labo- 

 ratory it is known that a neutral mixture of the ferrous sulphate and nitrate 



