108 REPORT — 1859. 



1. That the action of the light on chloride of silver is to reduce it, in so 

 far as it is able to penetrate its substance, to the state of a subchloride. 



2. That in the presence of nitrate of silver, this deposit of subchloride is 

 necessarily more plentiful, while some part of the liberated chlorine passes 

 into an oxide, which prevents a portion of the chlorine set free from con- 

 ducing to the formation of fresh subchloride. 



From this point we may proceed to the discussion of the photographic 

 image in more complex, but, for the photographist, more available forms. 

 And in doing so, we must at the outset bear in mind that the image varies 

 in its character in different stages of the photographic process. The first 

 result obtained by the light, even if it be the same in all stages of the solari- 

 zation, is not the result which is in many cases left after the fixing solution 

 has performed its work; but it is perhaps more interesting, as indicating the 

 nature of the change effected by the light, independent of the chemical re- 

 agents which are afterwards applied. 



Iu endeavouring to reduce into orderly arrangement the great number of 

 photographic results which this inquiry involves, it seemed best to sever at 

 the outset two series of them which bear but little relation to each other, — 

 namely, the images obtained by development, and those which are formed 

 visibly by the light. Commencing with the latter of these, the attention is 

 at once arrested by the processes involving the use of chloride of silver in 

 conjunction with the nitrate of that metal. 



The rationale of the union of these two compounds for the production of 

 an effect far greater than that upon the chloride alone, has been shown ; but, 

 practically, in photographic processes there are other agents present in the 

 paper, or purposely introduced into it, which play a part in the photochemical 

 change hardly less important than that of the silver salts themselves. — ■ 



We may fairly inquire in the first instance whether the presence of the 

 fibre of the paper itself may not assist in effecting decompositions under the 

 influence of light. To determine this point, Swedish filtering paper, as the 

 type of the most uniform and pure fibre of paper that could be procured, was 

 treated with nitrate of silver alone : on being exposed for some hours, it 

 exhibited a pale-reddish stain, which after several days' insolation reached no 

 deeper tone than a brown. The substitution of ammonio-nitrate of silver for 

 the nitrate gave a rapidity to the change, and ultimately a depth of opacity 

 to the result, by affording an antagonism, as we suppose, to the influence of 

 the nitric acid. The reactions of the darkened ammonio-nitrate paper are 

 as follows : — Ammonia does not otherwise affect it, than that treatment there- 

 with (probably by action on the tissue of the paper) makes it slightly more 

 readily acted on by other reagents. Nitric acid, though exceedingly dilute, 

 rapidly dissolves it. Indeed an acid so far diluted that it took many hours to 

 destroy the substance left by treating with ammonia Swedish paper that had 

 been prepared with chloride of silver and subsequently darkened in the sun, 

 was able to destroy this bronzed image formed by the ammonio-nitrate in a 

 few minutes. Cyanide of potassium in presence of air rapidly destroys it, 

 but not so rapidly as it does the image on chloride of silver just alluded to. 



It would be difficult, from the above reactions, to come to any positive 

 opinion on the nature of the photochemically changed substance left by the 

 ammonio-nitrate of silver on pure tissue of paper. But that this tissue is 

 not without a part to play in the changes which the oxide of silver under- 

 goes, perhaps even a more important one than that of an absorber of oxygen, 

 seems indicated by one curious experiment. Swedish filtering paper treated 

 with nitrate of silver, and while still moist touched with a solution of proto- 

 sulphate of iron, gives a grey stain easily recognized as metallic silver. "When, 



