62 On the Destruction of the undeveloped Photographic Image, 



cess ; and that is, the danger of making emulsified silver bro- 

 mide which should give a veiled image. Various means of over- 

 coming this difficulty have from time to time been employed ; 

 and it was in my endeavour to discover the reason why they 

 were effective that I was led to make other experiments. 



Now the veil (or fog, as it is more commonly designated) 

 seemed to depend on similar chemical changes in the bromide 

 film to those on which the existence of the image itself de- 

 pended. By finding out the cause of one, it seemed probable 

 that the reason of the other might be discovered. 



It was found, if films made by the silver-bromide emulsion, 

 which on development gave an unveiled image, were exposed 

 to light and then treated with (1) bromine, (2) iodine, (3) 

 nitric acid, (4) sulphuric acid, (5) .cupric bromide, (6) ferric 

 chloride, and other similar bromides and chlorides which 

 easily part with a portion of their bromine or chlorine to 

 metallic silver, that on development no trace of the action of 

 light need be apparent. If we suppose that the photographic 

 image is dependent on the reduction of the argentic to a state 

 of argentous bromide, the action of (1), (2), (5), and (6) would 

 be easily accounted for : (1) and (2) would furnish one of the 

 atoms (which, for convenience sake, I will call the loose atom) 

 of silver with the necessary atoms to saturate it. Thus : — 

 2Ag 2 Br + 2I=Ag 2 Br 2 + Ag 2 I 2 . 



Similarly, by a decomposition of cupric bromide or the 

 ferric chloride, the same reaction would be obtained : — 

 2 Ag 2 Br + 2 Cu Br 2 = 2 Ag 2 Br 2 + 2 CuBr. 



The action of nitric acid and sulphuric acid was more 

 difficult of explanation, unless it were believed that one 

 atom of the argentous bromide was completely removed. 

 When testing silver-iodide films with these bodies, it was 

 found that nitric acid did not destroy the image, but that 

 it could be developed after its application. This experiment 

 seems to prove that the image on the iodide was not caused 

 by a separation of metallic silver. The question then arose as 

 to whether these images were formed by argentous iodide at 

 all, in which case it might be considered that the chemical 

 theory failed. In order to obtain more evidence, it was thought 

 advisable to try whether the loose atom of silver, if it existed, 

 could be oxidized. If this could be effected, it seemed pro- 

 bable that the image would be undevelopable. 



Silver bromide was first put to the test. Plates were treated 

 with potassium bichromate, potassium permanganate, or with 

 chromic acid. In every case the image was obliterated and the 

 film was in a state to receive another developable impression. 



