104 REPORT — 1859. 



after the light has completed the decomposition so far as it can go. Is the 

 result a subchloride of silver? or are the chlorine and the silver completely 

 dissevered, the gaseous element going away, and the metal remaining mixed 

 with, or rather encrusting, particles of unaltered chloride? 



Certainly the weight of authority is in favour of the latter view. Such, 

 at least, is to be gathered from papers by Dr. Draper of New York*, by 

 Mr. Guthrief, and more recently from a series of papers by MM. Davanne 

 and Girard, in France. 



In the first two memoirs referred to, an allotropic state of the metallic silver 

 is viewed as the only explanation of the reactions of the dark substance formed 

 by the light. No chemist, however, has yet produced this substance in such 

 a state of purity as to be able to subject it to an analysis ; and the only 

 arguments, therefore, which can be relied on in explanation of the change 

 are such as make the fewest assumptions and put the least strain on the 

 present experience of the chemist. 



There have been many methods proposed for the production of a sub- 

 chloride of silver by processes directly chemical. One of these consists in 

 the suspension of silver leaf in a dilute solution of sesquichloride of iron, 

 or of chloride of copper. But this experiment has been repeated by us, 

 and we are compelled to look upon the purple-tinted product as chloride of 

 silver accompanied by but a trace of a substance possessing a profoundly- 

 colouring power, which, as will presently be explained, we believe to be a 

 subchloride. 



In order to produce this substance with at all events a greater approach to 

 isolation, we endeavoured to avail ourselves of the possibility of a reaction 

 between chlorhydric acid and the suboxide of silver, and with this view in- 

 stituted many experiments for the production of this last body in a state of 

 chemical purity. Memoirs devoted to the chemistry of the suboxide of 

 silver are not rare. Professor Faraday J showed that the deposit formed by 

 the exposure to the air ot an ammoniacal solution of oxide of silver, consists 

 of a compound with a composition of 108 silver and 5*4 oxygen. This com- 

 position is incompatible with a formula Ag 2 O (supposing oxide of silver to 

 be AgO) ; but the physical characters of the body are interesting. It is grey, 

 and by reflected light is seen to possess a strong lustre. By transmitted 

 light a thin layer of it appears bright yellow. 



Rose § has called attention to various other reactions in which suboxide of 

 silver appears to be formed. Thus, if ammoniacal solution of nitrate of silver 

 be added to protosulphate of iron, a deep and intensely colorific black preci- 

 pitate is formed, consisting of a compound expressed by the formula Ag^ O, 

 2FeO, Fe 2 3 . Similar or analogous products of different composition are 

 formed by the use of salts of the manganous oxide, and by solutions of 

 cobalt; but in all these cases the suboxide of silver is associated in combi- 

 nation with other bodies, and does not present itself in a state from which it 

 would be easily convertible into a subchloride. Rose, indeed, has made one 

 remark, in connexion with these researches, which has a significance of 

 some value for the photographic chemist. He shows that, in the case of 

 adding the acetate of silver to a protoacetate of iron, the precipitate presents 

 the black tint and deeply colorific power which seem to characterize the com- 

 pounds of the suboxide of silver. When the salts used, however, contain 

 "strong" mineral acids, as when nitrate of silver and sulphate of iron are 

 the mutual precipitants, the deposit is grey and metallic — the reduction of 



* Phil. Mag. xiv. 322. t Chem. Soc. Quart. Joura. x. 74. 



t Quart. Journ. Sc. iv. 268. 



§ Journ. Pract. Chem. bud. 215, 407 et seq. ; see also Wohler, Pogg. Ann. xli. 344. 



