ON THE PHOTOGRAPHIC IMAGE. 105 



the silver is, in short, complete. The significance of this fact we shall here- 

 after recall. 



The processes which seemed to hold out the greatest prospect of success 

 for the production in the first place of a suboxide, and subsequently of a sub- 

 chloride, by the methods of the laboratory, and independently of the action 

 of the light, were those afforded by the reduction* of the citrate of silver, 

 and by the conversion of arsenite of silverf by the action of a caustic alkali 

 into alkaline arseniate, accompanied by a reduction of the oxide of silver to 

 a mixture of metallic silver and suboxide, thus: 



3AgOAs0 3 + 3NaOHO=3NaOAs0 5 + Ag 2 + Agt. 



Of the results yielded by the first of these, none were found that gave any 

 promise at all satisfactory. Hydrogen was passed through citrate of silver 

 suspended in hot water. The products, at first brown, and then black, and 

 finally grey, were examined at various stages of their progress in coloration, 

 citric acid being used as a solvent to remove the citrate and the oxide §, the 

 residuary product being examined by treatment with dilute chlorhydric acid 

 to convert it into chloride. The citric acid solution was found to contain 

 nothing capable of reducing permanganate of potash, and must therefore 

 have been free from suboxide. The result of treating the residue with chlor- 

 hydric acid, and then dissolving the silver by dilute nitric acid, was a rose- 

 tinted chloride of silver. 



On the supposition that this residue was a mixture of suboxide, or a salt 

 of it, with metallic silver, we are constrained to the view that the suboxide 

 of silver is not characterized by the property of entirely passing, under the 

 influence of chlorhydric acid, into subchloride. This seems to be confirmed 

 in some degree by the results with the arsenite, to which we now proceed. 

 To that reaction, which Wohler has described, much attention was devoted ; 

 and it was tried under several modifications ||. By forming a dilute solution 



* Wohler, Ann. Pharra. xxx. 3. 



t Wohler, Ann. Cheni. Pharm. cl. 363. 



% The formula for arsenite of silver usually accepted is 2AgO As0 3 , but we find Wohler's 

 formula as above given to be the correct one. 



§ The brown product became converted into the black one by the treatment with citric acid. 

 Both underwent similar changes under the successive action of chlorhydric and nitric acids, 

 and both previous to this treatment reduced the permanganate of potash powerfully. But it 

 was found that the citric acid alone was capable of reducing the deposit to the grey condition 

 of metallic silver, withdrawing from it at the same time (all the) oxide of silver, — a result which 

 6eemed to render almost hopeless the effort to form the suboxide by its means. 



Indeed the mere boiling of the citrate blackened it, producing a dark-coloured mixture of 

 silver with some compound of the suboxide, the citrate itself undergoing a transformation 

 which must have lowered its saturating power, as the solution remained neutral. The citrate, 

 however, when thus boiled with water through which a stream of hydrogen was passing, be- 

 came more darkly coloured, but imparted an acid reaction to the water. 



The black body that results from the reactions described, contains organic matter, as it 

 intumesces when heated. It cannot therefore be merely a mixture of metallic silver with the 

 suboxide. 



The dry citrate heated in a stream of hydrogen is very slowly affected at 212°, but passes 

 at length into a substance which produces on the one hand a dark- brown solution, and on the 

 other a brown residue which yields a very pale-red body on being transformed by chlorhydric 

 and nitric acids. 



|| It appeared, in trying Wohler's experiment in several ways, that on the one hand it was 

 extremely difficult to get rid of all the arsenic compound from the residue, and on the other 

 that the tendency of arsenic acid in solution was to further the breaking up of the suboxide 

 into oxide and metal. Lime- and baryta-water were therefore substituted for the soda, but 

 still arsenite of silver remained undecomposed. This seemed due to its solid condition. It 

 was to overcome this that the solution in nitric acid was adopted. 



It was found, however, that the chocolate-tinted compound of chlorine and silver, by what- 

 ever process it had been produced, became, by fresh treatment with chlorhydric acid, again 



