48 Captain Abney on the Alkaline Development 



reduce 4 equivalents of silver bromide to the metallic state if 

 sufficient alkali be present to combine with the bromine libe- 

 rated. The solution, after evaporation to dryness, was found 

 insoluble in benzole or ether &c, and only partially soluble 

 in alcohol. A part of that dissolved was further found to be 

 the bromide of the alkali; and the excess of alkali not in 

 combination with carbonic acid was also found in the solution. 

 The amount of carbonic acid was found to be considerable, 

 equal to 1 equivalent of pyrogallic acid. (It seems unneces- 

 sary to give the analysis of the organic residue. Apparently 

 the compound formed is totally different from that obtained by 

 Stenhouse * when investigating the action of bromine on alka- 

 line pyrogallate.) From the above experiment, it will be 

 noted that the reduction of the silver is independent of the 

 absorption of external oxygen, and that only a definite amount 

 of silver can be reduced by a given amount of pyrogallic acid. 

 When, in addition to the alkali, a large excess of soluble bro- 

 mide was added to the bulb B, the same results were obtained, 

 though the reduction of the silver bromide seemed to take rather 

 longer time to effect. The following is an example of the 

 quantities employed in these experiments: — 



In bulb A was placed . . 300 grains of AgBr and 



10 grains of C 6 H 6 3 ; 

 In the bulb B was placed . 300 grains of KHO 



dissolved in the least possible quantity of water. 



The amount of Ag found to be Reduced was 33*86 grains ; 

 and the amount of KBr found in solution was 36*97 grains. 



Layers of thoroughly exposed and unexposed silver bromide 

 were next treated with a rather weaker solution of the alkaline 

 developer; and when the restraining soluble bromide was 

 omitted, the reduction to the metallic state took place in each 

 layer almost equably. When the soluble bromide was present 

 in equivalent quantities with the alkali, the reduction took place 

 first in the layer that had been exposed to light, and spread 

 gradually into the other layer. From this we may gather that 

 the exposed silver bromide is more readily reduced than that 

 unexposed, and that the solution of soluble bromide and the 

 alkali acts less vigorously than the alkali alone. 



Another point to investigate was as to the means by which 

 the density in an alkaline-developed image was produced ; for 

 it could not be supposed that merely those atoms of silver bro- 

 mide which had been reduced to the state of subbromide would 

 be attacked, since the subbromide of silver is essentially a 

 coloured compound and can be distinguished even in small 

 * Journal of the Chemical Society, January 1875. 



