
hy 
q 

1905.] Silver Reaction in Animal and Vegetable Tissues. 221 
the hydroxylated benzol compounds have the same effect on silver salts. In 
these cases the metallic silver is black, and it is not soluble in the solutions 
(sodium thiosulphate, etc.), which dissolve argentic or argentous chloride. 
The reduction involving diminution in the quantity of the element 
combined with the silver is, of course, well known in the case of the haloid | 
salts, the chloride AgCl, for exampie, becoming converted by the action of light 
into the subchloride Ag,Cl, which is violet, reddish-violet, blueish-violet in 
mass, but it may also be reddish-brown when occurring in thin layers, mem- | 
branes, or deposits, the shade of colour apparently depending on the presence 
of the subhaloid in a finely divided form or otherwise. The quantity so con- 
verted is, according to Carey Lea,* not more than 1 per cent. of the total haloid 
salt, with a portion of which it combines, the compound formed not containing 
more than 9 per cent. of the subchloride. Hodgkinson,+ however, regards the 
salt formed by the action of light on silver chloride as an oxychloride, 
probably of the composition represented by the formula Ag,OCl,, in which 
case there is no reduction, the change merely involving a substitution of 
oxygen for half the chlorine. There is no doubt about the loss of chlorine, 
as simple experiments indicate such a loss, but whether the coloured salt is an 
oxychloride or a simple subchloride does not matter for the present, although 
Carey Lea is pronounced in the view that the compound is a simple subhaloid, 
while Guntzt was able to prepare the subfluoride of silver Ag,F, from which, 
through the action of chloride of carbon, of silicon, and of phosphorus he 
obtained Ag,Cl. He further obtained by the same method the subiodide Ag,I, 
the subsulphide Ag,S, and the suboxide Ag,O. 
There is also the difficulty presented by the proteids. The current view 
held, not only by the chemists, but also by scientific exponents of photo- 
graphy, is that proteids form coloured reduction products with silver, and that 
organic matter generally gives similar reduced compounds. Upon this point 
the evidence has appeared decisive, for if egg “albumen” or serum 
“albumen” be treated with acid nitrate of silver the result is a precipitate 
which in the sunlight quickly becomes coloured. Further, if gelatine in 
solution be similarly treated, a precipitate may not occur, but the colour 
reaction quickly appears, and is usually of a pronounced character. This 
summarises the evidence, and, taken in conjunction with the fact that certain 
organic compounds in neutral or alkaline solution, in sunlight, “reduce” salts 
* “On some Reactions of Silver Chloride and Bromide,” ‘Amer. Jour. of Science,’ 
3rd series, vol. 15, 1878, p. 189 ; also “On Red and Purple Chloride, Bromide and Iodide 
of Silver, on Heliochromy and on the Latent Photographic Image,” ‘Amer. Jour. of 
Science,’ vol. 33, p. 349, 1887. 
t Meldola, ‘The Chemistry of Photography,’ 1889, p. 56. 
t ‘Comptes Rendus,’ vol. 112, pp. 861 and 1212. 

a 
