

1905.|] Silver Reaction in Ammal and Vegetable Tissues. 223 
halogens affect the silver salt in light, I was led to take up the question anew 
on lines that would promise a definite solution of the problem. The results 
of these observations seem to be decisive, and therefore these and the methods 
by which they were reached may now be described. 
Il. Methods and Results. 
There are, beside the haloid salts of silver, other compounds of the same 
metal which undergo in the presence of light a change which is termed 
reduction. Some of these are important, but only those were examined 
which appeared likely to affect directly the question of the nature of the 
silver reaction in tissues. 
The phosphate when precipitated darkens on exposure to light, but if 
free nitric acid is present, the precipitate does not occur and the dark 
reaction fails to appear. The carbonate also darkens, but in the presence of 
nitric acid the nitrate is formed, and this does not undergo reduction. The 
sulphate is unaffected, but the sulphocyanide is “reduced,” undergoing a 
slight change in colour, even in the presence of nitric acid. The hippurate, 
the oxalate, the valerate, the palmitate, the oleate and the stearate are 
unaffected, while the glycero-phosphate acts like the phosphate in the 
presence of nitric acid, and lecithin also does not affect the silver salt. 
The tartrates and citrates in the presence of nitric acid are unaffected. 
Further, the acetate, the amido-acetate, the amido-propionate, the succinate 
and the lactate, when pure, give no reaction in the same acid medium. 
Of the more strictly physiological compounds, taurine and creatine act on 
the acid solution of the silver salt, producing in the light a coloured reduction 
compound, and cyanuric acid acts similarly, yet less readily, while alloxan 
and alloxantin immediately reduce the silver to the metallic condition, but 
purins, urea, leucine, tyrosine, indol, skatol, and their derivatives exercise no 
effect. 
This makes it certain that only a few compounds of the extractive class 
may affect the silver reaction in tissues, and, with the exception of creatine, 
their presence may be disregarded, for they are excessively small in amount 
when occurring at all in tissues. Creatine, on the other hand, is abundant 
in the striated muscle of vertebrates, while it is absent wholly from 
invertebrates. One may consequently avoid the difficulties presented by 
the occurrence of creatine simply by using for investigation the muscle tissue 
of invertebrates. 
That part of the result of the reaction in tissues is due to haloid salts of 
silver formed there seems to be beyond doubt. Sodium and potassium 
chlorides are constituents of all tissues, animal and vegetable, and it is 
