120 
Mr. Davy on some 
tanning principle in different astringent substances ; but for 
this purpose, as Sir H. Davy has shown,* many precautions 
are necessary ; and from his experiments it appears, that 
tannin may exist in a state of combination, in which its pre- 
sence cannot be made evident by means of a solution of 
gelatine. I have made several comparative experiments on 
the efficacy of those astringent infusions, and of the sulphate 
of platinum, as tests for gelatine ; and I think I may venture 
to conclude, that the sulphate is a test of superior delicacy, 
and more certain in its operation. Thus, in cases where the 
gelatine was in very minute quantity, or in a very diluted 
state, when no effect was produced by strong infusions of 
oak-bark, nutgalls, or catechu, there was an immediate pre- 
cipitate on adding sulphate of platinum. In instances also, 
when the quantity of gelatine was too minute to be readily 
detected by simply adding the sulphate, the effect was im- 
mediately produced on boiling the fluid. 
The effects of sulphate of platinum on solutions of the 
different Varieties of gelatine, as isinglass, glue, and size, 
appear to be precisely similar, and the precipitates obtained 
in such cases, seem to be uniform in their properties and 
composition ; nor are they affected by the presence of any of 
the mineral acids in excess. The operation of astringent 
infusions, as oak-bark, nutgalls, and catechu, on solutions of 
the different varieties of gelatine, is not uniform. According 
to Sir H. Davy, catechu contains a much larger quantity of 
the tanning principle than oak-bark ; yet I found that an in- 
fusion of it produced no precipitate in solutions of size, of 
different degrees of concentration. The size I employed was 
* Phil. Trans. 1803. 
