234 -Mr. Davy’s Experiments and Observations 
practical part of the art ; but, in pursuing it, I was necessarily 
led to general chemical inquiries concerning the analysis of the 
different vegetable substances containing tannin, and their pe- 
culiar properties. 
I. OBSERVATIONS ON THE ANALYSIS OF ASTRINGENT VEGETABLE 
INFUSIONS. 
The substances that have been supposed to exist most gene- 
rally in astringent infusions are, tannin, gallic acid, and extractive 
matter. 
The presence of tannin in an infusion, is denoted by the pre- 
cipitate it forms with the solution of glue, or of isinglass. And, 
when this principle is wholly separated, if the remaining liquor 
gives a dark colour with the oxygenated salts of iron, and an 
immediate precipitate with the solutions of alum and of muriate 
of tin, it is believed to contain gallic acid, and extractive matter. 
The experiments of MM. Fourcroy, Vauquelin, and 
Seguin, have shown that many astringent solutions undergo 
a change by exposure to the atmosphere ; an insoluble matter 
being precipitated from them. A precipitation is likewise occa- 
sioned in them by the action of heat ; and these circumstances 
render it extremely difficult to ascertain, with any degree of 
precision, the quantities of their constituent parts, as they exist 
in the primitive combination. 
After trying several experiments on different methods of 
ascertaining the quantity of tannin in astringent infusions, I was 
induced to employ the common process of precipitation by gela- 
tine, as being the most accurate. 
This process, however, requires many precautions. The tan- 
ning principle in different vegetables, as will be seen hereafter, 
demands for its saturation different proportions of gelatine ; and 
