2^2 Mr. Davy’s Experiments and Observations 
acting upon gelatine and the salts of iron. After being heated 
with carbonate of lime and carbonate of magnesia, they were 
found deeper coloured than before ; and, though they had lost 
their power of acting on gelatine, they still gave dense olive- 
coloured precipitates with the salts of iron. 
In all these cases, the earths gained tints of brown, more or 
less intense. 
When the compound of the astringent principles of the in- 
fusion of oak bark with lime, procured by means of lime-water, 
was acted on by sulphuric acid, a solution was obtained, which 
precipitated gelatine, and contained a portion of the vegetable 
principles, and a certain quantity of sulphate of lime; a solid 
fawn-coloured matter was likewise formed, which appeared 
to be sulphate of lime, united to a little tannin and extractive 
matter.* 
The solutions were copiously precipitated by solution of 
albumen. 
The precipitates they gave with gelatine were similar in their 
appearance ; their colour, at first, was a light tinge of brown, but 
they became very dark by exposure to the air. Their composition 
was very nearly similar; and, judging from the experiments 
on the quantity of gelatine employed in forming them, the 
compound of tannin and gelatine from "the strongest infusion 
of oak bark, seems to consist, in the 100 parts, of ,59 parts of 
* M. Merat Guillot proposes a method of procuring pure tannin, ( Annales de 
Chimie, Tome XLI. p. 325.) which consists in precipitating a solution of tan by 
lime-water, and decomposing it by nitric or muriatic acid. The solution of the 
solid matter obtained in this way in alcohol, he considers as a solution of pure tannin ; 
but, from the experiments abovementioned, it appears, that it must contain, besides 
tannin, some of the extractive matter of the bark ; and it may likewise contain saline 
matter. 
