on the constituent "Parts of astringent Vegetables. 273 
estimation, that from four to five pounds of common oak bark 
are required to produce one pound of leather, it appears, from 
the various synthetical experiments, that about half a pound of 
catechu would answer the same purpose.* 
Also, allowing for the difference in the composition of the 
different kinds of leather, it appears, from the general detail of 
facts, that one pound of catechu, for the common uses of the 
tanner, would be nearly equal in value to 2 ^ pounds of galls, 
to 7§- pounds of the bark of the Leicester willow, to 11 pounds 
of the bark of the Spanish chesnut, to 18 pounds of the bark of 
the elm, to 21 pounds of the bark of the common willow, and 
to 3 pounds of sumach. 
Various menstruums have been proposed for the purpose of 
expediting and improving the process of tanning, and, amongst 
them, lime-water and the solutions of pearl-ash : but, as these 
two substances form compounds with tannin which are not de- 
composable by gelatine, it follows that their effects must be 
highly pernicious ; and there is very little reason to suppose, that 
any bodies will be found which, at the same time that they 
increase the solubility of tannin in water, will not likewise di- 
minish its attraction for skin. 
* This estimation agrees very well with the experiments lately made by Mr. Purr is, 
upon the tanning powers of Bombay catechu in the processes of manufacture, and which 
he has permitted me to mention. Mr. Purkis found, by the results of different 
accurate experiments, that one pound of catechu was equivalent to seven or eight of 
oak bark. 
N n 
MDCCCIII. 
