S04 On the constdumf Farts ofastringenf Vegetalles^ 



lu examining astringent vegetables in relation to thcjr 

 powers ot" tanning skin, it is neces:?ary to take into account, 

 not only the quantity they contain of the substance preci- 

 pitable by gelatine, but likewise the (Quantity and the nar 

 fure of the extractive matter; and, in cases of comparison, 

 it is essential to employ infusions of the same degree of con- 

 centration. 



It is evident from the experiments detailed in the third 

 section, that of all the astringent suhstamces which have 

 been as yet examined, catechu is that which contains the 

 largest proportion of tannin ; and in supposing, according 

 to the common estimation, that from four to five pounds of 

 common oak hark are required to produce one pound of 

 leather, it appears, from tlic various svnthetical experi- 

 ments, 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 1 1 pounds of the iiark of the Spanish chestnut, 

 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 menstnnmis 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 decomposable by gelatine, it follows that 

 their effects must be highly pernicious ; and there is very 

 little reason to suppose that anybodies will be found which, 

 at the same time that thev increase the solubility of tannin 

 in water, will not likewise diminish its attraction for 

 skin. 



* This c'-timation agrees very v.cU with the expe-.iments lately made 

 by Mr. Puikis upon the tanning powers of Bombiy catechu in the pro- 

 cesses of manufacture, and which he has permitted me to mention. Mr- 

 Purkis founJ, by the results of different accurate experiments, that one 

 pouwl of catechu was equivalent to seven or eight of oak bark. 



LIV. Lctt(^r 



