CATECHU. 183 



haps be giving an erroneous view of the subject, if 

 another agent in the process of tanning were not 

 slightly noticed. 



There is another vegetable matter, called by che- 

 mists extractive, which is absorbed by the skins 

 together with the tannin, and which gives to them 

 softness and durability ; and therefore, in estimating 

 the relative value of vegetable substances as appli- 

 cable to tanning, it is necessary to take into account 

 not only the quantity of tannin they contain, but 

 likewise the quantity and nature of their extractive 

 matter. The leather obtained by means of an infu- 

 sion of galls, is generally found harder and more 

 liable to crack than that prepared with the infusion 

 of barks, from the circumstance of a less propor- 

 tionate quantity of extractive matter having entered 

 into chemical combination with the skin. Saturated 

 infusions are found to contain much less extractive 

 matter, in proportion to their tannin, than the weak 

 infusions of the bark ; and therefore, when skins are 

 quickly tanned in a strong astringent solution, they 

 absorb considerably less of extractive matter than 

 when very slowly tanned in a weak solution. 



The result of Sir H. Davy's examination of several 

 vegetable substances, as applicable to the common 

 uses of the tanner, shows that one pound of catechu 

 would be nearly equal in value to two and a quarter 

 pounds of galls, to three of sumach, to seven and a 

 half of the bark of the Leicester willow, to eight and 

 a half of oak bark, to eleven of the bark of the 

 Spanish chesnut, to eighteen of elm bark, and to 

 twenty-one of common willow bark. 



Of all the plants here mentioned, that from 

 which Catechu, or Terra japonica, is obtained, is 

 the only one which is not noticed in other parts of 

 this series. Catechu is a dry extract prepared from 



