117 



matter ; viz. sand, with a email quantity of calcareous and aluminous 

 earth. 



The importance of the object will, we trust, justify our inserting 

 here what our author has ascertained concerning the application of 

 this substance to tanning. Of two pieces of calf-skin, he tells us, 

 which weighed when dry 132 grains each, and which had been pre- 

 pared for tanning : one was immersed in a large quantity of the in- 

 fusion of extract of catechu from Bengal, and the other in an equal 

 portion of the infusion of the extract from Bombay. In less than a 

 month they were both found converted into leather. When freed from 

 moisture by long exposure in the sunshine, they were weighed. The 

 first piece had gained about 34 grains, and the second piece 35^ 

 grains. The colour of the leather was much deeper than that tanned 

 with galls, and on the upper surface it was of a reddish brown. It 

 was not acted on by hot or cold water ; and its apparent strength 

 was the same as that of similar leather tanned in the usual manner. 



4. On the Infusions of Barks, and other vegetable productions, 

 The experiments described in this section were chiefly made on the 

 strongest infusions of the barks of oak, Leicester willow, and Spanish 

 chestnut : each of them were nearly of the specific gravity denoted 

 by 1'05. Their tastes were alike, strongly astringent : 200 grains 

 of each, on being submitted to evaporation, yielded, the oak bark 1 7 

 grains, and the two other barks about 1 6^ grains of solid matter ; 

 and the tannin afforded by these substances were, the oak bark 14 

 grains, the willow bark 14^ grains, and the Spanish chestnut 13 grains. 

 These substances also gave by incineration only a very small quantity 

 of ashes, scarcely T -f o-th part of their original weights ; and these 

 ashes consisted chiefly of calcareous earth and alkali, the quantity 

 being greatest from the matter produced from the chestnut bark. 



These several infusions were acted on by the acids and pure alka- 

 lies in a manner very similar to that adopted with the infusion of galls. 

 No gallic acid whatever could be obtained from any of them ; and if 

 any be contained in them, it is imagined that it must be in a state 

 of intimate combination with extractive matter. The proportions of 

 the astringent principle in barks vary considerably according to the 

 age and size of the trees from whence they are taken, and probably 

 also according to the different seasons in which they are gathered. 

 In every astringent bark the interior white part (that is the part next 

 to the wood) contains the largest quantity of tannin : the proportion 

 of extractive matter is generally greatest in the middle or coloured 

 part ; but the epidermis seldom furnishes either tannin or extractive 

 matter. A few other circumstances are here added, which ought to 

 influence tanners in the choice of their barks. 



The other vegetable infusions examined by Mr. Davy were those 

 of the barks of elm and common willow, of sumach, Mirabola nuts, 

 tea, and some other vegetables of known astringent qualities. The 

 results offer no very material differences ; but in general the author 

 remarks, that in all substances possessed of an astringent taste, there 

 is great reason to suspect the existence of tannin ; that it may be 



