Page:Proceedings of the Royal Society of London Vol 1.djvu/137

Rh matter; viz. sand, with a small 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 qf 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 ['05. Their tastes were alike, strongly astringent: 200 grains of each, on being submitted to evaporation, yielded,;—the oak bark 17 grains, and the two other barks about 16-; 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 chestnutl3 grains. These substances also gave by incineration only a very small quantity of ashes, scarcely 1{ﬁrth 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; hut the epidermis seldom furnishes either tannin or extractive matter. A few other circumstances are here added, which ought to inﬂuence tanner-s 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 he