Page:Proceedings of the Royal Society of London Vol 1.djvu/136

Rh chemical agents, viz. the sulphuric, muriatic, and nitric acids, potash, soda, ammonia, and a variety of alkaline earths, magnesia, alumine, different solutions of neutral salts, metallic oxides and solutions, and other substances, the list of which is too long to be here inserted. Many of the results may be considered as insulated facts ; but those who attend to inquiries of this nature, will ﬁnd most of them con- nected with useful conclusions, tending to elucidate this obscure, but no doubt very interesting part of chemistry. The ultimate result of those experiments, as to the constituent parts of gall-nuts, is, that 500 grains of good Aleppo nuts gave by lixiviation with pure water, till all the soluble parts were taken up, 185 grains of solid matter, and that this matter, examined by analysis, consisted of 130 grains of tannin; 35 grains of gallic acid, with a little extractive matter; 12 grains of mucilage, and matter rendered insoluble by evaporation, and the remaining 8 grains of calcareous earth and saline matter.

3. On the Extracts of Catechu, or Terra Japonica.—This extract is said to be obtained from the wood of a species of the Mimosa, which is found abundantly in India, by decoction and subsequent evaporation. There are two kinds, the one sent from Bombay, and the other from Bengal. They somewhat differ from each other in their external appearance, but very little, it seems, in their chemical composition. The tastes of both are sensibly astringent; and neither of them deliquesces, or is apparently changed by exposure to air.

Our President was the ﬁrst who, noticing the more obvious qua- lities of this substance, suSpected that it contained the tanning prin- ciple; and being possessed of a sufﬁcient quantity, he was pleased to supply Mr. Davy with all he wanted for the purpose of a chemical examination. The ﬁrst experiments showed that its solution copi- ously precipitated gelatine, and that it speedily tanned skin. And hence he was encouraged to undertake the particular investigation of its properties, the account of which is the subject of the present section.

His mode of proceeding, of course, could not differ materially from that which he adopted in the analysis of the gall-nuts.» And indeed most of the same, and. some additional chemical agents, have been put to the test. The ultimate analysis has been attended with some diﬂiculty, different specimens of this substance, though to all ap- pearance ever so pure, differing materially among themselves; the natives, for the sake of proﬁt, being apt to adulterate what they sell, either with sand, earthy substances, or other extraneous matter.

Mr. Davy, in order to obviate this difﬁculty, selected a number of specimens, such as he had reason to think the least tainted, and having reduced them into powder, he found the two sorts to consist of the follong ingredients:—200 grains of the extract of catechu from Bombay consisted of 109 grains of tannin, 68 grains of a peculiar extractive matter, 13 grains of mucilage, and 10 grains of residual matter, chieﬂy sand and calcareous earth. The same quantity of the extract from Bengal yielded 97 grains of tannin, 73 grains of peculiar extractive matter, 16 grains of mucilage, and 14 of residual