Page:Proceedings of the Royal Society of London Vol 1.djvu/211

Rh supposed it could be produced by art, unless a fact observed by Mr. Chenevix,- namely, that a decoction of coffee-berries did not precipi- tate gelatine until they had been roasted, may be considered as an exception. Some recent experiments, however, have, he says, con- vinced him, that tan may be formed, not only from vegetables, but even from mineral and animal substances.

The powerful effects observed by Mr. Hatchett to be produced on resinous substances by nitric acid, and the discovery made by him of a natural substance, composed partly of resin and partly of asphal- tum, induced him to extend his experiments to the bitumens. He had found, that almost every species of resin is completely dissolved in nitric acid, and so totally changed that water does not cause any separation; and that, by evaporation, a deep yellow viscid substance is obtained, equally soluble in water or in alcohol: whereas, the ﬁrst effect of the above acid on some of the bitumens, for instance, asphal- tum and jet, was to form a dark brown solution, Whilst an orange- coloured mass was separated, which, by subsequent digestion in an- other portion of nitric acid, was completely dissolved, and, by eva- poration, afforded a yellow viscid substance, nearly similar to that obtained from the resins. But coals, which contained little or no bitumen, did not yield the yellow substance above mentioned.

Mr. Hatchett then made a similar experiment upon charcoal, and found it was more readily dissolved than the preceding substances; no residuum was left; and the solution was of a reddish brown colour.

All the above solutions, when carefully evaporated to dryness, left a brown glossy residuum, which exhibited a resinous fracture ; that left by the solution of charcoal having the most beautiful appearance.

The chemical properties of these residua were as follows :-

1. They were speecb'ly dissolved by cold water and by alcohol.

2. Their ﬂavour was highly astringent.

3. When exposed to heat, they smoked but little, swelled much, and afforded a very bulky coal.

4. Their solutions in water reddened litmus-paper.

5. These solutions copiously precipitated the metallic salts, espe- cially muriate of tin, acetite of lead, and oxysulphate of iron.

6. They precipitated gold from its solution in the metallic state.

'7. They also precipitated the earthy salts, such as the nitrates of lime, of barytes, &c.

8. The ﬁxed alkalies, as well as ammonia, deepen the colour of these solutions. and, after some hours, render them turbid.

9. Glue or isinglass was immediately precipitated from water by these solutions. These precipitates were, in every respect, similar to those formed by the solutions of tan hitherto known, excepting that this factitious tan appeared to be exempt from the extract, gallic acid, and mucilage, which commonly accompany natural tan.

Mr. Hatchett, having thus discovered that tan might be so readily formed from vegetable and mineral coals, was led to examine whether it could not also be formed from animal coal. For this purpose, be