effects of heat and cold upon the figure of the mirrors. This cir-
cumstance has called forth a particular investigation, in which heated
bodies were approached at different distances, both before and behind
mirrors, either of glass or metal; and it was remarkable how their
focal lengths were immediately affected by it. Hence it may reason-
ably be inferred, that the rays of the sun on a minor will produce a
similar distortion. That the dilatation occasioned by heat is the
cause of this defect, will easily be admitted; but our author does not
enter here upon the theory-of this influence, nor upon the remedies
that may be applied to its detrimental consequences. These points
he reserves for a future communication.
Parts of certain astringent Vegetables; and on their Operation in Tanning. By Humphry Davy, Esq. Professor of Chemistry in the Royal Institution. Communicated by the Right Hon. Sir Joseph Banks, Bart. K.B. P.R.S. Read February 24, 1803. [Phil.
Trans. 1803, p. 233.]The importance of the subject handled in this paper, which, as it particularly relates to the process of tanning leather, will be allowed to be of sufficient magnitude, has of late excited the attention of se- veral able philosophers, among whom Mr. Seguin was the first Who ascertained the peculiar vegetable matter which is essential to this process, and which is possessed of the characteristic property of pre- cipitating gelatine from its solutions. Mr. Proust has since investi- gated many other properties of this substance ; but neither these, nor any other chemists, have as yet carried their investigations so far as to determine the various affinities of tannin, and especially how its action upon animal matters is modified by combination with other substances. This task was reserved for our author, who during the two last years bestowed most of his leisure hours on a course of ex- periments on this subject; and he here lays before the SOciety an account of their general results. His chief design was to elucidate the practical part of the process; but in pursuing it he found himself necessarily led to general chemical inquiries concerning the analysis of the different vegetable substances containing tannin, and their peculiar properties.
The paper consists of five parts, the titles of which are as follows: ——~l. Observations on the analysis of astringent vegetable infusions. 2. Experiments on the infusions of galls. 3. Experiments and ob-_ servations on the extracts of Catechu, or Terra Japonica. 4. Ex- periments and observations on the astringent infusions of barks, and other vegetable productions; and 5. General observations.
And first, as to the analysis of astringent vegetable infusions; the substances that have been supposed to exist most generally in them are tannin, gallic acid, and extractive matter. The presence of tannin in an infusion is denoted by the precipitate it forms with gelatine, such as glue or isinglass; but the process requires many and
