114 



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 mirror 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. 



An Account of some Experiments and Observations on the constituent 

 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,;?. 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: 

 1. 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 tan- 

 nin in an infusion is denoted by the precipitate it forms with gela- 

 tine, such as glue or isinglass ; but the process requires many and 



