VOL. LXXXIX.] PHILOSOPHICAL TRANSACTIONS. 527 



irregular, and in a still greater degree than in any of the other teeth which have 

 been described. It is highly probable that there are many other varieties in the 

 structure of the grinding teeth of quadrupeds, but these will be sufficient to 

 illustrate the general principles on which such varieties depend. 



XV. On the Quantity of Tanning Principle and Gallic Acid contained in the Bark 

 of various Trees.* By George Biggin, Esq. p. 25 Q. 



The bark of trees contains the astringent principle, called gallic acid, and also 

 that principle which has a peculiar affinity to the matter of skin, and which, from 

 the use to which it is applied, is called the tanning principle. But in the pre- 

 sent mode of tanning bark is applied in mass to the skins; consequently both prin- 

 ciples are applied. It remains for examination, whether both principles are useful 

 in the process of tanning: for, if they are not both useful, probably one is detri- 

 mental. To a nobleman, whose zeal on every occasion by which the sciences or 

 arts may receive illustration or improvement is eminently conspicuous, and to 

 whose public energy, as well as private friendship, I feel myself much indebted, to 

 his Grace the Duke of Bedford, I owe the means of prosecuting some experiments 

 on this subject. His Grace, by collecting a variety of barks at Woburn, gave me 

 an opportunity of making some experiments to ascertain the quantity of tanning 

 principle and gallic acid each bark contained. For that purpose, 1 made use of the 

 following methods, according to the principles laid down by M. Seguin. 



By dissolving an ounce of common glue in 2 lb. of boiling water, I procured a 

 mucilaginous liquor, which, as it contains the matter of skin in solution, is a test 

 for the tanning principle. By a saturated solution of sulphate of iron, I obtained 

 a test for the gallic acid. I then took 1 lb. of the bark I meant to try, ground as 

 for the use of tanners, and divided it into 5 parts, each part being put into an 

 earthen vessel. To 1 part of this bark I added 2 lb. of water, and infused them 

 for J hour. Thus I procured an infusion of bark, which I poured on the 2d part 

 of the bark, and this strengthened infusion again on the 3d part, and so on, to 

 the 5th. But as a certain portion of the infusion will remain attached to the wood 

 of the bark, after the infusion is poured or drawn off, I added a 3d lb. of water to 

 the first part, and then followed up the infusion on the several parts, till the 3 lb. of 

 water, or so much of them as could be separated from the bark, were united in the 

 5th vessel; from which I generally obtained 1 pint of strong infusion of bark.-f~ To 

 a certain quantity of this infusion I added a given measure of the solution of glue; 

 which formed an immediate precipitate, that may be separated from the infusion by 

 filtering paper. When dried, it is a substance formed by the chemical union of the 

 matter of skin with the tanning principle, and is in fact a powder of leather. By 



* This inquiry has been further prosecuted by Mr. Davy in a paper inserted in the Phil. Trans, 

 for the year 1803. 



f The specific gravity of this infusion was ascertained by an hydrometer whose gradations are inverse 

 to those of a spirit hydrometer. — Orig. . 



