208 TANNING. 



with the tannin, into chemical combination with 

 the skin. In no case is there any reason to believe 

 that gallic acid is absorbed in this process ; and M. 

 Seguin's ingenious theory of the agency of this 

 substance, in producing the de-oxygenation of 

 skin, seems supported by no proofs. Even in the 

 formation of glue from skin, there is no evidence 

 which ought to induce us to suppose that it loses a 

 portion of oxygen ; and the effect appears to be 

 owing merely to the separation of the gelatine, 

 from the small quantity of albumen with which it 

 was combined in the organized form, by the sol- 

 vent powers of water. 



" The different qualities of leather made with 

 the same kind of skin, seem to depend very much 

 upon the different quantities of extractive matter it 

 contains. The leather obtained by means of in- 

 fusions of galls, is generally found harder, and more 

 liable to crack, than the leather obtained from the 

 infusion of barks ; and in all cases it contains a 

 a much larger proportion of tannin, and a smaller 

 proportion of extractive matter. 



" When skin is very slowly tanned in weak so- 

 lutions of the barks, or of catechu, it combines with 

 a considerable proportion of extractive matter ; and 

 in these cases, though the increase of weight of the 

 skin is comparatively small, yet it is rendered per- 

 fectly insoluble in water, and is found soft, and at 

 the same time strong. The saturated astringent 

 infusions of barks contain much less extractive 

 matter, in proportion to their tannin, than the weak 

 infusions; and when skin is quickly tanned in 

 them, common experience shows that it produces 

 leather less durable than the leather slowly formed. 



" Besides, in the case of quick tanning by means 

 of infusions of barks, a quantity of vegetable ex- 



