2J)8 Experiments and Olservattons on the 



hy being acted upon by a weak infusion of oak bark. Tlie 

 process was completed in about three months ; and it was 

 found that one piece of skin had gained in weight 14 grains^ 

 and the other piece about 16-j grains. This increase is pro- 

 portionally much less than that which took place in the 

 experiment on the process of quick tanning. The colour 

 of the pieces of leather was deeper than that of the pieces 

 which nad been quicklv tanned ; and, to judge from th» 

 properties of the residual liquors, more of the extractive 

 matters of the barks bad been combined with them. 



The experiments of Mr. Biggin* have shown that si- 

 milar barksj when taken from trees at different seasons, 

 differ as to tlie quantities of tannin they contain : and I 

 ^ave observed that the proportions of the astringent prin- 

 ciples in barks varv considerably according as their age an4 

 s\ze are different ; besides, these proportions are often in- 

 fluenced by accidental circumstances, so that it is extremely 

 difficult to ascertain their distinct relations to each other. 



In every astringent bark, the interior white bark (that is, 

 the part next to the alburnum) contains the largest quantity 

 <frf tannin- The proportion of extractive matter is generally 

 greatest in the middle or colouretl part : but the epidermis 

 jeldom furnishes either tannin or extractive matter. 



The white cortical layers are comparatively most abun- 

 dant in young trees ; and hence their barks contain, in the 

 fame weight, a larger proportion of tannin than the barks of 

 old trees. In barks of the same kind, but of different ages_, 

 which have been cut at the same season, the similar parts 

 contain always very nearly the same quantities of astringent 

 principles ; and the interior layers afford about equal por-t 

 tions of tannin. 



An ounce of the white cortical layers- of old oak bark, 

 furnished, by lixiviation and subsequent evaporation, 108 

 grains of solid matter; and of this, 72 grains were tannin. 

 An equal quantity of the white cortical layers of young oak 

 produced ill grains of solid matter, of which 77 were pre- 

 cipitated by gelatine. 



An ounce of the interior part of the bark of the Spanish 

 chestnut gave 89 grains of solid matter, containing 63 grains 

 of tannin. 



The same quantity of the same part of the bark of the 

 Leicester wilbw produced 117 grains, of which 79 were 

 tannin. 



An ounce of the coloured or externa] cortical layers fron^ 



• Philosophical Transactions for 1799, p. i9>. 



the. 



