1S2 VEGETABLE SUBSTANCES. 



wine*." Various experiments have been made with 

 a view of ascertaining the relative quantity of tannin 

 in different vegetable substances. 



Mr. Biggin t found that similar barks, when taken 

 from trees at different seasons, differ as to the quan- 

 tity of tannin which they contain. More than four 

 and a half times greater quantity of tannin was ob- 

 tained from oak bark, cut in spring, than from an 

 equal weight of the same bark cut in winter. Sir H. 

 Davy observed, that the proportion of the astringent 

 principles in barks varies considerably, according as 

 their age and size are different ; these proportions are 

 besides often influenced by accidental circumstances, 

 so that it is extremely difficult to ascertain their dis- 

 tinct relations to each other. 



" In every astringent bark," says this celebrated 

 philosopher, " the interior white bark, that is the part 

 next to the alburnum, contains the largest quantity of 

 tannin. The proportion of extractive matter is gene- 

 rally greatest in the middle or coloured part ; the epi- 

 dermis seldom furnishes either tannin or extractive 

 matter. The white cortical layers are comparatively 

 most abundant in young trees, and hence their bark 

 contains in the same weight a larger proportion of 

 tannin than the barks of old trees. In barks of the 

 same kind, but of different ajres, which have been cut 

 at the same season, the similar parts contain always 

 very nearly the same quantities of astringent prin- 

 ciple ; and the interior layers afford about equal por- 

 tions of tannin J." 



An interesting account of a series of experiments, 

 conducted by this eminent chemist, may be found in 

 the work from which we have just quoted. It does 

 not fall within the plan of the present volume to 

 enter into any chemical details ; but it would per- 



* Phil, Trans, for 1803. f Ibid, for 1799. 



J Ibid, for 1803. 



