158 Expenments and Remarks on a Substance 



forget, that even the varieties of tannin * do not accord in 

 the degree of destructibility. 



The second variety of the tanning substance is obtained 

 from a great number of vegetable bodies j such as indigo, 

 dragon's blood, common resin, 8cc. &c., by digesting and 

 distilling them with nitric acid. It is not, therefore, quite 

 so readily prepared as that which was first described, and its 

 relative quantity, when compared with that of the substance 

 employed to produce it, is less. 



As resin and some of the other bodies do not afford it 

 until they have been repeatedly treated with nitric acid, and 

 as during each operation nitrous gas is produced, whilst the 

 strength of the acid which comes over is diminished, it 



seems 



* I shall here venture to state some ideas which have occurred to me on 

 the probable cause and mode of the formation of tannin. 



Mr. Biggin has proved, that similar barks, when taken from trees at dif- 

 ferent seasons, ditTer as to the quantities of tannia contained in them.— 

 (Phil. Trans. 1799, p. ej9.) 



_Mr. Davy also ob^orves, " that the proportions of the astringent principles 

 iu barks vary considerably according as their age and size are different." 



" That in every astringent bark the interior white bark (which is the part 

 next to the alburnum) contains the largest quantity of tannin. The propor- 

 tion of extractive matter is generally greatest in the middle or coloured part ; 

 but the epidermis seldom furnishes either tannin or extractive matter." 



Moreover Mr. Davy remarks, " that the white cortical layers are compa- 

 ratively most abundant in young trees, and hence their barks contain in the 

 «ame weight a larger proportion of tannin than the barks of old trees."— 

 (Phil. Trans. 1803, p. '264.) 



XVe find, therefore, 



1st, That the proportion of tannin in the same trees is different at diffcreijt 

 (easons. 



2dly, That tannin is principally contained in the white cortical layers, or 

 interior white bark, which is next to the alburnum or new wood : and, 



3dly, That these white cortical layers are comparatively most abundant 

 !n young trees, and that their barks consequently contain in the same weight 

 more tannin than the barks of old trees. 



I shall not make any remark* on the first of these facts, as it accords with 

 Other similar effects, which are the natural consequences of the processes and 

 periods of vegetation ; but the second and third appear to be important ; 

 for they prove th:.t tannin is principally formed, or at least deposited, in the 

 interior white bark, which is next to the alburnum or new wood ; so that in 

 the very same part where the successive portions of new wood are to be ela- 

 borated and deposited, we find the principal portion of tannin. 



It should scemj therefore, ihat there is an intimate connection between tlie 



formation 



