on a Substance possessing the Properties of Tannin. 303 
this principle by its solubility in water and in alcohol, by its 
action upon gelatine and upon skin, by the effects which it 
produces upon metallic solutions, upon those of the earths, and 
of the alkalis. 
The sulphuric and muriatic acids also affect the solutions of it 
as they do those of tannin ; and the only marked difference 
which as yet has been found in the characters of the artificial 
substance and of tannin, is, that the former is produced, whilst the 
varieties of the latter are more or less destroyed by nitric acid. 
This, for the present at least, must draw a line of separation 
between them, but we must not forget, that even the varieties of 
tannin * do not accord in the degree of destructibility. 
* 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 different 
seasons, differ as to the quantities of tannin contained in them. (Phil. Trans. 1799, 
p. 259.) 
Mr. Davy also observes, “ that the proportions of the astringent principles in 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 proportion 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 comparatively 
most abundant in young trees, and hence their barks contain in the same weight a 
larger proportion of tannin than the barks of old trees.” Phil. Trans. 1803, p. 264. 
We find, therefore, 
1st. That the proportion of tannin in the same trees is different at different seasons. 
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 
jdly. That these white cortical layers are comparatively most abundant in 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 remarks on the first of these facts, as it accords with other 
similar effects, which are the natural consequences of the processes and periods of vegc- 
