84 
The quantity of the tannin principle in barks 
differs in different seasons; when the spring has 
been very cold the quantity is smallest. On an 
average, 4 or olbs. of good oak bark are required 
to form lib. of leather. The inner cortical layers 
in all barks contain the largest quantity of tannin. 
Barks contain the greatest proportion of tannin at 
the time the buds begin to open — the smallest 
quantity in winter. 
The extractive or colouring matters found in 
barks, or in substances used in tanning, influence 
the quality of leather. Thus skin tanned with 
gall-nuts is much paler than skin tanned with oak 
bark, which contains a brown extractive matter. 
Leather made from catechu is of a reddish tint. 
It is probable that in the process of tanning, the 
matter of skin, and the tanning principle first enter 
into union, and that the leather, at the moment of 
its formation, unites to the extractive matter. 
In general, skins in being converted into lea- 
ther increase in weight about one third # ; and the 
operation is most perfect when they are tanned 
slowly. When skins are introduced into very 
strong infusions of tannin, the exterior parts im- 
mediately combine with that principle, and defend 
the interior parts from the action of the solution : 
such leather is liable to crack and to decay by the 
action of water. 
The precipitates obtained from infusions con- 
taining tannin by isinglass, when dried, contain at 
a medium rate about 40 per cent, of vegetable 
* This estimation must be considered as applying to dry 
skin and dry leather. 
