C ^ ] 



cold the quantity is smallest. On an average, 4 or 

 5lbs. of good oak bark are required to form lib. of 

 leather. The inner cortical layers in all barks con- 

 tain the largest quantity of tannin. Barks contain the 

 greatest proportion of tannin at the time the buds be- 

 gin 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. Leader 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 leather 

 at the moment of its formation unites to the extractive 

 matter. 



In general, skins in being converted into leather 

 increase in weight about one third j* and the opera- 

 tion is most perfect when they are tanned slowly. 

 When skins are introduced into very strong infusions 

 of tannin, the exterior parts immediately 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 contain- 

 ing tannin by isinglass, when dried, contain at a medi- 

 um rate about 40 per cent, of vegetable matter. It 

 is easy to obtain the comparative value of different 

 substances for the use of the tanner, by comparing 



* This estimation must be considered as applying to dry skin and dry leather, 



