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SIR H. DAVY S AGRICULTURAL CHEMISTRY. 



Tannin is not a nutritive substance, but is 

 of great importance in its application to 

 the art of tanning. Skin consists almost 

 entirely of jelly or gelatine, in an orga- 

 nized state, and is soluble by the long con- 

 tinued action of boiling water. When 

 skin is exposed to solutions containing 

 tannin, it slowly combines with that prin- 

 ciple ; its fibrous texture and coherence 

 are preserved ; it is rendered perfectly 

 insoluble in water, and is no longer liable 

 to putrefaction ; in short, it becomes a sub- 

 stance in chemical composition, precisely 

 analogous to that furnished by the solution 

 of jelly and the solution of tannin. 



In general, in this country, the bark of 

 the oak is used for affording tannin in the 

 manufacture of leather ; but barks of some 

 other trees, particularly the Spanish ches- 

 nut, have lately come into use. 



The following table will give a general 

 idea of the relative value of different 

 species of barks. It is founded on the 

 result of experiments made by myself. 



Table of numbers exhibiting the quanti- 

 ty of tannin afforded by 480 lbs. of differ- 

 ent barks, which express nearly their rela- 

 tive values: 

 Average of entire bark of middle 



sized Oak, cut in spring. 

 Average of Spanish chesnut, 



Leicester willow, large 

 size. 



Elm, 



Common willow, large 11 



Ash, 



Beech, 



Horse chesnut, 



Sycamore, 



Lombardy poplar, 



Birch, 



Hazel, 



Black thorn, 



Cappice oak. 



Oak cut in autumn. 



Larch cut in autumn. 



White interior cortical 

 layers of old bark. 

 The quantity of the tanning principle 

 in barks, differs in diflerent seasons ; when 

 the spring has been cold, the quantity is 

 smallest. On an average, 4 or 5 lbs. of 

 good oak bark are required to form 1 lb. 

 of leather. The inner cortical layers in 

 all barks contain the largest quantity of 



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tamiin. Barks contain the greatest propor- 

 tion of tannin, at the time the buds begin 

 to open — the smallest quantity in winter. 



The extractive or coloring matters found 

 in barks, or in substances used in tanning, 

 influence the quality of the 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 reddish 

 tint. It is probable that in the process of 

 tanning, the matter of skin and the tanning 

 principle first enters into union, and that 

 leather, at the moment of its formation, 

 unites to the extractive matter. In general, 

 skins in being converted into leather, in- 

 crease in weight about one third*; and the 

 operation is most perfect when they are 

 tanned slowly. Whenskins 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 

 containing tannin by isinglass, when dried, 

 contain, at a medium 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 quantities of precipitate afford- 

 ed by infusions of given weights mixed 

 with solutions of glue or isinglass. 



To make experiments of this kind, an 

 ounce, or 480 grains of the vegetable sub- 

 stance in coarse powder, should be acted 

 upon by half a pint of boiling water. The 

 mixture should be frequently stirred, and 

 suffered to stand 24 hours ; the fluid should 

 then be passed through a fine linen cloth, 

 and mixed with an equal quantity of solu- 

 tion of gelatine, made by dissolving glue, 

 jelly, or isinglass in hot water, in the pro- 

 portion of a drachm of glue or isinglass, 

 or six table spoons full of jelly, to a pint 

 of water. The precipitate should be col- 

 lected by passing the mixture of the solu- 

 tion and infusion through folds of blotting 

 paper, and the paper exposed to the air 

 till its contents are quite dry. 



If pieces of paper of equal weights are 

 used, in cases of which different vegetable 



* This estimation must be considered as applying 

 to drif siviii and dry leather. 



