TANNIN. 



523 



Tannahill was small in stature, and in manners 

 diffident almost to bashfuliiess. In his disposition 

 he was tender and humane, and extremely attached 

 to his home, his kindred, and his friends. His life 

 was simple and unvaried in its details, but even the 

 uneventful character of his existence renders more 

 striking and more affecting its tragic close. 



Of his " Poems and Songs," which have been 

 often reprinted, the latter alone preserve his name. 

 His poems, indeed, properly so called, are few in 

 number, and do not rise much above that medio- 

 crity which the great success of Burns, to the mis- 

 fortune of many a rhymster, allured from its native 

 shades into publicity. But his songs are eminently 

 distinguished by elevation and tenderness of senti- 

 ment, richness of rural imagery, and simplicity of 

 diction. 



TANNIN; a peculiar vegetable principle, so 

 named because it is the effective agent in the con- 

 version of skin into leather. The oak and its 

 'products gall-nuts, &c. contain two kindred 

 matters, tannin and gallic acid, which seem, by the 

 powers of vegetation, mutually convertible. The 

 former is supposed to be characterized by its form- 

 ing, with gelatine, a flexible and unputrefiable com- 

 pound ; and by forming with oxide of iron a black 

 combination, which, having a strong affinity for cot- 

 ton, linen, silk and wool, is much used by the dyer. 

 Hitherto, tannin has been found only in perennial 

 plants, and chiefly in the more durable parts of 

 these. The barks of almost all trees and shrubs 

 contain it, principally in the parts nearest the wood, 

 because in the outer coats it is changed by the air. 

 It has never been met with in the poisonous plants, 

 nor in such as contain elastic, resinous and milky 

 juices. Decoction of nut-galls contains tannin 

 with a little gallic acid, some tannates and gallates 

 of potash and lime, tannin altered into the matter 

 commonly called extractive, and lastly a compound 

 (insoluble in cold water") of tannin with perhaps 

 some pectic acid, which is found especially in the 

 extract of oak bark. The purification of tannin, 

 or its separation from the principles with which it 

 occurs, may be effected as follows: Mix a filtered 

 infusion of nut-galls with a concentrated solution 

 of carbonate of potash, as long as a white precipi- 

 tate falls, but no longer, because the precipitate is 

 redissolved by an access of alkali. The precipitate 

 must be washed on a filter with ice-cold water, and 

 afterwards be dissolved in dilute acetic acid, which 

 removes a brown matter from it. This matter is 

 extractive, formed, during the washings, by the ac- 

 tion of the air. After filtering the solution, the 

 tannin is to be precipitated by acetate of lead ; and 

 the precipitate is to be well washed, although in 

 this operation its colour passes from white to yel- 

 low, and it is to be then decomposed by sulphureted 

 hydrogen. The filtered liquor is colourless, and 

 leaves, by evaporation in vacua over potash, tannin 

 in hard, light-yellowish, and transparent scales, 

 which, when exposed to the air, and particularly to 

 the sunbeam, assume a deeper yellow colour. It 

 is not deliquescent; dissolves in water with the 

 greatest facility, and may be readily reduced to 

 powder. Exactly saturated compounds of tannin 

 with acids have no sour taste, but a purely astrin- 

 gent one. In the pure state, they are usually very 

 soluble in water, and cannot, be precipitated from 

 it except by a great excess of acid. Tannin forms, 

 with the salifiable bases, very remarkable com- 

 pounds; that with potash or ammonia in the neu- 

 tral state is but slightly soluble in cold water, and 



may be precipitated in the form of a white earth ; 

 it dissolves in boiling water, and separates from it, 

 on cooling, in the shape of a powder, which, when 

 drained on a filter, pressed and dried, has quite the 

 aspect of an inorganic earthy salt, and is permanent 

 in the air. The compound with soda has the same 

 appearance ; but it is much more soluble. It is 

 known that tannin precipitates solution of tartar 

 emetic. This precipitate is remarkable from a por- 

 tion of the tannin taking, in the salt, the place of 

 the oxide of antimony. 



Proportion of Tannin in different vegetable products. 



The most important property of tannin, among 

 those above mentioned, is that displayed in its re- 

 lation to animal gelatine. They combine with 

 much facility, forming, from a state of solution, a 

 soft, flocculent precipitate, which, on drying, be- 

 comes hard and brittle : this has been called tanno- 

 gelatine. The combination is not always established 

 in the same proportions, but varies according to the 

 concentration of the solutions and the relative 

 quantities of the substances ; nor is the compound 

 in all cases insoluble in water. When the gelatine 

 is only slightly in excess, it consists of fifty-four 

 gelatine and forty-six tannin : when there is a large 

 excess of gelatine, the compound is redissolved. 

 On the formation of this combination, the art of 

 tanning depends. The skin of an animal, when 

 freed from the hair, epidermis and cellular fibre 

 (which is done principally by the action of lime), 

 consists chiefly of indurated gelatine. By immer- 

 sion in the tan liquor, which is an infusion of bark, 

 the combination of the tannin with the organized 

 gelatine, which forms the animal fibre, is slowly 

 established; and the compound of tannin and gela- 

 tine not being soluble in water, and not liable to 

 putrefaction, the skin is rendered dense and imper- 

 meable, and not subject to the spontaneous change 

 which it would otherwise soon undergo. To ren- 

 der it equal throughout the whole substance of the 

 skin, the action of the tan liquor must be gradual ; 

 and hence the tanning is performed by successive 



