CHEMISTRY. 



Element* 

 of 



Pheini!rv. 



Natural 



How ob- 

 tained. 



I is proper 

 ties. 



thod of making it artificially from almost ail animal 

 and vegetable substances. As Mr Hatchctl's tannin 

 differ* in several respects from the tannin of nutgalls 

 and otli.-r astringent substances, it will be proper to 

 divide this section into two parts. 



1. Xntttral Tnnnin. 



No unexceptionable method of obtaining tannin 

 from nutgalls, in a slate of complete purity, has yet 

 been discovered. The best method is this : 



Make an infusion of nutgalls in water, evaporate 

 the infusion to dryness, pulverize the residuum, and 

 digest the powder in repeated portions of pure alcohol 

 till that liquid ceases to dissolve any thing. The re- 

 sidue is tannin tolerably pure. It may be dissolved 

 in water, and precipitated by acetate of lead. The 

 edulcorated precipitate being mixed with water, and 

 a current of sulphuretcd hydrogen passed through it, 

 the lead combines with sulphur and remains insoluble ; 

 while the tannin, thus set at liberty, dissolves in the 

 water, and may be obtained by evaporating the li- 

 quid. 



Tannin, thus obtained, is a brittle substance of a 

 brown colour, with an astringent taste like that of 

 nutgalls. It dissolves readily in water, and the so- 

 lution, according to Trommsdorf, is not liable to be- 

 come mouldy. Pure alcohol does not dissolve it ; 

 but it is soluble in alcohol diluted with a little water, 

 as for example in alcohol of 0.818 of specific gravity, 

 which contains -^th of its weight of water. 



It seems capable of combining with oxygen, but 

 its properties are, by that means, completely alter- 

 ed ; being, according to Proust, a species of extrac- 

 tive. 



The action of the simple combustibles on tannin is 

 unknown. The action of the metals is probably 

 small, but it combines with most of the metallic ox- 

 ides, and forms compounds which, for the most part, 

 are insoluble in water. Thus it strikes a deep blue 

 or black with solutions of iron, and if the solutions 

 be diluted, the compound of tannin and the oxide of 

 iron precipitates. 



When a solution of glue, or gelatine as chemists 

 term it, is poured into an aqueous solution, a preci- 

 pitate immediately falls. This precipitate consists of 

 tanniu and gelatine combined together. It is inso- 

 luble in water, and composed, according to Davy, of 

 54 gelatine and 46 tannin. This property renders 

 gelatine a very delicate test of tannin, which it detects 

 when it exists, even in small proportion, in vegetable 

 liquids. 



The alkalies combine with tannin, and prevent it 

 from precipitating gelatine, till they are saturated 

 with an acid. 



The earths combine with tannin, and form with it 

 compounds nearly insoluble. Hence they precipitate 

 it from the infusion of nutgalls. 



Most of the acids combine with tannin, and form 

 soluble compounds with it. Arsenic, muriatic, and 

 sulphuric acids precipitate it from water. The sul- 

 phuric acid gradually decomposes it. Nitric acid al- 

 so decomposes it, and a substance is formed having 

 the properties of malic acid. 



Such are the properties of the tannin of nutgalli, 

 M far as they have been ascertained. The difficulty 



Elements 

 of 



of obtaining it in a rtate of purity renders some of 

 them ambiguous, and has induced chemists to employ 

 the infusion of nutgalls in their experiments rather 

 than tnuiiiii. 



This infusion is employed in considerable quantity 

 by dyers, and it forms the principal ingredient of 

 common writing ink. This liquor consists of a wlu- Writing 

 tion of sulphate of iron in the infusion of nutgalls. '"'" 

 No other -.alt of iron tried answers so well as the sul- 

 phate. The deepest black is formed when equal 

 weights of nutgalls and of sulphate of iron are em- 

 ployed. But it is not permanent unless the nutgalls 

 amount to about thrice the weight of the sulphate. 

 The be.t ink, according to Dr Lewis, who made 

 many experiments on the subject, may be made by 

 means of the following formula. 



Logwood, 1 ounce. 



Nutgalls, in powder, 3 



Green vitriol, .... 1 



Water 1 or 2 quarts. 



Boil the logwood and the nutgalls in water, adding 

 new liquid in proportion to the evaporation, then 

 strain through a cfotli and dissolve the green vitriol, 

 adding at the same time one ounce of gum arabic and 

 a little sugar. Some recommend the addition of a 

 little cloves to prevent the ink from moulding. 



Tannin exists in many other substances besides nut- 

 galls. The barks of many trees, the substances cal- 

 led catechu and kino, logwood, brazil wood, fustick, 

 and many other vegetable bodies yield it in abundance. 

 From the experiments of Proust, it appears that it 

 varies in its qualities in these bodies, or that there 

 are different species of tannin varying from each other 

 in several respects, especially in the colour which they 

 strike with iron. Some precipitate that metal black, 

 some green, and some flesh-red. 



2. Ar/i/iciul Tannin. 



The important discovery, that a substance capable Artificial 

 of tanning leather like the tannin of nut-galls, may Tauuiii. 

 be formed artificially, was made by Mr Hatchett in 

 the course of a set of experiments on the slow car- 

 bonization of vegetable bodies, and detailed by him 

 in various papers read to the Royal Society in 180.5. 



To form it, we have only to digest diluted nitric How oh- 

 acid on charcoal, till the whole, or nearly the whole, tamed, 

 is dissolved, and evaporate the solution to dryness ; 

 a brown coloured mailer remains, which is artificial 

 tannin. 100 grains of charcoal, by this process, were 

 converted into 120 grains of artificial tannin. A 

 part of this increase is moisture, and it is very difficult 

 to get red of the whole of the nitric acid, a portion 

 of which adheres to the tannin with great obstinacy. 



Tannin, thus prepared, is a substance of a brown Iisproper* 

 colour, it has considerable lustre, and breaks with a l ' et * 

 vitreous fracture. Its taste is very bitter and highly 

 astringent. It has no smell. It dissolves readily in 

 cold water, forming a brown coloured solution. Al- 

 cohol, of the specific gravity 0.800, also dissolves it. 



The solution is precipitated by gelatine very rea- 

 dily. The precipitate i.^ brown, and composed, ac- 

 cording to Hatchett, of 36 tannin and 64 gelatine. 



Sulphuric and muriatic acids form a precipitate 

 when poured into solutions of artificial tannin. Nitrio 



