18?8] 



FARMERS' REGISTER 



599 



substance consists principally of astiinijcnt matter, I 

 and extract ; by the action of water upon it, the j 

 astringent matter is first dissolved, and may be | 

 separated from tiie extract. Extract is always j 

 more or less colored : it is soluble in alcohol and , 

 water, but not soluble in ether. It unites with [ 

 alumina, when that earth is boiled in a solution of ^ 

 extract ; and it is preci[)itated by the salts of alu- 

 mina, and by many metallic solutions, particularly 

 tlie solntion ot' muriate of tin. 



From the f)roducts ol" its distillation, it seems to 

 be composed principally of hydrogen, oxygen, 

 carbon, and a little azote. 



There appears to be almost as rnany varieties of 

 extract as there are species of plants. ThediHer- 

 ence of their properties probably in many cases 

 depends upon their being combined with small 

 quantities of other vegetable principles, or to their 

 containing different saline, alkaline, acid, or earthy 

 ingredients. Many dying substances seem to be 

 of the nature of extractive principle ; such as the 

 red coloring matter of madder, and the yellow 

 dye, procured li-om weld. 



Extract has a strong attraction for the fibres of 

 cotton or linen, and combines with these sub- 

 stances when they are boiled in a solution of it. 

 The combination is made stronger by the inter- 

 vention of mordants, which are earthy or metallic 

 combinations that unite to the cloth, and enable 

 the coloring matter to adhere more strongly to its 

 fibres. 



Extract, in its pure form, cannot be used as an 

 article of food, but it is probably nutritive when 

 united to starch, mucilage, or sugar. 



8. Tannin, or the tanning principle, may be 

 procured by the action of a small quantity of cold 

 water on bruised grape-seeds, or pounded gall- 

 nuts ; and by the evaporation of the solution to 

 dryness.* It appears as a yellow substance, pos- 

 sessed of a highly astringetit taste. It is difficult 

 of combustion. It is very soluble, both in water 

 and alcohol, but insoluble in ether. When a so- 

 lution of glue, or isinglass, (gelatine,) is mixed 

 with an aqueous solution of tannin, the two sub- 

 stances, i. e, the animal and vegetable matters, 

 fall down in combination, and Ibrm an insoluble 

 precipitate. 



When tannin is distilled in close vessels, the 

 principal products are charcoal, carbonic acid, and 

 inflammable gases, with a minute quantity of vo- 

 latile alkali. Hence its elements seem the same 

 as those of extract, but probably in different pro- 

 portions. The characteristic property of tannin is 

 its action upon solutions of isinslass or jelly; this 

 particularly distinguishes it from extract, with 

 which it agrees in most other chemical qualities. 



There are many varieties of tannin, which pro- 

 bably owe the difference of their properties to 

 combinations with other principles, especially ex- 

 tract, from which it is not easy to free tannin. The 

 purest species of tannin is that obtained from the 

 seeds of the grape ; this forms a white precipitate, 

 with solution of isinglass. The tannin from gall- 

 nuts resembles it in i:s properties. That from 

 sumach affords a yellow precipitate; that from 

 kino a rose-colored ; that from catechu a fawn- 



colored one. The coloring matter of Brazil- 

 wood, which M. Chevreul considers as a peculiar 

 princijjle, and which he has called Ihmatinc, dit- 

 lers from other species of tannin, in allbrdmg a 

 |)recipitate with gelatine, which is soluble in abun- 

 dance of hot water. Its taste is much sweeter 

 than that of the other varieties of tannin, and it 

 may perh.aps be regarded as a substance interme- 

 diate between tannin and extract. 



Tannin is not a nutritive substance, but is of 

 great importance in its application to the art of tan- 

 ning. Skin consists almost entirely of jelly or 

 gelatine, in an organized state, and is soluble by 

 the long-continued action of boiling water. When 

 skin is exposed to solutions containing tannin, it 

 slowly combines with that principle ; its fibrous 

 texture and coherence are preserved ; it is render- 

 ed perfectly insoluble in water, and is no longer 

 liable to putrefiiction: in short, it becomes a sub- 

 stance in chemical composition precisely analo- 

 gous to that (iirnished by the solution of jelly and 

 the solution ol tannin. 



In general, in this country, the bark of the oak 

 is used for affording tannin in the manufacture of 

 leather: but the barks of some other trees, parti- 

 cularly the Spanish chestnut, 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 quantity of Tan- 

 nin afforded by 480 lbs. of different Barks, which 

 express nearly their relative values. 



* In the purest form in which it has been obtained 

 by M. Pelouze, it is colorless, has distinct acid proper- 

 ties, and is converted into tj&llic acid by an absorption 

 of oxyp^en on exposure to the atmosphere. The term 

 tannic acid has recently been applied to it.— J. D. 



The quantity of the tannin principle in barks 

 differs in different seasons ; when the spring iias 

 been very 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 lay- 

 ers 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 (juantity in winter. 



The extractive or coloring matters found in 

 barks, or in substances used in fanning, 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 mat- 

 ter of skin and the tanning principle first enter into 

 union, and that the leather, at the moment of ita 

 Ibrmaiion, unites to the extractive matter. 



In general, skins in being converted into leather 

 increase in weight about one-third; * and the ope- 



* This estimation must be considered as applying to 

 dry skin and dry leather. 



