TANNIN. 



obffrvcd fome wliitc fumes, which ri'fuhed from the nitric 

 acid uniting with the ammonia. 



On replacing the jar, he continued the procefs. The 

 future gas which came over was carbonic acid, and a little 

 of another, which he fuppofed to be nitrogen. 



Although the artificial tannin appears in almoft every re- 

 fpeft fimilar to the natural tannin, it might be expefted that 

 eacli would have been fimilarly affected by nitric acid, yet 

 Mr. Hatchctt found that the one was dellruftible by that 

 acid, while the other was not in the lead affected by it/ He 

 frequently diftillcd nitric acid from the artificial tannin, 

 without producing any change upon it. 



The fame acid being added to infufion of galls, fumach, 

 Pegu cutch, and kafcutti, completely deftroyed the tannin. 

 The common catechu and oak bark treated in the fame way, 

 had not the whole of their tannin deftroyed. He then made 

 infufions of equal ftrength of nutgalls, ■ fumach, fhavings of 

 oak wood, oak bark, and the artificial tainiitig fubftances. 

 To half an ounce of each of thcfe, one drachm in meafure 

 of ftrong nitric acid was added. The tannin of all were 

 deftroyed, with the exception of the oak bark and the ar- 

 tificial tannin. Mr. Hatchett obferves that the tannin of 

 thofe bodies containing much mucilage is the moft hable to 

 dcftruftion by the nitric acid ; and that in the cafes where 

 the tannin is deftroyed, the oxalic acid is moftly formed. 

 The oak bark and the common catechu contain little or no 

 mucilage, and hence have lefs of their tannin deftroyed. 



Thefe fafts feem to prove that pure tannin, unmixed with 

 otlier matter, is not affefted by nitric acid, and that in thofe 

 inftances in which it is deftroyed, the effeft muft arife from 

 fome new fubftance being formed by the aftion of the acid 

 upon the extraneous matter, and alfo that mucilage contri- 

 butes the moft to this change. 



Mr. Hatchett has not given us any precife analyfis of this 

 fubftance, but concludes, from its formation, that it has car- 

 bon for its bafe'. From the circumftance of its being formed 

 with greater facility when the nitric acid is diluted, he thinks 

 that the water is decompofed, and that the hydrogen enters 

 into its conipofition. 



In the cafe of its decompofition by heat, the formation 

 »>f ammonia led him to believe that azote was alfo one of its 

 elements. 



This, however, does not agree with the faft of its being 

 formed with fulphuric acid and vegetable matter only. Al- 

 though this objeftion feems partly to be removed by the cir- 

 cumilance of the tannin from fulphuric acidteing different in 

 fome of its properties from that produced from the nitric 

 acid and charcoal, it may, however, be here obferved that 

 the natural tannin, which is not known to contain azote, 

 more nearly refemblcs that formed with nitric acid, than that 

 from the fulphuric acid. 



It does not appear probable that azote is a component 

 part, even of the natural tannin. The ammonia which Mr. 

 Hatcliett obtained might be in combination with the tannin, 

 and may perhaps always be formed when the tannin is formed. 

 Tile circumftance of the tannin not being changed by re- 

 peatedly digefting it with nitric acid, would feem to favour 

 the idea of its confifting of carbon and oxygen only. The 

 importance of this fubftance in the art of tanning, ought to 

 be a fufficient ftimulus to future experiments. 



If in the procefs of forming artificial tannin, the nitric 

 acid is decompofed merely into nitrous gas, it might be 

 pratticable to bring it back to its original ftate by the oxy- 

 gen of the atmofphere only, and thus produce a valuable 

 produft, which is now obtained at confiderable expence from 

 certain vegetables. 



. The following table flievvs the proportions prcfent of na- 

 tural tannin in a variety of fubftances. 



Catechu from Bombay 



Bengal 



Aleppo nutgalh 



Sumach - - - 



Souchong ti.a 



Green tea 



Leiccfterfhire willow baik 



Oak bark 



Spaiiifti chefnut bark 



Elm bark 



Common willow bark 



54-5 

 48.5 



26 



18 



1 1 

 9.2 

 7.5 

 6.6 

 5 ■ 

 3 

 2.5 



in the Philofophical 



Mr. Biggin alfo has publifticd 

 Tranfaftions for 1799, the rcfult of fome experiments on 

 the proportions of tannin in various kinds of bark, which 

 nearly correfponds with the preceding flatement, as will be 

 feen by the following table, in which the maximum of the 

 tanning principle is luppofed to be 20. 



Tanning Principle. 



Huntingdon or Leieefter willow lo.i 109 



. Sumach .... 16.2 158 



It appears from this table that fimilar barks, when taken 

 from trees at different feafons of the year, differ as to the 

 quantities of tannin contained in them. In the winter they 

 contain the leaft, and in the fpring (which is the ufual time 

 of felling oak -timber), if the feafon is very cold and un- 

 genial, the quantity is diminifhed. This is well known to 

 praftical men, as likewife that all barks contain the greatefl 

 proportion of tannin ; at the time when the buds of the trees 

 begin to germinate. It is found by experience that, on an 

 average, four or five pounds of good oak bark are required 

 to form one pound of leather. 



Sir Humphrey Davy obferves that the aftringent princi- 

 ples in barks vary cbnfiderably, according as their age and 

 fize are different. That in every aftringent bark, the in- 

 terior white bark (which is the part next to the alburnum) 

 contains the largefl quantity of tannin. The proportion of 

 extraftive matter is generally greateft in the middle ; but 

 the epidermis feldom furnifties either^ tannin or extraftive 

 matter. The white cortical layers are, comparatively, moll 

 abundant in young trees ; and hence their barks contain, in 

 the fame weight, a larger proportion of tannin than the 

 7 ' barks 



