TAN

TAN

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The fubftance called tan or tannin, is diftin-guiflied by its ftrong aflringent tafte, by a pe-culiar fmell, by forming immediately with afolution of gelatin a whitilh compound infolu-ble in water and alcohol, and by uniting toanimal (kin (which is chiefly gelatine in a con-denfed date) immerfed in it, rendering itharder, lefs pervious to water, and no longerfufceptible of putrefaction. Thefe are the pe-culiar advantages procured by the art of tan-ning which has been deferibed under the articleLeather.

It is not certain whether we have ever pro-cured pure tannin, (at lead the natural tannin]for all the parts of vegetables that contain italfo give out on infufion a quantity of extract,fometimes mucilage, often gallic acid, befideslome'faline fubltances, and as the chemical dif-ference between fome of thefe bodies appeal'sbut flight, and as all foluble vegetable matterpoflefles many properties in common, it is ex-tremely difficult to obtain any one of themfeparate.

Proud* who has made many refearches ontannin, gives the following method of obtain-ing it from the decoCtion of galls.

Pour into the decoCtion a folution of muriatof tin, which will give a copious yellow preci-pitate. Separate this by filtration, and whenwell wafhed it will confid (as he afferts) of ailthe tannin united to the oxyd of tin. To fepa-rate thefe two, diffufe the precipitate in water,and pafs a current of fulphuretted hydrogengas through the liquid. By degrees an infolu-ble hydro-fulphuret of tin will be precipitated,and the tannin, now feparated from the oxyd,will refume its folubiiity and diffolve in thefluid, giving it the acerb fade, and peculiarfmell of the decoCtion of galls after the excefsof fulphuretted hydrogen has been expelled byboiling. This folution lathers like foap wateron agitation, and when concentrated by boilingit depofits a brown powder on cooling, whichis re-diflolved by heat. When evaporated todrynefs it leaves a dry, brown, friable, refinouslooking mafs like aloes, which does not deli-quefee, has an intenfely acerb tafte, rediflolvesin water and alcohol, and then gives an imme-diate precipitate with the dilutions of gelatine,which is the mod charatferidic property oftannin.

The clear liquor that remains in the firftpart of this procefs (that is after adding muriatof tin to the decoCtion of galls) contains gallicacid and muriat of tin.

* An. Chim. tom. *5. k Ibid. tom. 4 ®-

Such was the original procefs of this ingeni-ous chemid, 1 ’ but fubfequent experiments con-vinced him that it was by no means lo accu-rate as he had at fird fuppofed, for the oxyd oftin has fo ftrong an affinity for other vegetableprinciples and particularly tor extract, that thisfubftance when prefent (which it generally is)accompanies the tannin in its precipitation withthe oxyd of tin, and alfo in its fubfequent folu-tion in water after the oxyd has been removedby fulphuretted hydrogen. A part alfo of thecompound of tannin and oxyd of tin remainsin folution inftead of precipitating, fo that thismethod of eftimating the ’proportional quantityof tannin in any folution would be liable to adouble e-ror.

M. Proud alfo tried the effeCt of fimply agi-tating the decoCtion of galls with the oxyd oftin , made by nitric acid and kept under water)which in a few days abforbed all the tannin,but it equally took away the extract, leavingthe fuper.natant liquor nearly clear. He alfofound that in this way the gallic acid, and partof the tannin were deftroyed, that is, changedin fuch a manner, that their refpeCtive diftin-guilhing properties were loft; fo that on thewhole not much advantage can be expected inthefe experiments, from the ufe of this power-ful chemical agent.

Another method 0 propofed by the fame che-mift of obtaining what he conceived to be puretannin, was to pour into an infufion of galls, afolution of potaffi thoroughly faturated withcarbonic acid, fuch as the cryftallized carbonatof potafh is, which produces a yellowilh whitecurdy precipitate of the fubftance in quettion.The liquors Ihould not be too dilute, nor theprecipitate wafhed with too much water, as itis foluble in water, though fparingly. Thisprecipitate dries but (lowly, but when fpreadover any fmooth furface, in thin layers, andftoved, it becomes a brittle refinous yellowmafs. "I his fubftance diftiiled per je gives afaline liquor with an ammoniacal fmell, whichblackens the folutions of iron, and alfo a littlethick butyraceous oil, and leaves a bulky coal. d

Mr. Davy however, on repeating thefe expe-riments, found that this precipitate did not ex-hibit the properties of pure tannin, as it wantsthe peculiar aftringent tafte, is but flightiy fo-luble in cold water and alcohol, and the folu-tion is not precipitated by gelatin till it is fatu-rated with an acid. It alfo gives by incinerationa confiderable quantity of carbonat of potafh,

c Ibid. tom. 25, < Phil. Tranf. for 1805.