338 ExperlmtKii en the Tanning Prtnc'tpk and Add of Galls. 



luted tills mixture with a quantity of water fuflicient to feparate the lad portions of preci- 

 pitate which the acids may hold in folution, it is filtered, and the liquor contains the acids 

 of galls, muriatic acid, and muriate of tin. The precipitate is a combination of the tanning 

 principle, and the oxyde of tin. It cannot contain the gallic acid, becaufe this acid is not 

 precipitated by tin. The folutions of lead, on the contrary, precipitate the gallic acid, as 

 •well as the tanning principle ; and on this account it was that Scheele could not feparate 

 the acid of galls from the principle of aftringency. 



In order to obtain the acid of galls, the tin mud firft be feparated by fulphurated hydro- 

 genous gas. The fulphurated oxyde falls down in the form of a brown powder. After 

 having left the fluid expofed to the fun, with a covering of paper, in order that the excefs 

 of fulphurated hydrogen gas may be diflipated or entirely decompofed, it is to be filtered, 

 and then evaporated, in a vefTel of filver, to the reqiiifite point for the crydallization of the 

 gallic acid by cooling. The cryftals being then thrown on a filter muft be wafhed with a 

 fmall quantity of cold water. The remaining fluid is to be treated by evaporation, and 

 cooling in the fame manner, in order to deprive it as much as poffible of gallic acid. At 

 the end of the operation, it is neceflary to ufe capfules of glafs, becaufe the muriatic acid 

 becomes concentrated. The earths which fome chemifts have pretended to exift in the 

 decodtion of nut-galls ought to be found in this refidue. Mr. Proud; met with none ; but 

 he propofes to examine the refidue with more particular attention to this point. 



To feparate the tanning principle from its combination with the oxyde of tin, tliis lad 

 powder is difFufed in a quantity of water, through which a current of fulphurated hydro- 

 genous gas is pafled. The fulphurated oxyde falls down, and the pure tanning principle at 

 the fame time becomes diflblved in the water. After filtration, the folution Is evaporated in 

 a filver bafon, in which it acquires the deep colour of a decoftion of nut-galls, and its pe- 

 culiar fmell, which is one of the diftindive charaQers of the aflringent principle. Its tafte 

 is exceedingly acerb, and rather bitter, without being difagreeable. It lathers like foap- 

 water, without feeling unftuous to the touch. It becomes turbid by cooling, and lets fall 

 a light brown powder, which is diflblved again by heat. It refembles the laft decodtions of 

 nut-galls, which do not become mouldy, and are thought by Mr. Prouft to contain the 

 tanning principle nearly in a ftate of purity. 



The fubflance which remains after its evaporation is dry, brown, friable, breaks with a 

 vitreous fra£lure, like aloes, and does not attract the humidity of the air. Its tafte is 

 very rough and harfh : it is totally foluble in hot water, and flill more readily in alcohol. 



All the acids precipitate the tanning principle from its aqueous folution, by uniting with 

 it. As this fad belongs to the analyfis of galls, which Mr. Prouft has made by means of 

 the fulphuric and muriatic acids, he promifes to treat of it again in the fecond part of his 

 work. 



The aqueous folution of the tanning principle, poured into a folution of glue, imme- 

 diately converts it into a magma, which pofl"efles the elaftic properties of the gluten of 

 wheat. This fubflance, when left to dry, contrafls in its dimenfions, and, when perfectly 

 dry, has the appearance of a brown mafs, vitreous in its fradture, not capable of putre- 

 faction, perfedly infoluble in water, yielding little to alcohol, and capable of refuming its 

 elaftic property by foftening it in hot vi'ater. It is, in a word, favs the author, the com- 

 bination dlfcovered by Seguin, the preferving principle of tanned leatlier. It is alfo ob- 

 tained with the folution of the nut-gall. 



Albuminous 



