8j« On the Nature of the tanning Principle. 



The oil which tannin affords by diftillation is very fraall in quantity, and fo thick, 

 that it remains attached to the neck of the retort. Its coal is bulky, and forms ^ or 

 0.026 part of the diftilled mafs. 



The liquid, from which the tannin has been feparated by the carbonate of pot-afb, foon 

 becomes green in the air, the oxygen of which it abforbs. 



Sulphurated hydrogen deftroys this green colour by uniting itfelf, in its turn, to the 

 oxygen. Though the gallic acid renders tannin more foluble in water, as was remarked by 

 Citizen Deyeux, neverthelefs M. Prouft does not believe that it is by combination with 

 this acid that alkaline carbonates precipitate the tannin, fince they alfo fepatate this matter 

 from a fimple folution in water, and becaufe mod of the very foluble neutral falts alfo pre- 

 cipitate it. It is, therefore, fimply from a ftronger affinity of thefe falts to water that thefe 

 different fubftances feparate the tannin from that fluid. The gallic acid pofleffing, how- 

 ever, the faculty of diffolving a certain quantity of this matter, and the carbonate of pot- 

 afh poffeffing, at the fame time, the double power of faturating this acid and of uniting 

 itfelf clofely to the water, it ought, under like circumftances of folubility, to leave lefs 

 tannin in folution than the other falts. 



M. Prouft obferves, that thefe fafts, unimportant as they may feem to be, fhould put us 

 upon our guard againft the effedls which take place between the folution of the tanning 

 principle and animal fluids ; indeed, fince the faline materials, of which thefe liquids are 

 never deprived, are able to precipitate the tanning principle, it ought not henceforth to be 

 concluded from the appearance of the precipitate, at leaft not till the nature of the depofi- 

 tion has been examined, that animal fluids contain gelatin. 



A piece of the mufcle of an ox, or of raw hide, fpeedlly difcolours the infufion of gall- 

 nut, and leaves nothing but gallic acid in the liquid. When thefe fubftances arc faturated 

 with the tanning principle, they eafily dry without putrefaftion ; their fibres then feparate 

 and crumble into duft between the fingers, like wood which is worm-eaten. M. Prouft 

 very rationally confiders the galling of wool, filk, and cotton, as a fpecies of tanning ; in 

 which, by taking care not to carry the combination too far, the animal fubftance is made 

 to acquire confiderable hardnefs and refiftance to the a6lion of water, without lofing too 

 much of its flexibility, befides rendering it unfit to become the food of infe£ts. He alfo 

 demands whether the folution of tan would not anfwer the purpofes of embalming better 

 than any of the ingredients which have yet been made ufe of. 



Citizen Dizc is the firft perfon who obferved that by pouring fulphuric or muriatic acid 

 upon a rather ftrong decoftion of galls, a confiderable depofit is formed. M. Prouft 

 availed himfelf of this property to feparate the tanning principle from the gallic acid with 

 which it is mixed in the galls. It is, in fa£V, this fubftance which, by combining itfelf 

 with the acids, becomes infoluble, and is precipitated from the water in the form of an 

 adhefive pitch. When the precipitation is complete, the liquid which contains the gallic 

 acid is decanted ; the precipitate is walhed with cold water ; it is then dlflblved in boiling 

 water j the fulphuric acid Is faturated with carbonate of pot-afli, and the tanning principle 



becomes 



