i^^ Siluhle Pniiciphs of AJir'wgent Vegetables. 



thefe llqnors, a precipitate more or lefs abundant, according to tlic quantity of gelatin 



they contain. 



This peculiar property of the tanning principle affords an application which may become 

 of great importance in the art of treating difcafe«, to determine the nature of urine, and to 

 afcertain fome of its changes. In the healthy fubjecf , all whofe fundions are duly exercifed, 

 the urine docs not cont.iin gelatin, nor aflbrd a precipitate with the infufion of tan : on the 

 contrary, in all the gaftric alTcaions, the urine is more or lefs charged with gelatin, and 

 forms, with the infufion of tan, a precipitate more or lefs abundant. The fame obfervation 

 is applicable to acute and chronical difcafes, in which the aOimilating or digcftive forces arc 

 troubled, deranged, or perverted. 



4. The gallic acid, or, if other terms be preferred, the principle which precipitates thff 

 fulphate of iron, is often found alone, or at leafl without being accompanied by tlie tanning 

 principle. Thus quinquina, crude or torrefied coffee, the roots of the ftrawberry-plant, 

 fcrofularia, milfoil, arnica, the flowers of Roman camomile, and all the multitude of plants 

 vaguely comprifed under the title of aftringents, contain the gallic acid only. All thefc 

 form with the fulphate of iron a precipitate more or lefs coloured and abundant -, but none 

 of them produce the flighteft change in the folutlon of animal glue. On.the contrary, the 

 tanning principle has never been found alone, but always united or combined with the gallic 

 principle. It was long fuppofcd to exift exclufively in the oak, the nut-gall, and fumac, the 

 only fubftances ufed at the tan-works ; but it is found more or lefs abundantly in the fili- 

 quaftrum, the rofe-tree, tlie larix, f<j»eral fpecies of pines, the acacias, the lotus, the fquil!, 

 the roots of biftort, of rluibarb, of parella, and feveral other plants, of which we fliall here- 

 after give a lift. We have alfo found this principle in the produQs of diftillation of dif- 

 ferent vegetable fubftances, where it was in fome meafure formed during the operation. 



From thefe diffirrent confiderations, founded on experiment, the following general prin- ■ 

 ciples may be deduced : 1. Every fubftance of which the infufion is capable of precipitating 

 animal jelly, poITefles the tain.lng property. 2. Every fubftance which poflelTes the tanning 

 property, likewife precipitates the fulphate of iron black. 3. Every fubftance which preci- 

 pitates the fulphate of iron, but not the folution of glue, does not poftcfs the tanning pro- 

 perty *. 



We fliall difmifs the confideration of the tanning principle, by flicwlng fome of tlie re- 

 markable changes to which its infufion is fubje£l. 



I. A few days after its preparation a yellowifli precipitate is fpontaneoufly formed^ 

 which is more abundant in proportion as the liquor is more faturated, and the time of in- 

 fufion longer. This precipitatt, when feparated, is converted by drying into a very fine, 

 light, afti-coloured powder, totally infoluble in water, oil, alcohol, and ether, even at the 

 temperature of ebullition. When thrown on burning coals, it readily takes fire, and emits 

 a thick fmoke. By deftruilive diftillation it affords a confiderable quantity of carbonic 

 acid gas, an aqueous acid, and a brownifli oil, leaving in the retort a light fpongy coal. 



• The foluticn of glue is a convenient re-agent to afcertain the prefencc of the tanning principle. It is there- 

 fore advantageous to have it in reaiiincfs in the laboratory. To prevent the fpecdy puircfaflion to which ani- 

 mal fubfonccj arc liable, we have added one-twcnticth part of alcohol, which anfwcis tlic purpofe very well, 

 witltout altering the properties of the fluid. C. 



6 A» 



