30f Experiments and Olservatlons on the 



III making some experiments upon the affinities of the 

 tanning principle, I found that all the earths were capable 

 of attracting it froin the alkalis : and so great is their ten- 

 dency to combine with it, that, by means of them^ the com- 

 pound of tannin and gelatine may be decomposed without 

 much difficulty; for, after pure magnesia had been boiled 

 for a few hours with this substance diffused through water, 

 it became of a red brown colour, and the fluid obtained by 

 iiltration produced a distinct precipitate with solution of 

 galls. The acids have less affinity for tannin than for gela- 

 tine ; and, in cases where compounds of the acids and tan- 

 nin arc acted on by solution of gelatine, an equilibrium of 

 affinity is established, in consequence of which, by far the 

 greatest quantity of tannin is carried down in the insoluble 

 rombination. The different neutral salts have, compara- 

 tivelv, feeble powers of attraction for tjie tanning principle; 

 but that the precipitation they occasion in astringent solu- 

 tions is not simply owing to the circumstance of their uni- 

 tmg to a portion of the water which held the vegetable sub- 

 stances in solution, is evident from many facts, besides 

 tliose which have been already stated. The solutions of 

 aluni, iand of some other salts which are less soluble In 

 water than tannin, produce, in many astringent infusions, 

 precipitates as copious as the more soluble saline matters ; 

 and sulphate of lime, and other earthy neutral compounds, 

 which are, comparatively speaking, insoluble in water, 

 speedily deprive them of their tanning principle. 



From the different facts that have been stated, it is eivi- 

 dent that tannin may exist in a state of combination in dif- 

 ferent substances, in which its presence cannot be made 

 evident by means of solution of gelatine ; and in this case, 

 to detect its existence, it is necessary to have recourse to 

 the action of the diluted acids. 



In considering the relations of the different facts that 

 have been detailed, to the processes of tanning and of lea- 

 tlier-nuiking, it will appear sufficiently evident, that v/hen 

 skin is tanned in astringent infusions that contain, as well 

 is tsLnnIn, extractive matters, portions of these matters 

 enter, With the tannin, into chemical combination with the 

 skin. In no case is there any reason to believe that gallic 

 acid is absorbed in this process ; and M. Seguin's ingenious 

 theory of the agency of this substance in producing the de- 

 oxygcnation of skin, seems supported by no proofs. Even 

 in the formation of glue from skin^ there is no evidence 

 which ought to induce tis to suppose that it loses a portion 

 9f oxygen; and the effect appears to be owing merely to 



the. 



