TEGETABLE ACIDS. 159 



entirely in producing an union of the astringent princi- 

 ple with the gelatine, which constitutes nearly the whole 

 substance of skins : since this fact has been ascertained, 

 tanners make use of the liquor of tan which has been 

 once applied, but of which the strength is not exhausted, 

 to moisten the bark in the pits, by which the operation is 

 accelerated, without any injury to the product, and leath- 

 er is thus formed in three or four months, as completely 

 as it would be in eighteen by the use of the bark in a 

 nearly dry powder. 



Dry skins generally increase about one third part of 

 their weight by tanning. The different kinds of bark 

 used in tanning bestow various shades of color upon the 

 leather produced. - 



Tannin has a strong arffinity for coloring principles, so 

 that in many cases it serves as a mordant in stamping; 

 it is not then surprising that leather should retain the col- 

 ors it receives so strongly. 



ARTICLE X. 



The Vegetable Acids, 



I HAVE already observed, that when the proportions of 

 ©xygen combined with hydrogen are more than sufficient 

 for the formation of water, the vegetable product will have 

 an acid character. It can therefore be a matter of but 

 little surprise, that we find acids so abundant in the vege- 

 table kingdom. 



The quantity of acid contained in plants varies greatly 

 during the several stages of vegetation, and according to 

 the circumstances by which the developement of the in- 

 dividual is influenced. Plants raised in the shade, or 

 which grow in cloudy, cold, or rainy seasons, when the 

 transpiration of carbonic acid by the leaves cannot be 

 carried on for want of the action of the direct solar rays, 

 by which a^Ione it is produced, accumulate the acid i^ 

 their vessels, and consequently all their products partake 

 of the same general character. The greater part of fruits 

 are sour before arriving at maturity ; but this is owing to 

 the fact, that the mucilage and su^ar, which are afterwards 



