132 VEGETABLE ACIDS. 



Th€ substances, the chemical constitution of which we have stDI 

 to examine, may in general be obtained in the cry'stallized state ; 

 their individuality seems more decided ; they are more stable, better 

 characterized, and their specific properties often assimilate them to 

 inorganic bodies. Such, for example, are the acids formed in the 

 course of vegetable existence. 



Vegetable acids present all the general characters of mineral 

 acids, while they participate in the properties inherent in organic 

 substances. Thus they form salts by uniting with bases ; with 

 potash, soda, ammonia, they form sahs soluble in water ; the other 

 bases produce compounds that are soluble or insoluble, according to 

 the nature of the acid. These acids, free or uncombined, are very 

 frequently met with in fruit, sometimes in the leaves, more rarely 

 in the seeds and roots ; but in combination with bases they are met 

 with in almost all parts of plants. Already very numerous, they are 

 increasing rapidly with the progress of discovery ; with the excep- 

 tion of a very few employed in the arts, their study forms a subject 

 of no great interest. I shall therefore confine myself to a few of thf. 

 most extensively distributed of these acids. 



Oxalic acid. This acid exists free in the hairs of the cicer or 

 chick-pea, and united with potash constituting an acid salt, the bin- 

 oxolate of potash in the w ood sorrel and the common or garden sor- 

 rel. It is from the former of these plants that the salt called salt 

 of lemons, but which is, in fact, the binoxolate of potash, is still ex- 

 tracted in some countries. The juice of the wood sorrel is expressed 

 and yields about 0.003 of its weight of the salt, from which, by or- 

 dinary chemical manipulation, the oxalic acid is readily obtained. 

 At the present time this acid is prepared artificially by the action of 

 nitric acid upon starch ; it is a powerful acid, and its atfinily for lime 

 is such that it takes this base even from its union with sulphuric 

 acid. 



Tartaric acid is met with above all in the grape in the state of 

 bit;irtrate of potash, a salt which is deposited upon the sides of the 

 casks ill which the wine is kept. After having been pn)perly puri- 

 fied, it is known in commerce under the name of cream of tartar, 

 from which the tartaric acid can readily be obtained. Another par- 

 ticular acid, the raccmic acid, the composition of whicH is identical 

 with that of the tartaric acid, has been discovered in the tartar of 

 the wines grown on the Upper Rhine. 



Citric acid. This acid is found in the juice of many plants, and 

 abundantly in the juice of lemons, oranges, currants, <S:c. It is from 

 the lemon and the lime that the citric acid employed in the arts is 

 generally obtained. 



Tannic acid. A certain substance which is met with in the bark 

 of particular trees, and which has the valuable property of rendering 

 the hides of animals with which it is combined insusceptible of pu- 

 trefaction, is familiarly known under the name of tannin. The art 

 of the tanner is founded upon this property of tannm. A solution 

 of gelatine being poured into an infusion of tannic acid, an insoluble 

 precipitate, t'nrmrd by thr> union ot ih'^ arid with the animal matter 



