ORGANIC ACIDS. 89 



is owing to the generation of this acid, that milk, when kept, becomes 

 curdled,— the acid which is formed coagulating the casein. It is mo- 

 nobasic. 



Tartaric acid, CgH_jO^,3-{-2HO. — This is the acid of grapes, tama- 

 rinds, and several other fruits, in which it exists combined with 

 potassa. The tartaric acid of commerce is prepared from the tartar 

 or argot, ciw impure acid tartrate of potash, which is deposited from 

 the grape-juice during the process of fermentation. The argol, when 

 purified and deprived of its colour, constitutes cream of tartar. The 

 acid is obtained from this salt by saturating it with carbonate of 

 lime, by which it is converted into a tartrate of lime, and a tartrate 

 of potassa; the latter is separated from the former, which is insoluble, 

 by filtration ; the tartrate of lime is then decomposed by sulphuric 

 acid. It forms colourless, transparent crystals, freely soluble in 

 water, of a sour taste. As it is bibasic, it requires two equivalents 

 of a base to form with it a neutral salt ; hence, the salts which it 

 forms with a single atom of a fixed base, have an acid reaction, and 

 require the presence of an atom of basic water. Thus, the salt 

 heretofore known as bitartrate of potash (cream of tartar) must now 

 be considered as an acid tartrate of ])otash ajid water. This salt 

 forms small transparent crystals ; it is tolerably soluble in boiling 

 water, sparingly so in cold water ; has an acid reaction and sour 

 taste. 



There is also a neutral tartrate of potash, called soluble tartar, 

 which contains two equivalents of the base united to one of acid. 



Tartrate of potash and soda, — Rochelle salts ; — made by neutra- 

 lizing a solution of cream of tartar with carbonate of soda. It forms 

 large prismatic, transparent crystals, freely soluble in water. Acids 

 precipitate cream of tartar from its solution. 



Tartrate of antimony and potassa, — Tartar emetic ; — made by 

 boiling sesquioxide of antimony in a solution of cream of tartar ; the 

 excess of acid unites with the base. Tartar emetic crystallizes in 

 octohedrons with a rhombic base ; very soluble in boiling water ; 

 has an austere metallic taste. Its solution is decomposed by both 

 acids and alkalies ; the former throws down a mixture of cream of 

 tartar and oxide of antimony ; the latter, the oxide. Sulphuretted 

 hydrogen precipitates the sulphuret of antimony. Tartaric acid is 

 distinguished by forming with any salt of potash the well-known 

 cream of tartar. 



Heat converts tartaric acid into pyrotartaric acid. 



Tannic acid, C^gH-Og+SHO. This is the astringent principle 

 found in many vegetables, as the oak, gall-nut, &c. ; it is generally 

 associated with gallic acid. It is best procured by pouring commer- 

 cial sulphuric ether on coarsely powdered galls, and allowing it to 

 percolate slowly. The water, which always exists in combination 

 with common ether, dissolves out the tannic acid, while the ether 

 V 8* 



