932 ACIDS DERIVED FROM CARBONIC OXIDE. 



of potash, in a great number of plants, particular!} 7 in the spe- 

 cies of Oxalis and Rumex ; combined with lime it also forms 

 a part of several lichens. Oxalate of lime occurs likewise as a 

 mineral, humboldite, and forms the basis of a species of urinary 

 calculus. This acid is also produced by the oxidation of carbon 

 in combination, in a variety of circumstances, being the general 

 product of the oxidation of organic substances by nitric acid, 

 hypermanganate of potash, and by fused potash. Those matters 

 which contain oxygen and hydrogen in the proportion of water, 

 furnish the largest quantity of oxalic acid. 



This acid has been derived in quantity from lichens, but it is 

 usually prepared by acting upon 1 part of sugar, or better, starch, 

 by 5 parts of nitric acid, of 1.42, diluted with ten parts of water 

 at a gentle heat till no gas is evolved, and evaporating to crys- 

 tallize. The crystals must be drained, and crystallized a second 

 time, as they are apt to retain a portion of nitric acid. 



It forms long, four-sided, oblique prisms, with dihedral sum- 

 mits, or terminated by a single face. These crystals contain 

 three atoms of water, one of which is basic, and the other two 

 constitutional, or water of crystallization.. The latter two may 

 be expelled at a temperature above 212, and the protohydrate 

 rises at the same time in vapour, and condenses as a woolly 

 sublimate. Heated in a retort, the hydrated acid undergoes 

 decomposition about 311, and is converted into carbonic oxide, 

 carbonic acid, and formic acid, without leaving any fixed residue. 

 Nitric acid, with heat, converts oxalic acid into water and car- 

 bonic acid. When heated with sulphuric acid, oxalic acid yields 

 equal volumes of carbonic oxide and carbonic acid ; C 2 O 3 being 

 equivalent to CO + CQ 2 , (page 308). No charring, nor evolu- 

 tion of any other gas occurs, so that the action of concentrated 

 sulphuric acid affords the means of recognising oxalic acid or 

 any oxalate. Crystallized oxalic acid is soluble in 8 parts of 

 water, at 59, in its own weight of boiling water, and in 4 parts 

 of alcohol, at 59. 



Oxalates. With potash, and with ammonia, oxalic acid forms 

 neutral oxalates, binoxalates, and quadroxalates. Oxalate of 

 potash, KO,C 2 O3-fHO, contains 1 atom of water of crystal- 

 lization, which it loses a little above 212; it crystallizes 

 generally in prisms of six unequal sides, terminated by oblique 

 dihedral summits ; these crystals are soluble in 3 parts of water, 



