OXALIC ACID. 43 



semblance to crystals of phosphate of ammonia-magnesia, which in 

 the projection resemble a square octahedron ; but a more accurate 

 microscopic examination and the solubility of the triple phosphate in 

 acetic acid enable us to discriminate between these crystals and 

 those of oxalate of lime. GoldingBird* also describes crystals of oxalate 

 of lime shaped-like dumb-bells or rather like two kidneys with their 

 concavities opposed, and sometimes so closely approximating as to 

 appear circular, the surface being finely striated. These crystals 

 are produced, in all probability, by a zeolitic arrangement of minute 

 acicular crystals presenting a physical structure resembling that of 

 spherical crystals of carbonate of lime. [Dr. Golding Birdf has 

 recently shown that in all probability these dumb-bell crystals con- 

 sist of oxalurate of lime. G. E. D.] 



Other oxalates have at present excited no physiological in- 

 terest. 



Preparation. Oxalic acid is a final product of the oxidation of 

 most animal and vegetable bodies ; hence it may be prepared from 

 very different substances by strong oxidising agents : it is most 

 commonly obtained by the decomposition of sugar by not too con- 

 centrated nitric acid, by evaporation to crystallisation, and finally 

 by recrystallisation in water. 



Tests. Oxalic acid and its salts are so well characterised that 

 it is hardly possible to mistake them for any other bodies. In the 

 animal organism oxalic acid is almost always combined with lime, 

 and with a little practice this salt may be readily discovered by the 

 microscope, and by the insolubility of its crystals in acetic acid. 

 Should a further investigation appear necessary, the presence of 

 oxalic acid might be determined by its property of reducing gold from 

 its solutions, and by its not charring either in the free or in the 

 combined state when heated, or on the application of sulphuric acid. 

 Oxalate of lime can be separated from most of the sub stances with 

 which it is likely to be mixed either by acetic acid or by dilute 

 solution of potash. 



Physiological Relations. 



Occurrence. Frequently as oxalic acid, combined either with 

 the alkalies or with lime, occurs in the vegetable kingdom (Schlei- 

 den,J Carl Schmidt, and others), it is very seldom found in the 



* Urinary Deposits ; their diagnosis, pathology, and therapeutical indications. 

 Third edition, p. 208. 

 f Op. cit. p. 212. 



Grundziige der Botanik. 2 Aufl. 1846. 

 Entwurf u. s. w. 



