1252 CARBAMIC ACID. 



and in gravel, and is occasionally found in urine, from which it 

 separates out as a greyish sediment on standing. It is prepared 

 from this sediment, or better still from cystic calculi, by solution 

 in ammonia. This solution is then allowed to evaporate spon- 

 taneously and yields the cystine in regular, colourless, six-sided 

 tables of very characteristic appearance. Cystine may be sep- 

 arated from urine by taking advantage of the formation of a 

 sodium salt of benzoyl-cystine when it is shaken up with a few 

 drops of benzoyl chloride. 



Cystine is insoluble in either water, alcohol or ether, readily 

 soluble in ammonia, differing in this respect from uric acid, 

 also in many alkaline carbonates and in mineral acids. Its 

 solutions are strongly Isevorotatory. 



Cystine is one of the few crystalline substances, occurring 

 physiologically, which contain sulphur, hence its detection is 

 easy. 



AMIDO-ACIDS OF THE OXALIC SERIES. 



1. Carbamic acid. 



Carbamic acid is a substance of peculiar interest to the physi- 

 ologist on account of the important part it is frequently sup- 

 posed to play in the formation of urea in the animal body. 



Carbamic acid is unknown in the free state ; its best known 

 salt is that with ammonium, but many others have been pre- 

 pared. It further appears that some of its salts occur in serum 

 and its calcium salt in the urine of the horse, and it is also 

 stated to be formed during the oxidation of glyciiie, leucine, 

 and tyrosine by means of potassium permanganate in alkaline 

 solution. Ammonium carbamate is extremely soluble in water, 

 in which solution it is gradually converted into the carbonate. 

 At ordinary pressures when heated to 60 it is decomposed into 

 ammonia and carbon dioxide, but under pressure at 130 140 

 it yields urea. When electrolyzed in cold aqueous solution 

 by a rapidly and continuously commutated current the salt 

 similarly loses water and yields urea. 



2. Aspartic (or asparaginic) acid. C 4 H 7 NO 4 . 

 [COOH.CH 2 .CH(NH 2 ).COOH]. (Amido-succinic acid.) 



This acid is chiefly obtained from plant extracts, and occurs 

 notably in beet-sugar molasses. It may be synthetized, but is 

 most conveniently prepared by boiling asparagine with caustic 

 alkalis or mineral acids. It is also a typical product of the 

 action of boiling mineral acids and caustic baryta on both 

 vegetable and animal proteids and of acids on gelatin, being 

 usually accompanied by its homologue, glutamic acid. It is 

 also now recognized as a product in minute quantities of the 





