152 CARBAMIC ACID. 



the remaining acids of this group since they are all monobasic, whereas 

 carbonic acid is dibasic. It may therefore be more appropriately classed 

 with the dibasic acids of the oxalic series. In virtue of the two replace- 

 able hydroxyls which it contains, it yields two amido-derivatives, of which 

 the first is carbamic acid, the second urea (NH 2 ) 2 CO or carbamide. 

 Carbamic acid is a substance of peculiar interest to the physiologist on 

 account of the important part it is frequently supposed to play in the 

 formation of urea in the animal body. It is formed by the direct union 

 of equal molecules of dry ammonia and carbonic anhydride, a second 

 molecule of ammonia uniting with it at the same time to yield the 

 ammonium salt or ammonium carbamate. Thus 2 NH 3 + C0 2 = NH 4 

 NH 2 C0 2 : simple dehydration of this salt yields urea (NH 2 ) 2 CO. 

 This point will be returned to further on when discussing the probable 

 mode of formation of urea in the body. 



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

 that with ammonium, but many others have been prepared. It further 

 appears that some of its salts occur in serum, and it is also stated to be 

 formed during the oxidation of glycin, leucin and tyrosin by means of 

 potassium permanganate in alkaline solution 1 . 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 carbonic anhydride, but under 

 pressure at 130 140 it yields urea. When electrolysed in cold 

 aqueous solution by a rapidly and continuously commutated current 

 the salt similarly loses water and yields urea (Drechsel). The dehy- 

 dration may be represented as taking place in the following way : 

 i. NH 2 . CO . . NH 4 + = NH 2 . CO . O . NH 2 + H 2 O. 

 ii. NH 2 . CO . O . im 2 + H 2 = NH 2 . CO . NH 2 + H 3 O ; 

 or by the action first of H 2 and then of O. 2 



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 synthetised, but is most conveniently 

 prepared by boiling asparagin 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 (antea p. 49) 

 and of acids on gelatin 3 , being usually accompanied by its homologue, 



1 Drechsel, Ber. d. k. s. Gesell. d. Wiss. Leipzig. Math, naturwiss. Cl. Juli, 1875. 

 Jn.f. prakt. Chem. (2) Bd. xn. (1875), S. 417; xvi. (1877), S. 180; xxn. (1880), S. 

 476. Arch. f. Physiol. Jahrg. 1880, S. 550. But see also Hofmeister, Pfliiger's 

 Arch. Bd. xn. (1876), S. 337. 



2 Cf. Ludwig's Festschrift, 1887, S. 1. 



3 Horbaczewski, Sitzb. d, k. Akad. d. Wiss. Wien, Bd. LXXX. (2 Abth.) Juni- 

 Hft. 1880. 



