CHEMICAL BASIS OF THE ANIMAL IIODV. 151 



are strongly la vnmtatury, (a) D =-205 < 9 in hydrochloric acid 

 112 j>. c. 1 or if the acid is dilute (a) D = -214. 2 



Apart from the characteristic crystalline form and its solubility 

 in ammonia, the fact that cystin is one of the few crystalline sub- 

 stances, occurring physiologically, which contain sulphur, ren- 

 ders its detection \vi Thus when boiled with caustic 

 alkalis a sulphide of the alkali is obtained which gives a dark 

 stain on silver foil; also a brown or black colouration appear- 

 when cystin is boiled in a test-tube with a solution of oxide of 

 lead in caustic soda. 3 



AMIDO-ACIDS OF THE OXALIC SERIES. 



1. Carbamic acid. NH,(COOH). 



Carbonic acid is more usually classed at the head of the acids 

 of the glycolic (lactic) series. It exhibits however a remarkable 

 difference from 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 replaceable hydroxyls which it con- 

 tains, it yields two amido-derivatives, of which the first is car- 

 bamic 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 witli it at the 

 same time to yield the ammonium salt or ammonium carl a mate. 

 Thus 2NH 8 + CO 8 = NH 4 NH,CO, : simple dehydration of this 

 salt yields urea (NH 2 ) 3 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 i- that with ammonium, but many others have been prepared. 

 It further apjM-ars that some of its salts occur in serum, and it is 

 ^tate.l to I..- formed during the oxidation of glycin, leucin. 

 ami tyrosin by means of potassium permanganate in alkaline 

 solution. 4 Ammonium carbamate i- extremely soluble in water. 



i Mautlm..,-. 7.i / pkgM. '(.'hem. Bd. vn. (1883), S. 225. Cf. Drechael, .1 



i: 



- Bamnanii, iif. <-,!. S. :tn:t 



8 The following literatim' may ho a<Mitimally con.siiltoil on the occurrence of 

 cystin in iiriii.-. /.< t. ;././.</../. ft,', m. 15.1.- l\. I _>'.. -_'M>: \ll --'.VI; \n 

 Vin-how's .liW/. I 3 Lit M,; - Ja 1884 S. 465. Berl. klin. 



tck. I**'.*. No. If,. '/.< /. /.//. .\f"l. ll.'l. \^ - .325. 



1 Dn-. hsrl, II, r . il.l:. s. (,'. |'/itr M-illi. nntum-iss. CI. .Tllli. I -7 " 



.//. /'. prnk-t. Chrm. (>) H.I. XII. ( 175). - I. (1877), 8. 180 ; XXII. (1M 



\>-rh. f. rii*il. Jahrjf. 1880, S. 550. But see Uo ll.,fin. i-t.-r. 1: 

 .1 1M \n (1871 8 



