CHEMICAL BASIS OF THE ANIMAL BODY. 247 



readily obtained by suspending bilirubin in alcohol and oxidising it 

 by passing the fumes of nitrous acid into the mixture. As soon as the 

 play of colours is complete and the solution is of a pure yellow colour, 

 it is poured into a large excess of water, from which on more or less 

 prolonged standing choletelin separates out as a flocculent mass, whicfr 

 if washed and dried yields a brown powder 1 . It is readily soluble in 

 alkalis, as also in either alcohol, chloroform or ether, but least so in the 

 two last solvents. None of the solutions exhibit any fluorescence even 

 after the .addition of zinc chloride. In this it differs markedly from 

 urobilin, a well-known yellow urinary pigment. The above statements 

 scarcely provide any certain means of identifying choletelin as a 

 chemical substance, and no specific test for it has as yet been 

 described. Neither is it quite certainly characterised by its absorption 

 spectrum, so far at least as any specific bands are concerned. Indeed 

 there has been very great difference of opinion as to whether it ever 

 gives any band at all, and if it does, where this band is situated. 

 With our existing knowledge it seems safe to say that in alkaline 

 solutions choletelin shows no absorption band, and that in acid 

 solutions a band may be, and frequently is seen, lying between b 

 and F. The uncertainty as to its spectroscopic properties led some of 

 the older observers 2 to regard choletelin as identical with hydrobilirubin 

 (urobilin). This view is however quite untenable both as the result of 

 purely chemical investigations 3 and of spectrophotometric determina- 

 tions of the optical properties of the two substances 4 . 



5. Hydrobilirubin. C 32 H 40 N 4 O 7 . 



When bilirubin is dissolved in dilute caustic potash or soda or 

 suspended in water and treated with sodium-amalgam in successive 

 portions, air being at the same time carefully excluded, it is observed 

 that at first no hydrogen is evolved ; the dark-coloured solution becomes 

 gradually lighter in colour and more transparent, until at the end of 

 two or three days it is bright yellow or brownish-yellow, and now 

 hydrogen begins to come off from the mixture. At this stage the 

 supernatant fluid should be poured off from the metallic mercury which 

 has accumulated, and if it is now acidulated strongly with either 

 hydrochloric or acetic acid, it yields a more or less copious flocculent 

 precipitate of a dark reddish-brown colour. This precipitate is impure 



1 Maly, Sitz. d. k. Akad. d. Wiss. Wien, Bd. LVII. (1868), 2 Abth. Feb.-Hft. 

 LIX. (1869), 2 Abth. Ap.-Hft. 



2 Heynsius u. Campbell, loc. cit. Stokvis, Centralb. f. d. med. Wiss. 1873, S 

 211, 449. 



3 Maly, Ibid. S. 321, and more particularly Liebermann, Pfliiger's Arch. Bd 

 xi. (1875), S. 181. 



4 Vierordt, Zt. f. Biol. Bd. x. (1874), S. 399. 



