382 CHEMISTRY OF THE DIGESTIVE PROCESSES. 
The acids formed as cleavage products from human bile are cholalic, 
choleic, and fellic acids. 
Cholalic acid and its allies, on boiling with acids, on heating in the 
dry state, or by putrefaction, lose water, and become converted into 
anhydrides, or, as they are called, dyslysins. The dyslysin corresponding 
to cholalic acid has the formula C^H^O^ ; it is found in faeces ; is a white 
amorphous substance, insoluble in water and alcohol, soluble in ether 
and melting at 140 c C. Another compound, choloidinic acid, is formed, 
as an intermediate stage, of the formula C 24 H 3S 4 . On boiling with 
alkalies, dyslysin takes up water, and is reconverted into cholalic acid. 
The bile pigments and their derivatives. — The variations in the 
colour of bile early attracted attention, and Gmelin, 1 in 1826, first 
obtained proof of a relationship between these colours, and described 
the test which still bears his name. He was aware that the play of 
colours was due to a process of oxidation, and made an experiment to 
illustrate this by acidifying bile with hydrochloric acid, and enclosing 
it in a tube from which the air was shut oft' by a mercury trap. Under 
these circumstances no change in colour took place ; but on exposing 
the acidified bile to the air, a green colour slowly developed. He also 
accurately described the play of colours obtained on oxidising with 
nitric acid. 
Berzelius 2 precipitated biliverdin from ox bile with barium chloride, 
purified it to some extent, and described its properties, but he fell into 
the error of supposing that it was identical with chlorophyll 3 
Heintz, 4 preventing oxidation by exclusion of ah', extracted from 
gallstones a brown amorphous pigment, which he named biliphain. 
He analysed it, and converted it by dissolving in sodium carbonate, and 
leading oxygen through the solution into a green pigment, biliverdin. 
His biliphain corresponded to the bilirubin of the present day. and his 
experiment shows well the connection between the two pigments. 
Valentiner, 5 in 1859, was the first to obtain bilirubin in a crystalline 
form, by dissolving in chloroform, from winch, on evaporation of the 
solvent, it crystallises in microscopic crystals. From this discovery 
onwards, research on the bile pigments took a more exact form, as 
methods for the isolation of the pigments were discovered and perfected. 6 
Although a considerable number of more or less well-characterised 
bile pigments have been described, only two are found under normal 
conditions in the bile, these are bilirubin and biliverdin ; the others are 
obtained by artificial means from these, are found under pathological 
conditions only in the body, or are formed after death. The colour of 
the bile is a compound of the colour of these two pigments, and varies 
with the varying ratio of their amounts through all shades between 
1 Tiedemann and Gmelin, " Verdauung naeh Versuchen," 1826. 
2 "Chemie," S. 281. 
3 The spectra of phylloporphyrin and hsematoporphyrin and their derivatives are almost 
identical, and in other respects the substances closely resemble each other, so that there is 
undoubtedly a relationship between them (Schunck and Marohlawski, Proc. Roy. Soc. 
London, Jan. 1896). Now, phylloporphyrin is a derivative of chlorophyll, and hsemato- 
porphyrin is isomeric with bilirubin, so that there may be some remote connection between 
biliverdin and chlorophyll. 
4 Jahrcsb. ii. d. Fortschr. d. ges. Med., Erlangen, 1851, Bd. ii. S. 59; Ann. d. Phys. 
u. Chem., Leipzig, 1851, Bd. lxxxiv. S. 106. 
5 Jahresb. ii. d. Fortschr. d. <jcs. Med.. Erlangen, 1859, Bd. ii. S. 87. 
6 Brucke, Untersuch. z. Naturl. d. Mensch. it. d. Thiere, Bd. vi. S. 173; Stadeler, 
Vrtljschr. d. naturf. Gesellsch. in Zurich, 1863, Bd. viii. S. 1 ; Ann. d. Chem., Leipzig 
1864. Bd. exxxii. S. 323. 
