622 THE CHEMISTRY OF THE URINE. 
when applied to tissues stained with urobilin, and may thus be used as 
a test for such staining (Adolf Schmidt). 
When to a concentrated solution of nearly pure urobilin in sodic or 
potassic hydrate, sufficient sulphuric or hydrochloric acid is added to 
render the liquid faintly acid, a slight turbidity is observed, due to the 
liberation of the free pigment from its more soluble alkaline combination. 
If the turbid liquid be examined with the spectroscope, there is seen, in 
addition to the ordinary acid band between b and F, a sharply-defined 
narrow band in the green, enclosing, and being almost bisected by the 
Fraunhofer line E (Fig. 57, Spectrum 6). This extra band is most probably 
due to the special light absorption exercised by the impalpable particles of 
solid urobilin in suspension. It wholly disappears when the precipitate 
is filtered off, or when it is redissolved, the ordinary band alone being 
then visible. 1 
Solid urobilin is an amorphous red-brown substance, which, when 
isolated and dry, may be kept without decomposition. It is not 
deliquescent, but fuses at comparatively low temperatures, afterwards 
solidifying to a brittle transparent shellac-like form. It has a slight 
but peculiar and characteristic odour. 
Physiological relations. — Urinary urobilin is identical with the chief 
pigment of faeces (stercobilin). So certain is the identity of these two 
substances, that it is undesirable to retain separate names for them. 
Urobilin is closely related to the pigments of the bile. This was 
from the first recognised by Jaffe ; and shortly after the discovery of 
the pigment, Maly prepared a substance (hydrobilirubin) by the 
reduction of bilirubin with sodium amalgam, which he held to be 
identical with urobilin. That the urinary pigment is a reduction pro- 
duct of bilirubin is likely, but it is probable that hydrobilirubin, as 
described by Maly, represents an intermediate stage in the reduction. 
It differs at any rate somewhat from urobilin as it occurs naturally. 
Urobilin is formed, however, when bile decomposes out of contact 
with the air, and it may be extracted from the bile removed post- 
mortem from the gall bladder. 
Several observers have shown that intestinal micro-organisms can 
effect the reduction of bilirubin to urobilin. 
This pigment, or substances closely allied to it, can be prepared direct from 
haemoglobin derivatives — haematin and haematoporphyrin — by reduction pro- 
cesses. It has been stated that oxidation is also capable of yielding urobilin 
from bile and blood pigments respectively, but it is not conceivable that 
both reduction and oxidation could lead to the same chemical result, and there 
is in this matter an anomaly which requires explanation. It must not be 
forgotten that peroxides (peroxide of hydrogen and peroxide of lead, have 
been employed in this connection) may in a sense act as reducing agents, free 
oxygen being given off by the interaction of the peroxide and any easily 
reducible compound with which it is brought in contact. 
Urinary urobilin has not yet been analysed. If the formula of 
hydrobilirubin be compared with those of the related pigments, it will 
be seen that both reduction and hydration probably occur in its 
formation. 
Haematin . . . C 32 H 32 N 4 4 Fe 
Bilirubin . . . C^H.^X 4 O 
Hydrobilirubin . . C 32 H 40 N 4 O 7 
1 Garrod and Hopkins, Journ. Physiol., Cambridge and London, 1896, vol. xx. p. 125. 
