6 22 THE CHEMISTRY OF THE URINE. 



when applied to tissues stained with nrobilin, 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 & 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 hsematoporphyrin 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. 



Hasmatin . . . C 32 H 32 N 4 4 Fe 

 Bilirubin . . . C 32 H 36 N 4 6 

 Hydrobilirubin . . C 32 H 40 N 4 T 

 1 Garrod and Hopkins, Journ. PhysioL, Cambridge and London, 1896, vol. xx. p. 125. 



