206 THE BILE. 



and sheep and ox-bile of a greenish-olive, or more frequently of a pure 

 green. As a general rule, the bile of the herbivorous animals is more 

 decidedly green in hue, that of the carnivora and omnivora orange or 

 brown. All these differences may be referred to two main classes of 

 tints, corresponding with two different coloring matters; in one of which 

 the predominating color is red or reddish-brown, dependent on biliru- 

 bine, while in the other it is green owing to the presence of biliverdinc. 

 As the proportion of these two substances varies in any given specimen, 

 it will exhib^ a corresponding color of the pure or mingled tints. 



The color of the bile is also modified by oxidizing agents, which pro- 

 duce a green hue in bile which was originally olive or brown, and 

 increase the intensity of the green tint when this color is already pre- 

 sent. If brown or olive-colored bile be exposed to the air for a short 

 time, its surface becomes green by contact with the atmosphere. The 

 same change may be instantly produced by adding to the bile a few 

 drops of a watery solution of iodine ; and a little nitric acid acts with 

 great energy, developing a bright grass-green hue. These changes 

 depend upon the oxidation of the bilirubine, and its consequent con- 

 version into biliverdine. 



The green color of bile also disappears rapidly when excluded from 

 all sources of oxidation. If ox-bile, of a pure green or olive-green hue, 

 be inclosed in a perfectly full and securely stoppered vessel, so as to be 

 entirely protected from the air, it gradually loses its green color and 

 becomes of a dull yellow. This change progresses from the external 

 parts of the liquid toward its centre, until at the end of twelve, twenty- 

 four, or thirty-six hours the whole of it has become of a light yellow or 

 yellowish-brown. In this condition the green hue may be again restored 

 by the addition of iodine, or by exposing the bile in thin layers to the 

 air. The green color of the bile accordingly appears to be dependent 

 on continued oxidation. 



The bile presents, in addition, certain remarkable optical properties 

 which distinguish it from other animal fluids. 



In the first place, it is dichroic ; that is, it has two different colors 

 by transmitted light, according to the thickness of its mass. If ox-bile, 

 which is of a pure transparent green by ordinary daylight in layers of 

 two or three centimetres, be viewed by strong sunlight in a thickness 

 of five or six centimetres, it is red. In this respect it resembles a solu- 

 tion of chlorophylle, which presents the contrast of the same two colors 

 in a very marked manner. 



Secondly, the bile is fluorescent? that is, it becomes faintly luminous 



1 This property, so called from fluor spar, in which it was first observed, is 

 shown by various transparent substances, when illuminated by solar light, or by 

 that of certain parts of the spectrum. Thus a solution of quinine sulphate, which 

 is perfectly colorless by ordinary diffused daylight, becomes blue at any spot where 

 the sun's rays are concentrated upon it by a lens ; and it exhibits a distinct lumi- 

 nosity in both the violet and ultra-violet parts of the spectrum. 



