178 FUNCTIONS OF NUTRITION. 



sion of the atmosphere. If ox-bile of a pure green or olive-green hue 

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

 from the air, it gradually loses its green color, becoming a dull yellow. 

 The alteration 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 has become light yellow or yellowish-brown. The green 

 hue may then be restored by the addition of iodine, or by exposing 

 the bile in thin layers to the air. This change depends on the conver- 

 sion of bilirubine by oxidation into biliverdine. 



The bile exhibits a peculiar reaction with nitroso-nitric acid, due to 

 the effect on its coloring matter. If bile be brought in contact, in a 

 cylindrical glass vessel, with a layer of this acid, and allowed to remain 

 without agitation, a series of colored rings are produced at the surface 

 of contact, following each other in definite order, from the bile to the 

 nitric acid, as green, blue, violet, red, and yellow. The colors repre- 

 sent successive^ stages of the oxidation and final destruction of the color- 

 ing matter. This test, known as " Gmelin's bile test," may be applied 

 to other animal fluids in which bilirubine is supposed to be present. 



The bile presents, also, certain optical properties which distinguish 

 it from other animal fluids. 



First, it is dichroic ; that is, it has two different colors by trans- 

 mitted light, according to its mass. If a specimen of ox-bile, which 

 appears of a pure transparent green color by ordinary daylight in layers 

 of two or three centimetres, be viewed by strong sunlight in a thick- 

 ness of five or six centimetres, it is red. In this respect it resembles 

 a solution of chlorophylle, which presents the same contrast of colors 

 in a very marked manner. 



Secondly, it is fluorescent ; * that is, it becomes faintly luminous 

 with a color of its own, when viewed by the more refrangible rays of 

 the solar spectrum. If a specimen of clear greenish bile be. placed in 

 the track of either the violet or the blue ray of the solar spectrum, it 

 becomes visible with a light yellowish-green tint. In the green it is 

 more yellowish ; and in the yellow it has a tinge of red. Thus in all 

 parts of the spectrum where it exhibits this property, it emits a light 

 of less refrangibility than that of the ray by which it is illuminated. 

 Fluorescence is also manifested, to a remarkable degree, by solutions 

 of chlorophylle, which, although of a clear green color by diffused day- 

 light, are pure red, when viewed by either the violet, blue, green, or 

 yellow ray of the spectrum. 



The fluorescence of bile does not depend on its coloring matter, but 

 is due mainly to the biliary salts, since it is also exhibited by their 



* 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 cer- 

 tain parts of the spectrum. Thus a solution of quinine sulphate, which is colorless 

 in ordinary daylight, becomes blue where the sun's rays are concentrated upon it by 

 a lens ; and it exhibits a distinct luminosity in both the violet and ultra-violet parts 

 of the spectrum. 



