ELEMENTARY CHEMICAL PHYSIOLOGY 237 



of colour following the gradual oxidation of the pigments by means 

 of nitrous acid. 1 



EXPERIMENT V. Dilute some ox bile with an equal volume of 

 water. Hold the test tube as nearly horizontal as possible, and 

 allow some fuming nitric acid to run down it, so that this forms a 

 layer under the bile. Where the two fluids are in contact, a play 

 of colours is produced. This test can be rendered still more delicate 

 by filtering a little diluted bile through white filter paper, then 

 removing and opening out the filter paper and placing a drop of 

 fuming nitric acid on it. 



Bilirubin is the least oxidised bile-pigment, and its empirical formula is 

 C 32 H 36 N 4 O 6 . If we compare this with the formula of hsematin C 32 H 32 N 4 O4Fe 

 we see that it must be from this body that it is derived, the change being 

 the abstraction of iron and the addition of two molecules of water. This is 

 also the formula of iron-free haematin or haematoporphyrin, and of haematoidin, 

 a pigment which crystallises out in old blood clots in the tissues. Although 

 the same empirically, these bodies vary somewhat in their physical behaviour 

 and neither of them gives Gmelin's test, so that we may assume that they have 

 different constitutional formulae. 



EXPERIMENT VI. Bilirubin can be extracted from pigmented 

 gall-stones. The gall-stones are ground to a rough powder and 

 extracted by heating with 95 per cent, alcohol, to which a few drops 

 of strong hydrochloric acid have been added. (The acid is necessary 

 to decompose the compound of bile pigment with calcium present 

 in the stones.) The coloured extract is then cooled. The crystals 

 of cholesterol, which separate, are filtered off, washed with alcohol 

 and examined (see p. 225). The filtered extract is placed in a 

 dish, and pure nitric acid run in, drop by drop, when a brilliant 

 Gmelin's test is obtained. 



EXPERIMENT VII. Place some bile in a test tube, and add one or two 

 crystals of cholesterol to it and gently warm. The cholesterol dissolves. 

 Before doing this show that the crystals will not dissolve in water. 



Inorganic Salts. These are chiefly sodium carbonate and disodium 

 hydrogen phosphate. 



The Uses of the Bile in Intestinal Digestion. (1) It is an alkaline 

 fluid, containing a viscid substance (mucin, etc.) ; consequently, it 

 assists in the emulsification of fats. 



EXPERIMENT VIII. Shake up some rancid oil with bile in a test 

 tube. Notice that a very stable emulsion is formed (see Fats, 

 p. 223). 



(2) It causes a precipitate when added to an artificial peptic 

 digest. 



EXPERIMENT IX. Add some bile to a sample of a twenty-four 

 hours' peptic digestion of egg-white. A precipitate of proteins is 

 produced. 



1 This test depends on the various colours of the oxidation products of 

 bilirubin. The first oxidation product is biliverdin (green) ; the next bili- 

 cyanin (blue) ; the next bilipurpurin (purple) ; and, finally, choletelin which 

 is yellow. 



