PRACTICAL EXERCISES 431 



the bile-acids, and succeeds also in a solution of bile-salts. The test 

 is very sensitive. But in stomach contents, vomit, or stools, it 

 rarely gives good results, since alcohol or acetic acid is often present 

 in the gastric liquid, and phenol and its derivatives in intestinal 

 contents, and all of these cause such an alteration in the surface 

 tension that the sulphur sinks. Ether, chloroform, turpentine, 

 benzine and its derivatives, anilin and soaps, also vitiate the test 

 in the same way. 



(e) Add yellow nitric acid (containing nitrous acid) to a little bile 

 on a white porcelain slab. A play of colours, beginning with green 

 and running through blue to yellow and yellowish-brown, indicates 

 the presence of bile-pigment (Gmelin's reaction). The reaction may 

 also be obtained by putting some yellow nitric acid into a test-tube, 

 and then running a little bile from a pipette on to the surface of the 

 acid. The play of colours is seen at the surface of contact. Where 

 the bile-pigment is present only in traces, some of the liquid may be 

 filtered through white filter-paper, and the test applied by putting 

 a drop of the nitric acid on the paper. 



(/) Cholesterin (Fig. 159) Preparation. Extract a powdered 

 gall-stone (preferably a white one) with hot alcohol and ether in a 

 test-tube. Heat the test-tube in 

 warm water, not in the free flame. 

 Put a drop of the extract on a slide. 

 Flat crystals of cholesteria, often 

 chipped at the corners, separate out. 

 (a) Carefully allow a drop of strong 

 sulphuric acid and a drop of dilute 

 iodine to run under the cover-glass. 

 A play of colours violet, blue, green, 

 red is seen. 



(/3) Evaporate a drop of the solu- 

 tion of cholesterin in a small porce- 

 lain capsule, add a drop of strong 

 nitric acid, and heat gently over a 



flame. A yellow stain is left, which FlG - W- CRYSTALS OF CHOLES- 

 becomes red when a drop of ammonia 

 is poured on it while it is still warm. 



(7) Dissolve a little cholesterin in chloroform. Add an equal bulk 

 of strong sulphuric acid, and shake gently. The solution turns red 

 and the subjacent acid shows a green fluorescence. 



(8) Put a drop or two of water in a watch-glass, and add a drop 

 of an ethereal solution of cholesterin. The cholesterin is precipi- 

 tated, and will not dissolve in the water even on heating. Repeat 

 the observation with bile instead of water. The cholesterin dis- 

 solves in the bile. 



(g) To a little of the filtrate from a peptic digest (e.g., fibrin which 

 has been digested for twenty-four hours with pepsin and hydro- 

 chloric acid) add some bile. A precipitate is thrown down, which 

 is redissolved in excess of the bile (p. 341). 



(h) Shake up a little bile with some rancid olive-oil in a test-tube. An 

 emulsion is formed . Repeat the experiment with the same quantities 

 of bile and oil, but use perfectly fresh oil. Compare the stability of 

 the two emulsions, allowing the tubes to stand together for a while. 



(i] To some starch mucilage, shown to be free from sugar, add a 

 little bile, and place in a bath at 40 C. After a time test for reducing 

 sugar. Report the result. Bile often has a slight diastatic power. 



