PRACTICAL EXERCISES 457 



through white filter-papsr, and the test applied by putting a drop of 

 the nitric acid on the paper. 



(/) Cholesterin or Cholesterol (Fig. 176) 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 cholesterin, 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. 



() Evaporate a drop of the solution 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 becomes red when a drop of am- 

 monia is poured on it while it is still warm. 



(y) 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. 



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

 ethereal solution of cholesterin. The cholesterin is precipitated, and 

 will not dissolve in the water even on heating. Repeat the observation 

 with bile instead of water. The cholesterin dissolves in the bile. 



(g) To a little of the filtrate from a 

 psptic digest (e.g., fibrin which has been 

 digested for twenty-four hours with 

 p2psin and hydrochloric acid) add some 

 bile. A precipitate is thrown down, 

 which is re dissolved in excess of the 

 bile (p. 364). 



(h] Shake up a little bile with some 

 rancid olive -oil in a test-tube . An emul- 

 sion 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 A* 

 the tubes to stand together for a while. Fig. 176. Crystals of Cholesterin 



(i) To some starch mucilage, shown (Frey). 



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. 



(/) To demonstrate the Presence of Iron in the Liver Cells. Steep sec- 

 tions of liver in a solution of potassium ferrocyanide, and then in dilute 

 hydrochloric acid. Or a 1-5 per cent, solution of potassium ferrocyanide 

 in 0-5 per cent, hydrochloric acid may be used. (The iron may pre- 

 viously be fixed in the tissue by hardening it in a mixture of alcohol 

 and ammonium sulphide.) The sections become bluish from the 

 formation of Prussian blue. A fine-pointed glass rod or a platinum 

 lifter should be used in manipulating them. A steel needle cannot bo 

 employed. Mount in glycerin or Farrant's solution, or (after dehy- 

 drating with alcohol and clearing in xylpl) in xylol-balsam. Blue 

 granules may be seen under th? micrpscop3 in some of the hepatic cells. 

 Sections of spleen may also be examined for this reaction. 



12. Microscopical Examination of Faeces. Examine und?r the micro- 

 scope the slides provided. Draw, and as far as possible d3termine the 

 nature of, the objects seen (p. 418). 



13. Absorption of Fat. (a) Feed a rat or frog with fatty food; kill 

 the rat in throe or four hours, the frog in two or three days. Imme- 

 diately after killing the rat open the abdomen, carefully draw out a 

 loop of intestine, and look through the thin mesentery. The white 



