12 A MANUAL OF PHYSIOLOGY 



4. Fatty Acids. Heat some 20 per cent, sulphuric acid in a small 

 flask nearly to boiling, and drop into it some of the soap obtained 

 in 3. The fatty acids separate out and rise to the top as an oily 

 layer. Cool, skim off the fatty acid, and wash it with distilled 

 water till the wash- water is no longer acid. 



(a) Dissolve a little of the washed fatty acid in ether. Add a few 

 drops of an alkaline solution of phenolphthalein to a few c.c. of 

 water in a test-tube. Drop into this the ethereal solution of fatty 

 acid. The red colour is discharged. 



(b) Put a small portion of the fatty acid on a glass slide resting 

 on a piece of white paper. Place on it a drop or two of a i per cent, 

 solution of osmic acid (osmium tetroxide). The osmic acid is 

 reduced to a lower oxide (which is black) by the action of oleic acid 

 present in the fatty acid mixture, which abstracts some of the oxygen. 

 Any fat which contains olein or oleic acid, as body-fat does, is 

 therefore blackened by osmic acid. 



(c) Add to a portion of the fatty acid some sodium hydroxide 

 solution, and warm. Sodium soap is formed. Add warm water 

 and shake up. A lather is produced. Keep the soap solution 

 for 6. Keep a little of the fatty acid for 5 (b) and 6 (b). 



5. Glycerin. (a) Add to a little glycerin in a dry test-tube a 

 few crystals of potassium bisulphate (KHSO 4 ), and heat over the free 

 flame. Acrolein is given off, which is recognised by its pungent 

 odour, and by blackening a piece of filter-paper moistened with 

 ammoniacal silver nitrate solution, and held over the mouth of the 

 test-tube. The paper is blackened owing to the reducing action of 

 the vapour on the silver nitrate. 



(b) Repeat this test with lard, and with a portion of the fatty 

 acid from 4. Acrolein will be given off by the lard because glycerin 

 is contained in neutral fat, but not by the fatty acid if it has been 

 properly separated from the glycerin. 



6. Emulsification. (a) Take three test-tubes and label them A, 

 B, and C. Put a few c.c. of water in A, a solution of soap in B, 

 and a dilute solution of sodium carbonate or sodium hydroxide in C. 

 To each add a few drops of fresh olive-oil and shake. An emulsion 

 will be formed in B, but not in A. Probably there will be some 

 emulsification in C also, owing to the presence in the oil of some 

 fatty acid, which forms soap with the alkali. But if the oil is free 

 from fatty acid no emulsion will be formed. 



(b) Repeat (a) with rancid olive-oil, which contains much fatty 

 acid, or with fresh olive-oil to which some of the fatty acid obtained 

 in 4 has been added. A good emulsion will be produced in C as 

 well as in B. 



7. Melting-point of Fat. Put into a very narrow test-tube or a 

 short piece of narrow glass tubing some finely divided mutton fat, 

 freed as far as possible from connective tissue. Fasten the test-tube 

 on to the bulb of a thermometer with a rubber band, and immerse 

 the thermometer and tube in a beaker filled with water and standing 

 on a water-bath which is gradually heated. Observe the tempera- 

 ture at which the fat melts. Repeat the experiment with hog's lard 

 and dog's fat. 



