98 THE CHEMICAL COMPOSITION OF THE BODY 



teids are insoluble in the usual solvents, though fibrin is slightly soluble in 

 10 per cent sodium chloride. 



Carbohydrate Reactions. The carbohydrate representatives that 

 should be examined are: 



8. Starch. Make a solution of starch by boiling i gram of 

 starch in 100 c.c. of distilled water and test. 



a. Iodine test. Shake up three or four drops of dilute iodine solution 

 with 2 c.c. starch. A deep blue color appears. The color is discharged in 

 dilute alkali and reappears on acidifying again. Heat also discharges the color. 



b. Fehling's test. Commercial starch often contains reducing sugar. 

 Boil 2 c.c. of starch solution with i c.c. of fresh Fehling. If a reddish-yellow 

 precipitate settles on standing, the starch contains reducing sugar as an 

 impurity. Starch does not reduce copper in the presence of an alkali. 



c. Hydrolysis of starch. Boil starch solution with 5 per cent sulphuric 

 acid for fifteen minutes. Test with Fehling's solution, first neutralizing the 

 excess of acid. A copious precipitate of cuprous oxide shows that the starch 

 has been converted to reducing sugar. 



9. Dextrin. Make a 5 per cent solution of dextrin in distilled 

 water and test: 



a. Iodine. This gives a rich reddish-brown color which is characteristic. 



b. Fehling. Not reduced by dextrin. 



10. Dextrose. Test a 5 per cent solution of dextrose: 



a. Iodine test. No reaction. 



b. Trommer's test. Add caustic soda in excess and a few drops of 2 per 

 cent copper sulphate and boil, or use Fehling's solution. A reduction of the 

 copper takes place. Barfoid's solution also is reduced by dextrose, but not 

 by maltose. 



11. Glycogen. Make up 10 c.c. of a i per cent solution of gly- 

 cogen and repeat the tests: 



a. Iodine. This gives a wine-red color very much like that given by 

 dextrin. The color is discharged by heating, but reappears on cooling. 



b. Lead acetate. It gives a precipitate, but one must guard against 

 the presence of proteid as an impurity. 



c. Trommer's test. Glycogen does not reduce copper. 



The Fats. The common fats are the oleins, palmitins, and 

 stearins. These are glycerin salts of the fatty acids. The animal fats are 

 mixtures of these fats in different proportions. 



12. Neutral Fat. a. Melting-point. Compare neutral olive oil, 

 some fresh rendered lard, and some tallow. The former is fluid at ordinary 

 room temperature. Determine the melting-points of the lard and of the 

 tallow by the method of Wiley. Fill a test tube, one-half full of water and 

 add a two-inch top layer of alcohol. Prepare a thin flake of fat and suspend 

 it in the test tube at the dividing line of the water and alcohol. Insert the 



