COMPOSITION OF LIFELESS AND LIVING THINGS 15 



chemical elements, namely, nitrogen, sulphur, and phos- 

 phorus; in fact, proteins are the most complex of all chemical 

 substances known. 



Following is the composition of the various nutrients stud- 

 ied thus far : 



Starch, composed of C, H, O (in the proportion of CeHioOs). 



Sugar, composed of C, H, 0. (Grape sugar = C 6 H 12 O 6 .) 



Fat, composed of C, H, 0. 



Protein, composed of C, H, 0, N, S, P (and sometimes of 

 other elements). 



22. Tests for the food substances. Having demon- 

 strated that the various food substances are chemical com- 

 pounds, each composed of several chemical elements, we are 

 now to carry on experiments by which it will be possible to 

 test for each of these food substances. By this means we 

 shall be able to prove the presence or absence of starch, grape 

 sugar, protein, fat, mineral matters, and water in the foods 

 used by plants, animals, and man. 



23. To test foods for starch. Laboratory Study No. 8. 



Materials: Corn starch, grape sugar, white of egg, fat or oil, 

 salt, water; various foods in the home kitchen; iodine solution 

 (see below) ; test tubes; gas burner or alcohol lamp. 



Preparation of iodine solution: A quart (1000 cc.) of iodine solu- 

 tion is made by dissolving in 5 teaspoonfuls (40 cc.) of water, one- 

 half teaspoonful (4 grams) of potassium iodide, and one-fourth 

 this amount of iodine (1 gram). This solution, when thoroughly 

 mixed, should be diluted to make one quart (1000 cc.). In a clean 

 bottle this mixture will keep indefinitely. 1 



1. Put a small amount (size of a pinhead) of corn starch 

 in a test tube, add water, shake the mixture, and boil 

 it over a gas flame. Pour into the starch mixture 



1 From Peabody's " Laboratory Exercises." Henry Holt & Co. 



