FOODS AND THEIR USES 47 



small amount into a test tube ; add a little strong 

 nitric acid, and boil. 



a. Describe what was done. 



b. What change in the color of the milk do you observe ? 



c. What food substance do you therefore conclude to be 



present in milk? 



2. Place a drop of the " mixed milk," used in 1 above, on 



paper, and allow the paper to dry over a warm 

 radiator. Hold the paper to the light. What kind 

 of food substance is present in considerable quantity 

 in the milk? How do you know ? 



3. Add a few drops of iodine to some milk. What is the 



result, and what is your conclusion ? 



4. Test another sample of milk with Fehling's solution. 



State the result and your conclusion from the ex- 

 periment. The sugar found in milk is known as 

 milk sugar, and when it is heated with Fehling's 

 solution, it is changed to grape sugar. 



5. Heat a half spoonful of milk, and hold over it a clean, 



dry tumbler. What nutrient does this experiment 

 prove to be present? Why? 



6. (Optional.) Evaporate to dryness the spoonful of milk, and then 



burn the solid residue over a very hot flame. Does all 

 the solid disappear, or is something left on the spoon ? 

 What is your conclusion as to the presence or absence of 

 mineral matter in milk ? 



7. As a conclusion from all your experiments, state what 



food substances or nutrients^ are present in milk, 

 and what food substances are absent. 



51. The composition of other foods. — Our study of 

 milk has shown us that this food is composed of the same 



1 In our study of plant biology we called the oompounds named in 

 this paragraph food substances rather than nutrients, for botanists 

 regard the simpler compounds (carbon dioxid, water, and mineral 

 matters) that plants obtain from the water and air as the nutrients 

 of the plants. By some writers water is not regarded as a nutrient ; 

 since, however, it is an essential constituent of protoplasm, it may 

 well be named among the nutrients. 



