_Laboratory exercise._--Composition of common foods. The series of food charts supplied by the United States Department of Agriculture makes an excellent basis for a laboratory exercise to determine common foods rich in (_a_) water, (_b_) starch, (_c_) sugar, (_d_) fats or oils, (_e_) protein, (_f_) salts, (_g_) refuse. _Demonstration._--Method of using bomb calorimeter. _Laboratory and home exercise._--To determine the best individual balanced dietary (using standard of Atwater, Chittenden, or Voit) as determined by the use of the 100-Calorie portion. _Demonstration._--Tests for some common adulterants. _Demonstration._--Effect of alcohol on protein, _e.g._ white of egg. _Demonstration._--Alcohol in some patent medicines. _Demonstration._--Patent medicines containing acetanilid. Determination of acetanilid. Why we Need Food.--A locomotive engine takes coal, water, oxygen, from its environment. A living plant or animal takes organic food, water, and oxygen from its environment. Both the living and nonliving machine do the same thing with this fuel or food. They oxidize it and release the energy in it. But the living organism in addition may use the food to repair parts that have broken down or even build new parts. _Thus food may be defined as something that releases energy or that forms material for the growth or repair of the body of a plant or animal._ The millions of cells of which the body is composed must be given material which will form more living matter or material which can be oxidized to release energy when muscle cells move, or gland cells secrete, or brain cells think. [Illustration: The composition of milk. Why is it considered a good food?] Nutrients.--Certain nutrient materials form the basis of food of both plants and animals. These have been stated to be _proteins_ (such as lean meat, eggs, the gluten of bread), _carbohydrates_ (starches, sugars, gums, etc.), _fats_ and _oils_ (both animal and vegetable), _mineral matter_ and _water_. Proteins.--Protein substances contain the element nitrogen. Hence such foods are called nitrogenous foods. Man must form the protoplasm of his body (that is, the muscles, tendons, nervous system, blood corpuscles, the living parts of the bone and the skin, etc.) in part at least from nitrogenous food. Some of this he obtains by eating the flesh of animals, and some he obtains directly from plants (for example, peas and beans). Proteins are the only foods available for tissue building. They may be oxidized to release energy if occasion requires it.