HOME ECONOMICS AND EDUCATION 545 



ing a ventilating current of air, for sampling and analyzing this 

 air, for removing and measuring the heat given off within the cham- 

 ber, arid for passing food and other articles in and out. It is fur- 

 nished with a folding bed, chair, and table, with scales and with ap- 

 pliances for muscular work, and has telephone connection with the 

 outside. Here the subject stays for a period of from three to twelve 

 days, during which time careful analyses and measurements are 

 made of all material which enters the body in the food and of that 

 which leaves it in the breath and excreta. Record is also kept of the 

 energy given off from the body as heat and muscular work. The dif- 

 ferences between the material taken into and that given off from the 

 body is called the balance of matter, and shows whether the body 

 is "gaining or losing material. The difference between the energy 

 of the food taken and that of the excreta and the energy given off 

 from the body as heat and muscular work, is the balance of energy, 

 and if correctly estimated should equal the energy of the body ma- 

 terial gained or lost. 



With such apparatus it is possible to learn what effect different 

 conditions of nourishment will have on the human body. In one 

 experiment, for instance, the subject might be kept quite at rest, 

 and in the next do a certain amount of muscular or mental work, 

 with the same diet as before". Then by comparing the results of the 

 two the use which the body makes of its food under the different 

 conditions could 'be determined. Or the diet may be slightly 

 changed in one experiment and the effect of this on the balance 

 of matter and energy observed. Such methods and apparatus are 

 very costly in time and money, but the results are proportionately 

 more valuable than those from similar experiments. (Dept. Agr,. 

 F.- B. 142; Dept. Agr. Off. of Exp. St. Buls. 17, 66, 69, 109, 123 : , 

 136.) 



CHEMICAL COMPOSITION OF FOOD. . ; 



The value of food for nutriment depends mainly upon its com- 

 position and digestibility. The composition of foods is determined 

 by chemical analysis. The first effective impulse to the systematic 

 study of the chemistry of foods was given by Liebig somewhat over 

 50 years ago, but nearly all of our definite knowledge of the chemical 

 composition of food materials has accumulated within compara- 

 tively a few years past. 



Until about the year 1880 most of the food analyses had been 

 made in German laboratories and during the last two decades, the 

 results of over 4,000 analyses of food materials from.. different parts 

 of the United States are available. A large proportion of these 

 analyses have been made during the last few years in connection 

 with nutrition investigations under the auspices of the Department 

 of Agriculture. 



Table I shows the average composition of ordinary American 

 food materials as calculated from the analyses now available. The 

 value of food as influenced by digestibility is discussed later. 



