HOME ECONOMICS AND EDUCATION 541 



and is taken as a measure of its latent or potential energy. The unit 

 commonly used is the calorie, the amount of heat which would raise 

 the temperature of 1 kilogram of water 1 Centigrade, or 1 pound 

 of water 4 Fahrenheit. Instead of this unit of heat a unit of me- 

 chanical energy may be used for instance, the foot-ton, which 

 represents the force required to raise 1 ton 1 foot. One calorie is 

 equal to very nearly 1.54 foot-tons; that is to say, 1 calorie of heat, 

 when transformed into mechanical ,power, would suffice to lift 1 

 ton 1.54 feet. 



The Conservation of Energy in the Body. The amounts of 

 energy transformed in the body when food and its own material 

 are burned within it are measured with the respiration calorimeter. 

 Food is not completely oxidized in the body. Experiments have 

 shown that the material which is oxidized yields the same amount 

 of energy as it would if burned with oxygen outside the body. The 

 experiments show also that when a man does no muscular work 

 (save, of course, the internal work of respiration, circulation, etc.), 

 all the energy leaves his body as heat; but when he does muscular 

 work, as in lifting weights or driving a bicycle, part of the energy 

 appears in the external work thus done, and the rest is given 

 off from the body as heat. The energy given off from the body as 

 heat when the man is at rest, or as heat and mechanical work to- 

 gether when he is working, exactly equals the latent energy of the 

 material burned in the body. This is in accordance with the law of 

 the conservation of energy which dominates the physical universe. 



The fuel value of protein is 4 calories per gram or 1820 calories 

 per pound. The fuel value of fats is 9 calories per gram or 440 

 calories per pound. The fuel value of carbohydrates is 4 calories 

 per gram or the same as that for protein. So a pound of protein 

 of lean meat or albumen of eggs is just about equivalent to a pound 

 of sugar or starch, and a little over 2 pounds of either would be 

 required to equal a pound of the fat of meat or butter or of body 

 fat. Fat is the most concentrated form of body fuel. There has 

 thus far been considered food only as a source of heat and muscular 

 power. There is no doubt that intellectual activity, also, is some- 

 how dependent upon the consumption of material which the brain 

 has obtained from the food, but concerning this humanity does 

 not know it all yet. (Dept. Agr. F. B. 142.) 



FUNCTIONS AND NUTRITIVE VALUE OF FOOD. 



The principles above explained are based upon a great deal of 

 experimentation and observation, but of especial importance is the 

 work with the respiration (breathing out) apparatus and respira- 

 tion calorimeter. These will be described later. With such appa- 

 ratus it is possible to learn what effect different conditions of nour- 

 ishment will have on the body. 



FOOD AND FOOD ECONOMY. 



What has thus far been said about the ingredients of food and 

 the ways they are used in the body may be briefly summarized in 

 the following manner: 



