THE WOBK OF LEAVES 173 



Food is used in two ways: (1) as a source of 

 material for growth, (2) as a source of energy for do- 

 ing work and keeping the living machine in motion. 

 The energy is obtained from the food by burning it 

 (just as in an engine: see page 35). The burning or 

 oxidation is accomplished by means of ferments of 

 various kinds which abound in both the plant and 

 the animal organism. 



The amount of energy furnished by a food is called 

 the fuel value, and may be ascertained by measuring 

 the heat produced by burning it directly or by feeding 

 it to an animal and measuring the amount of heat 

 given off by the body during a given time. The latter 

 method does not give as high results as the former, 

 since the body resembles other machines in not getting 

 the full theoretical amount of energy out of its fuel 

 (it is, however, a far more perfect machine in this 

 respect than most machines devised by man) : more- 

 over, not all the energy of the fuel is set free at once 

 as heat, since a certain part is used in doing mechanical 

 work, chemical work, etc. The fuel value of proteids 

 and carbohydrates is in general about the same, while 

 that of fats is about two and one-half times as great. ^ 



Since the energy is obtained from the food by burn- 



1 See Peabody, "Physiology and Anatomy," Chaps. II and IV. Also the 

 following bulletins of the U. S. Department of Agriculture. "Foods; Nutri- 

 tion and Cost," "Meats: Composition and Cooking," "Milk as a Food," "Sugar as 

 a Food," "Fish as a Food," "Food and the Principles of Nutrition." Also arti- 

 cles in the Year-Book of the U. S. Dept. of Agriculture for 1894, by Atwater 

 (see also p. 547 ff.); for 1902, by Milner '.. for 1903, by Snyder and Woods. 



