ITS SCOPE AND METHOD 65 



than can be neutralized in the chemical laboratory of 

 the body. 



The heat of the body is traceable to chemical decom- 

 position of its substance. The material whence the 

 energy has been thus far extracted is no longer useful 

 to the body. It is got rid of, for instance, in each 

 breath we exhale. Yet our weight keeps up from week 

 end to week end. While that is maintained and the 

 body's average composition kept unaltered, every mani- 

 festation of activity by the body is traceable to the 

 energy of its food. The body is thus a fuel-fed machine. 

 The energy we dispense each day, not only as bodily 

 warmth but in all our various activities, walking, talking, 

 climbing omnibuses and taking cabs, comes in as the 

 chemical energy of food. And the principle which is 

 termed that of the ' equivalence of energy' allows us a 

 precise quantitative idea of the value of food. 



We can measure the energy that the food taken by 

 the body sets free when burnt in the presence of oxygen. 

 A representative sample of the total diet is placed in 

 a steel shell which contains compressed oxygen. This 

 shell is immersed in water contained in a vessel so 

 jacketed as to avoid loss or gain of heat from the 

 external air. When the temperature of the whole is 

 constant the sample of food is fired electrically. In 

 being totally burnt in the oxygen, it produces a certain 

 amount of heat. This is caught by the surrounding 

 water. The temperature of the water is taken before 

 and after the combustion. The quantity of water is 

 measured. The amount of heat produced is thus known 

 from the amount communicated to the water. A mea- 

 sure is thus obtained of the quantity of energy available 

 in the food. From this we subtract the energy of a 

 residuum of the food not fully oxidized in its passage 



