THE CARBON OF THE FOOD. 15 



that, in the same individual, the quantity of nourishment 

 required must vary with the force and number of the 

 respirations. 



A child, in whom the organs of respiration are natural- 

 ly in a state of great activity, requires food oftener, and in 

 greater proportion to its bulk, than an adult, and bears 

 hunger less easily. A bird, deprived of food, dies on 

 the third day, while a serpent, which, if kept under a 

 bell-jar, hardly consumes in an hour so much oxygen as 

 that we can detect the carbonic acid produced, can live 

 without food three months and longer. 



The number of respirations is smaller in a state of 

 rest than during exercise or work. The -quantity of food 

 necessary in both conditions must vary in the same ratio. 



An excess of food is incompatible with deficiency 

 in respired oxygen, that is, with deficient exercise ; just 

 as violent exercise, which implies an increased supply 

 of food, is incompatible with weak digestive organs. In 

 either case the health suffers. 



But the quantity of oxygen which an animal takes up 

 by the lungs, depends not only on the number of respi- 

 rations ; it is also affected by the temperature and densi- 

 ty of the atmosphere. 



The capacity of the chest in an animal is a constant 

 quantity. At every respiration a quantity of air enters, 

 the volume of which may be considered as uniform ; but 

 its weight, and that of the oxygen it contains, is not 

 constant. Air is expanded by heat, and contracted 

 by cold, and equal volumes of hot and cold air contain 

 unequal weights of oxygen. In summer air contains 

 aqueous vapor, in winter it is dry ; the space occupied 



