16 EFFECT OF HEAT ON THE 



just as violent exercise, which implies an increased 

 supply of food, is incompatible with weak diges- 

 tive organs. In either case the health suffers. 



But the quantity of oxygen inspired is also af- 

 fected by the temperature and density of the atmos- 

 phere. 



The capacity of the chest in an animal is a con- 

 stant quantity. At every respiration a quantity of 

 air enters, the volume of which may be considered as 

 uniform ; but its weight, and consequently that of the 

 oxygen it contains, is not constant. Air is expanded 

 by heat, and contracted by cold, and therefore equal 

 volumes of hot and cold air contain unequal weights 

 of oxygen. In summer, moreover, atmospherical 

 air contains aqueous vapour, while in winter it is 

 dry ; the space occupied by vapour in the warm air 

 is filled up by air itself in winter ; that is, it con- 

 tains, for the same volume, more oxygen in winter 

 than in summer. 



In summer and in winter, at the pole and at the 

 equator, we respire an equal volume of air ; the cold 

 air is warmed during respiration, and acquires the 

 temperature of the body. To introduce into the 

 lungs a given volume of oxygen, less expenditure of 

 force is necessary in winter than in summer ; and for 

 the same expenditure of force, more oxygen is in- 

 spired in winter. 



It is obvious, that in an equal number of respira- 

 tions we consume more oxygen at the level of the 

 sea than on a mountain. The quantity both of 



