268 



EFFECT OF HUMIDITY OF AIR 



and contents of the intestines. Whether the relation of heat 

 production to surface holds good in comparing animals of different 

 species is a matter of controversy. 



THE EFFECT OF HUMIDITY OF THE AIR ON HEAT Loss 



Rubner has studied the effect of altering (1) the humidity 

 of the air with constant temperature ; (2) the temperature with 

 constant humidity. 



The effect on the dog of altering the relative dryness of the 

 air with constant temperature is seen in the following table. 



The water output thus varies inversely as the humidity of 

 the air, 1 and may be reduced 75 per cent, in wet air without 

 sign of disturbance, so perfectly is the body heat adjusted by 

 other means. The adjustment is brought about by nervous 

 regulation, and its perfection is well illustrated by the following 

 observation. A fasting dog with a daily expenditure of 250 Calories 

 diminished its water output by 53J grm. when the relative 

 wetness of the air increased 35 per cent. Now the evapora- 

 tion of this quantity of water requires 32 Calories. Nevertheless 

 as no noteworthy change occurred in heat production -the 

 nitrogen and carbon output remaining the same it is evident 

 that the heat loss remained undiminished. A compensatory 

 increase occurred in radiation and conduction. Vaso-dilatation 

 took the place of evaporation. The actual difference in heat 

 production which resulted from varying the wetness of the air 

 35 per cent, was not equal to that produced by altering the 



1 In marching soldiers Zuntz found increased dryness of the air actually 

 lessened the amount of sweat which was produced per 1000 Calories of energy 

 output owing to the more perfect evaporation from the clothes. 



