600 ANIMAL HEAT. [CH. XL. 



the animal, the heat production of which is to be ascertained, is 

 placed within the cage C. In the other, hydrogen is burnt (H). 

 Both chambers are shut, the tubes AA, A'A' being clamped. 

 The heat given off from the animal warms its chamber, and thus 

 increases the pressure of the air in the air-space between the two 

 copper walls of the chamber. This would lead to movement of 

 the fluid in the manometer but that the heat given off by the 

 burning of the hydrogen increases at the same time the pressure 

 in the air-space between the walls of its chamber. This latter 

 increase of pressure tends to make the fluid in the manometer 

 move in the other direction. If the fluid in the manometer 

 remains stationary, the amount of heat given off by the animal is 

 equal to that produced by the burning hydrogen ; and during 

 an experiment the fluid in the manometer is kept stationary 

 by turning the hydrogen flame up and down. The amount of 

 hydrogen burnt is estimated by the amount of water formed, and 

 the heat of combustion of hydrogen being known, it is perfectly 

 easy to calculate the calories produced, which equal those given 

 off by the animal. 



Regulation of the Temperature of Warm-blooded 

 Animals. 



We have seen that heat is produced by combustion processes, 

 and lost in various ways. In order to maintain a normal 

 temperature, both sides of the balance-sheet must be equal. This 

 equalisation may be produced by the production of heat adapting 

 itself to variations in discharge, or by the discharge of heat 

 adapting itself to variations in production, or lastly, and more 

 probably, both sets of processes may adapt themselves mutually 

 to one another. We have, therefore, to consider (i) regulation 

 by variations in loss and (2) by variations in production. 



Regulation by Variations in Loss. The two means of loss 

 susceptible of any amount of variation are the lungs and the skin. 

 The more air that passes in and out of the lungs, the greater will 

 be the loss in warming the expired air and in evaporating the 

 water of respiration. In such animals as the dog, which perspire 

 but little, respiration is a most important means of regulating the 

 temperature ; and in these animals a close connection is observed 

 between the production of heat and the respiratory activity. The 

 panting of a dog when overheated is a familiar instance of this. 

 A dog also, under the same circumstances, puts out its tongue, 

 and loses heat from the evaporation that occurs from its surface. 

 The great regulator, however, is undoubtedly the skin, and this 



