ANIMAL HEAT 



385 



season, and severe cold may even arouse a hibernating animal 

 from its state of torpor. 



The Amount of Heat produced by animals depends upon the 

 rate of their metabolism and the surface area of their bodies ; the 

 latter factor determines the loss of heat, and hence its production 

 if the temperature of the body is to be kept constant. A large 

 animal produces actually, but not relatively, more heat than a 

 small one ; a small animal, as has been previously stated, has a 

 greater body surface relative to its weight than a large animal, 

 and in this way its loss is more rapid. As heat production must 

 balance heat loss, the small animal must lose more heat, and 

 therefore produce relatively more heat, than a large animal. 



In the following table the body surface has been calculated by 

 a formula, and the heat given off per unit of surface is seen to be 

 verv close in all animals : 



The heat produced is measured as heat-units or calories — 

 viz., the amount of heat required to raise 1 gramme of water i° C, 

 known as the ' small calorie,' or the amount required to raise 

 1 kilogramme of water i° C. This is the large calorie sometimes 

 spoken of as the ' kilogramme-calorie.' 



The method by which the heat given off by a body is ascertained 

 is by means of a calorimeter (Fig. 109). This is a chamber with a 

 double wall containing air or water, which absorbs the heat given 

 off, say, from an animal in the chamber. This chamber is 

 contained within another, and the two separated by non-con- 

 ducting material in order to prevent loss of heat by radiation. 

 The weight of the water in the calorimeter is known, and the 

 extent to which its temperature is raised during the experi- 

 ment ascertained ; a simple calculation shows the number of 

 calories given off during the observation. The temperature of 

 the animal's body before and after the experiment shows whether 

 the loss of heat has been made good or not, or whether, on the 

 contrary, the body has produced more heat than it got rid of. 

 By the process of calorimetry the heat value of food is also 

 ascertained. In the Respiration Calorimeter described at p. 346, 



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