CHAPTER XII 

 ANIMAL HEAT 



FROM the earliest ages it must have been noticed that the bodies of 

 many animals, and particularly of men, are warmer than the air 

 and than most objects around them. The ' vulgar opinion ' of 

 Bacon's time, ' that fishes are the least warm internally, and birds 

 the most,' if it does not imply a very extensive knowledge of animal 

 temperature, at least shows that the fundamental distinction of 

 warm and cold-blooded animals, which is to-day more accurately 

 expressed as the distinction between animals of constant tempera- 

 ture (homoiothermal) and animals of variable temperature (poikilo- 

 thermal), had been grasped, and was even popularly known. Since 

 that time the accumulation of accurate numerical results, and the 

 advance of physical and physiological doctrine, have given us 

 definite ideas as to the relation of animal heat to the metabolic 

 processes of the body. It is impossible to understand the present 

 position of the subject without an elementary knowledge of the 

 science of heat. For this the student is referred to a textbook of 

 physics. All that can be done here is to preface the physiological 

 portion of the subject by a few remarks on the physical methods and 

 instruments employed: 



SECTION I. THERMOMETRY AND CALORIMETRY. 



Temperature. Two bodies are at the same temperature if, when 

 placed in contact, no exchange of heat takes place between them. 

 They are at different temperatures if, on the whole, heat passes from 

 one to the other, and that body from which the heat passes is at the 

 higher temperature. It is known by experiment that if two bodies of 

 different temperature are placed in contact, heat will pass from one to 

 the other till they come to have the same temperature. If, then, we 

 have the means of finding out the temperature of any one body, we 

 can arrive at the temperature of any other by placing the two in con- 

 tact for a sufficiently long time, under the proviso that the quantity of 

 heat necessary to bring the temperature of the first body, which may be 

 called the ' measuring ' body, to equality with that of the second is so 

 small as not to make a sensible difference in the latter. This is the 

 principle on which thermometric measurements depend. A mercurial 

 thermometer consists of a quantity of mercury ordinarily contained in 

 a thin glass bulb, the cavity of which is continued into a tube of very 



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