CHAPTER VIII 

 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 intern- 

 ally, 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 temperature (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 pro- 

 cesses 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 text-book 

 of physics. All that can be done here is to preface the physio- 

 logical portion of the subject by a few remarks on the physical 

 methods and instruments employed : 



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 contact 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 



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