408 TEMPERATURE AND LIFE. 



to that of the water or air surroiiiiding them. All animals except 

 mammals aud birds are cold-blooded auimals. It is to be noted how- 

 ever that certain mammals, usually rodents, are in turn warm- 

 blooded and cold blooded. These are hibernating animals, which after 

 the fall of the external temperature below a certain point, become tor- 

 pid and fall asleep, their own temperature being hardly higher than 

 that of the air about them. Of these we shall speak again later. 



Without the aid of certain instruments heterothermic animals would 

 appear to generate no heat whatever; for to our senses, their tempera- 

 ture is the same as that of their surroundings. In the case of reptiles, 

 however, the temperature exceeds that in which they are placed, (the 

 difference being estimated when the external temperature was at a 

 point between 5° and 15° C.) as much as C, 7, and 8 degrees, though it 

 more frequently varies from 1 to 4 degrees. In the case of batrachians 

 it is less, scarcely exceeding 2 or 3 degrees under the same conditions. 

 The difference is still less appreciable in fishes, and it reaches its low- 

 est j)oint in invertebrates, in which the temperature only occasionally 

 shows an excess of one-fourth or one-half of a degree centigrade over 

 the temperature of their surroundings. Insects, particularly those 

 which live in communities, generate at times considerable heat. Thus 

 Reaumur observed the temperature in a beehive raised to 12^.5 C. 

 when the external air was at — 3°.7 C. In short, heterothermic animals 

 generate little heat, but its production is constant. 



What is the cause of this calorification? This is the question into 

 which we are now to inquire. The strangest ideas have been enter- 

 tained in regard to it. One investigator makes a mysterious principle 

 of animal heat, the seat of which is the heart, where it develops so high 

 a degree of temperature that touching this organ by chance results in 

 a painful burn. The author of this theory has evidently never prac- 

 tised vivisection, for, as a matter of fact, the heart is one of the coldest 

 of the organs, in mammals its temperature rarely exceeding 39° or 40° 

 0. According to J. Hunter, the celebrated surgeon and anatomist, this 

 mysterious principle of animal heat resides in the stomach. Barthez 

 and his followers attribute it to an entirely different cause; more rea- 

 sonable (in that it excludes the supernatural and mysterious), but no 

 less erroneous. Their belief is that it is due to the commingling of the 

 several liquid and solid portions of the organism. It was Lavoisier 

 who laid the foundations of the true theory of calorification. Having 

 nuide an exact calculation of the nature aud properties constituting the 

 atmosphere in its normal condition, he demonstrated in an irrefutable 

 manner that air, expelled by a living (;reature, contains carbonic acid 

 in larger quantities than the air which he inhales. A combination has 

 therefore taken place between the oxygen in the air and the carbon 

 contained in the organism. "Pure air, in passing through the lungs, 

 effects a combination aiuilogous to tlmt Avliich takes i)lace in the com- 

 bustion of (iliarcoal. Now, in the combustion of charcoal there is a 



