Animal bodies do not depend for the quantity of caloric necessary to their existence upon the sun. By chemical changes, not yet well understood by philosophers, depending upon that subtle ethereal principle which we call _life_,[3] a sufficient quantity of animal or vital heat, as it is called, is evolved within the body itself. As this heat is constantly generated, it is necessary, in order that the body may not acquire too high a temperature, that it be as constantly conducted or radiated off. When the atmosphere contains too little caloric, its power of absorbing heat is so great as to deprive the animal body of it more rapidly than it is generated; thus producing the sensation of cold. On the contrary, when the weather is too warm, the air and other surrounding bodies, having less attraction for caloric, do not withdraw it as fast as it is generated; thus producing the feeling of heat. Perhaps, however, this is scarcely a scientific method of stating the fact in question. It is generally supposed by philosophers, that all bodies, whether in equilibrium, as it regards temperature, with surrounding substances, or not, are constantly radiating and absorbing caloric. When equally heated, the cause of their continuing so is, that they receive as much as they give off. When unequally heated, that which contains most caloric radiates more than the rest, and, of course, absorbs less than it parts with. By this means, an equilibrium of temperature is after a time brought about. Now, in cold weather, the heat which an animal body radiates is greater in quantity than the sum of what it generates itself, and absorbs from the sun and other bodies. The consequence is, it experiences the feeling of cold. In warm weather, the caloric radiated is less than that absorbed and generated; in which case, the animal suffers from heat. The vital heat of the generality of quadrupeds and other warm-blooded animals is several degrees greater in intensity than that of the atmosphere, during the warmest season in the tropics. The temperature of the human body is about ninety-eight degrees. The mean equatorial temperature Humboldt proved by repeated experiment to be eighty-one and a half degrees. It is evident, therefore, that in warm regions it is more important that the physical state and constitution of animal bodies should be adapted to the radiation of internal, than to the reflection of external heat, since the intensity of the former exceeds that of the latter.