These various symptoms are simply results of the excess of caloric, which excites universal irritation, and, if prolonged, destroys the tissues. This fact I have verified by three series of experiments, by the first of which it was shown that the general application of external heat so as to raise the bodily temperature produces all the phenomena of fever; by the second, that the local application of heat to the brain and to the heart causes the nervous and circulatory disturbances so universally seen in fever; and by the third that the abstraction of heat in fever is followed by immediate subsidence of the other symptoms. If a small animal, such as a dog, cat or rabbit, be placed in a chamber heated by means of the sun's rays falling upon a slanting glass roof or by some artificial method to a temperature of considerably over one hundred degrees, a very constant series of phenomena is developed. The breathing becomes hurried and the pulse greatly quickened, whilst the restless movements of the body indicate nervous distress. After a time, if the exposure be continued, the symptoms are intensified, and restlessness passes into the weakness of partial paralysis; then suddenly or gradually, with or without convulsions, stupor sets in, deepening into coma, and death from arrested respiration is the final result. If the temperature of the animal be tested from time to time during the exposure, it will be found to rise steadily, and the severity of the symptoms will be directly, and in any one species constantly, proportional to the intensity of the bodily heat. The nervous system of man apparently resists the action of heat, but in reality it does not do so. Man, it is true, is the only animal that can thrive almost equally amidst arctic snows and in tropical jungles. This is not, however, because his nervous system lacks sensitiveness, but because he has the power of heating or cooling his body in such a manner that its temperature is comparatively unaffected by that of the surrounding air. Man might be well defined as the naked sweating animal. In the north he strips the bear and the fox of their coat to keep him warm; in the south his own skin acts as a refrigerator. The dog has a few sweat-glands about the mouth--man has two millions densely covering his body. In the horse exposed to heat the hair soon becomes wet and matted, interfering very greatly with evaporation; in man the bare skin offers an excellent surface, from which the perspiration passes off almost as fast as formed. Evaporation, conversion of a liquid into a vapor, means a steady conversion of sensible heat into what was formerly called latent heat, but what we now know to be repulsive force: the heat-energy of the body is lost in driving the particles of sweat asunder in the form of vapor.