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.

It is possible, however, to have a temperature which even a Hindoo cannot resist.  When a man is exposed to such a heat his bodily temperature rises, and as it rises the symptoms of fever develop precisely as they do in the lower animals—­sometimes slowly, sometimes suddenly—­with disturbances of the respiration, circulation and innervation precisely similar to those already noticed as occurring in the dog, the cat and the rabbit.  Sunstroke, or thermic fever, is generally believed to be instantaneous in its onset, but the wide experience of the English in India has shown that whilst in some cases it is thus sudden in its development, in others it is a slow process, and probably in almost all cases close observation would have revealed the existence of premonitions.