Page:Popular Science Monthly Volume 23.djvu/810

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per cent of the increase of heat which it produces under a sustained effort. The problem we have to solve is not to seek for a way of producing less heat, but to find a means of getting rid of that which we do produce.

Water is a much more effective refrigerant than air, because of its much greater conductibility; at the same temperature, a bath of water will refresh one more than a bath of air; but baths are necessarily of limited use. The important matter should be to diminish the temperature of the air that comes in contact with the body.

We have next to consider the effect of evaporation through the lungs and the skin. When the thermometer indicates more than 98° in the shade, the body can no longer be cooled by contact or by radiation, and only a single way is left by which the surplus heat can be dissipated. It can only expend itself in vaporizing the water which transpiration carries to the skin and to the mucous membrane of the respiratory apparatus. The lungs, as a rule, exhale about half as much water as is excreted by the skin. Both together remove about a kilogramme of water every twenty-four hours, disposing of as much heat as would boil five quarts of water; but the quantity of water and of heat removed in this way may be doubled and even tripled when all the channels of transpiration are fully opened under the pressure of an excess of internal heat. The vapor disengaged by these operations is absorbed by the surrounding air with a facility proportioned to the dryness of the atmosphere, or to the degree in which it is removed from the point of saturation. There is a limit at every degree of temperature to the proportion of vapor which the air can contain; and the interval between the points of dryness and of saturation increases with the temperature. An atmosphere at the same time very moist and very hot seems heavy to us because it hinders the evaporation of the water that transpiration brings to the surface of the body. This is why hot and moist climates are so much more unhealthy than hot and dry ones.

When the internal calorification is increased in consequence of violent exercise, the excess of sensible heat is eliminated by a more intense radiation, by ascending air-currents, and by a more abundant transpiration; it thus happens that after several hours of sustained effort we sometimes observe a slight cooling of the body, an effect which is the result of a too rapid using up of disposable materials. Hence, to cite the illustrations given of this fact by M. Bouchardat, dogs, which have run long at the hunt, and the overworked and exhausted children in the Belgian coal-mines, returning to the lodge or to their home, first of all things, before even satisfying their hunger, stretch themselves before the bright fire for warmth.

Thus the means of refrigeration at the disposal of Nature are quite varied; they complement and replace each other according to circumstances. But it is necessary to avoid the too abrupt changes which