Page:Popular Science Monthly Volume 3.djvu/512

498 do with the regulation of the body-temperature. If, by exposure to cold, for example, heat is lost more rapidly from the surface than it is supplied within, its warmth must decrease, and there needs but a few degrees of fall to induce disease; on the other hand, if, by exposure to external heat, loss from the body is impeded, its temperature rises, and disease again results.

In hot climates, or during the hot season in temperate latitudes, we are in contact with an atmosphere which frequently attains a temperature nearly or quite equal to that of the body; and, at such times, our sensations tell us that we are losing heat less easily than during the cooler portions of the year. But, when thus exposed, the extensive evaporating surfaces of the lungs and skin are called into more active play; and, if the air is of average dryness, heat is rapidly thrown off by perspiration, the loss being so nicely adjusted that a uniform temperature of 99° Fahr. is continuously maintained. This heat-regulating function of the organism is one of the marvels of physiology. In spite of the vicissitudes of climate and season, it ever holds the temperature at a uniform degree, and, even under the stress of high artificial heat, is able to keep the balance comparatively unchanged. Blagden and Fordyce exposed themselves in an oven to a heat of 260° Fahr., without serious inconvenience, and with but a slight rise of temperature. But the air was dry, and the heat was kept down by perspiration. The substitution of moist for dry air in the oven hindered evaporation, and the temperature of the body rose rapidly. With this hasty sketch of the heat-producing and heat-regulating functions of the animal economy, we pass to a brief consideration of some of the more important ill-effects resulting from the action of excessive heat on the conditions of health and on the body itself.

High temperature is a powerful aid to decomposition. Dead organic matter, under its influence, speedily decays, giving rise to foul and poisonous products which, escaping into the atmosphere, find their way, sooner or later, into the system. Here they work various forms of mischief. Like sulphuretted hydrogen gas, they may be directly poisonous, or, like carbonic-acid gas, may act by diluting the air, and so, by reducing the normal supply of oxygen, interfere with the due oxygenation and purification of the blood. The filth of towns is always highly charged with organic matter, and this, when rapidly decomposing in the heats of summer, has long been recognized as a prolific source of disease.

Decaying vegetable matter in the soil is held to be the source of malarial poison; and here, again, if moisture is present, decay is always hastened and increased by excessive heat. So great, in the opinion of Dr. Parkes, is the influence of heat on the development and spread of malarious disease, that, in selecting a residence with a view to escaping its effects, he recommends that, in tropical countries, the point be 1,000 or 1,500 feet higher above the source of the poison