Page:Popular Science Monthly Volume 23.djvu/808

788 acid. This combustion raises the temperature of the blood, and the warm liquid, which penetrates everywhere, warms the organism almost in the same way that a house is heated by hot-water pipes. The activity of respiration and the consumption of oxygen are diminished during sleep, but increased in taking exercise, when a part of the heat produced is transformed into mechanical work; but, from birth to death, man continues, without ever wholly resting, to draw the breath that keeps up the fire of life.

Notwithstanding this incessant production of heat, which may be increased or diminished, according to circumstances, by as much as fifty per cent, the temperature of the body continues almost invariable. In health it is always about 98°, and seldom varies as much as 2°; and yet we know that in some regions of the globe the monthly means of external temperature present variations rising to more than 115°, with much wider divergences in extreme cases. In parts of Siberia the extremes range from 70° or 80° below zero to 80° or 90° above; temperatures of from 120° to 130° have been remarked in hot regions in Australia and Asia; and men have been able to support much higher temperatures than these for a short time—Blagden 259° for seven minutes, and a certain Martinez, by wrapping his head in cloth, 338° for a quarter of an hour. Under such excessive heats, the temperature of the blood may rise a few degrees higher than its ordinary extreme; but such cases are abnormal.

Constancy of bodily temperature is an indispensable condition of health to warm-blooded animals. By what means does Nature supply deficiencies of internal heat and eliminate an injurious excess, and, in either case, restore the organs to the temperature which is most agreeable to the regular performance of the functions of life? The means are various. When food becomes insufficient, calorification is effected at the expense of the tissues of the animal, and it grows lean. When heat is produced in excess, the organism rids itself of it speedily by several outlets. The body may be cooled by radiation, by evaporation, or by conduction or convection. It is estimated that radiation generally carries off half, and the other two ways a quarter each, of the surplus heat. These ratios are, however, far from being constant; they vary with external circumstances. Evaporation is the valve that regulates the loss of heat, by completing, at a given point, the action of conduction and radiation.

The intensity of radiation, by which heat is dissipated from the body around, is proportional to the difference between the normal temperature of the body and that of the surrounding medium, and increases in the neighborhood of a very cold body. We may in this way explain the chilly sensation we feel and which persists in a room that has not been used for a long time, after the fire has been kindled, and even after the air in the room has become quite warm; while, after the room has been well warmed up, we may feel quite