COOLING AGENCIES. ' 185 



organic mass, loses or gains heat, as the case may be, by radi- 



~. 3 . ,_ 11 Of radiation, 



ation. bince, however, in man, the temperature is usually 



higher than that of the surrounding medium, the result of this action is 

 that cooling takes place. With regard to loss of heat by warming the 

 expired air, it may be observed that, whatever the temperature of the ex- 

 ternal air may be, it is raised to that of the lungs after it has been brought 

 into the respiratory passages. This constitutes, therefore, a cooling 

 agency of variable power, for the loss will be greater as the external heat 

 is lower : if the atmospheric temperature rose to 98, loss in this manner 

 would cease. Becalling what has been said respecting the mode in which 

 air is introduced, it is plain that this loss will chiefly fall Heat given to 

 upon the nasal passages, the trachea, and larger ramifications the expired air. 

 of the bronchial tubes ; for, by the time the volume inspired has made its 

 way beyond that limit, its temperature must be nearly that of the body. 

 The contact of the cold surrounding air, and more particular- contact of the 

 ly of currents which may be occurring in it, act chiefly upon surrounding 

 the skin, and it is in preventing this loss that clothing be- 

 comes so efficient. The difference we so frequently notice between the 

 indications of the thermometer and our own sensations are, for the most 

 part, dependent on these currents. A temperature of 50 below zero can 

 be sustained without much inconvenience if the air is perfectly calm, 

 but not so if there is any wind. Of all the cooling agencies, evaporation 

 is, however, by far the most energetic. From the skin and Coolino . b 

 the air cavities, large quantities of the vapor of water are ex- evaporation of 

 haled. As the external heat rises, the sudoriparous tubes act* water< 

 with increased energy, and pour out their excretion as drops of sweat 

 faster than it can be removed. Their length has been estimated at 28 

 miles. Since, at the temperature of the body, the heat of elasticity of the 

 vapor of water is 1114, this continued vaporization from the skin and 

 lungs is one of the most powerful sources of refrigeration. 



It may be well to direct a closer attention to the special action of the 

 air passages and skin as concerned in these cooling process- Variabilit in 

 es. The diurnal loss of water, by both organs conjointly, is the action of 

 usually estimated at 3 Ibs., of which the pulmonary exha- theskin - 

 lation constitutes about one third, and the cutaneous about two thirds. 

 The skin acts in a variable manner, losing more or less water as the ex- 

 ternal air is dryer or more damp. The removal of water therefore be- 

 comes a complex operation, in which three different organs are concerned 

 the skin, the lungs, and the kidneys. Of these, the skin acts meteoro- 

 logically and variably, as has been just remarked, and the respiratory or- 

 gans for the most part uniformly. But since it is requisite, in the nor- 

 mal operations of the system, that the diurnal average of water should be 

 removed, the variable action of the skin throws a variable action upon 



