214 PHYSIOLOGY AND HEALTH. 



the rapid evaporation of this cools the water within. Even 

 ice may be thus produced. We are therefore cooler when 

 we sweat. Blagden found great relief, in his oven, from the 

 profuse perspiration which was rapidly evaporated. 



511. The cooling power of the air is influenced by other 

 states besides its temperature. A dry atmosphere, by in- 

 creasing evaporation, cools the body more rapidly than air 

 saturated with vapor. Winds have the same effect. Even 

 if the air is warmer than the body, if it is in motion and dry. 

 it cools us ; so that a lady's fan, at summer's noon, when the 

 thermometer stands at 100, two degrees warmer than the 

 flesh, affords a pleasant coolness, by moving the air, and 

 hastening the evaporation. So slight a motion of the air is 

 thus perceptible by the increasing coolness, that men, when 

 they cannot distinguish the direction of the wind by its force 

 upon their bodies, or even by the movements of leaves of 

 trees, often wet a finger, and, holding it up to the air, dis- 

 cern, by their sensations, which is the colder side. This 

 determines the course in which the air is moving. 



512. In order that the' skin should passively permit the 

 heat to pass off by radiation, or actively throw it off by 

 perspiration, it must itself be in good health. It must be 

 able to prepare within itself just as much fluid as will, by its 

 evaporation, carry off the surplus heat, and no more ; other- 

 wise we may be too hot or too cold. 



513 In some states of disease, men suffer from a con- 

 stantly 'dry and parched skin. Their flesh burns within, and 

 the accumulating heat finds no outlet, for the skin affords no 

 relief. In other disorders, they are prostrated with a pro- 

 fuse and cold sweat. The skin pours out the perspiration 

 like water, and this, by evaporating, creates a constant and 

 painful demand for heat ; and then the patient finds it im- 

 possible to keep warm. 



514. Although it is through the sensibility of the skin 

 that we perceive things to be hot or cold, yet this is by no 

 means an exact measurement of the degree of heat; for 

 the apparent and sensible temperature of any substance is 



