COOLING EFFECT OF CUTANEOUS EXHALATION. 313 



perspiration, is not the same, therefore, with the difference 

 between simple evaporation and exhalation from the skin ; for 

 a part of the latter is commonly insensible ; and the degree 

 in which it is so depends upon the amount of fluid exhaled, 

 and the state of the surrounding atmosphere. If the fluid 

 thus poured forth be allowed to remain upon the surface of 

 the skin, it produces a very oppressing effect ; most persons 

 have experienced this, when walking in a mackintosh cloak or 

 coat, on a damp day. The waterproof garment keeps in the 

 perspiration, almost as effectually as it keeps out the rain; 

 and consequently the air within it becomes loaded with fluid, 

 and the skin remains in a most uncomfortable as well as pre- 

 judicial state of dampness. 



372. The purpose of this watery exhalation, and of its 

 increase under a high temperature, is evidently to keep the 

 heat of the body as near as possible to a uniform standard. 

 By the evaporation of fluid from the surface of the skin, a 

 considerable quantity of heat is withdrawn from it, becoming 

 latent (according to ordinary phraseology) in the change from 

 fluid to vapour : of this we make use in applying cooling 

 lotions to inflamed parts. The more rapid the evaporation, 

 the greater is the amount of heat withdrawn in a given time ; 

 hence, if we pour, on separate parts of the back of the hand, 

 small quantities of ether, alcohol, and water, we shall find 

 that the spot from which the ether is evaporating feels the 

 coldest, that which was covered by the alcohol less so, whilst 

 the part moistened with water is comparatively but little 

 chilled. The greater the amount of heat applied to the body, 

 then, the more fluid is poured out by the perspiratory glands ; 

 and as the air can carry it off more readily in proportion to 

 its own heat, the evaporation becomes more rapid, and its 

 cooling effect more powerful. It is in this manner that the 

 body is rendered capable of sustaining very high degrees of 

 external heat, without suffering injury. Many instances are 

 on record, of a heat of from 250 to 280 being endured in 

 dry air for a considerable length of time, even by persons 

 unaccustomed to a peculiarly high temperature ; and indi- 

 viduals whose occupations are such as to require it, can sustain 

 a much higher degree of heat, though perhaps not for any 

 great length of time. Thus, the workmen of the late Sir F. 

 Chantrey were accustomed to enter a furnace in which his 



