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 fiuid 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 owt 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considerable quantity of heat is withdrawn from it, becoming 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the greater is the amount of heat withdrawn in a given time ; 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 
