76 EFFECTS OF CUTANEOUS EVAPORATION. 



Liquids If therefore, we moisten with a liquid the sur- 



placed on 



the skin are face of the evaporating skin at any point, the liquid 



by the eva- is forced inwards by the external pressure. 



poration 



of other Let us suppose any part of the skin to be rubbed 



Effect of w ^h ^ at tne transpiration ceases at that part. If 



f now ^ ie S ^ n around the part is in its normal 

 the skin activity, if, therefore, in the surrounding parts 

 liquid is constantly passing off by evaporation, the 

 fat must be urged, by the unequal pressure thus 

 arising, towards these parts, or it is absorbed, just 

 as water, in the apparatus, Fig. 12, is absorbed, 

 when in consequence of evaporation a difference 

 between the internal and external pressure has 

 or on the arisen. If the whole skin were covered with fat, 

 the absorption would be effected by the pulmonary 

 evaporation. 



The blistering of the skin, and the sun-burning, 

 to which men are exposed at great elevations, arise 

 from the extraordinary dryness of the air, the in- 

 creased evaporation, and the pressure by which the 

 fluids filling the vessels are forced towards the 

 surface. 

 Causes of Several causes contribute iointly to the appear- 



the efflux 



of sweat, ance of the sweat, to the efflux of fluid from the 

 pores of the skin. One of these obviously depends 

 on the velocity, which the fluid set in motion by 

 evaporation or by a mechanical cause, acquires from 

 the accelerated motion of the blood. In conse- 

 quence of this velocity, the fluid moves out beyond 

 the limits of the absorbing membrane or skin. 



