THE EFFECTS OF VARIATIONS OF TEMPERATURE. 57 



oration from the surface. To change the water into vapor, 

 has been shown to require a large expenditure of heat; and 

 precisely the same heat is drawn from the soil when vapor 

 rises from it as is imparted by the fuel of a fire which pro- 

 duces the same amount of evaporation. This heat drawn 

 from the soil necessarily reduces its temperature. An ex- 

 periment which exemplifies this result may be made as 

 follows : A few drops of ether are placed upon the skin ; 

 and the breath is blown upon it. The current of air evap- 

 orates the volatile ether quickly, and causes a large absorp- 

 tion of heat. The abstraction of the heat from the skin to 

 supply this requirement of the evaporation causes a sensa- 

 tion of cold upon the skin. This is precisely the effect up- 

 on the soil, when the warm air blowing over wet clay or 

 swampy land causes copious evaporation and is all the 

 greater as the evaporation is excessive. 



This effect operates to relieve persons from the results of 

 excessive heat. When the temperature rises to 90 degrees 

 and over, the animal system becomes oppressed. The blood 

 w 7 hose normal heat is 98 degrees, rises in temperature and 

 produces serious disturbance of the nervous system, which if 

 not relieved quickly ends in what is known as sunstroke, 

 and speedy death. But the evaporation of the water of the 

 system in the form of perspiration relieves the oppression; 

 carries off the heat; cools the blood and the skin; and prevents 

 the fatal results of the unrelieved heat. When an incau- 

 tious person suddenly plunges into cold water, or drinks 

 cold water to excess, the pores of the skin are closed in the 

 one case and a chill is produced in the other; either of 

 which checks the perspiration ; and prevents the escape of 

 the internal heat; when fatal results are often produced. 

 So the wearing of wet clothes abstracts heat from the body 

 and thus produces pernicious effects; while the use of wet 

 sheets in which fever patients are wrapped; rapidly cools 

 the parched skin ; induces natural perspiration; and saves the 

 sufferer. 



The abstraction of heat by evaporation is so great under 

 some circumstances that water can be frozen by it. This 



