cloud, { 



EVAPORATION. 



427 



cloud, and can scarcely be seen, the vapour having been con- 

 densed by the cold air. 



A very familiar instance of this sudden condensation may be 

 seen in the streets of London on any winter day. There may 

 be a couple of omnibus horses, nearly at the end of their day's 

 work, and quite tired out. Suddenly they are pulled up by the 



HEATED HORSE. 



PERFUME SPRAY. 



FREEZING BY EVAPORATION. 



WET-BULB 



THERMOMETER. 



driver, and as suddenly disappear for a moment or two, being 

 concealed in a cloud of moisture proceeding from their bodies. 

 Of course in a hot day there is more of the moisture, but the 

 warmth of the atmosphere prevents it from condensation, and 

 so it is not visible. 



One valuable property of the system of evaporation and 

 condensation is its cooling power. Thus it is that a person who 

 is ill with fever tosses about with a burning skin until the 

 pores of the body act, and allow the normal moisture to pass 

 through them. Then the body cools by evaporation, and the 

 patient begins to amend. 



So it is that the bather can endure in the Turkish bath a 

 heat so great that a glass of water, if held in the hand, would 

 speedily boil, and a piece of meat be cooked in about the same 

 period. But, if the air were not dry enough to carry off the 

 perspiration, the bather would be scalded to death. 



A most valuable adaptation of the principle is shown in the 



