ICE, WATEE, AND STEAM 111 



the wet bathing suit was drawn from the water remaining in 

 the suit and from the body of the bather, thus giving him the 

 sensation of cold. As soon as all the water had evaporated 

 from the suit, this withdrawal of heat ceased, and the usual 

 air temperatures were felt. 



Substances which evaporate more rapidly than water may 

 cause greater lowering of temperature. Ether and alcohol 

 evaporate so much more rapidly than water that they feel 

 very cold when poured over the hand. Ether may produce 

 so low a temperature that water will be frozen by its use. 

 Surgeons sometimes use ether to partly freeze the surface of 

 a portion of the body upon which a surgical operation is to 

 be performed, since the frozen tissues are so numb that the 

 pain is lessened. 



In many hot, dry countries use is made of the cooling effect 

 of evaporation to secure a supply of cool water for drinking 

 purposes. In such countries the water from wells is usually 

 warm and needs cooling. Often ice cannot be had, and other 

 methods of cooling must be resorted to. The water is put into 

 porous earthenware jars or into canvas bags, and these are 

 placed where the dry winds can blow over them. The water 

 soaks through the canvas or the porous jar and evaporates so 

 rapidly in the dry, hot air that the contents of the vessel are 

 made pleasantly cool. Travelers in such regions have learned 

 to cover their canteens with layers of woolen cloth, and to 

 wet this at every possible opportunity. So long as the cloth 

 cover is wet, the water in the canteen will be kept cool enough 

 to be palatable. 



125. Ice machines and cold storage. Cooling by evaporation 

 is applied in a practical way in plants for the artificial pro- 

 duction of ice and in cold-storage plants. In the operation 

 of these plants advantage is taken of the fact that the 

 boiling point of ammonia at .atmospheric pressure ( 28 F. 

 or 33 C.) is so little below atmospheric temperature that 

 the boiling point can be raised above the usual temperature 



