ICE, WATER, AND STEAM 109 



and from liquid to gaseous condition. There is one important 

 exception, however. Few expand at the time of freezing, as 

 water does, and these expand much less. 



Substances such as glass and wax do not have a definite 

 melting point. They gradually soften, becoming first like a 

 very thick liquid, and finally flow freely. 



122. Evaporation. Even when the temperature of water is 

 not high enough to cause it to boil, the water changes into 

 vapor, but not rapidly. This is shown by the drying of wet 

 objects when exposed to the air. 



Water will slowly change into vapor (evaporate) at any 

 temperature. Even if water is frozen, evaporation continues. 

 This is shown when wet clothing is hung out of doors on a 

 cold day. It immediately freezes, and remains frozen until it 

 has become dry by evaporation of the frozen water. 



123. Laws of evaporation. The rate of evaporation depends 

 upon several factors. For instance, it is commonly known 

 that things dry more rapidly when the air is warm than when 

 it is cold, and that drying is assisted by wind. There are 

 three conditions that strongly affect the rate at which water 

 evaporates, and the facts may be stated thus : Rapid evapora- 

 tion is favored by high temperature, dry air, and winds. 



124. Cooling effects. If one stands on the beach in wet cloth- 

 ing after bathing in a lake or in the ocean, he soon becomes 

 chilled, even though it is a warm day. He is chilled more when 

 the wind is blowing than when the air is quiet. When the 

 clothing has become dry, the air seems warm again. One is 

 cooled as long as water is evaporating from the surface of his 

 body or from his clothing. Whenever water or any other 

 liquids evaporate, the temperature is lowered, and the more 

 rapidly evaporation occurs the more the temperature is lowered. 



Under ordinary conditions water does not boil unless con' 

 siderable quantities of heat are supplied to it. If heat is sup- 

 plied more rapidly, it results in more rapid formation of steam, 

 but there is no change in temperature of either the water or the 



