120 ELEMENTS OF GENERAL SCIENCE 



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. Rapid evaporation is favored by high 

 temperature, dry air, and winds. 



139. Cooling effects. If one stands on the beach in wet 

 clothing 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 other 

 liquids evaporate, the temperature is lowered; and the more 

 rapidly evaporation occurs, the more the temperature is lowered. 



When water boils, considerable quantities of heat are used. 

 If heat is supplied more rapidly, it results in the more rapid 

 formation of steam, but there is no change in temperature 

 of either the water or the steam. At the end of half an hour's 

 vigorous boiling, a kettle of water will have the same tempera- 

 ture as at the beginning of the period, but the amount of 

 water in the kettle will have decreased. The effect of the 

 heat, therefore, has been to evaporate the water, but it has 

 had no effect upon the temperature. The fire or stove has 

 lost the heat which has passed into the water. 



The total amount of heat that disappears when a gram 

 weight of water changes into steam is the same whether the 

 steam forms rapidly, as in boiling, or slowly, as in evaporat- 

 ing from wet clothing. In the latter case there is usually no 

 source of intense heat, such as the fire, and the heat which 

 disappears during evaporation is taken from the water itself 

 and from the surrounding objects. Since heat disappears 

 from the water and its surroundings, they are cooler (fig. 61). 

 In the case of the bather on the beach, the heat which was 

 used in evaporating the water from the wet bathing suit was 

 drawn from the water remaining in the suit and from the 



