44 A YEAR IN SCIENCE 



raise the melting point, and an increase in pressure 

 will usually lower the melting point. 



Quantity of heat used in changing water to steam. 

 The quantity of heat necessary to change 1 c.c. of 

 water into water vapor, or steam, can be determined. 

 (See Laboratory Manual, Exercise 18.) Experiments 

 have proved that it takes about 536 calories of heat to 

 convert 1 c.c. of water into vapor. This is about 5J times 

 as much heat as is necessary to raise the same amount of 

 water from to 100 C. 



Heat withdrawn in evaporation of liquids. If chloro- 

 form or ether is left exposed to the air it evaporates 

 very rapidly. Likewise, if water is left exposed to 

 the air it evaporates. The liquids, which have appar- 

 ently disappeared, have passed into the surrounding 

 air in the form of vapor. When water is converted 

 into vapor by ordinary evaporation, heat is consumed 

 in the same manner as when Avater is converted into 

 steam by boiling, but the process is very much slower. 

 The heat, which is used in evaporation, is withdrawn 

 from the surrounding objects. 



Evaporation is a slow process, occurring at all tem- 

 peratures. It is hastened in the summer because of 

 the large amount of heat in the atmosphere. 



After a shower of rain, the water evaporates and 

 cools the air. When streets are sprinkled, the water 

 not only lays the dust, but in evaporating cools the 

 atmosphere. When perspiration evaporates from the 

 skin it cools the surface of the body. In tropical 



