SOME EFFECTS OF HEAT 143 



(or the reverse) is called the heat of fusion of the substance. 

 The heat of fusion of ice is eighty calories; i. e., eighty cal- 

 ories of heat are absorbed in the melting of each gram of 

 ice; also for each cc. (gram) of water that freezes, eighty 

 calories are given out to its surroundings. Here again we 

 get an idea of why large bodies of water in cold weather 

 have such a warming effect upon the surrounding regions. 

 Since the change of a liquid to a solid involves, as you 

 have just noted, the giving off of heat, what heat effect 

 would you expect in connection with the change of a 

 liquid to a gas; that is, in connection with evaporation? 

 The change of liquid to solid is a process of condensation; 

 that is, a process of reducing molecular motion. But 

 the change of a liquid to a gas is a process of expansion, 

 a process of increasing molecular motion. Since the 

 former process releases heat, it follows that the latter 

 process absorbs it. This we find to be the fact. Evapora- 

 tion is a cooling process; it absorbs heat from its sur- 

 roundings. This you have already learned, both in this 

 book and elsewhere. The added discomfort we feel on hot 

 days when the atmosphere is humid (full of moisture) is 

 due not only to the perspiration which stays on our skins; 

 it is due even more to the failure to have heat drawn 

 off from our skins by the process of evaporation. Hot 

 climates where the air is dry are even more comfort- 

 able than cooler climates where the humidity of the air 

 interferes with evaporation. In southwestern United 

 States it is common to keep drinking water cool by placing 

 it in porous vessels and hanging these up in the air; the 

 water will become cooler than the air which surrounds it 

 on account of the heat carried off in the process of rapid 

 evaporation in the dry air. Canteens are usually wrapped 



