EFFECTS OF HEAT 75 



upon it heavily, the ice beneath the dish melting faster 

 than the other parts. 



85. Evaporation We know that a moist cloth soon 



dries if hung in warm air; also that a thin layer of 

 water in a dish or on some hard surface soon disappears. 

 Clearly the liquid must have gone somewhere. But did 

 it pass off in a liquid state? If so, we should probably 

 have seen it go. We have to suppose, then, that it 

 changes into a gas and passes off" into the air, and we say 

 that it has evaporated. Evaporation may be denned as 

 that sort of vaporization which goes on quietly at ordi- 

 nary temperatures. 



Note that evaporation, not being produced necessarily 

 by boiling, depends partly upon the ability of the atmos- 

 phere to receive the vapor. Of course some substances 

 vaporize more easily than others, but in general the 

 conditions which aid evaporation are conditions in the 

 air surrounding the liquid. Warm air can hold more 

 vapor than cold; dry air can naturally take on more 

 than that which is moist or humid; and evaporation 

 goes on faster when the atmosphere is in motion. Thus 

 the best conditions would be warm, dry, moving air. 



Experiment 67. Try these different conditions with small 

 amounts of water. Also use such liquids as alcohol, ether, and 

 naphtha. Blow upon them, and see if there is any faster evapora- 

 tion. Why put damp clothes in a warm place to dry? Will 

 clothes dry when frozen ? Do they dry better on windy days ? 



86. Condensation. The amount of vapor which air 

 can hold varies with its temperature; other things being 

 equal, the warmer the atmosphere, the more vapor it can 



