EVAPORATION 115 



evaporating, but there are a few solids which change 

 directly into gas. Camphor is an example. 



Evaporation is a process with which all living things 

 are much concerned. It occurs at the surface of your 

 body as well as at the surface of a plant's body. 



You have noticed on hot summer days that you are 

 much more uncomfortable when perspiration sticks to 

 you than you are when it quickly evaporates. People 

 speak of " sticky heat " and of " dry heat." The former 

 is much more uncomfortable than the latter. This illus- 

 trates a matter which affects plant life as well as our own. 



On those " sticky hot " days of August you may have 

 heard the expression that the " humidity is great." That 

 expression refers to the air. It refers to the fact that in 

 the air there is an unusual amount of water. This water 

 is, of course, in the gaseous state. The humidity of air 

 is the quality it obtains from the presence of water par- 

 ticles in it. The more water particles there are in the air, 

 the less rapidly will evaporation take place. 



This reminds you of solution and osmosis. There is the 

 same principle involved. The rate of diffusion of particles 

 in both solution and evaporation depends on the difference 

 in their abundance. Solution is more rapid when the 

 particles of the solute are less abundant in the solvent; 

 diffusion of sugar in tea is most rapid when it is first put 

 in; it gradually slows down. Evaporation is more rapid 

 when the air has in it few particles of the substances evapo- 

 rating than it is when the air has in it many of such par- 

 ticles ; water evaporates more rapidly when the humidity 

 of the air is slight than when it is great. This is a matter 

 which affects plant life as well as our own, though in dif- 

 ferent ways. 



