RELATIONS BETWEEN WATER AND LIFE n 



The third way in which the changing of water from liquid 

 into gas affects us is by keeping us cool in hot weather. This 

 is not quite so easy to understand, and yet it is a thing 

 that has happened to you hundreds of times. Why is it 

 that it cools your face to fan it ? Chiefly because the stir- 

 ring of the air makes the moisture on your face evaporate 

 more rapidly. As the tiny particles pass into the air, 



FIG. i. Principle of the steam-engine: fire at H heats water in boiler A; this 

 produces steam which travels, as indicated, into cylinder A', pushes piston P, 

 and operates the machinery to which the engine is attached. 



they carry heat away with them. Evaporation is a cooling 

 process. This is a matter we shall have to discuss more 

 fully a little later. Here again we find that we cannot 

 study water without studying heat. 



Now about the changing of water from liquid to solid. 

 That is what gives us ice, and it would be very inconvenient 

 for us not to have ice. Then there is another interesting 

 thing about the changing of water from liquid to solid, 

 a thing that affects our lives, a thing that seems to be an 

 exception to one of the "rules" of nature. That rule is 

 that heat causes expansion and cold causes contraction. 



