RELATIONS BETWEEN WATER AND LIFE 13 



far too small to be seen, scientists have been able to mea- 

 sure the size of molecules. The water molecule is so small 

 that there are millions of them in a single drop. Since 

 they are so wonderfully small, it is not hard to understand 

 that they may fly upward from the surface of a liquid. 

 The surface of a liquid may seem to be very quiet, yet 

 something is going on there all the time. 



Think of the surface of a pond on a summer day. The 

 warm sun shines on it. The air above it is dry. A breeze 

 blows over it. Under these conditions evaporation is 

 rapid. But if the air were cold and moist and still, evapora- 

 tion would be very slow. You see the air can hold just 

 so much water and no more. We say of a sponge that is 

 full of water that it is saturated. Air that has taken up 

 all the water it can is also said to be saturated. 



But to go back to our pond. The heat and the wind 

 and the dryness of the air make millions upon millions of 

 invisible molecules keep rising from the pond. Up and 

 up they go, until at last they come where the air is cooler. 

 Then what happens? Then these tiny molecules of water 

 begin to condense. It is as though they huddled together 

 in crowds. These crowds of molecules are tiny droplets, 

 so small that you could not see one of them, and yet when 

 billions and billions of them are near each other they form 

 what we call clouds. 



Here for the third time in studying water we have come 

 to heat. Cold means simply less heat, and you have just 

 noted that heat causes water molecules to evaporate, and 

 then less heat causes them to condense again. If it were 

 not for heat, all the water in the world would change to 

 ice, and would remain in that form. Water seems to be 

 quite an active substance, changing constantly as it does 



