ICE, WATER, AND STEAM 



119 



137. Boiling and melting points. Every substance has its 

 boiling and melting point, but these may be at temperatures 

 very different from those of water. The approximate melting 

 and boiling points of a few substances are given below : 



Excepting for differences in temperature, most substances 

 behave very much like water in changing from solid to liquid 

 and from liquid to gaseous condition. There is one impor- 

 tant exception, however. Few expand at the time of freezing, 

 as water does, and these few expand much less. 



Substances such as glass and wax do not have a definite 

 melting point. They gradually soften, becoming first like a 

 very thick liquid, and finally flow freely. 



138. Evaporation. Even when the temperature of water 

 is not high enough to cause it to boil, the water changes into 

 vapor, but not rapidly. This is shown by the drying of wet 

 objects when exposed to the air. Water will slowly change 

 into vapor (evaporate) at any temperature. Even if water 

 is frozen, evaporation continues. This is shown when wet 

 clothing is hung out of doors on a cold day. It immedi- 

 ately freezes, and remains frozen until it has become dry 

 by evaporation of the frozen water. 



