104 ELEMENTS OF GENERAL SCIENCE 



In the first place, the temperature at which water freezes 

 is perfectly definite, and under ordinary circumstances it is 

 always the same. You may perhaps remember that in mak- 

 ing the centigrade thermometer the freezing point of water is 

 taken as the starting point in making the scale, and that point 

 is called zero. What is the freezing point on the Fahrenheit 

 thermometer ? 



There are some very peculiar facts about the behavior of 

 water in connection with its freezing. If a quantity of water 

 in liquid form is cooled slowly, it will be found that as it 

 cools it contracts. Perhaps that is as we should have expected, 

 since most substances contract when cooling. As the water 

 approaches the freezing point, however, it begins to expand, 

 and very quickly expands more than the whole amount that 

 it has contracted in cooling. After it has become solid, if it 

 is cooled still more it continues to contract, but it never con- 

 tracts to so small a volume as it had when it was a liquid. 

 You should determine for yourselves the temperature at which 

 water begins to expand and at which it freezes. 



The temperature while freezing behaves almost as pecul- 

 iarly as the volume. If the thermometer is observed during 

 the experiment, it will be found that the temperature falls 

 regularly until the water begins to freeze ; it then remains 

 stationary until all of the water is frozen, after which it begins 

 to fall again. If a substance (salt, for instance) is dissolved 

 in the water, the freezing point is much lower just how 

 much lower depends on the amount of salt that is dissolved. 



118. Changing water to steam. When water is heated, as 

 over a lamp or a stove, it finally gets hot enough so that some 

 of the water nearest the flame (that is, at the bottom) is 

 changed to steam. The steam rises in bubbles through the 

 remaining water, and the bubbles burst at the surface. The 

 disturbance which results is called boiling (fig. 56). 



The temperature at which boiling begins is called the boil- 

 ing point On the centigrade scale the point is 100. What 



