114 ELEMENTS OF GENERAL SCIENCE 



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 cool- 

 ing. After it has become solid, if it is cooled still more it 

 continues to contract, but it never contracts to so small a 

 volume as it had when it was a liquid. See if you can deter- 

 mine for yourselves the temperature at which water begins 

 to expand and that at which it freezes. 



The temperature during 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 the water is frozen, after which it be- 

 gins to fall again. If a substance (salt or sugar, for instance) 

 is dissolved in the water, the freezing point is much lower 

 just how much lower depends upon the amount of the 

 substance dissolved. 



132. Changing water to steam. When water is heated 

 over a lamp or a stove, it finally gets so hot 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 

 is called boiling (fig. 58). 



The temperature at which boiling begins is called the boil- 

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

 is it on the Fahrenheit scale ? After a mass of water has 

 reached the boiling point, its temperature does not rise as 

 long as any of the water remains in the liquid form. In your 

 experiment you will find the boiling point somewhere near 

 100 C., but it is not probable that it will be exactly that. 

 The boiling point is raised if solid substances such as salt or 

 sugar are dissolved in the water. 



