I 4 4 ELEMENTARY SCIENCE 



in cloth for the same purpose of keeping the water within 

 at a temperature which makes it agreeable for drinking. 



Your attention has already been called to the exceptional 

 character of the expansion of water when it freezes, and 

 you have considered the effects which this exception pro- 

 duces that are beneficial to man. Now, if we begin with 

 steam, we note that reduction of the temperature of water 

 is accompanied by contraction, just as in other substances. 

 In the change of steam to liquid there is a very con- 

 siderable reduction of volume, and if you go on lowering 

 the temperature, this contraction continues. At what 

 point then does this normal contraction cease and the ab- 

 normal expansion of water begin?* We find that this oc- 

 curs at 4 C. ; that is, at four degrees above freezing-point 

 on a centigrade thermometer, water ceases to contract and 

 begins to expand. 



Now let us consider what effects this produces upon a 

 body of water in cold weather. As the air over the sur- 

 face of the water loses its heat, the heat of the water 

 passes up into it. As the surface-water loses heat, it in- 

 creases in weight and sinks toward the bottom, while 

 the warmer and, necessarily, lighter water beneath rises 

 to the surface. So, on account of these movements of 

 the water induced by the differences in density which 

 accompany changes of heat, the whole body of water tends 

 to become equally chilled throughout. This process con- 

 tinues until the whole body of water reaches a temper- 

 ature of 4 C. Then, evidently, something different is 

 sure to occur, inasmuch as at this point of temperature 

 the water begins to expand instead of contract; it begins 

 to gain in lightness instead of in weight. So the surface- 

 water which first cools below 4 C. does not sink; it re- 



