FREEZING OF WATER. 49 



in the case of water ; when water is cooled it con- 

 tracts, and this goes on till very near the freezing 

 point ; but then the water begins to expand, and in 

 place of continuing to contract, as all other liquids 

 do, it becomes larger; this leads to a very important 

 result in Nature. When the air above the surface of 

 a lake or pond becomes cold, as towards the end of 

 autumn, it gradually cools the surface of the water ; 

 the upper part becoming cold, shrinks, and conse- 

 quently becomes heavier, it therefore sinks through the 

 warmer "water. This circulation or gradual sinking 

 of the cooled water goes on, if the air continues cold, 

 until the whole of the water is very near freezing, 

 but then it stops ; because if the surface still goes on 

 cooling, the water begins to expand, becoming larger 

 again, and consequently lighter ; the surface, there- 

 fore, gets colder and freezes, whilst the lower part of 

 the water remains considerably above the freezing 

 point. If it were not for this curious fact, water 

 would continue to become colder, until the whole of 

 it froze together. 



64. In passing from the liquid to the solid state, 

 some substances contract, such as melted lead, for ex- 

 ample, whilst others expand; thus ice-cold water, in 

 freezing, expands very considerably, and therefore ice 

 is even lighter than the water on which it is formed. 

 It is for this reason that ice floats on water, and if 

 the ice did not expand in forming, the curious fact just 

 mentioned would not prevent lakes and ponds from 

 freezing entirely, because the ice, if it contracted in 

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