THE EFFECTS OF HEAT. 231 



law that bodies expand by heat and contract by cold, or 

 by a withdrawal of heat.* 



As the temperature of water is lowered, it continues to contract until it 

 arrives at a temperature of 39° F., when all further contraction ceases. The 

 volume or bulk is observed to remain stationary for a time, but on lowering 

 the temperature still more, instead of contraction, expansion is produced, and 

 this expansion continues at an increasing rate until the water is congealed. 

 At the moment also of its conversiou into ice, it undergoes a still further 

 expansion. 



528. Water attains its greatest density, or 

 of the greatest the greatest quantity IS contained in a given 

 bulk, at a temperature of 39' F. 



As the temperature of water continues to decrease below 39°, the point of 

 its greatest density, its particles, from their expansion, necessarily occupy a 

 larger space than those which possess a temperature somewhat more elevated. 

 The coldest water, therefore, being lighter, rises and floats upon the surface 

 of the warmer water. On the approach of winter this phenomenon actually 

 takes place in our lakes, ponds and rivers. "^Vhen the surface-water becomes 

 sufQcientl}'- chilled to assume the form of ice, it becomes still lighter, and con- 

 tinues to float. By this arrangement, water and ice being almost perfect 

 non-conductors of heat, the great mass of the water is protected from the 

 influence of cold, and prevented from becoming chillod throughout. 



If water constantly grew heavier as its temperature diminished (as is the 

 case with most liquids), the colder particles at the surface would constantly 

 sink, until the whole body of water was reduced to the freezing point. Again, 

 if ice was not lighter than water, it would sink to the bottom, and by the 

 continuance of this operation, a river or lake would soon become an immense 

 solid mass of ice, which the heat ot' summer would be insufficient to dissolve. 

 The temperate regions of the earth would thus be rendered uninhabitable. 

 Among all the phenomena of the natural world, there is no more striking 

 illustration of the wisdom of the Creator, and of the evidences of design, than 

 in this wonderful exception to a great general law. 



,^ , The expansion of water at the moment of freezing is attrib- 



Why does wa- ' , . . ? , _ 



ter expand in uted to a new and peculiar arrangement of its particles. Ice 

 freezing ? jg^ jj^ reaUty, crystallized water, and during its formation the 



particles arrange themselves in ranks and lines which cross each other at 

 angles of 60° and 120°, and consequently occupy more space than whea 

 liquid. This may be seen by examining the surface of water in a saucer whOe 

 freezing. 



A beautiful illustration of this crystallization of water in freezing is seen in 

 the frost-work upon windows in winter, caused by the congelation of the 

 vapor of the room when it comes in contact with the cold surface of the glass. 



• A few other liquids besides watgr expand with a reduction of temperature. Fused 

 Iron, antimony, zinc, and bismnth, are examples of such expansion. Mercury is a re- 

 markable instance of the reverse, for when it freezes, it suffers a very great contraction. 



