The most conspicuous effect of the expansive power of freezing water is seen in rocky or mountainous regions (Fig. 21).  Water easily finds entrance into the cracks and crevices of the rocks, where it lodges until frozen; then it expands and acts like a wedge, widening cracks, chiseling off edges, and even breaking rocks asunder.  In regions where frequent frosts occur, the destructive action of water works constant changes in the appearance of the land; small cracks and crevices are enlarged, massive rocks are pried up out of position, huge slabs are split off, and particles large and small are forced from the parent rock.  The greater part of the debris and rubbish brought down from the mountain slopes by the spring rains owes its origin to the fact that water expands when it freezes.

[Illustration:  FIG. 21.—­The destruction caused by freezing water.]

34.  Heat Necessary to Dissolve a Substance.  It requires heat to dissolve any substance, just as it requires heat to change ice to water.  If a handful of common salt is placed in a small cup of water and stirred with a thermometer, the temperature of the mixture falls several degrees.  This is just what one would expect, because the heat needed to liquefy the salt must come from somewhere, and naturally it comes from the water, thereby lowering the temperature of the water.  We know very well that potatoes cease boiling if a pinch of salt is put in the water; this is because the temperature of the water has been lowered by the amount of heat necessary to dissolve the salt.

Let some snow or chopped ice be placed in a vessel and mixed with one third its weight of coarse salt; if then a small tube of cold water is placed in this mixture, the water in the test tube will soon freeze solid.  As soon as the snow and salt are mixed they melt.  The heat necessary for this comes in part from the air and in part from the water in the test tube, and the water in the tube becomes in consequence cold enough to freeze.  But the salt mixture does not freeze because its freezing point is far below that of pure water.  The use of salt and ice in ice-cream freezers is a practical application of this principle.  The heat necessary for melting the mixture of salt and ice is taken from the cream which thus becomes cold enough to freeze.

CHAPTER IV

BURNING OR OXIDATION

35.  Why Things Burn.  The heat of our bodies comes from the food we eat; the heat for cooking and for warming our houses comes from coal.  The production of heat through the burning of coal, or oil, or gas, or wood, is called combustion.  Combustion cannot occur without the presence of a substance called oxygen, which exists rather abundantly in the air; that is, one fifth of our atmosphere consists of this substance which we call oxygen.  We throw open our windows to allow fresh air to enter, and we take walks in order to breathe the pure air into our lungs.