[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.
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