rature from 0 deg. C. to 100 deg. C. Since, with a
steady flame, it takes 5-1/4 times as long to change water into steam
as it does to change its temperature from 0 deg. C. to the boiling point,
we conclude that it takes 5-1/4 times as much heat to convert water at
the boiling point into steam as it does to raise it from the
temperature of ice water to that of boiling water.

The amount of heat necessary to raise the temperature of 1 gram of
water 1 deg. C. is equal to 1 calorie, and the amount necessary to raise
the temperature 100 deg. C. is equal to 100 calories; hence the amount of
heat necessary to convert 1 gram of water at the boiling point into
steam at that same temperature is equal to approximately 525 calories.
Very careful experiments show the exact heat of vaporization to be
536.1 calories. (See Laboratory Manual.)

22. General Truths. Statements similar to the above hold for other
liquids and for solutions. If milk is placed upon a stove, the
temperature rises steadily until the boiling point is reached; further
heating produces, not a change in temperature, but a change of the
water of the milk into steam. As soon as the milk, or any other liquid
food, comes to a boil, the gas flame should be lowered until only an
occasional bubble forms, because so long as any bubbles form the
temperature is that of the boiling point, and further heat merely
results in waste of fuel.

We find by experiment that every liquid has its own specific boiling
point; for example, alcohol boils at 78 deg. C. and brine at 103 deg. C. Both
specific heat and the heat of vaporization vary with the liquid used.

23. Condensation. If one holds a cold lid in the steam of boiling
water, drops of water gather on the lid; the steam is cooled by
contact with the cold lid and _condenses_ into water. Bottles of water
brought from a cold cellar into a warm room become covered with a mist
of fine drops of water, because the moisture in the air, chilled by
contact with the cold bottles, immediately condenses into drops of
water. Glasses filled with ice water show a similar mist.

In Section 21, we saw that 536 calories are required to change 1 gram
of water into steam; if, now, the steam in turn condenses into water,
it is natural to expect a release of the heat used in transforming
water into steam. Experiment shows not only that vapor gives out heat
during condensation, but that the amount of heat thus set free is
exactly equal to the amo