If the flame is removed from the boiling water for but a second, the
boiling ceases; if the flame is replaced, the boiling begins again
immediately. Unless heat is constantly supplied, water at the boiling
point cannot be transformed into steam.

_The number of calories which must be supplied to 1 gram of water at
the boiling point in order to change it into steam at the same
temperature is called the heat of vaporization_; it is the heat
necessary to change 1 gram of water at the boiling point into steam of
the same temperature.

21. The Amount of Heat Absorbed. The amount of heat which must be
constantly supplied to water at the boiling point in order to change
it into steam is far greater than we realize. If we put a beaker of
ice water (water at 0° C.) over a steady flame, and note (1) the time
which elapses before the water begins to boil, and (2) the time which
elapses before the boiling water completely boils away, we shall see
that it takes about 5-1/4 times as long to change water into steam as
it does to change its temperature from 0° C. to 100° 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° 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° C. is equal to 1 calorie, and the amount necessary to raise
the temperature 100° C. is equal to 100 calories; hence the amount of
heat necessary to convert 1 gram of water at the boiling point into