nd gas.
The heat of our bodies comes from the foods which we eat. Fruits,
grain, etc., could not grow without the warmth and the light of the
sun. The animals which supply our meats likewise depend upon the sun
for light and warmth.
The sun, therefore, is the great source of heat; whether it is the
heat which comes directly from the sun and warms the atmosphere, or
the heat which comes from burning coal, wood, and oil.
CHAPTER III
OTHER FACTS ABOUT HEAT
20. Boiling. _Heat absorbed in Boiling_. If a kettle of water is
placed above a flame, the temperature of the water gradually
increases, and soon small bubbles form at the bottom of the kettle and
begin to rise through the water. At first the bubbles do not get far
in their ascent, but disappear before they reach the surface; later,
as the water gets hotter and hotter, the bubbles become larger and
more numerous, rise higher and higher, and finally reach the surface
and pass from the water into the air; steam comes from the vessel, and
the water is said to _boil_. The temperature at which a liquid boils
is called the boiling point.
While the water is heating, the temperature steadily rises, but as
soon as the water begins to boil the thermometer reading becomes
stationary and does not change, no matter how hard the water boils and
in spite of the fact that heat from the flame is constantly passing
into the water.
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 deg. 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 tempe
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