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.