General Science | Page 9

great as that present in the
earth. In winter, long after the rocks and sand have given out their heat
and have become cold, the water continues to give out the vast store of
heat accumulated during the summer. This explains why lands situated
on or near large bodies of water usually have less variation in
temperature than inland regions. In the summer the water cools the
region; in the winter, on the contrary, the water heats the region, and
hence extremes of temperature are practically unknown.
19. Sources of Heat. Most of the heat which we enjoy and use we owe
to the sun. The wood which blazes on the hearth, the coal which glows
in the furnace, and the oil which burns in the stove owe their existence
to the sun.
Without the warmth of the sun seeds could not sprout and develop into
the mighty trees which yield firewood. Even coal, which lies buried
thousands of feet below the earth's surface, owes its existence in part to
the sun. Coal is simply buried vegetation,--vegetation which sprouted
and grew under the influence of the sun's warm rays. Ages ago trees
and bushes grew "thick and fast," and the ground was always covered
with a deep layer of decaying vegetable matter. In time some of this
vast supply sank into the moist soil and became covered with mud.
Then rock formed, and the rock pressed down upon the sunken

vegetation. The constant pressure, the moisture in the ground, and heat
affected the underground vegetable mass, and slowly changed it into
coal.
The buried forest and thickets were not all changed into coal. Some
were changed into oil and gas. Decaying animal matter was often
mixed with the vegetable mass. When the mingled animal and
vegetable matter sank into moist earth and came under the influence of
pressure, it was slowly changed into oil and 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° 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