58 PHYSICAL GEOGRAPHY.

 

 

of steam, its temperature can be raised very high. In
such cases great pressure is exerted on the walls of the
vessel. Conversely, on high mountains, where the pres-
sure of the atmosphere is lower than at the level of the
sea, water boils at temperatures lower than 212° Fahr.

148. Maximum Density of Water.—A pint of
cold water is heavier than a pint of warm water,
because as water is cooled it contracts and grows
denser. The coldest pint of water, however, is
not the heaviest. -The heaviest pint of water is
water at the temperature of 39.2° Fahr, This
temperature is therefore called the temperature
of the maximum density of water. If water at
this temperature be heated, it becomes lighter, or
expands; if water at this temperature be cooled,
it also becomes lighter or expands until ice is
formed, which Aone on the water. When at the
temperature of its maximum density, water is
7.2° warmer than the freezing-point.

149. Effect of the Maximum Density of Water
on its Freezing.—If water continued to contract
indefinitely while cooling until freezing began,

the ice first formed would sink to the bottom, and, :

this process continuing, the entire mass would soon
become solid. In this manner all bodies of fresh
water, in times of great cold, might freeze through-
out; when, not even the heat of a tropical sun
could entirely melt them.

But for this curious exception in the physical ponenics
of water, at least three-fourths of the globe would be in-
capable of sustaining its present life.

The entire floor of the ocean, both in the tropics and in
the temperate and the polar regions, is covered with a layer

of cold, salt water at nearly the temperature of its maxi-"

mum density. In the tropics the surface-water is warmer
and lighter than this dense layer, and in the polar re-
gions it is colder and lighter.

150. Specific Heat of Water.— Another re-
markable property of water—its specific heat—
enables it to play an important part in the
economy of the world.

The specific heat of a body is the quantity of
heat-energy required to produce a definite in-
crease of temperature in a given weight of that
body.

Water has a very great specific heat; that is,
a given quantity of water requires more heat-energy
to warm it, and gives out more heat-energy on cool-
ing, than an equal quantity of any other common
substance.

The quantity of heat required to raise a pound of ice-
cold water to 212°, would heat a pound of ice-cold iron to a
bright red heat, or to about 1600° Fahr.; or, conversely, a
pound of boiling water cooling to the freezing-point, would

give out as much heat as a pound of red-hot iron cooling
to 32° Fahr.

 

 

 

The enormous capacity of water for heat is of
great value to the life of the earth. The oceanic
waters are vast reservoirs of heat, storing heat in
summer and giving it out in winter. The great
specific heat of water prevents it from either heat- ‘
ing or cooling rapidly. Large bodies of water,
therefore, prevent great extremes of heat and
cold.

151. Heat Absorbed or Emitted during Change
of State—During the conversion of a solid into
a liquid, or a liquid into a vapor, a large quantity
of heat-energy is absorbed. This heat-energy does
not increase the temperature of the body, and
therefore cannot be detected by the thermometer.
The heat-energy is then in the condition of stored
or potential energy, sometimes called latent heat.
When the vapor condenses into a liquid, or the
liquid freezes, the stored heat- energy again becomes
sensible as heat.

In freezing, water gives out heat and raises the
mean temperature of the atmosphere.

In melting, ice takes in heat and lowers the mean
temperature of the atmosphere.

Water has a higher latent heat than any other
common substance. ;

Stored Heat-Energy of Ice-Cold Water—In
order to heat a pound of water 1° Fahr. an
amount of heat called a heat-unit, or a pound
degree is required. Before one pound of ice at
32° Fahr. can melt and form one pound of water
at 32° Fahr., zt must take in 142 heat units; and
yet a thermometer plunged in the water from
melting ice will indicate the same temperature as
when entirely surrounded by lumps of the un-
melted material.

The great latent heat of ice-cold water has an important
influence on the freezing of large bodies of water, since,
after the surface-layers have reached the temperature of
the freezing-point, they have still 142 heat-units to lose be-
fore they can solidify. Again, when ice reaches a tempera-
ture of 32° Fahr., it has still 142 heat-units to absorb before
it can melt. Were it not for this fact destructive floods
would often result from the rapid melting of the winter’s
accumulation of snow and ice.

Stored Heat-Energy of Water-Vapor.—Before
one pound of water can pass off as vapor, it
must take in sufficient heat to raise nearly 1000
pounds of water'1° Fahr. The vapor which then
escapes is still at the same temperature as the
water from which it came. The 1000 heat-units,
or pound-degrees of heat, have been rendered latent,
and have no influence on the thermometer.

When the vapor in the air is condensed as rain,

_ snow, hail, fog, or cloud, the stored heat-energy