The Elk County advocate. [volume] (Ridgway, Pa.) 1868-1883, 1871-07-27
How Bodies May be Frozen by Heat.

The fact that there now exists several
machines which through the consump-
tion OF coal produce ice, is one quite in
explicable to many; and perhaps while
we are enjoying our iced drinks, so
grateful in the hot weather suddenly
come upon US, an explanation of this
apparent paradox may not be unaccept-
able. That heat should directly or in
directly produce cold seems, at first
thought, an impossibility nevertheless,
in the laboratory of nature this is an
operation constantly going on and it
is in this wise:
Whenever a body changes from a
solid to liquid state, or from liquid
to vaporous condition, large amounts
of sensible heat disappear. Either the
temperature sensible heat) of the body
itself falls very much lower than it was
before in its change of state, or sensible
heat is distracted from surrounding
| bodies to maintain the expanding sub
stance to its former temperature. The
heat abstracted and stored up i'll the
body, so that it no longer produces the
effects popularity included in thu term
heating" has been called latent heat.
Its amount varies greatly in different
solids, liquids, and vapors.
Now there are two ways in which
bodies may be expanded, namely: by
adding to their heat-sensible or latent,
or both-or by removing the pressure
their surfaces sustain. Or we may, if
we choose, both impart heat and remove
pressure simultaneously.
Thus the gas chlorine, when submit
Tod 10 oe
pheres, becomes a liquid, and will re-
main so as long as the pressure is con-
tinued. During the act of compression,
it gives off certain amount of heat,
which is the exact equivalent of the
mechanical power employed in reducing
its volume. When the pressure is re-
moved, it expands to its original bulk as
a gas, and in so doing takes the same
amount of heat, from other bodies, as it
lost when compressed. Air, when COM
pressed, gives off heat, and absorbs the
same amount again when it expands.
In reducing the volume of bodies, we
may not only use compression, but we
may also abstract heat by bringing
them into contact with colder bodies,
thus powerfully aiding the mechanical
power in bringing about the desired re-
suit.
But mechanical poweris only another
name for heat, the terrestrial power. If
we employ water wheel to generate our
power, we find this possible only because
heat has raised the water for us. If we
use wind as a motor, it is heat that puts
the air in motion; and if we employ
steam, we must do the same thing. five
use an electromotor we find our materials
prepared for us through the same agency.
The various ice machines employ
volatile materials such as expand into
gas at ordinary temperatures, or at
least do so when atmospheric pressure
is removed from their surfaces. In thus
expanding they abstract heat from water
placed in suitable vessels, brought in
contact with the absorbing bodies. The
expanded gases are next compressed,
the heat given off during the compress
sion being absorbed by some other body
generally water. The condensed
and cooled materials are then allowed
to expand in contact with the vessels
containing the water to be frozen again,
and so on repeatedly until ice is pro-
duced.
Thus we see that heat indirectly pro-
duces cold, and this is only an expression
of general law. Nothing can gain
heat without loss of heat in something
else, and though the gain or loss may be
latent and not appear in the tempera-
ture, yet we may be sure that the sum
total is always the same.