Year: 1871

TIE fact that there now exist several
machines which through the consumption
of coal produce ice, is one quite inexpli-
cable to many; and perhaps while VIC are
enjoying our iced drinks, so grateful in
The hot weather suddenly come upon us,
an explanation Of this apparent paradox
may not bc unacceptable. That heat should
directly or Indirectly produce cold seems,
at first thought, an impossibility; never
theless, in the laboratory of nature this is
an operation constantly going on; and it
is in this wise:


Whenever body changes from solid
to liquid state, or from a liquid
to a vaporous condition, large amounts
of sensible heat disappear Either
the temperature (sensible heaD of
the body itself falls very much
lower than it was before its change of
state, or sensible heat is abstractcd from
surrounding bodies to maintain the ex-
panding substance of its former tempe.
rature. The heat abstractcd and stored
upin the body, so that it no longer pro-
duces the effects popularity included in the
term heating" has been called latent
heat Its amount varies greatly in diff
ferent 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
itnpart heat and remove pressure stimuli
taneously.


Thus the gas chlorine, when submitted
to pressure of about four atmospheres,
becomes liquid, and will remain so as
long as the pressure is continued. During
the act of compression, it gives off Ger
tain amount of heat, which is the exact
equivalent Of the mechanical power en-.
ployed in reducing its volume. When the
pressure is removed, it expands to its orig.
inal bulk as gas and in so doing takes
bodies, as it lost when impressed. Air,
when compressed, gives off heat, and ab-
serbs the same amount again when it ex-
pands. In reducing the volume of bodies,
we may not only use compr. ssion, bu, we
may also abstract heat by bringing them
into contact with colder bodies, thus pow
erfully aiding the mechanical power in
bringing about the desired result.


But mechanical power is only another
name for heat, the sourcs Of all terrestrial
power. II 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 en-
ploy steam, we must do the same thing.
If we use an elcctrometer we find our ma
tcrials prepared for us through the same
agency.


The various ice machines employ vola-
tile 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 compression being absorbed
by some other body-most generally
water The condensed and cooled mate
rials arc then allowed to expand in con
tact with the vessels containing the water
to be frozen again, and SO on repeatedly
until ice is produced.


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 temperature, yet we
may be sure that the sum total Is always
the same.-Scientifc american.