Year: 1871

TEE fact that there now exist several


machines which through the consumption


of coal produce ice, Is one cutie inexpli.


cable 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 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 goifg on; and it


ie in this wise:


Whenever body changes from solid


to a liquid state, or from a LIQUID


to a vaporous condition, large amounts


of sensible heat disappear. Either


the temperature Sensible heat of
the body itself falls very much
lower than il was before its change of
state, or sensible heat is distracted from


surrounding bodies to maintain the ex-


panding substance of its former tempe


rature. The heat distracted 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
AAhJ OO JdHOSNJ 1uulUR 10
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 stimuli
taneously


Thus the gas chlorine, when submitted
to pressure of about four atmospheres.
becomes liquid, and will remain so IS
long as the pressure is continued. During
the act of compression, it gives OF a 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 |
lnal bulk as q sand in SO doing takes |
the same amount OF heat from other |
bodies, as it lost when compressed. Air, |
when compressed, gives off heat, and ab- /
sOrbs the same amount again when il ex-
pands. In reducing the volume of bodies, |
we may not only use comprs ssion, but we |
may also abstract heat by bringing them |
into contact with colder bodies, thus DOW
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. If we employ a water wheel to
generate our power, we find this possible
only because heat has raised the water for
US. IL We use VINO as motor, IL Is heat
that puts the air in motion; and if we en-
ploy steam, we must do the same thing.
If we use an electrometer we find our ma
terials prepared for US through the same
agency.


The various ice machines employ vola-
tile materials such as expand into gss 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
rails are then allowed to expand in con.
act with the vessels containing the water
10 be frozen again, and so on repeatedly
Until ice is produced.


Thus we see that heat indirectly pro
iuces cold, and this is only an exr7ession
If general law. Nothing can gain heat
without loss of heat in something else, ,
md though the gain or loss may be latent j
md not appear in the temperature, yet We
nay be sure that the sum total IS alWsys E
he Same-Scientifc Ag1eyienA :