Year: 1871

The fact that there now exists scy.
eral machines which through the
consumption Of coal produce ice, is
one quite inexpljcable to many; and
perhaps while we are enjoying oul
iced drinks, so grateful in the hot
weather suddenly come upon us, al
explanation ofthis apparent paradox
may not be unacceptable. That
heat should directly or indirectly
produce cold seems, at first thought,
an impossibility; nevertheless, i'll the
laboratory of nature this is an opera
ton constantly going on; and it is in
this wise:


Whenever a body changes from a
solid to Il liquid state, or from a
liquid to a VapOrous condition.
large amounts of sensible heat disap-
pear. Either the temperature (sen-
slble heat of the body itself falls
very much lower than it was before
it's change Of state, OF sensible heat
is abStracted from surrounding bod
les to maintain the expanding sub
stance G it's tarmcr temperature.
The heat abstlacted and stored up
i'll the body, so that it no longer pro-
iuces the effects popularity included
i'll the term " heating," has been
called latent heat. sameunt varies
greatly i'll different solids, liquids,
and vapors.


Now there are too ways in which
bodies may be expanded, namely :
by adding to their heat-sensible or
latent, or botll-or by removing the
pressure their surfaces sustain. Or
we may, il we choose, both impart
heat and remove pressure silnulta-
ncously.


Thus the gas chlorine, when sub
mitted to pressure of about four
atmospheres, becomes a liquid, and
will remain so as long as the pres
sure is continued. During the act
of compression, it gives off a certain
amount of heat, which is the exact
equivalent OF the mechanical power
employed i'll reducing it's volume.
When the pressure is removed. it ex-
pands to it's original bulk as gas,
and i'll SO doing takes the same
amount of heat, from other bodies,
as it lost when compressed. Air,
when compressed, gives off heat, and
absorbs the same amountagain when
it expands. In reducing the volume
OF bodies, we may not only use COM
preston, but we may also abstract
heat by bringing them into contact
with colder bodies, thus powerfully
aiding the mechanical power in
bringing about tie desired result.


But mechanical power is only all
other name for heat, the source of all
terrestrial power. If we employ q
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 III motion and if
we employ steam, we must do the
same thing. If We use an electro
meter we find our materials prepared
for lis through the same agency.
The various ice machines employ
volatile materials such as expand
into gas at ordinary temp eratures,
or at least do so when atmospheric
pressure is removed from their sur
faces. In thus expanding they alb.
stract heat from water placed in
suitable vessels, brought in contact
with the absorbing bodies. The ex-
panded gases are next compressed,
the heat given off during the COM
preston being absorbed by some
other body-most generally water.
The condensed and cooled materials
are then allowed to expand in con
tact with the vessels contQining the
water to be frozen again, and so on
repeatedly until ice is produced.
Thus we see that heat indirectly
produces cold, and this is only an
expression of a general law. Moth
ing 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 Scientific Amer
lean.


The way to quarrel with a wife is
10, l. H. l.