Rock Island Argus. (Rock Island, Ill.) 1893-1920, 1911-09-30
PRESSURE OF AIR.
1t May Readily Turn the Thermom-
eter Into q Fibber.


On an ordinary Fahrenheit thermom-
eter there is written opposite 212 de-
grees Boiling point of water" and op-
posite 81 degrees Freezing point of
water" Neither of these is correct ex-
cepr for a certain condition Of the at
mosphere. and that is when IL gives on
the barometer about thirty inches, or
fifteen pounds pressure to the square
inch. This is the ordinary pressure at
what is known as sea level, and to this
all thermometers are calibrated- In
mountainous region the pressure Is
hardly ever so much as fifteen pounds.
and water bOils at sometimes as low as
200 degrees.
II water is boiled in q diving bell.
where the pressure is forty or fifty
pounds a square inch. its temperature
will be several hundred degrees in
stead Of 212. If water is boiling in
near vacuum the temperature is so
small that the hand thrust into the
water would actually feel cold
What has been said about the boiling
point applies to some extent to the
freezing point, but here it differs for
different materials. whereas the re-
marks about the boiling point Of water
apply to the boiling points of all
liquids.
Some substances when they freeze
become larger. while others become
smaller. On this depends the freezing
point at different pressures of atmos-
phere. Water expands on freezing; so
do type metal and some other things.
All other substances become smaller
on freezing Water pipes burst when
the water freezes. Coins Of gold and
silver are stamped instead of being
nodded, for the metals grow smaller on
freezing or sOlidifying. and consequent
" the coin would be wabbly.
1t has been found that the things
that expand on solidifying, as water,
freeze at a lower temperature when
the pressure is increased, while the
others freeze at q higher. When
substance that expands freezes under
higher pressure than usual ii has TO
exert more force to shove the pressure
away, and consequently has 10 use up
more Of its heat energy, thus losing
more heat and becoming colder.
Take the substance that contracts
when it solidifies. The pressure will
help it to get smaller, and consequent
ly the greater the pressure the less
heat it has to lose on attaining the
solid state. so it will freeze at higher
temperature. If the pressure is great
enough it may freeze or solidify at a
thousand degrees temperature, which
is high enough to change most sub
stances to vapor under ordinary at
mOSDheric pressure
This is one of the reasons advanced |
to prove that the interior of the earth |
is solid, for the assumption is that the .
core is made up of substances that |
contract when freezing, and there is, |
hundreds of miles below the surface.
In regard to the boiling points of /
liquids, there is an upper limit to the |
point at which a thing boils-that is, |
changes to the state of vapor. 1t is |
called the critical temperature. No |
matter how great pressure exists on |
a substance, if it is at a temperature !
greater than its critical it will change .
to vapor anyhow.
The ignorance of this point held .
back the making of Iiguefied gases
such as air. carbon dioxide, etc.-for
many years. The experimenters tried
to Hquefy gases at ordinary tempera-
lures by enormous pressures, whereas
If they had just cooled the gases be
low their critical temperatures before
applying the pressure HQuefaction
would have ensued Immediately.
This is the method employed today
in making liquid air. The air is com
pressed at first and then allowed to
issue from d small orifice, thus ex-
pansing and cooling, is then pumped
back and compressed by the pump,
allowed TO go through the orifice
again. thus cooling still more, until at
last it is below the critical tempera-
ture. when the compression caused by
the pump liquefies IT. - Lawrence
Hodges in Chicago Record Herald.