Evening capital and Maryland gazette. (Annapolis, Md.) 1910-1922, 1911-10-06
PRESSURE OF AIR.
1t Hay Readily Turn the Their
mometer into a Fibber.
THE BOILING WATER POINT.
On the Scale This Is Marked at PIP
Degrees, but Under Certain Condi-
tiens It May Be Several Hundred

Degrees, but Under Certain condi-
tiens IT May Be Several Hundred
Desrees-The Critical Temperature.
On an ordinary Fahrenheit thermom-
eter there is written opposite 212 de-
grees Rolling point of water" and OPP
posite 80 degrees Freezing point Of
water" Neither of these is correct ex-
cept for a certain condition of the at
mosphere. and that Is Zen 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 a
mountainous region the pressure is
hardly ever so much as fifteen pounds,
and water boils at sometimes as low as
200 degrees.
If water is boiled in a diving bell,
where the pressure is forty or fifty
pounds a square inch. its temperature
will be several hundred degrees in
stead OF DO. If water is boiling In a
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 n lower temperature when
the pressure is increased, while the
others freeze at a higher. When a
substance that expands freezes under
higher pressure than usual it has to
exert more force to shove the pressure
away, and consequently has to 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 viii freeze at higher
temperature. If the pressure is great
enough it may freeze or solidify at q
thousand degrees temperature, which
is high enough to change most sub
stances to vapor under ordinary at
mosphGric 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,
of course, an enormous pressure n few
hundreds of miles below the surface.
In regard to the boiling points of
liquids, there Is an upper limit to the
point at which n thing boils-that is,
changes to the state of vapor. 1t is
called the critical temperature. No
matter how great pressure exists on
n substance. if it is at temperature
greater than its critical it will change
TO vapor anyhow.
The ignorance Of this point held
back the making Of liquefed gases
such as air, carbon dioxide, etc.-for
many years The experimenters tried
to liquefy 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 liquefaction
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 small orifce, thus ex-
panding 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 1t is below the critical tempera-
ture. when the compression caused by
the pump liquefies 1t. Lawrence
H., TTnra16