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Mercury Can Be Frozen
into a Serviceable
Hammer by Immersion
in Liquid
Air
FOUR HUNDRED DEGREES BELOW ZERO 73
that is, when heated they tend to assume
the gaseous or vaporous state and when
cooled they return to the solid or liquid
state. Of course it is extremely difficult
to solidify or liquefy some gaseous substances,
while others change their state
very quickly.
Water offers a perfect illustration.
Naturally it is a liquid. If we boil it, it
becomes a vapor and if we cool this
vapor, it returns to a liquid state. What
happens if we cool it further? It becomes
ice. This law is followed strictly
in nature, although there are certain substances
that pass directly from the solid
to the gaseous state and vice versa. In
that case they entirely avoid the intermediate
liquid state.
We will start with a simple example
of super-cooling. If we pucker our lips
(as in whistling) and blow our breath
upon the hand, we will find that the
gaseous matter emanating from our
mouth actually is cool when it strikes
our hand, although it was quite warm
before leaving our body. We find that
the temperature of this exhaled breath is
much lower than that of the surrounding
atmosphere.
The simple little experiment proves a
great law that was laid down by scientists
some years ago. It states, in its
most simple form, that if a gas, when
stored under pressure, is allowed to
expand freely through a small orifice,
it actually cools itself. This is called
the "self-intensifying method" of producing
low temperatures.
It is by this method that every gaseous
substance upon our planet has been
brought into the liquid and solid state.
If it is desired to liquefy air, the air is
forced by a pressure pump into a small
coil of pipe arranged in the form of a
helix; at the end of this coil of pipe
there is placed what is known as an expansion
valve. The air upon emanating
from the pipe expands freely. The
coiled pipe is housed in a durable steel
cylinder and the air expands in this.
Now it readily will be understood that
the air that is coming out of the valve
will be colder (contain less heat preferably)
than the air that is flowing further
back in the pipe. Then, at the expansion
valve, the gas cools and as the ex-