plants is by
the familiar iron pipes, not dissimilar in appearance (when not in
operation) to the familiar gas, water, and steam pipes. When operating,
however, the pipes themselves are soon hidden from view by the thick
coating of frost which forms over them. In a moist beer-cellar
this coating is often several inches in thickness, giving a very
characteristic and unmistakable appearance.

Another commercial use to which refrigerator machines are now put is in
the manufacture of various drugs, where absolute purity is desirable.
As different substances congeal at different temperatures, but the same
substances at uniform pressure always at the same temperature, a
means is afforded of freeing a drug from impurities by freezing, where
sometimes the same result cannot be accomplished with like thoroughness
by any other practicable means. Indeed, by this means impurities have
been detected where not previously suspected. And Professor Ramsay has
detected some new elementary substances even, as constituents of the
air, which had previously not been dissociated from the nitrogen with
which they are usually mixed.

Such applications of the refrigerator principles as these, however,
though of vast commercial importance, are held by many enthusiasts to
be but a bagatelle compared with other uses to which liquefied gases
may some time be put. Their expectations are based upon the enormous
potentialities that are demonstrably stored in even a tiny portion of,
say, liquefied air. These are, indeed, truly appalling. Consider, for
example, a portion of air at a temperature above its critical point, to
which, as in Thilorier's experiments, a pressure of thirty-one tons to
the square inch of the encompassing wall is being applied. Recall that
action and reaction are equal, and it is apparent that the gas itself is
pushing back--struggling against being compressed, if you will--with an
equal power. Suppose the bulk of the gas is such that at this pressure
it occupies a cubical space six inches on a side--something like the
bulk of a child's toy balloon, let us say. Then the total outward
pressure which that tiny bulk of gas exerts, in its desperate molecular
struggle, is little less than five thousand tons. It would support an
enormous building without budging a hair's-breadth. If the building
weighed less than five thousand tons it would be lifted by the gas; if
much less it would be thrown high into the air as the gas expanded. It