gives one a new sense of the power of numbers to feel that infinitesimal
atoms, merely by vibrating in unison, could accomplish such a result.

But now suppose our portion of gas, instead of being placed under our
hypothetical building, is plunged into a cold medium, which will permit
its heat-vibrations to exhaust themselves without being correspondingly
restored. Then, presently, the temperature is lowered below the critical
point, and, presto! the mad struggle ceases, the atoms lie amicably
together, and the gas has become a liquid. What a transformed thing
it is now. Instead of pressing out with that enormous force, it has
voluntarily contracted as the five thousand tons pressure could not
make it do; and it lies there now, limpid and harmless-seeming, in the
receptacle, for all the world like so much water.

And, indeed, the comparison with water is more than superficial, for
in a cup of water also there are wonderful potentialities, as every
steam-engine attests. But an enormous difference, not in principle but
in practical applications, exists in the fact that the potentialities
of the water cannot be utilized until relatively high temperatures are
reached. Costly fuel must be burned and the heat applied to the water
before it can avail to do its work. But suppose we were to place our
portion of liquid air, limpid and water-like, in the cylinder of a
locomotive, where the steam of water ordinarily enters. Then, though no
fuel were burned--though the entire engine stood embedded in the snow
of an arctic winter--it would be but a few moments before the liquid air
would absorb even from this cold medium heat enough to bring it above
its critical temperature; and, its atoms now dancing apart once more and
re-exerting that enormous pressure, the piston of the engine would be
driven back and then the entire cylinder burst into fragments as the
gas sought exit. In a word, then, a portion of liquid air has a store
of potential energy which can be made kinetic merely by drawing upon
the boundless and free supply of heat which is everywhere stored in the
atmosphere we breathe and in every substance about us. The difficulty
is, not to find fuel with which to vaporize it, as in case of water,
but to keep the fuel from finding it whether or no. Were liquid air in
sufficient quantities available, the fuel problem would cease to
have any significance. But of course liquid air is not indefinitely
available, and exactly here com