If either of those gases becomes liquid at a lower pressure than
the other, and if those pressures are within the limits of our
present powers of compression, the object might be accomplished.
Let us assume, for example, that oxygen becomes liquid under
a pressure of 200 atmospheres, whilst azote requires a pressure
of 250. Then if atmospheric air be condensed to the two hundredth
part of its bulk, the oxygen will be found in a liquid state at
the bottom of the vessel in which the condensation is effected,
and the upper part of the vessel will contain only azote in the
state of gas. The oxygen, now liquefied, may be drawn off for the
supply of the furnace; but as it ought when used, to have a very
moderate degree of condensation, its expansive force may be
previously employed in working a small engine. The compressed
azote also in the upper part of the vessel, though useless for
combustion, may be employed as a source of power, and, by its
expansion, work another engine. By these means the mechanical
force exerted in the original compression would all be restored,
except that small part retained for forcing the pure oxygen into
the furnace, and the much larger part lost in the friction of the
apparatus.
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