Provided the second gas is well chosen, such mixtures exhibit
properties which render them peculiarly valuable for special purposes.
They have, usually, a far lower upper limit of explosibility than that of
neat acetylene, and they admit of safe compression to an extent greatly
exceeding that of acetylene itself, while they do not lose illuminating
power on compression. The second characteristic is most important, and
depends on the phenomena of "partial pressure," which have been referred
to in Chapter VI. When a single gas is stored at atmospheric pressure, it
is insensibly withstanding on all sides and in all directions a pressure
of roughly 15 lb. per square inch, which is the weight of the atmosphere
at sea-level; and when a mixture of two gases, X and Y, in equal volumes
is similarly stored it, regarded as an entity, is also supporting a
pressure of 15 lb. per square inch. But in every 1 volume of that mixture
there is only half a volume of X and Y each; and, ignoring the presence
of its partner, each half-volume is evenly distributed throughout a space
of 1 volume. But since the volume of a gas stands in inverse ratio to the
pressure under which it is stored, the half-volume of X in the 1 volume
of X + Y apparently stands at a pressure of half an atmosphere, for it
has expanded till it fills, from a chemical and physical aspect, the
space of 1 volume: suitable tests proving that it exhibits the properties
which a gas stored at a pressure of half an atmosphere should do.
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