As
is well known, roughly four-fifths by volume of the air consist of
nitrogen, which is non-inflammable and accordingly devoid of danger-
conferring properties; but in all flames the presence of nitrogen is
harmful by absorbing much of the heat liberated, thus lowering the
temperature of that flame, and reducing its illuminating power far more
seriously. On the other hand, a certain quantity of air in acetylene
helps to prevent burner troubles by acting as a mere diluent (albeit an
inferior one to methane or marsh-gas), and therefore it has been proposed
intentionally to add air to the gas before consumption, such a process
being in regular use on the large scale in some places abroad. As Eitner
has shown (Chapter VI.) that in a 3/4-inch pipe acetylene ceases to be
explosive when mixed with less than 47.7 per cent. of air, an amount of,
say, 40 per cent. or less may in theory be safely added to acetylene; but
in practice the amount of air added, if any, would have to be much
smaller, because the upper limit of explosibility of acetylene-air
mixtures is not rigidly fixed, varying from about 50 per cent. of air
when the mixture is in a small vessel, and fired electrically to about 25
per cent.
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