e._, the
construction of a trustworthy burner of the Bunsen type.
This has been exceedingly difficult to achieve for two reasons: first,
the wide range over which mixtures of acetylene and air are explosive;
secondly, the high speed at which the explosive wave travels through such
a mixture. It has been pointed out in Chapter VIII. that a Bunsen burner
is one in which a certain proportion of air is mixed with the gas before
it arrives at the actual point of ignition; and as that proportion must
be such that the mixture falls between the upper and lower limits of
explosibility, there is a gaseous mixture in the burner tube between the
air inlets and the outlet which, if the conditions are suitable, will
burn with explosive force: that is to say, will fire back to the air jets
when a light is applied to the proper place for combustion. Such an
explosion, of course, is far too small in extent to constitute any danger
to person or property; the objection to it is simply that the shock of
the explosion is liable to fracture the fragile incandescent mantle,
while the gas, continuing to burn within the burner tube (in the case of
a warming or cooking stove), blocks up that tube with carbon, and
exhibits the other well-known troubles of a coal-gas stove which has
"fired back.
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