, sufficient air has entered the vessel to
form, with the remaining gas, a mixture lying within the explosive
limits, when an explosion will occur. Again, if a gaseous mixture
containing less of its combustible constituent than is necessary to
attain the lower explosive limit escapes from an open-ended pipe and a
light is applied to it, the mixture will not burn as a useful compact
flame (if, indeed, it fires at all); if the mixture contains more of its
combustible constituent than is required to attain the upper explosive
limit, that mixture will burn quietly at the mouth of the pipe and will
be free from any tendency to fire back into the pipe--assuming, of
course, that the gaseous mixture within the pipe is constantly travelling
towards the open end. If, however, a gaseous mixture containing a
proportion of its combustible constituent which lies between the lower
and the upper explosive limit of that constituent escapes from an open-
ended pipe and a light is applied, the mixture will fire and the flame
will pass back into the pipe, there to produce an explosion, unless the
orifice of the said pipe is so small as to prevent the explosive wave
passing (as is the case with a proper acetylene burner), or unless the
pipe itself is so narrow as appreciably to alter the range of
explosibility by lowering the upper explosive limit from its normal
value.
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