If the
combustible gas is present in the mixture with air in less than that
percentage explosion is impossible. The upper explosive limit is the
highest percentage of combustible gas in the mixture of it and air at
which explosion will occur in the mixture if a light or spark is applied
to it. If the combustible gas is present in the mixture with air in more
than that percentage explosion is impossible. Mixtures, however, in which
the percentage of combustible gas lies between these two limits will
explode when a light or spark is applied to them; and the comprehensive
term "range of explosibility" is used to cover all lying between the two
explosive limits. If, then, a naked light is applied to a vessel
containing a mixture of a combustible gas and air, in which mixture the
proportion of combustible gas is below the lower limit of explosibility,
the gas will not take fire, but the light will continue to burn, deriving
its necessary oxygen from the excess of air present. On the other hand,
if a light is applied to a vessel containing a mixture of a combustible
gas and air, in which mixture the proportion of combustible gas is above
the upper limit of explosibility, the light will be extinguished, and
within the vessel the gaseous mixture will not burn; but it may burn at
the open mouth of the vessel as it comes in contact with the surrounding
air, until by diffusion, &c.
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