5 inches, which it was almost impossible to induce to fire
back to the jets however slowly the cock was manipulated, provided the
pressure of the gas was maintained well above the point specified. But
ordinarily a pressure of about 4 inches is used with incandescent
acetylene burners.) Secondly, it is necessary that the acetylene shall at
all times be free from appreciable admixture with air, even 0.5 per cent,
being highly objectionable according to Caro; so that generators
introducing any noteworthy amount of air into the holder each time their
decomposing chambers are opened for recharging are not suitable for
employment when incandescent burners are contemplated. The reason for
this will be more apparent later on, but it depends on the obvious fact
that if the acetylene already contains an appreciable proportion of air,
when a further quantity is admitted at the burner inlets, the gaseous
mixture contains a higher percentage of oxygen than is suited to the size
and design of the burner, so that flashing back to the injector jets is
imminent at any moment, and may be determined by the slightest
fluctuation in pressure--if, indeed, the flame will remain at the proper
spot for combustion at all.
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