Thirdly, the fact that the acetylene which is
to be consumed under the mantle must be most rigorously purified from
phosphorus compounds has been mentioned in Chapter V. Impure acetylene
will often destroy a mantle in two or three hours; but with highly
purified gas the average life of a mantle may be taken, according to
Giro, at 500 or 600 hours. It is safer, however, to assume a rather
shorter average life, say 300 to 400 burning hours. Fourthly, owing to
the higher pressure at which acetylene must be delivered to an
incandescent burner and to the higher temperature of the acetylene flame
in comparison with coal-gas, a mantle good enough to give satisfactory
results with the latter does not of necessity answer with acetylene; in
fact, the authors have found that English Welsbach coal-gas mantles of
the small sizes required by incandescent acetylene burners are not
competent to last for more than a very few hours, although, in identical
conditions, mantles prepared specially for use with acetylene have proved
durable. The atmospheric acetylene flame, too, differs in shape from an
atmospheric flame of coal-gas, and it does not always happen that a coal-
gas mantle contracts to fit the former; although it usually emits a
better light (because it fits better) after some 20 hours use than at
first.
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