e._, be as light in weight as possible), and should be
constructed of a bad heat-conductor. But if a small mass of a material
very deficient in heat-conducting properties comes in contact with a
flame, its temperature rises seriously and may approach that of the base
of the flame itself. In the case of coal-gas this phenomenon is not
objectionable, is even advantageous, and it explains why a burner made of
steatite, which conducts heat badly, in always more economical (of heat
and therefore of light) than an iron one. In the case of acetylene the
same rule should, and undoubtedly does, apply also; but it is
complicated, and its effect sometimes neutralised, by a peculiarity of
the gas itself. It has been shown in Chapters II. and VI. that acetylene
polymerises under the influence of heat, being converted into other
bodies of lower illuminating power, together with some elemental carbon.
If, now, acetylene is fed into a burner which, being composed of some
material like steatite possessed of low heat-conducting and radiating
powers, is very hot, and if the burner comprises a tube of sensible
length, the gas that actually arrives at the orifice may no longer be
pure acetylene, but acetylene diluted with inferior illuminating agents,
and accompanied by a certain proportion of carbon.
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