But in one most important respect carburetted
acetylene is totally different from air-gas: partial precipitation of
spirit from air-gas removes more or less of the solitary useful
constituent of the material, reducing its practical value, and causing
the residue to approach or overpass its lower explosive limit (_cf._
Chapter I.); partial removal of spirit from carburetted acetylene only
means a partial reconversion of the material into ordinary acetylene,
increasing its natural illuminating power, lowering its calorific
intensity somewhat, and causing the residue to have almost its primary
high upper explosive limit, but essentially leaving its lower explosive
limit unchanged. Thus while air-gas may conceivably become inefficient
for every purpose if supplied from any distance in very cold weather, and
may even pass into a dangerous explosive within the mains; carburetted
acetylene can never become explosive, can only lose part of its special
heating value, and will actually increase in illuminating power.
It is manifest that, like air-gas, carburetted acetylene is of somewhat
indefinite composition, for the proportion of vapour, and the chemical
nature of that vapour, may vary.
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