These hydrocarbons are known to chemists as "paraffins,"
"naphthenes" being occasionally met with; while a certain proportion of
unsaturated hydrocarbons is also present in most petroleum spirits. The
hydrocarbons of coal-tar are "aromatic hydrocarbons," their generic
formula being C_nH_2^n-6, where n is never less than 6.] are allowed to
vaporise in a room in which a light may be introduced. Less of the vapour
of these hydrocarbons than of acetylene in the air of a room brings the
mixture to the lower explosive limit, and therewith subjects it to the
risk of explosion. This tact militates strongly against the use of such
hydrocarbons within a house, or against the use of air-gas, which, as
explained in Chapter I., is air more or less saturated with the vapour of
volatile hydrocarbons. Conversely, a combustible gas, such as acetylene,
may be safely "carburetted" by these hydrocarbons in a properly
constructed apparatus set up outside the dwelling-house, as explained in
Chapter X., because there would be no air (as in air-gas) in the pipes,
&c., and a relatively large escape of carburetted acetylene would be
required to produce an explosive atmosphere in a room.
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