C., and the critical
pressure, _i.e._, the pressure under which that change takes place
at that temperature, is nearly 68 atmospheres. Below the critical
temperature, a lower pressure than this effects liquefaction of the gas,
_i.e._, at 13.5 deg. C. a pressure of 32.77 atmospheres, at 0 deg. C.,
21.53 atmospheres (Ansdell, _cf._ Chapter XI.). These data are of
comparatively little practical importance, owing to the fact that, as
explained in Chapter XI., liquefied acetylene cannot be safely utilised.
The mean coefficient of expansion of gaseous acetylene between 0 deg. C.
and 100 deg. C., is, under constant pressure, 0.003738; under constant
volume, 0.003724. This means that, if the pressure is constant, 0.003738
represents the increase in volume of a given mass of gaseous acetylene
when its temperature is raised one degree (C.), divided by the volume of
the same mass at 0 deg. C. The coefficients of expansion of air are: under
constant pressure, 0.003671; under constant volume, 0.003665; and those
of the simple gases (nitrogen, hydrogen, oxygen) are very nearly the
same. Strictly speaking the table given in Chapter XIV., for facilitating
the correction of the volume of gas measured over water, is not quite
correct for acetylene, owing to the difference in the coefficients of
expansion of acetylene and the simple gases for which the table was drawn
up, but practically no appreciable error can ensue from its use.
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