Moreover, it has been found convenient to discuss
fully in other chapters certain properties of acetylene, and in regard to
such properties the reader is referred to the chapters mentioned.
PHYSICAL PROPERTIES.--Acetylene is a gas at ordinary temperatures,
colourless, and, when pure, having a not unpleasant, so-called "ethereal"
odour. Its density, or specific gravity, referred to air as unity, has
been found experimentally by Leduc to be 0.9056. It is customary to adopt
the value 0.91 for calculations into which the density of the gas enters
(_vide_ Chapter VII.). The density of a gas is important not only
for the determination of the size of mains needed to convey it at a given
rate of flow under a given pressure, as explained in Chapter VII., but
also because the volume of gas which will pass through small orifices in
a given time depends on its density. According to Graham's well-known law
of the effusion of gases, the velocity with which a gas effuses varies
directly as the square root of the difference of pressure on the two
sides of the opening, and inversely as the square root of the density of
the gas. Hence it follows that the volume of gas which escapes through a
porous pipe, an imperfect joint, or a burner orifice is, provided the
pressure in the gas-pipe is the same, a function of the square root of
the density of the gas.
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