The " particles " of which we have spoken must not be taken
to be individual molecules of the gas, but small volumes each
containing a large number of molecules. According to the
Kinetic Theory of Gases each molecule is in irregular and rapid
motion, even though the gas as a whole is at rest. The elastic
vibrations of which we have spoken are regular periodic
motions, which are superposed on the irregular motions of the
molecules. If we take as our "particles" small cubes with
edges of, say, -001 millimetre, each cube will contain so many
molecules that its centre of gravity may be taken to be at rest,
when the gas as a whole is undisturbed; and yet the cube will
be so small that we need not consider the difference between
the displacements of two opposite faces, but can regard it as
moving bodily to and fro without change of shape or size.
