The kinetic model of gases (Section 1.2b) and the equipartition theorem (Section F.5)
jointly imply that the mean kinetic energy of molecules in a gas is proportional to the
temperature. It follows that reducing the average speed of the molecules is equivalent
to cooling the gas. If the speed of the molecules can be reduced to the point that neighbours
can capture each other by their intermolecular attractions, then the cooled gas
will condense to a liquid.