If two gases are in separate containers that share a common movable wall (a
‘piston’, Fig. 1.1), the gas that has the higher pressure will tend to compress (reduce
the volume of ) the gas that has lower pressure. The pressure of the high-pressure
gas will fall as it expands and that of the low-pressure gas will rise as it is compressed.
There will come a stage when the two pressures are equal and the wall has
no further tendency to move. This condition of equality of pressure on either side
of a movable wall is a state of mechanical equilibrium between the two gases. The
pressure of a gas is therefore an indication of whether a container that contains the
gas will be in mechanical equilibrium with another gas with which it shares a movable
wall.