Vacuum systems often contain stationary columns of vapour. A general expression is derived for the conductance of such a column for gases at low pressures.A rough experiment using columns of mercury vapour yielded values for the conductance for air and hydrogen which were of the same order of magnitude as the values predicted from the expression given.It is shown that the conductance of the vapour can be less than that of the pipe which contains it. This becomes increasingly probable as the pipe diameter and the vapour pressure are increased.