A hollow container defining a sample chamber is formed of at least two parts (30, 40)which fit together and have apertures (34, 36 44, 46) therein. One of the parts (40)is displaceable relative to the other part (30) between a first relative dispositionof the parts in which the apertures are open so as to provide a clear path for airto pass through the container and a second relative disposition of the parts inwhich the apertures are closed so that there is no path for air to pass through anda volume of air is trapped in the chamber. In one version the part (40) is axiallymoveable relative to the other part (30). In another version where the parts aretubular one of the parts is rotatable relative to the other. In other versionsthe apertures are in respective end closures and end caps located thereover andthe end caps are rotatable to align the apertures and open the chamber for air topass through. In other versions, one of the parts is pivotable or bendable relative tothe other to open or close the respective apertures. In yet other versions one partis rotatable on an axis transverse to the axis of the other part to open or close therespective apertures.
