A condenser and a containment vessel adapted to efficiently condense a gas out of a mixture of condensing and non-condensing gases. In condensers, the fraction of gas within a condenser made up of non-condensing gases can be significantly reduced by withdrawing gas from localised regions of relatively low temperature where the mass fraction of non-condensing gases will be relatively high. In containment vessels, pressures can be reduced by providing a large surface area of the liquid into which the condensing gas condenses in a relatively cool region of the containment vessel. Both these effects result from an appreciation of the manner in which non-condensing gases tend to accumulate in regions which are relatively cold.