Liquefaction is the process of change from the solid or gaseous into a liquid state. Heat has to be given to a solid to cause it to melt, but when a gas is liquefied heat is taken away from it. [Latent Heat.] Until comparatively few years ago it was supposed to be impossible to liquefy many gases, but the recent experiments with liquid oxygen show how much can be done under favourable conditions. It is not sufficient to have a low temperature in order to liquefy a gas; the change of state will not occur unless the pressure be sufficiently high. The effect of pressure on the liquefactions of a solid is not so marked, although it has some slight result.