ONE of the most important generalisations which has been obtained in recent years from the study of the effect of temperature and pressure on the volume of stable liquids and gases may be expressed by the law that if the volume of a given mass of substance be kept constant, increase of pressure is proportional to increase of temperature. This relationship was proposed as early as 1878 by Lévy, who was indeed anticipated to some extent by Dupré in 1869, but was first set upon a firm experimental basis, at least for vapours, by Ramsay and Young in 1887. They represent it algebraically by the equation p=bt-a, in which p is the pressure, t the temperature, and b and a are constants which vary with the volume and the chemical nature of the substances employed, and the curve corresponding to this equation they term an isochor. The law may therefore be shortly expressed by stating that for stable substances the isochors are straight lines. This generalisation leads, as Fitzgerald has shown, to the significant conclusions that specific heat at constant volume must be a function of the temperature only, and internal energy and entropy must be expressible as the sum of two functions, one of which is a function of the temperature only, and the other a function of the volume only.