The sublimation curve of all substances, so far as yet found, has its upper limit at the melting point (triple point), although the possibility of the existence of a superheated solid is not excluded. The lower limit is, theoretically at least, at the absolute zero, provided no new phase, _e.g._ a different crystalline modification, is formed. If the sublimation pressure of a substance is greater than the atmospheric pressure at any temperature below the point of fusion, then the substance will _sublime without melting_ when heated in an open vessel; and fusion will be possible only at a pressure higher than the atmospheric. This is found, for example, in the case of red phosphorus (p. 47). If, however, the sublimation pressure of a substance at its triple point S-L-V is less than one atmosphere, then the substance will melt when heated in an open vessel. In the case of the vaporization curve, the upper limit lies at the critical point where the liquid ceases to exist;[114] the {66} lower limit is determined by the range of the metastable state of the supercooled liquid. The interpolation and extrapolation of vapour-pressure curves is rendered very easy by means of a relationship which Ramsay and Young[115] found to exist between the vapour-pressure curves of different substances. It was observed that in the case of closely related substances, the ratio of the absolute temperatures corresponding to equal vapour pressures is constant, _i.e._ T_{1}/T'_{1} = T_{2}/T'_{2}. When the two substances are not closely related, it was found that the relationship could be expressed by the equation T_{1}/T'_{1} = T_{2}/T'_{2} + _c_(_t_' - _t_) where _c_ is a constant having a small positive or negative value, and _t_' and _t_ are the temperatures at which one of the substances has the two values of the vapour pressure in question. By means of this equation, if the vapour-pressure curve of one substance is known, the vapour-pressure curve of any other substance can be calculated from the values at any two temperatures of the vapour pressure of that substance. Fusion Curve--Transition Curve.--The fusion curve represents the conditions of equilibrium between the solid and liquid phase; it shows the change of the melting point of a substance with change of pressure.