The explanation of such a behaviour is, that if the determination of the melting point is carried out rapidly, the point O_{1}, the melting point of the metastable solid form, may be realized. At this temperature, however, the liquid is metastable with respect to the stable solid form, and if the temperature is {46} not allowed to rise above the melting point of the latter, the liquid may solidify. The stable solid modification thus obtained will melt only at a higher temperature. D. _Phosphorus._ An interesting case of a monotropic dimorphous substance is found in phosphorus, which occurs in two crystalline forms; white phosphorus belonging to the regular system, and red phosphorus belonging to the hexagonal system. From determinations of the vapour pressures of liquid white phosphorus, and of solid red phosphorus,[68] it was found that the vapour pressure of red phosphorus was considerably lower than that of liquid white phosphorus at the same temperature, the values obtained being given in the following table. VAPOUR PRESSURES OF WHITE AND RED PHOSPHORUS. ------------------------------------------------------------------------- Vapour pressure of liquid white phosphorus. | Vapour pressure of red | phosphorus. -------------------------------------------------+----------------------- Temperature.| Pressure | Temperature.| Pressure | Temperature.| Pressure | in cm. | | in atm. | | in atm. ------------+----------+-------------+-----------+-------------+--------- 165° | 12 | 360° | 3.2 | 360° | 0.1 180° | 20.4 | 440° | 7.5 | 440° | 1.75 200° | 26.6 | 494° | 18.0 | 487° | 6.8 219° | 35.9 | 503° | 21.9 | 510° | 10.8 230° | 51.4 | 511° | 26.2 | 531° | 16.0 290° | 76.0 | -- | -- | 550° | 31.0 -- | -- | -- | -- | 577° | 56.0 ------------------------------------------------------------------------- These values are also represented graphically in Fig. 10. [Illustration: FIG. 10.]