At all temperatures above about 260°, transformation of the white into the red modification takes place with appreciable velocity, and this velocity increases as the temperature is raised. Even at lower temperatures, _e.g._ at the ordinary temperature, the velocity of transformation is increased under the influence {47} of light,[69] or by the presence of certain substances, _e.g._ iodine,[70] just as the velocity of transformation of white tin into the grey modification was increased by the presence of a solution of tin ammonium chloride (p. 40). At the ordinary temperature, therefore, white phosphorus must be considered as the less stable (metastable) form, for although it can exist in contact with red phosphorus for a long period, its vapour pressure, as we have seen, is greater than that of the red modification, and also, its solubility in different solvents is greater[71] than that of the red modification; as we shall find later, the solubility of the metastable form is always greater than that of the stable. The relationships which are met with in the case of phosphorus can be best represented by the diagram, Fig. 11.[72] In this figure, BO_{1} represents the conditions of equilibrium of the univariant system red phosphorus and vapour, which ends at O_{1}, the melting point of red phosphorus. By heating in capillary tubes of hard glass, Chapman[73] found that red phosphorus melts at the melting point of potassium iodide, _i.e._ about 630°,[74] but the pressure at this temperature is unknown. At O_{1}, then, we have the triple point, red phosphorus, liquid, and vapour, and starting from it, we should have the {48} vaporization curve of liquid phosphorus, O_{1}A, and the fusion curve of red phosphorus, O_{1}F. Although these have not been determined, the latter curve must, from theoretical considerations (_v._ p. 58), slope slightly to the right; _i.e._ increase of pressure raises the melting point of red phosphorus. [Illustration: FIG. 11.] When white phosphorus is heated to 44°, it melts. At this point, therefore, we shall have another triple point, white phosphorus--liquid--vapour; the pressure at this point has been calculated to be 3 mm.[75] This point is the intersection of three curves, viz. sublimation curve, vaporization curve, and the fusion curve of white phosphorus. The fusion curve, O_{2}E, has been determined by Tammann[76] and by G. A. Hulett,[77] and it was found that increase of pressure by 1 atm. raises the melting point by 0.029°. The sublimation curve of white phosphorus has not yet been determined.