32 



which starts from h at about 4G0° must meet tlie line OV, as the 

 transition point between red and violet phosphorus lies at this tem- 

 perature. So a triple point occurs again in U, and tlie only stable 

 unary equilibrium below this point is that betvyeen violet phosphorus 

 and vapour. 



A similar PJ'-figure holds for cyane, as Fig. 2 shows. Here it 

 is remarkable that it was not necessary to iind the critical point 

 7^1 by calculation as for white phosphorus, but that it could be 

 easily observed. On the other hand, the triple point of the red modi- 

 fication of phosphorus could be determined by experiment ; the 

 triple point of paracyanc, on the contrary, is not to be realised 

 because the substance decomposes before that time. As we know 

 only one kind of liquid cyane below the temperature of the ci'itical 



Fig. 2. 

 point A^i, it is assumed here too that the metastable prolongation 

 of the vapour tension line of molten paracyane must end in a lower 

 critical end point A^ 



We see therefore that according to the earlier view not a single 

 connection could be indicated between the different phases of the 

 white phosphorus on one side and the violet and the red phosphorus 

 on the other side; nor could any be given between cyane and para- 

 cyane, though some connection must exist, because the different 

 forms of the phosphorus resp. cyane can be converted into each 

 Other by some means or other. 



The theory of allotropy demonstrates this connection in a very 

 simple way, as Figs. 1 and 2 show. 



A deeper insight is obtained when the projection of the three- 

 phase regions of the pseudo-binary and that of the two-phase regions 

 of the unary system on the r-Xsurface are drawn. These projec- 



