31 



the three-phase line ob, which holds for the coexistence between 

 violet mixed crystals, liquid, and vapour. 



If these latter three-phase lines are prolonged towards lower tem- 

 peratures, this line must terminate in a metastable critical endpoint 

 5'. From the triple point two more three-phase lines start, of 

 which OU relates to the equilibrium between violet mixed crystals, 

 red mixed crystals, and \apour. We have now discussed wiiat is 

 peculiar in the P, 7^-figure of the pseudo-system. If in this tigure we 

 now draw the P, 7^-lines of the unary system, we get what follows. 

 The vapour-tension line of the liquid white phospliorus, which must 

 be considered as a supersaturate solution, lies under the three-phase 

 line fp, and meets the metastable part of the plaitpoint curve at 

 /Vj. If the critical temperature calculated from the surface tension 

 is accepted as correct, this point lies at 422° and 18 atmospheres. 

 At the triple point of the red phosphorus, which was found at 588°, 5 

 and 42 atmospheres by Mr. Bokhokst ^), liquid is formed again, and 

 it is now clear that the vapour tension line for liquid i)hosphorus 

 starting from this point, has nothing to do with the vapour tension 

 line of molten white phosphorus. 



Now it is interesting to question how the metastable part of the 

 vapour tension line of liquid red phos|)horus prolonged towards 

 lower temperatures will run. In fig. 1 one of the possibilities 

 is drawn. In this it is assumed that the said vapour tension 

 line meets the metastable intermediate part of tiie plaitpoint cui've 

 in /v,, and will therefore end in a /ö^cer-critical endpoint. As 

 was stated before unmixing can also occur in the metastable 

 region, and in this case when the plait is closed the point A', can 

 occur at much lower temperature and pressure than A^j, and then 

 the line liK^ will lie under gK-^- This line cannot extend far towards 

 low temperature, however, for then there would ha\'e to exist two 

 different kinds of liquid white phosphorus, of which nothing has 

 appeared. If the plait was open, the metastable part of the vapour 

 tension line of the liquid red phosphorus would also proceed to 

 lower temperatures under gK^, but the objection advanced just now, 

 is then of force, so that the most probable assumption by far is that 

 the said vapour tension Hue terminates in a metastable lower-critical 

 endpoint K.^. We shall get perfect certainty when the [)rolongations 

 of the two vapour tension lines gK^ and liK^ intersect. Investigations 

 specially undertaken for this purpose may soon decide this. It must 

 further be noted that the vapour tension line of red phosphorus, 



1) The former delerminations were mado with an apparatus which appeared to 

 yield inaccurate results at liigh temperatures. 



