1002 
clear that we shall get a solid substance containing more of the 
B-pseudo component than corresponds to the internal equilibrium, 
and this involved a smaller vapour tension. 
If this substance is examined, a vapour tension will be found 
at lower temperature which lies below that of the unary stable 
state. If, however, we heat up to temperatures at which the internal 
equilibrium begins to set in with appreciable velocity, the vapour 
tension will rise and become equal to that of the stable unary state. 
There is, however, still another case possible. 
It is possible that the black phosphorus is not a state to which 
the violet phosphorus is converted at higher pressure in a continuous 
way, but a new modification, which is stable at higher pressure. 
If this is really the case the black phosphorus will owe its origin 
to a kind of molecule which has not been taken into account in 
our representation fig. 3. To keep the representation as simple as 
possible, however, we shall assume that the P,X-section at 200° is 
given by fig. 4. 
When the substance behaves as a unary one S,, deposits from 
the vapour on diminution of volume. The line S,, S’u indicates the 
change of the internal equilibrium with the pressure in the solid 
unary phase. 
In the point S’,, this line meets the region where unmixing takes 
place in the pseudo-binary system, in consequence of which on further 
diminution of volume the solid phase S’, is converted to S’,, in 
case of unary behaviour. Here a transition point therefore occurs, 
and at higher pressures the internal equilibrium moves along the 
line S’,, S",, in the new phase. 
If now the mixed crystals Sar are supposed to be violet and $ 
ay 
to be black, then a transition point between violet and black phos- 
phorus is assumed here. If we suppose this transition point to lie 
e.g. at 10000 atmospheres at the assumed temperature, black phos- 
phorus is metastable under the vapour pressure. 
Now the unary equilibrium between black phosphorus and vapour 
is indicated in fig. 4, by the points G’, and S’,,, the corresponding 
equilibrium for the violet phosphorus being denoted by the points 
G, and S,,, from which it therefore appears that the vapour tension 
of the metastable phosphorus is greater than that of the stable violet 
phosphorus. 
These considerations in themselves are therefore not sufficient to 
account for the fact that the black phosphorus, as we jirst get it, 
at first possesses a vapour tension smaller than the violet. To explain 
this behaviour we must, just as in our discussion of fig. 3, come 
