V 
( 38 ) 
of the mixture (in consequence of changes of the values ot #or T), 
which phases differ in concentration etc. from the original homogeneous 
phase; for a simple phase with internal equilibrium a new phase 
simply exists by the side of the original phase, e. g. a solid phase by 
the side of a liquid one, and that only with change of the temperature 
(or of the pressure). And these two phases can only coexist at the 
transition point, not with a series of concentrations x of the original 
homogeneous phase, as in the case of the ordinary imperfect misci¬ 
bility. As soon as the temperature (or the pressure) differs however 
slightly from that of the transition point, one of the two phases 
vanishes at once. 
And this, we think, sufficiently refutes the “longitudinal plait 
argument” raised by some. 
So a new (third) phase for a simple substance — and this phase 
can hardly be another than the third phase, which we know for 
nearly all simple substances: the solid one. Below the triple-point 
temperature it can either be in equilibrium with the vapour or with 
the liquid (for high pressures, if Lb is negative); only in the triple 
point it can be in equilibrium both with the vapour and the liquid. 
But why does the new phase possess the properties of the “solid” 
state ? 
We first put the question: Why does the second phase, which 
may exist by the side of the gaseous one in the ordinary equation 
of state of van der Waals, possess the properties of the “liquid” state? 
Because its density is so much greater. And from this ensue all the 
properties which we observe in the liquid state. These properties 
cannot be directly derived from the equation of state itself: the fact 
of the greater internal friction e. g. follows only from the smaller 
mutual distance of the molecules; it is not to be derived from the 
equation of state, which remains very simple. Also the internal 
friction for gases can only be derived from kinetic considerations in 
a very intricate way, and it is really not to be deduced from the 
exceedingly simple relation pv = RT\ 
And now the answer to our first question. The third phase is 
solid, because in the formation of larger molecule-complexes the 
mobility decreases to such a degree that the molecules no longer can 
pass over and past each other as in the liquid state, but will neces¬ 
sarily be fixed in their positions. The fact of this slighter mobility 
is quite independent of the value of Lb, which is mostly very slight. 
We have namely exclusively to deal with the fact, that while the 
total volume remains almost unchanged, about the same intramolecular 
space v—b is found both for the smaller simple molecules and for 
