(1345) 
another. But if we continue to call those phases represented by the 
lower sheet, vapour phases, and those phases represented by the 
higher sheet, liquid phases, then the vapour phases do not reach 
the point, where the connection of the two branches has taken 
place (the plaitpoint), but only the point where the value of »,, 
is zero, i.e. the point, where two successive curves of equal 
pressure intersect. For all points lying on one of the sides of that 
point of intersection, — e.g. on the side where the plaitpoint occurs, — v‚, 
is positive. 
These points will be displaced towards the conjugated point, when 
the pressure is increased; all points on the other side of the point 
of intersection will move away from the points, representing coexisting 
phases. If we, therefore, continue to use the expressions “liquid 
phasis” and “vapour phasis” with the same meaning as we have 
done till now, we must say that for points between the plaitpoint 
and the point for which v,,= 0 two liquid phases coexist. If for 
the two pairs of the ternary system we had a course of the pressure 
as is represented in Cont. Il, p. 135, fig. 12, the above rules would 
continue to hold; but in this case we find a series of vapour phases 
coexisting with vapour phases between the plaitpoint and the point, 
for which v,,=0. For these points we have then retrograde con- 
densation of the second kind. We may expect that it will be easier 
to observe this phenomenon for a ternary system, than for a binary 
one. In order that retrogade condensation may be easily observed a 
rather great distance between the two sheets of the surface of satur- 
ation is required; and the distance between the sheets will be 
more considerable in the middle than at the ends, where we have 
to deal with a binary mixture, because the requirements for sta- 
bility and coexistence for a ternary mixture are stricter than those 
for a binary mixture (See Vol. IV, p. 577). But then we have to 
avoid the case that a real maximum pressure occurs, for in that case 
we have also in the middle of the figure a point in which the two 
sheets touch each other. 
c. Curves of slope and nodal envelopes. 
If for a binary mixture we have construed the curves p= ACs 
and p= f(w,), we have at the same time answered the question, 
what phases may coexist with each other. Every line parallel to the 
X-axis joins a pair of coexisting phases. If on the other hand we 
have construed the two sheets of the surface of saturation for a ternary 
system, this is not sufficient in order to answer the question which 
