( 456 ) 
0 x 
Fig. 3. 
EN ees re ; 
region —; Is positive, and in the unstable region negative.!) Between 
x 
E and Z' the S-curve must turn its concave side downwards, and 
outside EH and £' its convex side. 3° that between D and D' the 
value of « decreases. The place of the points B and #' is found 
by tracing the double tangent of the ¢-curve, as follows from the 
conditions of the equilibrium. According to these conditions p and 
T are in the first place equal for the pair of coexisting phases, and 
d 
Pe ate es = Ms us — M, wu, equal and ¢— 2 6 eae. The double 
OepT 0% 
tangent therefore cuts off from the vertical axis oe 0) a portion 
equal to the molecular potential of the first substance, and a portion 
from the vertical axis at z= 1 equal to the molecular potential of 
the second substance. 
If we now increase the pressure, so that it varies in the 
direction towards the plaitpoint pressure, we could find from 
di =tdp 
the modification in the traced curve for every value of z. 
In our figure this could only lead to a correct result, if it were 
not schematical, but if it were numerically accurate down to the 
smallest peculiarities. 
2 
az. eb. ge 
is negative, as long as — > is positive; so between 
du? 2 >i) et 
1) In the unstable region 
’ i Pi Pa 8 
the points D and # and D' and £', If sa negative, as will be the case along the 
v 
. Gee ae Me : 
curve DD', then a is positive, In fig. 3 the branch DD' has wrongly been drawn 
av 
as being concave. The points D and D' must therefore be ordinary cusps. 
