(539 ) 
Physics. — Prof. J. D. vAN DER WAALS on: “ Ternary Systems.” 
IT. (Continued from page 465). 
It occurs frequently in a binary mixture that two phases coexist 
which have the same concentration. In this case the pressure when 
we move along the connodal curve is either maximum or minimum. 
An isobar may then be drawn, touching both the liquid- and the 
vapour branch, and the isobars of other values of p cut then both 
branches twice. The ¢-curves for such a binary mixture will have 
a much more complicated shape than I have given up to now, and 
if we should now have a ternary system in which one or two or 
the three pairs which may be formed from it show this particularity 
then the ¢-surface will also show particularities whose main fea- 
tures we shall examine. As a minimum pressure has not yet been 
observed for normal substances, we shall only discuss the case of 
maximum pressure. 
The property that for a bivary system the concentration of liquid 
and vapour are the same, coheres with another property for such 
systems, which I have discussed, Cont. II, p. 86, though I have 
neglected to point out the relation between these two properties. 
This second property and the relation is found from the following 
formula. One of the conditions for coexistence is: 
(*) = (),. 
We write 
y= f(r) — | pd, 
eS re) =i 5) Ak 
For the case that «j = #2 we derive from the above furmulae: 
and so 
which equation occurs already in Théor, Mol. Arch. Néerl. XXIV, 
