109 
above, by the absence of accurate numerical values of all the a’s, 
b’s, and even the molecular weights. 
So summarizing we must say that a// the experimentally found 
particularities can appear exactly in the same way for mixtures of 
perfectly normal substances which behave according to the sim- 
plest theory. 
§ 6. Deviation from the law of van ’t Horr even in case of 
extreme dilution. There is another particularity in connection with the 
absence or presence of unmixing, to which it may be desirable to 
draw attention. I mean departures from the well-known vapour- 
pressure formula of van ‘rt Horr for extreme dilutions 
d, 
ee 
pde, 
This formula, which may be expressed geo- 
A metrically by saying that the vapour-pressure 
line *) in its limiting direction points to the 
opposite angle (direction AB in figure 3), is 
considered of general validity for systems 
whose components differ widely in volatility. 
And indeed if we understand by this latter 
bre condition that —? — 0, at the limiting value, 
Fig. 3. ©, 
from a hyperbola that the property mentioned in the last lines of § 2 can be 
cal. 
20 
10 
OO 
0.50 1.00 
Fig. 2. 
proved in a simple way for the volume contraction, but not for the heat of mixing. 
1) Of course the total vapour-pressure line is meant here. For the partial vapour 
