1097 
and —147°.13, 33.49 atm. for nitrogen, it appears, that the critical 
curve for the oxygen-nitrogen mixtures differs very little from a 
straight line. On a straight line a temperature of — 140°.73 would 
correspond to a pressure of 37.16 atm. whereas the plait-point pres- 
sure is 37.25 atm. The critical point appears to change approxi- 
mately proportionally to the composition in weight: tj, calculated 
according to the composition would be — 140°.56. PawrewskKr’s rule 
thus holds approximately for mixtures of oxygen and nitrogen. 
In connection with the very small distance between plait-point 
and point of contact, already referred to, a further conclusion may 
be drawn from this result. The latter fact involves that the critical 
temperature which air would have, if it condensed without change 
of composition, deviates very little from the two other critical 
points and that this temperature — the so-called critical temperature 
of the undivided mixtures — thus also changes proportionally to 
the composition, with a certain degree of approximation. If this 
proportionality held for the molar composition, we might, as the 
theory of mixtures shows, infer from this, that at low temperatures 
the vapour-pressure of these mixtures would change about linearly 
with the composition of the liquid’). Evidently with mixtures of 
oxygen and nitrogen this is not the case and in agreement with 
this Bauy’s experiments *) gave a vapour-pressure line which is very 
distinctly curved. 
The critical density of air calculated from those of oxygen and 
nitrogen, 0.43 and 0.31 respectively, according to the simple rule 
of mixtures by weight, is 0.34, a value which as the table shows 
is intermediate between the density at the critical point of contact 
0.31 and that of the plait-point 0.36. Assuming that the rule of 
mixtures would hold approximately for the mixtures of constant 
composition and that therefore the critical density of undivided air 
would be 0.34, a somewhat higher density might actually be expected 
at the plait-point. For the connodal curve lies entirely outside the 
saturation-curve for the mixtures of constant composition and on 
that curve the plaitpoint lies in this case on the side towards the 
smaller volumes or higher densities. The somewhat smaller density 
at the critical point of contact is also as might be expected. 
To Professor H. KAMERLINGH ONNes our sincere thanks are due 
for his continual interest in our research, many helpful suggestions 
1) Comp. e.g. J. P. Kuenen. Handb. ang. Phys. Ch. IV p. 126 1906. 
2) E C. C. Baty, Phil. Mag. (5) 49 p. 517 1900. 
