1092 
capillary can then be taken very narrow, so that the influence of 
the diffusion will be imperceptible and the determinations of the 
volume will obtain greater accuracy. But the rule, that in a set of 
readings, after the condensation has once begun, no expansion must 
be applied, will always remain valid. 
Finally it may be remarked that, whereas air is, properly speaking, 
a ternary mixture of nitrogen, oxygen and argon, the amount of 
the latter gas is so small and its properties differ so little from those 
of the other two components, that a perceptible influence on the 
phenomena cannot be assumed and our mixture may thus be 
actually looked upon as being a binary mixture. 
Apparatus. The apparatus which we have used agree in the 
main with former apparatus in use in the cryogenic laboratory : we 
may therefore refer to previous communications (for the piezometer 
compare Comm. 69). Between the observation-tube and the com- 
pression-apparatus a steel three-way stopcock was inserted by means 
of which the apparatus could be connected with a separate reser- 
voir (pipette) filled with pure air: by this means, if required, measured 
quantities of air could be introduced into or removed from our 
piezometer; this arrangement was chiefly made with a view to 
density-determinations of liquid air at temperatures far below the 
critical, where the quantity present in the compression-tube would 
not have been sufficient. But as we were obliged to confine ourselves 
to experiments in liquid ethylene, there was after all no necessity 
for drawing on the pipette. 
The pressures were measured on a closed hydrogen-manometer 
(Comm. 78), a metal gauge being used as a control during the mea- 
surements. Two platinum-thermometers (Comm. 141a) served for 
reading the temperatures. The cryostat was described in Comm. 83. 
Critical point aud critical phenomena. As shown by the theory 
of mixtures a distinction has to be made between two different 
critical points: the “plait-point”, where the two coexisting phases 
become identical and where thus the critical phenomena will be 
most conspicuous and secondly, corresponding to a somewhat higher 
temperature, the “critical point of contact”, ie. the limit for the 
separation into two phases. In the temperature-range between those 
two points the condensation is “retrograde”, in this case of the first 
kind. We have succeeded in confirming these several theoretical con- 
clusions for air and have thus been able to show, that at these 
very low temperatures the phenomena are no other than what theory 
leads to expect. The experiments were far from easy, as the two 
points lie very close together, which is connected with the circum- 
