385 
intersects it in ils P,7-projection. With regard to this last case again 
three cases may be distinguished. We may, namely, imagine that the 
three-phase line without previous intersection terminates on the cri- 
tical line, that intersection takes place before this end-point (see 
fig. 48), and thirdly that the intersection takes place exactly in the 
end-point, i.e. that in the critical end-point the three-phase line touches 
the critical line (fig. 49). This last case is again to be considered as 
the transition case between the two first-mentioned ones. 
5. To enable us to decide which of the possible cases discussed 
in the preceding paragraph presents itself in the system ether-water, 
the P,7-projections of the plaitpoint line and the three-phase line 
had to be experimentally determined. For it is possible to derive 
from the situation of the plaitpoint line whether or no a minimum 
critical temperature occurs (vertical tangent), and from the relative 
situation of the said lines a conclusion may be drawn as to whether 
or no an intersection occurs. So the determination of the P,7- 
projections of the two lines might suffice; the three-phase line can 
be determined by means of one mixture, provided it do not possess 
a concentration that lies too much on one side. For the determina- 
tion of the eritical line the 7; and P, found for different mixtures 
should be combined together to one line. So for every mixture 
practically nothing but the critical phenomenon need be observed, 
and of a single one the three-phase line in the neighbourhood of 
the critical end-point. 
However, I have not been satisfied with this. To obtain as much 
certainty as possible I have observed part of the three- and two- 
phase equilibria of every mixture. The great advantage yielded by 
these observations, is the following. If a mixture should contain a 
slight quantity of admixtures, and the presence of air is the most 
probable, this mixture would bring about an error in 7; and Px, 
and so it might furnish a point which might cause the course of 
the critical line to deviate from the correct one. In the determina- 
tion of the three-phase pressure, which must show the same value 
for all the mixtures used, we have, however, a criterion of purity. 
With none of the mixture, for which this test could be applied, a 
deviation was found exceeding 0,1 atmosphere. 
But besides this, the observation of the two-phase equilibria furnished 
another advantage. We want to decide, among others, whether inter- 
section takes place between the three-phase line and the critical curve. 
If we now put the case that this intersection really occurs, the part 
of the critical line between the point of intersection in question and 
