1896 - 97 .] Prof. J. P. Kuenen on Ethane and Butane. 
439 
The peculiarities of the condensation at temperatures below the 
•critical temperature do not come out properly in a pv diagram. 
These phenomena, as deduced from Yan der Waals’ theory of 
mixtures, are found in previous papers already quoted. 
Below tC (39 - 25 for b, 35’7 for a) the condensation is retrograde, 
that is to say, liquid is formed by compression, the quantity of 
which first increases and then diminishes and disappears, dust 
below the critical temperature the quantity of liquid formed is 
only very small, but the lower the temperature the larger the 
maximum quantity of the liquid becomes. At last temperatures 
are reached where the “ critical phenomenon ” takes place, first at 
the bottom of the column and then at higher levels as the tem- 
perature falls. The critical phenomenon is here meant to compre- 
hend all the phenomena which are displayed when the coexisting 
phases approach each other and finally coincide : they consist 
chiefly in the disappearance of the liquid surface and the formation 
of a characteristic bright blue mist in the substance. The tempera- 
ture at which the critical phenomenon occurs about at the middle 
of the tube is the plait-point temperature tV (38*35 for b, 35*25 
for a). A few tenths of a degree lower the condensation is normal, 
the quantity of the liquid increasing during compression until the 
whole of the substance is liquid. The critical phenomenon would 
be confined to the plait-point temperature, if it were not for gravi- 
tation. Gravitation makes the density and composition of the 
mixture different at different levels, and the consequence is a small 
range of temperatures (38*2-38*7 for 6), at which the critical 
phenomenon takes place at different levels, the higher the tem- 
perature the lower the level. 
The point indicating the end of the condensation at the plait- 
point temperature corresponds to the plait-point in Yan der Waals’ 
theory. This point (P in the diagram) does not coincide with the 
top of the border curve M, as will be seen even better in the 
tables, especially for mixture b. Evidently P is to the left of M, C 
being to the right of M. 
In the pt diagram (fig. 3) are given the vapOur-pressure curve 
for ethane and the two looped vapour-pressure curves for the 
mixtures. The curve which envelops the loops is the plait- 
point curve, the point of contact P corresponding to the plait- 
