( ^^^' ) 



of pure ethane (31°. 88), the connodal line consists of another part, 

 Avhich I have not drawn, howeA'er, in order not to make the figure 

 uselessly intricate. This second piece should have its apex, the critical 

 point of contact, at about .rj^. = 0.92, and Vf/, = 006S, and would 

 intersect the axis a' = 1 at 27 = 0.00472 and v = 0.01031. 



To this second piece of the connodal a second border curve cor- 

 responds which would begin at a height /; = 46.1 (maximum tension 

 of ethane at 29°.0 C.) and terminate at the plaitpoint 2)Tpi = 51.2, 

 V2'jjI = 0.0063. But this border curve too I have omitted like the 

 isothermal of pure ethane. 



At the lower part of the figure it maj be seen that the isobars 

 in the neighbourhood of the critical point C, indeed to the first 

 approximation, are hyperbolae of which one of the asj'mptotes, which 

 agree with the critical pressure, is parallel to the ?;-axis, the other 

 cuts this axis at a given angle. To the second approximation the 

 first asymptote is a parabola which coincides with the projection of 

 the connodal line. 



It were useless to investigate whether indeed the border curve is 

 of the fourth degree and the connodol of the second degree ; for this 

 the data are not numerous enough and the drawing not sufficiently 

 accurate. But it is obvious why the bordei- curve should be of a 

 higher order than the connodal. The j;, v, ^I'-surface, of which the 

 projections on the surfaces p, v and x, v are shown in the fig., is in 

 the neighbourhood of the critical point a saddle-shaped surface, which 

 at the upper part of the figure is seen parallel to the tangent surface 

 at the point C The isothermals of the mixtures ,v, ^ 0, x ^0.14 

 and x = 0.40 are situated on the slope turned towards us ; the latter 

 over a fairly extensive range (of large volumes to about ,?• = 0.006) 

 forms nearly the border of the surface; the critical isothermal lies 

 just beyond that border, but becomes visible at Tand remains visible 

 foi- small volumes. The isothermals ,r =: 0.62, ,/' = 0.71 and .r = 1 

 occur on the back of the p, v, ,i'-surface, yet for small volumes they 

 become visible. The parabola : 



1 Wjj" 



p - pTk = - -, (t' - vtl)\ 



which envelops the isothermals in the neighbourhood of the point C 

 (1. c. p. 344 and fig. 16) is the api)arent outline of the surface in 

 that neighbourhood. 



The lower part of the figure re^jresent the surface seen from above ; 

 the isobars are there level-curves. The critical isobar forms a loop 

 which agrees with the described shape of the surface. A section of 

 a horizontal surface situated a little higher consists of two pieces, 



