( 446 ) 



And this different beliavioni- i)f tlip plait] loiiit will necessarily be 

 n,cc'oinpiiiiic(l liy a dilforeiit i)elia\ ioiir of llio coiniodal and spinodal 

 curves. For they must always cut the r-axis at decrease of temper- 

 ature, the comiodal in the jtoints of contact of the double tangent 

 of the i|%r-curve of the principal c(MU|ionent, the spinodal in its two 

 points of inflcclion; at increase of teuiperature above the critical 

 temperature of the principal component, however, they get fjuile 

 detached from the r-axis. In connection with this they turn in tlie 

 first four cases of flg. i their convex sides, in the last four cases 

 their concave sides tow\ards the side .n = of the i|j-surface as is 

 also indicated in the figure, where the connodal curves are traced, 

 the spinodal cui-ves dotted. 



^^' "' At decrease of temperatui'e a figure originates in Uiefwdt 



■^ four cases as is scheuiaticaliy given here in Fig. a. At 

 < increase of temperature, on tiie contrary, in the /ast four 



cases, the spinodal and connodal curves disappear from tlie 

 surface at the same time with the plait point itself. 



Besides to this different behaviour it appeared however 

 V. * desirable, to pay attention to two other circumstances. First 

 to the direction of the tangent in the plaitpoint, whether 

 if prolonged towards the side of the large volumes, it 

 inclines to the inner side of the i|'-surface, as in cases 1, 2, 5 and 

 6 of fig. 1, or whether it inclines to the outer side, as in the 

 remaining four cases. For on this it will tlepend which of the two 

 kinds of retrograde condensation will eventually appear ^). Hut besides 

 we have to pay attention to the question whether the plaitpoint, 

 entering the if'-^^ii'^^i'C^' either at decrease or increase of temperature, 

 will move towards the side of the larger volumes as in cases 1, 3, n 

 and 7, or whether it will move towards that of the smaller volumes 

 as in the other cases. In connection with this question we may point 

 out here that the line KP in fig 1 of the plate may everywhere be 

 considered as a small chord of the plaitpoint curve of the y;,./>diagram 

 and accordingly indicates the initial direction of that curve, which it 

 has when starting from point K. 



The three different alternatives, Avhich we ha\e distinguished in 

 this way, give rise to the eight cases represented in fig. J, and we 

 may now raise the question on what it will depend which of these 

 eight cases will occur at a given i)rincipal conqjonent with a given 



1) See ÜU these two kind:; of retmgiade cundeusutiuii inter alia, llie paper of van der 

 Waals: "Statique des tluides (Mélanges)": in Tome 1 of the "Rapports présentés 

 au cougrès inleruational de physique, réuui a Paris en I'JUU", page üOÜ— (309. 



