174 Dr. Kuenen on the Critical Phenomena of 



there is only one plait on the surface, the vapour-liquid plait, 

 which determines the coexistence of a vapour-phase and a 

 liquid-phase. The plait, roughly speaking, has the direction 

 of the .r-axis ; its plaitpoint, which will make its appearance 

 if only we raise the temperature to a sufficient height, need 

 not coincide with the end of the plait, i. e. with that point of 

 the connodal curve which is nearest to one of the ylrv planes 

 (in the projection = i?-axis).» The latter point was called 

 critical point of contact*. Now t in deriving the critical 

 phenomena two cases have to he distinguished depending on 

 the relative position of these two points, which we shall call 

 P and R. Either P is situated nearer to the yjra plane (#-axis) 

 or this is the case with R. 



3. In the first case the condensation of a mixture has the 

 following character. Below a definite temperature (plaitpoint- 

 temperature = Tp), different for every mixture, the quantity 

 of the denser phase increases regularly during compression ; 

 the pressure rises, and both the composition and the volume 

 of both phases change continuously. Above another tempe- 

 rature (temperature of the critical point of contact = critical 

 temperature = T R ) there is no condensation possible. At 

 temperatures ranging between T P and T R the condensation 

 will have the following character : — With compression the 

 quantity of the appearing liquid first increases, reaches a 

 maximum value, and after that decreases till it disappears. 

 This was called retrograde condensation of the first hind 

 (r.c. I.)t» The nearer we get to the plaitpoint-temperature 

 the larger becomes the quantity of the liquid and the smaller 

 the difference between the phases, apparent from a less 

 distinct liquid surface between the phases. A mixture, there- 

 fore, has a critical region instead of one critical point, lying 

 between two critical temperatures. 



4. The phenomena predicted in this manner were then 

 observed by me in mixtures of carbonic acid and methyl 

 chloride t, and afterwards of carbonic acid and air§. The 

 deviation from what had been observed by former experi- 

 menters, viz., the flattening and disappearing of the liquid 



surface throughout the critical region without retrograde 

 © © t> 



* Kuenen, Archives Neerl. xxvi. p. 379 ; Communications from the 

 Lab. of Physics, Leiden, No. 4, pp. 7 sqq. ; Zeitschrift phys. Chemie, xi. 

 p. 44. 



t Kuenen, Archives Neerl. xxvi. pp. 378-379 ; Commiin. No. 4 ; 

 Zeitschr. phys. Chemie, xi. p. 44. 



X Kuenen, Archives Neerl. xxvi. pp. 382-3S5 ; Commun. No. 4 ; 

 Zeitschr. phys. Chemie, xi. p. 47. 



§ Commun. No. 7, p, 6. 



