[ 173 ] 



XL On the Condensation and the Critical Phenomena of 

 Mixtures of Ethane and Nitrous Oxide. By Dr. Kuenen*. 



1- "OEFORE relating the results arrived at in this investi- 

 -L* gation I shall have to give a short account of the 

 course of thought which led to the choice of the subject. 



My experiments on the conduct of mixtures f are based 

 upon the theory of Prof, van der Waals J. Yan der Waals 

 " describes " the properties of mixtures by means of an iso- 

 thermal surface, the coordinates of which are the composition 

 of the mixture x (0<x<l), the volume v, and the free energy 

 ^(^ = 6-^77; e = energy, 77= entropy, t— temperature). On 

 this surface there appear plaits^, which determine the coexist- 

 ence of two or more phases. Those points of these plaits which 

 have a common tangent plane represent the phases which 

 may coexist at the temperature for which the surface is con- 

 structed, and at a pressure indicated by the slope of the 



tangent plane lp= — -^-l The double curves traced out by 



rolling the tangent planes over the plaits are the so-called 

 connodal curves. It may happen that the plaits terminate on 

 the surface itself (i. e. between the planes for which x = and 

 x=-l). In that case the coexisting phases approach each 

 other and at last coincide in a so-called plaitpoint. In study- 

 ing the plaits on the surface, one may use the projection upon 

 the x-v plane. This projection consists of the ^-axis between 

 x = and x = l, the two v-axes for x=0 and x=l, the con- 

 nodal curves with or without the plaitpoints. The points of 

 the connodal curves belonging together may be joined by 

 straight lines. The ratio of the parts into which these lines 

 are divided by a line perpendicular to the .r-axis, say for 

 x = Xi, represents the ratio of the quantities of the two phases 

 which will be formed when the mixture x x is taken at the 

 pressure at which the two phases may coexist. Keeping this 

 in view, it is easy to derive the condensation and critical 

 phenomena of a mixture. 



2. We shall only discuss the very simple case here, that 



* Communicated by the Physical Society : read May 24, 1895. 



t Kuenen, Archives Neerl. xxvi. pp. 354-422 ; Zeitschr. phys. Clxemie, 

 xi. pp. 38-48; Verhand. Kon. Akad. Amsterdam, passim; Communica- 

 tions from the Labor, of Physics, Leiden, Nos. 4, 7, 13. 



% Yan der Waals, Archives Neerl. xxiv. pp. 1-56 ; Zeitschr. phys. 

 Chemie, v. pp. 133-173. 



§ For the nomenclature used in this treatise, cf. Korteweg, Wien. Ber. 

 xcviii. pp. 1154-1191 ; Archives Neerl. xxiv. pp. 57-98. 



