( 231 ) 



Physics. — "Contributions to the knowledgt of th ty-surface of 

 van der Waals. XV. The cast' that one component is a gas 

 without cohesion with molecules which have extension. Limited 

 miscibility of two gases." (Continuation). By Prof. H. Kamek- 

 i.ixgh Onnes and Dr. W. H. Keesom. Supplement N°. 15 

 to the Communications from the Physical Laboratory at 

 Leiden. 



(Communicated in tbe meeting of May 2i. 1907). 



§ 8. On the temperatures and the pressures on the gas-gasplait. In 

 order to form a provisional opinion as to the experimental conditions 

 which must be satisfied that limited miscibility in the gas state may 

 be observed, and to be able to derive what pairs of substances 

 must be considered suitable for this purpose, it is desirable to examine 

 for some cases what temperatures and pressures occur on the gas- 

 gasplail '). In the case § '2, March '07 of a componenl without 

 cohesion') or almost without cohesion (m<^m l , see § 7), the gas- 

 gasplail will occur for all temperatures between the critical tempe- 

 rature of the less volatile component and the critical temperature 

 of complete miscibility '). Its pressures will then be larger than 



!) In our opinion Mr. van Laak (These Proc. May ' n 7, p. 35, note 2) is 

 wrong in thinking that in the case of a three-phase-equilibrium, as e.g. in the 

 system water- ether, our terminology with regard to the distinction between gas 

 and liquid does not agree with that used by van deb Waals. According to what 

 has been said about this distinction in g 4, for a three phase equilibrium of a 

 system of the type water-ether the denser phase which is rich in water, must be 

 called liquid as belongirg to the liquid branch of the connode, just as the less 

 dense phase which is rich in ether, must be called gas phase; whether the denser 

 phase which is rich in ether is to be called liquid or gas, is not determined by 

 the principle of the continuity of the phase along the connode; if tlie reduced 

 temperatures at which this phase appears, are taken into account, also the last 

 mentioned phase will be called liquid for the system ether-water in accordance 

 with what has been said in § 4. 



-) The second branch of the plaitpoint eurve in Fig. 1 § 2 (These Proc. March '07, 

 p. 787), about which Van Laar speaks in These Proc. May '07, p. 36, has 

 there (a 12 = 0) contracted to a point x=l, V = b 23 . It is true that in the case 

 of our g 7 (Plate II, These Proc. March '07) a second branch of the plaitpoint 

 curve occurs, but it has been explicitly stated there (p. 795 at the bottom and 

 p. 797) that we did not discuss the spinodal curves at the lower temperatures 

 at which this branch of the plaitpoint curve makes its influence felt, referring inter 

 alia with a view to these temperatures to Van Laae's papers. 



3 ) As mentioned in g 1 this idea was introduced by Van der Waals, who 

 also gave the formulae for the calculation of this temperature (Van Laar calls 

 it "third critical temperature''). 



