( 664 ) 



p. <>60) indicates, thai Tb P i$ would have (o lie still pretty much 

 higher, and therefore 7Vh.- pretty much lower (probably < about 4°) ! ). 

 Of this result we availed ourselves in the treatment of the estimation 

 of the critical temperature of He in Comm. N°. 966. 



(To be continued). 



Physics. — "Contributions to the knowledge of the xr-surface of 'van 

 der Waals. XIV. Graphical deduction of the results of 

 Kubnen's experiments on mixtures of ethane and nitrous oxide." 

 Supplement 14 to the Communications from the Physical 

 Laboratory of Leiden. By Prof. H. Kamerlingh Onnes and 

 Miss T. C. Jolles. 



(Communicated in the meeting of Januari 26, 1007). 



§ 1. Introduction. In what follows we have endeavoured to 

 derive quantitatively by first approximation the behaviour of the 

 mixtures of N,0 and C a H 8 (mixtures of the II type 5 )), which has 

 become known through Kuenen's experiments 8 ), by the aid of van der 

 Waals' free-energy surface. The ip-surfaces construed for this purpose 

 (see plate I) are the counterparts of those construed in Comm. N°. 59 

 (These Proc. Sept. 1900) and Comm. N°. 64 4 ) for the derivation 

 of the results of Kuenen's and Hartman's experiments on mixtures 

 of CO, and CH 3 C1 (mixtures of the I type). In the graphical treat- 

 ment 5 ) of our problem we have chiefly followed the method given 

 in Comm. N°. 59, where the critical temperature and pressure of 

 some mixtures were borrowed from Kuenen's determinations, and 

 then the results of another group of experiments — those referring 

 to the conditions of coexistence of two phases at a certain tempera- 

 ture — were deduced by the aid of van dek Waals' theory. 



Kuenen's results for N,0 and C 2 H B are principally laid down in 



!) If b -2-i ! b u is taken larger than Vb (Gf. Footnote 3 p. 660) this supposition too 

 makes the upmost limit for T. , f on the said supposition smaller. This is seen 



when we compare with table I that we obtain T b Jrp =0.679 for 6 »/& n = 1 /4 

 with T k f T =0.15. 



2 ) Hartman, Leiden Comm. Suppl. no. 3, p. 11. 



3 ) Kuenen, Leiden Comm. no. 16, Phil. Mag- 40, p. 173, 1895, cf. also Kamer- 

 lingh Onnes and Zakrzewski, Leiden Comm. Suppl. no. 8. (These Proc. Sept. 1904). 



It is remarkable that the possibility of this case was foreseen by van der Waals, 

 Gontin. II, p. 49 [added in the -English translation]. 



4 ) Arch. Néerl. Serie II, Tome V, p. 636. 



5 ) Only graphical solutions for definite cases are here possible. (Gf. Suppl. 8, 

 These Proc. Sept. 1904. § 1). 



