194 Critical Phenomena of Ethane and Nitrous Oxide. 



the ends of the tube. In this manner the process of 

 retrograde condensation is cut off halfway ; and if the dis- 

 tance between the two critical temperatures is very small, 

 the phenomenon may perhaps disappear entirely*. This sug- 

 gests the use of horizontal, or at least nearly horizontal, tubes 

 for experiments on the critical phenomena. 



45. Now in this region, where gravitation plays an im- 

 portant part, the stirring of the substance cannot be of any 

 avail ; on the contrary, the equilibrium, which is very much 

 influenced by gravitation, is spoilt rather by mixing together 

 the different layers. So in this region the only thing we can 

 do is, to wait a long time, and Gouy's experiments with an 

 almost pure substance show that " long " means " days " 

 here ; and of course that would be no use unless the tempera- 

 ture and the volume could be kept absolutely constant, which 

 is practically impossible. 



46. Though the special critical phenomena predicted by 

 means of van der Waals's theory have escaped observation so 

 far, there is no reason to doubt their reality or to see any 

 contradiction between them and my experimental results, I 

 believe. 



47. Some of the results arrived at in this investigation may 

 be enumerated here once more : — 



1. The mixtures of C 2 H 6 and N 2 containing more than 

 OT of C 2 H 6 have critical temperatures lying beneath those of 

 the substances (§ 23). 



2. The pressures of the mixtures lie partly above those of 

 N 2 0, and accordingly show a maximum situated near 0*2 

 C 2 H 6 (§24). 



3. The maximum does not disappear with increase of 

 temperature, but remains up to the critical region : the maxi- 

 mum-curve reaches the plaitpoint-curve (§ 26). 



4. The mixtures ranging between 0*2 and 0*5 C 2 H 6 have 

 r.c. II., the rest r.c. I. 



I intend now to investigate mixtures of acetylene and 

 carbonic acid, a group of which according to Dewarf has 

 critical temperatures lying above those of the components. 

 If I should be able to confirm this result, I shall treat that 

 combination in the same manner experimentally and theo- 

 retically as I have done the combination of C 2 H 6 and HN 2 

 in this paper. 



The experiments described in this paper have been carried 

 out in the Physical Laboratory at Leiden, to the Director of 

 which, Professor Kamerlingh Onnes, I am indebted for con- 

 tinuous and invaluable advice and help. 



* Kuenen, Verslagen kon. Akad. Amsterdam, 25 Mei, 1895. 

 f Proc. Roy. Soc. of Lond. xxx. p. 543. 



