Phenomena of some Substances and Mixtures. 183 



17. The diagram (fig. 1) shows that the plaitpoint curve 

 is a line with small curvature and that mixtures of carbonic 

 acid and acetylene have critical temperatures between those 

 for the pure gases. The border-curve for the mixture inves- 

 tigated is relatively narrow. This entails small differences in 

 composition between the coexisting phases, the vapour con- 

 taining only a little more carbonic acid than the liquid. It 

 also follows that the critical phenomena will closely resemble 

 those of a pure substance. This was found to be the case, 

 the plaitpoint temperature and critical temperature nearly 

 coinciding. 



18. An instance of a critical temperature above those for 

 the components has therefore not been discovered as yet. 

 From the theory it may be deduced that this phenomenon 

 will probably occur for mixtures which have a minimum 

 vapour-pressure at low temperature ; in the same way as the 

 critical temperatures below those for the components seem 

 to occur for mixta res which have a maximum vapour-pressure 

 (as for nitrous oxide and ethane). 



The law of the connexion between the two phenomena will 

 be again referred to in connexion with the application of 

 Prof, van der Waals's theory. 



G. Mixtures of Acetylene and Ethane. 



19. The relative position of the vapour-pressure curves 

 and critical points in the t-p diagram of these gases was such 

 as to promise interesting results for mixtures of them : in 

 fact the same reasoning holds for this combination which led 

 me originally to the choice of nitrous oxide and ethane. If 

 we consider the curves for the four gases the critical tem- 

 peratures of which are between 30° and 40° — C0 2 , C 2 H 6 , 

 C 2 H 2 , and N 2 — we find that besides with the combinations 

 C 2 H 6 -N 2 and C 2 H 6 -C 2 H 2 we may expect similar pheno- 

 mena with C 2 H 2 -N 2 0, a combination which I have not been 

 able to try. Results obtained with the couple C 3 H 6 -C0 2 

 will be described later on. They agree with those of N 2 0- 

 C 2 H 6 , and C 2 H 2 -C 2 H 6 . The case C 2 H 2 -C0 2 was discussed 

 above. The combination C0 2 -N 2 offers no interest a priori. 

 Moreover, some time ago 1 investigated a mixture of these 

 two gases, but did not obtain any remarkable results. 



Mixtures of some of these four gases with hydrochloric 

 acid (51°) or methyl fluoride (44 0, 9) also promise new results. 

 Mixtures of hydrochloric and carbonic acids were investigated 

 by Dewar * and Ansdell f. Their figures differ from each 



* Proc. R. S. of London, xxx. p. 543. 



t Ibid, xxxiv. pp. 113-119. 



