124 Prof. Kuenen and Mr. Robson on Mixtures with 



3. Carbon Dioxide and Ethane at Low Temperatures. 



These mixtures, as was discovered before *, have a mini- 

 mum critical temperature and a maximum vapour-pressure,, 

 but had only been examined at temperatures above 0° C. 



In extending the investigation towards low temperatures, 

 we wanted to trace the maximum vapour-pressure more 

 fully than could be done in the small range between 0° C. 

 and the critical region, and to ascertain whether the maxi- 

 mum was in any way connected with a splitting into two 

 liquids at low temperatures. We have found that no such 

 separation takes place up to the region where solidification 

 sets in. This solidification of the carbon dioxide afforded 

 an opportunity for the study of the influence of the second 

 substance on the phenomenon. 



The minimum critical temperature belongs to a mixture 

 containing 45 per cent, of ethane ; the maximum vapour- 

 pressure, near the critical point, to a mixture containing 30 

 per cent. According to the theory, the maximum at low 

 temperatures ought to belong to the 45 per cent, mixtures. 



This point was tested in the following manner : — Two 

 strong glass tubes closed at the bottom were connected with 

 each other at the top by means of flexible high-pressure 

 piping, and with the two supply vessels containing the purified 

 carbon dioxide and ethane. A mixture was let into one of 

 the tubes cooled for the purpose to — 50°C and then dis- 

 tilled into the second tube, and so on backwards and forwards. 

 The last portion in every distillation was not caught in the 

 glass tube, but was blown off and collected over mercury 

 and analysed. From the theory of mixtures of maximum 

 vapour-pressure, it follows that the mixture coming over must 

 gradually approach the maximum mixture at the temperature 

 of the liquid. The pressure will therefore gradually increase. 

 This appeared to be actually the case. The initial pressure 

 was 7*8 atmospheres, and after successive distillations the 

 readings were 8*5, 8*8, 8*5, 8'6, the temperature being kept 

 roughly constant. The analysis of two samples after theithird 

 and the last distillations gave as the compositions 42 and 44 

 per cent, ethane respectively. The mixture has approached 

 and practically reached the theoretical value of 45 per cent. 

 The predictions of the theory had thus been completely verified. 



We then proceeded to measure vapour-pressures of two 

 different mixtures by the accurate method described for pure 

 carbon dioxide and ethane in a former paper f. The change 

 of the vapour-pressure with volume is comparatively small 

 for both mixtures, owing to the narrowness of the conden- 



* Phil. Mag. [5] xliv. p. 187. f Ibid. [6] iii. p. 149. 



