38 THE ROYAL SOCIETY OF CANADA 



tures until on expansion a slight cloud was seen. The temperature 

 was still a little too high. As would be expected, a cloud appears on 

 expansion before liquid can be seen on compression, owing to the fact 

 that in one case the mixture is cooled adiabatically and in the other 

 it is heated. Since the substance is very compressible and the pressure 

 is considerable, a large amount of work is done in compressing and the 

 change in temperature is noticeable, even when the pressure is applied 

 very slowly. The two operations of compression serve to determine 

 upper and lower limits of temperature, so that the final result is very 

 good. The temperature of the upper critical point is —140-63° C, 

 and the pressure 37-17 atmospheres. Results taken at different 

 times agreed very closely. 



After this point was determined, the temperature was lowered a 

 few hundredths of a degree and the operations repeated. If the 

 reduction is slight so that the temperature does not fall below the 

 plait-point temperature, the phenomena of retrograde condensation 

 may be observed, provided the mixture is stirred. When the pressure 

 is decreased slowly with stirring, the liquid appears somewhere below 

 the middle of the tube, with meniscus nearly, if not quite, flat. With 

 further expansion, the meniscus rises and reaches a maximum height 

 and then slowly falls again and disappears at the bottom of the tube. 

 If the mixture is not stirred, the meniscus rises and either fades out 

 without sinking again, or sinks very slightly before disappearing. When 

 the substance is stirred, both liquid and vapor phases are uniform and 

 they preserve their individuality even to the surface of separation, 

 while if not stirred the two phases possess very different proportions of 

 the components at different levels and merge into each other near the 

 meniscus. With compression and stirring, the liquid appears at the 

 bottom of the tube, increases in amount to a maximurti, and then 

 sinks again and finally the meniscus fades out after sinking a short 

 distance. If the substance is not stirred, the meniscus fades out 

 without falling. 



These operations were repeated at lower temperatures until 

 on expansion the meniscus appeared at the middle of the tube. This 

 is the plait-point temperature and the pressure when the tube is half 

 full is the plait-point pressure. Here the two phases are identical 

 and ordinary critical phenomena occur. Compression at this tempera- 

 ture results in the formation of liquid at the bottom of the tube, 

 which increases in amount until the meniscus reaches the middle 

 of the tube. Further application of pressure results in fading out 

 of the meniscus. The temperature for the plait-point is — 140 -73° C, 

 and the pressure 37 -25 atmospheres. 



