[kuenen-clark] critical PHENOMENA OF AIR 37 



the air very slowly indeed, in order to remove the condensable 

 components. 



Even with the extraordinary care taken, there was evidence that 

 the last traces of carbon dioxide (?) had not been removed. This was 

 shown by the sticking of the glass thiead of the stirrer in the capillary 

 through which it runs. This sticking increased progressively as the 

 air was forced into the observation tube and disappeared upon removal 

 of the pressure. In most cases it could be reduced or obviated en- 

 tirely by operating the stirrer while compressing the air into the 

 observation tube. 



After the apparatus was filled with air, the glass reservoir tube 

 was placed in the steel pressure-pot in the usual manner and the obser- 

 vation tube placed in the cryostat. Since the operations of filling, 

 inserting in the pot and mounting of the cryostat are standardized 

 in the Leiden Laboratory, there was very little difficulty in managing 

 these otherwise difficult and complicated operations. The methods 

 of measuring pressures and temperatures are also so well developed 

 that no difficulty was experienced in obtaining reliable results. Our 

 thanks are due to Dr. Crommelin for assistance with the pressure read- 

 ings and to Mr. Burgers for looking after the temperature readings. 

 After the minor mechanical and manipulatory difficulties were mas- 

 tered, the only difficulty that remained was the securing of a uniform 

 mixture in the observation tube. When air has been liquefied and is 

 re-evaporated, the nitrogen boils away first so that the mixture becomes 

 richer and richer in oxygen. Owing to the small size of the capillary 

 tubes, the rich oxygen mixture drives the nitrogen ahead of it, as 

 it boils away, and when pressure is applied again this mixture is 

 driven back into the compression chamber. The only way by 

 which thorongh mixing can be accomplished is by lowering the pres- 

 sure in the apparatus to normal and recompressing several times. 

 If this is done, the mixing is fairly complete. The importance of this 

 was not realized in the earlier experiments, so many of the earlier 

 results have been rejected. 



Our first object was the determination of the upper limit of 

 temperature and the corresponding pressure at which air may be 

 liquefied or the second critical point.^ This was done by holding 

 the temperature constant at a point slightly above that at which 

 condensation was expected, and compressing very slowly, watching 

 the tube very carefully. Then the mixture was expanded slowly. 

 No liquid having appeared, the operations were repeated at a slightly 

 lower temperature. This was continued at lower and lower tempera- 



* Kuenen. loc. cit. p. 60. 



