iKUENEN-CLARKl CRITICAL PHENOMENA OF AIR 41 



rises in the tube and fades out without reaching the top of the tube, 

 as theory would have it do. But the range of temperature in which 

 this occurs is very small indeed. 



It will be noticed that the two critical points are very close to- 

 gether, and that the region in which retrograde condensation is possible 

 is very narrow. So without the perfect temperature control of the 

 Leiden Laboratory, it would be impossible to see this interesting 

 and rarely seen phenomena. 



In conclusion, we wish to thank Mr. Van Urk for his assistance 

 in preparing and assembling the apparatus as well as for his help 

 in making observations. We wish to express our indebtedness to 

 Professor Kamerlingh Onnes for his unfailing and stimulating interest, 

 and many helpful suggestions, and for placing the unsurpassed facili- 

 ties of his famous laboratory at our disposal. 



