[kuenen-clark] critical PHENOMENA OF AIR 39 



The substance is very sensitive to changes of temperature and 

 pressure in the critical region as is shown by the following. At a 

 temperature of — 140-64° and pressure of 37 -26 atmospheres, the stirrer 

 was raised from the bottom of the observation tube, and the usual 

 cloud appeared below it, and disappeared in the normal manner, but 

 reappeared and disappeared periodically, with a period of ap- 

 proximately one second. This phenomena was caused probably 

 by the upward motion of the stirrer creating a partial vacuum below 

 it which resulted in the cloud. Above the stirrer, air was forced 

 up into the capillary driving cold air ahead of it out into the warmer 

 portion of the capillary above the cryostat. This resulted in an 

 increase in pressure above, so the air flowed down by the stirrer, its 

 inertia carrying it beyond equilibrium until there was a slight excess 

 of pressure below the stirrer. This was sufficient to dissipate the 

 cloud. The air flowed back up again and oscillation was set up until 

 after about three distinct periods became too feeble to create the cloud. 

 No doubt, the motion of the mercury in the pressure apparatus con- 

 tributed to this motion. The phenomena was most striking and 

 beautiful to watch. Opalescence was observed frequently when the 

 pressure was slightly lower than that at which the cloudy conden- 

 sation would appear, and in some cases persisted for some time, even 

 when the mixture was stirred. Opalescence can be easily changed to 

 cloud by a quick motion of the stirrer. 



Next a study of condensation phenomena below the plait-point 

 was begun. It was found for the reasons already given that expan- 

 sion results are very unreliable, and consequently all such have been 

 rejected. At and above the plait-point, this difficulty is not present 

 since the substance passes from the liquid to vapor state without 

 boiling and very quickly. The pressures for initial appearance of 

 liquid in the tube, and also for the complete filling of the tube were 

 found for a number of temperatures, and with the data thus obtained 

 and with those previously mentioned, the curve of figure I was drawn. 

 Of course, above the plait-point there are two pressures for the same 

 temperature for which the tube is full of vapor. 



