262 Sir James Deivar [Jan. 20, 



(in millimetres of mercury), and the corresponding temperatures (in 

 degrees absolute) for the chief gases : — 



Helium 

 Neou . 

 Argon . 

 Krypton 

 Xenon . 

 Hydrogen 

 Nitrogen 

 Oxygen 



Any good air-pump, capable of maintaining an exhaust below 

 • 1 mm. with a reasonable leak, can easily cause oxygen to solidify 

 if thermally isolated. 



Sepaeation of Mixed CtASes. 



The easy application of low temperatures, combined with the use 

 of charcoal, has led to a means of separating mixed gases by the use 

 of various liquids as cooling agents. When the least volatile gas 

 is condensed, the residual pressure is due to the more volatile gases, 

 and finally to helium, which is the most difficult to condense. 



The use of liquid hydrogen and charcoal as agents in the 

 separation of gaseous mixtures may be shown as follows : — Two 

 long glass tul)es, A and B (Fig. 6), each originally full of ordinary 

 air, are joined, B to a charcoal bulb dipped into liquid air, and 

 A to a plain tube cooled in liquid hydrogen. Under the electric 

 discharge, both tubes show the ruddy glow of mixtures of neon and 

 helium. When the two cooling vessels are removed the discharge 

 in A, cooled by liquid hydrogen, stops almost instantaneously ; the 

 discharge in the other tube, B, cooled by liquid air and charcoal, 

 continues almost unchanged for some time. The reason being that 

 the cooling effect of the liquid hydrogen disappears almost instan- 

 taneously, while in the other tube the gases are only very slowly 

 liberated from the charcoal after the cooling by liquid air is stopped. 



In the following table the results of the separation of the rare 

 gases in the atmosphere by different methods are given, the figures 

 in the last column being those obtained by using liquid hydrogen as 

 the cooling agent. 



Rare Gases in the Atmosphere, 



Per Million Volumes. 



