'produced hy Cooling Gases to Low Temperatures. 533 



on, the glass tube leading from R to the " tester " and the 

 tester itself were well heated with a flame so as to dry them 

 thoroughly. The method of filling the apparatus with the 

 gas preparatory to making a test has already been described. 

 The following table gives the " critical temperature " at 

 different pressures for air obtained as described from boiling- 

 liquid air. 



Pressure in centims. 

 of Mercury. 



" Critical Temperature " 

 (approximate). 



101 



80 

 41 



-140° C. 

 -145° 0. 

 -160° C. 



Ordinary Air from outside. 



The ordinary air was drawn slowly into the apparatus 

 through long tubes containing calcium chloride, solid potash, 

 phosphorus pentoxide, and cotton-wool. 



The results obtained were practically identical with those 

 tabulated above for air derived from liquid air. 



Hydrogen. 



The hydrogen was prepared by the action of KOH on pure 

 aluminium. The gas passed from the generating vessel to 

 the apparatus entirely through glass, no rubber connexions at 

 all being used. On its way the gas was dried and purified 

 by passing through sulphuric acid, calcium chloride, KOH, 

 P 2 5 , and finally through a glass spiral about 2 feet long 

 immersed in liquid air. A plug of glass-wool was inserted 

 in the tube leading from the spiral to the main apparatus. 

 No nuclei were ever detected in hydrogen until it had been 

 cooled to — 175° C. Generally, when hydrogen was cooled 

 by means of liquid air, many nuclei were produced ; but on 

 one or two occasions the gas was obtained in such a state 

 (possibly depending on the degree of purity; that no nuclei 

 were formed at this low temperature, the pressure of the 

 gas being approximately atmospheric. The " critical tem- 

 perature " for hydrogen is therefore uncertain, but it is 

 certainly very much lower than that obtained with all the 

 other gases tried. 



