726 Professor Sir Jdnies Dewar [June 11, 



large diminution of pressure is produced, showing that even helium is 

 condensed to a considerable extent by charcoal at 20° absolute. As 

 the charcoals warm up, the condensed gases are again expelled ; the 

 helium very rapidly, the hydrogen somewhat more slowly, and the air 

 after some few minutes. 



[Brief explanation of low temperature researches with helium, 

 illustrated by slides of the Royal Institution and Onnes Helium 

 Plants ; Reference Diagrams.] 



Composition of Bath Gas. 



Hydrogen Nitrogen Inert Gas 



Liquid Bath Gas (most volatile part) 15 30 55 



Helium Neon 



Inert Gas (Bath) 88 17 



(Air) 16 84 



Vapour Pressures op Liquid Gases. 



A B 



1. Oxygen .. . .. .. log. P = 7-012 — 378- 3/T 



2. Hydrogen „ = 5-981 — 63-6/T 



3. Gas same volatility as 1 to 2 .. „ =5-182— 10/T 



4. Helium deduced from charcoal 



tensions „ = 5-324 — 11/T 



5. Helium (Onnes) „ = 6-496 — 16- 3/T 



Critical constants and boiling points used in calculating 1, 2, 3, 5. 



T is the Absolute Temperature, and P is Pressure in ram. of INIercury. 

 The B constant is proportional to the Molecular Latent Heat. 



The boiling point of hydrogen is about 20° abs., its critical tem- 

 perature 32° abs., and the critical pressure lo atm. The corresponding 

 figures for helium are approximately : boiling point 4^°, critical tem- 

 perature 5J°, critical pressure about :> atm. Onnes has, by evapora- 

 ting liquid helium under reduced pressure, reached a temperature 

 of 3° abs. With the aid of some hypothetical element related in 

 volatility to helium, as helium is to hydrogen, we should be able to 

 reach 1° or possibly even h° abs., but not the absolute zero. The 

 experimental approacli to the absolute zero has practically been 

 made during the last thirty years. Far greater advances have been 

 made during this period than in the previous 300 years, yet all the 

 new knowledge acquired only shows the need for further research. 



PhospJiorescence of Gases. — Twenty years ago, I endeavoured in 

 a Friday Evening Discourse to demonstrate the phosphorescence of 

 ozone and oxygen compounds. The effect of the impurities in the 

 air was not then fully recognized. Geissler was the first to discover 

 that phosphoresence may be produced in vacuum tubes. Becquerel 

 considered that oxygen was essential to the production of such 

 phenomena. The new apparatus may be understood by referring to 

 Fig. 3. On the brass cover, A, ground to fit air-tight to a glass 



