1909] on Problems of Helium and Radium. 725 



spiral of worsted cord. This enables a silvered glass vacuum vessel 

 to fit tightly over it and form a kind of open regenerator covering, 

 through which the cold air coming from the evaporating liquid is 

 directed spirally round the neck of the flask, thereby greatly 

 retarding the heat conduction into the inner vessel, which chiefly 

 arises from the outer metallic tube. 



Comparative Condensability of Gases by Charcoal at 

 Liquid Air and Liquid Hydrogen Temperatures. 



The relative condensability by charcoal of the gases, air, hydrogen, 

 and helium, can be shown by filliug three similar sets of double tubes, 

 shown in Fig. 2, respectively with samples of these gases. The two 

 tubes of each set are filled with the particular gas at atmospheric 

 pressure and the ordinary temperature, and dip into a little bottle of 

 mercury, A. The tubes are bent twice at right angles and sealed, up, 

 as shown in the figure. Thus the closed end of either tube of each 

 set can be cooled by immersion in a vacuum vessel containing liquid 

 air or hydrogen. A gramme of charcoal is placed in the sealed end 

 of one tube in each pair. For convenience of pressure observa- 

 tions, an ordinary barometric tube C accompanies each pair of tubes. 

 When the closed end of the air tube is cooled in liquid air, only a 

 small contraction is observed, as shown by a slight rise of the 

 mercury in the tube. If, however, the air tube which contains the 

 charcoal is cooled all the air is thereby condensed, and the mercury 

 quickly rises to the barometer height in the tube. Now take the 

 pair of hydrogen tubes. As before, on cooling the tube without 

 charcoal, only a slight contraction is shown, due to the cooling, but 

 when the charcoal tube is immersed in the liquid air, a quantity of 

 the hydrogen is condensed in the charcoal, and the mercury rises in 

 the tube. The height it attains, however, is noticeably less than in 

 the air-charcoal tube, showing the smaller condensability of hydrogen 

 in the charcoal at liquid air temperature. On cooling either of the 

 helium tubes with liquid air, practically no condensation is shown. 

 Now instead of liquid air, let us use liquid hydrogen, and cool first 

 the air tube. Immediately the mercury rushes up to the barometric 

 height, all the air being condensed into a solid of inappreciable tension 

 of vapour, and the charcoal tube behaves in the same way. Now pass 

 to the hydrogen tubes. When the plain gas tube is cooled, quite a 

 noticeable contraction is visible, because the temperature of liquid 

 hydrogen is so low compared to the hydrogen gas inside at the room 

 temperature. On now cooling the charcoal-hydrogen tube, complete 

 absorption is produced, and the mercury rises to the barometric height. 

 The hydrogen, at the temperature of its own boiling point, is com- 

 pletely absorbed in the charcoal. Now compare this with the set 

 containing helium. On cooling the helium tube it behaves similarly 

 to the plain hydrogen tube, but on cooling the charcoal tube, quite a 



