444 Professor Sir James Deivar [June 8, 



discharge expelled all the phosphorous vapour from the discharge 

 tube, and that the moving phosphorescent stream is due to the 

 vapour of phosphorus coming back again. While the electric dis- 

 charge is passing in B, the spectroscope reveals only the oxygen 

 bands. 



Separation of the volatile gases Neon, Hydrogen, Helium. 



Attention has already been called to the selective nature of the 

 absorption when a mixture of gases is presented to cooled charcoal, 

 and it has been noted that the gas obtained from the charcoal after 

 absoi-ption of air has the quantity of oxygen increased from 21 to 

 50 or 60 per cent. The general law of this absorption is that the lower 

 the boiling point, in other words, the more volatile or less condensable 

 the gas, the less is the absorption. Thus for air, nitrogen with the 

 lower boiling point is not absorbed to so great a degree as oxygen, 

 which has the higher boiling point. This is beautifully shown in the 

 following experiment. A number of spectroscopic tubes connected 

 in series with a large charcoal U-tube, are highly exhausted by cooled 

 charcoal so that the discharge will hardly pass. The charcoal tube is 

 now placed in liquid air, and a slow current of air is allowed to 

 enter the system. The less volatile gases, oxygen, nitrogen, argon, 

 are absorbed by the charcoal, while the more volatile gases, helium, 

 neon, and hydrogen, are allowed to pass. In a short time, when the 

 pressure of the latter gases has risen sufficiently, the first tube begins 

 to glow with the well-known rich orange hue of neon. As tlie air- 

 absorption progresses, the characteristic discharge of neon and helium 

 gradually extends to the other tubes, the slowness of the current 

 displaying vividly the march of the neon and helium glow. In this 

 connection the sensitivity of the neon tube to induced electric oscilla- 

 tions from a coil of wire placed at right angles to the tube, was shown. 

 This property of neon Professor Fleming makes use of in his ingenious 

 wave-detector for wireless telegraphy — the cymometer. 



The most volatile gases being neon, hydrogen and helium, when 

 a charcoal bull) attached to a sparking tube filled with helium, was 

 placed in liquid air, the nature of the luminosity remained unchanged, 

 thus indicating tliat hardly any absorption had taken place at the 

 temperature of lifjuid air. But on replacing the liquid air by liquid 

 hydrogen under exhaustion, the pressure was so much reduced tliat 

 the discharge no longer passed. This observation corroborates the 

 statement already made that the boiling point of hydrogen is a tem- 

 perature for helium which corresponds with the boiling point of air 

 for hydrogen, and leads us to infer that the boiling point of helium 

 is about 5° to 0° a])solute. 



Measurements made in 1902, ])y what was called the float-method 

 of separation, showed that the atmosphere contained as a minimum 

 one part in 70,000 of neon, and one part in 862,000 of helium. 



