186 Professor Sir James Dewar [Jan. 20, 



concentration could have been reached by fractionating the gas as 

 it slowly leaves the charcoal on gradually increasing the temperature. 



Production of High Vacua and Spectroscopic Studies. Separation 



of Gases like Helium, Neon, and Hydrogen, from Air and 



other Gas Mixtures.* 



The high absorption of gases by charcoal suggested an inquiry 

 into the limits of gaseous pressure reached by such means of con- 

 densation. With this object, an ordinary spectroscopic sparking 

 tube A B was sealed to a narrow tube C E, the end of which was 

 blown into a bulb, D E, capable of containing a few grammes of 

 cocoanut charcoal. After the charcoal had been freed from 

 n gases by heating and exhaustion, and the poles cleared by 



M sparking during this operation, pure and dry gases like 



A)^ oxygen, nitrogen, air, carbonic oxide, hydrogen, neon, and 

 ^ helium, could be admitted at different pressures and the 



tube with its attached charcoal chamber sealed off. 



On placing the charcoal capsule in liquid air, the gas in 

 each case was rapidly absorbed, and the vacuum produced 

 reached the phosphorescent stage, except in the cases of 

 hydrogen, neon, and helium. 

 |, A large spectroscopic tube of 1300 c.c. capacity was 



^B sealed to a bulb containing 30 grammes of charcoal. When 

 the tube was filled with air at atmospheric pressure and the 

 charcoal cooled in liquid air, the pressure fell to 50 mm. of 

 mercury. On refilling the tube with air at the pressure of 

 half an atmosphere, and treating it as before, the exhaus- 

 tion reached beyond the striee stage ; and a final charge at 

 a quarter of an atmosphere gave a vacuum through which 

 no spark passed. When the experiment was repeated with 

 only 1 gramme of charcoal, and an initial pressure of 3 mm. 

 of mercury, the vacuum just reached the beginning of the 

 phosphorescent stage. 



When hydrogen was employed, in order to get a vacuum 

 well up in the stride stage, either a larger amount of 

 charcoal had to be employed or the initial pressure had to 

 be less than an atmosphere. But, if the liquid air bath 

 was cooled to —210° C. by exhaustion, the tube just reached 

 the beginning of phosphorescence round the cathodes. With 

 Fig. 2. ^^elium there was very slight absorption, but more appreciable 

 results were obtained with neon. Spectroscopic observa- 

 tions made during the condensation of the gas in the charcoal showed 

 the gradual disappearance of the characteristic spectrum of oxygen, 

 nitrogen, and air, as the high vacuum was reached and the discharge 

 passed with great difficulty. In tubes of this kind filled at atmo- 



* Proc. Roy. Soc, 1904. 



¥ 



