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XXIV. Separation and Striation of Rarefied Gases under the 

 Influence of the Electric Discharge. By E. C. C. Baly*. 



SOME time ago, on examining with a spectroscope a 

 vacuum-tube which happened to contain a small quantity 

 of hydrogen, during the passage of the electric spark I noticed 

 that the hydrogen lines, while strongly visible in the negative 

 glow, could not be seen in the body of the tube. The hydrogen 

 appeared to be, in fact, withdrawn from the tube and collected 

 about the negative pole. Finding the same result in a tube 

 which I fitted up for the purpose, it appeared to me to point 

 to a separation of the gases in the tube ; and I determined to 

 make a series of experiments with a view to investigating the 

 matter and the behaviour generally of different gases under 

 similar conditions. 



The tubes 1 employed were about 9 inches long and f inch 

 internal bore. The electrodes were of aluminium wire, and, 

 except in certain cases to which I shall refer, about li inch 

 in length. Two of these tubes were connected to the pump 

 at the same time, one direct and the other through tubes for 

 the absorption of mercury vapour ; so that in all the expe- 

 riments results were obtained in the presence and in the 

 absence of mercury vapour. Gas-generators, fitted with 

 purifying and drying apparatus, were connected so as to 

 allow of varying quantities of the particular gases under ex- 

 amination being admitted to the tubes as might be required. 

 The pump, I may mention, was a modified form of the 

 Geissler mercury-pump, to which I was able to attach an 

 automatic apparatus for working it — a very great saving of 

 labour. Measurements of the vacua were obtained by means 

 of an ordinary barometer-gauge, and varied from 15 millim. 

 to \ millim. 



I first worked with varying quantities of carbon dioxide 

 and hydrogen. On using certain mixtures of these gases 

 exhausted to about | millim., when the current was first passed 

 a white glow appeared throughout the tube, no strise being- 

 visible, giving a mixed spectrum of the two gases. After a 

 few seconds the negative glow changed to a pink colour, and 

 well-defined striae, whiter than the preceding glow, began to 

 appear. On watching this change with the spectroscope, 

 the hydrogen lines in the tube were seen to become fainter 

 and gradually to disappear, leaving only the spectrum of 

 carbon dioxide, while those in the negative glow became 



* Communicated by the Physical Society : read Feb. 10, 1893. 



