﻿290 



Mr. E. J. Evans on the 



to note that it is comparatively easy to obtain a helium 

 spectrum showing no trace o£ the hydrogen lines of the 

 Balmer series if an ordinary discharge is sent through a 

 spectrum tube containing purified helium. With the con- 

 denser discharge, however, the hydrogen lines usually appear 

 in the bulbs, although they are often not present in the 

 capillary. This was also observed by Curtis * during a 

 research on the band spectrum of helium. 



The arrangement of the apparatus and the method of con- 

 ducting the experiments are made clear by reference to the 

 accompanying diagram. 



In the diagram, E represents the spectrum tube and A, 

 0, F, and G bulbs containing carefully prepared coconut 

 charcoal. The tap B was placed at a distance of 1 metre 

 from the spectrum-tube, and the discharge scarcely ever 

 reached it. A Topler pump and a tube of P 2 5 were attached 

 to H, and the whole apparatus was evacuated to a pressure 

 of l/100th of a mm. of mercury. The charcoal bulbs C, F, 

 and Gr were then heated for several hours until no more gas 

 was given off'. Also the bulbs and the capillary of the 

 spectrum-tube were heated until the softening point of glass 

 was reached. The gases given off during the heating process 

 were absorbed by the charcoal bulb A, which was immersed 

 in liquid air. The tap B was then closed and the gas 

 absorbed by the charcoal in A removed by heating. 

 The bulb A was again immersed in liquid air, and the helium 

 containing a small quantity of impurities was then introduced 

 through the side-tube K and allowed to remain in that part 

 of the apparatus for over an hour. The charcoal bulbs C, F, 

 and Gr were now immersed in liquid air, and the tap B was 

 opened so that helium could enter the spectrum-tube. The 

 pressure of the gas was then adjusted to '.3 mm. by means of 

 the pump and McLeod gauge, and the ordinary' discharge 

 (without condenser) passed through the tube. The current 



* Cuvtis, Pron. Roy. Sop. A. vol. lxxxix. p. 146 (191 3). 



