[mclennan-petrie] spectra OF HELIUM, ETC. 17 



and 7.8 cms. long with 6273 lines per centimetre. The photographic 

 plates used were of the Schumann type and were made by The Adam 

 Hilger Co. 



In working with the gases helium and hydrogen the source of 

 light was a fused quartz Geissler tube of the form and dimensions 

 shown in Fig. 1, the electric discharge being obtained from a 15,000 

 volt yi kilowatt Clapp-Eastham Transformer in parallel with a yi 

 kilowatt condenser. The discharge in the Geissler tube was backed 

 up by a spark gap in air inserted in the circuit. 



The general arrangement of the equipment is that shown in 

 Fig. 2. The vacuum grating, the Geissler tube and two cocoanut 

 charcoal filled tubes which could be cooled with liquid air, were 

 joined in series with a Gaede mercury pump. Between the Geislers 

 tube and the slit of the spectrograph there was inserted a small 

 brass vessel containing a shutter which could be operated with an 

 electromagnet, but which, when either in the closed or in the open 

 position, did not interfere with the passage of gas in the circuit. 



In operating with this equipment care was taken to see that all 

 joints were gas tight. The photographic plate was first inserted and 

 then the whole system was repeatedly washed out with hydrogen and 

 thoroughly evacuated with a pair of Trimount pumps backed by a 

 Langmuir diffusion pump, a liquid air-cooled trap being inserted 

 between the latter and the system. The gas to be studied was then 

 admitted and its pressure reduced to the point where the discharge 

 gave the brightest illumination. 



In taking the photographs of the spectrum of hydrogen this gas 

 was carefully purified with charcoal cooled with liquid air before it was 

 admitted to the system. The pressure of the gas when the photo- 

 graphs were taken with it was from 5 to 6 cms. of mercury. In the 

 case of helium the gas was also highly purified before being admitted 

 to the apparatus and the exposures were made with the gas at a 

 pressure of 29 cms. of mercury. Prior to taking the photographs the 

 Gaede pump was maintained in operation for upwards of an hour 

 and the gas by means of it made to circulate through the system. 

 During this operation the tubes Q and R were kept surrounded with 

 liquid air and the shutter E was kept in the closed position. The 

 discharge was passed continuously (during this time) in the Geissler 

 tube in order to drive out any gases which might have been occluded 

 in the electrodes. 



When it became fairly certain that the gas had been purified as 

 highly as possible by means of the circulation through the charcoal 



