with Spectra of Helium and Hydrogen. 809 



times with pure dry hydrogen. The bulb was then pumped out 

 and the pressure — amounting to about "01 or '02 mm. — was 

 then measured with a McLeod gauge and a little helium then 

 admitted. After waiting a fair interval of time to allow the 

 pressure to become constant all over the apparatus, the 

 pressure was measured again, and the partial pressure of 

 the helium and its proportion calculated. As charcoal does 

 not absorb helium, the apparatus had to be pumped down 

 to a low pressure by hand. The new pressure was noted and 

 the pressure of helium calculated. Photographs of the 

 positive rays and of the spectrum of the positive column 

 were taken simultaneously. Some pure dry hydrogen was 

 then admitted, the pressure measured, and the proportion of 

 helium present calculated. The apparatus was then pumped 

 down to a low pressure and photographs taken as before. 

 This process was repeated several times, so that a series of 

 photographs was obtained; each member of the series being- 

 taken for a definite proportion of helium present in the 

 mixture. Altogether three sets of positive-ray photographs 

 were taken on the same plate, when the apparatus had to be 

 opened up to remove the plate and prepare for a new series. 

 Photographs of spectra were given from three to five hours 

 exposure, and this was by no means too much. Longer time 

 could not be given for fear of air leak. Photographs of the 

 positive rays were generally given about one and three- 

 quarter hours. 



Results. 



The photographs of the spectrum in residual air when the 

 pressure is sufficiently low to give good photographs of the 

 positive rays (*005 mm. say), show the mercury lines strongly 

 and the negative band of nitrogen at \ = 3911. I have not 

 been able to detect the positive bands or the line spectrum 

 of nitrogen, nor could I observe any of the spectra of oxygen. 

 The positive rays, besides showing the lines due to the 

 hydrogen atom and molecule, also show the line due to air 

 very strongly. The apparatus used was not powerful enough 

 to separate the lines due to nitrogen and oxygen, but other 

 photographs taken elsewhere in the laboratory show the 

 oxygen line frequently stronger than the nitrogen line. 

 The lines due to the molecules of oxygen and nitrogen also 

 occur. Thus while we get the positive rays of atomio oxygen 

 and nitrogen in residual air, as well as the lines due to their 

 molecules, we get only one of the spectra of molecular 

 nitrogen present in the light from the positive column. In 



Phil. Mag. S. 6. Vol. 30. No. 180. Dec. 1915. 3 G 



