810 On the Spectra of Helium and Hydrogen. 



this case no connexion can be traced between the positive 

 rays and the spectrum of the gaseons mixture. 



Helium and Hydrogen. 



It was not found possible to get rid entirely of the air line 

 from the positive rays, or of the nitrogen band from the 

 spectrum, though both conld be considerably diminished. In 

 the first experiment there was about 60 per cent, of helium 

 present, but the pressure of the gas was rather high, falling 

 from *048 mm. to "025 mm. during the exposure. As a 

 consequence, while the line due to the hydrogen atom was 

 strong, all other lines were weak; the lines due to the 

 hydrogen molecule and to helium could just be detected. The 

 spectrum of helium, however, was just as strong as the 

 spectrum of hydrogen. In the next photograph, with a 

 smaller percentage of helium at a lower pressure, a strong- 

 positive ray photograph was obtained in which the line due 

 to helium was as strong as the line due to the hydrogen 

 molecule. The spectrum, unfortunately, gave no trace of 

 either helium or hydrogen. A series of photographs was 

 taken in which the amount of helium was varied from 

 30 per cent, to 2 per cent, of the mixture. In the positive 

 ray photographs I could not detect any helium at a smaller 

 proportion than 10 per cent., though here the line due to it 

 was as strong as the line due to the hydrogen molecule. In 

 the spectra oblained not a trace of the helium spectrum 

 appeared in any, though the spectrum of hydrogen was 

 clearly visible. In a final case, where the proportion of 

 helium was so small that I could not estimate the amount, 

 I obtained a positive ray photograph of four hours - ' exposure. 

 The line due to helium, though faint, was visible. The 

 spectrum, as was to be expected, showed only the hydrogen 

 and mercury lines with a faint negative band of nitrogen. 



As far as these experiments go, it would appear that the 

 helium spectrum disappears at a much earlier stage than the 

 helium positive rays, which are remarkably persistent. It is 

 difficult to obtain satisfactory results, as the pressure must 

 be kept so low to produce good positive ray photographs 

 that the light from the positive column becomes too faint to 

 render good spectrographs work possible. 



Conclusion. 



No simple connexion between the spectra of mixed gases 

 and their positive rays has been observed. It would seem 

 that a gas, whose spectrum in the positive column is more or 



