with Spectra of Helium and Hydrogen. 807 



spectra produced are the band spectra of the molecule* of 

 nitrogen; if there were no nitrogen present, the series-line 

 spectrum of the oxyen atom* would be produced. Conse- 

 quently we have here the masking effect of the heavier 

 constituent. With the condensed discharge, however, or in 

 the case of the spark in air at atmospheric pressure, the 

 line spectrum of the nitrogen atom* occurs. In this case 

 there should be no masking' effect; and we find that the 

 elementary line spectrum of oxygen appears as brightly as 

 the spectrum of nitrogen. 



The case of a mixture of hydrogen and helium is somewhat 

 of an exception of the rule given above. Ramsay and Collie 

 have shown that at a pressure of about 3 mm. as much as 

 30 per cent, of helium is necessary before the spectrum of 

 helium appears in the mixture. At the lowest pressures the 

 amount necessary falls as low as 10 per cent. For the 

 spectrum of hydrogen to be detected in the mixture as low a 

 percentage as '001 per cent, is sufficient for all pressures. 



Nature of the Problem. 



In the case of a gas whose spectrum is masked by that of 

 another, it is interesting to inquire to what extent it takes 

 part in the discharge. In the case of air cited above, is the 

 current so far as it is due to the motion of the positive ions, 

 carried by the nitrogen molecules alone ? It seems unlikely 

 that this should be so, but it would leave one without an 

 explanation for the absence of the oxygen spectrum. 



The present research was undertaken to compare the 

 spectra of various mixtures of helium and hydrogen with 

 the relative intensity of the positive rays of the same 

 mixture. As a large proportion of helium is necessary 

 before its spectrum appears, it would be possible to observe 

 whether the positive rays of helium appeared just when the 

 same proportion was present. If there were a direct con- 

 nexion between the spectrum of helium and its positive rays 

 in this manner, it would be strong argument for supposing 

 that a gas whose spectrum is completely masked takes no part 

 in the discharge. 



Apparatus. 



The type of apparatus was the usual type used for producing 

 positive rays (fig. 1). The bulb and camera were connected 

 by a side tube so that both could be exhausted to a low 



* It is generally supposed that band spectra are produced by molecules, 

 line spectra by atoms. 



