156 Liveing and Dewar — Speetrum of the more 



some of these lines were used as lines of reference. In general, 

 the iron spark spectrum was the standard of reference. 



The tubes when first examined showed the lines of the first 

 spectrum of hydrogen vividly, and the earlier photographs of 

 the spectrum of the negative pole contained not only the violet 

 lines of hydrogen, but also the ultra-violet series as far up as 

 \ 337. In order to get impressions of the fainter rays, expo- 

 sures of half an hour or more were required, and a succession 

 of photographs had to be taken so as to get different sections 

 of the spectrum into the middle of the field, where measure- 

 ment of the deviations would not be impeded by the double 

 refraction of the calc spar. As the light of the negative pole 

 only was required, the electric discharge was made continu- 

 ously in one direction only, with the result that the hydrogen 

 lines grow fainter in each successive photograph, and soon dis - 

 appeared altogether. Along with the ultra-violet rays, the 

 less refrangible rays of hydrogen also disappeared, so that no 

 trace of the C or F line could be seen, nor yet of the second 

 spectrum, so long as the current passed in the same direction 

 as before. Reversal of the current soon made the F line show 

 again, so that it seems that the whole of the hydrogen was 

 driven by the current to the positive pole. The conditions 

 under which this ultra-violet series shows itself are a matter of 

 interest. It appears here in the midst of a brilliant spectrum 

 due to gases other than hydrogen, and yet it is very difficult to 

 obtain a photograph of it when no gas but hydrogen is known 

 to be present, or, at least, to become luminous in the electric 

 discharge. 



We have had an opportunity of comparing the spectrum of 

 the volatile residue of air with that of the more volatile part 

 of gas from the Bath spring. The tube did not admit of the 

 separate examination of the light from the negative pole, but 

 was examined end on, so that the radiation probably included 

 rays emitted from the neighborhood of the negative pole. The 

 whole of the hydrogen had been removed from the Bath gas, 

 but not all the argon. In the spectrum of this gas the rays of 

 helium are dominant, decidedly stronger than those of neon, 

 although the latter are very bright. In the spectrum of the 

 residue of atmospheric air, the proportion of helium to neon 

 seems reversed, for in this the yellow neon line is as much 

 more brilliant than the yellow helium line as the latter is the 

 more brilliant in the spectrum of Bath gas. All the promi- 

 nent lines in the spectrum of the volatile residue of Bath gas 

 were also in that of the residue of atmospheric air except the 

 argon lines. There were, on the other hand, many lines in 

 the latter not traceable in the former, some of them rather 

 conspicuous, such as the ray at about \ 4:664:. It is, of course, 



