26 Effect of Mercury Vapour on the Spectrum of Helium. [June 3, 



plicated, a large number of other and fainter lines appearing as well. 

 If mercury vapour be introduced into such a tube filled with helium at 

 from 2 — 5 mm. pressure, the first change noticed is the simplification 

 of the spectrum of this negative glow ; and at the same time three of 

 the helium lines disappear from the spectrum of the negative glow, 

 the red 7065, the blue 4713, and the violet 4472, whilst the yellow 

 5876 becomes very feeble. In the orange part of the spectrum a 

 brilliant new line can now be seen, namely, the mercury line 6151, a 

 line which as a rule does not appear in a Pliicker's tube containing 

 mercury vapour when an electric discharge from an induction coil is 

 passed through it. The presence of the orange line of mercury is of 

 interest, as other gases, argon, krypton, hydrogen, &c, mixed with 

 mercury vapour, when examined under the same conditions fail to 

 give it. 



Whether this orange line is shown in a neon tube where mercury 

 vapour is present is difficult to say, as one of the bright orange lines of 

 neon almost coincides with it.* 



Besides this alteration of the helium spectrum at the negative 

 electrode another curious change can be observed, if a piece of tubing 

 (whose internal diameter is about 4 mm.) be introduced into the 

 centre portion of the Pliicker's tube. The spectrum of the gas in this 

 central portion invariably consists of the mercury lines in the yellow, 

 green, and violet, together with one and one only of the helium lines, 

 namely, the green line 4922. This seems to be a most delicate test 

 for minute traces of helium in other gases, and this line can be easily 

 seen when no other helium lines can be detected elsewhere in the tube. 

 A large excess of mercury vapour will also produce the same result in 

 the spectrum of the glowing vapour in the capillary bore tube. 



Thus in the same Pliicker's tube, containing helium and mercury 

 vapour, helium may be made to yield three distinct spectra : — 



(1.) In the narrow bore capillary portion the full spectrum of eight 

 lines, 



(2.) At the negative electrode, three lines disappear, one red, one 

 blue, one violet, and the yellow line becomes very faint. 



(3.) In the wider bore central portion only one green line is visible. 



Of course the mercury spectrum is visible in all portions, but at the 

 negative electrode the orange mercury line becomes the most brilliant. 



Another interesting fact is the great purity of the spectrum. In 

 every case there is a marked absence of light between the lines. 



Eunge and Paschen pointed out in 1895f that the helium spectrum 

 seemed to belong to two systems. 



The lines which persist and those which disappear in the negative 



* The author has to thank Professor Ramsay for kindly helping him and for 

 supplying the rare gases neon and krypton for these experiments, 

 f ' Phil. Mag.,' vol. 40, p. 297. 



