ON THE SPECTRUM OF HYDROGEN. 
38 
Q 
») 
' 
SA (found). 
8A (found) 
(mean value). 
8A calculated at 
456° Abs. for— 
Atom. 
Molecule. 
6018 0-037, 0-036, 0-034, 0-030 . . . . 
0-034 
0-046 
0-032 
6028 0-032,0-033,0-032 . 
0-033 
0-046 
0-032 
6225 0 • 034, 0 • 033, 0 • 038, 0 • 039, 0-032 . 
0-035 
0-048 
0-034 
5015He 0-024,0-022 . 
0-023 
0-019 
— 
It will be seen that the results are uniformly in agreement with a molecular origin for 
the secondary lines, the half-widths found being very close to the values calculated for 
the molecule. In the case of the Helium line the fact that the half-width found is 
slightly greater than the calculated value is perhaps to be explained in part by the 
conservative estimate of the temperature in the case of the Helium tube, but it is 
believed that the limiting theoretical widths of the lines are more nearly attained in 
the tubes with wide capillaries and in which the current density is consequently lower, 
than in the tubes with narrow capillaries of the conventional Plucker form. The lines 
A A 6018 and 6225 are both “Fulcher” lines, are enhanced at low pressures, and are 
weakened by the condensed discharge. Neither of them shows the Zeeman effect. The 
line A6028 belongs to an entirely different class, being a high-pressure line which shows 
the Zeeman effect. Since these are the two most important classes of lines it is probable 
that the whole of the secondary spectrum is due to the Hydrogen molecule. 
(10) The Separation of Gases in Vacuum Tubes. 
In a previous communication (Merton, ‘ Roy. Soc. Proc.,’ A, vol. 98, p. 255, 1920) 
an account has been given of a curious effect, which, on further investigation, seems to 
throw some light on the phenomena observed in vacuum tubes containing Hydrogen. 
It was found that when a vacuum tube containing Helium at a comparatively high 
pressure, and also a little Hydrogen, was excited by an uncondensed discharge and was 
observed through a direct-vision prism, the lines of both Helium and Hydrogen appeared 
with uniform intensity throughout the capillary. On putting a condenser and a spark 
gap in the electrical circuit the Hydrogen lines became much weaker in the centre of 
the capillary, but showed brightly at the two ends. This is in agreement with an 
observation of Curtis (‘ Hoy. Soc. Proc.,’ A, vol. 89, p. 146, 1914) ; but it was found 
that on cutting out the condenser the Hydrogen lines did not immediately reappear with 
uniform brightness, but gradually extended from bright spots at the ends of the capillary 
until the intensity became uniform, which took a considerable time to occur, depending 
