244 SCIENCE PROGRESS 



subject has recently been treated at considerable length in a 

 monograph by Prof. Fowler. 



The source of the Balmer series is known very definitely to 

 be the hydrogen atom. This conclusion was at first merely a 

 judicious guess from the conditions of excitation of the spectrum 

 in the stars and the laboratory. It received experimental sup- 

 port later, from Fabry and Buisson's measurements of the 

 broadening of the lines by the Doppler effect. 



The hydrogen atom, according to modern conceptions, 

 consists of a central positive nucleus, round which rotates a 

 single electron. It is the simplest known atom, and it is not 

 surprising that the spectrum it emits was the first to be explained 

 on theoretical grounds by Bohr. 



The secondary spectrum has hitherto gained only a modest 

 degree of prominence. For a long time it was even suspected of 

 an origin other than hydrogen, but this doubt was removed by 

 an elaborate investigation of Dufour's, early in this century. 

 The spectrum has been observed only in vacuum tubes. ^ It is 

 an interesting fact that the purer the hydrogen in a vacuum 

 tube, the stronger is the secondary spectrum relative to the 

 Balmer lines. The latter never disappear, but in pure hydrogen 

 with an uncondensed discharge the absolute intensity of even 

 Ha may be less than that of neighbouring secondary lines. 

 The enhancement of the Balmer lines by impurities is an effect 

 possibly closely allied to chemical catalysis. The pure hydrogen 

 used in such spectroscopic experiments is the gas obtained 

 by diffusion through palladium. The vacuum tube is first 

 brought to an X-ray vacuum-^— by charcoal immersed in liquid 

 air or some other means — and then a palladium tube attached 

 to it is heated in the base of a Bunsen burner, where the partial 

 pressure of hydrogen is high. Hydrogen then flows into the 

 tube through the metal, which is, of course, impermeable to all 

 other gases. If the palladium is heated in the very tip of the 

 flame, where the hydrogen has all been oxidised, the gas which 

 has passed into the vacuum tube may be extracted again, leaving 

 a vacuum as good as the original. Palladium acts as a reversible 

 hydrogen valve. 



To return to the secondary spectrum, most of this is now 

 definitely known to originate from the hydrogen molecule, H^. 

 It is possible that other complexes of hydrogen atoms may be 

 represented among the weaker lines. Such complexes as the 

 H3 of Sir J. J. Thomson, and the active hydrogen described by 

 Wendt, may be present in the tubes, but their contributions to 



1 Prof. Merton and the writer have attempted, but unsuccessfully, to 

 obtain evidence of secondary hydrogen lines in the solar spectrum, by a 

 comparison of Rowland's solar wave-length tables with their own wave-length 

 measurements in the hydrogen spectrum. 



