1922] on Problems in the Variability of Spectra 527 



regions two band spectra — one known as the positive band spectrum, 

 which appears in the capillary of a vacuum tube of the conventional 

 type ; and the negative band spectrum, which is found in the neigh- 

 bourhood of the cathode, and which constitutes an important part 

 of the spectrum of the aurora. Both these band spectra, and indeed 

 all band spectra, are generally attributed to molecules rather than 

 atoms ; but if a condensed discharge is passed through nitrogen the 

 spark spectrum associated with the nitrogen atom is obtained, and 

 this is capable of further modification when discharges of great 

 intensity are employed. The action of the condensed discharge is 

 almost certainly due to the greatly increased current density which 

 obtains during the very brief periods while the discharge is passing, 

 and its first effect is to break up the molecules into atoms, and the 

 further stages brought about by an increase in the intensity of the 

 discharge are generally supposed to be due to the removal of succes- 

 sive electrons from the atoms. There are other methods by which 

 the current density can be increased with similar changes in the 

 spectrum, the effect of an increase in the current density being to 

 increase the number of charged particles in a given volume of the gas, 

 with the result that a large number of the radiating atoms are sub- 

 jected to intense electric fields due to neighbouring charged particles. 



Similar results are observed in the spectra associated with carbon. 

 There are at least six spectra due to compounds of carbon with 

 hydrogen, oxygen and nitrogen, and special experimental conditions 

 are necessary for the production of some of these spectra. In addi- 

 tion to these band spectra carbon shows line spectra, and with the 

 most intense discharges which can be employed in the laboratory a 

 number of new lines appear which are also found in the spectra of 

 the hottest type of stars, known as the Class 0, or Wolf-Rayet stars. 

 All these changes can be reasonably accounted for, but there are a 

 number of other changes which are more difficult to explain. For 

 many reasons the spectrum of hydrogen is of particular interest, 

 because the atom of hydrogen is the simplest known atom, and is 

 supposed to consist of a positive nucleus and a single electron. 

 There are two spectra associated with hydrogen, one of which is 

 known as the Balmer series and is found in almost all celestial 

 spectra, and also in vacuum tubes in the laboratory unless the most 

 rigorous precautions are taken to exclude all traces of hydrogen. 

 The explanation of the origin of this spectrum has been one of the 

 most striking successes of the quantum theory of spectra developed 

 by Bohr and by Sommerfeld. The other spectrum of hydrogen, 

 known as the secondary spectrum, consists of an enormous number 

 of lines, and differs in its mode of production from the Balmer series 

 in that the secondary spectrum is characteristic of pure hydrogen. 

 In the purest hvdrogen obtainable the secondary spectrum may be as 

 bright as the Balmer series, but if the smallest trace of impurity is 

 present the Balmer series gains in intensity and the secondary spec- 



Vol. XXIII. (Xo. 116) 2 p 



