138 Multiple Spectra of Gases. 



intensities*, among which the fonr usual hydrogen lines, 

 although present, are by no means specially prominent. A 

 large capacity is required to change this spectrum into tho 

 familiar four-line spectrum which is comparable with the 

 blue spectrum of argon. The change is marked by a sharp 

 alteration in the colour of the glow from white to a deep red. 

 In the process, the bluish-green line (H/3),as well as the two 

 in the violet, which retain their early position unaltered, 

 becomes nebulous at its edges f ; while the red H-line 

 remains sharp and clear. The most marked change in the 

 spectrum, however, is the complete obliteration of all the host 

 of other lines covering the whole spectrum ; and the obvious 

 contrast between the oscillatory and non-oscillatory spectra 

 of this gas is quite as striking as in the case of nitrogen, 

 although somewhat different in nature. This four-line deep 

 red glow appears satisfactorily in a tension of gas of about a 

 millimetre — when the tension of the gas is much higher or 

 lower the resistance is increased, the oscillations are damped, 

 and other lines begin to appear. Curiously enough, however, 

 the damping of the oscillatory discharge does not at first 

 replace all the fines which were extinguished by the intro- 

 duction of the condenser. At first only a sharp line in the 

 yellow and one in the green begin to appear, and gradually 

 others are added as the impedance is increased. 



The relation of these conclusions to the varying spectra of 

 hydrogen observed in stars leads to interesting speculations 

 regarding the nature of the electrical and thermal conditions 

 in the photospheres of these bodies J. 



Each of the halogens gives two spectra, one with, and 

 one without the condenser. With iodine, if any of the solid 

 itself is present in the tube, the vapour-tension is so soon 

 altered by the heat of the discharge that the oscillatory dis- 

 charge is damped and the non-oscillatory substituted. Hence 

 the former can be obtained only for a few moments. 



A tube of helium made by Professor Ramsay, the kind 

 gift of the Hon. R. J. Strutt, gave a brilliant yellow glow 

 under the influence of the continuous discharge, and a brilliant 

 blue with the condenser discharge; but since the bright helium 

 lines remained in each, and every other important line in the 

 blue spectrum proved to be an argon line, it is evident that 

 the oscillations produced no considerable effect upon the 

 helium. 



* Smyth, Pogg. Ann., Beiblatter [2] vii. p. 286. Wiillner observed 

 this spectrum but did not measure the lines. 

 t E. Villari, Fievez, and Salet. 

 % E. Ebert, Wied. Ann. liii. 1894. 



