Second Spectrum of Hydrogen. 331 



different ; and, indeed, the employment of more powerful 

 instruments does not reveal in its whole extent a single line 

 of similar nature to the band characteristic of the so-called 

 spectra of the first order of other gases. On the contrary, 

 the spectrum consists entirely of sharply-defined lines distri- 

 buted upon a feebly-illuminated background without any 

 definite order, which neither in appearance nor in intensity 

 differ essentially from the four characteristic lines of hydrogen. 

 If, then, we take into account only the appearance of the 

 spectrum, we find no reason for distinguishing it as a separate 

 spectrum of hydrogen from the four lines H a . . . H§. It would 

 be more simple to assume the existence for hydrogen of only 

 one line-spectrum, consisting of all these lines together with 

 the lines H a . . . H$, always assuming that they can be shown 

 to possess identical origin. This view of the matter is, to a 

 certain extent, justified by the circumstance that with hydro- 

 gen, in opposition to what is found with other gases giving 

 double spectra, the two spectra almost always appear together, 

 whereas in other cases the appearance of the one spectrum is 

 accompanied by the disappearance of the other, or at least 

 by its greater or less diminution. We may, for example, recall 

 the behaviour of a tube filled with nitrogen the pressure of 

 which is caused to vary: as long as the pressure is small or 

 there is no air-break in the circuit, the well-known band- 

 spectrum only is seen; whereas if the pressure be increased, 

 or if an air-break be introduced, the spectrum gives place to 

 the line- spectrum. Between these two cases there is always 

 a transition-point at which a sort of conflict takes place be- 

 tween the two spectra, in which sometimes the one, and some- 

 times the other, spectrum partially starts up ; but we never 

 have the two spectra completely developed at the same time. 

 Although these conditions clo assume somewhat different 

 forms with hydrogen, yet one difference between the two 

 spectra, based upon the different temperatures at which they 

 are produced, may be pointed out. Pllicker has already 

 shown that in wide tubes, where the temperature of the dis- 

 charge is certainly less than in capillary tubes, H^ is the 

 only one of the chief lines of hydrogen which makes its 

 appearance, whilst the groups of the second spectrum in the 

 red and orange are still distinctly to be seen. On the other 

 hand, by the introduction of an air-break or Leyden jar, 

 even at extreme rarefaction, it is possible to entirely extin- 

 guish the second spectrum, whilst the characteristic lines at 

 the same time assume a more or less expanded and diffuse 

 appearance. Now, since this arrangement involves an in- 

 crease in the potential of the quantities of electricity which 



Z 2 



