86 M. E. Wiedemann's Investigations on 



whose great capacity of reaction indicates that their molecules 

 are easily broken up, almost constantly show line spectra. That 

 different substances at the same temperature possess different 

 kinds of spectra we might also be inclined to infer from the 

 fact that in the same Geissler tube, which contains at the 

 same time hydrogen and nitrogen, the nitrogen bands appear 

 together with the hydrogen-lines. This inference, however, 

 is not absolutely correct, since, as we shall subsequently see, 

 in a mixture of gases the passage of electricity is brought 

 about with different degrees of facility by the molecules of 

 different substances. 



In complete accordance with this are the results obtained 

 by M. Wulluer, who found, when he caused discharges to pass 

 through gases at a determined pressure, that now line and 

 now band spectra appeared, according as the equalization of 

 the electricities ensued as a tuft or a spark. The latter ex- 

 tended to only a few, the former to great number of particles. 

 As in general at each discharge of a definite collecting-appa- 

 ratus with an equal charge an equal quantity of electricity 

 passes, the temperature in the spark must be much higher 

 than in the tuft ; hence in the spark discharge a breaking-up 

 of the molecules into their parts is much more probable than 

 in the tuft discharge *. Moreover the behaviour of mercury 

 vapour proves that a band spectrum is not constantly com- 

 bined with the tuft discharge, nor a line spectrum with the 

 spark discharge. If mercury is introduced into wide tubes, 

 and heated, and the discharge of the induction-coil passed 

 through, the luminous particles of mercury fill the entire tube ; 

 and yet it shows only the line spectrum. Sodium behaves in 

 just the same manner. 



Disagreeing with the views we have now unfolded, some 

 have supposed that the differences between band and line 

 spectra could be explained by mere alterations of pressure or 

 equivalent alterations in the thickness of the luminous layer 

 of gas. This idea, however, is hardly tenable in the face of 

 Lockyer's experiments, who introduced into a glass tube, 5 

 feet in length, through which a slow current of hydrogen was 

 passing, a small piece of sodium, heated the entire tube to red- 

 ness, and let the light of an electric lamp pass through its 

 length, which he then examined with a spectroscope. The 



* To decide the question why in certain gases which principally give 

 line spectra (as chlorine, bromine, iodine) spark discharges are much more 

 readily formed than tuft discharges, further experiments are still required. 

 Probably a much higher potential is necessary for the discharge in them 

 than in the other gases, to which, perhaps, the shortness of the path-length 

 of chlorine points. 



