414 Prof. Pliicker on the Electrical Discharge 



The middle piece is connected with the end ones by means of 

 two tubes of different diameter, the narrower being a rather wide 

 thermom eter-tubc. 



64. One of the tubes just described contained fluoride of sili- 

 cium. The electrical light-stream was diffused through the wide 

 middle cylinder with a faded colour, while the light in the con- 

 necting tubes was far more intense, and more intense in the 

 narrower than in the vsddcr of these tubes. In the bulb contain- 

 ing the negative electrode, a beautiful atmosphere of magnetic 

 light was diffused. 



65. On bringing the hand towards the middle cylinder, the 

 light was deviated even while the hand was at a considerable 

 distance. On touching the cylinder with the finger, the light 

 was sometimes attracted to the point touched ; sometimes it was 

 repelled from this point, the light-current in the cylinder being 

 broken by dark strata. On inverting the RuhmkorfF's apparatus, 

 an inversion of the phajnomena invariably occurred. 



The very remarkable phsenomena described would be contra- 

 dictory if everything had been symmetrical on both sides of the 

 middle cylinder. The fi'ee tension at the two ends of the induc- 

 tion wire is very different ; but it is easy to convince oneself that 

 the cause could not consist in this. Nothing therefore remained 

 but to seek the cause in the difference of width of the two con- 

 necting tubes; and it was actually found that the light in the 

 middle cylinder was either attracted or repelled by the hand, accord- 

 ing as the positive light-current passed into this cylinden' by the 

 wider or narrower of the connecting tubes. 



Spectra of the Electrical Light-currents. 



66. It was to be confidently expected that, on looking through 

 a prism at a tube which showed clearly the black lines of Ruhm- 

 korff, these lines would be elongated to longitudinal ones tra- 

 versing all the colours of the spectrum. This conjecture I im- 

 mediately found confirmed on trying the first Geissler tubes in 

 the physical cabinet of this town (Bonn). I convinced myself 

 also on the same occasion that such tubes show beautiful spectra 

 of the most varied kind, according to the nature of the traces of 

 gases or vapours which they contain. All these spectra have 

 this in common, that the colours do not merge into one another 

 as in the ordinary solar spectrum. They are, on the contrary, 

 sharply demarcated; and the separate spaces of colour again are 

 also divided into well-defined lighter and darker strips. Each 

 gas, moreover, has a characteristic spectram. 



67. On discharging Ruhmkorff^s apparatus through one of 

 the tubes before described (63), not only the intensity, but the 

 colour of the light is different in different parts of the tube. 



