412 Prof. Pliicker on the Electrical Discharge 



a cylindrical envelope of light of the well-known light blue colour, 

 surrounding the narrower tube. When this system of tubes was 

 laid in an equatorial position upon the two approximated arma- 

 tures, and the electro-magnet excited, no action was observed 

 upon the cylindrical envelope of fluorescent light. 



As yet, therefore, the magnetic light appears only to occur 

 under very limited circumstances ; and electrical conditions ap- 

 pear essential to its production. 



Electro -magnetic Action upon the Electric Light-current. 



57. In the observations already described (9 to 15), it has 

 been shovm that, apart from secondary phsenomena, the mag- 

 net acts upon the electric light-current in tubes of rarefied 

 gases in the same manner as upon the electric current in me- 

 tallic conductors. For although hitherto practical difficulties 

 have prevented the construction of apparatus to reproduce with 

 the electric light the rotation of Barlow's wheel and similar phe- 

 nomena, still, even in the incomplete results obtained, we recog- 

 nize a new confirmation of the above assertion. 



58. The following observation may be not unworthy of being 

 mentioned. The fact that, in consequence of the magnetic 

 attraction and repulsion of the light-current, the latter was 

 sometimes increased, sometimes diminished, in intensity in dif- 

 ferent places, was at once and clearly recognized by me. Sub- 

 sequently, however, by the approximation of a tube through 

 which the discharge passed to the magnet, I have observed the 

 light-current become weaker and entirely disappear, and at the 

 same time the beating of the hammer in Ruhmkorfi^'s apparatus 

 underwent a change, showing an alteration in the conditions of 

 conduction. 



59. Further, the different kind of action upon differently- 

 coloured light, which is originally united in the electrical light- 

 current, deserves om* attention. This occurs, for instance, in 

 tubes containing hydrobromic or hydriodic acids ; and the dif- 

 ferent lights correspond probably with the different ponderable 

 substances in the tubes. The original light is thereby decom- 

 posed into light of different colour, which, under certain circum- 

 stances, appears as variously-coloured flashes of lightning (18). 

 Dependent upon this is also the phsenomenon, to be afterwards 

 described, that in narrow tubes the colour of the discharge-cur- 

 rent is temporarily changed by the magnet. 



60. The electric light of the discharge-current in Geissler's 

 tubes extends also to those parts which do not lie in the path of 

 discharge. This occurs, for instance, in the long narrow tubes 

 which, having been fused on to any point of the principal tube, 

 and having been used as evacuating tubes, have been fused off 



