392 Conduction in Solutions and Gases, 



investigations in this direction. " The results connected with 

 the different conditions of positive and negative discharge," he 

 wrote,* " will have a far greater influence on the philosophy of 

 electrical science than we at present imagine." 



Twenty more years, however, passed before another notable 

 advance was made. That a subject so full of promise should 

 progress so slowly may appear strange ; but one reason at any 

 rate is to be found in the incapacity of the air-pumps then in 

 use to rarefy gases to the degree required for effective study 

 of the negative glow. The invention of Geissler's mercurial 

 air-pump in 1855 did much to remove this difficulty; and it 

 was in Geissler's exhausted tubes that Julius Plticker,t of Bonn, 

 studied the discharge three years later. 



It had been shown by Sir Humphrey Davy in 1821 J that 

 one form of electric discharge namely, the arc between carbon 

 poles is deflected when a magnet is brought near to it. 

 Pliicker now performed a similar experiment with the vacuum 

 discharge, and observed a similar deflexion. But the most 

 interesting of his results were obtained by examining the 

 behaviour of the negative glow in the magnetic field ; when 

 the negative electrode was reduced to a single point, the whole 

 of the negative light became concentrated along the line of 

 magnetic force passing through this point. In other words, 

 the negative glow disposed itself as if it were constituted of 

 flexible chains of iron filings attached at one end to the 

 cathode. 



Pliicker noticed that when the cathode was of platinum, 

 small particles were torn off it and deposited on the walls of 

 the glass bulb. " It is most natural," he wrote, " to imagine 

 that the magnetic light is formed by the incandescence of these 

 platinum particles as they are torn from the negative electrode." 

 He likewise observed that during the discharge the walls of 



* Exper. Res., 1523. 



| Ann. d. Phys. ciii (1858), pp. 88, 151 ; civ (1858), pp. 113, 622 ; cv (1858), 

 p. 67; cvii (1859), p. 77. Phil. Mag. xvi (1858), pp. 119, 408; xviii (1859), 

 pp. 1, 7. 



J Phil. Trans., 1821, p. 425. 



