PHYSICAL SCIENCE 45 



cases at all events, the ions which were effective 

 electrically were also the agents in chemical activity. 

 This discovery gave precision to the connection 

 long suspected between electric forces and chemical 

 affinity, and led to a great development in the theory 

 of physical chemistry, which, during late years, has 

 been largely inspired by the ideas of the ionic hypo- 

 thesis. 



The conception of electric ions, originally due to 

 the investigation of the electrical properties of solu- 

 tions, has been used with striking success in the 

 explanation of the kindred phenomena of the con- 

 duction of electricity through gases. 



When the air or other gas in a glass vessel with 

 metallic electrodes is gradually pumped out, an elec- 

 tric spark passed between the electrodes becomes a 

 broadened band of luminosity, separated from the 

 negative electrode or cathode by a narrow, dark space. 

 As the exhaustion of the gas proceeds, this dark space 

 grows till it fills the vessel, the walls of which then 

 show green phosphorescent effects, and become the 

 source of the remarkable rays discovered by Rontgen 

 and now used with great success in surgery. 



At this stage the phosphorescence may be shown 

 to be due to the bombardment of the glass by streams 

 of minute particles, shot off from the cathode and 

 known as cathode rays. These particles may be 

 deflected by magnetic and also by electric forces, 

 and must therefore be charged electrically, the 



