394 Conduction in Solutions and Gases, 



rarefied gases were investigated by Sir William Crookes. 

 Influenced, doubtless, by the ideas which were developed in 

 connexion with his discovery of the radiometer, Crookes,* like 

 Varley, proposed to regard the cathode rays as a molecular 

 torrent : he supposed the molecules of the residual gas, coming 

 into contact with the cathode, to acquire from it a resinous charge, 

 and immediately to fly off normally to the surface, by reason of 

 the mutual repulsion exerted by similarly electrified bodies. 

 Carrying the exhaustion to a higher degree, Crookes was enabled 

 to study a dark space which under such circumstances appears 

 between the cathode and the cathode glow ; and to show that at 

 the highest rarefactions this dark space (which has since been gene- 

 rally known by his name) enlarges until the whole tube is occupied 

 by it. He suggested that the thickness of the dark space may 

 be a measure of the mean length of free path of the molecules. 

 " The extra velocity," he wrote, " with which the molecules 

 rebound from the excited negative pole keeps back the more 

 slowly moving molecules which are advancing towards that pole. 

 The conflict occurs at the boundary of the dark space, where the 

 luminous margin bears witness to the energy of the collisions."f 

 Thus according to Crookes the dark space is dark and the 

 glow bright because there are collisions in the latter and not in 

 the former. The fluorescence or phosphorescence on the walls 

 of the tube he attributed to the impact of the particles on 

 the glass. 



Crookes spoke of the cathode rays as an " ultra-gaseous " or 

 " fourth state " of matter. These expressions have led some 

 later writers to ascribe to him the enunciation or prediction of 

 a hypothesis regarding the nature of the particles projected from 

 the cathode, which arose some years afterwards, and which we 

 shall presently describe ; but it is clear from Crookes' memoirs 

 that he conceived the particles of the cathode rays to be 

 ordinary gaseous molecules, carrying electric charges ; and by 



* Phil. Trans, clxx (1879), pp. 135, 641 ; Phil. Mag. vii (1879), p. 57. 

 t Phil. Mag. vii (1879), p. 57. 



