Electrification of Air, Vapour of Water, and other Gases. 483 



7. It* thin metal screens are used to sift the cathode rays the 

 luminescent phenomena change. The rajs of least penetrating 

 power appear to be most susceptible to magnetic and electrostatic 

 forces. The various constituents of a heterogeneous cathode beam 

 are emitted in various proportions at different degrees of exhaustion. 

 In the cathode rays emitted at higher degrees of exhaustion there is 

 a greater proportion of the less-deflectable rays. The least-deflect- 

 able rays are those which most readily penetrate through a per- 

 forated screen when that screen is itself made cathodic. 



When ordinary cathode rays fall upon a perforated screen which 

 is itself made cathodic, or are attempted to be passed through a 

 tubular cathode, there emerge beyond the screen or tube some 

 rays, here termed dia-cathodic rays, which differ from the ortho- 

 cathodic, and also from the para-cathodic rays. These dia-cathodic 

 rays are not themselves directly deflected by a magnet. They show 

 themselves as a pale blue cone or streak. Where they fall on the 

 glass they do not excite the ordinary fluorescence of the glass. The 

 dia-cathodic rays excite, however, a different or second kind of fluor- 

 escence ; the tint in the case of soda-glass being a dark orange. 

 Intervening objects in the beam or cone of dia-cathodic rays cast 

 shadows. The orange fluorescence evoked on soda-glass by the 

 dia-cathodic rays shows in the spectroscope the D lines of sodium 

 only. The shadows cast b}* dia-cathodic rays are not deflected by 

 the magnet, nor do they change their size when the object is elec- 

 trified. 



" Electrification of Air. of Vapour of Water, and of other Gases." 

 By Lord Kelvin, G.C.V.O., F.R.S., Magnus Maclean, 

 D.Sc, F.R.S.E., and Alexander Galt, B.Sc, F.R.S.E. 

 Received and Read June 17, 1897. 



(Abstract.) 



§ 1. In this paper the authors describe a long series of experi- 

 ments on the electrification of air and other gases with which they 

 have been occupied from May, 1894, up to the present time (June, 

 1897). Some results of earlier experiments, and of preliminary 

 efforts to find convenient methods of investigation, have from time 

 to time been communicated to the Royal Society, the British Asso- 

 ciation, and the Glasgow Philosophical Society.* 



* " On the Electrification of Air," by Lord Kelvin and Magnus Maclean, ' Roy. 

 Soc. Proc.,' May 31, 1894, and 'Phil. Mag.,' 1894; "Preliminary Experiments to 

 find if Subtraction of Water from Ah* electrifies it," by Lord Kelvin, Magnus 

 Maclean, and Alexander Gait, ' Brit. Assoc. Report,' 1894, p. 554 ; " Electrification 

 of Air and other Grases by bubbling through "Water and other Liquids," by Lord 

 Kelvin, Magnus Maclean, and Alexander Gralt, f Eoy. Soc. Proc.,' February, 1895; 



