Mr. J. C. McLennan. 



(5) biscussion of methods for measuring the ionisations produced 



in different gases. 



(6) Description of apparatus used. 



(7) Explanation of the method adopted for comparing ionisations. 



(8) lonisation in different gases at the same pressure. 



(9) lonisation in air at different pressures. 



• (10) lonisation in a gas independent of its chemical composition. 

 (11) Comparison of ionisations produced by cathode and by Eontgen 

 rays. 



.(12) Summary of results. 



The tube used for the production of cathode rays was similar in 

 form to that devised by Lenard,"^ but, as the brass plate carrying the 

 aluminium window was found to act very well as an anode, the 

 ordinary positive electrode in his apparatus was dispensed with. 



The paper commences with a series of experiments illustrating the 

 conductivity produced by cathode rays, and the various phenomena 

 met with are shown to be fully explained on the supposition that 

 positive and negative ions are produced in a gas by the radiation, and 

 that the conductivity arises from the motion of these ions under the 

 action of an electric force. 



This view of the conductivity is also shown to explain the loss of 

 charge sustained by a conductor upon which the rays fall. Lenard's 

 experiments! in this connection were repeated, and, contrary to his 

 observations, negative charges were not in any case found to be com- 

 pletely dissipated by the rays, but were reduced, at atmospheric pres- 

 sure, to small limiting values of the order of 0*25 volt. These values 

 were found to be slightly increased when a blast was directed so as to 

 remove the air close to the conductor, and when the latter was placed 

 in a vacuum, the limiting charge rapidly assumed a very high value. 

 The value of the limiting charge was found to be affected also by the 

 proximity of conductors whose potentials were different to that of the 

 one upon which the rays fell. 



Conductors initially unelectrified gained the limiting negative charge 

 under the action of the rays, and positive electrifications upon con- 

 ductors, surrounded by air at normal pressure, were completely dis- 

 charged. 



The explanation offered regarding this limiting or steady state is 

 that it represents a condition of equilibrium in which the electric con- 

 vection by the rays to the conductor is just equal to the conduction by 

 the ionised gas away from it. 



It has been thought by some that the ionisation under consideration 

 may be due to Eontgen rays sent out from the aluminium window at 



* ' Wied. Ann.,' vol. 51, 1894, p. 225. 

 t ' Wied. Ann./ vol. 63, 1897., p. 253. 



