1898.] 



Electrification by Ultra-violet Light. 



415 



These experiments on the variation of the velocity with the 

 pressure afford very strong evidence that the carrier of the charge 

 is of molecular dimensions. 



According to the kinetic theory of gases the velocity of a 

 charged particle of molecular dimensions varies inversely as the 

 pressure 1 . If however the particle is large compared with a 

 molecule, the velocity of the carrier is dependent only on the 

 viscosity of the gas which is independent of the pressure within 

 limits. The conclusion to be drawn from these results is that the 

 carriers must be very small and comparable in size with a 

 molecule. 



Many of the interesting results of Stoletow 2 on the relation 

 between the current and the E.M.F. for different pressures on 

 ultra-violet light conduction can be generally explained on the 

 hypothesis that the rate of production of the carriers is a function 

 of the pressure, and that the rate of escape of the negative ions 

 is dependent on the velocity of the carrier. 



It is not my intention here to enter into a detailed discussion 

 of his valuable results but the fact that the current through the 

 gas at very low pressures (about "002 mm.) is independent of the 

 pressure seems to show that either there is a slight disintegration 

 of the electrodes by ultra-violet light or that the presence of 

 mercury vapour is responsible for the action. 



Velocity in different gases. 



The velocity of the carrier depends on the gas surrounding the 

 plates. Hydrogen and carbonic acid were used and were well 

 dried before passing into the apparatus. 



These results were obtained with the use of the arc light when 

 the heating of the gas prevented the velocities being obtained 

 with as much accuracy as desired. The hydrogen ion in hydrogen 



1 J. J. Thomson. Brit. Assoc. Report, 1894. 



2 Journ. de Phys. ix. p. 468. 1890. 



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