vi PEEFACE 



experiments on ultra-violet light which were made in 

 1888, as considerable increases in the current of electri- 

 city through a gas were obtained with small voltages ; 

 but this method of explaining these experiments does 

 not seem to have been suggested until (Nature, August 9, 

 1900) the theory had been substantially verified by the 

 results of experiments on the conductivity produced by 

 Eontgen rays. The explanation previously given of 

 Stoletow's experiments was founded on a theory of 

 electrical surface layers, but the fatal objection to it is 

 that it fails to explain the fact that the current in the 

 gas increases in geometric proportion as the distance 

 between the plates increases in arithmetic proportion 

 when the electric force remains constant. The agree- 

 ment on the other hand between the experiments and 

 the numbers calculated on the collision theory is so 

 accurate that the phenomena are now attributed to the 

 effects produced by collisions. 



It is obvious from the large effects that may be 

 obtained by the multiplication of ions with comparatively 

 small electric forces that the process of ionization by 

 collision is of fundamental importance in the develop- 

 ment of large currents, and affords an explanation of 

 many phenomena in connection with the discharge of 

 electricity through gases. In particular, I may mention 

 the application of these principles to account for spark- 

 ing in gases. The exact value of the sparking potential, 

 agreeing with the experimental determinations within a 

 few per cent., may be calculated for a uniform field for 

 different pressures of the gas, and the leading features of 

 the more complicated phenomena obtained with large 

 currents may also be accounted for. 



The various points in connection with the theory which 



