RELATIONS TO OTHER SCIENCES 151 



whether the differences which appear to exist are real and are 

 caused by the difference in the molecules which adhere to them, or 

 are due to the electrified centres which serve as the nuclei for them. 



(44) Mobility and Recombination. It is equally important to be 

 able, by measurement of mobility, to follow the modification which 

 a change of temperature produces in the size of the agglomeration 

 and to connect the ions observed at ordinary temperatures with 

 the incomparably more mobile ions which we observe in flames, and 

 which appear to be made up of single electrical centres, cathode cor- 

 puscles and perhaps a particles. 



The rate of recombination of ions is as yet not well known in 

 respect to the variations with pressure and temperature, although 

 it certainly plays an essential part in the phenomena of disruptive 

 discharge through gases at low pressures; it would be desirable if 

 this point were better fixed. 



(45) lonization by Impact. Every actual theory of the disrup- 

 tive discharge rests on the conception that the impact of an electri- 

 fied particle in sufficiently rapid motion against a molecule can cause 

 corpuscular dissociation. 



This idea was a natural consequence of the known fact that cathode 

 and Becquerel rays, made up of similar particles, make a gas through 

 which they pass a conductor. If the corpuscular dissociation pro- 

 duces in the gas, separated from the molecule, a cathode corpuscle 

 and a positive residue, these fragments can, if a sufficiently intense 

 electric field exists in the gas, acquire a velocity great enough to act 

 as /? or a rays and cause from point to point a rapid increase in con- 

 ductivity. 



Townsend has shown how this consequence is capable of exact 

 experimental verification, and he has found that between certain 

 limits of velocity, each impact between the cathode corpuscle and 

 a molecule results in a corpuscular dissociation of the same kind. 

 The velocity acquired ought not, however, to exceed a certain limit 

 beyond which the negative corpuscle or /? particle passes through 

 the atomic edifice without producing a sensible disturbance in it. 



In order that a disruptive discharge may exist without an external 

 cause to maintain the production of the first electrified centres, it is 

 necessary that the positive centres should be able, like the negative, 

 although with more difficulty, to produce corpuscular dissociation 

 at the moment of their impact with the molecules, as this latter 

 causes the conductivity produced in gases by the a rays. 



Townsend has been able, in support of this hypothesis, to deter- 

 mine the exact moment when the disruptive phenomenon is pro- 

 duced, and to analyze the mechanism of it. 



In addition to this fundamental conception of ionization by impact, 

 the theory of the disruptive discharge has yet much progress to make. 



