126 PHYSICS OF THE ELECTRON 



charges are distributed in a continuous manner in space, a mass of 

 gas electrically neutral could furnish a limited quantity of electricity 

 of each kind, decreasing with the time by progressive recombination 

 if one delays the establishment of the electric field in the gas. 



It is indeed necessary to admit, for the two electricities, a discon- 

 tinuous structure in order to allow their coexistence without com- 

 pletely neutralizing one another. The progressive recombination of 

 the charged particles or ions of two kinds would produce this neutral- 

 ization at the moment of their mutual collisions. 



The phenomena of the saturation current, of the limited quantity 

 of free electricity in a gas, were obtained under conditions most favor- 

 able to experimental study, when, immediately after the discovery 

 of Roentgen rays and like radiations, one had recognized their property 

 of making the gas they traversed a conductor of electricity. The 

 limited charge which we can extract from a gas thus modified, the 

 velocity, finite and easily measured, with which they move under 

 the action of an electric field, their progressive recombination, are 

 interpreted in an admirable manner on the hypothesis that the radi- 

 ations, as well as the intense heat agitations in a flame, dissociate 

 a certain number of the molecules of the gas into electrified parts 

 carrying charges of opposite, kinds. 



(8) The Phenomena of Condensation. We know how the phe- 

 nomena of condensation of supersaturated water vapor in the pre- 

 sence of a conducting gas, already referred by R. von Helmholtz to 

 the presence of ions, has given the preceding hypothesis a brilliant 

 confirmation. As a result of the researches of J. J. Thomson, Town- 

 send, C. T. R. Wilson, and H. A. Wilson, these droplets of visible 

 water, each formed by condensation around an electrified centre, 

 bring forward a tangible witness to the existence of these centres, and 

 furnish a means of measuring the individual charge, present on each 

 drop of water formed, and equal to about 3.4xlO~ 10 electrostatic 

 units of electricity according to the recent measurements of J. J. 

 Thomson and H. A. Wilson. 



The fundamental idea in these kinds of measurements, applied 

 for the first time by Townsend to the charged drops which are pro- 

 duced in the presence of saturated water vapor in recently prepared 

 gases, consists in deducing the mass of each drop from its velocity 

 of fall under the action of gravity by means of Stokes's formula, which 

 gives the frictional resistance of a sphere moving through a viscous 

 medium, and which expresses the velocity of fall in terms of the 

 radius of the drop and consequently of its mass. We can obtain 

 from this the electric charge carried by each drop if we know the 

 ratio of this charge to the mass. 



This ratio can be obtained, as was done by Townsend and J. J. 

 Thomson, by measuring or calculating the total mass of water carried 



