RELATIONS TO OTHER SCIENCES 127 



by the droplets, considered as uniform, as well as the total quantity 

 of electricity carried by the ions which have served as centres for 

 the formation of the drops. The charge thus obtained by Townsend 

 was found to be 3X 10~ 10 electrostatic units for each centre in the 

 case of gases of electrolysis, and to 6.5 X 10~ 10 by J. J. Thomson 

 from the first series of measurement on gases ionized by Roentgen 

 rays. 



H. A. Wilson obtained the ratio of charge to the mass of a drop 

 more simply by comparing the velocity of fall under the action of 

 gravity alone with the velocity of fall in a vertical electric field. He 

 obtained thus directly the ratio sought for. This method has the 

 advantage of showing that the electric charges are really carried by 

 the drops, and of separating those drops which carry a single ele- 

 mentary charge from those which, by diffusion of the ions toward 

 one another, carry a double or triple charge. 



Wilson gives as the mean result of his measurements 3. IX 10~ 10 , 

 a value very near to that of Townsend. 



A second series of experiments by Professor J. J. Thomson, in 

 which he used radioactive substances as sources of ionization more 

 constant than the Crookes tube, and in which he took care to cause 

 the drops to form on all the ions present in the gas, by producing a 

 supersaturation of the water vapor by a rapid expansion of sufficient 

 magnitude to cause the condensation on the ions of both kinds, gave 

 as a mean result 3.4 X 10~ 10 , a value in complete agreement with 

 the other two experimenters. The principles of thermodynamics 

 account perfectly for the influence of electrified centres on the con- 

 densation of water vapor : the electric charge of a drop in fact dimin- 

 ishes the pressure of water vapor in equilibrium with it. Moreover, 

 the least supersaturation found necessary, by C. T. R. Wilson, for 

 the formation of drops of water on the ions, which are the same what- 

 ever may be the means of producing them (Roentgen rays, Becquerel 

 rays, brush discharge, action of ultra-violet light on metal negatively 

 charged), allows us by purely thermodynamical reasoning to calcu- 

 late approximately the charge carried by each of the ions, and this 

 calculation, entirely distinct from direct measurement, gives in the 

 case of the positive centres a value of 4X 10~ 10 E. S. units. 



(9) The Radiation Integral. More surprising still is the result 

 recently obtained by H. A. Lorentz, who succeeded in basing a pre- 

 cise measurement of the elementary charges carried by the electrified 

 centres present in metals on the experimental study of the radia- 

 tion integral or black body radiation. 



We will see how the emission and absorption of heat- and light- 

 waves by matter are dependent on the presence in it of electrons 

 in motion. The ratio, for a radiation of given wave-length, between 

 the emissive and absorptive power, a ratio independent of the nature 



