52 



SCIENCE. 



[N. S. Vol. XXIII. No. 576. 



countered in a large number of more re- 

 condite phenomena, and often apparently 

 free, i. e., not attached to any matter in 

 the ordinary sense, are the electrons. Thus 

 physicists have been led to return in a cer- 

 tain sense to atomistic conceptions, without 

 however, abandoning the idea of the propa- 

 gation of electric, magnetic and optical 

 disturbances through the ether in time. 

 Lord Kelvin, in his Baltimore lectures in 

 1884, gave expression to this tendency so 

 largely developed in the succeeding twenty 

 years. The very first words of his first 

 lecture are: "The most important branch 

 of physics which at present makes demands 

 upon molecular dynamics seems to me to be 

 the wave theory of light." 



Without discussing the experimental 

 basis of the electron theory it must here 

 suffice to say that on the one hand the dis- 

 persion and diffraction of light, on the 

 other the phenomena exhibited by cathode 

 and canal rays, Rontgen rays, the Becquerel 

 rays emitted by radium, etc., all find their 

 ready interpretation in this theory.* At 

 the same time, the electron theory as de- 

 veloped by Lorentz, Wiechert, Drude and 

 others seems to furnish an excellent basis 

 for the whole theory of electricity, mag- 

 netism and light.^ Indeed, attempts have 

 already been made of interpreting matter 

 itself as an electromagnetic phenomenon 

 and of explaining gravitation by means of 

 this electron theory of matter. 



* See, for instance, W. Kaufmann, Physikalische 

 Zeitsohrift, 3 (] 901), pp. 9 sq., translated in The 

 Electrician, 48 (1901), pp. 95-97; 0. Lodge, 

 Journal of the Institute of Electrical Engineers, 

 32 (1902-3), pp. 45-115; P. Langevin, Revue 

 gen4rale des sciences, 16 (1905), pp. 257-276; 

 H. A. Lorentz, ' Ergebnisse und Probleme der 

 Elelctronentheorie,' Berlin, Springer, 1905. 



° It will be sufficient to mention Lorentz's arti- 

 cles in the Eneylclopiidie der mathematischen 

 Wissenschaften, V., 13, 14, where full refei'ences 

 are given, and to the systematic work of M. 

 Abraham, ' Theorie der Elektrizitiit,' I. (1904), 

 II. (1905), Leipzig, Teubner. 



It should be observed that the electron 

 theory does not upset that beautiful struc- 

 ture known as the electromagnetic theory 

 of Maxwell and Hertz. It merely modifies- 

 it to a certain extent so as to give a more 

 detailed account of electromagnetic phe- 

 nomena in ordinary matter. It is related 

 to the older theory somewhat as the kinetic 

 theory of gases is related to the theory of 

 heat and of ordinary matter in general. 

 The kinetic theory assumes the laws of 

 ordinary mechanics for the motion of the 

 hypothetical molecule and then tries to de- 

 termine the average effects arising from the 

 motion of very large numbers of such 

 molecules, these averages being the only 

 thing actually observable. Similarly the 

 electron theory must begin with postulating 

 laws of motion for the single electron in 

 the electromagnetic field and try to deduce 

 the average effects due to swarms of elec- 

 trons; the comparison of these calculated 

 average effects with the results of observa- 

 tion and experiment must serve as verifica- 

 tion of the postulated laws. 



If, then, observation leads us to the as- 

 sumption that electric charges may exist 

 and move about without being attached to, 

 or carried by," ordinary matter, what are 

 the 'laws of motion' of such an electron? 

 As the moving object is not ordinary matter 

 we must not be astonished to find that New- 

 ton's laws of motion can not be applied 

 blindly. The electron moves according to 

 the laws of electrodynamics. We are thus 

 confronted with the question as to the rela- 

 tion of the fundamental postulates of this 

 science to those of ordinary mechanics. 



An electric charge at rest manifests its 

 presence only by the field which it excites 

 in its vicinity, by the sheaf of lines of force 

 issuing from it. To take a simple concrete 

 example, a small charged sphere has lines 

 of force radiating as if from its center in 

 all directions, and the electric force, or in- 

 tensity of the field, ■ iE, at any point P, at 



