June 10, 1904.] 



SCIENCE. 



897 



The electron itself, although it has some 

 very definite claims to objective existence, is 

 not an entirely clear idea. The electric stress 

 which radiates from an electron can, indeed, 

 be thought of in mechanical terms, and the 

 manner in which these lines of stress sweep 

 through space when the electron moves, and 

 the way in which they build up a mag- 

 netic field by their motion, can be thought of 

 in a precise way, but no one has at present any 

 definite idea of the nucleus of an electron nor 

 of the way in which the nucleus moves. The 

 mechanical analogue outlined below is mis- 

 leading in this respect in being free from 

 some of the essential difficulties which arise 

 in the electrical case. 



Professor Thomson's explanation of zero 

 valency (which is exhibited by such elements 

 as helium, neon, argon, etc.) does not appear 

 quite satisfactory. He represents zero valency 

 as an extremely evanescent and unstable case 

 of monovalency. 



I fail to see how, in Professor Thomson's 

 theory, to explain the rise of valency of a 

 given element by twos, for example the rise 

 of valency of nitrogen from 1 to 3 to 5. This 

 mode of rise of valency has suggested to 

 chemists the idea of the neutralization of 

 valency bonds by each other in pairs. How- 

 ever, this difficulty may be met, perhaps, by 

 using the notion of subsidiary groups of 

 electrons. 



On the other hand. Professor Thomson's 

 hypothesis, that the atom consists of a num- 

 ber of extremely minute negative electrons 

 moving about in a small spherical region 

 containing uniformly distributed positive 

 charge, meets a fundamental difficulty, namely, 

 that experiment has hitherto failed to give 

 any evidence of the existence of concentrated 

 positive charges corresponding to the exces- 

 sively concentrated negative charges which 

 constitute cathode rays. In conformity with 

 this hypothesis as to the structure of the atom, 

 the mass of an atom is to be taken as pro- 

 portional to the number of negative electrons 

 the atom contains, regardless of the extent 

 and value of the distribiited positive charge. 

 This is explained later in mechanical terms. 



A clear idea of the behavior of a system of 



electrons depends upon an understanding of 

 the dynamics of a single electron, and the 

 dynamics of an electron is very different from 

 the dynamics of a material particle. 



The dynamics of an electron depend pri- 

 marily upon the fact that kinetic energy is 

 associated with a moving electric charge inde- 

 pendently of the ' material ' mass of the body 

 upon which the charge resides. That is to 

 say, electric charge has inertia or mass. This 

 association of kinetic energy with an electric 

 charge is a phenomenon of the electric field, 

 not of the charge itself and the mode of 

 association is precisely (not necessarily ac- 

 curately) understood. Stated in terms of a 

 mechanical analogue, it is as follows: Im- 

 agine a great lake of jelly with a mass-less 

 cylinder pressed down upon its surface. Un- 

 derneath the cylinder the jelly will be under 

 stress and strain and this stress and strain 

 will represent energy, corresponding to the 

 purely electrical energy associated with the 

 electrical stress surroimding a charged body. 

 If this mass-less cylinder be rolled along over 

 the jelly surface the strain figure underneath 

 the cylinder will travel with the cylinder, and 

 each successive portion of the jelly will move 

 as it is twisted into conformity with the ap- 

 proaching strain figure and as it is again 

 twisted into its unstrained condition after the 

 strain figure is passed. This motion repre- 

 sents Tcinetic energy corresponding to the 

 magnetic energy of a moving electron, and the 

 strain figure, therefore, has inertia. 



If the cylinder is small in diameter the 

 strain which is associated with it is greatly 

 concentrated and for a given integral amount* 

 of stress or strain (given force pushing the 

 cylinder down) a much greater amount of 

 kinetic, energy would be associated with a 

 given velocity of the rolling cylinder, inas- 

 much as the successive portions of the jelly 

 would move with increased velocity as they 

 are twisted into conformity with the ap- 

 proaching strain figure, and as they are again 

 twisted into an unstrained condition after 

 the strain figure has passed. Therefore, a 

 strain figure having a given integral amount 

 of strain has greater and greater inertia the 

 more the strain figure is concentrated. At 



