THE STRUCTURE OF THE WORLD-STUFF 275 



in highly rarefied gases — the so-called kathode rays. These had been 

 proved to be negatively charged particles, and were supposed to be 

 atoms or molecules of the gas. Thomson showed that they were 

 identical in their properties whatever was the gas used; and while he 

 was not able to determine their mass nor charge directly he succeeded 

 in measuring the ratio of their charge to their mass and showed that 

 either their charge was much greater than the atomic charge, or the 

 mass much less than that of an atom. Of these alternatives he chose 

 the latter as the simpler and more probable — a choice justified by 

 subsequent research. Later investigations showed that particles 

 identical in properties were emitted from metals under the in- 

 fluence of light, and from incandescent solids. If we assume that 

 the unit charge is carried by these particles the mass is calculated to 

 be about an eighteen-hundredth part of that of a hydrogen atom. 

 They are also shown to be enormously more concentrated, as their 

 diameter is estimated to be only a few millionths of the atomic 

 diameter. 



Let us note as the result of these investigations three highly sig- 

 nificant facts. First, that here we are presented with bodies smaller 

 — much smaller — than atoms. Secondly, that from whatever source 

 they are derived — gas, metal or hot lime — their properties are identical. 

 Finally that they are associated with a definite charge of electricity, 

 and that this charge is negative. Here is the electron, the atom of 

 electricity demanded by the theory of Lorentz. 



But what of the positive electron? Search for this has often been 

 made, but the mass of the positively charged particles has always proved 

 to be comparable with that of an atom. There are some indications 

 that positive electrons may be identical with positively charged atoms 

 of hydrogen or of the gas helium whose atomic mass is four times 

 that of hydrogen; but for the present we can say only that positive 

 electricity is never found dissociated from matter of atomic dimensions. 



These conclusions received confirmation from two other sources. 

 The year before Thomson's measurement of the electron marks the 

 discovery of radioactivity and the beginning of the researches which 

 speedily led to the discovery of radium and similar substances. The 

 properties of the rays emitted by these substances were carefully in- 

 vestigated and it was proved that they consist partly of negatively 

 charged particles identical with the electrons, and partly of positively 

 charged particles having a mass equal to that of a helium atom. 



The other confirmation to which I alluded, while less direct, pene- 

 trates even more deeply into the structure of matter. In this same 

 year (1896) a minute effect of magnetism upon light, discovered by 

 Zeeman, was shown to be completely concordant with the theory of 

 Lorentz, and to lead to the conclusion that the light of a luminous 

 vapor was due to negatively charged particles circling in or about the 



