The Theory of Electrons 567 



order of rising atomic weight, but leaving a gap where necessary to 

 bring similar elements into vertical columns, he obtained a periodic 

 table with natural vacancies to be filled as new elements were dis- 

 covered, and with a certain amount of flexibility at the ends of the 

 horizontal lines. From the position of the vacancies, the general 

 chemical and physical properties of undiscovered elements could be 

 predicted, and the success of such predictions gave a striking proof 

 of the usefulness of Mendele'eff's generalisation. 



When the chemical and physical properties of the elements were 

 known to be periodic functions of their atomic weights, the idea of a 

 common origin and common substance became much more credible. 

 Differences in atomic weight and differences in properties alike might 

 reasonably be explained by the differences in the amount of the 

 primordial substance present in the various atoms; an atom of 

 oxygen being supposed to be composed of sixteen times as much stuff 

 as the atom of hydrogen, but to be made of the same ultimate material. 

 Speculations about the mode of origin of the elements now began to 

 appear, and put on a certain air of reality. Of these speculations 

 perhaps the most detailed was that of Crookes, who imagined an 

 initial chaos of a primordial medium he named protyle, and a process 

 of periodic change in which the chemical elements successively were 

 precipitated. 



From another side too, suggestions were put forward by Sir 

 Norman Lockyer and others that the differences in spectra observed 

 in different classes of stars, and produced by different conditions in 

 the laboratory, were to be explained by changes in the structure of 

 the vibrating atoms. 



The next step in advance gave a theoretical basis for the idea of 

 a common structure of matter, and was taken in an unexpected 

 direction. Clerk Maxwell's electromagnetic theory of light, accepted 

 in England, was driven home to continental minds by the confirmatory 

 experiments of Hertz, who in 1888 detected and measured the electro- 

 magnetic waves that Maxwell had described twenty years earlier. 

 But, if light be an electromagnetic phenomenon, the light waves 

 radiated by hot bodies must take their origin in the vibrations of 

 electric systems. Hence within the atoms must exist electric charges 

 capable of vibration. On these lines Lorentz and Larmor have 

 developed an electronic theory of matter, which is imagined in its 

 essence to be a conglomerate of electric charges, with electro- 

 magnetic inertia to explain mechanical inertia 1 . The movement of 

 electric charges would be affected by a magnetic field, and hence the 



1 Larmor, Aether and Matter, Cambridge, 1900. 



