THE ELECTRON— COMPTON 207 



The existence of electrons had been foreshadowed for a century by 

 the facts of electrolysis, which led Davy and Berzelius to conclude 

 that chemical forces were electrical in nature, and Faraday to conclude 

 that electric charges exist only in multiples of some fundamental unit. 

 For chemical acids and salts, dissolved in water, tend to spKt up into 

 ions, that is, atoms or groups of atoms which move in an electric field 

 in such directions as to indicate that they carry either positive or 

 negative electric charges. Furthermore, it is found that the amounts 

 of these ions which carry equal amounts of electricity are exactly 

 proportional to the chemical combining weights of the ions. Faraday 

 saw that this fact would be simply explained by assuming that every 

 ion carries a charge proportional to its chemical valency, that is, the 

 valency times a fundamental unit charge. But Faraday could not, 

 from these facts, deduce the size of this unit of charge; he could only 

 state the ratio of tliis charge to the mass of the chemical substance 

 with which the charge was associated. Hydrogen, being the Ughtest 

 of all ions, had of all known substances, therefore, the largest value of 

 this ratio of charge to mass. 



The first real evidence of particles of larger ratio of charge to mass 

 than hydrogen ions came from the field of optics. Ever since Max- 

 well's equations of electromagnetism had predicted the existence of 

 electromagnetic waves with the velocity of light, and Hertz, 17 years 

 later, had discovered them experimentally, physicists had felt sure 

 that light must be caused by some sort of oscillations of electricity 

 within atoms. But only the vaguest and most unsatisfactory specu- 

 lations, such as whirling vortices or pulsating spheres of electricity, 

 had been suggested. 



In 1896, however, Zeeman tried the experiment of examining the 

 spectrum of a light source placed in a strong magnetic field, and dis- 

 covered that the spectrum lines thus became spUt into components of 

 slightly differing wave length, and that these components of the light 

 showed characteristic types of polarization depending on the direction 

 in which the light emerged from the magnetic field. Almost at once, 

 in January 1897, Lorentz showed that this experiment proved that 

 light is caused by the oscillation of electric charges, the motions of 

 wliich are afl'ected by the magnetic field in the manner required to 

 explain Zeeman's experiments. This much was not unexpected, but 

 what was startling was Lorentz's proof that the Zeeman efl^ect could 

 only have been produced by electrified particles whose ratio of charge to 

 mass is nearly 2,000 times larger than that of a hydrogen ion, and whose 

 mass is therefore presumably nearly 2,000 times lighter than hydrogen. 



Almost at once this conclusion was confirmed in a more dramatic 

 and understandable way by J. J. Thomson, the then youthful director 

 of the Cavendish Laboratory. But let me first pick up this thread 

 of the story a little farther back. 



