568 The Evolution of Matter 



discovery by Zeeman that the spectral lines of sodium were doubled 

 by a strong magnetic force gave confirmatory evidence to the theory 

 of electrons. 



Then came J. J. Thomson's great discovery of minute particles, 

 much smaller than any chemical atom, forming a common constituent 

 of many different kinds of matter 1 . If an electric discharge be passed 

 between metallic terminals through a glass vessel containing air at 

 very low pressure, it is found that rectilinear rays, known as cathode 

 rays, proceed from the surface of the cathode or negative terminal. 

 Where these rays strike solid objects, they give rise to the Rontgen 

 rays now so well known ; but it is with the cathode rays themselves 

 that we are concerned. When they strike an insulated conductor, 

 they impart to it a negative charge, and Thomson found that they 

 were deflected from their path both by magnetic and electric forces 

 in the direction in winch negatively electrified particles would be 

 deflected. Cathode rays then were accepted as flights of negatively 

 charged particles, moving with high velocities. The electric and 

 magnetic deflections give two independent measurements which 

 may be made on a cathode ray, and both the deflections involve 

 theoretically three unknown quantities, the mass of the particles, 

 their electric charge and their velocity. There is strong cumulative 

 evidence that all such particles possess the same charge, which is 

 identical with that associated with a univalent atom in electrolytic 

 liquids. The number of unknown quantities was thus reduced to 

 t W o— the mass and the velocity. The measurement of the magnetic 

 and electric deflections gave two independent relations between the 

 unknowns, which could therefore be determined. The velocities of 

 the cathode ray particles were found to vary round a value about 

 one-tenth that of light, but the mass was found always to be the same 

 within the limits of error, whatever the nature of the terminals, of the 

 residual gas in the vessel, and of the conditions of the experiment. 

 The mass of a cathode ray particle, or corpuscle, as Thomson, adopting 

 Newton's name, called it, is about the eight-hundredth part of the 

 mass of a hydrogen atom. 



These corpuscles, found in so many different kinds of substance, 

 are inevitably regarded as a common constituent of matter. They 

 are associated each with a unit of negative electricity. Now elec- 

 tricity in motion possesses electromagnetic energy, and produces 

 effects like those of mechanical inertia. In other words, an electric 

 charge possesses mass, and there is evidence to show that the effective 

 mass of a corpuscle increases as its velocity approaches that of light 

 in the way it would do if all its mass were electromagnetic. We 



1 Thomson, Conduction of Electricity through Gases (2nd edit.), Cambridge, 

 1906. 



