568 The Evolution of Matier 
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’. 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 Réntgen 
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 which 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 
two—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. 
