a subject. I must exhibit a few diagrams, and briefly summarize a few main facts:—
Electrons have been shown to be shot off from any negatively charged body, especially from negatively electrified metals, when exposed to ultra-violet light.
When shot into a mass of air they dissociate and ionize that air for a time, and render it electrolytically conducting; also of course they can discharge positively electrified bodies themselves, and can thus be most readily detected in small numbers.
Electrons in orbital motion have been shown to constitute the mechanism by which atoms are able to radiate light; and a great mass of semi-astronomical facts concerning these orbits and their perturbations have been obtained by immersing the source of light in a strong magnetic field, and observing the minute but very definite changes of spectra thereby produced: a branch of science with which the names of H. A. Lorentz of Leyden, and Zeeman of Amsterdam, will be inseparably associated.
In all these and other ways the electron has become a familiar object. It constitutes the ionic charge of matter. Multiples of it, but no fractions, are possible. Its mass, its charge, and its speed have been frequently measured by different processes, and always with consistent results. It is the most definite and fundamental and simple unit which we know of in nature.
It has thus displaced the so-called atom of matter from its fundamental place of indivisibility. The atom of matter has been shown capable of losing an electron, of having at least one chipped off it. The electron has
