instead of two, showing that in these gases we have atoms but no molecules.
But when we analyse in this way a gas through which an electric discharge is passing, we find along with the atoms and molecules particles of an altogether different type; these particles are always charged with negative electricity, and their mass is an exceedingly small fraction, 1/1700, of that of the smallest atom known, the atom of hydrogen. They are so small that their volume bears to that of the atom much the same proportion as that between a small pellet and this room. These particles are called electrons or corpuscles, and no matter what the nature of the gas may be, whether it is hydrogen, helium, or mercury vapour, the electrons or corpuscles remain unchanged in quality; in fact, there is only one kind of electron, and we can get it out of every kind of matter. The conclusion is irresistible that the electron or corpuscle is a constituent of every atom, and that we are able, by forces which we have even now at our command, to detach it from the atom.
Though the electrons were first detected under the somewhat artificial and sophisticated condition of a rarified gas traversed by an electric current, yet, as so often happens in such cases when once they had been detected, they were found to be of quite common occurrence, and to occur in many familiar phenomena. They are found, for example, round a red-hot piece of metal, the filament of an electric lamp gives out large quantities; they come out of metals, whether hot or cold, when these are reflecting ultra-violet light; they are given out spontaneously by radio-active substances; and Haber has described experiments which indicate that they are given out during some chemical reactions. There are, however, many chemical reactions which are not accom-
