while the two possess the same charge, the inertia or mass which, so far as we know, is inseparably associated with a positive unit is that of a hydrogen atom, while that inseparably associated with the negative unit is 1/1.760th as much. The negative units, furthermore, or electrons, are so small in volume and are separated from one another within the atom by so large spaces, that one of them can shoot through hundreds and thousands of atoms without hitting anything or doing anything whatever to these atoms. Its diameter is about one one-hundred-thousandth of that of the atom. It is the smallest thing we know anything about so far—probably the smallest thing in existence. Such an enumeration of properties is as near to a definition of electricity as we can come now or are ever likely to be able to come. For, since electricity is the most fundamental thing thus far known to us, it is obviously incapable of definition in terms of anything more fundamental. Its elementary unit, according to the best determination which we have yet been able to make, is 4.80 times 10-10 so called electrostatic units, a quantity so small that the electrical charge produced by a single stroke of a cat's back contains billions of them, while the number which courses each second through the filament of a common 16 candle power incandescent lamp is about a billion billion. The electron is thought by many reputable scientists of the present day to be the primordial thing out of which all matter is built up, so that from this point of view the different atoms of ordinary matter are merely different groupings of these fundamental electrical units.
Turning next to the kinetic theory of matter, what have the present experiments to do with it? There are three different ways in which they bring to it powerful support. When these experiments were begun it was anticipated that positively charged ions would be caught by negatively charged oil drops and negatively charged ions by positively charged oil drops, but it had not been predicted that positively charged drops would catch positive ions and negatively charged drops negative ions; for since electrical charges of like sign always repel each other, it might be thought that positive drops would push away positive ions and negative drops negative ions. As a matter of fact, however, positive ions were found to be caught by positive drops about as readily as the negative ions and vice versa. The above table shows several catches of this kind. Whence, then, do positive ions obtain the energy which enables them to push themselves up to the surface of a positive drop against the electrical repulsion existing between the two? This energy could not have been obtained from the field, since the capture of the ions occurred when the field was not on. It could not have been obtained from any explosive process which frees the electron from the molecule at the instant of ionization, since in this case, too, ions would have been caught as well, or nearly as well, when the field was on as when
