panied by any emission of electrons. Whatever the source of the electrons may be, they are always the same; some may be moving faster than the others, but that is the only difference. By observing the behaviour of the electron under electric and magnetic forces, the values of its mass and electric charge—the quantities which determine its behaviour under specified conditions—have been measured; indeed, though the electron has only lately come under our notice, we know a good deal more about it than we do of many things which have been discovered centuries ago. One important result of these measurements is that the electron or corpuscle is of the same type when it is ejected with enormous velocities from radio-active substances, as when it oozes out of a hot body; this is very strong evidence that it cannot be broken up by any forces we can apply, as these would be insignificant in comparison with those called into play when it is ejected from radium. Since the electron can be got from all the chemical elements, we may conclude that electrons are a constituent of all atoms. We have thus made the first step towards a knowledge of the structure of the atom and towards the goal towards which since the time of Prout many chemists have been striving, the proof that the atoms of the chemical elements are all built up of simpler atoms—primordial atoms, as they have been called.
As we have proved that the atoms contain these electrons, the next step is to find out how many there are in any particular kind of atom. This was first done by the following method. When Röntgen rays fall on an electron, the rays are scattered just as light is scattered by the small particles of carbon in the smoke from a peat fire, or by the molecules of air in the upper regions of the atmosphere producing the blue of the sky; this, by
