electricity; so that if the atoms of all elements are made up of these constituents there is no room for the existence of an atom lighter than hydrogen, such as that which has sometimes been suspected to exist in the sun's corona. The properties of hydrogen are well known and show no very exceptional simplicity; thus, for example, one of its spectra—the second spectrum—is so complicated that many thousand different lines have been detected, and apparently there is no simple relation between the frequencies of the lines to indicate that they are the members of a single series like the lines in the first spectrum. Is it likely, it may be urged, that such a simple structure as a single electron and one positive charge could give rise to a complication as great as this? But is the system so very simple after all? We must distinguish between arithmetical and physical simplicity. The electron and the positive charge produce an electric field all round them, and an electric field is probably a very complicated piece of mechanism. We may picture it in this case as consisting of a large number of lines of force, with one end on the electron and the other on the positive charge, spreading out into the space round the atom, and we may also suppose that these lines of force may move about even though their ends are at rest, and thus vibrate independently of the electrons. We can easily realize that a bundle of lines of force of this kind could vibrate in a very great number of ways, far more than would be necessary to account for the most complicated spectrum yet observed.
Before we can get very far in explaining the structure of the atom, we shall, I am convinced, have to deal with the question of the structure of the electric field.
It is, I think, possible that an atom may be able to give out vibrations of almost any period if these are
