radioactive matter, or of inducing similar effects in ordinary matter, does not at present seem at all promising.
We have seen that in recent years a number of methods have been devised for determining with precision the actual weight of any atom of matter. If it be assumed that in the solid state the atoms, or molecules, of matter are in close contact, it is a simple matter to deduce the diameter of the atom. This varies slightly for different atoms, but on an average comes out to be about one hundred-millionth of a centimeter. It is necessary, however, to be cautious in speaking of the diameter of the atom. The term "diameter of the sphere of action" of the atom is preferable, for it is not at all certain that the actual atomic structure is nearly so extensive as the region through which the atomic forces are appreciable.
Even before the discovery of the electron, the general idea had been suggested that the atom was an electrical structure composed of negatively and positively charged particles held in equilibrium by electrical forces. Such ideas had been proposed and developed by Larmor and Lorentz in order to explain the electrical and optical properties of the atom. The proof that the negative electron was an independent unit of the structure of the atom gave a great impetus to the formation of more concrete ideas on atomic structure. There was one important difficulty, however, that arose at the outset. While negative electricity had been shown to exist in independent units of very small apparent mass, the corresponding unit of positive electricity was never found associated with a mass less than the atom of hydrogen. All attempts to show the existence of a positive electron of small mass, which is a counterpart of the negative electron, have resulted in failure, and it seems doubtful whether such a positive electron exists. The rôle played by positive electricity in the atom was thus a matter of conjecture. In a paper called "Æpinus Atomized," the late Lord Kelvin considered an atom to consist of a uniform sphere of positive electrification, throughout which negative electricity was distributed in the form of discrete electrons. In order to make such an atom electrically neutral, it is, of course, necessary that the positive charge should be equal and opposite to the charge carried by the electrons. This idea of the structure of the atom was taken up and developed with great mathematical skill by Sir J. J. Thomson. He investigated the constitution of atoms containing different numbers of electrons, and showed that such model atoms possessed properties very similar to those shown by the actual atoms. The Thomson atom proved for many years very useful in giving a concrete idea of the possible structure of the atom, and had the great advantage of being amenable to calculation.
The rapid advance of science in the last decade has provided us