As concerns the spontaneous transmutations undergone by the radioactive elements, the facts appear to indicate (or, at least, can be brought into some sort of order by supposing) the atom to consist of a central nucleus and an outer shell, as suggested by Sir Ernest Rutherford. The nucleus may be compared to the sun of a solar system. It is excessively small, but in it the mass of the atom is almost entirely concentrated. It is positively charged, the charge being neutralised by that of the free electrons which revolve like planets about it, and which by their orbits account for the volume of the atom. The atomic weight of the element depends upon the central sun; but the chemical properties of the element are determined by the number of electrons in the shell; this number is the same as that representing the position of the element in the periodic system. Radioactive change originates in the atomic nucleus. The expulsion of an α-particle therefrom decreases the atomic weight by 4 units, necessitates (since the α-particle carries two positive charges) the removal of two electrons from the shell in order to maintain electrical neutrality, and hence changes the chemical nature of the body, transmuting the element into one occupying a position two places to the left in the periodic system (for example, the change of radium into niton). But radioactivity sometimes results in the expulsion of a β-particle from the nucleus. This results in the addition of an electron to the shell, and hence changes the chemical character of the element, transmuting it into one occupying a position one place to the right in the periodic system, but _without altering its atomic weight_. Consequently, the expulsion of one α- and two β-particles from the nucleus, whilst decreasing the atomic weight of the element by 4, leaves the number of electrons in the shell, and thus the chemical properties of the element, unaltered. These remarkable conclusions are amply borne out by the facts, and the discovery of elements (called “isobares”) having the same atomic weight but different chemical properties, and of those (called “isotopes”) having identical chemical characters but different atomic weights, must be regarded as one of the most significant and important discoveries of recent years. Some further reference to this theory will be found in §§ 77 and 81: the reader who wishes to follow the matter further should consult the fourth edition of Professor Frederick Soddy’s _The Interpretation of Radium_ (1920), and the two chapters on the subject in his _Science and Life_ (1920), one of which is a popular exposition and the other a more technical one.