considered as definitely established. It is very remarkable that a chemically inert element like helium should play such a prominent part in the constitution of the atomic systems of uranium and thorium and radium. It may well be that this property of helium of forming complex atoms is in some way connected with its inability to enter into ordinary chemical combinations. It must not be forgotten that uranium and thorium and each of their transformation products must be regarded as distinct chemical elements in the ordinary sense. They differ from ordinary elements in the comparative instability of their atomic systems. The atoms break up spontaneously with great violence, expelling in many cases an atom of helium at a high speed. All the evidence is against the view that uranium or thorium or radium can be regarded as an ordinary molecular compound of helium with some known or unknown element, which breaks up into helium. The character of the radioactive transformations and their independence of temperature and other agencies have no analogy in ordinary chemical changes. Apart from their radioactivity and high atomic weight, uranium, thorium, and radium show no specially distinctive chemical behaviour. Radium for example is closely allied in general chemical properties to barium. It is consequently not unreasonable to suppose that other elements may be built up in part of helium, although the absence of radioactivity may prevent us from obtaining any definite proof. On this view, it may prove significant that the atomic weights of many elements differ by four--the atomic weight of helium--or a multiple of four. Time is too limited to discuss in greater detail these and other interesting questions which have been raised by the proof of the chemical nature of the α-particle. [Footnote 1: E. Rutherford, Uranium radiation and the electrical conduction produced by it, _Phil. Mag._, 47 (1899) 116.] [Footnote 2: E. Rutherford and F. Soddy, _Phil. Mag._, 5 (1903), 106.] [Footnote 3: E. Rutherford and F. Soddy, _Phil. Mag._, 4 (1902), 582.] [Footnote 4: E. Rutherford and F. Soddy, _Phil. Mag._, 5 (1903), 453.] [Footnote 5: E. Rutherford, letter in _Nature_, _69_ (Aug. 20, 1903).]