their distinct physical and chemical properties. The extraordinary differences in properties which sometimes exist between a product and its parent substance are well illustrated by the comparison of radium and its product, the emanation. Radium is a solid element of atomic weight 226, which has chemical properties allied to barium but is capable of separation from it. The emanation is a heavy monatomic gas of atomic weight 222, which by its absence of chemical properties is allied to the well known group of rare gases, helium, argon, neon, xenon and krypton. In some cases, the elements show almost identical physical and chemical properties with those of known elements, although they differ from them in their atomic weight and radioactivity. For example, radium B appears to be identical in ordinary chemical and physical properties with lead although its atomic weight, 214, is quite distinct from lead, 207. The probable explanation of this, at first sight, remarkable identity will be discussed later.
It is of interest to note that in the majority of cases a radioactive element breaks up in only one way which is characteristic for all the atoms of that element, and gives rise to only one new product. The work of Fajans and Marsden, however, has clearly shown that in the case of radium and the corresponding products in the thorium and actinium series, the atoms break up in two distinct ways and give rise to two distinct radioactive elements. It has already been pointed out that actinium is in reality one of these side or branch products. It is supposed that uranium breaks up in two distinct ways, the smaller fraction giving rise to actinium. The evidence, however, on this point, is not yet complete.
The radioactive elements are in some respects more interesting and important than stable elements, for, in addition to the ordinary physical and chemical properties, they possess the radioactive property which allows us to study the mode and rate of transformation of their atoms.
It may be asked what is the essential difference between radioactive changes and ordinary chemical changes. In the radioactive changes we are not dealing with the dissociation of molecules into atoms but an actual disruption of the chemical atom. The disintegration of any given element appears to be a spontaneous and uncontrollable process which, unlike ordinary chemical changes, is quite unaffected by the most drastic changes in temperature or by any other known physical or chemical agency.
The radioactive changes differ entirely from chemical changes not only in the peculiar character of the emitted radiations but also in the enormous emission of energy. It can be simply shown that the energy emitted from a radioactive substance which expels alpha particles is several million times greater than the energy emitted from an equal weight of matter in any known chemical reaction. This emission of energy is mainly to be ascribed to the conversion of the energy of