It is convenient to distinguish between the properties assigned to the nucleus and the planetary electrons. The movements of the outer electrons are responsible for the X-ray and optical spectra of the elements and their configuration for the ordinary physical and chemical properties of the element. On the other hand, the phenomena of radioactivity and all properties that depend on the mass of the atom are to be definitely assigned to the nucleus. From a study of the radioactive transformations, we know that the nucleus of a heavy atom not only contains positively charged bodies but also negative electrons, so that the nuclear charge is the excess of positive charge over negative. In recent years, the general idea has arisen that there are two definite fundamental units that have to do with the building up of complex nuclei, viz., the light negative electron and the relatively massive hydrogen nucleus which is believed to correspond to the positive electron. This view has received very strong support from the experiments of Aston on Isotopes in which he has shown that the masses of the various species of atoms are represented nearly by whole numbers in terms of O = 16. From the general electric theory, it is to be anticipated that the mass of the hydrogen nucleus in the nucleus structure will be somewhat less than its value 1.0077 in the free state on account of the very close packing of the charged units in the concentrated nucleus. From Aston's experiments, it appears that the average mass of the hydrogen nucleus, or proton as it is now generally called, is very nearly 1.000 under these conditions. We should anticipate that the whole number rule found by Aston would hold only to a first approximation, since the mass of the proton must be to some extent dependent on the detailed structure of the nucleus. In the case of tin and xenon Aston has already signalized a definite departure from the whole number rule, and no doubt a still more accurate determination of the masses of the atoms will disclose other differences of a similar kind.