of the units, possibly secondary, of which the nuclei of the heavy atoms are built up. In course of its successive transformations, a uranium atom loses eight helium atoms, a thorium atom six, and an atom of actinium five. The probability that helium is one of the units of atomic structure not only in the case of radioactive atoms but for ordinary atoms is strengthened by the fact that the atomic weights of a number of elements differ by about four units.
The fact that the helium nucleus survives the intense disturbance resulting in its violent ejection from a radioactive atom suggests that it is a very stable configuration. On the views discussed, it is natural to suppose that the helium nucleus of atomic weight about four is made up of four positive electrons united with two negative electrons. No doubt it is difficult to understand why such a system should hold together, but it must be remembered that we have no information as to the nature of magnitude of the forces existing at such minute distances as are involved in the structure of the nucleus.
We have so far assumed without proof that while the nucleus of an atom carries a resultant positive charge, negative electrons are also present. The main evidence on this point comes from a study of the radioactive elements. A substance which breaks up with the emission of swift electrons (beta rays), but no alpha particles, suffers disintegration according to the same laws and gives rise to a new element in the same way as when an alpha particle is lost. It seems necessary to suppose from a number of lines of evidence that a transformation which is accompanied by the emission of primary beta particles must have its origin in the ejection of a negative electron from the nucleus itself, or from a point very close to the nucleus.
There are no means at present of deciding definitely the relative number of positive and negative units composing the nucleus, except possibly from a consideration of the atomic weight of the atom in terms of hydrogen. It is, however, premature to discuss such questions until more information is obtained as to the structure of the nucleus and the effect of concentration and distribution of the component electrical charges on its apparent mass.
Charge Carried by the Nucleus
We are now in a position to consider a very important question, viz., the magnitude of the positive charge carried by the atomic nucleus. Since an atom is electrically neutral, the negative charge carried by the exterior distribution of electrons in the structure of the atom must be equal and opposite to the resultant positive charge carried by the nucleus. The electrical charge is most conveniently expressed in terms of the number of the fundamental units of charge in the nucleus. Since the charge carried by the electron is one unit, the charge on the nucleus of the atom may be expressed numerically by the number