of electrons exterior to the nucleus. Several methods of attack on this problem have been suggested. Sir J. J. Thomson showed that the scattering of Röntgen rays in passing through the atoms of matter must depend on the number of electrons composing the atom. By assuming that each electron scattered is an independent unit, an expression for the scattering was found in terms of the number of electrons in the atom. By comparison of the theory with experiment, Barkla deduced that for many elements the number of electrons in an atom was approximately proportional to its atomic weight and numerically equal to about one half of the atomic weight in terms of hydrogen.
The charge in the nucleus can also be directly determined from the experiments on scattering of alpha rays, to which attention has previously been drawn. Geiger and Marsden found that the large angle scattering of alpha rays in passing through different substances was proportional per atom to the square of its atomic weight. This showed that the positive charge on the nucleus was approximately proportional to the atomic weight at any rate for elements of atomic weight varying between aluminium and gold. By measuring the fraction of the total number of alpha particles which were deflected through a definite angle in passing through a known thickness of matter, the charge on the nucleus was deduced directly. The number of positive units of charge on the nucleus, which is equal to the number of external negative electrons, was found to be expressed by about one half of the atomic weight in terms of hydrogen. The results obtained by two entirely distinct methods of attack are thus in good accord and give approximately the magnitude of this important atomic constant.
It is obvious, however, that the deduction that the number of units of charge on the nucleus is half the atomic weight, must be only a first approximation to the truth even in the case of the heavier atoms. It has already been pointed out that the nucleus of the helium atom of atomic mass four must carry two unit charges, for it is difficult to believe that any of the exterior electrons of helium can remain attached after its violent expulsion from the atom and its subsequent passage through matter. If this be the case, the nucleus of the hydrogen atom of atomic mass one, must carry one unit charge. Van den Broek and Bohr have suggested that the charge on the nucleus might be equal to the actual number of the element when all the known elements are arranged in order of increasing atomic weight. This is in excellent accord with the experiments of scattering and removes a difficulty in regard to the lighter atoms. Taking this view, the nucleus charge is for hydrogen 1, helium 2, lithium 3, carbon 6, oxygen 8, etc. The simplicity of this conception has much to commend it.
During the last year a new and powerful method of attack on this fundamental problem has been developed by Moseley by the study of X-ray spectra. In 1912, Laue found that X rays showed obvious in-
