CERTAIN facts concerning the structure of atoms are now generally accepted. Atoms are known to be aggregations of positive and negative electricity in which the positive electricity is concentrated into an extremely minute central nucleus with the negative electricity spatially distributed in discrete equal portions --electrons--around it. The net positive charge on the nucleus, called the atomic number, and therefore the total number of electrons in the normal atom, is known for each of the chemical elements. It is admitted that these electrous are arranged in shells or layers, although there is no accurate knowledge of the dynamics of such systems excepting for the hydrogen atom. The differences in properties which distinguish the elements are probably determined by the mode of arrangement of the electrons in these shells, but it is not completely known how the electrons are distributed in the shells or what the motions of the systems may be. Of the theories which have been advanced to cover these points, that of Bohr 1 has the greatest strength at the present time. This theory gives a connected picture of the mode of building up of the atoms from hydrogen to uranium by the successive additions of electrons one by one affording a plausible explanation of the changes in the physical and chemical properties even through the rare earth and other anomalous groups and predicting to a large extent the constitution of the X-ray spectra. Although at present almost entirely without any mathematical basis, the theory is in many particulars so in accord with the X-ray measurements of Siegbahn and his associates and others 2 that the probability of its truth increases day by day.
* Communicated by Professor Bazzoni. Contribution from the Randal