part in the spectra of all atoms. In a similar way, the helium atom is supposed to consist of a nucleus of two unit charges surrounded by two electrons. On the theory, the spectrum of helium is connected in a very simple way with that of hydrogen. Bohr also pointed out that the Pickering series of spectral lines observed in certain stars which were originally attributed to hydrogen must be ascribed to helium. This conclusion has since been strongly supported by the direct experiments of Fowler and Evans, In a similar way, Bohr described the possible distribution of electrons in several of the lighter atoms and also discussed the structure of the hydrogen molecule, which is composed of two hydrogen nuclei and two electrons. The heat of combination deduced for the theoretical molecule is in fair accord with experiment. He found that two helium atoms were unable to unite to form a molecule—in agreement with a well-known property of this gas.
While there is room for much difference of opinion as to the interpretation of the rather revolutionary assumptions made by Bohr to explain the structure of the simple atoms and molecules, there can be no doubt of the great interest and importance of this first attempt to deduce the structure of the simple atoms and to explain the origin of their spectra. The agreement of the properties of such theoretical structures with the actual atoms is in several cases so remarkable that it is difficult to believe that the theory is not in some way an expression of the actual facts. While much work will be necessary before we can hope to understand the structure of any but the simplest atoms, a promising beginning has been made in the attack on this most difficult and fundamental of problems.
There seems to be little doubt that the more marked physical and chemical properties of an atom are to be attributed to a few outlying electrons in the atomic structure. The position and number of these "valency" electrons, as they have been termed by Stark, are defined by the magnitude of the nucleus charge. It has previously been pointed out that the loss of an alpha particle from a radioactive atom changes the position of the element two groups lower in the periodic table, while the loss of a beta particle raises it one group higher. Consequently it follows that the loss or gain of a unit charge from the nucleus of an atom causes it to change its position from one group to the next. If, for example, we follow the chemical properties of successive elements when the nucleus charge increases by unity, we soon reach an element which belongs to the same group as the first, although of much higher atomic weight. We must consequently conclude that the number and position of the outlying electrons in the structure of the atom passes through successive changes which are regularly repeated with increasing atomic weight. Quite apart from any detailed knowledge of the electronic distribution of atoms, the regular recurrence of elements of
