to the conclusion that the atoms of the crystal are arranged according to a definite system, which is characteristic of the particular crystalline form, and the unit of structure is repeated indefinitely with continued growth of the crystal. In fact, if we had no other evidence, the crystalline form of matter would itself point to the necessity of an atomic structure of matter. While many attempts have been made to explain the grouping of the atoms in a crystal, there has been on the whole little success with the exception, possibly, of Pope and Barlow's theory that the atoms take up the positions of closest packing, the dimensions assigned to the atom depending on a quantity connected with its chemical valency. It is only within the last year that a new and powerful method of attack of this problem has been developed, largely through the experiments of Professor Bragg and his son, W. L. Bragg. On account of the definite ordering of the atoms in a crystal, it acts like an almost perfect optical grating, only in three dimensions, where the grating space is exceedingly small—in most cases about one hundred millionth of a centimeter. Laue showed that when Röntgen rays passed through a crystal, definite interference patterns were observed. This result was of great importance, as it showed that Röntgen rays must consist of very short transverse waves akin to those of light. Bragg showed that the reflection, or rather diffraction, of Röntgen rays incident on the face of a crystal, afforded a very simple method of determining the wave length of the bright lines generally present in an X-ray spectrum. By a study of the position and intensity of the spectra
Fig. 1. Arrangement of Atoms in a Rock Salt (NaCl) Crystal, White Circles represent Sodium Atoms, Black Chlorine.
in different orders thrown by the crystal, it was possible to examine in detail the structure of the crystal, and to deduce the grating space, i.e., the distance between successive planes of atoms. The subject is so large and the discovery of this method so recent, that so far only a few of the typical crystals have been examined, but in these cases we are able to obtain most positive evidence of the grouping of the atoms in the crystal. The results indicate that the atom and not the mole-
