CONSTITUTION OF MATTER — RtJ.THERFORD. 171 



definite geometrical form, wliicli is cliaracteristic of the particular 

 atoms or groups of atoms. The great variety of crystal forms that 

 are known have all been classified as belonging to one or more of the 

 230 forms of point symmetry wliich are theoretically possible. 

 While considerations of symmetry are a sufficient guide to the classi- 

 fication of crystals, they offer no explanation of the definite archi- 

 tecture of the crystal nor of the nature of the forces that cause the 

 atoms or molecules to arrange themselves in such definite geometric 

 patterns. We are inevitably led to the conclusion tliat 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 crys- 

 tal. 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. 



Wliile 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 ex- 

 periments of Prof. 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 one-hundred-mil- 

 lionth of a centimeter. Lane showed that when Eontgen rays passed 

 through a crystal definite interference patterns w^ere observed. This 

 result was of great importance, as it showed that Rontgen rays must 

 consist of ^-ery short transverse waves akin to those of light. Bragg 

 showed that the reflection, or rather diffraction, of Rontgen 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 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 molecule is the unit of the crystal structure. Con- 

 sider the structure of the simple cubic crystal of rock salt (sodium 

 chloride). The structure of the crystal deduced by Bragg is shown 

 in figure 1. The sodium atoms are marked by black spheres, the 



