THE CONSTITUTION OF MATTER 



109 



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 crys- 

 talline 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 chem- 

 ical 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 ac- 

 count 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 Rontgen rays 

 passed through a crystal, definite interference patterns were observed. 

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

 must consist of very 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 de- 

 termining 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- 



