weight: crystals and crystal forces 327 



rendered rigidity in a crystal a less fundamental characteristic 

 than it was formerly supposed to be; while the common occur- 

 rence of zonal growth in natural mix-crystals has taken away 

 some of the emphasis which used to be placed on chemical and 

 physical homogeneity. In general, crystals are rigid and show a 

 high degree of homogeneity and are bounded by flat faces; but 

 the fact that these characteristics are lacking in certain indivi- 

 vidual units has an important bearing on the theory of crystal 

 growth and crystal forces. 



In crystals there is a regular periodic arrangement in space of 

 the component atoms. All theories of crystal structure are 

 based on this postulate which in recent years has been con- 

 firmed by the brilliant investigations, especially, of Professor 

 W. H. Bragg and his son, W. L. Bragg, on the phenomena of 

 diffraction and reflection of characteristic X-rays from oriented 

 crystal plates. From this it may be inferred that the orderly 

 arrangement of the atoms in interpenetrating space lattices is 

 the result of the action of interatomic forces which are spa- 

 cially vectorial in character. Little is known, however, of the 

 order of magnitude of these forces and of the law of their varia- 

 tion with distance. The problem of measuring these forces and 

 of ascertaining the laws describing their behavior may be treated 

 from different viewpoints. 



Development of crystal forms. The development of crystal 

 forms as it has been stated by Victor Goldschmidt 2 in his law of 

 complication is a remarkable expression of the action of crystal 

 forces; this law may, in a given crystal substance, enable us to 

 ascertain the directions of the primary poles of attraction in the 

 crystal structure. 



Crystal growth. Studies on the rate and character of crystal 

 growth have added much to our knowledge of the individuality 

 of a particular crystal group and also of the mechanism of crystal 

 growth. The importance of diffusion and adsorption in crystal 

 growth is now clearly recognized; Marc 3 and others have shown 



2 Uber Entwickelung der Kristallformen. Zeitschrift fur Kristallographire, 

 28: 1. 1897. 



3 Zeitschrift fur Physikalische Chemie, 67: 470. 1909; 67: 640. 1909; 68: 104. 

 1909; 73: 685. 1910; 75: 710. 1911; 79: 71. 1912; 81: 641. 1912. 



