I9I2.] 



THE LAW OF RATIONAL INDICES. 



105 



arrangement. According to Hauy the omission is usually of one, 

 two or three, rarely of four or five rows of molecules. Fig. 2 shows 

 the production of an (no) face in this manner. If the cubes were 

 very minute the (no) face would appear to be smooth. This 

 epoch-making discovery laid the foundation of crystallography as an 

 exact science and entitles Haiiy to the title " father of crystal- 

 lography." With some modification it has been the guiding prin- 

 ciple in crystallography since that time and should not be abandoned 

 unless the evidence is clearly against it. 



Fig. 2. The production of an (no) face. 



Some authors express the fundamental law of crystallography as 

 the law of simple mathematical ratio. Thus Williams'* says: "Ex- 

 perience has shown that only those planes occur on any crystal whose 

 axial intercepts are either infinite or small even multiples of unity." 

 Tutton^ also says : " The indices of any and every face on a crystal 

 are three small numbers." Small in these quotations is usually inter- 

 preted as not more than six. Faces with indices larger than six, 

 according to this view, are accidental and are usually relegated to 

 the list of uncertain forms. There is a tendency to consider forms 



■"'Elements of Crystallography," 3rd ed., p. 26 (1901). Similar state- 

 ments are also made in the text-books of Bayley, Brush-Penfield, Moses and 

 Parsons, Patton, and Van Horn. 



'"Crystallography and Practical Crystal Measurement," p. 70, 1911. 



