231 



CHAPTER III. y 



RECEPTION AND CORRECTIONS OF THE HAITIAN 

 CRYSTALLOGRAPHY. 



I HAVE not hitherto noticed the imperfections of 

 the crystallographic views and methods of Haiiy, 

 because my business in the last section was to mark 

 the permanent additions he made to the science. 

 His system did, however, require completion and 

 rectification in various points; and in speaking of 

 the crystallographers of the subsequent time, who 

 may all be considered as the cultivators of the 

 Hau'ian doctrines, we must also consider what they 

 did in correcting them. 



The three main points in which this improve- 

 ment was needed were ; a better determination of 

 the crystalline forms of the special substances; 

 a more general and less arbitrary method of con- 

 sidering crystalline forms according to their sym- 

 metry ; and a detection of more general conditions 

 by which the crystalline angle is regulated. The 

 first of these processes may be considered as the 

 natural sequel of the Hau'ian epoch : the other two 

 must be treated as separate steps of discovery. 



When it appeared that the angle of natural or 

 of cleavage faces could be used to determine the 

 differences of minerals, it became important to 

 measure this angle with accuracy. Haiiy's mea- 



