10 



MINERALOGY 



triangular faces of equal size, Fig. 8, or by three large faces and three 

 smaller faces, as b, or the point may be stretched out to a straight 

 line, as in c. If a section be cut directly through the apex of a 



FIG. 8. Distorted Crystals of Quartz, Herkimer County, New York. 



quartz crystal at right angles to the opposite faces, the outline of 

 the section in every case will be different, but the corresponding 

 angles, Fig. 9, a, or b, are always equal. This law may be stated 

 P. Q El as follows : the dihedral 



solid angle between simi- 

 lar faces of crystals of 

 the same substance is 

 constant, provided al- 

 ways that the substance 

 is chemically pure and 

 that the angle is measured 

 at the same temperature. 

 Crystal angles are as 

 characteristic of chemi- 

 cally pure compounds 

 as their chemical or 



FIG. 9. Parallel Sections of Quartz Crystals. 



a b 



physical properties ; not 

 only may they be iden- 

 tified by the angles, but 

 they are an index to the 

 purity of compounds. 



The constancy of the 

 interfacial angle is a di- 

 rect result of the regular molecular network or point-system. In 

 Fig. 10 a the round dots represent one sheet of molecules of the point- 



FIG. 10. 



