36 Mr. William Phillips's Description 



FIFTH MODIFICATION. 



This interesting modification is found in combination with each 

 of the preceding, but is rarely seen displayed by itself on the primi- 

 tive crystal as in fig. 74. It arises from a decrease of the crystalline 

 laminae on that part of each face forming the solid angle, by which 

 each is replaced by an obtuse quadrilateral pyramid ; the faces of 

 which incline on the axis that passes through the angles. The 

 angle formed by the meeting of P and 5 is about 160 d. and of 1 

 on 5 about 144 d.* 



Fig. 78 seems to be the result of a partial after-deposition on the 

 faces of the primitive crystal, by which each face of it (such parts of 

 of them excepted as contribute to form the solid angle,) is brought 

 in form to resemble fig. 77, except only that in this the solid angles 

 of the primitive crystal remain. 



It may perhaps be imagined that some of the latter figures in the 

 series of this modification, exhibiting its combination with some of 

 the preceding modifications might have been omitted ; but as the 

 term variety is used to signify combination of two or more modifi- 

 cations, as well as those differences in crystals which arise from 

 the various proportions of the faces to each other, by which vari- 

 ations in the form of the face are produced ; and as each of these 

 differs in one or more of these respects, and actually exists, it 

 seemed proper not to omit them. 



* There are other modifications, and their combinations delineated in the scries of the 

 crystallization of this substance, the angles of which it would have been desirable to have 

 given; in most, if not all of which, it is, from their exceedingly small size, to be regretted 

 that it would have been difficult, if not impossible, even for the most skilful and prac- 

 tised hand, to have subjected them to the goniometer. 



