TERMINOLOGY. . 159. 



sceles, the form will be the right rectangular four-sided 

 prism of the pyramidal system ; and should they be scalene, 

 we have reason to suppose (. 150.), that the form in ques- 

 tion will be the right rectangular four-sided prism of the 

 prismatic system. 



In similar cases, the forms of cleavage (. 167-) allow 

 very often the same application as crystalline forms. The 

 octahedral Fluor-haloide may serve as an example; the 

 angles of a right rectangular four-sided prism of this spe- 

 cies may be taken away, or broken off, by equilateral tri- 

 angles, which are faces of cleavage. The figure of these 

 triangles proves the form to be the hexahedron, although 

 perhaps not one of its faces is a square. The following 

 chapter will contain farther observations on cleavage. 



In the third place, the quality of the faces (Chap. III.) 

 must be considered. Nothing is more easy than to decide 

 whether all the faces under which a form is contained, are 

 of the same quality, or whether they differ from each other 

 in this respect. If this quality is the same in all the faces, 

 the form may be a simple one ; if it is different, the form 

 must be a combination of at least as many simple forms, or 

 symmetrical halves and fourths, as there are differences ex- 

 isting in the qualities of the faces. A right rectangular four- 

 sided prism, contained under faces of three different quali- 

 ties, must belong to the prismatic system. If the faces pre- 

 sent only two different qualities, the form may belong to the 

 pyramidal system, though by this description of the faces it 

 is not yet excluded from the prismatic system ; and it may 

 belong to the tessular system, if all its forms are exactly 

 of the same quality. In this last case, however, the form 

 is not excluded from any one of the other two systems. 



The same is evidently applicable to combinations. It 

 sometimes happens that some of the faces belonging to the 

 simple forms which the combination contains, are irregu- 

 larly increased, whilst other faces belonging to the same 

 forms, diminish till even they entirely disappear. This 

 will naturally produce differences in the figure of the faces. 

 The method to be followed in occurrences of this kind con- 



