CHAPTER VII. 



CRYSTALLIZATION. 



MY purpose in this chapter is not to write an elementary- 

 treatise on Crystallography, but merely to give such 

 an account of the principal facts of that science as may 

 Contrasts enable me to show the remarkable contrast, and the yet 

 and resem- jj^Qj.g remarkable points of resemblance, between crystals 



blances ... 



between and Organisms. I will avoid technical language as much 



aSd or^an- ^^ possible, but I fear I shall not be able to make myself 

 isms. intelligible, except to readers who are acquainted with the 

 fundamental conceptions of solid geometry. 



The first and most obvious point of resemblance between 

 crystals and organisms is, that every specimen of either a 

 Species Crystal or an organism belongs to a species, having its 

 and classes, gpegific characters ; and the species of both group them- 

 selves into genera, and the genera into classes. 

 Crystal- A Crystalline species is defined as a body of definite 



^"fi'^rr^^'* cJ^<^'>niccd constitution, from %cliicli, in virtue of unknown 

 formative principles, rcstdts a form having definite geome- 

 trical proijcrties. I say a form having definite geometrical 

 properties, rather than a definite form ; because the same 

 crystalline species often contains specimens having very 

 different forms, the " crystallographic elements " of which, 

 however, are the same, as is hereafter to be explained. 



There are, however, qualifications to be made in asserting 



the invariabihty of either the chemical constitution, or the 



crystallographic elements, among all the specimens of a 



Foreign crystalline species. The presence of foreign substances, or 



mil ►^1"3 Ti pp^ 



modify of (what is practically a foreign substance) an excess of 

 forms. Qj^e of the elements of a compound, often causes a sensible 



