74 



MINERALOGY. 



for these individual substances, when submitted to a further 

 analysis, are shown by the chemist to be composed of elements, 

 not mingled as are the minerals in a rock, but united according 

 to the laws of chemical combination. Furthermore, in common 

 with the products of chemical processes, minerals very usually 

 present themselves in more or less symmetrical polyhedra, at 

 times eminently complex, yet always fashioned in obedience to 

 a geometry at once simple, exact, and universal. Such minerals 

 are said to be crystallised, and the investigation of the geo- 

 metrical law which all c^stals obey, the determination of the 

 character of the symmetry of each particular crystallised 

 mineral, the accurate measurement of the angles between the 

 faces, and thus the assignation of the specific geometrical cha- 

 racter and crystallographical constants of every such mineral, 

 are among the preliminary duties of the mineralogist. 



Mineralogy is thus a science which deals with the description 

 of the chemically distinct substances which form the material of 

 the globe ; its task, however, also extends to the classification 

 of these compounds. When, with the latter view, it is at- 

 tempted to assign to a certain mineral species those minerals 

 which present at once identity in chemical composition and 

 crystalline form, other minerals force themselves on the attention 

 which, though corresponding very closely in their crystallo- 

 graphic features and constants, yet present considerable diversity 

 in chemical composition. Many of these anomalies are explained 

 hy the principle termed isomorphism, according to which diffe- 

 rent elements belonging to the same chemical group may replace 

 each other and play a similar part in the compound ; that is to 

 say, a mineral may contain one or another, or several different 

 members of the same group of elements, provided that the 

 chemical type as expressed bya general chemical formula remains 

 the same ; and in such cases it is usually found that the charac- 

 ter of the crystalline symmetry is the same for all the minerals of 

 the group, the crystallographic constants themselves differing 

 in only a minute degree. An excellent example of this 

 diversity of composition in minerals belonging to the same 

 group and having the same crystalline form is presented by the 

 Garnets, which crystallise in the Cubic system and have an 

 identical development of crystal. The typical formula of 



