CLASSIFICATION. 359 



Classification in Crystallography. 



One of the most perfect and instructive instances of 

 classification which we can find is furnished by the science 

 of crystallography, already briefly noticed (vol. i. p. 153). 

 The system of arrangement now generally adopted is 

 conspicuously natural, and is even mathematically perfect. 

 A crystal consists in every part of similar molecules simi- 

 larly related to the adjoining molecules, and connected 

 with them by forces the nature of which we can only 

 learn by their apparent effects. But these forces are 

 exerted in space of three dimensions, so that there is a 

 limited number of suppositions which can be entertained 

 as to the relations of these forces. In one case each mole- 

 cule will be similarly related to all those which are next 

 to it ; in a second case, it will be similarly related to those 

 in a certain plane, but differently related to those not in 

 that plane. In the simpler cases the arrangement of 

 molecules is rectangular ; in the remaining cases oblique 

 either in one or two planes. 



In order to simplify the explanation and conception of 

 the complicated phenomena which crystals exhibit, an 

 hypothesis has been invented which is an excellent illus- 

 tration of the class of Descriptive Hypotheses before men- 

 tioned (vol. ii. p. 153). Crystallographers imagine that 

 there are within each crystal certain axes, or lines of 

 direction, by the comparative length and the mutual 

 inclination of which the nature of the crystal is deter- 

 mined and recorded. In one somewhat exceptional class 

 of crystals there are three such axes lying in one plane, 

 and a fourth perpendicular to that plane ; but in all 

 the other classes there are imagined to be only three axes. 

 Now these axes can be varied in three ways as regards 



