OF NATURAL PHILOSOPHY. 239 



Crystallography, 



(261.) It cannot be supposed that these and 

 many other tangible qualities, as they may be called, 

 should subsist in solids without a corresponding 

 mechanism in their internal structure. That they 

 have such a mechanism, and that a very curious and 

 intricate one, the phenomena of crystallography 

 sufficiently show. This interesting and beautiful 

 department of natural science is of comparatively 

 very modern date. That many natural substances 

 affected certain forms must have been known from 

 the earliest times. Pliny appears to have been ac- 

 quainted with this fact, at least in some instances, 

 as he describes the forms of quartz and diamond. 

 But till the time of Linnaeus no material attention 

 seems to have been bestowed on the subject. He, 

 however, observed, and described with care, the 

 crystalline forms of a variety of substances, and even 

 regarded them as so definite a character of the 

 solids which assumed them, that he supposed every 

 particular form to be generated by a particular salt. 

 Rome de 1'Isle pursued the study of the crystalline 

 forms of bodies yet farther. He first ascertained 

 the important fact of the constancy of the angles 

 at which their faces meet ; and observing further 

 that many of them appear in several different shapes, 

 first conceived the idea that these shapes might 

 be reducible to one, appropriated in a peculiar 

 manner to each substance, and modified by strict 

 geometrical laws. Bergmann, reasoning on a fact 

 imparted to him by his pupil Gahn, made a yet 

 greater step, and showed how at least one species 



