Mr. J. F. Daniell on Crystallization. 31 



corrected of things in the farthest distance. A more extended 

 view of this description of the ultimate structure of crystalline 

 bodies appears to me to be afforded by the experiments of 

 Professor Mitscherlich upon the expansion of certain bodies 

 by heat ; as an introduction to which, a connected retrospect of 

 the two theories of crystallization, although presenting but 

 little new, will, I trust, not be considered as misplaced. 



The observation upon which M. Haiiy founded his beautiful 

 theory of the structure of crystals is well known. He took a 

 six-sided prism of calcareous spar, and, in attempting to split it, 

 he found that of the six edges of the superior base, three alter- 

 nate edges only yielded to the blow, and that the division there 

 took place at a certain determinate angle. The three inter- 

 mediate edges resisted this division ; but, in applying the same 

 force to the inferior base of the crystal, the intermediate edges 

 alone yielded. By following up this cleavage in the natural 

 directions thus pointed out, the new-formed planes met toge- 

 ther, and he at length obtained an obtuse rhombohedron of 

 definite angles ; which was further divisible in the direction of 

 faces into, apparently, an infinite number of similar smaller 

 rhombohedrons. 



To this invariable solid he gave the title of the PRIMITIVE 

 FORM of calcareous spar, and he supposed it to be the form of 

 its ultimate molecules; from aggregations of which, externally 

 modified, according to geometric laws, he conceived all 

 secondary forms of the same substance to be produced. 



This conclusion seemed to derive much force from the ob- 

 servation, that any crystal of calcareous spar, of whatever form, 

 carefully broken, may always be resolved into an infinitude of 

 small rhombohedrons, and that this form persists to the utmost 

 limit to which we can carry mechanical division. 



The same observation is applicable to other substances ; 

 but the primitive form is, in many cases, peculiar to the sub- 

 stance examined : thus, for instance, a crystal ofjulphuret of 

 lead is resolvable by mechanical force into a number of small 

 cubes, in the same manner that calcareous spar is resolvable 

 into small rhombohedrons. 



M. Haiiy calculated the secondary forms of crystals, by 

 decrements of particles taking place on different edges and 



