THE PROBLEM OF CRYSTALLIZATION. 



49 



In the three oblique systems the axes are partly or altogether obliquely 

 inclined to one another, while their magnitudes are unequal. Fig. 11 

 is a crystal of the monoclinic system, and Fig. 12 of the triclinic sys- 

 tem. The names of the different oblique systems indicate the mutual 



inclinations of the axes. Fig. 13 

 represents a crystal of the hexa- 

 gonal system, which is allied in 

 symmetry to the dimetric sys- 

 tem ; but there are four lines of 

 symmetry, of which the three 

 A A', B B', and C C, lying in the 



Fig. 10. 



Fig. 11. 



Fig. 12. 



same horizon, are equal in their mutual inclination and magnitude, 

 while the fourth axis, D D', is at right angles to these but different in 

 magnitude. 



The reader will now have formed a tolerably correct idea of a crys- 

 tal, and when it is borne in mind that crystallization is a widely dif- 

 fused and essential property of matter, and also that the solution of 

 this question has engaged some of the ablest minds of the century, 

 the high purpose and importance of this investigation will perhaps be- 

 come evident to him. 



Now, the invariability of certain relations existing between the 

 axes and the planes bounding crystal forms are geometrically similar, 

 and are effects produced by causes similar to D 



those which occasion the constancy of the slopes 

 in heaps of the same material. In the heap of 

 gravel considered above, the horizon was chosen 

 as the reference plane in the crystal the planes 

 containing the lines of symmetry are selected 

 as reference planes, whereby to gauge the in-c 

 clination of the bounding surfaces. From our 

 considerations of the heap of gravel, the reader 

 will perceive the intimate connection between 

 outward form and internal structure, and is in a 

 measure prepared to follow deductions made from 

 the one upon the other. Already in the remote in- 

 fancy of mineralogy assumptions as to the internal structure of crystals 

 were made to explain the axial relations alluded to. The assumption 

 that the internal structure of a crystal is similar to, and in a measure 

 identical with, the internal structure of a cannon-ball pile, is sufficient to 

 vol. xxix. 4 



