CRYSTALLOGRAPHY. 



smooth surfaces, a fact long known by 

 those who work on the perns. Suppose 

 we have a crystal of calcareous spar, a 

 regular hexaedral prism, represented in 

 plate crystallography, h'g. 5 and 6, if we 

 endeavour to divide it parallel to the 

 edges which form the outlines of the 

 basis of the prism, we shall find that three 

 of these edges, taken alternately, are the 

 upper extremity of the edges lf,dc, b, m, 

 readily yield to this division by a knife 

 struck in the proper direction; but that 

 the other three, those which are interme- 

 diate,/^ cb, and m L, cannot be divided 

 in a similar manner : and if broken by a 

 greaterforce, the^fracture, instead of be- 

 ing polished like the others, is rugged 

 and uneven. If we repeat the experi- 

 ments at the under extremity of the 

 prism, we shall find here, also, that seg- 

 ments of three only of the edges can be 

 detached ; but these edges, instead of be- 

 ing the corresponding one with those de- 

 visable at the upper extremity, that is, 

 //, c dy b m, are the intermediate ones 

 f d, c ti, b m, are the intermediate ones df, 

 c A, and m I. 



The six divisions compose so many 

 trapeziums. Three of these are repre- 

 sented in fig. 6, namely, the two which 

 cut off the edges, //, c d represented by 

 the dotted lines/)/;, o o and a a, h h, and 

 that which cuts oft" the inferior edge df, 

 and which is marked by the dotted lines 

 n n, i i. 



Each of these trapeziums will have a 

 smoothness and lustre, from which it can 

 be perceived that it coincides with one 

 of the natural joinings, the assemblage of 

 which form the prism. The prism can- 

 not be divided in any other directions 

 than these. But if the division be con- 

 tinued parallel to the first segments, it 

 necessarily happens, that on one hand 

 the surfaces of the bases of the prism be- 

 come narrower, and that on the other 

 hand the heights of the sides diminish : 

 and at the point at wliich, continuing the 

 section, the bases disappear, the prism 

 will be changed into a dodecaedron, 

 with pentagonal faces (fig. 7) ; six of 

 which, o o i O e, o I hi i, 8cc. are the re- 

 mains of the sides of the prism, andfhe 

 other six, EAIoo, OAKtt, 8tc. are the 

 immediate results of the mechanical divi- 

 sion. 



In this, and the two succeeding figures, 

 the hexaedral prism, which circumscribes 

 the solid extracted from it in the division, 

 is still represented, to shew better the 

 progress of the operation. 



Beyond this point, the planes at the 

 extremity preserve their figure and di- 



mensions, while the lateral planes conti- 

 nue to diminish in height, until the 

 points o, h of the pentagon o I h i i coin- 

 ciding with the points ', ', and also the 

 otlu-r points similarly situated having a 

 like coincidence, each pentagon is reduc- 

 ed to a simple triangle, as is represented 

 in fig. 8. 



Lastly, by continuing the section the 

 triangles are made to disappear, so that 

 there remains no vestige of the surface of 

 the original prism; but in place of it we 

 have the obtuse rhomboid E A I O (fig. 

 9), which is therefore the nucleus, or 

 primitive form. 



This discovery of the method of divid- 

 ing a crystal was made by Hauy, in 

 examining a crystal of calcareous spar 

 which had been detached from a group 

 of which it formed a part. He observed 

 that the fracture had happened at one of 

 the edges of the base of the prism, und 

 that its surface was perfectly smooth and 

 regular. Attempting to detach a seg- 

 ment in a similar direction from the con- 

 tiguous edge, he could not succeed, but 

 the one next to it was easily divided ; and 

 proceeding in this manner, he was able 

 to effect the mechanical division of the 

 crystal in the manner already explained. 

 Struck with the important result of the 

 experiment, he applied the same method 

 to other crystalline forms of the same 

 substance, and obtained from them the 

 same result; the crystal, whatever was 

 its figure, being by this mechanical divi- 

 sion converted into a rhomb. Thus, in 

 the dodecaedron, composed of two six- 

 sided pyramids joined by the base, the 

 primitive form may be obtained at once 

 by making a first section, on the edges 

 E O, O I, fig. 10 ; a second, on the edges 

 I K, G K ; a third, on G H, E H ; a fourth, 

 on O I, I K ; a fifth, on G K, G H : and 

 lastly, a sixth, on E H, E O ; and the re- 

 sult is, that these edges become the same 

 with the lateral edges of the primitive 

 form, us may be perceived from mere in- 

 spection of fig. 11, which represents this 

 primitive form described in the dodecae- 

 dron. He then applied it to other crys- 

 tulline substances, and found, that from 

 these also, by discovering the joints 1>\ 

 which the laminoe composing the crys- 

 tals were united, a certain primitive form 

 might be extracted. That of fluor spar is 

 an octaedron; and that of the heavy spar, 

 a prism with rhomboidal bases; of corun- 

 dum, a rhomboid somewhat acute : ot 

 beryl, a hexaedral prism ; and of the elba 

 iron-ore, a cube. Each of these forms is 

 constant with regard to the species, and 

 is that from which all the forms of the 



