CRYSTALLOGRAPHY. 



varieties, often extremely numerous, are 

 derived. The latter are denominated, by 

 Hauy, secondary forms. Sometimes, 

 though rarely, the primitive and secon- 

 dary forms are the same. 



It is not every crystallized substance, 

 however, that admits of this mechanical 

 analysis. But with regard to those that 

 have hitherto refused it, Hauy has re- 

 marked, that their surface striated in a 

 certain direction, or the relation subsist- 

 ing among the different secondary forms 

 of the same substance, afford indications 

 which lead to the determination, with at 

 least much probability, of their primitive 

 forms. 



Such is the process, by which Hauy es- 

 tablishes what he names the " Primitive 

 Form of Crystals," and which he defines, 

 "A solid of a constant form, inserted 

 symmetrically in all the crystals of the 

 same species, and the faces of which ob- 

 serve the directions of the layers which 

 compose these crystals." The primitive 

 forms hitherto observed are reducible 

 to six ; the parallelopipedon, which in- 

 cludes the cube ; the rhomb, and all the 

 solids which are terminated by six faces 

 parallel two and two ; the tetraedron ; 

 the octaedron ; the regular hexaedral 

 prism ; the dodecaedron, with equal and 

 similar rhomboidal planes ; and the dode- 

 caedron with triangular planes. 



Hauy carries the division of crystals 

 still further, however, than the primitive 

 forms. The solid which constitutes it is 

 not the last term of the mechanical ana- 

 lysis ; it may always be still further sub- 

 divided parallel to its different faces, and 

 sometimes even in other directions. All 

 the enveloping matter is equally divisible 

 by sections parallel to the faces of the 

 primitive forms : and the only limit to 

 this possible division is that placed by the 

 composition of the substance. The cal- 

 careous spar, to take it as an example, 

 may be reduced to a particle, beyond 

 which the division cannot be carried, 

 without resolving it into its elements, 

 lime and carbonic acid ; or at least it may 

 be reduced to a particle, beyond which, 

 if its minuteness allowed us to operate 

 upon it, it is demonstrable its figure 

 would not change. To these last parti- 

 cles, the result of the mechanical ana- 

 lysis, Hauy gives the name of integrant 

 particles, and their union constitutes the 

 crystal. Their forms, so far as experi- 

 ment has been carried, are three : the 

 tetraedron, the simplest of the pyramids ; 

 the triangular prism, the simplest of 

 prisms; and the parallelopipedon, the 

 simplest of solids, which have their faces 



parallel, two and two. There is little 

 doubt that it is between these that the at- 

 traction of cohesion is immediately ex- 

 erted. 



The primitive forms, and the figures of 

 the integrant particles, being determin- 

 ed, it remains to complete the theory of 

 the structure of crystals, to shew by what 

 arrangements the secondary forms, in 

 other words, the actually existing crys- 

 tals, are produced. 



The nucleus of the crystal is the sym- 

 metrical solid which constitutes its primi- 

 tive form, arising from the union of the 

 integrant particles, either by their faces 

 or their edges ; and the additional matter, 

 which forms the crystal, consists of lay- 

 ers of these particles superadded to that 

 nucleus, and arranged on its faces ; and 

 to account for the formation of the crystal 

 under a figure different from that of its 

 primitive form, these layers, as they re- 

 cede from it, are supposed to decrease, 

 in the space they occupy, from the regu- 

 lar abstraction of one or more ranges of 

 the integrant particles. This decrease 

 may take place in various modes ; and ac- 

 cording to these, different figures of crys- 

 tallization will be produced. 



Thus, to take the simplest example, 

 let us suppose the primitive form is a 

 cube ; it is easy to conceive that on each 

 of its six sides may be reared a series of 

 decreasing layers, or laminae, composed 

 entirely of cubical particles, each layer 

 diminishing on each of its edges by one 

 row of the minute cubes of which it 

 consists. The lamina thus decreasing as 

 they recede from* the base on which they 

 rest, until the apex consists of a single 

 particle, it is obvious, that on each side 

 of the cube a four-sided pyramid will be 

 formed. Two of these are represented, 

 (fig. 12.) A B C D, G B C G. 



We shall thus have, then, six four- 

 sided paramids, and of course 24 trian- 

 gles, such as A B C, B C E, C E G, &c. 

 But since the decrease is uniform on all 

 the sides, as from the line B C to A, and 

 from the same line to E, it must also be 

 uniform from A to E ; it is obvious, there- 

 fore, that the side A B C of the one py- 

 ramid will be found exactly in the same 

 plane as the side B C E of the adjacent 

 pyramid; so that the entire surface of 

 these will be the rhomb A B E C. The 

 case must be the same with all the others. 

 The 24 triangles will therefore be reduc- 

 ed to twelve rhombs, and the figure will 

 be a dodecaedron, very remote from the 

 primitive form. Now a crystal of this 

 figure, and having this primitive form, 

 would be resolved into that form, merely 



