2-22 On Crystallography. 



trianoles joined base lo base, or a quadrilatcr. "We see 

 (fig. 24) one of these qiiadrilaters, and in which the in- 

 ferior angle o is l)lended witli the angle O of the nucleus 

 (fig, -20) and the diagonal t x represents the edge H K 

 (fig. 93 C) of the lannna B K HD, which is the most ex- 

 tensive in the direction of this ^aine edge. As the num- 

 ber of tlie laminae of superposition producing the tri- 

 angle tax (ficr. 24) is less thun that of the laminae con- 

 stituting tht triangle t o x, and as there is here only a sitigle 

 lamina which precedes the lamina B K H D, (fig. C) 

 while there are six which follow it as far as the cube z 

 (fig. I) inclusively, the triangle t .s .r, ((ig. '24) composed of 

 the atrgreorate of the borders of these last lamina;, will be 

 much higher than the mterior triangle / o.r, as expressed 

 by the figure. 



The sM'-face of the secondary solid will therefore be 

 formed of 24 qnadrilalers, disposed tluee and three around 

 each solid angle of the nucleus : but as, in decrements by a 

 simple range on all the edges, the faces produced on both 

 sides of each edge are on the same plane; so in decrements 

 bv a range on all the angles, ihe faces which originate in 

 the three" sides of each solid angle, such as (fig. 20), are 

 on a level so as to form but one face: and pince the cube 

 has eitht solid angles, each composed of three plain angles, 

 the secondary crystals will have eight faces, which, on ac- 

 count of the regularity of the nucleus, will be equilateral 

 triangles, i. e. the secondary crystal will be a regular octa- 

 hedron. One of these triangles is represented at fig. 26, 

 so as to enable us to judge, at a single glance, of the ar- 

 rangement of the cubes which concur in forming it. 



This level of faces produced by subtractions of a range 

 from both sides of the same edge, or around the same 

 solid angle, is a general result of the crystallization which 

 takes place for any primitive form whatever. 



The circumstance just considered, and which occurs in 

 muriated soda, sulphuretted iron, sulphuretted lead, &c., 

 affords a new example of a form uhich, although primitive 

 in certain species, performs in others the function of a 

 secondary one. Theory thus traces the limit that separates 

 objects which the eye would be tempted to confound. 



If decrements had not their complete eflect, that is to 

 say, if they stopped short of the limit where the faces they 

 produce incline to unite in a point, some faces parallel to 

 those of the nucleus would remain on the secondary crystal. 

 The first would then have fourteen faces, namely, six ar- 

 ranged like those of a cube, and eight situated like those c^f 



a regular 



