IN Mr. Brooht on tht [Oct. 



Article IX. 



Oh the Crystalline Forms of Artificial Salts. 

 By H.J. Brooke, Esq. FRS. 



{Continued from p. 121.) 



In order to apply successfully the tables of modifications 

 referred to in my last communication, or indeed to compare 

 crystals with any of the engraved figures by which they are 

 represented, it is necessary to acquire a habit of what may be 

 tfermed reading crystals. This is not difficult of attainment when 

 they are regularly formed, but when they are distorted in their 

 shape, and some of the planes which are represented in the 

 drawings as equal and symmetrically placed, are disproportion- 

 ately enlarged at the expense of others, it requires a Uttle more 

 consideration to trace the character of the true form in the 

 impeifect crystal. It is convenient to attach the crystal we 

 are examining to the end of a bit of wax taper two or three 

 inches long, by means of which we may hold it in any position. 

 Our first object should be to discover some symmetrical lateral 

 or terminal planes, and when we have discovered these, the cry^-" 

 tal should be placed on the wax so as to enable its being con- 

 veniently held with its lateral planes vertical. 



The cube, tetrahedron, and all the octahedronsy may be easily 

 recognised. The right square prism may be distinguished from 

 the cube by not having its lateral audits terminal edges similarly 

 modified. In the light rectangular prism, the lateral planes 

 incline to each other at an angle of 90", but in the right oblique- 

 angled prism those planes incHne alternately at a greater and 

 less angle than 90° ; the terminal plane in both is perpendicu- 

 lar to the lateral planes, and the planes which replace the solid 

 angles incline unequally on the three adjacent primary planes. 

 The right rhombic prism is distinguishable from the oblique by 

 the inclination of the terminal or the lateral planes being 90^ in 

 the right prism, and being greater and less alternately in the 

 oblique. The planes marked c in the right prism incline equally 

 on the two adjacent lateral planes, while those marked e in the 

 obhque incline unequally on the adjacent planes. The rhomboid 

 may be distinguished from the oblique rhombic prism, to which 

 it bears a great analogy in its general form, by the symmetry of 

 its modifying planes when held with its axis vertical ; and by the 

 equal inclination on the three adjacent lateral planes, of a plane 

 replacing its terminal solid angle ; whereas an apparently corres- 

 ponding plane on the oblique rhombic prism will measure 

 unequally on the adjacent lateral planes. But it will be well to 

 procure regularly formed crystals of some of the substances 

 described, and by holding these in the positions in which they 

 are represented m the drawings, the relations of the several 



