SULPHURET Of LEAD, COPPER, AND ANTIMONY. 0<*" 



which at the points A, T, B, C represent the extremities of Mode in which 

 the several edges of the component crystals. From the ex- the P rimitive 

 tremity C of this I let fall the perpendicular Ch, repve- certakusdT * 

 senting the direction of the side, or altitude, of the prism, 

 and the length of which I leave undetermined. Through 

 the extremity of this line, C, I draw the lines G C P, 

 F C O, E C N, D C M, forming with it angles of 130°, 

 135°, 149% aad 171° 30', which I produce indefinitely above 

 the point C. From the point B, the extremity of the side 

 of the first molecule, I erect the indeterminate perpendicu- 

 lar BG, cutting all the preceding lines. It is evident, that 

 the lines CG, CF, C E, and CD, will indicate the direction 

 of the planes derived from the four different retrogradations, 

 that take place along the edges of the terminal faces ; and; 

 that, if one of them be made by a single row, the part of the 

 perp< ndieular B G, included between the line of the direc- 

 tion of the plane derived from this retrogradation and the 

 line A C, will represent the height of the molecule of the 

 last lamina placed on it, and consequently that of the pri- 

 mitive crystal. 



It remains now to inquire, which of these retrogradations 

 was "most probably made by a single row ; and whether, 

 after having determined this, all the others will agree with 

 it. The angle F C B, or that of inclination between F C 

 and A C, being 45°, or the supplement of B C O, which 

 was by construction 135°, would indicate a height equal to 

 the edges of the terminal faces, and consequently the cube 

 as the primitive crystal; and the observation already made - 

 militates against the choice of this, unless the farther ob~ 

 servations, in which we are engaged, oblige us to adopt it. 

 The angle of inclination, G C B, of the line G C, would 

 indicate a height greater than that of the cube; and in all 

 the crystals of this substance the longitudinal edges of the 

 prism being constantly shorter than those of the terminal 

 faces, I am led rather to reject than adopt this height, 

 which would give 28*6, the edges of the terminal faces be- 

 ing supposed 24. The choice then remains between the 

 two retrogradations represented by the two lines E C and 

 D C, the first indicating E B for the height, and the second 

 D B ; and the resolution of the two rectangular triangles 



EBC 



