SULPJIURET OF LEAD, COPPER, AXD ANTIMONY, 2,57 



science, the application of which is more easy, than crystal- 

 lography. 



From the angle of incidence of 135° between the termi- Circumstances 

 nal faces and one of the planes that are substituted for the *£ SSir! ** 

 edges of those faces in the rectangular tetraedral prism of 

 the endellion; from that nearly of the same number of de- 

 grees, which one of the planes substituted for their angles 

 makes with the same faces; and lastly, from another of 

 135°, which one of the planes substituted for the longitu- 

 dinal edges makes with the sides of the prism ; we may be 

 very easily led, if we confine our observations to these facts, 

 to consider the primitive crystal as a cube. These angles of 

 incidence however, either exact, or so near it that the in- 

 strument cannot detect the difference, may be produced by 

 retrogradations of a number of rectangular tetraedral prisms 

 by no means of a cubical figure. I have assigned the rea- 

 sons, which have appeared to me to militate against our ac- 

 ceding to this first attempt. I am persuaded we much too 

 readily yield to an inclination to consider as cubical the 

 primitive ciystals of substances, the secondary forms of 

 which indicate a rectangular tetraedral prism. There are 

 already a sufficient number of substances, that really have 

 the cube for their primitive crystal, though their integrant 

 molecules are of a different form, without our enlarging it 

 unnecessarily. By doing thus we afford an additional han- 

 dle to those persons, who, seeing in every primitive crystal 

 nothing but the form of the integrant molecules, from 

 which however it is frequently very remote, make of the 

 numerous repetitions of this form in several mineral sub- 

 stances, that are totally different, the grounds of a very un- 

 founded objection to crystallography. 



There is a fact relating to this substance worthy of re- 

 mark, whjch is the equality of the number of retrograda- 

 tions made both along the edges and at the angles of the 

 terminal faces, and the great analogy between the planes 

 owing to them. 



Vol. XXIV— Dec. 1809. B TABLE 



