SULPHURET OF LEAD, ANTIMONY, AND COPPER. 337 



Of the Form of this Substance. 



Of the seventeen figures which have been given, as of the Form of the 

 crystals of this compound sulphuret, in Part II of the vo- compound sul- 

 1 f xi rr. . » phuret. 



lume ot the Iransactions for 1804, great part are acknow- 

 ledged to have no existence, nor are indeed any of them 

 consistent with nature. 



This substance seems to have y&t offered but one form, 

 which is represented in Plate 9 under its two principal 

 appearances; that is, having the primitive faces the pre- 

 dominant ones of the prism; and having the secondary ones 

 such, and which will be fully sufficient to make it known. 

 In the first infancy of the study of crystals, it might be ne- 

 cessary to attend to every, the most trifling, variation of 

 them, to trace each of their changes step by step, to spell 

 as it were, the subject; but in the state to which the science 

 has now attained, to continue to do so would be not only 

 superfluous, but most truly puerile. 



I have a very small, but very regular, crystal of the form 

 of Fig. 1. 



By mensuration the faces a and m appear to form together 

 an angle of about 135'', and the faces c and b an angle of 

 about 125°. 



It is said in the account above quo'ed, that the primitive Dimensions of 



form of this matter is a rectangular tetraedral prism, but no ^^° P"^"^ V^' 



° ^ ' cessarv to deter- 



proofs of this have been offered ; nor have the dimensions of mine the primi- 



this prism been given, a circumstance of the first moment to '^'^'^ ^°'^'"* 



the determination of true or primitive form, nor have any 



quantities been assigned to the decrements supposed. I will, 



therefore, supply these very important omissions. 



That the atom of this substance is a rectangular tetraedral It is a cube. 

 prism, is inferable, not from the striae on the crystals, for 

 stride are by no means invariably indicative of a decrement in 

 the direction of them ; but from the angles which the faces a 

 and c make with the faces m and b ; and these angles also 

 prove, that the height of this prism is equal to the side of 

 its base, that is, that it is a cube. 



Hence the face a is produced by a decrease of one row of I^s decrements. 

 atoms along the edge of the cube, and the angle it forms 

 •with the face m is really of 135°. 



Vol, XX, — Supplbmext. Z The 



