**5& ftULPHURET OF LEAD, COPPER, AND ANTIMONY. 



tetraedral prism, as I could not sufficiently depend on the 

 angles of incidence, which the secondary faces, I had then 

 seen, enabled me to take; accordingly I contented myself 

 with giving a very near approximation to the measure of 

 these angles, without subjecting them to the scrutiny of cal- 

 culation *. 



Mctfein which Circumstances having since enabled me to acquire per- 

 the primitive , . * 



■crystal was a«- *ect certainty with respect to the measures of these angles, 



stained. jt now remains for me to determine the dimensions of the 

 tetraedral prism. To effect this, after recognizing four dif- 

 ferent retrogradations along the edges of the terminal faces 

 of the prism, as well as four others at the angles of the 

 same faces, I observe, that the dimensions of the crystal 

 may be determined either by the first of these retrograda- 

 tions, or by the second: I observe too, that each will serve 

 reciprocally as a support or confirmation of the other. 



Directing my attention in the first place to the four re- 

 trogradations, that take place along the edges of the ter- 

 minal faces, I begin by taking as accurately as possible the 

 angle formed by the planes arising from each of these re- 

 trogradations with the terminal faces of the prism, and 

 fimd one angle about 130°, another about 135°, a third 

 about 149% and the fourth between 171* and 172°. I then 

 draw a horizontal line, ABC, fig. 28, representing one of 

 the edges of the terminal face of a crystal perfectly resem- 

 bling the primitive one, but composed of a certain number 

 of crystalline molecules united, and the equal divisions of 



• Additional note. It was not from omission therefore, but for valid 

 reasons, that I did not give the cube as the primitive crystal of this sub- 

 stance. Mr Smithson, to whom I am very well known, might have 

 done me the justice to suppose, that, if the determination of this crys- 

 tal, ftom the facts I could then observe, had been as simple as his cal- 

 culation indicates, my eyes had too much experience in crystallography 

 for it to have escaped me. If however he had entertained any doubts on 

 this puint, he was sufficiently acquainted with me, to have communicated 

 them to me in a less hostile manner; when 1 would with great pleasuie 

 have submitted to him the reasons, that had determined me to act as I 

 did. By this science would have lost nothing, and I should have gained 

 much, in probably not experiencing the extraordinary, and I will boldly 

 say unmerited conduct, that has been held toward me in the name of the 

 Koyal Society, cf which 1 am proud to call myself a member, and fc; 

 -,vhlch J shall always feel the highest respect. 



which 



