254 SULPHURET OF LEAD, COPPER, AND ANTTMONV. 



Mo rie in which EBC and DBC will very readily give the value of these 

 tiie primitive ,. . , , * , ".j, . „ 



rVKstal was a&- two '' MMes vv,t " res P ect tn the edge oi the terminal fares re- 

 ;;'..i;ned. presented by B C, and which we have already supposed to 



be 24. But as the height D B, which then would be about 

 3*58, would not agree with the inclination of the planes 

 belonging to the other statements, the choice cannot remain 

 doubtful, it falling necessarily on E B, which is of 14'4, 

 and consequently to the edge of the terminal faces in the ra- 

 tio of 14-4 to 24, or of 3 to 5. And in fact, on fixing at this 

 the height of the primitive rectangular tetraedral prism, 

 the determination of the other statements by calculation 

 agrees perfectly with the inclination found by measuring 

 the planes arising from them. Besides, the height D B of 

 3*58 would be much too small with respect to what all the 

 crystals of this substance exhibit; while on the contrary that 

 of 14*4 agrees with every thing found by observation in these 

 crystals* 



It remains now to examine, whether the retrogradatious 

 at the angles of the terminal faces agree with this height; 

 and, if they should not agree with it, whether they do not 

 point to one more natural, and more lit to be adopted. 

 To proceed on this examination, I take with the instru- 

 ment, as accurately as possible, the angle of incidence be- 

 tween the terminal faces and the four planes which take the . 

 places of their angles. Their measurement gives me 125° 

 for one, between 134° and 135° for another, between 150° 

 and 151° for the third, and about 1?2° for the fourth. 



The terminal faces being a perfect square, fig. 30, and 

 the side of the square being assumed 24, the diagonal R S 

 is 33*94, and consequently its half is lG*97. 



As every retrogradation at the angles of a polygon is 

 made on the diagonal passing through these angles, if we 

 suppose the primitive rectangular tetraedral prism, of which 

 fig. 30 represents the terminal face, cut by a plane passing 

 through the diagonal RS and that which is opposite to it 

 in the lower face, all the diagonals of the molecnles of the 

 superficial lamina, on which the retrogradation is made, as 

 well as of those superimposed on it, will be placed on the 

 diagonal R S, or parallel to it. Let Q S, therefore, fig. 

 29, representing this diagonal, be drawn horizontally, and 



divided 



