Measurement and Cleavage of certain Primitive Crystals. 259 



hammer. Instead of a table, however, I frequently place the crys- 

 tals of such substances as may be best divided by a blow, on a slab 

 of steel, polished on one side for the sake of a perfectly level sur- 

 face, which is advantageous because the resistance given by steel is 

 greater than that of wood. A lighter blow is therefore effectual, 

 and hence there is less danger of cracking the crystal in directions 

 opposed to its natural joints. 



Sulphate of Lead. 

 Fig. 9, 10, 11,12, 13. 



The form of the primitive crystal of this substance is considered 

 by Raiiy to be a rectangular octohedron (fig. 9.) the angle formed 

 by the meeting of one plane of the upper pyramid, with the adja- 

 cent plane of the lower, being in one direction (P on P) 109° 18' 

 and in the other direction (P'onP') 78° 28' ; and he says, * Cet 

 octaedre se soudivise sur les aretes contigues.' 



The Count de Bournon, for reasons given in his ' Catalogue,' 

 &c. (p. 357) considers the primitive crystal to be a right rhom- 

 boidal tetraedral prism, with rhombic bases, of about 78° 30' and 

 101° 30' (fig. 11.) which are about the measurements of P on P, 

 and P over the elongated edge of a crystal represented by fig. 10. 

 He adds that the sulphate of lead is among those substances in 

 which there is no trace of natural joints, no possibility of obtaining 

 a cleavage.* 



* From the circumstance of the Count de Bournon having attributed to the angles of 

 what he conceives to be the primitive prism, measurements nearly approximating to those 

 of P on P, and of P on the opposed plane over the elongated edge of a crystal similar to fig. 



