260 Mr. William Phillips on the 



The crystals of the sulphate of lead, as well those from Anglesea 

 as those from Cornwall, have so great a tendency to become prismatic 

 by the lengthening of what Haiiy considers to be an octohedron, 

 and the Count de Bournon the primitive prism, that from among 

 several hundred crystals in my possession, I have been able to 

 select only one, having the appearance of an cctohedron with 

 pointed apices , and that this elongation always takes place in the 

 same direction is manifest both from the uniform position of the 

 secondary planes in relation to those which are elongated, and by 

 the measurements afforded by numerous crystals ; for although in 

 the latter respect there is not a perfect coincidence, the results are 

 sufficiently near to assure us of the fact. 



The common base formed by the meeting of the two pyramids 

 of the octohedron described by Haiiy as the primitive crystal, 

 (fig. 9.) though rectangular, is not square. The meeting of two 

 adjacent planes on one pyramid, with their continguous planes on 

 the other, will therefore be at different angles, as is the case also 

 in respect of the primitive octohedron of the carbonate of lead. 

 The angle formed by the meeting of P' with P' is according to 

 Haiiy 78° 28', and that of P with P 109° 18'. Six or seven crys- 

 tals submitted to the reflecting goniometer varied in P' on P' 

 from 76° 4' to 76° 20'. Nine crystals affording 16 measurements 

 of P with P varied from 101° 12' to 101° 28' ; and the same crys- 

 tals gave 17 measurements of P on the opposed plane of the same 



10, which is common to this substance, it may be assumed that he considers such a crystal 

 to be a rhomboidal prism with diedral terminations. It will appear that I agree with the 

 Count de Bournon in assuming the right rhomboidal prism with rhombic terminations as 

 the primitive form ; but differ from him in the manner in which that form lies (if it may 

 so be said) in a crystal represented by fig. 10. He conceives the planes P P, to be pri- 

 mitive : I am on the contrary induced to suppose the planes P' P' to be primitive planes ; 

 we consequently differ in the angles of the prism. 



