[ 175 ] 



XXI. On Murio-carbonate and Native Muriate of Lead. 

 By H. J. Brooke, Esq., F.R.S., $c* 



\ SPECIMEN I obtained some time since from Cornwall 

 ■** contains both these substances. I have not observed any 

 specimen of either of them in any of the collections that I 

 have seen in Cornwall, nor am I aware of the existence of 

 any others from that country, except a few single crystals of 

 the murio-carbonate in the collection of the late Wm. Phillips, 

 which came into his possession after the publication of his 

 " Mineralogy." 



The figure and measurements of this mineral given in page 

 343 of his work (edition of 1823)f do not afford data for 

 ascertaining the dimensions of the prism. 



I annex a figure of one of the crystals from Cornwall, from 

 which those elements may be deduced. 



The primary form is well known to be a square prism, and 

 from the inclination of P on a, the ratio of a terminal edge to 

 a lateral edge is found to be as 35 to 38 very nearly. There 

 are bright cleavages parallel to all the primary planes and to 

 both the diagonal planes of the prism. 



The symbols of the planes are A A 2 G G 

 The planes being a b d e 



M 



\^y 



P, M = 90° 

 P, a = 123 6' 

 P, b = 112 22 

 M, a = 126 20 

 M, b = 145 47 

 M, d = 1 35 

 M, e = 153 26 



Count Bournon gives 121° 52 ; as the inclination of P on 



* Communicated by the Author. 



f I cannot without injustice to the memory of my late friend William 

 Phillips refer to the recent edition of his Mineralogy, alterations of such 

 a nature having been made in it as to render it no longer his woik. 



The editor admits in his preface*, with equal candour and truth, his in- 

 capacity to do justice to the work, as indeed is apparent throughout the 

 book, for he has not only omitted to correct any of the errors, as far as 

 I have observed, but he has interpolated many additional mistakes, and in 

 a manner which might lead any reader to suppose them the blunders of 

 the author. Several however of these the editor might have avoided by 

 using the ordinary diligence of a compiler. 



The second angular measurement given in the work, Pon g 1 of quartz, 

 is more than 30° wrong, by a mistake of W. Phillips in copying from Haiiy, 



