360 Mr. W. Phillips on the [N<r 



Article IV. 



On the primary Form of the Bournonite, as deduced from ils 

 Cleavage and Planes of Modification. By William Phillips, 

 FLS. fee. 



(To the Editor of the Annals of Philosophy.) 



In the Transactions of the Royal Society for 1804 is a com- 

 munication by the Comte de Bournon on this substance, with a 

 plate of 17 of its crystalline forms. In the volume for 1808 there 

 is a paper by James Smithson, FRS. on the forms of the crystals 

 as they occur in nature, and on the primary form as deduced by 

 him from the planes of modification to which it appeared to him 

 liable. The strictures of the latter gentleman induced the Comte 

 de Bournon to re-examine the Bournonite, as to its primary 

 crystal and modifying planes, and an elaborate article on the 

 subject is inserted in Nicholson's Journal. 



The Comte de Bournon, in the firsC instance, was induced to 

 assume the rectangular tetrahedral prism as the primary form. 

 Mr. Smithson substitutes the cube, asserting at the same time, 

 though not in very courtly phrase, that "of the 17 figures given 

 by the Comte de Bournon, great part are acknowledged to have 

 no existence, nor indeed are any of them consistent with nature ;" 

 a remark which doubtless originated, in part, in the want of accu- 

 racy in the drawings, but chiefly in the errors of Mr. Smithson's 

 own admeasurements* The Comte de Bournon, in Nicholson's 

 Journal, and afterwards in his own " Catalogue," substitutes a 

 square prism as the primary form, and gives a more detailed 

 series of the crystalline forms of this substance. 



The last determination of the Comte de Bournon appears 

 rather to have receded from, than approximated to the truth, 

 •though at least as near to it as the cube adopted by his oppo- 

 nent. 



It does not seem to have been known to either of these gentle- 

 men, that this substance possesses regular cleavages. It is 

 -divisible into two prisms ; namely, a rectangular prism, and a 

 rhombic prism, owing to its possessing two sets of cleavages — 

 a circumstance not very uncommon among minerals. 



In my collection are several specimens of the Bournonite 

 from Huel Boys, in Cornwall, and a considerable number of 

 isolated crystals ; one of them near an inch in length, and half 



* Of the eight angles given by this author, only two are correct : it may, therefore, 

 he taken for gi anted that what he has given are not actual measurements, especially since 

 they are given to a second, which no goniometer will indicate, but rather assumptions 

 ibunded on the known laws of decrement belonging to the cube, which he erroneously 

 assumed as the primary form. 



