XIII. On the measurements , by the reflecting Goniometer, of certain 

 primitive Crystals ; with observations on the methods of obtaining 

 them by mechanical division along the natural joints of Crystals. 



By WILLIAM PHILLIPS, Esq. 



MEMBER OF THE GEOLOGICAL SOCIETY. 

 (Read 6th December, 1816.] 



J-N a communication read before the Society about the middle of 

 last year, I detailed some reasons for concluding that the angles of 

 some primitive crystals included in the terms parallelopiped, as well 

 as some varieties of the octohedron, had not been accurately ascer- 

 tained. Since that time, further attention to the subject has con- 

 firmed those observations. I proceed to lay before the Society the 

 results of investigations in regard to ten other substances, two 

 or three of which have been measured by the assistance of the 

 reflecting goniometer only upon their natural planes, on account 

 either of their not yielding to mechanical division with sufficient 

 freedom, or not yielding to it at all. The rest have been fractured 

 with exactness enough to allow the use of that instrument ; and for 

 that reason, the results allow of more complete confidence, than if 

 there had been a necessity for relying on their natural planes. 



It would have spared me much time and difficulty, if to the other 

 labours of the Abbe Haiiy and the Count de Bournon, they had added 

 some account of the means by which the mechanical division of 



