XII. An Account of some attempts to ascertain the angles of the Pri- 

 mitive Crystals of Quartz and of the Sulphate of Barytes, by means 

 of the reflecting Goniometer ; together with practical reasons for 

 presuming that the admeasurements assigned by Hauy to several 

 varieties of the parallelopiped and of the octohedron are inaccurate* 



By WILLIAM PHILLIPS, Esq. 



MEMBER OF THE GEOLOGICAL SOCIETY. 

 [Read 16th June, 1815.] 



XHE primitive crystal of quartz is considered to be an obtuse 

 rhomboid, of which the angles are given by Hauy in his ' Tableau 

 comparatif,' &c. as being 94° 24' and 85° 36' : that of the sulphate 

 of barytes is a quadrangular prism with rhombic terminations, the 

 angles of which according to the same authority are 101° 32' 13" 

 and 78° 27' 47". The results of some attempts to verify these ad- 

 measurements by subjecting the natural planes of the crystals of 

 both these substances, as well as some regular fragments of the latter, 

 to the reflecting goniometer, form the particular object of the pre- 

 sent communication. 



The first attempts to ascertain by this means the angles of the 

 rhomboid of quartz, were made upon some minute primitive crys- 

 tals from Bristol : seven of these gave incidences on the one angle 

 varying from 94° 12' to 94° 17', and on the other from 85° 44' to 

 85° 52' ; not more than two or three agreed. But the reflections 



