Primitive Crystals of Qttartz y and the Sulphate of Barytes. 235 



so perfect, that the expectation of actual agreement in the admea- 

 surement to be obtained from them, naturally arose ; but in this I 

 was disappointed on finding them to vary on the one angle from 

 101°. W to 101°. 51'; and on the other, from 78°. 10' to 78°. 28'. 



This want of co-incidence in the admeasurements obtained from 

 crystals which, on account of their brilliancy, seemed unexception- 

 able, induced me to submit to the reflecting goniometer some 

 cleavages made in the direction of the primitive planes, and selected 

 with the utmost care. Six of these agreed perfectly in affording 

 on each of the acute angles of each fragment the incidence of 

 70°. 18', and on each of the obtuse angles that of 101°. 42'; the 

 former being 9'. 47" less, and the latter, in being 9'. 47" more, 

 than the value assigned to these angles by Haiiy. 



This remarkable agreement afforded by the cleavages of the sul- 

 phate of barytes, induces regret that it is not in like manner practi- 

 cable to obtain regular fractures of quartz. The evidence above 

 cited in regard to the difficulties attending the measurement of that 

 substance, by means of the reflections afforded by the natural planes 

 of its crystals, tend, it must be owned, to render it somewhat un- 

 certain whether the only co-incidences obtained, viz. those 94°. 15' 

 85°. 45', are the true angles of its primitive rhomboid; but the 

 co-incidences given by the fracture of the sulphate of barytes, seem 

 to authorize the conclusion that the true value of the angles of its 

 primitive crystals are 78°. 18' and 101°. 42'. 



It would not perhaps be reasonable to draw, from evidence rela- 

 ting only to two substances, the inference that the actual value of 

 the angles of mineral substances in the general cannot by any means 

 be obtained by admeasurements attempted upon the natural planes 

 of their crystals. The difficulties attendant on many of them will 

 be found to be very great ; in regard to some they seem insur- 



