234 Mr. W. Phillips, on the angles of the 



afforded by these crystals were not perfectly clear, probably owing 

 to their enclosing a multitude of very minute diverging fibres, per- 

 haps of the oxide of iron. Numerous attempts were afterwards 

 made upon some crystals obtained under the name of Bornholm 

 diamonds; many of which exhibit, on the primitive crystal, the 

 planes of the modifications producing the pyramids and the prism, 

 not in a very advanced state ; the measurements they afforded were 

 much the same as, and by no means more regular than, those ob- 

 tained from the primitive crystals from Bristol. 



I next carefully examined about 300 very small, brilliant, and 

 colourless crystals of which I became possessed under the name of 

 Gibraltar diamonds, and selected a large number presenting the 

 most perfect reflections. These crystals generally shew both pyra- 

 mids, separated by a very short intervening prism. But even those 

 that were found to reflect the best, did not all present similar 

 results. 



Several of these however afforded perfect coincidences of 94°. 15' 

 on the one angle, and 85°. 45' on the other. Two of the seven 

 crystals from Bristol, and two or three from Cornwall, with brilli- 

 ant reflections, gave exactly the same admeasurements : and since 

 these are the only incidences that were found to agree, I am in- 

 duced to suppose that they approach, if not actually constitute, 

 the true value of the angles of the primitive rhomboid of quartz. 

 It will be observed that the former of these differs from that given 

 by Haiiy, in being 9' less ; the latter, in being 9' more. It seems 

 requisite to add, that among the very numerous crystals that I 

 have attempted to measure by means of the reflecting goniometer, 

 not one has afforded results agreeing with Haiiy. 



From among several hundreds of detached crystals of the sul- 

 phate of barytes, eight were selected, which afforded reflections 



