256 Mr. William Phillips on the 



the measurements of the obtuse angle vary from 103°. 45'. to 

 104°. 17', the greater part of them being 31' less than that ob- 

 tained by Haiiy, as I presume also on the natural planes. 



These disagreements induced me to attempt the splitting them 

 parallel with the natural planes of the prism ; for which purpose 

 those from the neighbourhood of Bristol, being flat, nearly trans- 

 parent, and almost always exhibiting the directions of their natural 

 joints, seemed well adapted. But the result was not at first equal 

 to the promise. For though they were readily split, the fragments 

 first obtained yielded results agreeing scarcely better than those pro- 

 cured from the natural planes ; the cause of which, not being then 

 able to discover, I was compelled to forego the hope of determin- 

 ing the point by such means. On resuming these fragments some- 

 time afterwards, their examination induced the suspicion, that the 

 differences in their results under the reflecting goniometer, arose 

 from the numerous crevices observable, when a fragment was held 

 between the eye and the light, in almost all the flat crystals from 

 the neighbourhood of Bristol ; but which were no doubt increased 

 in the fragments just alluded to by the manner of cleaving them ; 

 not having then discovered the best mode of effecting it. I deter- 

 mined therefore to reduce these fragments, until small portions with 

 splendid surfaces should be obtained, nearly or wholly free from 

 any crevice. And as the crystals are at once soft and very brittle, 

 the utmost care was requisite. When laid on a table, with the edge 

 of a sharp penknife placed in the direction of their natural joints, 

 the pressure or slight blow requisite to divide the laminae, injures 

 the crystal by increasing the crevices. The only way in which I 

 could succeed was by holding the specimen flat between the left fore- 

 finger and thumb, and applying without much force a sharp pen- 

 knife to the thinner edge of the tabular crystal, pressing at the same 



