Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/266

 the measurements of the obtuse angle vary from 103°. 45′. to 104°. 17′, the greater part of them being 31′ less than that obtained by Haüy, 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 transparent, 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 procured from the natural planes; the cause of which, not being then able to discover, I was compelled to forego the hope of determining the point by such means. On resuming these fragments sometime 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 determined 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 laminæ, injures the crystal by increasing the crevices. The only way in which I could succeed was by holding the specimen flat between the left forefinger and thumb, and applying without much force a sharp penknife to the thinner edge of the tabular crystal, pressing at the same