afforded by these crystals were not perfectly clear, probably owing to their enclosing a multitude of very minute diverging fibres, perhaps 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 obtained 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 I most perfect reflections. These crystals generally shew both pyramids, 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 co-incidences 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 brilliant reductions, gave exactly the same admeasurements: and since these are the only incidences that were found to agree, I am induced 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 Haüy, 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 Haüy.
From among several hundreds of detached crystals of the sulphate of barytes, eight were selected, which afforded reductions
