Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/264



Corundum.

Fig. 3.

The form of the primitive crystal of corundum is a slightly acute rhomboid. From among the numerous fragments in my possession, two were selected, in the form of the primitive crystal, which, from the unusual splendour of some of their planes, gave reason for supposing they might be adapted to the use of the reflecting goniometer; one of these yielded the incidences of 86° 18′ and 93° 45′. The other one incidence of 94° 3′. These angles are given by Haüy 86° 38′ and 93° 22′, and by the Count de Bournon as 86° and 94°.

In searching for the cause of so great difference in fragments of the same substance, it occurred to me that it arose from the nature of the substance itself, or rather from the peculiar aggregation of its laminæ. Though the corundum is one of the hardest substances in nature, it is well known that its laminæ may be separated without the application of any violent mechanical force, and in some specimens, even with ease. Hence it occurred to me that this must be the consequence of some foreign substance being interposed between the layers of the substance itself; and that, if this were the case, it could not be expected to give coinciding measurements, because of the doubt whether the interposed body could be disposed with perfect regularity. There seemed therefore no hope of attaining the desired object, unless the specimen could be reduced into laminæ so extremely thin that there should remain nothing but corundum. Finding that this was not to be expected from the common varieties, I sought, and fortunately found a small fragment, nearly colourless