IX. On the Sublimation of Silica.
By J. Mac Culloch, M.D. F.L.S. Chemist to the Ordnance, and Lecturer on Chemistry at the Royal Military Academy at Woolwich.
V. Pr. Geo. Soc.
SOME years ago, being in pursuit of another object, a mixture of the oxides of tin and lead was exposed to the heat of an air furnace in an English crucible, to the top of which was luted another of the same sort. This apparatus was allowed to remain in the fire for some hours. No account of the heat was taken, but I have on former occasions produced in the same furnace a heat sufficient to contract one of Mr. Wedgwood's original clay pieces to the 130th and 140th degree of his scale. On removing the crucibles, the insides of both from the bottom of the lowermost to within a third part of the top of the uppermost, were found covered with white brilliant filamentous crystals crossing each other in all directions. I concluded that they consisted of the oxide of tin, or perhaps that of lead, and subjected them to the obvious experiments necessary for ascertaining this circumstance. Failing to confirm this supposition, I then conjectured that they might consist of silex. The quantity I procured scarcely amounted to half a grain, and I therefore divided it into two parts, that I might have the satisfaction of confirming or refuting my own trials by comparison with those of some chemical friend. Mr. Aikin was so good as to undertake the examination of the reserved portion, and from his well known accuracy, the Society will naturally place confidence in our mutual results. On igniting