Additional observations on the optical properties and structure of heated glass and unannealed glass drops. By David Brewster, LL. D. F. R. S. Edin. and F. S. A. Edin. In a letter addressed to the Right Hon. Sir Joseph Banks, Bart. K. B. P. R. S

In this continuation of the author’s remarks on Rupert’s drops, he observes, that when they are made of colourless glass, and are held before a sheet of white paper, even the unbroken drops exhibit some appearances which he considers as marks of a crystalline texture; and when they are broken in such a manner that the parts are retained in their relative situation, the structure shows a cleavage in the direction of lines diverging from the apex, or rather in the form of conical layers, having a common axis, with their apex towards the blunt end of the drop. Since the surface of these drops is cooled suddenly by plunging into water, and fixed before the central parts are contracted, the author conceived that their specific gravity might be less than that of annealed drops; and indeed a difference was found upon trial, amounting to 1/45th part in the aggregate bulk of its external dimensions; but it appeared, on examination, that the contraction of the glass had taken place from within outwards, so as to leave cavities in the interior sufficient to account for the difference of weight, without supposing any intrinsic difference of density in the glass itself.