I. 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

Dear Sir In a former paper on the optical properties of heated glass and unannealed glass drops, I have briefly described the leading phenomena which they exhibit in their action upon polarised light. These experiments I have frequently repeated with the same results, and I have the satisfaction also of stating, that as soon as they were known in France, they were repeated and verified by M. Biot of the National Institute, to whose active genius this branch of optics owes great obligations. Having ascertained that glass melted and suddenly cooled, possessed all the optical properties of crystallized bodies, I was anxious to determine if it exhibited any other marks of a crystalline structure. Upon examining the bulb of an unannealed drop AB, Pl I., fig. 1, by holding it between the eye and a sheet of white paper, I observed a number of lines converging to the vertexa , as represented in fig. 2. This structure was more or less apparent in every bulb which I examined, but never appeared in annealed drops. It exhibited itself even on the surface, and seemed to be owing to an imperfect crystalline form, yet it was not marked with sufficient distinctness to entitle me to consider it as the effect of crystallization. In one specimen, however, where the bulb AB remained unshattered, while all the rest of the drop was burst in pieces, the lines diverging froma were most distinctly marked, and the bulb was actually cleft in the direction of these lines, so as to produce a real dislocation at thesurface of the drop. We may therefore consider the drop as possessing that crystalline structure which gives cleavages in the direction of lines diverging from its apex. By examining the fragments of the drop after it is burst, another cleavage is distinctly perceptible: it is parallel to the outer surface, and produces a concentric structure like that of an onion. This cleavage also shows itself in the splinters which are detached from the surface of the drop when it is ground upon freestone. A third cleavage is visible in the direction of lines inclined to the axis of the drop, as represented in fig. 3 ; but it is not so distinct as the two first.