The experiments have been recently made on these drops by Dr Brewster, and published in the Phil. Trans. for 1814, Part II. and 1815, page 1.
Haying ascertained that glass melted, and suddenly cooled, possessed all the optical properties of crystallized bodies, 1 was anxious to determine if it exhibited any other marks of a crystalline structure. Upon examining the bulb of an unannealed drop AB, Plate CCLXXIV. Fig. 1. by holding it between the eye and a sheet of white paper, I observed a number of lines converging to the vertex a, as represented in Fig. 2. This structure was more or less apparent in every bulb which 1 examined, but never ap peared in annealed drops. It exhibited itself even on the surface, and seemed to be owing to an imperfect crystal line form, yet it was not marked with sufficient distinct ness to entitle me to consider it as the effect of crystalliza tion. In one specimen, however, where the bulb AB re maihed unshattered, while all the rest of the chop was burst in pieces, the lines diverging from a were most dis tinctly marked, and the bulb was actually cleft in the direction of these lines, so as to produce a real dislocation at the surface 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 surface, and produces a concentric structure like that of on onion. A third cleavage is visi ble 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. These cleavages are represented in section in Fig. 4.
As it appeared probable that the glass drops possessed a less degree of density than if they had been annealed, I attempted to ascertain this point by measuring their spe cific gravities in these two different states. The unan 'waled drops, however, had always one or more vacuities, such as E, F, Fig. 1. so that I was able to obtain only ap proximate results by estimating the magnitude of these cavities.
The following specific gravities were measured by my friend Air Jardine, with his usual correctness.
Unannealed flint glass drop, Fig. 1 3 20405 Annealed flint glass from the same pot . . . 3.2763 In order to correct the first of these measures, I mould ed a piece of bees' wax into the size and form of the cavities E, F, Fig. I. by examining them under a fluid of the same refractive power as the glass. I then formed the two pieces of wax into a sphere, and thus ascertained, with tolerable accuracy, the weight of a quantity of water of the same magnitude as the cavities. By this means, I obtained the corrected specific gravity of the unannealed drop 5.264.
'With the view of obtaining some farther insight into the structure of the crystallized drop, I brought the one, repre sented in Fig. 1, nearly to a red heat. Its shape suffered no change at this temperature, and the vacuities E, F, still remained ; but it had now lost the faculty of depolarisation, and the particles had therefore assumed a new arrange ment. By increasing the temperature, the cavities E, F, disappeared : the lower side of the drop, upon which it rested, was indented by the bottom of the crucible ; but it had in no other respect lest its external shape, the ap pearance of the cleavage in Fig. 2. remaining unaltered. In this state Mr Jardine measured the specific gravity of the di op, and found it to be 3.278, which is almost exactly the same as that of the annealed drop.
In order to observe the manner in which the cavities disappeared, I suspended one of the drops by a wire, and viewed it Nviti a telescopic microscope when exposed to a strong heat. Soon after the drop became red hot, the carries gradually contracted, and at last vanished, the cen tre of the cavity being the part that was last filled up. The drop had begun to melt at its smaller extremity, hut the lines represented in Fig. 2. were still visible, the heat probably not haying been sufficiently intense to affect its superficial structure.