Glass may also be cut under water by a pair of scissars, but in an imperfect manner. If the operation were per formed under a thick viscid fluid, the effect would be still more complete.
There is perhaps no substance to which the progress of the has been so much indebted as glass; and there is none which has contributed more to the splendour and the comfort of civilized society.
We do not propose to enter at present into any detailed account of the electrical, the chemical, or the optical pro perties of glass, which the reader will find fully discussed under our treatises on CHEMISTRY, ELECTRICITY, and OrTIcs. We intend merely to enumerate some of the physical properties, which either distinguish it from other bodies, or which could not with propriety be noticed under other heads.
Glass possesses the remarkable property of suffering no change by the application of the most intense heat. The effect of great heats is only to melt the glass, or to dissi pate it in vapour ; but as long as any of the glass remains, it still preserves its transparency, and other distinguish ing properties. The conversion of glass into porcelain by long continued cementation with other materials, happens only to that particular kind which is made of alkaline salt and sand.
Of all the solid substances whose expansibility has been accurately examined, glass possesses the property of be ing the least affected by heat and cold. Its expansion, according to General Roy, with an increase of heat equal to 180^ of Fahrenheit's thermometer, is only 0.000776, while that of platina is 0.000856, and that of hammered zinc 0.003011. On account of this property, glass is pe culiarly fitted for containing fluids whose expansions are under examination, as its own change of form may in ordi nary cases be neglected. For the same reason, it is better than any other substance for the simple pendulum of a clock. See EXPANSION.
The great ductility of glass is one of its most remarka ble properties. When heated to a sufficient degree, it may be moulded into any possible form with the utmost facility, and it can be drawn out into the finest fibres. The method of spinning glass is very simple. The operator
holds a piece of glass over the flame of a lump with one hand ; he then fixes a hook to the melted mass, and with drawing it, he obtains a thread of glass attached to the hook. The hook is then fixed in the circumference of a cylindrical drum, which can be turned round by the hand ; and a rotatory motion being given to the drum, the glass is drawn in the finest threads from the fluid mass, and coil ed round the cylindrical circumference. Al. Reaumur supposed, with great probability, that the flexibility of glass increased with the fineness of the threads, and he therefore conjectured, that if they were drawn to a suffi cient degree of fineness, they might be used in the fabri cation of stuffs. He succeeded in making them as fine as a spider's web, but he was never able to obtain them of a sufficient length when their diameter was so much reduc ed. The circumference of these threads is generally a flat oval, about three or four times as broad as it is thick. By using opake and transparent glass of different colours, artists have been enabled to produce the most beautiful ornaments.
When glass has been annealed or cooled slowly, it is able to resist very considerable force without being broken; but when it has been cooled suddenly, either by exposure in the open air, or by immersion in water, it exhibits very remarkable properties. These properties are shewn in what are called Prince Rupert's drops, and glass cups.
The phenomena and the formation of Prince Rupert's drops, and the theory of their explosion, have already been explained in our article ANNEALING. The earliest ex periments upon glass tears were made in 1656, both in London and Paris ; but it is not certain in what country they were invented. They were first brought to England by Prince Rupert, third son of the riector Palatine, Fre derick V. and the Princess Elizabeth, dauzhter of James I. and experiments were made upon them by tn., Hon. Sir Robert Moray, in 1661, by the command of ma jesty. An account of these experiments is to be found in the Registers of the Royal Society, of which he was oae of the founders.