Glass of the class called Prince Rupert Drops exhibits another striking property. Let the small point be broken, and the whole flies with a shock into powder. Writers have endeavored to solve the philosophy of this phenomenon; some by attributing it to percussion putting in motion some subtle fluid with which the essential substance of glass is permeated, and thus the attraction of cohesion being overcome. Some denominate the fluid electricity, and assert that it exists in glass in great quantities, and is capable of breaking glass when well annealed. These writers do not appear to have formed any conclusion satisfactory to themselves, and fail to afford any well-defined solution to the mystery.
Another phenomenon in connection with glass tubes is recorded in the "Philosophical Transactions," No. 476:—
"Place a tube, say two feet long, before a fire, in a horizontal position, having the position properly supported, say by putting in a cork at each end supported by pins for an axis; the rod will acquire a rotary motion round the axis, and also a progressive motion towards the fire, even if the supporters are declined from the fire. When the progressive motion of the tube towards the fire is stopped by any obstacle, the rotation is still continued. When the tubes are placed in nearly an upright position, leaning to the right hand, the motion will be from east to west; but if they lean to the left hand, their motion will be from west to east; and the nearer they are placed to an upright position the less will be their motion either way. If the tubes be placed on a sheet of glass, instead of moving towards the fire they will move from it, and about the axis in a contrary direction from what they did before; nay, they will recede from the fire, and move a little upwards when the plane inclines towards the fire."
Glass is used for pendulums, as not being subject to affections from heat or cold. It is, as is well known, a non-conductor. No metallic condenser possesses an equal power with one of glass. In summer, when moisture fails to collect on a metallic surface, open glass will gather it on the exterior; the slightest breath of air evidently affecting the glass with moisture. Dew will affect the surface of glass while apparently uninfluential upon other surfaces.
The properties of so-called "musical glasses" are strikingly singular. Glass bowls, partly filled with water, in various quantity, will, as is well known, emit musical sounds, varying with the thickness of their edges or lips. When rubbed, too, with a wet finger, gently, the water in the glass is plainly seen to tremble and vibrate.
Bells manufactured of glass have been found the clearest and most sonorous; the vibration of sound extending to a greater degree than in metallic bells.
Glass resists the action of all acids except the "fluoric." It loses nothing in weight by use or age. It is more capable than all other substances of receiving the highest degree of polish. If melted seven times over and properly cooled in the furnace, it will receive a polish rivalling almost the diamond in brilliancy. It is capable of receiving the richest colors procured from gold or other metallic coloring, and will retain its original brilliancy of hue for ages. Medals, too, embedded in glass, can be made to retain forever their original purity and appearance.
Another singular property of glass is shown in the fact, that when the furnace, as the workmen term it, is settled, the metal is perfectly plain and clear; but if by accident the metal becomes too cool to work, and the furnace heat required to be raised, the glass, which had before remained in the open pots perfectly calm and plain, immediately becomes agitated or boiling. The glass rises in a mass of spongy matter and bubbles, and is rendered worthless. A change is however immediately effected by throwing a tumbler of water upon the metal, when the agitation immediately ceases, and the glass assumes its original quiet and clearness.
All writers upon the subject of glass manufacture fail to show anything decisive upon the precise period of its invention. Some suppose it to have been invented before the flood. Nervi traces its antiquity to the yet problematical time of Job.
It seems clear, however, that the art was known to the Egyptians thirty-five hundred years since; for records handed down to us in the form of paintings, hieroglyphics, &c., demonstrate its existence in the reign of the first Osirtasen, and existing relics in glass, taken from the ruins of Thebes, with hieroglyphical data, clearly place its antiquity at a point fifteen centuries prior to the time of Christ.