CURIOUS PROPERTIES OF GLASS.
Out of these elements, slightly varied according to circumstances, are produced the whole miracles of the glass-house. To any one, not previously acquainted with the component ingredients, the surprise which this information must naturally excite will be much increased upon being apprised of a few of the peculiarities or properties of glass. Transparent in itself, the materials of which it is composed are opaque. Brittle to a proverb when cold, its tenuity and flexibility when hot are so remarkable that it may be spun into filaments as delicate as cobwebs, drawn out like elastic threads till it becomes finer than the finest hair, or whisked, pressed, bent, folded, twisted or moulded into any desired shape. It is impermeable to water, suffers no diminution of its weight or quality by being melted down, is capable of receiving and retaining the most lustrous colors, is susceptible of the most perfect polish, can be carved and sculptured like stone or metal, never loses a fraction of its substance by constant use, and, notwithstanding its origin, is so insensible to the action of acids that it is employed by chemists for purposes to which no other known substance can be applied.
The elasticity and fragility of glass are among its most extraordinary phenomena. Its elasticity exceeds that of almost all other bodies. If two glass balls are made to strike each other at a given force, the recoil, by virtue of their elasticity, will be nearly equal to the original impetus. Connected with its brittleness are some very singular facts. Take a hollow sphere, with a hole, and stop the hole with your finger, so as to prevent the external and internal air from communicating, and the sphere will fly to pieces by the mere heat of the hand. Vessels made of glass that has been suddenly cooled possess the curious property of being able to resist hard blows given to them from without, but will be instantly shivered by a small particle of flint dropped into their cavities. This property seems to depend upon the comparative thickness of the bottom. The thicker the bottom is, the more certainty of breakage by this experiment. Some of these vessels, it is stated, have resisted the strokes of a mallet, given with sufficient force to drive a nail into wood; and heavy bodies, such as musket-balls, pieces of iron, bits of wood, jasper, bone, &c., have been cast into them from a height of two or three feet without any effect; yet a fragment of flint, not larger than a pea, let fall from the fingers at a height of only three inches, has made them fly. Nor is it the least wonderful of these phenomena that the glass does not always break at the instant of collision, as might be supposed. A bit of flint, literally the size of a grain, has been dropped into several glasses successively, and none of them broke; but, being set apart and watched, it was found that they all flew in less than three-quarters of an hour. This singular agency is not confined to flint. The same effect will be produced by diamond, sapphire, porcelain, highly-tempered steel, pearls, and the marbles that boys play with.[6]
Several theories have been hazarded in explanation of the mystery; but none of them are satisfactory. Euler attempted to account for it on the principle of percussion; but if it were produced by percussion the fracture would necessarily be instantaneous. The best solution that can be offered, although it is by no means free from difficulties, refers the cause of the disruption to electricity. There is no doubt that glass, which has been suddenly cooled, is more electric than glass that has been carefully annealed—a process which we will presently explain; and such glass has been known to crack and shiver from a change of temperament, or from the slightest scratch. The reason is obvious enough. When glass is suddenly cooled from the hands of the artificer, the particles on the outer side are rapidly contracted, while those on the inner side, not being equally exposed to the influence of the atmosphere, yet remain in a state of expansion. The consequence is that the two portions are established on conflicting relations with each other, and a strain is kept up between them which would not exist if the whole mass had undergone a gradual and equal contraction, so that when a force is applied which sets in motion the electric fluid glass is known to contain, the motion goes on propagating itself till it accumulates a power which the irregular cohesion of the particles is too weak to resist. This action of the electric fluid will be better understood from an experiment which was exhibited before the Royal Society upon glass vessels with very thick bottoms, which, being slightly rubbed with the finger, broke after an interval of half an hour.[7] The action of the electric fluid in this instance is sufficiently clear; but why the contact with fragments of certain bodies should produce the same result, or why that result is not produced by contact with other bodies of even greater size and specific gravity, is by no means obvious.
Among the strangest phenomena observed in glass are those which are peculiar to tubes. A glass tube placed in a horizontal position before a fire, with its extremities supported, will acquire a rotatory motion round its axis, moving at the same time toward the fire, notwithstanding that the supports on which it rests may form an inclined plane the contrary way. If it be placed on a glass plane—such as a piece of window-glass—it will move from the fire, although the plane may incline in the opposite direction. If it be placed standing nearly upright, leaning to the right hand, it will move from east to west; if leaning to the left hand, it will move from west to east; and if it be placed perfectly upright, it will not move at all. The causes of these phenomena are unknown, although there has been no lack of hypotheses in explanation of them.[8]
It is not surprising that marvels and paradoxes should be related of glass, considering the almost incredible properties it really possesses. Seeing that it emits musical sounds when water is placed in it, and it is gently rubbed on the edges; that these sounds can be regulated according to the quantity of water, and that the water itself leaps, frisks, and dances, as if it were inspired by the music; seeing its extraordinary power of condensing vapor, which may be tested by simply breathing upon it; and knowing that, slight and frail as it is, it expands less under the influence of heat than metallic substances, while its expansions are always equable and proportioned to the heat, a quality not found in any other substance, we can not be much astonished at any wonders which are superstitiously or ignorantly attributed to it, or expected to be elicited from it. One of the most remarkable is the feat ascribed to Archimedes, who is said to have set fire to the Roman fleet at the siege of Syracuse by the help of burning-glasses. The fact is attested by most respectable authorities,[9] but it is only right to add, that it is treated as a pure fable by Kepler and Descartes, than whom no men were more competent to judge of the possibility of such an achievement. Tzetzez relates the matter very circumstantially; he says that Archimedes set fire to Marcellus's navy by means of a burning glass composed of small square mirrors, moving every way upon hinges; which, when placed in the sun's rays, directed them upon the Roman fleet, so as to reduce it to ashes at the distance of a bow-shot. Kircher made an experiment founded upon this minute description, by which he satisfied himself of the practicability of at least obtaining an extraordinary condensed power of this kind. Having collected the sun's rays into a focus, by a number of plain mirrors, he went on increasing the number of mirrors until at last he produced an intense degree of solar heat; but it does not appear whether he was able to employ it effectively as a destructive agent at a long reach. Buffon gave a more satisfactory demonstration to the world of the capability of these little mirrors to do mischief on a small scale. By the aid of his famous burning-glass, which consisted of one hundred and sixty-eight little plain mirrors, he produced so great a heat as to set wood on fire at a distance of two hundred and nine feet, and to melt lead at a distance of one hundred and twenty, and silver at fifty; but there is a wide disparity between the longest of these distances and the length of a bowshot, so that the Archimedean feat still remains a matter of speculation.