Читать книгу Harper's New Monthly Magazine, No. IX.—February, 1851.—Vol. II. - Various - Страница 5

Upward of two thousand years ago, perhaps three, a company of merchants, who had a cargo of nitre on board their ship, were driven by the winds on the shores of Galilee, close to a small stream that runs from the foot of Mount Carmel. Being here weather-bound till the storm abated, they made preparations for cooking their food on the strand; and not finding stones to rest their vessels upon, they used some lumps of nitre for that purpose, placing their kettles and stew-pans on the top, and lighting a strong fire underneath. As the heat increased, the nitre slowly melted away, and flowing down the beach, became mixed up with the sand, forming, when the incorporated mass cooled down, a singularly beautiful, transparent substance, which excited the astonishment and wonder of the beholders.

Such is the legend of the origin of Glass.

A great many centuries afterward – that is to say, toward the close of the fifteenth century of the Christian era – when some of the secrets of the glass-house, supposed to have been known to the ancients, were lost, and the simple art of blowing glass was but scantily cultivated – an artificer, whose name has unfortunately escaped immortality, while employed over his crucible accidentally spilt some of the material he was melting. Being in a fluid state it ran over the ground till it found its way under one of the large flag-stones with which the place was paved, and the poor man was obliged to take up the stone to recover his glass. By this time it had grown cold, and to his infinite surprise he saw that, from the flatness and equality of the surface beneath the stone, it had taken the form of a slab – a form which could not be produced by any process of blowing then in use.

Such was the accident that led to the discovery of the art of casting Plate-Glass.

These are the only accidents recorded in the History of Glass. For the rest – the discovery of its endless capabilities and applications – we are indebted to accumulated observation and persevering experiment, which, prosecuting their ingenious art-labors up to the present hour, promise still farther to enlarge the domain of the Beautiful and the Useful.

The importance of glass, and the infinite variety of objects to which it is applicable, can not be exaggerated. Indeed it would be extremely difficult to enumerate its properties, or to estimate adequately its value. This thin, transparent substance, so light and fragile, is one of the most essential ministers of science and philosophy, and enters so minutely into the concerns of life, that it has become indispensable to the daily routine of our business, our wants, and our pleasures. It admits the sun and excludes the wind, answering the double purpose of transmitting light and preserving warmth; it carries the eyes of the astronomer to the remotest region of space; through the lenses of the microscope it develops new worlds of vitality which, without its help, must have been but imperfectly known; it renews the sight of the old, and assists the curiosity of the young; it empowers the mariner to descry distant ships, and to trace far-off shores, the watchman on the cliff to detect the operations of hostile fleets and midnight contrabandists, and the lounger in the opera to make the tour of the circles from his stall; it preserves the light of the beacon from the rush of the tempest, and softens the flame of the lamps upon our tables; it supplies the revel with those charming vessels in whose bright depths we enjoy the color as well as the flavor of our wine; it protects the dial whose movements it reveals; it enables the student to penetrate the wonders of nature, and the beauty to survey the marvels of her person; it reflects, magnifies, and diminishes; as a medium of light and observation its uses are without limit; and as an article of mere embellishment, there is no form into which it may not be moulded, or no object of luxury to which it may not be adapted.

Yet this agent of universal utility, so valuable and ornamental in its applications, is composed of materials which possess in themselves literally no intrinsic value whatever. Sand and salt form the main elements of glass. The real cost is in the process of manufacture.

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.

WHY IS NOT GLASS MALLEABLE?

In the region of glass, we have a puzzle as confounding as the philosopher's stone (which, oddly enough, is the name given to that color in glass which is known as Venetian brown sprinkled with gold spangles), the elixir vitæ, or the squaring of the circle, and which has occasioned quite as much waste of hopeless ingenuity. Aristotle, one of the wisest of men, is said, we know not on what authority, to have originated this vitreous perplexity by asking the question. "Why is not glass malleable?" The answer to the question would seem to be easy enough, since the quality of malleability is so opposed to the quality of vitrification, that, in the present state of our knowledge (to say nothing about the state of knowledge in the time of Aristotle) their co-existence would appear to be impossible. But, looking at the progress of science in these latter days, it would be presumptuous to assume that any thing is impossible. Until, however, some new law of nature, or some hitherto unknown quality shall have been discovered, by which antagonist forces can be exhibited in combination, the solution of this problem may be regarded as at least in the last degree improbable.

Yet, in spite of its apparent irreconcilability with all known laws, individuals have been known to devote themselves assiduously to its attainment, and on more than one occasion to declare that they had actually succeeded, although the world has never been made the wiser by the disclosure of the secret. A man who is possessed with one idea, and who works at it incessantly, generally ends by believing against the evidence of facts. It is in the nature of a strong faith to endure discouragement and defeat with an air of martyrdom, as if every fresh failure was a sort of suffering for truth's sake. And the faith in the malleability of glass has had its martyrology as well as faith in graver things. So far back as the time of Tiberius, a certain artificer, who is represented to have been an architect by profession, believing that he had succeeded in making vessels of glass as strong and ductile as gold or silver, presented himself with his discovery before the Emperor, naturally expecting to be rewarded for his skill. He carried a handsome vase with him, which was so much admired by Tiberius that, in a fit of enthusiasm, he dashed it upon the ground with great force to prove its solidity, and finding, upon taking it up again, that it had been indented by the blow, he immediately repaired it with a hammer. The Emperor, much struck with so curious an exhibition, inquired whether any body else was acquainted with the discovery, and being assured that the man had strictly preserved his secret, the tyrant instantly ordered him to be beheaded, from an apprehension that if this new production should go forth to the world it would lower the value of the precious metals.10 The secret, consequently, perished. A chance, however, arose for its recovery during the reign of Louis XIII., a period that might be considered more favorable to such undertakings; but unfortunately with no better result. The inventor on this occasion submitted a bust formed of malleable glass to Cardinal Richelieu, who, instead of rewarding him for his ingenuity, sentenced him to perpetual imprisonment, on the plea that the invention interfered with the vested interests of the French glass manufacturers.11 We should have more reliance on these anecdotes of the martyrs of glass, if they had bequeathed to mankind some clew to the secret that is supposed to have gone to the grave with them. To die for a truth, and at the same time to conceal it, is not the usual course of heroic enthusiasts.

Many attempts have been made to produce a material resembling glass that should possess the quality of malleability, and respectable evidence is not wanting of authorities who believed in its possibility, and who are said to have gone very near to its accomplishment. An Arabian writer12 tells us that malleable glass was known to the Egyptians; but we must come closer to our own times for more explicit and satisfactory testimony. Descartes thought it was possible to impart malleability to glass, and Boyle is reported to have held the same opinion. But these are only speculative notions, of no further value than to justify the prosecution of experiments. Borrichius, a Danish physician of the seventeenth century, details an experiment by which he obtained a malleable salt, which led him to conclude that as glass is for the most part only a mixture of salt and sand, he saw no reason why it should not be rendered pliant. The defect of his logic is obvious; but, setting that aside, the fallacy is practically demonstrated by his inability to get beyond the salt. Borrichius also thought that the Roman who made the vase for Tiberius, may have successfully used antimony as his principal ingredient. Such suppositions, however, are idle in an experimental science which furnishes you at once with the means of putting their truth or falsehood to the test. There is a substance known to modern chemistry, luna cornea, a solution of silver, which resembles horn or glass, is transparent, easily put into fusion, and is capable of bearing the hammer. Kunkel thought it was possible to produce a composition with a glassy exterior that should possess the ductile quality; but neither of these help us toward an answer to Aristotle's question. Upon a review of the whole problem, and of every thing that has been said and done in the way of experiment and conjecture, we are afraid we must leave it where we found it. The malleability of glass is still a secret.

DESCRIPTION OF A GLASS-HOUSE

Dismissing history and theory, we will now step into the glass-house itself, where the practical work of converting sand into goblets, vases, mirrors, and window-panes is going forward with a celerity and accuracy of hand and head that can not fail to excite wonder and admiration. As the whole agency employed is that of heat, the interior of the manufactory consists of furnaces specially constructed for the progressive processes to which the material is subjected before it is sent out perfected for use. Look round this extensive area, where you see numbers of men in their shirt-sleeves, with aprons before them, and various implements in their hands, which they exercise with extraordinary rapidity, and you will soon understand how the glittering wonders of glass are produced. Of these furnaces there are three kinds, the first called the calcar, the second the working furnace, and the third the annealing oven, or lier.

The calcar, built in the form of an oven, is used for the calcination of the materials, preliminary to their fusion and vitrification. This process is of the utmost importance: it expels all moisture and carbonic acid gas, the presence of which would hazard the destruction of the glass-pots in the subsequent stages of the manufacture, while it effects a chemical union between the salt, sand, and metallic oxides, which is essential to prevent the alkali from fusing and volatilizing, and to insure the vitrification of the sand in the heat of the working furnace, to which the whole of the materials are to be afterwards submitted.

The working furnace, which is round, and generally built in the proportion of three yards in diameter to two in height, is divided into three parts, each of which is vaulted. The lower part, made in the form of a crown, contains the fire, which is never put out. Ranged round the circumference inside are the glass-pots or crucibles, in which the frit, or calcined material, is placed to be melted; and from several holes in the arch of the crown below issues a constant flame which, enveloping the crucibles, accomplishes the process of melting. Round the exterior of the furnace, you perceive a series of holes or mouths; these are called boccas, from the Italian, and it is through them the frit is served into the crucibles and taken out when melted. The volume of heat is here so intense, that the boccas are provided with movable collars or covers, generally composed of lute and brick, to screen the eyes of the workmen who stand outside in recesses formed for the purpose in the projections of the masonry. The severest part of the work arises when any of the pots, or crucibles, happen to become cracked or worn out, in which case the bocca must be entirely uncovered, the defective pot taken out with iron hooks and forks, and a new one substituted in its place through the flames by the hands of the workman. In order to enable him thus literally to work in the fire, he is protected by a garment made of skins in the shape of a pantaloon, and heavily saturated with water. This strange garment completely covers him from head to foot, all except his eyes, which are defended by glasses.

The material being now melted is fashioned into the desired forms by the hands of the workmen while it is yet hot, and then placed to cool gradually in the third furnace, or annealing oven, called the lier. This oven is a long, low chamber, heated at one end, and furnished with movable iron trays or pans, called fraiches (from the French), upon which the various articles are set down, and finally removed, when they are sufficiently cold, through an opening which communicates with the sarosel, or room where the finished articles are kept.

The intensity of the fire requires that the furnaces and crucibles, should be constructed of materials the least fusible in their nature, and the best calculated to resist the violent and incessant action of heat; or the manufacturer will incur the most serious losses and delays from casualties which, even after the most careful and costly outlay, can not be always averted. The crucibles especially demand attention in this respect, in consequence of the solvent property of some of the materials which are melted in them. These crucibles are deep pots, varying in size according to the extent or objects of the manufacture; and some notion may be formed of the importance attached to them from the fact, that they are not unfrequently made large enough to contain individually not less than a ton weight of glass. Great skill and care are requisite in their structure, so as to adapt them to the temperature in which their qualities are to be tested; and even with the utmost attention that can be bestowed upon them, they are often found to break soon after they are exposed to the furnace, by which heavy losses are entailed upon the manufacturer. Nor is this the only point which must be considered. The size of the crucible should bear a proportionate relation to that of the furnace, or one of two consequences, equally to be avoided, will ensue; either that there will be a waste of fuel, if the crucibles are too small, or an inadequate heat, if they are too large.13

We have now before us the three principal processes – the calcination, by which the materials are prepared in the first instance – the melting down of these materials into glass in the great working furnace, and the annealing of the finished article after it has been fashioned by the workmen. These processes are broad and simple; but that part of the manufacture which is, probably, most calculated to surprise the uninitiated, is the manner in which the red-hot mass of glass, as it is taken out of the crucible, is instantly, so to speak, shaped into form by the dextrous hands and practiced eyes of those men whom you see standing about at tables and stools, twisting long iron rods called pontils, blowing through pipes, and performing mysterious evolutions with scissors, pronged sticks, compasses, and other instruments, with a rapidity that baffles the most vigilant observer. From the infinite diversity of objects into which glass is thus moulded, it must be obvious that the operations of these artificers embrace a variety of curious details which it is impossible to enter upon here; but a glance at some of them will enable the reader to form a general notion of the curious manipulations upon which they are so actively employed.

The initial movement of the glass-blower is to dip a hollow iron rod or tube, about five feet long, through the bocca, into one of the crucibles containing the melted glass. Having collected at the end of the tube a sufficient quantity of material for the article he is about to fashion – a drinking-glass, finger-glass, jug, or whatever it may be (which requires, perhaps, two or three dips according to the quantity he wants), he withdraws the tube, and holds it perpendicularly for a few seconds with the heated mass downward, till the fluid drops and lengthens by its own momentum beyond the end of the tube. He then quickly raises it, and rolls it on a smooth horizontal plate till it acquires a cylindrical form. When he has got it into this shape, he applies his mouth to the opposite end of the tube, and blows into the heated mass which swiftly becomes distended into a sphere. But as the globe thus obtained is not rendered sufficiently thin for his purpose by a single blowing, he reheats it by holding it within the furnace, and then blows again, repeating the operation till he brings it to the desiderated size and consistency. Thus prepared, he swings it in the air like a pendulum, or twirls it round and round rapidly, according to the elongated or circular form he requires, the molten particles obeying the tendency of the force and motion employed.

Having advanced to this stage, and the mass being ready for fashioning, a new instrument is brought to bear upon it. This is a small, solid, round iron rod, called the pontil, upon one end of which a lesser portion of material has been collected by another workman, and this portion being applied to the extremity of the globe already formed rapidly adheres to it. The whole is now detached from the tube, or blowpipe, by simply damping the point of contact, which causes the glass to crack, so that a stroke upon the tube separates it safely, leaving a small hole in the globe where the tube had originally entered.

By this time the temperature of the mass has cooled down, and it becomes necessary to reheat it, which is done as before. The artificer next seats himself on a stool with elevated arms, upon which he rests the pontil, which he grasps and twirls with his left hand, having thus a command over the red-hot glass with his right hand, in which he holds a small iron instrument called a procello, consisting of two blades with an elastic bow, similar to a sugar-tongs. With this little instrument the whole work of fashioning is performed, and as it must be completed while the glass is yet ductile (having always, however, the power of reheating it when necessary), the process is effected with wondrous celerity. By the aid of the procello he enlarges or contracts the mass, which he adapts to its motions with his left hand, and where any shapeless excrescences appear he instantly cuts them off with a pair of scissors as easily as if they were so much lace or cotton. And thus, almost in less time than it has occupied us in the description, articles of the most exquisite form and delicacy are created by the art-magic of these Vulcans of the glass-furnace.

That which chiefly excites astonishment and admiration in the spectator is the ease and security with which a material so fragile is cut, joined, twirled, pressed out and contracted, by the hands of the workmen. Long practice alone can insure the requisite certainty and quickness of manipulation, and the eye must be highly educated to its work before it can achieve off-hand, and, by a sort of accomplished instinct, the beautiful shapes which are thus rapidly produced.

The moment the article is finished it is detached from the pontil and dropped into a bed of ashes, from whence it is removed while it is yet hot, by a pronged stick or wooden shovel, to the tray to be deposited in the annealing oven where it is gradually cooled.

HOW CROWN, PLATE, AND WATCH GLASSES ARE MADE

In making crown-glass, which is used for windows, a slight alteration in the process is observed. When the globe is prepared as before at the end of the tube, it is flattened at its extremity by pressure against a plain surface; the new material at the end of the pontil is then attached to the flattened side, and the whole mass detached from the tube, leaving a circular hole at the point of separation. The mass is now twirled round and round, at first slowly, then more quickly, till its diameter, obeying the centrifugal force, becomes wider and wider, the hole expanding in proportion. At last, as the motion increases in velocity, the double portion suddenly bursts open, the whole forming a plain disc of uniform density throughout, except at the spot in the centre where the pontil is attached to it, and where there is accumulated that small lump which is vulgarly called a bull's eye. The most surprising incident in this process is the bursting open of the flattened globe, a circumstance which would shiver the entire mass if it were not kept up at a certain heat.

The mode of casting plate-glass presents a remarkable illustration of the skillful adaptation of means to ends. When the glass is melted in the crucible, a portion of it is transferred to a smaller crucible, called a cuvette, which contains the exact quantity requisite for the size of the plate about to be formed. The cuvette is then raised by means of a crane, and lifted over the casting table. These tables have smooth metallic surfaces which are carefully ground and polished, and wiped perfectly clean, and heated before they are used. Formerly they were made of copper, but the British Plate Glass Company have found that iron slabs answer the purpose better. The table used by them is fifteen feet long, nine feet wide, and six inches thick, and weighs fourteen tons. For the convenience of moving it to the annealing ovens it is placed upon castors. The cuvette being swung over the casting table, is gradually turned over, and a flood of molten glass is poured out upon the surface, and prevented from running off by ribs of metal. As soon as it is entirely discharged, a large hollow copper cylinder is rolled over the fluid, spreading it into a sheet of equal breadth and thickness. When the glass is sufficiently cool to bear removal it is slipped into the annealing oven, where it is placed in a horizontal position,14 great care having been taken to exclude the external air, it being indispensable to the beauty of these plates that the process of cooling should be regular and gradual.

No less than twenty workmen are engaged in these operations, and during the whole time the apartment is kept perfectly still, lest a motion of any kind should set the air in motion, the slightest disturbance of the surface of the plate being calculated to impair its value. "The spectacle of such a vast body of melted glass," observes Mr. Parks, "poured at once from an immense crucible, on a metallic table of great magnitude, is truly grand; and the variety of colors which the plate exhibits immediately after the roller has passed over it, renders this an operation more splendid and interesting that can possibly be described."15

To attempt the briefest outline of the vast number of objects that are composed of glass, and the variety of processes to which the material is subjected in their production, would carry us far beyond the limits within which we are unavoidably confined. Even the most trifling articles of daily use, apparently very simple in their formation, involve many elaborate details. Take a watch, for example. The history from the furnace to the workshop, of those parts of a watch which are composed of glass, is full of curious particulars. The watch-glass maker exercises a function distinct from any one of those we have hitherto been considering. He receives from the blower an accurate hollow globe of glass, measuring eight inches in diameter, and weighing exactly twelve ounces, which is the guarantee at once of the regularity and thinness of the material. Upon the surface of this globe the watch-glass maker traces with a piece of heated wire, sometimes with a tobacco pipe, as many circles of the size he requires as the globe will yield, and wetting the lines while they are yet warm, they instantly crack, and the circles are at once separated. He finds the edges rough, but that is got rid of by trimming them with a pair of scissors. The circles thus obtained are deficient, however, in the necessary convexity; he accordingly reheats them, and, with an instrument in each hand, beats or moulds them into the precise form desired, much in the same manner as a dairy-maid, with her wooden spoons beats a pat of butter into shape. The edges are now ground off, and the watch-glass is complete. The preparation of the dial, which is composed of opaque white glass, ordinarily known as enamel, is a much more complicated work, involving several minute processes and a larger expenditure of time. Upon both sides of a thin plate of slightly convex copper, bored with holes for the key, and the hour and minute hands, is spread with a spatula a coat of pounded glass which has gone through several stages of solution and purification before it is ready for application. In the management of this operation, and the absorption of any moisture that may linger in the enamel, considerable care and delicacy of hand are necessary. As soon as the dial-plate is perfectly dried it is put into the furnace to be heated gradually. These processes of firing and enameling must be repeated altogether three times before the work is finished; after which the lines and divisions for the hours and minutes are marked upon the surface by a totally different process. We have here merely touched the principal points in the formation of dial-plates; the details are too complex for enumeration.

If we find in such articles as these the employment of numerous chemical agencies, special tools, and peculiar manipulation, we may easily give credit to the greater wonders that remain to be developed in more costly processes; such as the composition of artificial gems, of the pastes that are made to resemble diamonds and pearls, amethysts, emeralds, and precious stones of all colors and degrees of brilliancy, beads, bulbs, striped tubes, and a hundred other fanciful toys and ornaments; the formation of lenses and eye-glasses; the coloring of glass for various purposes; and the arts of staining and painting, silvering, gilding, cutting, engraving, and etching, each of which has its own mysteries, and has been prosecuted in different ages by different means. When it is said that some of these arts are lost, the fact must be taken in a restricted sense, as merely implying that certain chemical combinations, formerly in use, are unknown to us; but the same arts are still practiced by other means. It is a peculiarity in the manufacture of glass that almost every establishment has its own receipts, and, consequently, its own secrets. Even in the materials employed in the first process of calcination – not to speak of subsequent working processes – there is an infinite diversity of choice in the ingredients, and the proportions in which they are combined; and such is the jealousy of the great manufacturers respecting these matters, that they never admit visitors into their establishments except under the seal of the strictest confidence.16 It is not surprising, therefore, that while the elementary principles of the art have descended to us, particular combinations and processes should have died with their discoverers, or be still kept shut up in the manufactories where they are successfully practiced.