a transparent, brittle, fictitious body, produced from sand melted in a strong fire with fixed alkaline salts, lead, flax, &c. till the whole becomes perfectly clear and fine. The word is formed of the Latin gla'fium, a plant called by the Greeks gla'tis, by the Romans vitrum; by the ancient Britons guadum, and by the English wood. We find frequent mention of this plant in ancient writers, particularly Caesar, Vitruvius, Pliny, &c. who relate that the ancient Britons painted or dyed their bodies with glaftum, guadum, vitrum, &c. i.e. with the blue colour procured from this plant. And hence the fictitious matter we are speaking of came to be called glass; as having always something of this bluishness in it.

At what time the art of glass-making was first invented, is altogether uncertain. Some imagine it to have been invented before the flood: but of this we have no direct proof, though there is no improbability in the supposition; for we know, that it is almost impossible to excite a very violent fire, such as is necessary in metallurgic operations, without vitrifying part of the bricks or stones wherewith the furnace is built. This indeed might furnish the first hints of glass-making; though it is also very probable, that such imperfect vitrifications would be observed a long time before people thought of making any use of them.

Neri traces the antiquity of glass as far back as the time of Job. That writer, speaking of the value of wisdom (chap. xxviii. verse 17.), says, that gold and crystal cannot equal it. But this word, which Neri will have to signify fictitious glass, is capable of a great many different interpretations, and properly signifies only whatever is beautiful or transparent. Dr Merret will have the art to be as ancient as that of pottery or the making of bricks, for the reasons already given, viz. that by all vehement heat some imperfect vitrifications are produced. Of this kind undoubtedly was the foil glass mentioned by Ferar. Imperator, to have been found under ground where great fires had been. But it is evident, that such imperfect vitrifications might have passed unnoticed for ages; and consequently we have no reason to conclude from thence, that the art of glass-making is of such high antiquity.

The Egyptians boast, that this art was taught them by their great Hermes. Aristophanes, Aristotle, Alexander Aphrodites, Lucretius, and St John the divine, put it out of all doubt that glass was used in their days. Pliny relates, that it was first discovered accidentally in Syria, at the mouth of the river Belus, by certain merchants driven thither by a storm at sea; who being obliged to continue there and dress their victuals by making a fire on the ground, where there was great plenty of the herb kali; that plant, burning to ashes, its salts mixed and incorporated with the sand, or stones fit for vitrification, and thus produced glass; and that, this accident being known, the people of Sidon in that neighbourhood essayed the work, and brought glass into use; since which time the art has been continually improving. Be this as it will, however, the first glass-houses mentioned in history were erected in the city of Tyre, and here was the only staple of the manufacture for many ages. The sand which lay on the shore for about half a mile round the mouth of the river Belus was peculiarly adapted to the making of glass, as being neat and glittering; and the wide range of the Tyrian commerce, gave an ample vent for the productions of the furnace.

Mr Nixon, in his observations on a plate of glass found at Herculaneum, which was destroyed A.D. 80, on which occasion Pliny lost his life, offers several probable conjectures as to the uses to which such plates might be applied. Such plates, he supposes, might serve for specula or looking glasses; for Pliny, in speaking of Sidon, adds, "siquidem etiam specula excoctavat:" the reflection of images from these ancient specula being effected by befleeming them behind, or tinging them through with some dark colour. Another use in which they might be employed, was for adorning the walls of their apartments, by way of wainscot, to which Pliny is supposed to refer by his vintre camerae, lib. xxxvi. cap. 25. § 64. Mr Nixon farther conjectures, that these glass plates might be used for windows, as well as the lamine of lapis specularis and phengites, which were improvements in luxury mentioned by Seneca and introduced in his time, Ep. xc. However, there is no positive authority relating to the usage of glass windows earlier than the close of the third century: Mansuetus efl (says Laetantius*), mentem esse, que per oculos ea quae sunt opposita, * De opif. transpiciat, quasi per fenestras lucente vitro aut speculari Dei, cap. 5. lapide obdules.

The first time we hear of glass made among the Romans was in the reign of Tiberius, when Pliny relates that an artist had his house demolished for making glass malleable, or rather flexible; though Petronius Arbiter, and some others, assure us, that the emperor ordered the artist to be beheaded for his invention.

It appears, however, that before the conquest of Britain by the Romans, glass-houses had been erected in this island, as well as in Gaul, Spain, and Italy.—Hence, in many parts of the country are to be found amulets of glass, having a narrow perforation and thick rim, denominated by the remaining Britons gleneu neidreedh or glass adler, and which were probably in former times used as amulets by the druids†. It can scarcely be questioned that the Britons were sufficiently well versed in the manufacture of glass, to form out of it many more useful instruments than the glass beads. History indeed assures us, that they did manufacture a considerable quantity of glass vessels. These, like their amulets, were most probably green, blue, yellow, or black, and many of them curiously streaked with other colours. The proceeds in the manufacture would be nearly the same with that of the Gauls or Spaniards. The sand of their shores being reduced to a sufficient degree of fineness by art, was mixed with three-fourths of its weight of their nitre (much the same with our kelp), and both were melted together. The metal was then poured into other vessels, where it was left to harden into a mass, and afterwards replaced in the furnace, where it became transparent in the boiling, and was afterwards figured by blowing, or modelling in the lath, into such vessels as they wanted.

It is not probable that the arrival of the Romans would improve the glass manufacture among the Britons. The taste of the Romans at that time was just the reverse of that of the inhabitants of this island. The former preferred silver and gold to glass for the composition of their drinking vessels. They made indeed great improvements in their own at Rome, during the government of Nero. The vessels then formed of this metal rivalled the bowls of porcelain in their clearness, and equalled the cups of crystal in their transparency. But these were by far too costly for common use; and therefore, in all probability, were never attempted in Britain. The glass commonly made use of by the Romans was of a quality greatly inferior; and, from the fragments which have been discovered at the stations or towns of either, appear to have consisted of a thick, sometimes white, but mostly blue green, metal.

According to venerable Bede, artificers skilled in making glass for windows were brought over into England in the year 674, by Abbot Benedict, who were employed in glazing the church and monastery of Weremouth. According to others, they were first brought over by Wilfrid, bishop of Worcester, about the same time. Till this time the art of making such glass was unknown in Britain; though glass windows did not begin to be common before the year 1180; till this period they were very scarce in private houses, and considered as a kind of luxury, and as marks of great magnificence. Italy had them first, next France, from whence they came into England.

Venice, for many years, excelled all Europe in the fineness of its glasses; and in the thirteenth century, the Venetians were the only people that had the secret of making crystal looking glasses. The great glass works were at Muran, or Murano, a village near the city, which furnished all Europe with the finest and largest glasses.

The glass manufacture was first begun in England in 1557: the finer sort was made in the place called Crutched Friars, in London; the fine flint glass, little inferior to that of Venice, was first made in the Savoy house, in the Strand, London. This manufacture appears to have been much improved in 1635, when it was carried on with sea coal or pit coal instead of wood, and a monopoly was granted to Sir Robert Mansell, who was allowed to import the fine Venetian flint glasses for drinking, the art of making which was not brought to perfection before the reign of William III. But the first glass plates, for looking glasses and coach windows, were made, 1673, at Lambeth, by the encouragement of the duke of Buckingham; who, in 1679, introduced the manufacture of fine glass into England, by means of Venetian artists, with amazing success. So that within a century past, the French and English have not only come up to, but even surpassed the Venetians, and we are now no longer supplied from abroad.

The French made a considerable improvement in the art of glass, by the invention of a method to cast very large plates, till then unknown, and scarce practised yet by any but themselves and the English. That court applied itself with a laudable industry to cultivate and improve the glass manufacture. A company of glassmen was established by letters patent; and it was provided by an arrest, not only that the working in glass should not derogate any thing from nobility, but even that none but nobles should be allowed to work therein.

An extensive manufactory of this elegant and valuable branch of commerce was first established in Lancashire, about the year 1773, through the spirited exertions of a very respectable body of proprietors, who were incorporated by an act of parliament. From those various difficulties constantly attendant upon new undertakings, when they have to contend with powerful foreign establishments, it was for some time considerably embarrassed; but government, of late, having taken off some restrictions that bore hard upon it, and made some judicious regulations relative to the mode of levying the excise duty, it now bids fair to rival, if not surpass, the most celebrated continental manufacturies, both with respect to the quality, brilliancy, and size of its productions.

With regard to the theory of vitrification, we are almost totally in the dark. In general, it seems to be uncertain that state in which solid bodies are, by the vehement action of fire, fitted for being distillated or carried off in vapour. In all vitrifications there is a plentiful evaporation: and if any solid substance is carried off in vapour by the intense heat of a burning speculum, a vitrification is always observed previously to take place. The difference, then, between the state of fusion and vitrification of a solid body we may conceive to be, that in the former the element of fire acts upon the parts of the solid in such a manner as only to disjoin them, and render the substance fluid; but in vitrification the fire not only disjoins the particles, but combines with them in a latent state into a third substance; which, having now as much fire as it can contain, can receive no further change from that element except being carried off in vapour.

But though we are unable to effect this change upon solid bodies without a very violent heat, it is otherwise in the natural processes. By what we call crystallization, nature produces more perfect glasses than we can make with our furnaces. These are called precious stones; but in all trials they discover the essential properties of glass, and not of stones. The most distinguishing property of glass is its resisting the force of fire, so that this element cannot calcine or change it as it does other bodies, but can only melt it, and then carry it off in vapours. To this last all the precious stones are subject. The diamond (the hardest of them all) may be distillated in a less degree of heat than what would distillate common glass. Nor can it be any objection to this idea, that some kinds of glass are capable of being converted into a kind of porcelain by a long-continued cementation with certain materials. This change happens only to those kinds of glass which are made of alkaline salt and sand; and Dr Lewis hath shewn that this change is produced by the distillation of the saline principle, which is the least fixed of the two. Glass, therefore, we may still consider as a substance upon which the fire has GLA

has no other effect than either to melt or dissipate it in vapour.

The other properties of glass are very remarkable, some of which follow:

1. It is one of the most elastic bodies in nature. If the force with which glass balls strike each other be reckoned 16, that whereewith they recede by virtue of their elasticity will be nearly 15.

2. When glass is suddenly cooled, it becomes exceedingly brittle; and this brittleness is sometimes attended with very surprising phenomena. Hollow balls made of unannealed glass, with a small hole in them, will fly to pieces by the heat of the hand only, if the hole by which the internal and external air communicate be stopped with a finger. Some vessels, however, made of such unannealed glass have been discovered, which have the remarkable property of resisting very hard strokes given from without, though they shiver to pieces by the shocks received from the fall of very light and minute bodies dropped into their cavities. These glases may be made of any shape: all that needs be observed in making them is, that their bottom be thicker than their sides. The thicker the bottom is, the easier do the glases break. One whose bottom is three fingers breadth in thickness flies with as much ease at least as the thinnest glass. Some of these vessels have been tried with strokes of a mallet sufficient to drive a nail into wood tolerably hard, and have held good without breaking. They have also resisted the shock of several heavy bodies, let fall into their cavities, from the height of two or three feet; as musket balls, pieces of iron or other metal, pyrites, jasper, wood, bone, &c. But this is not surprising, as other glases of the same shape and size will do the fame: but the wonder is, that taking a thiver of flint of the size of a small pea, and letting it fall into the glass only from the height of three inches, in about two seconds the glass flies, and sometimes at the very moment of the shock; nay, a bit of flint no larger than a grain, dropped into several glasses successively, though it did not immediately break them, yet when set by, they all flew in less than three quarters of an hour. Some other bodies produce the same effect with flint; as sapphire, diamond, porcelain, hard tempered steel; also marbles such as boys play with, and likewise pearls.

These experiments were made before the Royal Society; and succeeded equally when the glases were held in the hand, when they were rested on a pillow, put in water, or filled with water. It is also remarkable, that the glases broke upon having their bottoms slightly rubbed with the finger, though some of them did not fly till half an hour after the rubbing. If the glases are everywhere extremely thin, they do not break in these circumstances.

Some have pretended to account for these phenomena, by saying, that the bodies dropped into the vessels cause a concussion which is stronger than the cohesive force of the glass, and consequently that a rupture must ensue. But why does not a ball of iron, gold, silver, or copper, which are perhaps a thousand times heavier than the flint, produce the same effect? It is because they are not elastic. But surely iron is more elastic than the end of one's finger. Mr Euler has endeavoured to account for these appearances from his principles of percussion. He thinks that this experiment entirely overthrows the opinion of those who measure the force of percussion by the vis viva, or absolute apparent strength of the stroke. According to his principles, the great hardness and angular figure of the flint, which makes the space of contact with the glass extremely small, ought to cause an impression on the glass vastly greater than lead, or any other metal; and this may account for the flint's breaking the vessel, though the bullet, even falling from a considerable height, does no damage. Hollow cups made of green bottle glass, some of them three inches thick at the bottom, were instantly broken by a fliver of flint weighing about two grains, though they had resisted the shock of a musket ball from the height of three feet.

That Mr Euler's theory cannot be conclusive more than the other, must appear evident from a very slight consideration. It is not by angular bodies alone that the glases are broken. The marbles with which children play are round, and yet they have the same effect with the angular flint. Besides, if it was the mere force of percussion which broke the glases, undoubtedly the fracture would always take place at the very instant of the stroke; but we have seen that this did not happen sometimes till a very considerable space of time had elapsed. It is evident, therefore, that this effect is occasioned by the putting in motion some subtle fluid with which the substance of the glass is filled; and that the motions of this fluid, when once excited in a particular part of the glass, soon propagate themselves through the whole or greatest part of it, by which means the cohesive power becomes at last too weak to resist them. There can be little doubt that the fluid just now mentioned is that of electricity. It is known to exist in glass in very great quantity; and it also is known to be capable of breaking glases even when annealed with the greatest care, if put into too violent a motion. Probably the cooling of glass hastily may make it more electric than is consistent with its cohesive power, so that it is broken by the least increase of motion in the electric fluid by friction or otherwise. This is evidently the case when it is broken by rubbing with the finger; but why it should also break by the mere contact of flint and the other bodies above mentioned, has not yet been satisfactorily accounted for.

A most remarkable phenomenon also is produced in rotation of glass tubes placed in certain circumstances. When these glass tubes are laid before a fire in a horizontal position, having before a their extremities properly supported, they acquire a rotary motion round their axis, and also a progressive motion towards the fire, even when their supports are declining from the fire, so that the tubes will move a little way up hill towards the fire. When the progressive motion of the tubes towards the fire is stopped by any obstacle, their rotation still continues. When the tubes are placed in a nearly upright posture, 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 the perfectly upright posture, the less will the motion be either way.

If the tube is placed horizontally on a glass pane, the fragment, for instance, of coach window-glass, inste Gla. Head of moving towards the fire, it will move from it, and about its axis in a contrary direction to what it had done before; nay, it will recede from the fire, and move a little up hill when the plane inclines towards the fire. These experiments are recorded in the Philosophical Transactions*. They succeeded best with tubes about 20 or 22 inches long, which had in each end a pretty strong pin fixed in cork for an axis.

The reason given for these phenomena, is the swelling of the tubes towards the fire by the heat, which is known to expand all bodies. For, say the adopters of this hypothesis, granting the existence of such a swelling, gravity must pull the tube down when supported near its extremities; and a fresh part being exposed to the fire, it must also swell out and fall down, and so on.—But without going farther in the explanation of this hypothesis, it may be here remarked, that the fundamental principle on which it proceeds is false; for though fire indeed make bodies expand, it does not increase them in weight; and therefore the sides of the tube, though one of them is expanded by the fire, must still remain in equilibrium; and hence we must conclude, that the causes of these phenomena remain yet to be discovered.

4. Glass is less dilatable by heat than metallic substances, and solid glass sticks are less dilatable than Phil Trans. tubes. This was first discovered by Col. Roy, in making experiments in order to reduce barometers to a greater degree of exactness than hath hitherto been found practicable; and since his experiments were made, one of the tubes 18 inches long, being compared with a solid glass rod of the same length, the former was found by a pyrometer to expand four times as much as the other, in a heat approaching to that of boiling oil.—On account of the general quality which glass has of expanding less than metal, M. de Luc recommends it to be used in pendulums: and he says it has also this good quality, that its expansions are always equable, and proportioned to the degrees of heat; a quality which is not to be found in any other substance yet known.

5. Glass appears to be more fit for the condensation of vapours than metallic substances. An open glass filled with water, in the summer time, will gather drops of water on the outside, just as far as the water in the inside reaches; and a person's breath blown on it manifestly moistens it. Glass also becomes moist with dew, when metals do not. See Dew.

6. A drinking glass partly filled with water, and rubbed on the brim with a wet finger, yields musical notes, higher or lower as the glass is more or less full; and will make the liquor frisk and leap. See Harmonica.

7. Glass is possessed of very great electrical virtues. See Electricity, passim.

Materials for Making of Glass. The materials whereof glass is made, we have already mentioned to be salt and sand or siliceous earth.

1. The salt here used is procured from a sort of ashes brought from the Levant, called polverine, or rochetta; which ashes are those of a sort of water plant called kali†, cut down in the summer, dried in the fun, and burnt in heaps, either on the ground or on iron grates; the ashes falling into a pit, grow into a hard mass, or stone, fit for use. It may also be procured from common kelp, or the ashes of the fucus vesiculosus. See Kelp.

To extract the salt, these ashes, or polverine, are powdered and sifted, then put into boiling water, and there kept till one-third of the water be consumed; the whole being stirred up from time to time, that the ashes may incorporate with the fluid, and all its salts be extracted: then the vessel is filled up with new water, and boiled over again, till one half be consumed; what remains is a sort of ley, strongly impregnated with salt. This ley, boiled over again in fresh coppers, thickens in about 24 hours, and shoots its salt; which is to be ladled out, as it shoots, into earthen pans, and thence into wooden vats to drain and dry. This done, it is grossly pounded, and thus put in a sort of oven, called calcar, to dry. It may be added, that there are other plants, besides kali and fucus, which yield a salt fit for glass: such are the common way thistle, bramble, hops, wormwood, wood, tobacco, fern, and the whole leguminous tribe, as peas, beans, &c.

Pearl ashes form a leading flux in the manufacture of glass, and mostly supply the place of the Levant ashes, the barillas of Spain, and many other kinds, which were formerly brought here for making both glass and soap.

There are other fluxes used for different kinds of glass, and for various purposes, as calcined lead, nitre, sea salt, borax, arsenic, smiths clinkers, and wood-ashes, containing the earth and lixiviate salts as produced by incineration. With regard to these several fluxes, we may observe, in general, that the more calx of lead, or other metallic earth, enters into the composition of any glass, so much the more fusible, soft, coloured, and dense this glass is, and reciprocally.

The colours given to glass by calces of lead, are shades of yellow: on the other hand, glasses that contain only saline fluxes partake of the properties of salts; they are less heavy, less dense, harder, whiter, more brilliant, and more brittle than the former; and glasses containing both saline and metallic fluxes do also partake of the properties of both these substances. Glasses too saline are easily susceptible of alteration by the action of air and water: especially those in which alkalies prevail; and these are also liable to be injured by acids. Those that contain too much borax and arsenic, though at first they appear very beautiful, quickly tarnish and become opaque when exposed to air. By attending to these properties of different fluxes, phlogistic or saline, the artist may know how to adjust the proportions of these to sand, or powdered flints, for the various kinds of glass. See the article VITRIFICATION.

2. The sand or stone, called by the artists tarfo, is the second ingredient in glass, and that which gives it the body and firmness. These stones, Agricola observes, must be such as will fuse; and of these such as are white and transparent are best; so that crystal challenges the precedence of all others.

At Venice they chiefly use a sort of pebble, found in the river Tefino, resembling white marble, and called cuogolo. Indeed Ant. Neri assures us, that all stones which will strike fire with steel, are fit to vitrify; but Dr Morret shows, that there are some exceptions from this rule. Flints are admirable; and when calcined, powdered, and feared, make a pure white crystalline metal; but the expence of preparing them makes the masters of our glafs-houses sparing of their use. Where proper stones cannot be so conveniently had, sand is used. The best for this purpose is that which is white, small, and shining; examined by the microscope, it appears to be small fragments of rock crystal. For green glafs, that which is of a soft texture, and more gritty; it is to be well washed, which is all the preparation it needs. Our glafs-houses are furnished with white sand for their crystal glafs from Lynn in Norfolk, and Maidstone in Kent, and with the coarser for green glafs from Woolwich.

Some mention a third ingredient in glafs, viz. manganese, a kind of pseudo loadstone, dug up in Germany, Italy, and even in Mendip hills in Somersetshire. But the proportion hereof to the rest is very inconsiderable; beside, that it is not used in all glafs. Its office is to purge off the natural greenish colour, and give it some other tincture required.

For this purpose it should be chosen of a deep colour, and free from specks of metallic appearance, or a lighter cast; manganese requires to be well calcined in a hot furnace, and then to undergo a thorough levigation. The effect of manganese in destroying the colours of glafs, and hence called the soap of glafs, is accounted for by M. Montamy, in his Traité des Couleurs pour la Peinture en Email, in the following manner: the manganese destroys the green, olive, and blue colours of glafs, by adding to them a purple tinge, and by the mixture producing a blackish brown colour; and as blackness is caused merely by an absorption of the rays of light, the blackish tinge given to the glafs by the mixture of colours, prevents the reflection of so many rays, and thus renders the glafs less coloured than before. But the black produced by this substance suggests an obvious reason for using it very sparingly in those compositions of glafs which are required to be very transparent. Nitre or saltpetre is also used with the same intention; for by destroying in a certain degree the inflammable or carbonaceous matter which gives a strong tinge of yellow to glafs prepared with lead as a flux, it serves to free it from this colour; and in saline glafs, nitre is requisite in a smaller proportion to render them sufficiently transparent, as in the case of looking glafs and other kinds of plates.

Kinds of Glass. The manufactured glafs now in use may be divided into three general kinds; white transparent glafs, coloured glafs, and common green or bottle glafs. Of the first kind there is a great variety; as the flint glafs, as it is called with us, and the German crystal glafs, which are applied to the same uses; the glafs for plates, for mirrors, or looking glafses; the glafs for windows and other lights; and the glafs for phials and small vessels. And these again differ in the substances employed as fluxes in forming them, as well as in the coarseness or fineness of such as are used for their body. The flint and crystal, mirror and best window glafs, not only require such purity in the fluxes, as may render it practicable to free the glafs perfectly from all colour; but for the same reason likewise, either the white Lynn sand, calcined flints, or white pebbles, should be used. The others do not demand the same nicety in the choice of the materials; though the second kind of window glafs, and the best kind of phial, will not be so clear as they ought, if either too brown sand or impure salts be suffered to enter into their composition.

Of coloured glafs there is a great variety of sorts, differing in their colour or other properties according to the occasions for which they are wanted. The differences in the latter kind depend on the accidental preparation and management of the artists by whom they are manufactured, as will be afterwards explained.

Furnaces for the Making of Glass. In this manufacture there are three sorts of furnaces; one called calcar is for the frit; the second is for working the glafs; the third serves to anneal the glafs, and is called the leer. See Plate CCXLVII.

The calcar resembles an oven ten feet long, seven feet broad, and two deep; the fuel, which in Britain is sea coal, is put into a trench on one side of the furnace, and the flame reverberating from the roof upon the frit calcines it. The glafs furnace, or working furnace, is round, of three yards diameter, and two high: or thus proportioned. It is divided into three parts, each of which is vaulted. The lower part is properly called the crown, and is made in that form. Its use is to keep a brisk fire, which is never put out. The mouth is called the bocca. There are several holes in the arch of this crown, through which the flame passes into the second vault or partition, and reverberates into the pots filled with the ingredients above mentioned. Round the inside are eight or more pots placed, and piling pots on them. The number of pots is always double that of the boccas or mouths, or of the number of workmen, that each may have one pot refined to work out of, and another for metal to refine in while he works out of the other. Through the working holes the metal is taken out of the pots, and the pots are put into the furnace; and these holes are stopped with moveable covers made of lute and brick, to screen the workmen's eyes from the scorching flames. On each side of the bocca or mouth is a bocarella or little hole, out of which coloured glafs or finer metal is taken from the piling pot. Above this oven there is the third oven or leer, above five or fix yards long, where the vessels or glafs are annealed or cooled: this part consists of a tower, besides the leer, into which the flame ascends from the furnace. The tower has two mouths, through which the glafs are put in with a fork, and set on the floor or bottom: but they are drawn out on iron pans called fracher, through the leer, to cool by degrees; so that they are quite cold by the time they reach the mouth of the leer, which enters the farofel or room where the glafs are to be flowed.

But the green-glafs furnace is square; and at each angle it has an arch for annealing or cooling glafses. The metal is wrought on two opposite sides, and on the other two they have their colours, into which are made linnet holes for the fire to come from the furnace to bake the frit, and to discharge the smoke. Fires are made in the arches to anneal the work, so that the whole process is done in one furnace.

These furnaces must not be of brick, but of hard sandy stones. In France, they build the outside of brick; and the inner part, to bear the fire, is made of a fort GLA

Glas. sort of fullers earth, or tobacco-pipe clay, of which earth they also make their melting pots. In Britain the pots are made of Stourbridge clay.

Mr Blancourt observes that the worst and roughest work in this art is the changing the pots when they are worn out or cracked. In this case the great working hole must be uncovered; the faulty pot must be taken out with iron hooks and forks, and a new one must be speedily put in its place, through the flames, by the hands only. For this work, the man guards himself with a garment made of skins, in the shape of a pantaloons, that covers him all but his eyes, and is made as wet as possible; the eyes are defended with a proper sort of glass.

Instruments for Making of Glass. The instruments made use of in this work may be reduced to these that follow. A blowing pipe, made of iron, about two feet and a half long, with a wooden handle. An iron rod to take up the glass after it is blown, and to cut off the former. Scissors to cut the glass when it comes off from the first hollow iron. Shears to cut and shape great glases, &c. An iron ladle with the end of the handle cased with wood, to take the metal out of the refining pot, to put it into the workman's pots. A small iron ladle cased in the same manner, to skim the alkaline salt that swims at top. Shovels, one like a peel, to take up the great glases; another like a fire-hovel, to feed the furnace with coals. A hooked iron fork, to stir the matter in the pots. An iron rake for the same purpose, and to stir the frit. An iron fork, to change or pull the pots out of the furnace, &c.

Compositions for White and Crystal Glass. 1. To make crystal glass, take of the whitest tarlo, pounded small, and seared as fine as flour, 200 pounds; of the salt of polverine 130 pounds; mix them together and put them into the furnace called the calcar, first heating it. For an hour keep a moderate fire, and keep stirring the materials with a proper rake, that they may incorporate and calcine together; then increase the fire for five hours; after which take out the matter; which being now sufficiently calcined, is called frit. From the calcar put the frit in a dry place, and cover it up from the dust for three or four months. Now to make the glass or crystal: take of this crystal frit, called also bollito; set it in pots in the furnace, adding to it a due quantity of magnesia or manganese: when the two are fused, call the fluor into fair water, to clear it of the salt called sandover; which would otherwise make the crystal obscure and cloudy. This lotion must be repeated again and again, as often as needful, till the crystal be fully purged; or this scum may be taken off by means of proper ladles. Then set it to boil four, five or fix days; which done, see whether it have manganese enough; and if it be yet greenish, add more manganese, at discretion, by little and little at a time, taking care not to overdose it, because the manganese inclines it to a blackish hue. Then let the metal clarify, till it becomes of a clear and shining colour; which done, it is fit to be blown or formed into vessels at pleasure.

2. Flint glass, as it is called by us, is of the same general kind with that which in other places is called crystal glass. It has this name from being originally made with calcined flints, before the use of the white fand was understood; and retains the name, though no flints are now used in the composition of it. This flint glass differs from the other, in having lead for its flux, and white sand for its body; whereas the fluxes used for the crystal glass are salts or arsenic, and the body consists of calcined flints or white river pebbles, tarlo, or such stones. To the white sand and lead a proper proportion of nitre is added, to burn away the phlogiston of the lead, and also a small quantity of magnesia; and in some works they use a proportional quantity of arsenic to aid the fluxing ingredients. The most perfect kind of glass may be made by fusing with a very strong fire 120 pounds of the white sand, 50 pounds of red lead, 40 pounds of the best pearl ashes, 20 pounds of nitre, and five ounces of magnesia. Another composition of flint glass, which is laid to come nearer to the kind now made, is the following: 120 pounds of sand, 54 pounds of the best pearl ashes, 36 pounds of red lead, 12 pounds of nitre, and 6 ounces of magnesia. To either of these a pound or two of arsenic may be added, to increase the flux of the composition. A cheaper composition of flint glass may be made with 120 pounds of white sand, 35 pounds of the best pearl ashes, 40 pounds of red lead, 13 pounds of nitre, 6 pounds of arsenic, and four ounces of magnesia; or instead of the arsenic may be substituted 15 pounds of common salt; but this will be more brittle than the other. The cheapest composition for the worst kind of flint glass consists of 120 pounds of white sand, 30 pounds of red lead, 20 pounds of the best pearl ashes, 10 pounds of nitre, 15 pounds of common salt, and six pounds of arsenic. The best German crystal glass is made of 120 pounds of calcined flints or white sand, 70 pounds of the best pearl ashes, 10 pounds of saltpetre, half a pound of arsenic, and five ounces of magnesia. And a cheaper composition is formed of 120 pounds of calcined flints or white sand, 46 pounds of pearl ashes, 7 pounds of nitre, 6 pounds of arsenic, and 5 ounces of magnesia.

A glass much harder than any prepared in the common way, may be made by means of borax in the following method: Take four ounces of borax, and an ounce of fine sand; reduce both to a subtle powder, and melt them together in a large close crucible set in a wind furnace, keeping up a strong fire for half an hour; then take out the crucible, and when cold break it, and there will be found at the bottom a pure hard glass capable of cutting common glass like a diamond. This experiment, duly varied, says Dr Shaw, may lead to several useful improvements in the arts of glass, enamels, and facitious gems, and shows an expeditious method of making glass, without any fixed alkali, which has been generally thought an essential ingredient in glass, and it is not yet known whether calcined crystal or other substances being added to this salt instead of sand, it might not make a glass approaching to the nature of a diamond.

There are three principal kinds of glasses, distinguished by the form or manner of working them; viz. I. Round glass, as those of our vellies, phials, drinking glasses, &c. II. Table or window glass, of which there are divers kinds; viz. crown glass, jealous glass, &c. III. Plate glass, or mirror glass.

I. Working or Blowing Round Glass. The working furnace, we have observed, is round and has fix boccas or apertures; at one of these, called the great bocca, the furnace is heated, and the pots of frit are at this set in the furnace; two other small holes, called bocarellas, serve to lade or take out the melted metal, at the end of an iron, to work the glas. At the other holes they put in pots of fusible ingredients, to be prepared, and at last emptied into the lading pot.

There are fix pots in each furnace, all made of tobacco-pipe clay, proper to sustain not only the heat of the fire, but also the effect of the potherine, which penetrates every thing else. There are only two of these pots that work: the rest serve to prepare the matter for them. The fire of the furnace is made and kept up with dry hard wood, cast in without intermission at fix apertures.

When the matter contained in the two pots is sufficiently vitrified, they proceed to blow or fashion it. For this purpose the workman dips his blowing pipe into the melting pot; and by turning it about, the metal sticks to the iron more firmly than turpentine. This he repeats four times, at each time rolling the end of his instrument, with the hot metal thereon, on a piece of plate iron; over which is a vessel of water which helps to cool, and so to consolidate and to dispose that matter to bind more firmly with what is to be taken next out of the melting pot. But after he has dipt a fourth time, and the workman perceives there is metal enough on the pipe, he clasps his mouth immediately to the other end of it, and blows gently through the iron tube, till the metal lengthens like a bladder about a foot. Then he rolls it on a marble stone a little while to polish it; and blows a second time, by which he brings it to the shape of a globe of about 18 or 20 inches diameter. Every time he blows into the pipe, he removes it quickly to his cheek; otherwise he would be in danger, by often blowing, of drawing the flame into his mouth: and this globe may be flattened by returning it to the fire; and brought into any form by stamp irons, which are always ready. When the glas is thus blown, it is cut off at the collet or neck; which is the narrow part that fluck to the iron. The method of performing this is as follows: the pipe is refted on an iron bar, close by the collet; then a drop of cold water being laid on the collet, it will crack about a quarter of an inch, which, with a flight blow or cut of the shears will immediately separate the collet.

After this is done, the operator dips the iron rod into the melting pot, by which he extracts as much metal as serves to attract the glas he has made, to which he now fixes this rod at the bottom of his work, opposite to the opening made by the breaking of the collet. In this position the glas is carried to the great bocca or mouth of the oven, to be heated and scalded; by which means it is again put into such a soft state, that, by the help of an iron instrument, it can be pierced, opened, and widened, without breaking. But the vessel is not finihed till it is returned to the great bocca; where being again heated thoroughly, and turned quickly about with a circular motion, it will open to any size, by the means of the heat and motion.

If there remain any superfluities, they are cut off with the shears; for till the glas is cool, it remains in a soft flexible state. It is therefore taken from the bocca, and carried to an earthen bench, covered with brands, which are coals extinguished, keeping it turning; because that motion prevents any setting, and preserves an evenness in the face of the glas, where, as it cools, it comes to its consistency; being first cleared from the iron rod by a slight stroke by the hand of the work-man.

If the vessel conceived in the workman's mind, and whose body is already made, requires a foot, or a handle, or any other member or decoration, he makes them separately; and now essays to join them with the help of hot metal, which he takes out of the pots with his iron rod: but the glas is not brought to its true hardness till it has palled the lecr or annealing oven, described before.

II. Working or blowing of Window or Table Glass. The method of working round glas, or vessels of any sort, is in every particular applicable to the working of window or table glas, till the blowing iron has been dipt the fourth time. But then instead of rounding it, the workman blows, and so manages the metal upon the iron plate, that it extends two or three feet in the form of a cylinder. This cylinder is put again to the fire, and blown a second time, and is thus repeated till it is extended to the dimensions required, the side to which the pipe is fixed diminishing gradually till it ends in a pyramidal form; so that, to bring both ends nearly to the same diameter, while the glas is thus flexible, he adds a little hot metal to the end opposite the pipe, and draws it out with a pair of iron pincers, and immediately cuts off the same end with the help of a little cold water as before.

The cylinder being now open at one end, is carried back to the bocca; and there, by the help of cold water, it is cut about eight or ten inches from the iron pipe or rod; and the whole length at another place, by which also it is cut off from the iron rod. Then it is heated gradually on an earthen table, by which it opens in length; while the workman, with an iron tool, alternately lowers and raises the two halves of the cylinder; which at last will open like a sheet of paper, and fall into the same flat form in which it serves for use; in which it is preserved by heating it over again, cooling it on a table of copper, and hardening it 24 hours in the annealing furnace, to which it is carried upon forks. In this furnace an hundred tables of glas may lie at a time, without injury to each other, by separating them into tents, with an iron shiver between, which diminishes the weight by dividing it, and keeps the tables flat and even.

Of window or table glas there are various sorts, made in different places, for the use of building. Those most known among us are given us by the author of the Builder's Dictionary, as follows:

1. Crown, of which, says Neri, there are two kinds, distinguished by the places where they are wrought; viz. Ratcliff crown glas, which is the best and clearest, and was first made at the Bear garden, on the Bankside, Southwark, but since at Ratcliff: of this there are 24 tables to the cafe, the tables being of a circular form, about three feet fix inches in diameter. The other kind, or Lambeth crown glas, is of a darker colour than the former, and more inclining to green.

The best window or crown glas is made of white sand 60 pounds, of purified pearl ashes 30 pounds, of saltpetre. faltpetre 15 pounds, of borax one pound, and of arsene half a pound. If the glas should prove yellow, magnelia must be added. A cheaper composition for window glas consists of 60 pounds of white sand, 25 pounds of unpurified pearl ashes, 10 pounds of common salt, 5 pounds of nitre, 2 pounds of arsenic, and one ounce and a half of magnelia. The common or green window glas is composed of 60 pounds of white sand, 30 pounds of unpurified pearl ashes, 10 pounds of common salt, 2 pounds of arsenic, and 2 ounces of magnelia. But a cheaper composition for this purpose consists of 120 pounds of the cheapest white sand, 30 pounds of unpurified pearl ashes, 60 pounds of wood ashes, well burnt and sifted, 20 pounds of common salt, and 5 pounds of arsenic.

2. French glas, called also Normandy glas, and formerly Lorraine glas, because made in those provinces. At present it is made wholly in the nine glas works; five whereof are in the forest of Lyons, four in the county of Eu; the last at Beaumont near Rouen. It is of a thinner kind than our crown glas; and when laid on a piece of white paper, appears of a dirtyish green colour. There are but 25 tables of this to the cafe.

3. German glas is of two kinds, the white and the green: the first is of a whitish colour, but is subject to those small curved streaks observed in our Newcastle glas, though free from the spots and blemishes thereof. The green, besides its colour, is liable to the same streaks as the white; but both of them are straighter and less warped than our Newcastle glas.

4. Dutch glas is not much unlike our Newcastle glas either in colour or price. It is frequently much warped like that, and the tables are but small.

5. Newcastle glas is that most used in England. It is of an ash colour, and much subject to specks, streaks, and other blemishes; and besides is frequently warped. Leybourn says, there are 45 tables to the cafe, each containing five superficial feet: some say there are but 35 tables, and fix feet in each table.

6. Phial glas is a kind betwixt the flint glas and the common bottle or green glas. The best kind may be prepared with 120 pounds of white sand, 50 pounds of unpurified pearl ashes, 10 pounds of common salt, 5 pounds of arsenic, and 5 ounces of magnelia. The composition for green or common phial glas consists of 120 pounds of the cheapest white sand, 80 pounds of wood ashes well burnt and sifted, 20 pounds of pearl ashes, 15 pounds of common salt, and 1 pound of arsenic.

The common bottle or green is formed of sand of any kind fluxed by the ashes of burnt wood, or of any parts of vegetables; to which may be added the scorie or clinkers of forges. When the softest sand is used, 200 pounds of wood ashes will suffice for 100 pounds of sand, which are to be ground and mixed together. The composition with the clinkers consists of 170 pounds of wood ashes, 100 pounds of sand, and 50 pounds of clinkers or scorie, which are to be ground and mixed together. If the clinkers cannot be ground, they must be broke into small pieces, and mixed with the other matter without any grinding.

III. Working of Plate or Mirror GLASS. 1. The materials of which this glas is made are much the fame as those of other works of glas, viz. an alkali, falt and sand.

The falt, however, should not be that extracted from polverine or the ashes of the Syrian kali, but that from BARILLA, growing about Alicant in Spain. It is very rare that we can have the barilla pure; the Spaniards in burning the herb make a practice of mixing another herb along with it, which alters its quality; or of adding sand to it to increase the weight, which is easily discovered if the addition be only made after the boiling of the ashes, but next to impossible if made in the boiling. It is from this adulteration that those threads and other defects in plate glas arise. To prepare the falt, they clean it well of all foreign matters; pound or grind it with a kind of mill, and finally sift it pretty fine.

Pearl ashes, properly purified, will furnish the alkali falt requisite for this purpose; but it will be necessary to add borax or common falt, in order to facilitate the fusion, and prevent the glas from stiffening in that degree of heat in which it is to be brought into plates. For purifying the pearl ashes, dissolve them in four times their weight of boiling water, in a pot of cast iron, always kept clean from rust. Let the solution be removed into a clean tub, and remain there 24 hours or longer. Having decanted the clear part of the fluid from the dregs or sediment, put it again in the iron pot, and evaporate the water till the salts are left perfectly dry. Preserve them in stone jars, well secured from air and moisture.

Pearl ashes may also be purified in the highest degree, so as to be proper for the manufacture of the most transparent glas, by pulverizing three pounds of the best pearl ashes with fix ounces of faltpetre in a glas or marble mortar, till they are well mixed; and then putting part of the mixture into a large crucible, and exposing it in a furnace to a strong heat. When this is red hot, throw in the rest gradually; and when the whole is red hot, pour it out on a moistened stone or marble, and put it into an earthen or clean iron pot, with ten pints of water; heat it over the fire till the salts be entirely melted; let it then stand to cool, and filter it through paper in a pewter cullender. When it is filtered, put the fluid again into the pot, and evaporate the salt to dryness, which will then be as white as snow; the nitre having burnt all the phlogistic matter that remained in the pearl ashes after their former calcination.

As to the sand, it is to be sifted and washed till such time as the water come off very clear; and when it is well dried again, they mix it with the salt, passing the mixture through another sieve. This done, they lay them in the annealing furnace for about two hours; in which time the matter becomes very light and white: in this state they are called frit or fritta; and are to be laid up in a dry clean place, to give them time to incorporate: they lie here for at least a year.

When they would employ this frit, they lay it for some hours in the furnace, adding to some the fragments or shards of old and ill made glasses; taking care first to calcine the shards by heating them red hot in the furnace, and thus calling them into cold water. To the mixture must likewise be ad- Glas. ed manganese, to promote the fusion and purification.

The best composition for looking glass plates consists of 60 pounds of white sand cleansed, 25 pounds of purified pearl ashes, 15 pounds of saltpetre, and 7 pounds of borax. If a yellow tinge should affect the glass, a small proportion of magnesia, mixed with an equal quantity of arsenic, should be added. An ounce of the magnesia may be first tried; and if this proves insufficient, the quantity should be increased.

A cheaper composition for looking glass plate consists of 60 pounds of the white sand, 20 pounds of pearl ashes, 10 pounds of common salt, 7 pounds of nitre, 2 pounds of arsenic, and 1 pound of borax. The matter of which the glases are made at the famous manufacture of St Gobin in France, is a composition of folder and of a very white sand, which are carefully cleaned of all heterogeneous bodies; afterwards washed for several times, and dried so as to be pulverized in a mill, consisting of many peffles, which are moved by horses. When this is done, the sand is sifted through silk sieves and dried.

The matter thus far prepared is equally fit for plate glass, to be formed either for blowing or by casting.

The largest glases at St Gobin are run; the middle sized and small ones are blown.

2. Blowing the plates. The workhouses, furnaces, &c. used in the making of this kind of plate glass, are the same, except that they are smaller, and that the carquaffles are disposed in a large covered gallery, over against the furnace, as those in the following article, to which the reader is referred.

After the materials are vitrified by the heat of the fire, and the glass is sufficiently refined, the workman dips his in blowing iron, fix feet long, and two inches in diameter, sharpened at the end which is put in the mouth, and widened at the other, that the matter may adhere to it. By this means he takes up a small ball of matter, which sticks to the end of the tube by constantly turning it. He then blows into the tube, that the air may swell the annexed ball; and carrying it over a bucket of water, which is placed on a support at the height of about four feet, he sprinkles the end of the tube to which the matter adheres, with water, still turning it, that by this cooling the matter may coalesce with the tube, and be fit for sustaining a greater weight. He dips the tube again into the same pot, and proceeds as before; and dipping it into the pot a third time, he takes it out, loaded with matter, in the shape of a pear, about ten inches in diameter, and a foot long, and cools it at the bucket; at the same time blowing into the tube, and with the assistance of a labourer, giving it a balancing motion, he causes the matter to lengthen; which, by repeating this operation several times, assumes the form of a cylinder, terminating like a ball at the bottom, and in a point at the top. The assistant is then placed on a stool three feet and a half high; and on this stool there are two upright pieces of timber, with a cross beam of the fame, for supporting the glass and tube, which are kept in an oblique position by the assistant, that the smaller workman may with a puncheon set in a wooden handle, and with a mallet, make a hole in the mass: this hole is drilled at the centre of the ball that terminates the cylinder, and is about an inch in diameter.

When the glass is pierced, the defects of it are perceived; if it is tolerably perfect, the workman lays the tube horizontally on a little iron treffel, placed on the support of the aperture of the furnace. Having exposed it to the heat for about half a quarter of an hour, he takes it away, and with a pair of long and broad shears, extremely sharp at the end, widens the glass, by infinuating the shears into the hole made with the puncheon, whilst the assistant, mounted on the stool, turns it round, till at last the opening is so large as to make a perfect cylinder at bottom. When this is done, the workman lays his glass upon the treffels at the mouth of the furnace to heat it: he then gives it to his assistant on the stool, and with large shears cuts the mass of matter up to half its height. There is at the mouth of the furnace an iron tool called pontil, which is now heating, that it may unite and coalesce with the glass just cut, and perform the office which the tube did before it was separated from the glass. This pontil is a piece of iron fix feet long, and in the form of a cane or tube, having at the end of it a small iron bar, a foot long, laid equally upon the long one, and making with it a T. This little bar is full of the matter of the glass, about four inches thick. This red hot pontil is presented to the diameter of the glass, which coalesces immediately with the matter round the pontil, so as to support the glass for the following operation. When this is done, they separate the tube from the glass, by striking a few blows with a chisel upon the end of the tube which has been cooled; so that the glass breaks directly, and makes this separation, the tube being discharged of the glass now adhering to the pontil. They next present to the furnace the pontil of the glass, laying it on the treffel to heat, and redden the end of the glass, that the workman may open it with his shears, as he has already opened one end of it, to complete the cylinder; the assistant holding it on his stool as before. For the last time, they put the pontil on the treffel, that the glass may become red hot, and the workman cuts it quite open with his shears, right over against the aforementioned cut; this he does as before, taking care that both cuts are in the same line. In the mean time, the man who looks after the carquaffles comes to receive the glass upon an iron shovel two feet and a half long without the handle, and two feet wide, with a small border of an inch and a half to the right and left, and towards the handle of the shovel. Upon this the glass is laid, flattening it a little with a small stick a foot and a half long, so that the cut of the glass is turned upwards. They separate the glass from the pontil, by striking a few gentle blows between the two with a chisel. The glass is then removed to the mouth of the hot carquaffle, where it becomes red hot gradually; the workman, with an iron tool fix feet long, and widened at the end in form of a club at cards four inches long, and two inches wide on each side, very flat, and not half an inch thick, gradually lifts up the cut part of the glass to unfold it out of its form of a flattened cylinder, and render it smooth, by turning it down upon the hearth of the carquaffle. The tool already described being infinuated within the cylinder, performs this operation by being pushed hard against all the parts of the glass. When the glass is thus made quite smooth, it is pulled to the bottom of the carquaiife or annealing furnace with a small iron raker, and ranged there with a little iron hook. When the carquaiife is full, it is stopped and cemented as in the cafe of run glafts, and the glaft remains there for a fortnight to be annealed; after which time they are taken out to be polished. A workman can make but one glaft in an hour, and he works and rests for fix hours alternately.

Such was the method formerly made use of for blowing plate glafts, looking glafts, &c.; but the workmen, by this method, could never exceed 50 inches in length, and a proportional breadth, because what were larger were always found to warp, which prevented them from reflecting the objects regularly, and wanted substance to bear the necessary grinding. These imperfections have been remedied by the following invention of the Sieur Abraham Thevart, in France, about the year 1688.

3. Casting or Running of Large Mirror Glass Plates. The furnace is of a very large dimension, environed with several ovens, or annealing furnaces, called carquaiifes, besides others for making of frit and calcining old pieces of glafts. This furnace, before it is fit to run glafts, cofts 350l. It feldom lasts above three years, and even in that time it muft be refitted every fix months. It takes fix months to rebuild it, and three months to refit it. The melting pots are as big as large hogheads, and contain about 2000 weight of metal. If one of them burfts in the furnace, the losf of the matter and time amounts to 250l. The materials in these pots are the fame as defcribed before. When the furnace is red hot, thefe materials are put in at three different times, becaufe that helps the fufion; and in 24 hours they are vitrified, refined, fettled, and fit for casting. A is the boca, or mouth of the furnace; B is the ciftern that conveys the liquid glafts it receives out of the melting pots in the furnace to the casting table. Thefe cifterns are filled in the furnace, and remain therein fix hours after they are filled; and then are hooked out by the means of a large iron chain, guided by a pulley, placed upon a carriage with four wheels marked C, by two men. This carriage has no middle piece; fo that when it has brought the ciftern to the casting table D, they flip off the bottom of the ciftern, and out rushes a torrent of flaming matter upon the table: this matter is confined to certain dimensions by the iron rulers EE, which are moveable, retain the fluid matter, and determine the width of the glafts; while a man, with the roller F refling on the edge of the iron rulers, reduceth it as it cools to an equal thicknefs, which is done in the space of a minute. This table is supported on a wooden frame, with trutles for the convenience of moving to the annealing furnace; into which, ifrewed with fand, the new plate is hoved, where it will harden in about 10 days.

What is moft surprising throughout the whole of this operation, is the quicknefs and address wherewith fuch maffy cifterns, filled with a flaming matter, are taken out of the furnace, conveyed to the table, and poured therein, the glafts spread, &c. The whole is inconceivable to fuch as have not been eye witneffes of that surprising manufacture.

As faft as the cifterns are emptied, they carry them back to the furnace and take fresh ones, which they empty as before. Thus they continue to do fo long as there are any full cifterns; laying as many plates in each carquaiife as it will hold, and flopping them up with doors of baked earth, and every clink with cement, as soon as they are full, to let them anneal, and cool again, which requires about 14 days.

The firft running being difpatched, they prepare another, by filling the cifterns anew from the matter in the pots; and after the second, a third; and even a fourth time, till the melting pots are quite empty.

The cifterns at each running fhould remain at leaft fix hours in the furnace to whiten; and when the firft annealing furnace is full, the casting table is to be carried to another. It need not here be obferved, that the carquaiifes, or annealing furnaces, muft first have been heated to the degree proper for them. It may be obferved, that the oven full, or the quantity of matter commonly prepared, supplies the running of 18 glafts, which is performed in 18 hours, being an hour for each glaft. The workmen work fix hours, and are then relieved by others.

When the pots are emptied, they take them out, as well as the cifterns, to scrape off what glafts remains, which otherwife would grow green by continuance of fire, and fpoil the glafts. They are not filled again in lefs than 36 hours; fo that they put the matter into the furnace, and begin to run it every 54 hours.

The manner of heating the large furnaces is very singular; the two tifors, or perfons employed for that purpofe, in their shirts, run twifly round the furnace without making the leaft stop: as they run along, they take two billets, or pieces of wood, which are cut for the purpofe: thefe they throw into the firft tiffart; and continuing their courfe, do the fame for the fecond. This they hold without interruption for fix hours fucceffively; after which they are relieved by others, &c. It is surprising that two fuch fmall pieces of wood, and which are confumed in an instant, fhould keep the furnace to the proper degree of heat; which is fuch, that a large bar of iron, laid at one of the mouths of the furnace, becomes red hot in lefs than half a minute.

The glafts, when taken out of the melting furnace, needs nothing farther but to be ground, polished, and foliated.

4. Grinding and Polifhing of Plate Glass. Glafts is made tranfparent by fire; but it receives its lustre by the skill and labour of the grinder and polifher; the former of whom takes it rough out of the hands of the maker.

In order to grind plate glafts, they lay it horizontally upon a flat stone table made of a very fine grained freestone; and for its greater security they platter it down with lime or flucco; for otherwife the force of the workmen, or the motion of the wheel with which they grind it, would move it about.

This stone table is supported by a strong frame A, made of wood, with a ledge quite round its edges, rifing about two inches higher than the glafts. Upon this glafts to be ground is laid another rough glaft not above half fo big, and fo loofe as to flide upon it; but cemented to a wooden plank, to guard it from the injury it muft otherwife receive from the scraping of the wheel to which this plank is fafened, and from the weights laid upon it to promote the grinding or triture of the glafts. The whole is covered with a wheel B, made

PLATE CCXLVII.

FURNACE FOR ARTIFICIAL GEMS.

Fig. 1. BLOWING.

FURNACE FOR ARTIFICIAL GEMS.

Fig. 2. CASTING.

Fig. 3. POLISHING.

Fig. 4.

Fig. 5.

Fig. 6. made of hard light wood, about six inches in diameter, by pulling of which backwards and forwards alternately, and sometimes turning it round, the workmen, who always stand opposite to each other, produce a constant attrition between the two glases, and bring them to what degree of smoothness they please, by first pouring in water and coarse sand; after that, a finer sort of sand, as the work advances, till at last they must pour in the powder of saltnit. As the upper or incumbent glass polishes and grows smoother, it must be taken away, and another from time to time put in its place.

This engine is called a mill by the artists, and is used only in the largest sized glases; for in the grinding of the lesser glases, they are content to work without a wheel, and to have only four wooden handles fastened to the four corners of the stone which loads the upper plank, by which they work it about.

When the grinder has done his part, who finds it very difficult to bring the glass to an exact plainness, it is turned over to the polisher; who, with the fine powder of tripoli stone or emery, brings it to a perfect evenness and lustre. The instrument made use of in this branch is a board, &c, furnished with a felt, and a small roller, which the workman moves by means of a double handle at both ends. The artist, in working this roller, is assisted with a wooden hoop or spring, to the end of which it is fixed; for the spring, by constantly bringing the roller back to the same points, facilitates the action of the workman's arm.

Colouring of Glass. That the colours given to glass may have their full beauty, it must be observed, that every pot when new, and first used, leaves a foulness in the glass from its own earthy parts; so that a coloured glass made in a new pot can never be bright or perfectly fine. For this reason, the larger of these, when new, may be glazed with white glass; but the second time of using the pots lose this foulness. The glazing may be done by reducing the glass to powder, and moistening the inside of the pot with water; while it is yet moist, put in some of the powdered glass, and shake it about, till the whole inner surface of the pot be covered by as much as will adhere to it, in consequence of the moisture. Throw out the redundant part of the powdered glass; and the pot being dry, set it in a furnace sufficiently hot to vitrify the glass adhering to it, and let it continue there some time; after which, care must be taken to let it cool gradually. Those pots which have served for one colour must not be used for another; for the remainder of the old matter will spoil the colour of the new. The colours must be very carefully calcined to a proper degree; for if they are calcined either too much or too little, they never do well; the proper proportion, as to quantity, must also carefully be regarded, and the furnaces must be fed with dry hard wood. And all the processes succeed much the better if the colour be used dividedly, that is, a part of it in the frit, and the rest in the melted metal.

A hard glass, proper for receiving colours, may be prepared by pulverizing 12 pounds of the best sand, cleansed by washing in a glass or flint mortar, and mixing seven pounds of pearl ashes or any fixed alkaline salt purified with nitre, one pound of saltpetre, and half a pound of borax, and pounding them together. A glass less hard may be prepared of twelve pounds of white sand cleansed, seven pounds of pearl ashes purified with saltpetre, one pound of nitre, half a pound of borax, and four ounces of arsenic prepared as before.

Amethyst colour. See Purple below, and the article AMETHYST.

Balas colour. Put into a pot crystal frit, thrice washed in water; tinge this with manganese, prepared into a clear purple; to this add alumen catrum, sifted fine, in small quantities, and at several times: this will make the glass grow yellowish, and a little reddish, but not blackish, and always dissipates the manganese. The last time you add manganese give no more of the alumen catrum, unless the colour be too full. Thus will the glass be exactly of the colour of the balas ruby. See Ruby GLASS.

The common black colour. The glassmakers take old broken glass of different colours, grind it to powder, and add to it, by different parcels, a sufficient quantity of a mixture of two parts zaffer and one part manganese: when well purified, they work it into vessels, &c.

Glass beads are coloured with manganese only.

Black velvet colour. To give this deep and fine colour to glass, take of crystalline and pulverine frit, of each 20 pounds; of calx of lead and tin, four pounds; set all together in a pot in the furnace, well heated; when the glass is formed and pure, take steel well calcined and powdered, scales of iron that fly off from the smith's anvil, of each an equal quantity; powder and mix them well; then put fix ounces of this powder to the above-described metal while in fusion: mix the whole thoroughly together, and let them all boil strongly together; then let it stand in fusion 12 hours to purify, and after this work it. It will be a most elegant velvet black.

There is another way of doing this, which also produces a very fair black. It is this: take a hundred weight of rochetta frit, add to this two pounds of tartar and fix pounds of manganese, both in fine powder; mix them well; and put them to the metal while in fusion, at different times, in several parcels; let it stand in fusion after this for four days, and then work it.

A glass perfectly black may also be formed by adding to ten pounds of either of the compositions for hard glass above described, one ounce of zaffer, fix drachms of manganese, and an equal quantity of iron strongly calcined.

Blue colour. A full blue may be made by adding fix drachms of zaffer and two drachms of manganese to ten pounds of either of the compositions for hard glass, described above. For a very cool or pure blue glass, half an ounce of calcined copper may be used instead of the manganese, and the proportion of zaffer diminished by one half. Glass resembling sapphire may be made with ten pounds of either of the compositions for hard glass, three drachms and one scruple of zaffer, and one drachm of the calx cae/ii or precipitation of gold by tin; or, instead of this latter ingredient, two drachms and two scruples of manganese. Or a sapphire-coloured glass may be made by mixing with any quantity of the hard glass one-eighth of its weight of saltnit. A beautiful blue glass is also produced from the oxide of cobalt.

Venetian brown, with gold spangles, commonly called the philosopher's stone, may be prepared in the following manner: take of the second composition for hard glas above described, and of the composition for paste, of each five pounds, and of highly calcined iron an ounce; mix them well, and fuse them till the iron be perfectly vitrified, and has tinged the glas of a deep transparent yellow brown colour. Powder this glas, and add to it two pounds of powdered glas of antimony; grind them together, and thus mix them well. Take part of this mixture, and rub into it 80 or 100 leaves of the counterfeit leaf gold called Dutch gold; and when the parts of the gold seem sufficiently divided, mix the powder containing it with the other part of the glas. Fuse the whole with a moderate heat till the powder run into a vitreous mass, fit to be wrought into any of the figures or vessels into which it is usually formed; but avoid a perfect liquefaction, because that in a short time destroys the equal diffusion of the spangles, and vitrifies, at least in part, the matter of which they are composed; converting the whole into a kind of transparent olive-coloured glas. This kind of glas is used for a great variety of toys and ornaments with us, who at present procure it from the Venetians.

Chalcedony. A mixture of several ingredients with the common matter of glas, will make it represent the semi-opake gems, the jaspers, agates, chalcedonies, &c. The way of making these seems to be the same with the method of making marbled paper, by several colours dissolved in several liquors, which are such as will not readily mix with one another when put into water, before they are cast upon the paper which is to be coloured. There are several ways of making these variously coloured glasses, but the best is the following.

Dissolve four ounces of fine leaf silver in a glas vessel in strong aquafortis; stop up the vessel, and set it aside.—In another vessel, dissolve five ounces of quicksilver in a pound of aquafortis, and set this aside.—In another glas vessel, dissolve in a pound of aquafortis three ounces of fine silver, first calcined in this manner: amalgamate the silver with mercury, mix the amalgam with twice its weight of common salt well purified; put the mixture in an open fire in a crucible, that the mercury may fly off, and the silver be left in form of powder. Mix this powder with an equal quantity of common salt well purified, and calcine this for six hours in a strong fire; when cold, wash off the salt by repeated boilings in common water, and then put the silver into the aquafortis. Set this solution also aside.—In another vessel, dissolve in a pound of aquafortis three ounces of sal ammoniac; pour off the solution and dissolve in it a quarter of an ounce of gold. Set this also aside.—In another vessel, dissolve three ounces of sal ammoniac in a pound of aquafortis; then put into the solution cinnabar, crocus martis, ultramarine, and ferretto of Spain, of each half an ounce. Set this also aside.—In another vessel, dissolve in a pound of aquafortis three ounces of sal ammoniac; then put into it crocus martis made with vinegar, calcined tin, zaffer, and cinnabar, of each half an ounce; let each of these be powdered very fine, and put gently into the aquafortis. Set this also aside.—In another vessel, dissolve three ounces of sal ammoniac in a pound of aquafortis, and add to it brats calcined with brimstone, brats thrice calcined, manganese, and scales of iron which fall from the smith's anvil, of each half an ounce; let each be well powdered, and put gently into the vessel. Then set this also aside.—In another vessel, dissolve two ounces of sal ammoniac in a pound of aquafortis, and put to it verdigris an ounce, red lead, crude antimony, and the caput mortuum of vitriol, of each half an ounce; put these well powdered leisurely into the vessel, and set this also aside.—In another vessel, dissolve two ounces of sal ammoniac in a pound of aquafortis, and add orpiment, white arsenic, painters lake, of each half an ounce.

Keep the above nine vessels in a moderate heat for 15 days, shaking them well at times. After this pour all the matters from these vessels into one large vessel, well luted at its bottom; let this stand fix days, shaking it at times; and then set it in a very gentle heat, and evaporate all the liquor, and there will remain a powder of a purplish green.

When this is to be wrought, put into a pot very clear metal, made of broken crystalline and white glas that has been used; for with the virgin frit, or such as has never been wrought, the chalcedony can never be made, as the colours do not stick to it, but are consumed by the frit. To every pot of 20 pounds of this metal put two or three ounces of this powder at three several times; incorporate the powder well with the glas; and let it remain an hour between each time of putting in the powders. After all are in, let it stand 24 hours; then let the glas be well mixed, and take an assay of it, which will be found of a yellowish blue; return this many times into the furnace; when it begins to grow cold, it will show many waves of different colours very beautifully. Then take tartar eight ounces, foot of the chimney two ounces, crocus martis made with brimstone, half an ounce; let these be well powdered and mixed, and put them by degrees into the glas at fix times, waiting a little while between each putting in. When the whole is put in, let the glas boil and settle for 24 hours; then make a little glas body of it; which put in the furnace many times, and see if the glas be enough, and whether it have on the outside veins of blue, green, red, yellow, and other colours, and have, beside these veins, waves like those of the chalcedonies, jaspers, and oriental agates, and if the body kept within looks as red as fire.

When it is found to answer this, it is perfect, and may be worked into toys and vessels, which will always be beautifully variegated; these must be well annealed, which adds much to the beauty of their veins. Mafics of this may be polished at the lapidary's wheel as natural stones, and appear very beautiful. If in the working the matter grow transparent, the work must be flopped, and more tartar, foot, and crocus martis, must be put to it, which will give it again the necessary body and opacity, without which it does not show the colours well.

Chrysolite colour may be made of ten pounds of either of the compositions for hard glas described above, and fix drachms of calcined iron.

Red cornelian colour may be formed by adding one pound of glas of antimony, two ounces of the calcined vitriol called scarlet ochre, and one drachm of manganese or magnesia, to two pounds of either of the com- positions for hard glas. The glas of antimony and magnesia are first fused with the other glas, and then powdered and ground with the scarlet ochre: the whole mixture is afterwards fused with a gentle heat till all the ingredients are incorporated. A glas resembling the white cornelian may be made by two pounds of either of the compositions for hard glas, and two drachms of yellow ochre well waisted, and one ounce of calcined bones: grind them together, and fuse them with a gentle heat.

Emerald colour. See Green below.

Garnet colour. To give this colour to glas, the workmen take the following method. They take equal quantities of crystal and rochetta frit, and to every hundred weight of this mixture they add a pound of manganese and an ounce of prepared saffier: these are to be powdered separately, then mixed and added by degrees to the frit while in the furnace. Great care is to be taken to mix the manganese and saffier very perfectly; and when the matter has stood 24 hours in fusion, it may be worked.

Glas of this kind may be made by adding one pound of glas of antimony, one drachm of manganese, and the same quantity of the precipitate of gold by tin, to two pounds of either of the compositions for hard glas; or the precipitate of gold may be omitted, if the quantities of the glas of antimony and manganese be doubled.

Gold colour. This colour may be produced by taking ten pounds of either of the compositions for hard glas, omitting the saltpetre; and for every pound adding an ounce of calcined borax, or, if this quantity doth not render the glas sufficiently fusible, two ounces; ten ounces of red tartar of the deepest colour; two ounces of magnesia; and two drachms of charcoal of fallow, or any other soft kind. Precipitates of silver baked on glas will stain it yellow, and likewise give a yellow colour on being mixed and melted with 40 or 50 times their weight of vitreous compositions; the precipitate from aquafortis by fixed alkali seems to answer best. Yellow glases may also be obtained with certain preparations of iron, particularly with Prussian blue. But Dr Lewis observes, that the colour does not constantly succeed, nor approach to the high colour of gold, with silver or with iron. The nearest imitations of gold which he has been able to produce have been effected with antimony and lead. Equal parts of the glas of antimony, of flint calcined and powdered, and of minium, formed a glas of a high yellow; and with two parts of glas of antimony, two of minium, and three of powdered flint, the colour approached still more to that of gold. The last composition exhibited a multitude of small sparkles intersepered throughout its whole substance, which gave it a beautiful appearance in the mafs, but were really imperfections, owing to air bubbles.

Neri directs, for a gold yellow colour, one part of red tartar and the same quantity of manganese, to be mixed with a hundred parts of frit. But Kunckel observes, that these proportions are faulty; that one part, or one and a quarter, of manganese, is sufficient for a hundred of frit; but that six parts of tartar are hardly enough, unless the tartar is of a dark red colour, almost blackish; and that he found it expedient to add to the tartar about a fourth of its weight of powdered charcoal. He adds, that the glas swells up very much in melting, and that it must be left unfurled, and worked as it stands in fusion. Mr Samuel More, in repeating and varying this process in order to render the colour more perfect, found that the manganese is entirely unessential to the gold colour; and that the tartar is no otherwife of use than in virtue of the coaly matter to which it is in part reduced by the fire, the phlogiston or inflammable part of the coal appearing in several experiments to be the direct tinging sublance. Mr Pott also observes, that common coals give a yellow colour to glas; that different coaly matters differ in their tinging power; that caput mortuum of foot and lamp black answer better than common charcoal; and that the sparkling coal, which remains in the retort after the rectification of the thick empyretumatic animal oils, is one of the most active of these preparations. This preparation, he says, powdered, and then burnt again a little in a clofe vessel, is excellent for tinging glas, and gives yellow, brown, reddish, or blackish colours, according to its quantity; but the frit must not be very hard of fusion, for in this case the strong fire will destroy the colouring sublance before the glas melts; and he has found the following composition to be nearly the best; viz. and two parts, alkali three parts; or and two, alkali three, calcined borax one; or and two, alkali two, calcined borax one: and though saltpetre is hardly used at all, or very sparingly, for yellow glases, as it too much volatilizes the colouring sublance; yet here for the most part a certain proportion of it, easily determined by trial, is very necessary; for without it the concentrated colouring matter is apt to make the glas too dark, and even of an opake pitchy blackness. It does not certainly appear that there is any material diversity in the effects of different coals, the difference being probably owing to the different quantities of the inflammable matter which they contain; so that a little more shall be required of one kind than of another for producing the same degree of colour in the glas. Nor does the softness or fusibility of the frit appear to be in any respect neccessary.

Gold-coloured spangles may be diffused through the substance of glas, by mixing the yellow tales with powdered glas, and bringing the mixture into fusion.

Green. This colour may be imparted to glas by adding three ounces of copper precipitated from aquafortis, and two drachms of precipitated iron, to nine pounds of either of the compositions for hard glas. The finest method of giving this beautiful colour to glas is this: Take five pounds of crystalline metal that has been passed several times through water, and the same quantity of the common white metal of polverine, four pounds of common polverine frit, and three pounds of red lead; mix the red lead well with the frit, and then put all into a pot in a furnace. In a few hours the whole mass will be well purified: then cast the whole into water, and separate and take out the lead; then return the metal into the pot, and let it stand a day longer in fusion; then put in the powder of the residuum of the vitriol of copper, and a very little crocus martis, there will be produced a most lively and elegant green, scarce inferior to that of the oriental emerald. There are many ways of giving a green to glas, but all are greatly inferior to this.—To make a sea green, the finest crystalline glas only must be used, and no manganese must be added at first to the metal. The crystal frit must be melted thus alone; and the salt, which swims like oil on its top, must be taken off with an iron ladle very carefully. Then to a pot of twenty pounds of this metal add fix ounces of calcined brafs, and a fourth part of the quantity of powdered zaffer: this powder must be well mixed, and put into the glafs at three times; it will make the metal swell at first, and all must be thoroughly mixed in the pot. After it has flood in fusion three hours, take out a little for a proof: if it be too pale, add more of the powder. Twenty-four hours after the mixing the powder the whole will be ready to work; but must be well stirred together from the bottom, lest the colour should be deepest there, and the metal at the top left coloured, or even quite colourless. Some use for this purpose half crystal frit and half rochetta frit, but the colour is much the finest when all crystal frit is used.

Lapis lazuli colour. See Lapis LAZULI.

Opal colour. See OPAL.

Purple of a deep and bright colour may be produced by adding to ten pounds of either of the compositions for hard glafs, above described, fix drachms of zaffer and one drachm of gold precipitated by tin; or to the same quantity of either composition one ounce of manganese and half an ounce of zaffer. The colour of amethyst may be imitated in this way.

Red. A blood-red glafs may be made in the following manner: Put fix pounds of glafs of lead, and ten pounds of common glafs, into a pot glazed with white glafs. When the whole is boiled and refined, add by small quantities, and at small distances of time, copper calcined to a redness as much as on repeated proofs is found sufficient: then add tartar in powder by small quantities at a time, till the glafs is become as red as blood; and continue adding one or other of the ingredients till the colour is quite perfect.

Ruby. The way to give the true fine red of the ruby, with a fair transparency, to glafs, is as follows: Calcine in earthen vessels gold diffolved in aqua-regia; the menstruum being evaporated by distillation, more aqua-regia added, and the abstraction repeated five or fix times, till it becomes a red powder. This operation will require many days in a hot furnace. When the powder is of a proper colour, take it out: and when it is to be used, melt the finest crystal glafs, and purify it by often casting it into water; and then add, by small quantities, enough of this red powder to give it the true colour of a ruby, with an elegant and perfect transparency.

The procefs of tinging glafs and enamels by preparations of gold was first attempted about the beginning of the last century Libavius, in one of his tracts entitled Alchymia, printed in 1606, conjectures that the colour of the ruby proceeds from gold, and that gold diffolved and brought to redness might be made to communicate a like colour to facitious gems and glafs. On this principle Neri, in his Art of Glafs, dated in 1611, gives the procefs above recited. Glauber in 1648 published a method of producing a red colour by gold, in a matter which is of the vitreous kind, though not perfect glafs. For this purpose he ground powdered flint or sand with four times its weight of fixed alkaline salt; this mixture melts in a moderately strong fire, and when cool looks like glafs, but exposed to the air runs into a liquid state. On adding this liquor to solution of gold in aqua-regia, the gold and flint precipitate together in form of a yellow powder, which by calcination becomes purple. By mixing this powder with three or four times its weight of the alkaline solution of flint, drying the mixture, and melting it in a strong fire for an hour, a mafs is obtained of a transparent ruby colour and of a vitreous appearance; which nevertheless is soluble in water, or by the moisture of the air, on account of the redundancy of the salt. The Honourable Mr Boyle, in a work published in 1682, mentions an experiment in which a like colour was introduced into glafs without fusion; for having kept a mixture of gold and mercury in digestion for some months, the fire was at last immoderately increased, so that the glafs burnt with a violent explosion; and the lower part of the glafs was found tinged throughout of a transparent red colour, hardly to be equalled by that of rubics.

About the same time Cassius is said to have discovered the precipitation of gold by tin, and that glafs might be tinged of a ruby colour by melting it with this precipitate; though he does not appear, says Dr Lewis, from his treatise De Auro, to have been the discoverer of either. He describes the preparation of the precipitate and its use; but gives no account of the manner of employing it, only that he says one drachm of gold duly prepared will tinge ten pounds of glafs.

This procefs was soon after brought to perfection by Kunckel; who says, that one part of the precipitate is sufficient to give a ruby colour to 1280 parts of glafs, and a sensible redness to upwards of 1900 parts; but that the success is by no means constant. Kunckel also mentions a purple gold powder, resembling that of Neri; which he obtained by infipitating solution of gold to dryness; abstrating from it freih aqua-regia three or four times, till the matter appears like oil; then precipitating with strong alkaline ley, and washing the precipitate with water. By diffolving this powder in spirit of salt and precipitating again, it becomes, he says, extremely fair; and in this state he directs it to be mixed with a due proportion of Venice glafs.

Orschal, in a treatise entitled Solvire Veste, gives the following procefs for producing a very fine ruby. He directs the purple precipitate made by tin to be ground with fix times its quantity of Venice glafs into a very fine powder, and this compound to be very carefully mingled with the frit or vitreous composition to be tinged. His frit consists of equal parts of borax, nitre, and fixed alkaline salt, and four times as much calcined flint as of each of the salts; but he gives no directions as to the proportion of the gold precipitate or mode of fusion. Hellot describes a preparation, which, mixed with Venice glafs, was found to give a beautiful purple enamel. This preparation consists of equal parts of solution of gold and of solution of zinc in aqua-regia mixed together, with the addition of a volatile salt prepared from sal ammoniac by quicklime, in sufficient quantity to precipitate the two metals. The precipitate is then gradually heated till it acquires a violet colour. However, though a purple or red colour, approaching to that of ruby, may, by the methods above recited, be baked on glafs or enamels, and introduced into the mafs by fusion, the way of equally diffusing such such a colour through a quantity of fluid glass is still, says Dr Lewis, a secret. The following process for making the ruby glass was communicated to Dr Lewis by an artist, who ascribed it to Kunckel. The gold is directed to be dissolved in a mixture of one part of spirit of salt and three of aquafortis, and the tin in a mixture of one part of the former of these acids with two of the latter. The solution of gold being properly diluted with water, the solution of tin is added, and the mixture left to stand till the purple matter has settled to the bottom. The colourless liquor is then poured off, and the purple sediment, while moist and not very thick, is thoroughly mixed with powdered flint or sand. This mixture is well ground with powdered nitre, tartar, borax, and arsenic, and the compound melted with a suitable fire. The proportions of the ingredients are 2560 parts of sand, 384 of nitre, 240 of tartar, 240 of borax, 28 of arsenic, five of tin, and five of gold.

Topaz Colour. Glass resembling this stone may be made by pulverizing ten pounds of either of the compositions for hard glasses with an equal quantity of the gold-coloured glass, and fusing them together.

White opake and semi-transparent glass may be made of ten pounds of either of the compositions for hard glass, and one pound of well calcined horn, ivory, or bone; or an opake whiteness may be given to glass by adding one pound of very white arsenic to ten pounds of flint glass. Let them be well powdered and mixed by grinding them together, and then fused with a moderate heat till they are thoroughly incorporated. A glass of this kind is made in large quantities at a manufactory near London; and used not only for different kinds of vessels, but as a white ground for enamel in dial plates and snuff boxes, which do not require finishing with much fire, because it becomes very white and fusible with a moderate heat.

Yellow. See Gold colour above.

Painting in Glass. The ancient manner of painting in glass was very simple: it consisted in the mere arrangement of pieces of glass of different colours in some sort of symmetry, and constituted what is now called mosaic work. See Mosaic.

In process of time they came to attempt more regular designs, and also to represent figures heightened with all their shades: yet they proceeded no farther than the contours of the figures in black with water colours, and hatching the draperies after the same manner on glasses of the colour of the object they designed to paint. For the carnation, they used glass of a bright red colour; and upon this they drew the principal lineament of the face, &c. with black.

At length, the taste for this kind of painting improving considerably, and the art being found applicable to the adorning of churches, basilics, &c. they found out means of incorporating the colours in the glass itself, by heating them in the fire to a proper degree; having first laid on the colours. A French painter at Marseilles is said to have given the first notion of this improvement, upon going to Rome under the pontificate of Julius II.; but Albert Durer and Lucas of Leyden were the first that carried it to any height.

This art, however, has frequently met with much interruption, and sometimes been almost totally lost; of which Mr Walpole gives us the following account, in his Anecdotes of Painting in England.

"The first interruption given to it was by the reformation, which banished the art out of churches; yet it was in some measure kept up in the escutcheons of the nobility and gentry in the windows of their seats. Towards the end of Queen Elizabeth's reign it was omitted even there; yet the practice did not entirely cease. The chapel of our Lady at Warwick was ornamented anew by Robert Dudley earl of Leicester, and his counsellors, and the cipher of the glass-painter's name yet remains, with the date 1574: and in some of the chapels at Oxford the art again appears, dating itself in 1622, by the hand of no contemptible master.

"I could supply even this gap of 48 years by many dates on Flemish glass; but no body ever supposed that the secret was lost so early as the reign of James I. and that it has not perished since will be evident from the following series, reaching to the present hour.

"The portraits in the windows of the library at All Souls, Oxford. In the chapel at Queen's College there are twelve windows dated 1518. P. C. a cipher on the painted glass in the chapel at Warwick, 1574. The windows at Wadham's College; the drawing pretty good, and the colours fine, by Bernard Van Linge, 1622. In the chapel at Lincoln's Inn, a window, with the name Bernard, 1623. This was probably the preceding Van Linge. In the church of St Leonard, Shoreditch, two windows by Baptista Sutton, 1634. The windows in the chapel at University College, Hen. Giles pinxit, 1687. At Christ Church, Isaac Oliver, aged 84, 1700. Window in Merton Chapel, William Price 1700. Windows at Queen's New College, and Maunlin, by William Price, the son, now living, whose colours are fine, whose drawing is good, and whose taste in ornaments and mosaic is far superior to any of his predecessors; is equal to the antique, to the good Italian masters, and only surpassed by his own singular modesty.

"It may not be unwelcome to the curious reader to see some anecdotes of the revival of taste for painted glass in England. Price, as we have said, was the only painter in that style for many years in England. Afterwards one Rowell, a plumber at Reading did some things, particularly for the late Henry earl of Pembroke; but Rowell's colours soon vanished. At last he found out a very durable and beautiful red; but he died in a year or two, and the secret with him. A man at Birmingham began the same art in 1756 or 1757, and fitted up a window for Lord Lyttleton, in the church of Hagley; but soon broke. A little after him, one Peckitt at York began the same business, and has made good proficiency. A few lovers of that art collected some dispersed panes from ancient buildings, particularly the late Lord Cobham, who erected a Gothic temple at Stowe, and filled it with arms of the old nobility, &c. About the year 1753, one Alciotti, an Italian, who had married a Flemish woman, brought a parcel of painted glass from Flanders, and sold it for a few guineas to the Honourable Mr Bateman, of Old Windsor. Upon that I sent Alciotti again to Flanders, who brought me 450 pieces, for which, including the expense of his journey, I paid him thirty-six guineas. His wife made more journeys for the fame pur- pose; and sold her cargo to one Palmer a glazier in St Martin's lane, who immediately raised the price to one, two, or five guineas for a single piece, and fitted up entire windows with them, and with mosaics of plain glass of different colours. In 1761, Paterson, an auctioneer at Essex house in the Strand, exhibited the two first auctions of painted glass, imported in like manner from Flanders. All this manufacture consisted in rounds of Scripture stories, stained in black and yellow, or in small figures of black and white; birds and flowers in colours, and Flemish coats of arms.

The colours used in painting or staining of glass are very different from those used in painting either in water or oil colours.

For black, take scales of iron, once ounce; scales of copper, one ounce; jet, half an ounce: reduce them to powder, and mix them. For blue, take powder of blue, one pound; sal nitre, half a pound: mix them and grind them well together. For carnation, take red chalk, eight ounces; iron scales, and litharge of silver, of each two ounces; gum arabic, half an ounce: dissolve in water; grind all together for half an hour as stiff as you can; then put it in a glass and stir it well, and let it stand to settle 14 days. For green, take red lead one pound; scales of copper, one pound; and flint, five pounds: divide them into three parts; and add to them as much sal nitre; put them into a crucible, and melt them with a strong fire; and when it is cold, powder it, and grind it on a porphyry. For gold colour, take silver, an ounce; antimony, half an ounce: melt them in a crucible; then pound the mass to powder, and grind it on a copper plate; add to it yellow ochre, or brick dust calcined again, 15 ounces; and grind them well together with water. For purple, take minium, one pound; brown stone, one pound; white flint, five pounds: divide them into three parts, and add to them as much sal nitre as one of the parts; calcine, melt, and grind it as you did the green. For red, take jet, four ounces; litharge of silver, two ounces; red chalk, one ounce: powder them fine, and mix them. For white, take jet, two parts; white flint, ground on a glass very fine, one part: mix them. For yellow, take Spanish brown, ten parts; leaf silver, one part; antimony, half a part: put all into a crucible, and calcine them well.

In the windows of ancient churches, &c. there are to be seen the most beautiful and vivid colours imaginable, which far exceed any of those used by the moderns, not so much because the secret of making those colours is entirely lost, as that the moderns will not go to the charge of them, nor be at the necessary pains, by reason that this sort of painting is not now so much in esteem as formerly. Those beautiful works which were made in the glass houses were of two kinds.

In some, the colour was diffused through the whole substance of the glass. In others, which were the more common, the colour was only on one side, scarce penetrating within the substance above one-third of a line; though this was more or less according to the nature of the colour, the yellow being always found to enter the deepest. These last, though not so strong and beautiful as the former, were of more advantage to the workmen, by reason that on the same glass, though already coloured, they could show other kinds of colours where there was occasion to embroider draperies, enrich them with foliages, or represent other ornaments of gold, silver, &c.

In order to this, they made use of emery, grinding or wearing down the surface of the glass till such time as they were got through the colour to the clear glass. This done, they applied the proper colours on the other side of the glass. By these means, the new colours were hindered from running and mixing with the former, when they exposed the glasses to the fire, as will appear hereafter.

When indeed the ornaments were to appear white, the glass was only bared of its colour with emery, without tinging the place with any colour at all; and this was the manner by which they wrought their light and heightenings on all kinds of colour.

The first thing to be done, in order to paint or stain glass, in the modern way, is to design, and even colour, the whole subject on paper. Then they choose such pieces of glass as are clear, even, and smooth, and proper to receive the several parts; and proceed to distribute the design itself, or papers it is drawn on, into pieces suitable to those of the glass; always taking care that the glasses may join in the contours of the figures and the folds of the draperies; that the carnations, and other finer parts, may not be impaired by the lead with which the pieces are to be joined together. The distribution being made, they mark all the glasses as well as papers, that they may be known again: which done, applying every part of the design upon the glass intended for it, they copy or transfer the design upon this glass with the black colour diluted in gum water, by tracing and following all the lines and strokes as they appear through the glass with the point of a pencil.

When these strokes are well dried, which will happen in about two days, the work being only in black and white, they give a slight wash over with urine, gum arabic, and a little black; and repeat it several times, according as the shades are desired to be heightened; with this precaution, never to apply a new wash till the former is sufficiently dried.

This done, the lights and risings are given by rubbing off the colour in their respective places with a wooden point, or the handle of the pencil.

As to the other colours above mentioned, they are used with gum water, much as in painting in miniature; taking care to apply them lightly, for fear of effacing the outlines of the design; or even, for the greater security, to apply them on the other side; especially yellow, which is very pernicious to the other colours, by blending therewith. And here too, as in pieces of black and white, particular regard must always be had not to lay colour on colour, or lay on a new lay, till such time as the former are well dried.

It may be added that the yellow is the only colour that penetrates through the glass, and incorporates therewith by the fire; the rest, and particularly the blue, which is very difficult to use, remaining on the surface, or at least entering very little. When the painting of all the pieces is finished, they are carried to the furnace or oven to anneal or bake the colours.

The furnace here used is small, built of brick, from 18 to 30 inches square. At six inches from the bottom is an aperture to put in the fuel and maintain the fire. GLA

Glas. fire. Over this aperture is a grate made of three square bars of iron, which traverse the furnace, and divide it into two parts. Two inches above this partition is another little aperture, through which they take out pieces to examine how the coction goes forward. On the grate is placed a square earthen pan, fix or seven inches deep, and five or fix inches less every way than the perimeter of the furnace. On the other side hereof is a little aperture, through which to make trials, placed directly opposite to that of the furnaces defined for the same end. In this pan are the pieces of glas to be placed in the following manner: First, The bottom of the pan is covered with three strata or layers of quicklime pulverized; those strata being separated by two others of old broken glas, the design whereof is to secure the painted glas from the too intense heat of the fire. This done, the glasses are laid horizontally on the last or uppermost layer of lime.

The first row of glas they cover over with a layer of the same powder an inch deep; and over this they lay another range of glasses, and thus alternately till the pan is quite full; taking care that the whole heap always end with a layer of the lime powder.

The pan being thus prepared, they cover up the furnace with tiles, on a square table of earthen ware, closely luted all round; only leaving five little apertures, one at each corner, and another in the middle, to serve as chimneys. Things thus disposed, there remains nothing but to give the fire to the work. The fire for the first two hours must be very moderate, and must be increased in proportion as the coction advances, for the space of ten or twelve hours; in which time it is usually completed. At last the fire, which at first was charcoal, is to be of dry wood, so that the flame covers the whole pan, and even effuses out at the chimneys.

During the last hours, they make essays, from time to time, by taking out pieces laid for the purpose through the little aperture of the furnace and pan, to see whether the yellow be perfect, and the other colours in good order. When the annealing is thought sufficient, they proceed with great haste to extinguish the fire, which otherwise would soon burn the colours, and break the glases.

GLASS Balls, which are circular, or otherwise shaped hollow vessels of glas, may be coloured within, so as to imitate the femipellucid gems. The method of doing it is this: make a strong solution of ichthyocolla, or ifinglas, in common water, by boiling; pour a quantity of this while warm into the hollow of a white glas vessel; shake it thoroughly about, that all the sides may be wetted, and then pour off the rest of the moisture. Immediately after this, throw in red lead, shake it and turn it about, throw it into many places with a tube, and the moisture will make it flick and run in waves and pretty figures. Then throw in some of the painters blue smalt, and make it run in waves in the ball as the red lead; then do the same with verdigrise, next with orpiment, then with red lake, all well ground; always casting in the colours in different places, and turning the glas, that the moisture within may run them into the waves. Then take fine plaster of Paris, and put a quantity of it into the ball; shake it also nimbly about; this will everywhere stick firmly to the glas, and give it a strong inner coat, keeping all the colours on very fairly and strongly. These are set on frames of carved wood, and much esteemed as ornaments in many places.

GLASS Drop. See RUPERT's Drops.

Engraving on GLASS. Professor Beckmann has proved, that fo early as the year 1670 the art of etching upon glas was discovered by Henry Schwanhard, son of George Schwanhard, who was a celebrated glas-cutter, patronized by the emperor Ferdinand III. about the middle of the last century. At the time of his death, 1667, the father practised his art at Prague and Ratibon. Whether the son followed the same business at the same towns, or removed to Nuremberg, is not very evident; but in the year above mentioned, some aqua-regia (nitro-muriatic acid) having accidentally fallen on his spectacles, he was surprised to find the glas corroded by it, and become quite soft. He thus, it is said, found himself in possession of a liquid by which he could etch writing and figures upon plates of glas.

But it is probable, as Beckmann seems to think, that he had discovered the fluoric acid itself; for in the year 1725 there appeared in a periodical work the following receipt for making a powerful acid, by which figures of every kind can be etched upon glas.

"When the spiritus nitri per distillationem has passed into the recipient, ply it with a strong fire, and when well dephlegmated, pour it, as it corrodes ordinary glas, into a Weldenberg flask. Then throw into it a pulverized green Bohemian emerald, otherwise called helphorus, (which, when reduced to powder, and heated, emits in the dark a green light), and place it in warm sand for 24 hours. Take a piece of glas well cleaned, and freed from all grease by means of a ley; put a border of wax round it, about an inch in height, and cover it all over with the above acid. The longer you let it stand so much the better; and at the end of some time the glas will be corroded, and the figures which have been traced out with sulphur and varnish will appear as if raised above the pane of glas."

That the Bohemian emerald or helphorus mentioned in this receipt is green sparry fluor, cannot, says the professor, be doubted; and he seems to have as little doubt of the receipt itself having passed from Schwanhard and his scholars to the periodical work of 1725, from which it was inserted in the Oekonomische Encyclopedie of Krunitz. This supposition certainly acquires a considerable degree of probability from the similarity of Schwanhard's method of etching to that which is here recommended, and which is so different from what is now followed. At present, the glas is covered with a varnish either of ifinglas dissolved in water, or of turpentine oil mixed with a little white lead, through which the figures to be etched are traced as on copper; but Schwanhard, when he had drawn his figures, covered them with varnish, and then by his liquid corroded the glas around them. His figures, therefore, when the varnish was removed, remained smooth and clear, appearing raised from a dim or dark ground; and M. Beckmann, who persuaded some ingenious artists to make trial of this ancient method of etching, declares, that such figures have a much better effect than those which are cut into the glas.

Foliating of GLASS. See FOLIATING and LOOKING-glas. Glas.

Gilding of Glass. See Gilding.

Impressions of antique Gems taken in Glass. See Gems.

Glass of Lead, a glas made with the addition of a large quantity of lead, of great use in the art of making counterfeit gems. The method of making it is this: Put a large quantity of lead into a potter's kiln, and keep it in a state of fusion with a moderate fire, till it is calcined to a gray loofe powder; then spread it in the kiln, and give it a greater heat, continually stirring it to keep it from running into lumps; continue this several hours, till the powder become of a fair yellow; then take it out, and sift it fine: this is called calcined lead. Take of this calcined lead 45 pounds, and crystalline or other frit 12 pounds; mix these as well as possible together; put them into a pot, and set them in the furnace for ten hours; then cast the whole, which will be now perfectly melted, into water; separate the loofe lead from it, and return the metal into the pot; and after standing in fusion 12 hours more, it will be fit to work. It is very tender and brittle, and must be worked with great care, taking it slowly out of the pot, and continually wetting the marble it is wrought upon.

It is well known that ceruse or white lead, minium, litharge, and all the other preparations and calces of lead, are easily fused by a moderate fire, and formed into a transparent glas of a deep yellow colour. But this glas is so penetrating and powerful a flux, that it is necessary to give it a greater consistence, in order to render it fit for use. With this view, two parts of calx of lead, e.g. minium, and one part of sand or powdered flints, may be put into a crucible of refractory clay, and baked into a compact body. Let this crucible, well cloed with a luted lid, be placed in a melting furnace, and gradually heated for an hour, or an hour and a half; and afterwards let the heat be increased so as to obtain a complete fusion, and continued in that state for the same time: let the crucible remain to cool in the furnace; and when it is broken a very transparent yellow coloured glas will be found in it. Some add nitre and common salt to the above mixture, because these salts promote the fusion and the more equal distribution of the sand. This glas of lead has a considerable specific gravity, and its lowest part is always the heaviest. It is an important flux in the assays of ores to facilitate their scoriification.

Glas of lead is capable of all the colours of the gems in very great perfection. The methods of giving them are these: for green, take pulverine frit 20 pounds, lead calcined 16 pounds; sift both the powders very fine; then melt them into a glas, separating the unmixed lead, by plunging the mass in water; after this return it into the pot, and add brats thrice calcined fix ounces, and one pennyweight of crocus martis made with vinegar; put this in at fix different times, always carefully mixing it together, and take a proof of it; when the colour is right, let it stand eight hours, and then work it. If instead of the calcined brats the same quantity of the capit mortuum of the vitriolum veneris be used, the green is yet much finer.

For topaz colour, take crystal frit 15 pounds, calcined lead 12 pounds; mix them well together, by sifting the powders through a fine sieve; then set them in a furnace not too hot, and separate the superfluous unmixed lead, by calling the whole into water; repeat this twice: then add half gold yellow glas, and let them incorporate and purify, and they will be of the true and exact colour of the oriental topazes.

For sea green, take crystal frit 16 pounds, calcined lead 10 pounds; mix and sift them together, and set them in a pot in a furnace; in 12 hours the whole will be melted; then cast it into water, and separate it from the loose lead; put them into the furnace again for eight hours; then separate the loose lead by washing a second time, and return it to the pot for eight hours more.

Muscovy Glass. See Mica, Mineralogy Index. Painting on Glass by means of Prints. See Back-painting.

Glass Porcelain, the name given by many to a modern invention of imitating the china ware with glas. The method given by M. Reaumur, who was the first that carried the attempt to any degree of perfection, is shortly this: The glas vessels to be converted into porcelain are to be put into a large earthen vessel, such as the common fine earthen dishes are baked in, or into sufficiently large crucibles; the vessels are to be filled with a mixture of fine white sand, and of fine gypsum or plaster stone burnt into what is called plaster of Paris, and all the interstices are to be filled up with the same powder, so that the glas vessels may nowhere touch either one another, or the sides of the vessel they are baked in. The vessel is to be then covered down and luted, and the fire does the rest of the work; for this is only to be put into a common potter's furnace, and when it has stood there the usual time of the baking the other vessels, it is to be taken out, and the whole contents will be found no longer glas, but converted into a white opaque substance, which is a very elegant porcelain, and has almost the properties of that of China.

The powder which has served once will do again as well as fresh, and that for a great many times: nay, it seems, ever so often. The cause of this transformation, says Macquer, is probably that the vitriolic acid of the gypsum quits its basis of calcareous earth, and unites with the alkaline salt and saline earth of the glas, with which it forms a kind of salt, different from the calcareous felsite, by the interposition of which matter the glas acquires the qualities of porcelain.

Glass Pots, the vessels in the glas trade used for melting the glas. Those for the white glas works are made of a tobacco pipe clay, brought from the isle of Wight, which is first well washed, then calcined, and afterwards ground to a fine powder in a mill; which being mixed with water, is then trod with the bare feet till it is of a proper consistence to mould with the hands into the proper shape of the vessels. When these are thus made, they are afterwards annealed over the furnace. Those for the green glas works are made of the nonfuch, and another sort of clay from Staffordshire; they make these so large as to hold three or four hundred weight of metal. And besides these, they have a small font called piling pots, which they set upon the larger, and which contain a finer and more nice metal fit for the nicest works.

The clay that is used for this purpose should be of the purest and most refractory kind, and well cleansed from all sandy, ferruginous, and pyrites matters; and Glasf. to this it will be proper to add ground crucibles, white fand, calcined flints duly levigated, or a certain proportion of the fame clay baked, and pounded not very finely. The quantity of baked clay that ought to be mixed with the crude clay, to prevent the pots from cracking when dried, or exposed to a great heat, is not absolutely determined, but depends on the quality of the crude clay, which is more or lefs fat. M. D'Antic, in a memoir on this subject, proposes the following method of ascertaining it: The burnt and crude clay, being mixed in different proportions, should be formed into cakes, one inch thick, and four inches long and wide. Let these cakes be slowly dried, and exposed to a violent heat, till they become as hard and as much contracted as possible, and in this state be examined; and the cake, he says, which has suffered a diminution of its bulk equal only to an eighteenth part, is made of the best proportions. He observes, in general, that most clays require that the proportion of the burnt should be to the fresh as four to five.

Tim GLASS, the fame with Bismuth. See BISMUTH, CHEMISTRY Index.

GLASSES are distinguished, with regard to their form, use, &c. into various kinds, as drinking glases, optical glases, looking glases, burning glases, &c.

Drinking GLASSES, are simple vefels of common glasf or crystal, usually made in form of an inverted cone.

Each glas consists of three parts, viz. the bowl, the bottom, and the foot; which are all wrought or blown separately.

Nothing can be more dexterous and expeditious than the manner of blowing these parts: two of them opened, and all three joined together. An idea is only to be had thereof, by feeing it actually done. For the method of gilding the edges of drinking glases, see GILDING on Enamel and Glafs.

Optical GLASSES. See Optics.

The improvements hitherto made in telescopes by means of combining lenfes made of different kinds of glasf, though very great, are yet by no means adequate to the expectations that might reasonably be formed if opticians could fall on any method of obtaining pieces of glas sufficiently large for pursuing the advantages of Mr Dollond's discovery. Unfortunately, however, though the board of longitude have offered a considerable reward for bringing this art to the requisite perfection, no attempt of any consequence has hitherto been made. Mr Keir is of opinion, that the accomplishment of this is by no means an easy task; as it requires not only a competent knowledge of the properties of glas fitted for the purpose (the faults not being evident to common inspection), but a considerable degree of chemical knowledge is also necessary in order to invent a composition by which these faults may be avoided; and lastly, a kind of dexterity in the execution of the work, which can only be acquired by practice. Our author, however, thinks, that if the subject were more generally understood, and the difficulties more fully pointed out, for which purpose he makes the following remarks, the end may be more easily accomplished.

1. The rays of light passing through a glas lens or prifm, or through any other medium of unequal thickness, are refracted; but not in an equal manner, the blue, violet, &c. being more refracted than the red.

2. Hence it happens, that the rays of light, when refracted by a common lens, do not all unite in one focus, but in reality form as many different foci as there are colours; and hence arise the prismatic colours, or irises, which appear towards the borders of the image formed by the common convex lenfes, and which render the vision extremely indistinct.

3. The indistinctness of vision produced by this cause, which is sensible in telescopes of a small aperture, increases in fo great a proportion, viz. as the cubes of the diameters, that it seemed impossible to increase the power of dioptric telescopes greatly, without extending them to a very inconvenient length, unless this confusion of colours could be corrected.

4. It was known that different transparent bodies possessed different degrees of refractive power; and until Mr Dollond discovered the contrary, it was supposed, that the refractions of the coloured rays were always in a determined ratio to one another. On this supposition it seemed impossible to correct the faults of refracting telescopes: for it was supposed, that if the dispersion of light produced by a convex lens were counteracted by another lens or medium of a concave form, the refraction would be totally destroyed; and this indeed would be the case, if the two mediums were made of the same matter; and from some experiments made by Sir Isaac Newton, this was supposed to be actually the case in all substances whatever.

5. From considering that the eyes of animals are formed of mediums of different colours, it occurred first to Mr David Gregory, the celebrated professor of astronomy at Oxford, and then to Mr Euler, that, by a combination of mediums which had different refractive powers, it might be possible to remedy the imperfections of dioptric telescopes. It does not, however, appear, that either of these gentlemen underfood the true principle on which these phenomena depend. Mr Euler executed his idea by forming a compound object lens from two glas lenfes with water interposed, but his attempt was not attended with success. Mr Dollond, however, was led by some arguments adduced by Mr Klingenfierna of Sweden, to repeat one of Sir Isaac Newton's experiments, and which had induced even that great philosopher himself to suppose that the improvement afterwards executed by Mr Dollond was impossible. This experiment was made by Sir Isaac Newton, by placing a glas prifm within a prismatic vefel filled with water, in such a manner that the rays of light which were refracted by the glas prifm should pass through and be refracted in a contrary direction by the water prifm. In this manner the refraction of the light was entirely destroyed. But when Mr Dollond repeated the experiment, he found, that, contrary to his own expectations, when the angles of the two prisms were so proportioned that they counteracted each other's mean refraction, then colours appeared; and on the other hand, when they were so proportioned that the diffraction of the coloured rays was counteracted, the mean refraction still subsisted; which evidently proved, that the mean refractive and dispersive powers of glas and water were not proportional to one another.

6. To apply this to the proposed improvement, Mr Dollond examined several kinds of glas. Crown glas was found to possess the smallest dispersive power in proportion to its refraction; while flint glas possessed the greatest dispersive power in proportion to its refraction, which was also very great. On comparing these two exactly together, he found, that a wedge of white flint glass whose angle was about 25 degrees, and another of crown glass whose angle was 29 degrees, refracted very nearly alike. He found also, that, when the wedges were ground to such angles, the refraction produced by the flint glass was to that produced by the crown glass nearly as two to three; the refracted light was then free from colour. On measuring the general refracting powers of these two glasses, he found, that in flint glass, the fine of incidence of the rays was to the fine of mean refraction as 1 to 1.583; and that in crown glass, the fine of incidence was to the fine of mean refraction as 1 to 1.53.

The methods of determining the different refractive powers of glasses are given under the article Optics. Here we shall only observe, that two kinds of glass are necessary for the construction of achromatic telescopes; one of which shall possess as small, and the other as great, dispersive powers, relative to their mean refracting powers, as can be produced. The difference of glasses in this respect depends on the quality of the ingredients employed in their composition. Crown glass, which is composed of sand melted by means of the ashes of sea weeds, barilla, or kelp, both which fluxes are known to consist of vegetable earth, alkali, and neutral salt, is found to give the smallest dispersive power. Plate glass, which consists of sand melted by means of fixed vegetable alkali, with little or no vegetable earth, gives a greater dispersive power; but both these give much less than flint glass, which consists of sand melted by means of minium and fixed alkali. It appears, therefore, that the dispersion of the rays is greatest when minium, or probably other metallic calces, are made use of; and that alkalies give a greater power of dispersion than vegetable or other earths. Mr Zieher of Peterburgh, however, informs us, that he has made a kind of glass, much superior in this respect to flint glass; but it does not as yet appear whether it be more fit for optical purposes than that commonly made use of. There seems no difficulty in augmenting the dispersive power, as that is found to depend on the quantity of minium or other flux: but thus we unfortunately increase also the capital fault to which flint glass and all compositions of that kind are subject; namely, the being subject to veins or small threads running through it. By these, even when so small as to be imperceptible to the naked eye, the rays which fall on them are diverted from their proper direction, and thereby render the images confused. This is owing to the greater density of the veins, as appears by their image being received on white paper, when the glass is held between the paper and the sun or a candle at a proper distance. The rays of light being then made to converge by the superior density of the veins, their images will appear as bright lines bordered with obscure edges on the paper. Flint glass is so much subject to this kind of imperfection, that it is with difficulty the opticians can pick out pieces of the size commonly used from a large quantity of the glass. It is farther to be regretted, that the minium which produces the greatest dispersive power, is likewise the very substance which renders flint glass much more subject to these imperfections than any other. The reason is, that the sand and earthy matters mix uniformly in fusion; and having not only a considerable degree of affinity towards each other, but also being not much different from each other, they are not apt to separate. On the other hand, when such a heavy substance as minium is added to these earthy substances, though it has a pretty strong tendency to unite with the earthy substances, it has none with the fixed alkali, which is another ingredient in this glass. Hence some parts of the glass will contain more metallic matter than the rest; particularly that near the bottom of the pot, which is so full of large veins as to be applied only to the making of wares of little value. The veins in this case are formed by the descent of the minium at the bottom, which in its passage forms threads or veins by dragging other parts of the glass along with them.

The correction of this fault appears therefore to be very difficult. M. Macquer informs us, that he had in vain tried to remove it by very long fusion and a fierce fire; which indeed others have found by experience not to correct, but to augment the evil. Mr Keir is of opinion that some new composition must be discovered, which, along with a sufficient refractive power, should possess a greater uniformity of texture; but he is likewise of opinion, that scarce any alteration in this respect could be made without injuring the colour of the glass. For optical purposes, however, our author does not think that an alteration in the colour of the ingredients would be very detrimental. "I am convinced (says he), that glasses sensibly tinged with colour, might transmit as much or more light than the best flint glass. For the colourless appearance of flint glass is an optical deception. The minium gives it a considerable tinge of yellow, and the alkali inclines it to a bluish cast, besides the colour arising from a greater or less impurity of the materials; so that the glass would actually be very sensibly coloured, unless by the addition of manganese, which is known to give a purplish red. Thus the other tinges are counteracted, but not effaced or destroyed as has been frequently imagined. By the mixture of the three principal colours, red, yellow, and blue, more or less exactly counterpoised, a certain dark shade is introduced, in which, as not any one of the colours predominates, no coloured tinge appears, but the effect is merely a diminution of the transparency of the glass, which, however, is too small for ordinary observation." Mr Keir is even of opinion, that a certain tinge of yellow would in many cases be of service, because it would exclude some of the blue rays, which being most refrangible are most injurious to the distinctness of vision.

Very considerable difficulties, however, must arise in attempting improvements of this kind; as the experiments must all be tried on a very large scale. This is not only attended with a very heavy expense in itself on account of the quantity of materials employed, but from the heavy duty of excise which is rigorously exacted whether the glass be manufactured into saleable articles or not. It is observed in the manufacture of every kind of glass, that the glass in the middle of the area or transverse section of a pot is much purer and freer from veins and other imperfections than the part which is near the sides, and that the glass at the bottom is the worst of all. Consequently it is chiefly in large pots, such as are used in manufactures, that there is a probability of success. Very fine and beautiful glases, called pale and artificial gems, may be made in smaller pots or crucibles; but this glass is suffered to cool and subside in the vessel, by which means the contiguous parts are more uniform in their texture than can be expected in a piece of glass taken out of the pot while hot in the common way, by making it adhere and twist round an iron rod or pipe. But although the method of allowing the glass to cool in the pots is very advantageous for the purposes of the jeweller, it is by no means applicable to those of the optician. Glass cooled in that gradual manner, suffers some degree of crystallization or peculiar arrangement of its parts; the consequence of which is, that the rays of light undergo certain refractions independent on the form of the glass, which greatly affect the distinctness of vision in telescopes.

Musical GLASSES. See HARMONICA. Looking GLASSES. See LOOKING GLASS, MIRROR, and FOLIATING.

Burning GLASS. See BURNING GLASS. Weather GLASS. See BAROMETER. Cutting GLASS. See SURGERY. Hour GLASS. See HOUR GLASS. Watch GLASS. See WATCH. GLASS Wort. See SALSOBA, BOTANY Index.