largest glass-houses in Great Britain: Sand (well dried), from the neighbourhood of Leighton Buzzard, in Bedfordshire; sulphate of soda, ground; subcarbonate of soda, white oxide of arsenic, manganese, Welsh anthracite, chalk; limestone from Hopton Wood, Derbyshire; nitrate of soda; cullet, about as much as is equal to an eighth part of the other ingredients. The exact proportions are only known to the manufacturers. Each ingredient is carefully powdered before mixing, and they are afterwards calcined or fritted, except the anthracite, which is added in the pot for the purpose of decomposing the sulphate of soda, and dissipating its acid; and the manganese and arsenic, which are only added in very small quantities, to improve the colour; too much, however, of each is sure to injure the glass, and therefore these materials can only be safely used by experienced manipulators. The bulk of the glass, however, consists of the sand, and carbonate and sulphate of soda.

The arrangement of the window-glass houses is different, and on a much larger scale than in the houses for bottle-glass, and excepting in gathering and mavering, all the operations subsequent to the founding are different. Fig. 9 will give a general

and after turning it about for a minute or two in the air until sufficiently cooled, he then dips it in again, and over the first he makes a second gathering, which increases the weight to about three pounds weight; the same cooling process is repeated, and a third gathering is made, which brings up the weight to about nine pounds; he then holds his blow-pipe perpendicularly with the glass downward, so that it may by its own weight pull downward from the pipe in the form of a symmetrical pear-shaped bulb; he next takes it to the hollowed block before mentioned, and turns it round in the water placed in the cavity by which it is made ready for the mavering table. The workman, by skilful management, mavers the bulb of glass into the form b, fig. 11, and then forms a little knob at its apex, by turning it on a fixed bar of iron called the bullion bar; he then commences blowing, and soon the bulb of nearly solid glass is expanded into a large hollow sphere (c, fig. 11), still, however, with the little nipple made by the bullion bar. A little boy now comes forward with an iron rod, the pointel, upon the end of which has been gathered a small lump of metal, called the punty, about the size of a hen's egg, this he applies to the nipple, to which it firmly adheres, the workman meanwhile resting his blowpipe on a fixed rest called the casher-box, placed for the purpose; by the pressure of the pointil the globe of glass is flattened as in d, fig. 11. The application

Fig. 9.

plan of the house for crown window-glass, and fig. 10 gives an elevation of one side of the main furnace, with the three openings through which the glass is gathered from the pots. In fig. 9, a is the main furnace; b, b, two flashing furnaces; the projecting piece of brick-work, b, being the screen which protects the workman from the fire; and c, c are two annealing furnaces or ovens.

When the founding or melting and the skimming are completed, the workman takes his blow-pipe, which is about seven feet in length, heats it at the end, and dipping it into the pot of melted glass

Fig. 11.

of a piece of iron, cooled for the purpose by keeping it in water, to the junction of the glass with the blowpipe, detaches it instantly, and the globe of glass is now held with the pointil. The operator carries it next to the nose-hole (b, fig. 10), and presents the opening formed by the detachment of the blow-pipe, to the action of the furnace; this again softens the glass, which is kept continually revolving by turning the pointil on an iron rest or hook fixed to the masonry of the furnace. The revolutions are at first slow, but are gradually accelerated as the softening of the glass goes on, and the centrifugal force so produced throws the edges of the orifice outwards, as in e, fig. 11. As the glass flattens, it is revolved with greater rapidity, and advanced so near to the mouth of the nose-hole as to draw the flames outward, by contracting the draught. This completes the softening of the glass, which is done suddenly, with a rushing noise like the unfurling of a flag in the wind, caused by the rapid flying outward of the softened glass and the rush of the flames outwards. It becomes perfectly flat, and of equal thickness, except at the bullion or centre, formed, as before described, by the bullion-bar and the punty. The flashing is now complete; and the workman removes it from the nose-hole, and still continuing to turn it in his hands, in order to cool and harden it, as he walks along, carries it to the annealing oven, where another one receives it on a large flattened fork-like implement at the moment the flasher, who has hold of the pointil, suddenly detaches it by a touch of his shears. It is then passed through the long horizontal slit which forms the opening into the annealing oven, and when fairly in, it is dexterously turned on

its edge; here it remains at a temperature somewhat below that required to soften glass, until the oven is filled with these so-called tables of glass, when the heat is suffered to decline, until the whole is cold, when they are removed to the packing-room, to be packed in crates for sale.

Until lately, crown-glass was almost universally employed for windows, but now that which is called German sheet has become quite as common, besides which British sheet, which is the same glass polished, and plate-glass are much used. The operation of making the sheet-glass is very different from that employed in making crown-glass, inasmuch as a long and perfect cylinder is sought to be produced by the blower instead of a sphere of glass. This necessitates also a different arrangement of the glass-house, as is seen by the ground-plan shewn in fig. 12: aa is the furnace, b is the annealing oven,

heated by the flue ', which opens into the main furnace; the leer, or annealing oven, is often, however, an independent structure; c, c, c, c, c, c, c, c, are the eight pots, which is the number usually employed in these works. These, of course, are opposite to the openings for working them, and in front of each opening is a long opening in the ground, about eight feet deep and three feet in width; d, d, d, d, d, d, d, d. The workman stands on the edge of this pit, and having made his gathering, as in the crown-glass manufacture (a, fig. 13), he next mavers it, without, however, using the bullion-rod (b, fig. 13). He next proceeds to blow his glass, holding it downward whilst doing so, that its weight may widen and elongate the bulb, and from time to time dexterously swings it round, which greatly increases its length (c, d, fig. 13). As it cools rapidly in this operation, he from time to time places his pipe in the rest which is fixed before the furnacemouth, and gently turning it round, he brings it again nearly to the melting-point, then he repeats the blowing and swinging, standing over the pit, to enable him to swing it completely round as it lengthens out. These operations are continued until the cylinder has reached its maximum size, that is, until it is of equal thickness throughout, and sufficiently long and broad to admit of sheets of the required size being made from it (e, fig. 13). Sometimes these cylinders are made 60 inches in length, allowing sheets of glass 49 inches in length to be made from them. The next operation is to place the pipe in the rest, and apply the thumb so as to close

turns the cylinder, still with its end to the fire, and the softened edges of the opening, which at first are curved inwards, are flashed out until they are in a straight line with the sides of the cylinder (g, fig. 13). It is then removed, and placed on a rest or casher-box, when a small punty of melted glass at the end of a pointil is brought by a boy; this the workman applies to one side of the cylinder, just below the shoulder formed at the blow-pipe end (fig. 13, g), and drawing it out to a thin string, wraps it quickly so as to draw a line round the cylinder; after a second or two, he withdraws this line of red-hot glass, and touching it quickly with his cold shears, the shoulder and neck drop off as neatly as if cut with a diamond.

The cylinder (fig. 13, h) is now placed for a short time in the annealing oven (b, fig. 12), where it is prepared for cutting; it is next placed in a groove lined with green baize, and a diamond fixed to a sliding rule makes a perfectly straight cut from end to end. The split cylinder is then taken to the flattening arch or furnace, where it is laid on the bottom, with the diamond-cut upwards. The bottom is a perfectly smooth stone, kept constantly free from dust by the workman; here the heat is sufficient to soften without melting the glass, and the flattener, as it softens, opens the two edges of the crack until they fall outward flat on the stone; he then takes an implement in the form of a rake, made by placing a piece of charred wood transversely at the end of a long handle, and this is gently rubbed over the glass, producing a very smooth surface. At the back of the flattening arch is an annealing oven, communicating with the arch by a narrow horizontal slit, through which the sheet of glass is now pushed on to a plate of iron, which receives it; and as this plate is one of a series linked together so as to form an endless band, which can be turned round, the sheets move forward into the annealing oven, where the workman gently lifts them on edge until the oven is filled, when, as in the case of crownglass, the heat is allowed to decline until perfectly cool, the sheets are then ready for use. Very much larger sheets are obtained by this process than by the former one, hence it is becoming of great importance; but it is not easy to obtain workmen sufficiently powerful and dexterous to blow and twirl the largest-sized cylinders; at present, we obtain almost all the operatives so employed from Belgium.

Glass-shades are made in the same manner as above described; indeed, they are nothing more than the rounded ends of the cylinders before being burst. When wanted oval or square, these forms are produced by boxes of wood charred inside, of the size the shades are required, through which the

cylinder is passed when being blown, until the soft glass touches, and receives shapes from the inside of the box or mould: they are afterwards annealed, and cut to the lengths required. If of large diameter, they require immense strength and great skill in the operator, who sometimes aids the power of his breath by taking into his mouth a little spirit, which he blows down the pipe; this, of course, is instantly converted into vapour, when it reaches the red-hot cylinder, and by its expansion aids in distending the glass.

Plate-glass is made in a totally different manner; and as its value depends chiefly on its purity, the greatest possible care is taken to procure materials

of the best quality, and almost every manufacturer has his own private formula for the mixture. It may, however, be said to consist chiefly of sand and alkaline salts, as in other kinds of glass, and the following is one receipt known to be in use: Fine white sand well washed, to free it from impurities, 720 lbs.; sulphate of soda, 450 lbs.; slaked lime, 80 lbs. ; nitrate of potash, 25 lbs. ; and cullet of plateglass, 425 lbs. These ingredients, when melted and skimmed, should yield about 1200 lbs. of perfectly clear metal, which is the quantity usually required for a casting. When melted and ready for use, the pot is lifted out of the furnace (aa, fig. 14) by means of the forceps, and wheeled up to the casting-table

(cc, fig. 14); here it is seized by a crane and tackle, by which it is lifted, and so nicely poised over the table, that it can be easily tilted so as to pour out its contents. All this requires so much care and steadiness, that the men, impressed with the great danger of carelessness, usually preserve perfect silence during their work. The table is of large size -20 feet or more in length, by 8 or 10 feet in width. When the red-hot liquid glass is poured on, it immediately begins to spread; two bars of iron, a little thicker than the plate is intended to be, are quickly laid on each side of the table, and a steel roller is laid across, resting on these bars: this roller is worked by hand, and rapidly spreads the glass all over the table, the bars preventing it from running over the sides, and regulating its thickness. In a very short time, it begins to cool; the men then seize the end of it with pincers, and pull it forward with great dexterity on to an endless band of wire-gauze, which, being made to revolve, moves the immense plate forward to a slit-like opening to the annealing oven (fig. 14, ff), where it is worked on to another table on wheels, which is pushed forward to make room for another. The annealing oven is usually of immense length, as, in the case of plate-glass, the sheets cannot be set on edge. At the works at St Helen's, in Lancashire, where glass of all kinds is extensively made, there are usually two annealing ovens to each shed, the furnaces being placed between them; each oven runs to the end of the shed, and these sheds are usually over 300 feet in length. The ground-plan shewn in fig. 14 will give a general idea of the arrangement of one of these vast work-shops. The main building is a shed, with the doors at each end, and both doors and windows are made so as to exclude drafts of air, which, if admitted during the operation of casting, are highly injurious to the quality of the manufacture. a, a, are the two melting-furnaces; b, b, b, b, b, b, the pots; c, c, the casting-tables; d, d, the endless bands of wire-gauze for moving the plates to the annealing ovens ; e, e, where they enter

by the narrow openings, ff; and, after they have sufficiently cooled, are removed through the openings at each end, g, g.

The plates are next removed to the first polishingshed, where each is imbedded in a matrix of stucco, leaving one surface exposed; the whole is enclosed in a frame, which holds both glass and stucco securely. Two of these frames are placed one over the other, with the two exposed surfaces of glass in contact. The lower frame is fixed, and the upper is made to move by machinery with great rapidity backward and forward with a swinging motion, so as to describe an opposite curve with each backward and forward motion. Sand and water are continually thrown on the surface of the fixed plate, and thus the first stage of polishing is performed. The plates are then readjusted in the frames, and the other surfaces are brought upwards, and receive a similar rubbing down with sand and water. The plates are next removed to the second polishingroom, where women are usually employed; here they are again fixed on low tables, and each woman rubs the surface for a long time with a piece of plate-glass, covering from time to time the whole face of the plate with emery-powder and water. After both sides have received this hand-polishing, the plates are removed to a third room, where they are again imbedded on tables which are movable by machinery, so that the whole surface of the plate may be brought under the action of the polishers. These are large movable blocks, covered with woollen cloth and leather, and loaded so as to press on the glass; the polishing material used is colcothar, the red oxide of iron; this completes the polish which gives so much beauty to plate-glass. It is a long and laborious process, and is the chief cause of the high price of plate as compared with other sheet-glass. British plate is only the cylinder glass polished by the processes just described; its comparative cheapness is due to the rapidity with which the cylinder can be blown. Of this rapidity, the best estimate may be formed from

discolour the glass, an iron rod coated with platina is used. In the manufacture of this particular kind of glass, the Messrs Chance of Birmingham are unrivalled, and they have produced very perfect discs for lenses, weighing as much as two hundredweights each.

Flint-glass and Optical Glass.-The general principle of the manufacture of these two varieties of glass is identical with those already described, the chief difference consisting in the great care taken to insure perfect purity in the materials. The pots used are so made, that the metal is protected from the chance of being contaminated by any accidental impurities falling in or from the gases of the furnace; they are made with a dome-shaped roof and a lateral arch-shaped opening (fig. 7), which is placed opposite the furnace-mouth, so that the workman has easy access to the contents of the pot, which is necessarily smaller, otherwise the workman could not dip to the bottom.

The materials used for the best flint-glass are varied in their proportions, according to the judgment of the manufacturer; they consist of the whitest sand which can be procured, fine American pearl-ashes (impure carbonate of potash, which is purified by dissolving out the carbonate from its impurities, and evaporating it to dryness in leaden evaporating pans), red lead, or else litharge (the semi-vitrified protoxide of lead), and a small quantity of nitre (nitrate of potash). To these, according to their greater or less purity, the manipulator adds more or less of oxide of manganese and arsenic, as correctives; the former removes the green discoloration which the presence of even a small quantity of iron in the sand will produce; and the latter corrects the tendency the manganese has to give a purple tint to the glass. Both substances require the utmost care and judgment in their use, otherwise they are more injurious than beneficial. The following are the usual proportions: Sand, 51; pearl-ashes, prepared, 16; litharge, 28 (or red lead, 29); nitre, 4; white arsenic, ; peroxide of manganese,; cullet of flint-glass in any proportion the manufacturer thinks proper.

Formerly, the silica was obtained by calcining flints, hence the name applied to this kind of glass, but now sand is used instead; and although beautifully white sands are obtained from Lynn, in Norfolk, from the Isle of Wight and other parts of Hampshire, from Aylesbury, from France, and even from North America and Australia, it nevertheless requires most careful preparation by washing, calcining, and sifting.

But however carefully flint-glass is made, and however pure and transparent the crystal may be which is so made, it nevertheless possesses some defects, which interfere with its fitness for telescopes, microscopes, light-houses, and other optical purposes. These defects consist in almost imperceptible striæ in the material, which produce certain optical aberrations. These striæ are known to be caused by the imperfect mixture of the materials, and the want, consequently, of a uniform density. This has been obviated by M. Guinaud and his associate, M. Frauenhofer, by stirring the metal in the pot with an iron rod; but greater improvements have been effected by our own chemist Faraday, who not only improved upon the manipulation of Messrs Guinaud and Frauenhofer, but suggested also an improvement in the materials, by the addition of carbonate of baryta and a little carbonate of lime, which produces a glass of the greatest density and clearness that has ever been known before. Instead

Flint-glass is employed in the manufacture of all the articles of utility and ornament for table and other domestic uses; and as the manufacture of each article requires different management, it would be impossible here to give any satisfactory explanation of the manipulative processes. Suffice it to say, that at present Great Britain is unrivalled in the production of so-called crystal or flint-glass, which we manufacture of the greatest purity and brilliancy; but in the coloured kinds the Bohemians take the lead, and excel both in design and in the art of colouring.

Much flint-glass is now moulded into drinkingvessels, bottles, and other common articles; but these are always greatly inferior to those which are made by the handicraft of the regular glassblower.

Coloured glass is a general term which includes several distinct varieties: first may be mentioned the glass made for windows and other similar purposes. Coloured sheet-glass is made both by the crownglass and cylinder-glass processes. Sometimes it is of pot-metal-that is, the glass and the colouring materials are all melted and worked from one potgenerally, however, this glass is of too dark a colour, and the kind called flashed glass is most generally used; in this, two pots are employed, one containing the coloured glass, as if for pot-metal, the other colourless glass. The workman makes his first gatherings from the colourless glass, and the last only from the coloured pot; the consequence is that the glass when finished, although it cannot be perceived, has only a thin skin of the coloured material on one side, and the colour is thus as it were diluted. This has other advantages, because, by skilful grinding, the colour may be removed, and transparent patterns produced on the coloured ground; and the same may be done, and even delicate shading of the colour effected, by eating away the coloured side more or less by means of fluoric acid, which is frequently employed, and most beautiful effects are produced.

The colours usually employed consist of metallic oxides, other substances are, however, occasionally used. Gold, in the state called Purple of Cassius, invented by Dr Andrew Cassius of Leyden in 1632, and also in the state of a simple solution, without tin, yields the most beautiful ruby, crimson, rose, and purple colours. Copper, as a sub-oxide, yields a fine ruby red, and the black oxide gives an emerald green. Cobalt yields the rich deep blues. Iron, as a protoxide, gives a dull green; combined with alumina, it gives flesh colour, or pale rose, and combined with chloride of silver, it yields an orange yellow; as a peroxide, it gives a common red and a brownish red. Silver, with alumina, also yields a yellow colour of great beauty; and commoner and less beautiful yellow tints are produced by glass of antimony, and even by carbon, either in the form of soot or charcoal. Uranium gives the beautiful chrysoprase green and canary yellow, with a slight degree of opalescence; it also gives an emerald green. Arsenic, or arsenious acid, produces an opaque white. Manganese gives a purple or amethystine colour as an oxide; and as a peroxide, with a little cobalt, a fine garnet-red colour. These are some of the materials generally employed, but there are numerous others, the use of which depends upon the skill of the manufacturer.