If we attempt to lower the point of fusion by varying the composition, the glass then becomes so attractive of moisture as to be worthless as an object-glass. If, on the other hand, we attempt to make a very dry glass, there is a liability to crystallization and devitrification in the cooling.

M. Bontemps' is said to have overcome the difficulties of the manufacture of optical glass: he exhibited last year discs of flint-glass 29 inches in diameter, and of crown-glass of 20 inches, and others of smaller size, which are said to pass satisfactorily through the ordeal of polarized light, thus showing them to be of the same density throughout. The 29-inch disc, however, presented a few striæ near the surface, which, it was thought, would be ground out in giving the lens its curvature. These glasses were from the works of Messrs. Chance, who have engaged M. Bontemps to superintend this department of their large and varied establishment.

After this long detail, the reader will probably expect us to describe the process of making optical glass. We regret to say that we are unable to do so it is still a secret confined to the few whose labours suffice to supply the demand for large lenses. The circumstance of a very limited demand is a good reason why the possessors of the secret have been able to retain it so long. If optical glass were required in the large quantities in which other descriptions of glass are manufactured, there cannot be a doubt that its production would soon become one of the common operations of the glass-house.

Mr. James Nasmyth has recently proposed a method of making optical glass, which not only promises well, but may be the secret of his predecessors. Supposing the materials to be pure and in due proportions, and that the vitrification is perfect, he reasonably supposes that the variations in density arise from removing the glass from the melting-pot while in the fluid state. He therefore proposes to place a large pot of the most perfectly composed glass in the furnace, and to subject it to the highest degree of heat which it can stand, in order that, by rendering its contents as completely fluid as possible, the particles may arrange themselves in strata of perfect homogeneity and uniformity of density. In order to ensure this result, this high temperature is to be kept up for at least three days. A slight variation in density will be found in the top stratum of glass, as compared with that at the bottom of the pot; but the variation of the successive parallel strata would be so gradual, that one would not vary from another in any appreciable degree. The pot is then to be left perfectly quiet, and the furnace allowed to die out gradually, taking care that no draught have access to it, or any irregularity take place in the decrease of temperature. The cooling may thus require about twenty days. When the pot is cold, the glass is to be removed, and cut into slices parallel to its top surface, by the simple means employed by marble-cutters. The result would probably be a series

(1) This gentleman has just published an interesting "Examen des Verres, Vitraux, Cristaux, composant la Classe XXIV. de l'Ex; osition Universelle de 1851."

of discs of absolutely perfect glass. Mr. Nasmyth has tried this method on a considerable scale with crownglass with perfect success; masses of the most spiendid glass having been obtained, to all appearance perfectly free from striæ or other irregularity.

This ingenious method is evidently inapplicable to flint-glass: the long-continued application of heat would dissipate its alkali, and the long repose would cause the lead to subside, so that no two horizont:l layers, however thin, would possess the same density. Mr. Simms, the eminent optical instrument maker, states, that in only two instances has he succeeded in making unexceptionable object-glasses of English flintglass, and these had only 34 inches aperture. The flintglass as hitherto obtained from the Continent, in dises of from 4 to 9 inches diameter, is very costly; and the risk of its not turning out well after the labour of grinding has been bestowed upon it, has deterred the instrument-makers generally from making so costly an experiment. It is therefore to be hoped that Messrs. Chance will be encouraged in their undertaking, so important to the interests of science; and it is interesting to learn from Mr. Simms that, so far as he has tried it, their optical flint-glass is equal to the very best that was prepared by the elder Guinand.

SECTION VI.-CROWN-GLASS.

The glass commonly used for window-panes is a silicate of soda and lime: it is more difficult of fusion, and less easily worked, and much harder than flintglass, and hence does not well admit of being shaped into vessels, or ornamented by cutting and grinding. Crown-glass is manufactured in the form of flat dises or tables, 52 inches in diameter. At the Great Exhibition, Messrs. Chance displayed some tables 66 inches

in diameter.

The arrangement of the crown-glass furnace is similar to that of the flint: it is arranged in the centre of the cone, and contains from four to six pots, each of the capacity of half a ton. These pots are open at the top, not hooded as in the flint-glass pots. There are several subsidiary furnaces adjoining or round the inside of the cone; also an annealing furnace, or arch, and an oven for annealing the glass-pots before they are set. The fritting furnace, called colear, from the French calquaise, is reverberatory in its action, and is used for calcining or fritting the materials before they are placed in the glass-pots for vitrification; the object being to effect a partial union

between the silicic acid and the alkaline bases, so that

the latter may not be volatilized in the furnace; this alkaline vapour not only being a positive loss, but acting injuriously on the pots and sides of the furnace. The other furnaces are used for softening the glass while passing through the successive stages from a globe into a flat table:-the blowing furnace, used to facilitate the blowing of the glass into a large globe, the bottoming hole, the nose hole, and the flashing furnace, the use of which will be explained presently.

The materials consist of fine sand, chalk, soda-ash or crude carbonate of soda, or salt-cake, which is dry (2) Memoirs of the Royal Astronomical Society, vol. xvi. 1817.

sulphate of soda; also a little charcoal, black oxide of manganese, arsenious acid, and occasionally a little oxide of cobalt, to correct any defects in colour arising from the presence of oxide of iron. As the soda is liable to considerable fluctuations in its value, no fixed proportions of materials can be agreed upon, and the manufacturer has to determine the amount of real alkali in every fresh supply of ash. When all the alkali is soda-ash, the usual proportions are, 100 parts quartz sand, 35 to 40 chalk, a quantity of soda-ash containing about 8 parts soda, and from 150 to 200 old broken glass, or cullet. The proportion of arsenious acid and manganese can only be ascertained by trial. If the materials are to be calcined, they are put into the colcar, and well stirred with iron paddles and rakes, to expel water and burn off carbonaceous matters, as also to drive off carbonic acid from the chalk, and to unite the base of the alkaline carbonate with silicic acid, displacing the carbonic acid, and thus prevent the alkali from being volatilized in the glass-pot. The temperature of the colcar is gradually raised during three hours, and when the mixture becomes pasty it is diligently stirred, to facilitate the extrication of carbonic acid and the combustion of carbonaceous matters. Towards the end of the third hour, the temperature is considerably raised, and at the expiration of the fourth, while the mass is hot and soft, it is raked out into large castiron trays, and cut by a spade into square cakes, which are piled away for use. These frit-bricks are supposed to improve by age, and are sometimes kept for twelve months or more. Where the fritting is omitted, as it now often is, the materials must be thoroughly dried before the founding; for which purpose the sand is first calcined at a dull red heat in the colcar, then carefully sifted: the chalk is also raised to a more moderate heat, for the purpose of expelling moisture, not carbonic acid. The sand and chalk are finely sifted and thoroughly intermixed with the other materials, also in a dry and pulverulent form. The mixture is shovelled into the pots when they are at a white heat. The furnace being at its greatest heat, the first quantity thrown in fuses down in about 8 hours; another portion is then thrown in, and so on until the pot is sufficiently full. Vitrification is usually complete in about 18 hours. When gas ceases to be disengaged, the temperature is gradually lowered, by closing the draught-holes, and the chinks in the doors of the cave are stopped with mortar. In a few hours the glass is perfectly clear: the fire is then raised a little, and the glass-gall being skimmed off, the glass is ready for working. In order to preserve a clean surface, an earthenware ring of the same material as the pots, and about 2 feet in diameter and 3 inches thick, is floated on the melted glass after the surface is skimmed. The surface of the glass is much cleaner within the ring than without.

It is difficult to convey a complete idea in writing of the remarkable series of processes by which a lump of glass is expanded into a large sheet or table. In the Great Exhibition Messrs. Hartley & Co., the eminent crown-glass manufacturers of Sunderland, had

glass furnace, will be the better qualified to follow our description. Indeed, the general intelligence of the public would be promoted, writers on the useful arts assisted, and the cause of education forwarded, if an industrial museum, such as the Great Exhibition, could be made permanent, although on a much smaller scale; nor do we see any good reason why every town should not have its Useful Arts Museum, open to all, with competent persons to give instruction therein. Had such powerful educational means been in existence during the last half-century, many a poor inventor would have been spared much useless toil, anxiety, and expense.

The man collects on the end of his blowing-tube, from the inside of the ring, as much glass in successive layers as will form a disc or table of about 9 lbs. weight. An experienced workman seldom fails more than an ounce or two in the correct weight. After cooling the blowing-tube which has been heated in the melting furnace, by letting water drop upon it, he rolls this mass of glass, technically called the piece, upon the marver, into a pear-shaped lump, a, Fig. 1070, while a boy slightly distends it by blowing. It is next held in the mouth of the small blowing-furnace to soften: again rolled on the marver to correct inequalities in the thickness of the sides, and to collect the great mass of glass at the lowest point, thereby elongating the neck, as at b. In this marvering the outer extremity of the glass is made conical, and the extreme end is called the bullion. In again blowing out the bulb, the man supports it on a horizontal smooth iron rod, called the bullion-bar, placed across a stool, or across a pit in front of the marver, and he expands the bulb until it is nearly spherical, as at c: to preserve the perfect form of the sphere, the man,

with gradually increasing velocity, when the hole caused by the removal of the blowing-pipe enlarges. It is next heated more intensely before the flashingfurnace, and acquires the form shown in Fig. 1072.

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Fig. 1071. THE BOTTOMING-HOLE.

Hartley's works the bullion-bar is dispensed with; the glass is supported by a tube attached to the bullion, and held by a boy while the blower causes the tube to revolve. The boy is protected from the great mass of hot glass by a shield through which the supporting rod passes. The globe is next softened at the blowing-furnace and blown out until quite spherical. It is then conveyed to the bottoming-hole, Fig. 1071, which is similar to the flashing-furnace, but has a smaller opening: here it is exposed to the direct action of the flame, the man being screened from the heat by a wall in front and a little on one side of the furnace. The tube is supported on a hook projecting from the wall, and in revolving the globe the part opposite the tube becomes softened by the heat, and yielding to the action of centrifugal force, assumes the fo m shown at d, Fig. 1070. It is then removed, and another man applies a pontil tipped with melted glass to the centre of the flattened part, when the first man detaches the blowing-pipe by touching the neck of the globe with an iron wetted with cold water. The blowing-pipe carries with it a piece of glass, leaving a corresponding hole in the flattened sphere at a point exactly opposite the attachment of the pontil, as shown at e, Fig. 1070. By the action of heat and centrifugal force, the flattened sphere becomes more and more flat, while the hole becomes larger and larger, and at length entirely disappears on the glass flying out into a flat table. This final result is approached by gradual steps. The glass is held by the pontil and exposed to a

Fig. 1072. THE FLASHING-FURNACE.

By increasing the velocity of rotation, the flattened globe at length flies completely open with a noise something like that produced by quickly opening a wet umbrella: in this way a flat circular disc nearly 60 inches in diameter is produced, of uniform thickness except at the point of attachment to the pontil, where there is a swelling called the bull's-eye. A sheet of such dimensions would fold together while in its soft state if the man were to cease to rotate it; he therefore continues the motion until the table is sufficiently cool. It is then placed on a large iron fork, held by an assistant; the pontil is cracked off, and the plate transferred to the annealing arch Fig. 1073, where it is made to rest on edge in two strong parallel iron supports, running the whole length of the annealing kiln. The arch is of sufficient capacity to receive two rows of tables, as shown by the dotted circles: the annealing is continued for 24 hours, during which the whole arch is raised to a uniform heat, and allowed to cool gradually. A shorter time suffices when the arch is strongly heated at one end, and nearly cold at the other, while the tables are slowly moved forward towards the cold end.

Fig. 1073.

A pot containing half a ton of metal will produce. 100 tables. At an ordinary crown-glass house 4 such pots are emptied in three days every week. The glassgall, and the residue in the pots, are refined by being

(1) The glass at the edge of the disc is also in some cases a little

thickened. It is evident that the portion of the glass attached to the end of the blowing-tube afterwards becomes the edge or onter rim of the table: the thickening of this edge is prevented by a the concave of which the man places the neck of the piece or solid ball of glass, while the boy is distending it by blowing: it is thus heating and blowing, another form of cutter is used. The edge of prevented from thickening at that point. In subsequently re

cutter, attached to the marver, consisting of an iron crescent, into

a bar of wood attached to the marver will even answer the purpose.