closeness into water, and should be well agitated to separate all dirt and impurities. The alkali used is always soda: this is preferred to potash ; as glass made with the former substance is thinner, and flows better while hot, and yet is equally durable when cold. The quality of flowing freely is of the very first importance in casting large plates, which, to be perfect, require to be without streak or bubble. Another advantage attending the use of soda is this;-that the neutral salts of which it is the base, such as muriate and sulphate of soda, and which, in this instance, constitute the glass gall, are dissipated more readily by the heat of the furnace, than are the salts of which potash is the base. The soda must be used in a state of considerable purity; and is generally either that which is separated from the ashes of barilla, and other soda plants, by lixiviation, or is produced by the decomposition of common salt. Lime acts in promoting the fusibility of the silex and alkali, fulfilling thus the same office as is performed by litharge in the manufacture of flint glass. From one fifteenth to one twenty-fourth part of the whole materials is the largest proportion that can properly be used of lime; any greater quantity would impair both the colour and solidity of the glass. Manganese would have the effect of giving a slight tinge of red; but when mixed in a proper proportion with the blue of the cobalt, and both together are met by the natural slight yellow of the other materials, each neutralises the other, so that scarcely any definable tint remains. In addition to these ingredients, a considerable quantity of fragments of glass, or, as it is called, cullet, is used in combination with the fresh materials. Of these fragments there is always an abundant supply in the glasshouse, produced from what is spilt in casting, and from the ends and edges that are cut off in shaping the plates. This broken glass, or cullet, is previously made friable, by throwing it, while hot, into cold water. It is considered that the addition of one pound of pure soda is sufficient for four pounds of sand. But it is not enough, in the preparation of glass for casting, to apply the alkali only in the proportion necessary to produce good glass; much more than this must be used, in order to procure the requisite degree of fluidity. The composition given by Loysel as being used in the great works at St. Gobain is this: White sand, Carbonate of lime, Soda, 100 parts 45 to 48 Fragments of glass of like quality, 100 Parkes recommends the following proportions, as qualified to produce plates of glass of the best description :— Lynn sand, previously well washed and dried, 720 parts, Nitre, Cullet, or broken plate glass, 80 25 425 1700 These quantities are required to produce one pot of metal, which will yield 1200 pounds of good plate glass. Another author states the following proportions as being found to produce very fine glass:— The well known property of borax, as a powerful flux, has occasioned the suggestion that, by its means, glass made with potash might be caused to flow in fusion as freely as that wherein soda is employed. It has been asserted that small quantities of borax have always been used in the works at St.Gobain; but the secresy observed in regard to all the operations carried on in that establish ment renders it impossible to say what degree of truth there is in the assertion. Great care is required in mixing the materials; much more, indeed, than is called for in regard to other kinds of glass. The sand, lime, soda, and manganese, being properly intermingled, are fritted in small furnaces, wherein the temperature is gradually raised to a full red, or even to a white, heat, at which point it is maintained, and the materials are carefully stirred until vapour is no longer given off, and no further change is undergone by the materials. This process of fritting lasts about six hours; and when it is nearly completed, the remaining part of the ingredients, consisting of the cobalt and broken glass, are added. The latter, having already been perfectly vitrified, does not, consequently, require any lengthened exposure to the fire. The furnaces at St. Gobain, in which the perfect fusion and vitrification are accomplished, are eighteen feet long and fifteen wide. They contain two kinds of crucibles. The larger ones, wherein the glass is melted, are called pots, and are formed like inverted truncated cones; the other crucibles, which are smaller, are called cuvettes: these last are kept empty in the furnaces, exposed to the full degree of its heat, that when the glass is ready for casting, and is transferred to them, they may not injuriously lower its temperature. The comparative size of these cuvettes varies according to the dimensions of the plates which it is intended to cast: when these are very large, the cuvette will contain one third of the charge of the pot; but in other cases its capacity is not greater than a fourth, a fifth, or a sixth part of the contents of the crucible. The method used for regulating the supply of fuel to the furnace in the great works of St. Gobain, is at once so rude and so absurd, that one would hesitate to believe in the correctness of the narration, if it did not rest upon good authority. It is said that two persons are employed, who, being disencumbered of all superfluous clothing, incessantly run round the furnace with a speed "equal to seven leagues in six hours." These men on their circuit take up each two small billets of wood, cut to a certain size, and heaped together; these they deposit as they run, first one and then the other, in the openings of the furnace, which by such means is fed at regularly recurring intervals of time. Having continued this intellectual employment, without ceasing, during six hours, the men then surrender their " seven-league boots" to others, whose heels and hands are similarly employed during an equal period of time; after which they are in turn relieved by the first set of couriers. The From the time of filling the pots, it requires nearly forty hours' exposure to strong heat, ere the materials are properly vitrified and in a state fit for casting. processes of filling the pots, and of removing the glass gall, and the various appearances that ensue in the refining, are precisely similar to what has already been described in a former chapter. - When the glass is thoroughly refined, the cuvettewhich must be perfectly clean, and, as already mentioned, of a temperature equal with that of the glass — is filled in the following manner:—. -A copper ladle, ten to twelve inches in diameter, fixed to an iron handle seven feet long, is plunged into the glass pot, and brought up filled with melted glass, which is transferred to the cuvette; the ladle during this transference is supported upon a strong iron rest, placed under its bottom, and held by two other workmen. This precaution is necessary to prevent the bending and giving way of the red hot copper under the weight of fluid glass which it contains. When by successive ladings the cuvette is filled, it is suffered to remain during some hours in the furnace, that the air bubbles formed by this disturbance may have time to rise and disperse; an effect which is ascertained to have ensued by the inspection of samples withdrawn from time to time for the purpose. Another essential part of the apparatus consists in flat tables whereon the plates of glass are cast. These tables have perfectly smooth and level metallic surfaces, of suitable dimensions and solidity, supported by masonry. At St. Gobain, and formerly also at Ravenhead, these tables were made of copper; the reason assigned for preferring this metal being, that it does not discolour the hot melted glass, while the use of iron was thought to be accompanied by this disadvantage. These copper tables were very costly, both from the nature of their material, and the labour bestowed in grinding and polishing their surfaces; and as the sudden access of heat that accompanied the pouring over them of such a torrent of melted glass occasioned the metal frequently to crack, the tables were by such an accident rendered useless. The British Plate Glass Company having experienced several disasters of this nature, its directors determined upon making trial of iron; and they accordingly procured a plate to be cast, fifteen feet long, nine feet wide, and six inches thick, which has fully answered the intended purpose-having, during several years of constant use, stood uninjured through all the sudden and violent alternations of temperature to which it has been exposed. This table is so massive, weighing nearly fourteen tons, that it became necessary to construct a carriage purposely for its conveyance from the iron foundery to the glasshouse. It is supported on casters, for the convenience of readily removing it towards the mouths of the different annealing ovens. The foundery at Ravenhead wherein this table is used is said to be the largest room under one roof that has ever yet been erected in this kingdom; it is 339 feet long, 155 feet wide, and proportionately lofty. Westminster Hall, to which the superiority in this respect is so commonly ascribed, is smaller-its length being 300, and its breadth only 100 feet. The melting furnaces, which are ranged down the centre, occupy about one third of the whole area of this apartment. The annealing ovens are placed in two rows, one on each side of the foundery, and occupy the greatest proportion of the side walls. Each of these ovens is sixteen feet wide and forty feet deep. Their floors being level with the sur |