656 L A S S [...‘U1-'.c'rUmc. condition; and it is less subject to fracture on the applica- ' It is difficult to give a consistent and systematic view of tion of heat. According to Pelouze, Splittgerber, and the manufacture of glass, because not only chemical con- others, devitrification results simply fro111 a rearrangement stitution, but the mechanical operations by which glass is of the molecules into the crystalline form while the mass is prepared, and the purposes to which the material is ap- soft, no alteration in the proportions of the constituents ' plied, have also to be taken into consideration. A good taking place concurrently; but Dumas and his followers ' classification, from a chemical point of view, is that given maintain, on the contrary, that in undergoing the change by Stein (in Bolley’s Tec/nzologie), who distinguishes three the glass loses part of its alkali, and that crystallization classes :— takes place through the formation of compounds infusible Glass containing one or two bases belonging to the at the temperature existing at the moment of crystallization. same group. This class embraces only the soluble silicates These compounds may result either from the dissipation of which do not fulfil the ordinary functions of glass. alkaline matter or from the separation of the mass into two Glass with several bases which belong to different strata,—that most highly alkaline retaining its amorphous groups, comprehending two classes :—(1) calcium glass, condition more persistently than the other more siliceous under which come sodiu1n—calcium glass and potassium- portion. calcium glass; and (2) lead glass, which constitutes ordinary The physical properties upon which the great value and flint glass or crystal and strass. utility of glass principally depend are (1) its well-known (3.) Coloured and opaque glass, of which there are also prevailing transparency combined with a brilliant lustre and two classes,—the first embracing the varieties of transparent great hardness; its fusibility at a high temperature; coloured glass which may belong to any of the above classes. and (3) its softness and viscosity at a red heat, whereby it with the addition of colouring oxides, and the second being can be moulded and otherwise worked with facility into devoted to the various kinds of opaque glass. any shape desired. Of great value also is its resistance to The raw materials of the glass manufacture embrace the the influence of common solvents. Properly-made glass is ' following principal ingredients. Silica is used in the not sensibly acted on by any of the acids except hydro- forms of pure quartz (for very fine qualities of glass), crushed fluoric acid, which attacks it powerfully, combining with sandstone, pulverized flints, and especially sand of degrees and removing its silica. Water affects glass so feebly that of purity varying in proportion to the quality of the glass for practical purposes its action may be disregarded; but to be made. The finest iron—free sand in the United King- when it is submitted for a prolonged period to water at a I dom is obtained from Alum Bay in the Isle of Wight, from high temperature, it is slowly dissolved. Even prolonged Lynn, Norfolk, and from Leighton Buzzard, Bedfordshire ; exposure to moist air so acts on glass, particularly on highly but much pure sand is imported into the United Kingdom alkaline varieties, that the surface becomes clouded and from Fontainebleau in France, from Belgium, and other obscure, and the beautiful iridescent sealing off observable in localities. Lime is employed in the form of chalk or ancient glass is due to the exposure of the substance for marble, either burned or unslaked, and it also must for long ages to the influence of moist air or damp earth. colourless glass be free from iron impurities. Of potash Glass is an extremely bad_ conductor of heat, and from and soda any of the ordinary salts except chlorides, that property springs, in great measure, its brittleness. but especially the sulphates and carbonates, are inditlerently Owing to this imperfect conductivity it necessarily results utilized, the point of real importance being here also the free- that a mass of glass, or a glass object cooling from a state dom of the compound from contamination when fine glass is of fusion, becomes cooled and set or solidified on its outer being made. At no very remote date kelp was the principal surface before the internal molecules have parted with their source of soda alkali in glass, but this is now entirely dis- heat, contracted, and established themselves in a stable re- used, and the principal source of potash is the salt mines lation to each other. The solidification of the superficial of Stassfurt and Leopoldshall in Prussia, and at Kalusz in stratum thus necessarily hinders the contraction of the l Galicia. Both potash and soda are frequentlyconstituents internal portion of the mass, and as the internal molecules of the sa1ne glass ; but glass made from potash is free from cool down a state of tension is created, the central portion i the decided sea-green tinge which invariably is seen in soda tending to draw the surface stratum inwards with a force glass, although the latter is the more brilliant in lustre. held in check by the strain in the contrary direction of the Lead is the characteristic ingredient of a distinct class outer range of molecules. In this condition a very moderate of glass of which ordinary flint glass is the type. It is impact is sufficient to determine the fracture of the glass. .5 usually employed in the form of minium or red lead The high degree of brittleness which results from unequal 1 (2I’bO,PbO2), partly on account of its fine state of division cooling is exhibited in a very marked manner by the philo- I and partly because by giving off oxygen it helps to purify sophical toy known as “Ilupert’s drops.” Such pear—shaped the metal. (6.) Baryta and witherite or baric carbonate masses of glass are prepared by allowing molten glass to have been introduced with much success as a partial sub- fall, drop by drop, into cold water, when the drops assume stitute for alkali in soda or potash glass, and for a part a more or less spheroidal form, with a finely tapering point. of the lead in ordinary flint glass, and in all probability Of course a very sudden and rapid cooling of the surface barium compounds are destined to occupy a much more takes place, while the interior is still at a high temperature, important place in glass manufactures than hitherto they and correspondingly much dilated, the consequence of which have done. Cullet or waste and broken fragments of is that a state of great tension is established between the special kinds of glass to be made is an important and surface and centre. The breaking off of a small portion of I essential ingredient, being added to the extent of about one- the tail is sufficient to destroy the equilibrium established third of the whole charge in the melting and preparation of between the hard superficial and the dilate internal glass. These materials constitute the essential ingredients molecules; and immediately the whole mass is shattered which go to the formation of glass. In coarse varieties. to dust with explosive violence. Excessive brittleness is I such as bottle glass, alumina and iron are present, overcome by the operation of annealing to which glass is I but their presence simply results from the inferior and submitted,—a process which has been explained under impure nature of the raw materials employed, and are AI'2'EALI.*G, and which will be further referred to in deal- neither essential nor desirable. Some portion of alumina ing with a method of tempering or hardening glass which 1 too is taken up from the pots in which the materials has been introduced by M. de la Bastie within the last few are melted. Bleaching or oxidizing agents are also
3'€1I‘S- employed to produce a high degree of colourlessness in