Chap. VII.
731
into the atmosphere. The door of the chamber is then opened, and the lamp-black, beingswept up, is packed in small barrels made of the wood of the spruce fir for sale.
In the Landes the furnace and the chimney are in the open air, and the chamber onlycovered with a tile roof; but in Germany the whole apparatus is constructed in a barn-likebuilding, about 24 feet long, 12 feet wide, and 10 feet high.
Glass. — This transparent, impermeable, and brittle substance, consists of many varieties,which are differently composed, and applicable to as many purposes : the manufacture ofglass is of the highest importance, and now that all restrictions are withdrawn to itsimprovement, we may expect to find it rendered not only cheaper but better. It wasknown not only to the Egyptians, but also to the Phoenicians , whom Pliny says were theinventors of the manufacture ; both Sidon and Alexandria were famous for the productionof beautiful glass, which was cut, engraved, and tinted with a variety of colours, somespecimens of which equalled the precious stones in brilliancy of effect. Rome was alsocelebrated for its manufacture, and Nero is reported to have given as much as 6000sesterces for two glass cups: before Colbert introduced an establishment into France forblown mirror-glass, Europe generally was indebted to Venice for all that appeared incommerce: it was not made in London until 1557. Glass is made by fusing silica witha due proportion of alkali, which serves as a flux to the silica, and makes the wholetransparent: it may be said to consist of one or more salts, which are silicates, with basesof potash, soda, lime, oxide of iron, alumina, or oxide of lead.
The silica ordinarily made use of is sea-sand, which consists chiefly of quartz, and thefinest quality is obtained on the coast of Norfolk , near Lynn : the common black flint isalso used ; after it has been heated red-hot, and plunged into cold water, it breaks to pieces,and becomes so brittle that it is easily ground into a fine powder. The alkali is eitherpotash or soda in a state of carbonate : borax is the flux for the finer sorts of glass, butfor the more common kind lime is substituted : two oxides of lead are used, viz. lithargeand minium, which give to the glass greater powers of refracting light, and that of sud-denly changing its temperature without cracking. The white oxide of arsenic is also aflux, and in any large quantity will give the glass a milky hue, which increases with time.
Soluble Glass is a simple silicate of potash or soda, or of both these alkalies ; crown glassis a silicate of potash and lime; common window-glass is a silicate of soda and lime, or redpotash ; bottle glass is composed of silicate of soda, lime, alumina, and iron.
The common crown glass for ordinary windows is compounded after the following pro-portions ; 450 pounds of kelp, dried and grqpnd, 325 pounds of Lynn sand, and 25 poundsof slacked and sifted lime. The kelp, which is the alkali, differs so much in quality thatits proportions continually vary : arsenic is added to facilitate the fusion, and oxide ofcobalt with ground flint introduced to improve the colour. By measure, five parts of finesand and eleven parts of ground kelp or soda is another proportion for common crown glass.
Flint Glass is composed of silex, to which is added carbonate of potash and red lead orlitharge, the latter giving the glass its great specific gravity, its superior transparency, itsductility and powers of refraction. The materials after preparation are put into a crucibleof Stourbridge clay, which holds about 1600 lbs. weight of fused glass ; a double stoppercovers the mouth of the crucible, and as it is not luted, the carbonic acid gas, or excess ofoxygen, has the means of escaping : a strong, rapid, and intense heat for sixty hours isrequired to drive off the gases and to fuse the metal, during which process the surface isregularly skimmed of all the impurities that arise.
Flint plate glass , for optical purposes, is thus prepared ; seven pounds of the metal is takenout of the pot when at a certain point of fusion, in a conical-shaped ladle, and then blowninto a hollow cylinder, which is cut open and flattened into a sheet 20 inches by 14, andvarying in thickness from § to \ of an inch ; the plate is then annealed, and afterwards cutand ground into the required form.
When glass is not sufficiently annealed, it is put into tepid water, which is heated andmaintained for some hours at a boiling point, and is then suffered to become gradually cold ;unannealed flint glass, heated and suddenly cooled in water, resembles in its appearance amass of crystals, from whence it has been supposed that the process of annealing renders theglass incapable of polarisation, in consequence of its becoming more compact. A barometertube 40 inches in length, unannealed, heated, and suddenly cooled, would contract y s of aninch, and if done by the usual manner, its contraction would be double, or }. The red tintgiven to glass by manganese is destroyed by annealing, and the best fuel for melting glassin the furnace is oven-burnt coke mixed with screened coal: in plate glass there is nolead; in consequence it is purer, and more homogeneous than common flint glass.
Plate Glass. —Common salt or the muriate of soda is decomposed by the subcarbonateof potash, when both salts are in a state of solution and subjected to heat; an alkali soobtained is dried by being continually boiled: 1 pound of pure soda, and 4 pounds ofsand, produce a hard glass, which neither water nor the mineral acids will affect. The bestplates are formed out of 720 pounds of Lynn sand, 450 of alkaline salt, 80 of slacked quick-