GILDING DISSOLVENT. This liquid is used for obliterating the images which have been fixed by sel d'or upon dag,uerreotype plates. Its exact composition has not been published, but it, no doubt, contains nitro-hydrochloric acid, disguised in some way. A mix ture of salt and dilute nitric acid, or of nitre and dilute hydrochloric acid, will answer the purpose equally well.
GLA ss. Glass is a mixture of various insoluble silicates with excess of silica, and devoid of crystalline structure. The alkaline silicates, when in a state of fusion, have the power of dissolving a large quantity of silica.
There are two principal varieties of glass. The first comprises crown and plate glass, and is composed principally of silica, potass, and Hine ; the second, called flint glass, contains, in addition, silicate of lead ; the lead promoting fusibility, and increasing the density, lustre, and refractive power.
The principle of the glass manufacture is very simple. Silica, in the shape of fine sand, is heated with carbonate of potass or soda, and slaked lime, or oxide of lead. At a high temperature, fusion and combination occur, and carbonic acid is expelled. When the melted mass has become clear and free from air bubbles, it is left to cool slowly until it assumes the peculiar tenacious condition suitable for working.
Crown and plate glass have a greenish colour ; flint glass is colourless, and has a higher refractive index. Plate glass is cast upon a flat metal table, rolled, and, after very careful annealing, ground true and polished by machinery. (See "Plate Glass.") The large circular tables of crown glass are made by the glass blower. A long iron tube, called the blow-pipe, with a wooden mouth piece, has its end dipped into the tenacious, soft, semi-fused glass in the glass pot. The lump of glass thus removed is then blown into a flask. An iron rod, called a pontil, is then dipped into the glass pot, and applied to the bottom of the flask so as to form a handle to it. The blow-pipe is then detached from the neck of the flask, which is again heated in the furnace, and then suddenly turned with great rapidity, so that the centrifugal force causes it to assume the shape of a flat disc. This done it is removed from the pontil, leaving a great knob or bull's eye of glass in the centre of the sheet, and put into the annealing Oven, where it is allowed to cool slowly. Unless glass is annealed, or allowed to cool very slowly in this way, it becomes exceedingly brittle.
Tables of crown glass made in this way exhibit an exceedingly fine polished surface, although not strictly flat, like that of plate glass. Sheet glass is made by blowing a hollow cylinder, then cut ting it lengthwise with a red hot iron, and spreading it open upon a flat table.
Glass is coloured by the addition of various metallic oxides. Oxide of cobalt gives deep blue ; oxide of manganese, amethyst or black ; sub-oxide of copper, ruby-red ; black oxide of copper, green ; the oxides of iron, dull green or brown ; oxide of uranium, yellow ; oxide of silver, yellow ; purple of cassius, a ruby tint ; oxide of antimony, yellow ; oxide of chromium, fine reds and greens.
Opaque white glass, called white enamel, is made by adding oxide of tin.
The colouring oxide is either added to the glass in the glass pot, or applied afterwards to the surface, and burnt in.
Kane gives the following tabular view of the composition of several kinds of Glass. (Elem. Chem., 720.) No. 1, is the difficultly fusible Bohemian glass ; No. 2, ordinary Bohemian glass ; No. 3, English, and No. 4, German glass ; Nos. 5 and 6, French glass ; Nos. 7 and 8, English; No. 9, the celebrated optical glass of M. Guinaud, of Brennets, near Geneva.
All common glass, when reduced to fine powder is more or less acted on by boiling water, which separates the alkali ; glass cannot therefore be considered as insoluble in water. If finely powdered glass be laid upon a piece of reddened litmus paper and moistened with water, the moisture restores the blue colour of the paper by dis solving out the alkali in the glass. The power of glass to resist the action of water, alkalis, acids, air, and light, is in general greater the higher the temperature employed in its manufacture, the smaller the proportion of its fluxes, and the more exact the chemical ratios of its constituents. Most crystal glass is affected by having water boiled in it for a considerable time : but crown glass, being poorer in alkali, and containing no lead, resists that action much longer and is better adapted for chemical purposes.
Air and light act upon glass, probably by their oxidizing pro perty. Blueish and greenish coloured glasses become colourless by exposure, in consequence of the per-oxidation of the iron they contain. Glass containi g manganese becomes purple-red by the per-oxidation of that metal from the same cause. Flint glass, which contains lead, is acted on by sulphuretted hydrogen, and the surface rendered opaque and iridescent. Achromatic lenses should, therefore, be carefully preserved from the action of sulphur.
Charcoal colours glass of a yellow or brownish tint, so that it is impossible to make glass in furnaces which smoke.
The discovery of glass is said by Pliny to he due to the following accident :—A merchant ship laden with natron, (soda,) being driven in a gale upon the coast of Syria at the mouth of the river Belus, the crew were compelled to cook their victuals ashore, and having placed lumps of the natron upon the sand as supports to the kettles, found to their surprise masses of transparent stone among the embers. According to Pliny and Strabo the glass works of Sidon and Alexan dria were famous in their times, and produced beautiful articles which were cut, engraved, gilt, and stained of the most brilliant colours in imitation of gems. Some of the windows of Herculaneum appear to have been glazed.
For an account of the glass used in optical instruments See further particulars in the articles " Optical Glass," and " Plate Glass ;" and for the method of grinding lenses, See " Lens."