Elements of chemistry, in two volumes / by John Murray.

  • Murray, John.
Date:
1828
Catalogue details

Licence: Public Domain Mark

Credit: Elements of chemistry, in two volumes / by John Murray. Source: Wellcome Collection.

    122/768 (page 110)
    Glass in fusion has a great degree of tenacity and duc- tility, so that it can be easily cast in a mold, and cut and wrought in a variety of shapes. It loses this in cooling; and if cooled hastily is so brittle, that the slightest impulse or agitation is sufficient to cause it to fall into fragments. This is in part prevented by slow cooling, by which a dif- ferent arrangement of the particles of the glass is allowed to be assumed. This slow cooling, by which glass is ren- dered less brittle, is termed the annealing of glass. It appears even, that if the cooling be performed very slow- ly, the particles pass into such an arrangement that the glassy character is entirely lost, and the mass becomes opaque, and displays in its fracture a crystalline struc- ture. Glass thus devitrified may have its transparency and other usual properties restored by melting it, and cooling it more quickly. Glass when prepared without an excess of alkali, is permanent in the air, and is scarcely acted on by any che- mical agent. Fluoric acid, from its action on silex, erodes and even dissolves it. The fixed alkalis fused with it ren- der it soluble in water; and the metallic bases of the alka- lis* decompose it, by partially abstracting the oxygen of the alkali, which enters into its composition, and perhaps also of the silex itself. When glass is exposed to heat, imbedded in various earthy mixtures, it loses its trans- parency, and becomes like porcelain ; this is called Reau- mur’s porcelain ; it is more hard, less fusible, and better capable of bearing sudden alterations of temperature than transpai'ent glass, changes depending partly perhaps on the operation of slow cooling connected with the process. Glass made from very pure materials, with the addition of borax and oxide of lead, and coloured by the addition of other metallic oxides, forms imitations of the gems. The transparency and colours of these are often equal to