18 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Tablets", 668--626 B.C.E., appear to be factory records and give not only the Assyrian names for various kinds of glass, but also directions and for- mulae for manufacture. We may note that dictionary definitions are numerous, but in general are too restrictive and fail to bring out the essential continuity of condition from the fluid melt at high temperatures to the rigid glass at room tempera- tures. One of the better of these definitions is that given in Thorpe's Dictionary of Applied Chemistry, i.e., "an amorphous, hard, brittle substance, usually transparent but sometimes translucent or even opaque, breaking with a conchoidal fracture and consisting of a material obtained by the fusion of one or more of the oxides, silica, boric oxide, phosphoric oxide and the metallic oxides, followed by sufficiently rapid cooling of the fused mass to prevent crystallisation of the components. The glass may be colourless or coloured either by one or more of the constituent oxides or from the effects of neutral bodies such as carbon, sulphur or selenium. According to definition, glass, being amorphous, shall be isotropic." (In other words, being uncrystallised, its physical properties are the same in all directions, unlike many other transparent substances used, for instance, in optical instruments.) Transparency is frequently considered an essential characteristic of glass, yet much commercial glassware is either translucent or opaque. It is always true, however, that the lack of transparency is caused by the fact that some material, usually crystalline, is dispersed or suspended in a glassy matrix, itself transparent. Other products may contain much glassy matter, but are not considered as glasses. For example, glazes and enamels are essentially glassy, whilst practically all fired ceramics contain a proportion of glass, but there is no suggestion that these products should be included in the definition. Similarly excluded are the organic substitutes which differ so greatly in composition, methods of manufacture and properties from those substances which have always been known as glasses that they constitute a separate division of chemical technology. Discussion of the definition of glass usually centres around the argument whether, at ordinary temperatures, glass is an undercooled liquid or amor- phous solid, which places the emphasis on the definition of liquid and solid rather than on the properties of the substance to be defined. When a crystalline solid is heated to its melting point, it changes to a liquid, or a mixture of liquid and a different crystalline solid, and on cooling the reverse phase changes take place. The materials which have always been called glasses are characterised by the property that when melted at high tempera- tures and cooled, they do not devitrify, i.e., they do not undergo the dis- continuous change into the stable aggregates of crystalline phases which equilibrium would require. At high temperatures, during melting, they are

PRODUCTION AND PROPERTIES OF GLASS CONTAINERS 19 ordinary liquids, and like other liquids will flow under the influence of gravity or other small force. At the temperatures at which they should freeze or begin to crystallise, the glasses are viscous liquids and it is largely because of their great viscosity that glasses can be cooled through their freezing points without devitrification. They then become "undercooled liquids". As undercooling is continued to ordinary temperatures, the glasses become increasingly viscous. The increase in viscosity with decreas- ing temperature is a continuous process from the liquid above the melting or freezing point, to the rigid glass at ordinary temperature. From the freezing point to the ordinary temperatures the material remains as an undercooled liquid with respect to the process of crystallisation. Glass may then best be defined as "any inorganic substance in a condition which is continuous with and analogous to the liquid state of that substance but which, as a result of a reversible change in viscosity during cooling, has attained so high a degree of viscosity as to be for all practical purposes, rigid." This definition, which makes use of only those characteristics which are essential to glass, was oeornnfiated by George W. Morey, a leading American research worker and author on glass technology. COMPOSITION All known glasses are, then, supercooled liquids and that property which makes possible their manufacture and working, namely the property of remaining in the liquid condition while passing through the temperature range immediately below the freezing point and persisting in that condition indefinitely at ordinary temperatures, is the most important and character- istic property of glass. Some silicate glasses, for example those formed by fusing the alkali felspars (soda felspar or albite NasO.Al•03.6SiO• and potash felspar or orthoclose KO•.Al•03.6SiO•) are practically impossible to crystallise by heat treatment alone, while others, for example sodium metasilicate, can be obtained as a glass by rapidly cooling a melt of a few grams or less, whereas larger melts cannot be cooled without devitrification. Some elements can be obtained in the glassy form, notably selenium and tellurium. A number of inorganic salts have been obtained as a glass, notably beryllium fluoride. Zinc chloride, when quickly cooled, forms a glass, which gradually crystallises after standing some weeks or months in air, probably as a result of taking up moisture. Similarly sodium thiosulphite ("hypo"), alums, and some other hydrated salts form glass on rapid cooling. The majority of glasses may be regarded as composed of oxides, and the possibility of glass formation may be intimately connected with the oxygen atoms. Oxygen itself is highly viscous at its melting point and is stated to form a glass if cooled quickly. The outstanding glass forming oxides are