Ordinary glass has many valuable properties which make it of great importance in the arts and manufactures. Among these may be mentioned the fact that it can be made to take any shape with ease. It resists the action of all ordinary acids, and hence is of the utmost value to the chemist and the chemical manufacturer. Hydrofluoric acid alone attacks it, by combining readily with its silica and so dissolving it. For this reason, hydrofluoric acid is used in etching on glass. Again, glass is cheap, being literally made from the dust of the earth; it is transparent, and so can be used in buildings, transmitting light whilst protecting from the inclemency of the weather. Its transparency, too, combined with its high refractive power, make it of inestimable value in the manufacture of optical instruments. It is this high refractive power, too, which gives to cut glass its beautiful lustre and sparkle, and one aim of the glass-founder is to increase this refractive power and so enhance the brilliancy of his product. If glass could be made which would refract light to the same extent as the diamond does, it would exhibit the same “fire” as the king of gems. It is hard and close in texture, and so is capable of taking a high polish. Its great drawback is its brittleness, but this can be reduced to a great extent by immersing it, whilst red-hot, in a hot bath of paraffin oil, wax, or resin. A tumbler of glass so “tempered” may be dropped on the floor without breaking. It may be added, as a matter of common interest, that this brittleness is largely a result of the fact that glass is an extremely bad conductor of heat. Because of this, a mass of molten glass, when cooling, becomes set on its outside surface long before the interior has become solidified; hence the solid exterior prevents the molecules of the interior portion from contracting. As a result, a condition of strain is established, the interior molecules tending to contract, while the exterior tends in the opposite direction; consequently a very slight blow is enough to cause a fracture. _Varieties of Glass._--As we shall frequently find it necessary to refer to the various kinds of glass, it may be as well at the outset to attempt to give a clear idea of their differences and of the meanings of the various terms employed in describing them. As regards quality, the chief kinds are crown glass, flint glass, plate glass, bottle glass, and crystal glass, and the differences in composition may be conveniently expressed in the form of a table:--