All this time the art of glass-making on the Continent had been developing. In particular, the Venetian workers at Murano had perfected the art of colouring and enamelling glass--a result which was later to have its influence upon English artists. An admirable example of what they achieved in this direction is an old spinet in the South Kensington Museum, which once belonged to Queen Elizabeth of Bohemia, the daughter of James I. Whatever its merits as a musical instrument, its once gorgeous gilt crimson leather case hides an interior of the utmost interest to students of glass, for the interior of the lid is panelled into eighteen divisions, each representing some classical subject--Narcissus, Daphne, Andromeda, Argus, etc.--admirably done in coloured glass. The front of the keyboard, the stretcher bar and the keys themselves are also elaborately decorated in similar fashion with coloured glass, silver or enamel. The keys are covered with ornaments in coloured glass, the accidentals being faced with blue and white striped glass and the naturals being fronted with the same. Although it is no part of the purpose of this book to deal in detail with the technical side of the manufacture of glass, yet some few words as to the nature of the material with which we are dealing are not only desirable but essential to the proper understanding of its various qualities and kinds and the different stages of its manufacture. The scientist will tell us that glass is a double silicate, being compounded of a silicate of sodium (or potassium) and a silicate of lime. For the benefit of non-scientific readers, we may remark that a silicate is a chemical compound formed when silica combines with an alkaline substance like lime, soda, or potash. Silica is probably the most widely distributed substance in nature. Silicate of alumina is, for example, the basis of all clayey soils, and silica, in the pure form of quartz, is the chief constituent of the sand of the sea and of all those rocks which are known as sandstones. Rock-crystal, amethyst, agate, onyx, jasper, flint, etc., are all varieties of silica. Crystalline silica is hard enough to scratch glass--a fact utilised, as we shall see, in the sand-blast which is used for the purpose of engraving patterns on glass. Silica is fusible only at a very high temperature, but readily combines with alkaline substances to form soluble silicates, which are known in commerce as soluble glass, or water-glass, because it dissolves readily in hot water. Water-glass is used in making artificial stone, in coating stone surfaces, _e.g._ walls of buildings, etc., to preserve the stone from decay under the weathering influence of the atmosphere, and in the manufacture of cement.