Silica in any case is the essential element in glass, and in any normal glass there may be present from 60 to 75 per cent. If, however, the bases with which it is combined have a high combining number—and this is especially the case with lead—the percentage of silica may fall below the former figure. Thus, in a bottle glass with 12 per cent. of iron oxide and alumina[2] the proportion is reduced to 54 per cent., and in a flint glass with 43 per cent. of lead oxide there is only 45 per cent. of silica. It was once the fashion among English writers on glass to classify the substance under the heads of crown-glass, bottle-glass, broad-glass, plate-glass, flint-glass, etc.; but such a classification, not very logical in itself, would be of no use to us.[3] Glass, of course, varies in optical properties, in hardness, and in fusibility, but I do not think that any useful classification could be based directly on these properties. But there is one distinction of the greatest importance technically and geographically, and this is between the glass of maritime countries in which the alkali is soda, and that of inland and forest districts where the soda is replaced by potash. In the first group, by far the most important—I have indeed regarded such glass as the normal type—may, it would seem, be placed not only the ‘primitive’ glass of the Eastern Mediterranean, but probably all the glass of the Romans. To it belongs also the glass of the Saracens and the greater part of the artistic glass of the Renaissance, including the Venetian glass, although in this last the soda is often in part replaced by an appreciable quantity of potash. The potash group, on the other hand, includes the old _voirre à fougère_ of the French and the _wald-glas_ of the Germans. In addition, almost the whole of the glass of higher quality made in later days in Germany and in the Bohemian borderlands belongs essentially to this last class. Finally, it may be mentioned that in the case of the abnormal family where the lime is replaced by oxide of lead, the alkali is invariably potash. Of this family our English flint-glass is the most important member.