In many ways very similar to the processes employed in the production of bottles are those used in the manufacture of all hollow glass vessels that are produced by blowing, either with or without the aid of moulds. Apart from the actual shapes of the articles themselves, however, the principal difference between bottles and the better classes of hollow glass-ware lies in the composition and quality of the glass itself. In this respect all grades of manufacture are to be met with, from the light-coloured greenish or bluish glass used for medicine bottles to the most perfectly colourless and brilliant “crystal” or flint glass. This gradation in the perfection of the glass represents a corresponding gradation in the care bestowed upon the choice of raw materials and the various manipulations of melting the glass. As we have seen, for the commonest kinds of bottles, where colour and quality are immaterial, all kinds of fusible materials can be utilised, loamy or ferruginous sands and refuse glass of all kinds being employed. Where somewhat higher requirements have to be met, rather purer sands have to be used as sources of silica, while lime and alkali must be introduced in purer forms, the alkali in the shape of the cheapest qualities of salt-cake and the lime in that of lime-stones reasonably free from iron and magnesia. Finally, for the best qualities of glass the purest sand obtainable is used, being often specially washed to remove all loamy matter, while the alkali is introduced in the form of carbonate, a chemical product which in its better qualities is practically free from injurious impurities. In these high-class products two very distinct kinds of glass are met with. One class, of which the Bohemian “crystal” is the highest example, is chemically of the nature of an alkali-lime silicate, the alkali in the case of the Bohemian glass being potash; the other variety of glass contains no lime, its place being taken by lead, typical of this class being English flint glass. In some varieties of glass, lead is also replaced, partially or entirely, by barium, but this material is chiefly used for the manufacture of pressed glass.