[AS.] A substance composed of a mixture of two silicates-one being a silicate of an alkali metal, and the other a silicate of an alkaline earth. There are four different kinds of glass, each of which possesses special properties suited to the particular purpose for which it is used. 1. Crown glass, sheet glass, and plate glass are each composed of the same materials-namely, silicates of sodium and calcium ; but the method of manufacture is different in each case. Crown glass was at one time the only kind used in England for windows, but it has been superseded by sheet glass. For plate glass great care is taken in the selection of the materials, and the proportion of lime used is somewhat less than in the other two kinds. 2. Bohemian glass consists of silicates of potassium and calcium. This glass is very hard and difficult to melt, and is much used for chemical apparatus, or whenever a glass is required which can withstand heat. 3. Flint glass, or crystal, contains silicates of potassium and lead. It is employed for table glass, globes, ornaments, etc. Glass for optical purposes is made both of flint and crown glass. 4. Bottle glass is an impure mixture of various silicates, such as sodium, calcium, iron, and aluminium. In this variety the color and quality of the glass are not of the same importance as in the other three kinds. In glass manufacture the materials are melted together in a highly heated crucible. A portion of the melted mixture is then taken up by the glass-blower on the end of a long tube, and blown by him into a hollow pear-shaped bulb. It is then given the desired shape by various processes of handling. Many articles of. glass are formed in moulds, and other methods of manufacture are employed. The grinding and cutting of glass are subsequent processes for the purpose of ornamentation.