Glass is a mixture of metallic silicates, usually containing an alkali metal. It has no definite crystalline structure, absorbs, refracts, or transmits light, and becomes softer as its temperature increases. The main types of glass are soda lime glass, which is cheap and used for windows and containers; potash-lime glass, which fuses at high temperatures; and potash-lead glass, which has a high refractive power and is used for lenses and prisms. Each type has different chemical properties and melting temperatures depending on its composition and intended uses.