use in the production of even the most expensive kinds of glass; these substances are, however, obtainable in a state of considerable purity, although the hydrate has the inconvenient property of rapidly absorbing carbonic acid from the atmosphere, thus becoming converted into the carbonate. _Magnesia_ is another glass-forming base that is closely related, chemically, to calcium and barium. This element is usually introduced into glass mixtures in the form of either the carbonate or the oxide. The carbonate occurs in nature in a more or less pure state in the form of magnesite, and by calcination, the oxide is obtained. The natural mineral and its product are, of course, by far the cheapest sources of magnesia, but as the element is only used in comparatively small quantities, the artificial precipitated carbonate or calcined magnesia are frequently preferred. Magnesia is only introduced intentionally in notable quantities in special glasses where the properties it confers are of special value; in ordinary lime glasses this element, as has already been mentioned, is to be regarded as an undesirable impurity. _Zinc oxide_ lies, chemically, between the bases already discussed on the one hand, and lead oxide on the other. This element is only introduced into special optical glasses, a special “zinc crown” having found some application. Chemically prepared zinc oxide is almost the only form in which the element is used, but the very volatile character of this substance must be borne in mind when it is introduced into glass mixtures.