A further consideration in the choice of raw materials is facility of storage. Thus limestone in the shape of large lumps of stone which are only ground to powder as required, is readily stored, and undergoes no deleterious change even if exposed to the weather; on the other hand, sulphate of soda (salt-cake), if stored even in moderately dry places, rapidly agglomerates into hard masses, at the same time absorbing a certain percentage of moisture. Such properties are not always to be avoided, salt-cake for example being at the present time an indispensable ingredient in many kinds of glass-making, but the value of a substance is in some cases materially lessened by such causes. The raw materials ordinarily employed in glass-making may be grouped into the following classes:-- (1) Sources of silica. (2) Sources of alkalies. (3) Sources of bases other than alkalies. (1) _Sources of Silica._--The principal source of silica is sand. This substance occurs in nature in geological deposits, often of very considerable area and depth. These deposits of sand have always been formed by the disintegration of a siliceous rock, and the fragments so formed have been sifted and transported by the agency of water, being finally deposited by a river either in the sea (marine deposits) or in lakes (lacustrine deposits), while the action of the water, either during transport or after deposition, has frequently worn the individual particles into the shape of rounded grains. In consequence of this origin, the chemical composition of sand varies very greatly with the nature of the rock whose denudation gave rise to the deposit. Where rocks very rich in silica, or even consisting of nearly pure silica, have been thus denuded, the resulting sand is often very pure, deposits containing up to 99·9 per cent. silica being known. More frequently, however, the sand contains fragments of more or less decomposed felspar, which introduce alumina, iron and alkalies into its composition. Finally, “sands” of all ranges of composition from the pure varieties just referred to down to the clay marls, very rich in iron and alumina, are known.