_Soluble Silicates._--An article on glass in a modern scientific work like the present would not be complete without a notice of the manufacture of soluble glass and the uses to which it has been and may be applied. It has already been mentioned that when silica or sand is fused with an excess of alkali, the resulting glass is soluble in water. Soluble glass is made on a large scale in three different ways. First of all, if flints, that is, black flints, which are found in chalk, be heated to a white heat, they lose their black colour and their hardness, and are easily crushed to small pieces; and if flint in this condition be placed in a wire cage and put into a jacketed iron digester, that is, an iron digester which has an inner and an outer skin, with a free space between the two, so that steam may be forced into it from a boiler under pressure; and if the digester be screwed down tightly with an iron cover, and steam then be allowed to pass into the space between the two, the temperature can be raised at pleasure, according to the pressure under which the steam is introduced. If the valve of the boiler be loaded with a 60-lb. weight, the temperature of the water warmed by the steam will rise considerably higher than that of ordinary boiling water; and if this water be saturated with caustic soda, it will dissolve the flints slowly, forming silicate of soda, that is to say, the silicic acid of the flint will unite directly with the soda of the solution, and silicate of soda will thus be obtained. For certain applications, the silicate so formed is not sufficiently pure, because the soda used often contains a certain amount of sulphate, which will remain with it in the solution of silicate that is drawn off from the digester. This sulphate is very objectionable for certain applications of silicates, because it crystallizes out, and so destroys the substance, which the silicate is intended to preserve. Another and a much better method is to heat together the silica in the form of sand with alkali, either potash or soda, in a reverberatory furnace, and as the glass becomes formed, to rake it out into water, and then gradually to dissolve it by boiling in suitable vessels. Here the sulphate, if it existed in the alkali, is decomposed by the silicic acid, and the sulphuric acid passes off through the flues of the reverberatory furnace.