The material is, to all intents and purposes, a pure sand-stone, whose silicious particles are bound together by a cement of silicate of lime--a mineral substance well known to be of the most indestructible nature; its composition, mechanically and chemically, is precisely that of the Craigleith and other best quality building-stones. It can be moulded to any form while in a plastic state, and can be worked with the chisel the same as any natural stone. The process of manufacture is based upon one of the most beautiful of chemical reactions; flints are dissolved by means of caustic alkali under high pressure, so as to form silicate of soda, a kind of waterglass. This viscous and tenacious substance is then rapidly mixed with a proportion of very fine and sharp silicious sand in a pug mill, so as to form a soft, plastic mass, which can be moulded into any shape that is desired. The soft stone is next immersed in a bath of chloride of calcium solution, which is made to penetrate every pore by means of hydraulic or atmospheric pressure. Whenever this solution comes into contact with the silicate of soda the two liquids are mutually and instantaneously decomposed, the silica taking possession of the calcium and forming the hard, solid silicate of lime, and the soda uniting with the chlorine to form chloride of sodium in a small quantity. Instead, then, of the particles of sand being covered with a thin film of the liquid silicate of soda, they are covered and united together with a film of solid silicate of lime--one of the most indestructible substances known. The small quantity of soluble chloride of sodium, one of the results of decomposition, is then washed out of the stone by a douche of clean water, or by hydraulic pressure, its complete removal being ensured by chemical tests. The stone is then dried and is fit for use. [Illustration: Fig. 370.] [Illustration: Fig. 371.]