The apparatus for the manufacture of soda-water, as it is usually made on a large scale, consists of a strong vessel, furnished with a safety valve, in which the water is impregnated with gas. This vessel, containing about nine gallons, is made of thick wood, well seasoned and nicely fitted, and bound round with strong iron hoops, the heads of the cask being well secured by means of iron bolts and screw nuts. It is requisite that the receiver should be capable of bearing a pressure of at least six atmospheres, which is equal to 90 lbs. to the square inch. The carbonic acid gas is generated from chalk or whiting and diluted sulphuric acid. The materials are mixed together in a small closed wooden or leaden vessel, provided with an agitator, that can be worked by a handle fixed to a projecting axis at the top. The gas, as generated, enters by a bent tube into a gas-holder, the opening of the tube being under water. By this means the gas is freed from the fumes of sulphuric acid vapour, and from the fine particles of chalk that become mingled with it during its sudden liberation. The gas sometimes undergoes a further purification, by passing through a gas washer, before it is forced into the water. A small force-pump, worked by a crank, with the assistance of a fly-wheel, draws the carbonic acid from the gas-holder, and forces it into the water. The combination of the gas and water is facilitated by an agitator, the axle of which projects through a stuffing box, and it is worked either by hand, or is turned by means of a small cog-wheel, that works into the teeth of a larger one fixed to the crank axle, so as to produce rapid rotation. It is found requisite, in the first place, to expel the atmospheric air in the receiver; for which purpose the safety valve is left open for a short time after the gas is being forced in, otherwise it would retard the impregnation of the water by the gas. When the gas and water are well incorporated, the liquid will contain as many volumes of gas as there are atmospheres of pressure in the air-space above it in a state ready to effervesce, and one other volume, with water absorbs under the pressure of the atmosphere. Thus, when there are three atmospheres of gas under pressure, each bottle of soda-water contains four bottles full of gas, which are absorbed without perceptibly increasing its bulk. The perfect impregnation of the water with gas, however, requires time. The water will, indeed, become brisk almost as soon as two or three atmospheres of gas have been forced in, but it will not acquire the flavour of good soda-water until the gas and water have been allowed at least half an hour to digest; and it is improved by remaining in contact for several hours.