The lithograph thus prepared is given to the printer, who first etches it, in the manner originally practised by M. Senefelder. The nitric acid employed for the purpose is diluted with about thirty parts of water, and it is poured over the stone whilst it is inclined on one side. This process is repeated several times, the object of it being not so much to give relief to the lines, as to roughen the surface of the stone, and thus facilitate its absorption of water. The nitric acid also removes the alkali from the drawing ink. In printing, gum is added to the water with which the stone is moistened, as an additional preventive of the ink adhering to those parts not drawn upon. The printing ink is applied with large rollers, and the damped paper having been placed carefully upon the stone, with blankets at the back, it is passed through the press.
The lithographic press somewhat resembles in form an iron printing press, but differs from it greatly in its mode of action. Instead of the large flat plate that in a printing press is pulled down upon the whole surface of the types, a long, narrow arm, called a scraper, is brought to bear upon the stone, and the table whereon the stone is laid is pushed forcibly under it, by which means a great pressure is exerted on a smaller surface at successive times, instead of being brought to bear all at once. In the principle of its action, indeed, a lithographic press is like a printing machine, and steam lithographic presses have been invented to work in a similar manner, though the practical results have not generally been very successful.
Among the many applications of lithography, the transfer of copper-plate engravings is one of the most useful. An impression of the plate is taken on paper that is coated with a compound of flour, plaster of Paris, and glue, and from the paper it is transferred to stone. By this plan the original plate remains untouched, and the printing from the stone is much cheaper than from the copper. Tinted lithography and chromo-lithography, by which the beautiful effects of coloured drawings are produced in the manner indicated by M. Senefelder, have recently been applied very successfully.
[AERATED WATERS.]
The invention of soda-water, in the state in which it is now known, as an effervescing beverage impregnated with three or four times its volume of carbonic acid gas, is of very modern date. There are, indeed, to be found in most of the old works on chemistry descriptions of Nooth's apparatus for impregnating liquids with carbonic acid; but all that was attempted to be done by that apparatus was to produce an impregnation of the water with little more than the quantity of gas it will naturally absorb under the pressure of the atmosphere. It was not until about the year 1815 that mechanical pressure was applied to force a larger quantity of gas into combination with water, to imitate the briskly effervescing medicinal waters of Germany.
Mr. Schweppe and Mr. Paul were the first who introduced the manufacture of artificial effervescing waters into England, and soda-water was then considered, as tea was on its first introduction, entirely medicinal. Indeed, the quantity of soda which was at that time usually dissolved in the water gave it a disagreeable taste; but when the manufacturers diminished the quantity of alkali, and increased the volume of gas forced into the water, they produced a pleasant beverage, which soon became in request for its refreshing, wholesome qualities.
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.