Natural Soda. In Egypt, Thibet, and Utah, there are tracts of country where the soil is so impregnated with soda that the land is desert. The separation of the soda from the earth is a simple operation, for it is only necessary to agitate the soil with water and, after the insoluble matter has settled down, to evaporate the clear solution until the soda crystallizes out. In addition to alkali deserts, there are also alkali lakes. Those in Egypt are small, nevertheless, about 30,000 tons of soda per annum are exported from Alexandria. Owens Lake in California is said to contain sufficient soda to supply the needs of North America; while in the East African Protectorate, beneath the shallow waters of Lake Magadi (discovered in 1910), there is a deposit of soda estimated at 200,000,000 tons. The Leblanc Process. At the present time, the greater part of the world’s supply of soda is made from common salt by two processes. The older of these, which is known as the Leblanc process, was introduced in France towards the end of the eighteenth century. In those days soda was very dear, for the main supply came from the ashes of seaweeds; wherefore the French Academy of Sciences, in 1775, offered a prize for the most suitable method of converting salt into soda on a manufacturing scale. The prize was won by Nicholas Leblanc, who in 1791 started the first soda factory near Paris. These were the days of the French Revolution; the “Comité de Sûreté Général” abolished monopolies and ordered citizen Leblanc to publish the details of his process. [Illustration: Fig. 12. SALT CAKE FURNACE] The first alkali works were established in Great Britain in 1814. The total amount of soda now made in this country every year is about 1,000,000 tons, of which nearly one-half is still made by the Leblanc process. Salt Cake. The first stage of the Leblanc process consists in mixing a charge of salt weighing some hundredweights with the requisite amount of “chamber” sulphuric acid. The operation is carried out in a circular cast-iron pan (D, Fig. 12) about 9 ft. in diameter and 2 ft. deep. The pan is covered over with a dome of brickwork, leaving a central flue (E) for the escape of hydrochloric acid gas which is produced. At first, the reaction takes place without the application of heat, but towards the end the mass is heated for about one hour. The contents of the pan are then raked out on to the hearth of a reverberatory furnace (_a_, _b_) and more strongly heated. More hydrochloric acid gas is given off, and the reaction is completed. The solid product which remains is impure Glauber’s salt (sodium sulphate), and is known in the trade as “salt cake.”