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10 In an attempt to manufacture soda by LeBlanc's process without starting from sulfuric acid as one of the raw materials, Hargreaves in 1870 discovered a method of making sodium sulfate directly from sulfur dioxide and common salt. A mixture of sulfur dioxide, air, and steam is passed through layers of especially prepared salt blocks at a temperature of 400-500°C., when the following reaction takes place:

This is known as the Hargreaves process. As usual, hydrochloric acid is formed in the reaction, but without the use of sulfuric acid. In this country at present, because of the large demand for sodium sulfate for kraft paper manufacture and because of the introduction of the synthetic method for manufacturing hydrochloric and nitric acids, certain attempts have been made to revive the Hargreaves process for the production of sodium sulfate, which was formerly a by-product from the treatment of salt or Chile saltpetre with sulfuric acid for the production of hydrochloric or nitric acid.

All these processes were developed from the necessities of the LeBlanc soda industry, and their inventors were really pioneers in the application of chemical principles to industry, laying the foundation of what is now known as the chemical technology of heavy chemicals.

Although the LeBlanc process for soda ash manufacture has now been given up in favor of the newer ammonia process, the development of the sulfuric acid industry on the one hand, and the production of hydrochloric acid, bleaching powder, etc., on the other, are the direct outcome of the LeBlanc process. Heavy-chemical industries in general are lucky enough to have had a start in the LeBlanc industry. For, unfortunately or fortunately, the LeBlanc process involves the manufacture of all these chemicals, and what were originally by-products later became the principal products. It can be truly said that the history of modern inorganic chemical industry owes its beginning to the LeBlanc soda industry.

For the sake of clearness to the reader, the diagram in Fig. 3 is given to show how varied were the products from the LeBlanc industry. Products enclosed in rectangles were those appearing in commerce.

In contrast with these older methods, it may be mentioned briefly by way of digression that modern methods utilize these waste sulfur gases from pyrites roasters for the manufacture of concentrated sulfuric acid, not by the chamber process but by the vanadium contact process. For this purpose, mechanical roasters such as the Herreshoff furnace, the Wedge furnace, etc. are used, and the gases from these roasters are maintained at 7 per cent or higher SO2, by volume. This strength of sulfur gas is adequate for efficient conversion by the vanadium mass for the manufacture of contact acid. The waste gases are settled, cooled, and scrubbed by passing through an elaborate purification system to remove dust and