The first part of the process consists in the production of the sulphate from salt and sulphuric acid, during which hydrochloric acid is formed; this is carried out in two stages represented in the following formulæ: 1. NaCl + H₂SO₄ = NaHSO₄ + HCl. 2. NaCl + NaHSO₄ = Na₂SO₄ + HCl. The first stage in which bisulphate is produced is carried out at a moderate heat, the second requires a red heat. The reactions, therefore, are made in a furnace combining a pan and muffle furnace. This saltcake muffle furnace is so arranged that the pan can be shut off from the muffle by a sliding-door (D). The pan (A) and muffle (E) have separate flues for carrying off the hydrochloric acid developed (B, F). First, common salt is treated with sulphuric (Glover) acid in the cast-iron pan. When generation of hydrochloric acid vapour has ceased, the sliding-door is raised and the partly decomposed mixture is pushed through into the muffle, constructed of fire-resisting bricks and tiles, and surrounded by the fire gases. While the muffle is being raised to red heat, the sulphate must be repeatedly stirred with a rake in order, finally, while still hot and giving off acid vapour, to be drawn out at the working doors into iron boxes provided with doors, where the material cools. The acid vapour given off when cooling is drawn through the top of the box into the furnace. [Illustration: FIG. 3.—Saltcake Muffle Furnace—Section _(after Ost_) A Pan; B, F Pipes for hydrochloric acid vapour; D Shutter; E Muffle, O Coke fire.] Mechanical stirrers, despite their advantage from a health point of view, have not answered because of their short life. The valuable bye-product of the sulphate process, _hydrochloric acid_, is led away separately from the pan and the muffle, as is seen, into one absorption system. The reason of the separation is that the gas from the pan is always the more concentrated. The arrangement of the absorbing apparatus is illustrated in fig. 4. [Illustration: FIG. 4A.—Preparation of Hydrochloric Acid—Plan (_after Lueger_) A, A´ Earthenware pipes B, B´ Sandstone cooling towers C, C Series of Woulff’s bottles D, E Condenser wash towers FIG. 4B.—Elevation]