In the manufacture of washing soda, sulphate of soda, prepared as described in the fifth paragraph of p. 260, is mixed with coal slack and limestone and fed into a reverberatory furnace with a revolving bed. The mass fuses, and the principal products are carbonate of soda, sulphide of lime, and carbonic oxide. The fused mass is run into iron waggons and, alter cooling, the blocks are broken up, treated with water in large tanks, which are run in series, the water passing from one to the other so that the more concentrated liquor passes over new material. The concentrated solution of carbonate of soda is run into a pan placed over a reverberatory furnace, the heat from which serves further to concentrate the solution; from this tank it is gradually run on to the hearth of the furnace, where it forms a pool bounded by solid material. The flame of the furnace passes directly over the surface of the liquid and, as evaporation proceeds, the carbonate of soda separates in the solid form; it is raked out towards the fire, becoming hotter and hotter until finally it is drawn out at the furnace door, thoroughly calcined.

This is soda ash, which is dissolved in a very small quantity of water and the clear liquid run into large hemispherical iron pans, where it crystallises in a solid mass like ice; this is washing soda. As will be seen, a large and expensive plant is required.