The principal reaction is carried out in a tower (Fig. 14 (1), _a_, _a_) from 50 to 65 ft. in height and about 6 ft. in diameter. At intervals of about 3½ ft. throughout its length, the tower is divided into sections by pairs of transverse discs, one flat with a large central hole, and one hemispherical and perforated with small holes (Fig. 14 (2)). The discs are kept in position by a guide rod G. Fig. 14 (3) shows a better arrangement of the guide rods. In modern works, the space between the discs is kept cool by pipes conveying running water. The ammoniated brine is led into the tower near its middle point. The carbon dioxide is forced in at E in the lowest segment, and as it passes up the tower it is broken up into small bubbles by the sieve plates. Sodium bicarbonate separates out as a fine powder, which makes its way to the bottom of the tower suspended in the liquid. The perforated plates are necessary for the proper distribution of carbon dioxide through the brine. They are, however, a source of trouble, because the holes quickly become blocked up with sodium bicarbonate, and every ten days or so it is necessary to empty the tower and clean it out with steam or boiling water. Recovery of Ammonia. The production of 1 ton of soda ash by the Solvay process involves the use of a quantity of ammonia which costs about eight times as much as the price realized by selling the soda. It is evident that the success of the process as a commercial venture depends largely on the completeness with which the ammonia can be recovered. During the process, ammonia is converted into ammonium chloride, which remains dissolved in the residual liquor. From this ammonia gas is set free by adding quicklime and by blowing steam through the mixture. It is now claimed that 99 per cent. of the ammonia used in one operation is recovered. Soda Ash. The bicarbonate of soda produced by the Solvay process is moderately pure. For all ordinary purposes, it is only necessary to wash it with cold water to remove unchanged salt, and after drying, it is ready to be placed on the market if it is to be sold as bicarbonate. The greater part of the Solvay product, however, is converted into soda ash by the application of heat. If soda crystals are required, the soda ash is dissolved in water and crystallized. In many ways, the Solvay process compares very favourably with the older method. It is an advantage to start with brine, for that is the form in which salt is very often raised from the mines. The end product is relatively pure; moreover, it is quite free from caustic soda, which for some purposes for which soda ash is used is a great recommendation. There is no unpleasant smelling alkali waste. On the other hand, the efficiency of the Solvay process is not high, for only about one-third of the salt used is converted into soda. This would make the process impossible from the commercial point of view were it not for the cheapness of salt.