SODIUM CHLOKIDE-BERTHOLLET'S 14. WS 



449 



In chemical works the decomposition of sodium chloride by means of 

 sulphuric acid is carried on on a very large scale, chiefly with a view to 

 the preparation of normal sodium sulphate, the hydrochloric acid being 

 a bye-prbduct. 31 bii * The furnace employed is termed a, salt cake furnace. 

 It is represented in fig. 65, and consists of the following two parts the 

 pan B and the roaster C, or enclosed space built up of large bricks 

 a and enveloped on all sides by the smoke and flames from the fire 

 grate, F The ultimate decomposition ' of the salt by the sulphuric 

 acid is accomplished in the roaster. But the first decomposition 

 of sodium chloride by sulphuric acid does not require so high a 

 temperature as the ultimate decomposition, and is therefore carried 

 on in the front and cooler portion, B, whose bottom is heated by gas 

 flues. When the reaction in this portion ceases and the evolution 



FIG. 65. Section of a salt-cake furnace, B, pan in which the sodium chloride and sulphuric acid 

 are first mixed and he<tted. C, muffle for the ultimate decomposition. 



of hydrochloric acid stops, then the mass, which contains about 

 half of the sodium chloride still undecomposed, and the sulphuric 

 acid in the form of acid sodium sulphate, is removed from B and 

 thrown into the roaster C, where the action is completed. Normal 

 sodium sulphate, which we shall afterwards describe, remains in the 

 roaster. It is employed both directly in the manufacture of glass, and 

 in the preparation of other sodium compounds for instance, in the 



51 bu jn chemical works where sulphuric acid of 60 Baume* (22 p.c. of water) is 

 employed. 117 parts of sodium chloride are taken to about 125 parts of sulphuric acid 



