328 LECTURES TO SCIENCE TEACHERS. 



The acid vapours enter the tower (B) at (A) ; and in 

 passing up this tower, which is either filled with piled 

 bricks or with hard coke, they meet with a descending 

 current of water supplied from the cistern (F). The strongly 

 acid liquors flow away by a pipe at the bottom of the tower 

 (B), and are stored by the manufacturer for subsequent use. 

 Any hydrochloric acid vapours unabsorbed in the tower 

 (B) pass down the brick tunnel (c) into the second tower, 

 which they again ascend and meet another current of 

 falling water. When the vapours reach the top of this 

 tower they ought to be perfectly free from hydrochloric 

 acid gas, and are then allowed to pass away into the 

 chimney (E). 



After the mixture has been heated for about an hour 

 in the salt-cake pan and has become solid, it is raked through 

 the doors (a a) Fig. 8 on to the hearth of one of the fur- 

 naces or roasters at each side of the decomposing pan. 

 Here the hot air and flame from the fire at the end complete 

 the decomposition into sodium sulphate and hydrochloric 

 acid, as expressed by the following equation : 



NaHS0 4 + NaCl - Na 2 SO 4 + HC1. 



The acid vapours from the pans usually go into condensers 

 by themselves, those from the roasters or salt-cake furnaces 

 going into other condensers. The acid liquor obtained in 

 the latter case is weak, because large quantities of water 

 are needed to cool the heated air which passes along with 

 the hydrochloric acid gas into the towers. As soon as the 

 decomposition is complete the salt-cake (Na.,S0 4 ) is with- 

 drawn from the furnace and kept for the subsequent pro- 

 cess. Ten of the above charges are usually drawn in one 

 day, so that eight tons of salt and about the same weight 

 of oil of vitriol are used, whilst rather less than five tons of 

 gaseous hydrochloric acid and nearly twenty tons of salt- 

 cake are formed. 



Here you have a complete analysis of commercial salt- 

 cake, from which you will see that it contains about 3 '5 

 per cent, of impurity. 



