SODIUM CHLORIDE BERTHOLLET'S LAWS 441 



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. The hydrochloric acid gas evolved in this 

 reaction escapes through the tube O as will presently be described. 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 of hydrochloric acid stops, then the mass, which 

 contains about half of the sodium chloride yet 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 com- 

 pleted. Normal sodium sulphate, which we shall afterwards describe, 

 remains in the roaster. It is employed both directly in the manu- 

 facture of glass, and in the preparation of other sodium compounds for 

 instance, in the preparation of soda ash, as will afterwards be described. 

 For the present we will only turn our attention to the hydrochloric 

 acid evolved in B and C. In 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. 



The hydrochloric acid gas evolved is subjected to condensation by 

 dissolving it in water. 32 If the apparatus in which the decomposition 



apparatus is placed in order sulphuric acid mixed with water is poured down the thistle 

 funnel into the retort. About one and a half times the weight of the salt of strong sul- 

 phuric acid is usually taken, and it is diluted with a small quantity of water (half) if it be 

 desired to retard the action, as in using strong sulphuric acid the action immediately 

 begins with great vigour. The mixture, at first without the aid of heat and then at a 

 moderate temperature (in a water-bath), evolves hydrochloric acid. Commercial hydro- 

 chloric acid contains many impurities ; it is usually purified by distillation, the middle 

 portions being collected. It is purified from arsenic by adding FeCL>, distilling, and 

 rusting aside the first third of the distillate. If free hydrochloric acid gas be required, it 

 is passed through a vessel containing strong sulphuric acid to dry it, and it is collected 

 over a mercury bath. 



Phosphoric anhydride absorbs hydrogen chloride (Bailey and Fowler, 1888; 

 2P 2 O 5 + 8HC1 = POC1.-; + 3HPO 5 ) at the ordinary temperature, and therefore the gas cannot 

 l>c dried by this substance. 



52 As at works which treat common salt in order to obtain sodium sulphate, the 

 hydrochloric acid is sometimes held in no value, they would readily allow it to escape 

 into smoke and into the atmosphere, which would greatly injure the air of the neighbour- 

 hood and destroy all vegetation, and therefore in all countries there are laws forbidding 

 the works to proceed in this manner, and requiring the absorption of the hydrochloric acid 

 by water at the works themselves, and not permitting the solution to be run into rivers 

 and streams, whose waters it would spoil. It may be remarked that the absorption of 

 hydrochloric acid presents no particular difficulties (the absorption of sulphurous acid 

 is much more difficult), because hydrochloric acid has a great affinity for water and gives 

 a hydrate which boils above 100. Hence, even steam and hot water, as well as weaker 

 solutions, can be used for absorbing the acid. However, Warder (1888) showed that weak 



