512 PRINCIPLES OF CHEMISTRY 



and lastly to convert the sodium sulphate thus obtained into carbo- 

 nate and caustic soda. Hence these works prepare alkaline substances 

 (soda ash and caustic soda) and acid substances (sulphuric and hydro- 

 chloric acids), those two forms of chemical products which are dis- 

 tinguished for the greatest energy of their reactions, and which are 

 therefore frequently applied to technical purposes. And therefore the 

 works manufacturing soda are generally called chemical works (alkali 

 works). 



The process of the conversion of sodium sulphate into sodium 

 carbonate consists in strongly heating a mixture of the sulphate with 

 charcoal and calcium carbonate. The following reactions then t.-iko 

 place : the sodium sulphate is first deoxidised by the charcoal, forming 

 sodium sulphide and carbonic anhydride, Na 2 SO 4 + 2C = ]S"a. 2 S 4- 2C(X. 

 The sodium sulphide thus formed then enters into double decomposi- 

 tion with the calcium carbonate taken and gives calcium sulphide and 

 sodium carbonate, Na 2 S + CaCO 3 = Na 2 CO 3 + CaS. 



Besides which, under the action of the heat, a portion of the excess 

 of calcium carbonate is decomposed into lime and carbonic anhydride, 

 CaCO 3 =CaO + CO 2 , and the carbonic anhydride with the excess of 

 charcoal forms carbon monoxide, which towards the end of the opera- 

 tion shows itself by the appearance of a blue flame. Thus from a mass 

 containing sodium sulphate we obtain a mass which includes sodium 

 carbonate, calcium sulphide, and calcium oxide, but none of the sodium 

 sulphide first formed, or, strictly speaking, only a small portion. The 

 entire process, which proceeds at a high temperature, may be expressed 

 by a combination of the three above-mentioned formulae, if it be taken 

 into consideration that the product contains one equivalent of calcium 

 oxide to two equivalents of calcium sulphide. 12 The sum of the 

 reactions may then be expressed thus : 2ISra 2 S0 4 + 3CaCO 3 + 9C 

 = 2Na 2 CO 3 + CaO,2CaS + 10CO. Indeed, the quantities in which 

 the substances are mixed together at chemical works approaches to the 

 proportion required by this equation. The entire process of decompo- 

 sition is carried on in reverberatory furnaces, into which a mixture of 

 1000 parts of sodium sulphate, 1040 parts of calcium carbonate (as a 



18 Calcium sulphide, CaS, like many metallic sulphides which are soluble in water, is 

 decomposed by water (page 426), CaS + H 2 O-CaO + H 2 S, because hydrogen sulphide is 

 a very feeble acid. If calcium sulphide be acted on by a large mass of water, linn- may 

 be precipitated, a state of equilibrium will be entered on, when the system CaO-f '2< 'aS 

 remains unchanged. Lime, being the product of the action of water on CaS, limits 

 this action. Therefore, if in black ash the lime was not in excess, a part of the 

 sulphide compounds would be in solution (actually there is but very little). In this 

 manner in the manufacture of sodium carbonate the conditions of equilibrium which 

 enter into double decompositions have been made use of (see above), and the aim is to 

 form directly the unchangeable product CaO,2CaS. This was first regarded as a special 

 insoluble compound, but nothing points to its independent existence. 



