520 PRINCIPLES OF CHEMISTRY 



soda) and acid substances (sulphuric ,and hydrochloric acids), the two 

 classes of chemical products which are distinguished for the greatest 

 energy of their reactions and are therefore most frequently applied 

 to technical purposes. Factories manufacturing soda are generally 

 called 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 take 

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

 sodium sulphide and carbonic anhydride, Na 2 SO 4 -f- 2C = Na 2 S + 2CO 2 . 

 The sodium sulphide thus formed then enters into double decomposition 

 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 -f 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 which was formed on first heating the mixture. The entire 

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

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

 sidered 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 : 2Na 2 SO 4 + 3CaCO 3 + 9C = 2Na 2 C0 3 

 + CaO,2CaS + 10CO Indeed, the quantities in which the substances 

 are mixed together at chemical works approaches to the proportion re- 

 quired by this equation. The entire process of decomposition is carried 

 on in reverberatory furnaces, into which a mixture of 1 ,000 parts of 

 sodium sulphate, 1,040 parts of calcium carbonate (as a somewhat 

 porous limestone), and 500 parts of small coal is introduced from above. 

 This mixture is first heated in the portion of the furnace which is 



15 Calcium sulphide, CaS, like many metallic sulphides which are soluble m water, is 

 decomposed by it (Chapter X.), CaS + H^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, lime may 

 be precipitated, and a state of equilibrium will be reached, when the system CaO -f- 2CaS 

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

 action. Therefore, if in black ash the lime were not in excess, a part of the sulphide 

 would be in solution (actually there is but very little). In this manner in the manu- 

 facture 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 there is no evidence of its independent existence. 



