SODIUM 519 



sulphate thus formed, being a very sparingly-soluble salt, is obtained as 

 a precipitate, whilst the sodium hydroxide, or salt, NaX, is obtained in 

 solution, because all salts of sodium are soluble. Berthollet's doctrine 

 permits all such cases to be foreseen 



The reactions of decomposition of sodium sulphate are above all 

 noticeable by the separation of oxygen. Sodium sulphate by itself is 

 very stable, and it is only at a temperature sufficient to melt iron that it 

 is possible to separate the elements SO 3 from it, and then only partially. 

 However, the oxygen may be separated from sodium sulphate, as from- 

 all other sulphates, by means of many substances which are able to 

 combine with oxygen, such as charcoal and sulphur, but hydrogen is 

 not able to produce this action. If sodium sulphate be heated with 

 charcoal, then carbonic oxide and anhydride are evolved, and there is 

 produced, according to the circumstances, either the lower oxygen 

 compound, sodium sulphite, Na 2 S0 3 (for instance, in the formation 

 of glass) ; or else the decomposition proceeds further, and sodium 

 sulphide, Na 2 S, is formed, according to the equation Na 2 S0 4 4- 20 

 s= 2CO 2 + Na 2 S. 



On the basis of this reaction the greater part of the sulphate of 

 sodium prepared at chemical works is converted into soda ash that is, 

 sodium carbonate, Na 2 CO 3 , which is used for many purposes. In the 

 form of carbonates, the metallic oxides behave in many cases just as 

 they do in the state of oxides or hydroxides, owing to the feeble acid 

 properties of carbonic acid. However, the majority of the salts of 

 carbonic acid are insoluble, whilst sodium carbonate^ is one of the few 

 soluble salts of this acid, and therefore reacts with facility. Hence 

 sodium carbonate is employed for many purposes, in which its alkaline 

 properties come into play. Thus, even under the action of feeble 

 organic acids it immediately parts with its carbonic acid, and gives 

 a sodium salt of the acid taken. Its solutions exhibit an alkaline" 

 reaction on litmus. It aids the passage of certain organic substances 

 (tar, acids) into solution, and is therefore used, like caustic alkalis and 

 soap (which latter also acts by virtue of the alkali it contains), for 

 the removal of certain organic substances, especially in bleaching 

 cotton and similar fabrics. Besides which a considerable quantity 

 of sodium carbonate is used for the preparation of sodium hydroxide 

 or caustic soda, which has also a very wide application. In large 

 chemical works where sodium carbonate is manufactured from Na 2 SO 4 , 

 it is usual first to manufacture sulphuric acid, and then by its aid to 

 convert common salt into sodium sulphate, and lastly to convert the 

 sodium sulphate thus obtained into carbonate and caustic soda. Hence 

 these works prepare both alkaline substances (soda ash and caustic 



