SODIUM CHLORIDE- BERTHOLLET'S LAWS 421 



reaction. Thus table salt, like other salts, is decomposed by the action 

 of an electric current into a metal and a haloid, and presents a compo- 

 sition of great simplicity in comparison with the composition of many 

 salts containing oxygen, which fact supports the hydrogen theory of 

 acids discussed in Chapter III. Naturally, like all other salts, it may 

 be formed from the corresponding base and acid with the separation 

 of water. In fact if we mix caustic soda (alkali) with hydrochloric 

 acid (acid) then table salt is formed, NaHO + HCl=NaCl + H 2 O. 



AYith respect to the double decompositions of sodium chloride it 

 should be observed that they are most varied, and serve as the means 

 for obtaining nearly all the other compounds of sodium and chlorine. 



The double decompositions of sodium chloride, as an example of the 

 double decompositions of salts, are almost exclusively based on the pos- 

 sibility of the metal sodium being exchanged for hydrogen and other 

 metals. But neither hydrogen nor any other metal is able to directly 

 displace the sodium from table salt. This would result in the separation 

 of metallic sodium, which itself displaces hydrogen and the majority of 

 other metals from their compounds, and is not, as far as is known, ever 

 separated by them. The replacement, then, of the sodium in sodium 

 chloride by hydrogen and different other metals is accomplished by the 

 transference of the sodium into some other sodium compound. If 

 hydrogen or another metal, M, were combined with an element X, then 

 the double decomposition NaCl + MX=NaX + MCl takes place. Such 

 double decompositions proceed under particular conditions, sometimes 

 completely and sometimes only partially, as we shall endeavour to explain. 

 In order to acquaint ourselves with the double decompositions of sodium 

 chloride, we will follow the methods actually employed in practice to 

 procure compounds of sodium and chlorine from table salt. For this 

 purpose we will first describe the treatment of sodium chloride by sul- 

 phuric acid for the preparation of hydrochloric acid and sodium sulphate. 

 We will then describe the substances obtained from hydrochloric acid 

 and sulphate of sodium. Chlorine itself, and nearly all the other com- 

 pounds of this element, may be procured from hydrochloric acid, whilst 

 sodium carbonate, caustic soda, metallic sodium itself and all its com- 

 pounds may be obtained from sodium sulphate. 



Even in the laboratory of animal organisms table salt is subjected 

 to similar changes, furnishing the sodium, alkali, and hydrochloric acid, 

 which take part in the processes of animal life. 



Its necessity as a constituent in the food of both human beings and 

 animals becomes evident when we consider that both hydrochloric acid 

 and salts of sodium are found in the substances which are separated out 

 from the blood into the stomach and intestines. So, for example, 



