SODIUM CHLORIDE-BERTHOLLET'S LAWS 431 



With respect to the double decompositions of sodium chloride it 

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

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



The double decompositions of sodium chloride are almost exclusively 

 based on the possibility of the metal sodium being exchanged for 

 hydrogen and other metals. But neither hydrogen nor any other metal 

 can directly displace the sodium from sodium chloride, 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, so far as is known, ever separated by them. The replacement 

 of the sodium in sodium chloride by hydrogen and various metals 

 c'an only take place by the transference of the sodium into some 

 other combination. If hydrogen or a metal, M, be combined with an 

 element X, then the double decomposition NaCl + MX = NaX + MCI 

 takes place. Such double decompositions take place under special 

 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 of chlorine from common salt. For this purpose we 

 will first describe the treatment of sodium chloride with sulphuric 

 Acid for the preparation of hydrochloric acid and sodium sulphate. 

 We will then describe the substances obtained from hydrochloric acid 

 and sodium sulphate. Chlorine itself, and nearly all the compounds 

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

 carbonate, caustic soda, metallic sodium itself and all its compounds, 

 may be obtained from sodium sulphate. 



Even in the animal organism salt undergoes 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 both of human beings and 

 of 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.^ Sodium salts 

 are found in the blood and in the bile which is elaborated in the 

 liver and acts on the food in the alimentary canal, whilst hydro- 

 chloric acid is found in the acid juices of the stomach. Chlorides of the 

 metals are always found in considerable quantities, in the urine, and if 

 they are excreted they must be replenished in the organism ; and for 

 the replenishment of the loss, substances containing chlorine compounds 

 must be taken in food. Not only do animals consume those small 

 amounts of sodium chloride which are found in drinking water or in plants 



