422 PRINCIPLES OF CHEMISTRY 



sodium salts are found in the blood and in the bile elaborated in the 

 liver, and acting 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 tne loss substances containing chlorine compounds 

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

 of sodium chlorine which are found in drinking water or in plants or 

 other animals, but experience has shown that many wild animals travel 

 long distances in search of salt springs, and that domestic animals, 

 which in their natural condition do not require table salt, willingly 

 take it, and that the functions of their organisms become much more 

 regular from doing so. 



The action of sulphuric acid on sodium chloride, If sulphuric acid 

 be poured over table salt, then even at the ordinary temperature, as 

 Glauber observed, an odorous gas, hydrochloric acid, is evolved. The 

 reaction which takes place consists in the sodium of the table salt and 

 the hydrogen of the sulphuric acid changing places. 



NaCl + H 2 S0 4 HC1 + NaHS0 4 



Sodium chloride. Sulphuric acid. Hydrochloric acid. Acid sodium sulphate. 



At the ordinary temperature this reaction is not complete, but soon 

 ceases. If the mixture be heated, the decomposition proceeds until, if 

 there be sufficient table salt present, all the sulphuric acid taken is 

 converted into acid sodium sulphate. If there be an excess of acid it 

 will remain unaltered. If 2 molecules of sodium chloride (117 parts) 

 be taken per molecule of sulphuric acid (98 parts), then on heating the 

 mixture to a moderate temperature only one-half (58 '5) of the table 

 salt will suffer change. Complete decomposition, after which neither 

 hydrogen nor chlorine is left in the residual salt, proceeds (when 117 

 parts of table salt are taken per 98 parts of sulphuric acid) at a red heat 

 only. Then 



2NaCl + H 2 SO 4 = 2HC1 + Na 2 SO 4 



Table salt. Sulphuric acid. Hydrochloric acid. Sodium sulphate. 



This double decomposition is the result of the action of the acid 

 salt, NaHSO 4 , first formed on sodium chloride, because the acid salt, 

 as it contains hydrogen, itself acts like an acid, XaCl + JSTaHSO 4 = 

 HC1 + Na 2 S0 4 . By adding this equation to the first we obtain the 

 second, which expresses the ultimate reaction. Then all the hydrogen 

 of the sulphuric acid and chlorine of the salt are evolved as gaseous 



