122 METALS AND THEIR COMBINATIONS. 



ture, as these compounds are hygroscopic, whilst pure sodium 

 chloride is not. 



In the animal system, sodium chloride is found in all parts, it 

 being of great importance by aiding the absorption of albumi- 

 noid substances and the phenomena of endosmosis; also by fur- 

 nishing, through decomposition, the hydrochloric acid of the 

 gastric juice. 



Sodium chloride is soluble in about 2.8 parts of water, at all 

 temperatures. 



Sodium carbonate, Sodii carbonas, Na 2 C0 3 .10H 2 = 286 (Car- 

 bonate of sodium, Washing soda, Sal sodce). This compound is, of 

 all alkaline substances, the one which is manufactured in the 

 largest quantities, being used in the manufacture of many 

 highly important articles, as, for instance, soap, glass, etc. 



Sodium carbonate is made according to Leblanc's process 

 from the chloride by first converting it into sulphate (salt-cake) 

 by the action of sulphuric acid : 



2NaCl + H 2 S0 4 = 2HC1 + Na 2 SO 4 . 



The escaping vapors of hydrochloric acid are absorbed by 

 water, and this liquid acid is largely used in the manufacture 

 of bleaching-powder. The sodium sulphate is mixed with coal 

 and limestone (calcium carbonate) and the mixture heated in 

 furnaces, when decomposition takes place, calcium sulphide, 

 sodium carbonate, and carbonic oxide being formed: 



Na 2 S0 4 + 40 + CaCO 3 = CaS + Na 2 CO 3 + 4CO. 



The resulting mass, known as black-ash, is washed with water, 

 which dissolves the sodium carbonate, whilst calcium sulphide 

 enters into combination with calcium oxide, thus forming an 

 insoluble double compound of oxy-sulphide of calcium. 



The liquid obtained by washing the black-ash, when evapo- 

 rated to dryness, yields crude carbonate of sodium, or " soda-ash ;" 

 when this is dissolved and crystallized it takes up ten molecules 

 of water, forming the ordinary " soda." 



Sodium carbonate is also manufactured by a process which 

 depends on the decomposition of sodium chloride by ammonium 

 bicarbonate under pressure, when sodium bicarbonate and am- 

 monium chloride are formed. 



