142 METALS AND THEIR COMBINATIONS. 



obtained. Common table-salt frequently contains small quantities 

 of calcium and magnesium chlorides, the presence of which causes 

 absorption of moisture, 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 in aiding the absorption of albuminoid 

 substances and the phenomena of osmose; also by furnishing, 

 through decomposition, the hydrochloric acid of the gastric juice. 



Sodium chloride is soluble in 2.8 parts of cold water, and in 2.5 

 parts of boiling water ; almost insoluble in alcohol ; it crystallizes in 

 cubes and has a neutral reaction. 



Sodium hydroxide, Sodium hydrate, Soda, NaOH = 40, may 

 be obtained by the processes mentioned for potassium hydroxide, 

 which compound it closely resembles in its chemical and most of its 

 physical properties. 



Solution of soda is a 5 per cent, solution of the hydroxide in water. 



Sodium carbonate, Sodii carbonas, Na 2 CO 3 .10H 2 O = 286 

 ( Washing soda, Sal sodce). This compound is", of "all alKalme sub- 

 stances, the one 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 SO 4 = 2HC1 + Na 2 SO, 



The escaping vapors of hydrochloric acid are absorbed in water, 

 and this liquid acid is used largely in the manufacture of bleaching- 

 powder. The sodium sulphate is mixed with coal and limestone 

 (calcium carbonate) and the mixture heated in reverberatory furnaces, 

 when decomposition takes place, calcium sulphide, sodium carbonate, 

 and carbonic oxide being formed : 



Na 2 SO 4 + 40 + CaCO 3 = CaS + Na-jCOg + 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 evaporated 

 to dryness, yields crude sodium carbonate, or " soda ash -" when this 



