370 SMITH'S INTERMEDIATE CHEMISTRY 



308). On account of its solubility and its nitrogen content, it 

 is valuable as a fertilizer (p. 410). It is used also in the manu- 

 facture of cheap grades of gunpowder. Much of it is converted 

 into potassium nitrate, as this salt is less deliquescent, and the 

 gunpowder made from it keeps better. 



Other Salts of Sodium. Several of them, such as the 

 peroxide (p. 221), the silicate (p. 360), and the tetraborate (p. 

 363), have been described already. Sodium sulphate Na^SC^ 

 is used as a substitute for sodium carbonate in making cheap 

 glass. Sodium sulphite Na^SOs (p. 260), made by the action 

 of sulphur dioxide on an aqueous solution of sodium hydroxide, 

 is a convenient source of sulphur dioxide and is also used as a 

 preservative. Sodium thiosulphate Na 2 S 2 Ck (" hypo ") can be 

 obtained readily by boiling sodium sulphite solution with sulphur: 



+ S 



and is used in fixing photographs. .Sodium cyanide JNaNC 

 (preparation, see p. 393) is used in extracting gold from its ores. 



POTASSIUM K 



Potassium receives its symbol from the initial of the German 

 word for it, kalium (related to the word alkali). AU^compounds 

 of potassium confer a violet color upon the Bunsen flame. 



The metal itself may be prepared from its hydroxide, as in the 

 case of sodium (p. 164), and exhibits similar properties. It has 

 no uses. Certain of its salts, however, are of the greatest value, 

 chiefly as fertilizers (p. 412) and explosives. 



Occurrence. Silicates containing potassium, such as felspar 

 and mica, are common constituents of volcanic rocks. These 

 minerals have not yet been brought into commercial use as sources 

 of potassium compounds. Many salt deposits (p. 412) contain 

 potassium chloride, alone (sylvite) and in combination with 

 other salts, and most of the compounds of potassium are manu- 



