6 



air, and must be kept in oil or other substances that do not 

 contain oxygen. It does not occur in the free state, but is 

 always combined with other elements, as carbonates, sulphates, 

 silicates, and chlorides. Potash is an oxide of potassium, and 

 feldspar and mica consist largely of this element. Potassium 

 is present in the ash of all plants, is found most abundantly 

 in the growing parts, and is essential to plant life. 



Sodium (Na) is a soft, waxy, lustrous white metal much like 

 potassium in appearance and always occurs combined with 

 other elements. It oxidizes even in water and must be kept 

 in fluids lacking oxygen. Its various combinations are abun- 

 dant in the rocks. Sodium chloride (XaCl) is rock salt. What 

 we commonly call soda is an oxide or carbonate of sodium. 

 Chile saltpeter, or nitrate of soda, is largely used as a fertilizer. 

 Sodium, while usually found in plants, is known to be non- 

 essential, though it may take the place of potassium in neutral- 

 izing some of the acids formed in them. The chemical symbol is 

 taken from natrium, the name by which it was formerly known. 



Magnesium (Mg) is a light, silvery white, moderately hard 

 metal which is malleable but not very tenacious. It is not 

 found native, but forms many compounds. It is permanent in 

 dry air, but tarnishes in the presence of moisture. Burned in 

 oxygen, it produces a blinding light and forms magnesium 

 oxide (MgO), or magnesia. Dolomite and asbestos contain 

 much magnesium, and Epsom salts is the sulphate of this 

 element. Magnesium is found in most parts of^jre plant, but 

 less abundantly than calcium, except in tll9rtiRs, where it 

 is usually more abundant. 



Calcium (Ca) is a pale yellow, malleable, ductile, but some- 

 what brittle, metal. It is widely distributed, but never free. 

 It is the chief element in marbles, limestones, and dolomites. 

 Limestone is calcium carbonate (CaCO 3 ), and when carbon diox- 

 ide is driven off by burning, quicklime (CaO), which is used 

 in plastering, is left. Gypsum is calcium sulphate (CaSO 4 ). 



