CLASSES OF SUBSTANCES 187 



sources from which to obtain acids or metals. Salts are 

 used in many mechanic arts, in photography, in elec- 

 troplating, electrotyping, and batteries; in plaster, in 

 fertilizers, in explosives, and in other ways. 



214. Oxides. Nearly all elements combine directly 

 with oxygen (O) ; that is, each forms with O a com- 

 pound in which itself and oxygen are the only ele- 

 ments. The compound formed by the direct union of 

 an element with oxygen is commonly called an oxide. 



Some oxides are solids and are very hard, some are 

 gases, and still others are liquids. They occur very often 

 as powders, that is, masses of small particles. Iron 

 rust is an oxide of iron ; lead scraped bright and then 

 exposed to the air becomes covered with a thin, dull 

 coating of lead oxide. An oxide of carbon, CO 2 , is a 

 gas; it is found mixed with the air, and is formed 

 whenever C burns in O. 



Similarly, sulphur (S) combines directly with some 

 elements to form sulphides. Of these, iron sulphide 

 (FeS 2 ) is very common ; Cu, Pb, Sn, and Ag also form 

 common sulphides. 



215. Minerals. The earth, so far as we can discover, 

 is composed largely of rock masses (and soil on the sur- 

 face) which are either pure minerals or mixtures of min- 

 erals. Mineral substances are compounds, commonly 

 oxides, carbonates, or sulphates. The oxide of silicon, 

 SiO 2 , is very common we call it quartz ; other oxides 

 are those of Al, Ca, Mg, K, Na, and Fe. The important 

 carbonate is that of calcium, CaCO 3 , called limestone; and 

 the sulphate of calcium, CaSO 4 (gypsum), is also common. 



