26 The Realm of Nature CHAP. 



GRANITE 



'QUARTZ FELSPAR MICA" 



SILICA /S ilicon S!LICA 

 \ Oxygen 



MAGNESIA (gpi- 



IRONOXIDE { Oxygen 

 WATER (Hydrogen 

 I Oxygen 



44. Acids and Bases. Two classes of compounds 

 require to be specially mentioned. The non-metal oxygen 

 when it unites with a metal produces a compound called 

 a basic oxide^ and this is the case whether we consider the 

 gaseous metal hydrogen, the liquid metal mercury, or any 

 of the solid metals such as magnesium, calcium, or potas- 

 sium. When oxygen unites with another non-metal, such as 

 carbon, silicon, or sulphur, it produces an acid oxide. The 

 main characteristic of basic oxides and acid oxides is that 

 when brought together they unite to form more complicated 

 compounds called salts. A certain amount of each acid 

 oxide unites with a certain amount of each basic oxide to 

 form a compound showing neither acid nor basic properties, 

 but in many cases an additional definite amount of acid or 

 of basic oxide takes part in the compound which then 

 shows a more or less distinct acid or basic nature. Other 

 non-metals, such as sulphur and chlorine, unite with metals 

 to form compounds or salts termed sulphides and chlorides. 

 Energy in the form of light or heat is given out when 

 elements combine, and a precisely equal amount of energy 

 must be used up on the resulting compound in order to 

 decompose it. When much energy is involved in the trans- 

 action the compound is said to be a firm one. 



45. Elements. The process of analysis ceases when 

 we come to oxygen, silicon, aluminium, etc., for no 



