THE SUBSTANCES OF THE EAIMTI. 19 



The elements wliieh escape include the carbon, hydro- 

 gen, oxygen, and nitrogen, and sometimes the suli)hur. 

 The remaining elements are found in the ashes. The 

 former are called conlniatUdc or volatile, and th(5 latter 

 incombu-stlble or fixed. 



The following is a brief description of the more com- 

 lUDii elements : — 



Oxygen is the most common and interesting of the 

 elements. It forms about one half of the solid {)arts of 

 the earth, eight ninths, by weight, of all water, and one 

 fifth of the air. It lias powerful attractions for many 

 other elements. Tlie substances formed by its union 

 with these are generally called oxides. Water (HoO) 

 is sometimes called hydrie oxide. Lime (CaO) is cal- 

 cic oxide; iron rust (FeO), ferric oxide. There are 

 three familiar processes in nature in which oxygen 

 takes a leading part: — 



1. Combustion. — The ordinary process of liurning, or 

 combustion, consists of the nnion of the oxygen of the 

 air with carl^Du and some other elements of the fuel. 

 This union with carbon produces a gas (COo) called 

 carbon dioxide, or carbonic acid gas. 



Heat is a result of the union. It is regarded as a 

 kind of force. The force or clash with which atoms 

 come together in burning is converted into anothcn- kind 

 of force called heat. The degree of heat depends upon 

 the rapidity of the process. A draft through a fire in- 

 creases the heat, because it furnishes a larger supply of 

 oxygen. 



2. Oxidation. — lu the rusting of metals, and the de- 

 cay of wood, a process is going on precisely similar to 

 that of burning, except that it is much slow(>r. Xc^w 

 combinations arq formed, and the same amount of l;eat 



