THE SUBSTANCES OF THE 



The elements which escape include the carbon, hydro- 

 gen, oxygen, and nitrogen, and sometimes the sulphur. 

 The remaining elements are found in the ashes. The 

 former are called combustible or volatile, and the latter 

 incombustible or fixed. 



The following is a brief description of the more com- 

 mon elements : 



Oxygen is the most common and interesting of the 

 elements. It forms about one half of the solid parts of 

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

 fifth of the air. It has powerful attractions for many 

 other elements. The substances formed by its union 

 with these are generally called oxides. Water (H 2 0) 

 is sometimes called hydric 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 burning, or 

 combustion, consists of the union of the oxygen of 'the 

 air with carbon and some other elements of the fucL 

 This union with carbon produces a gas (C0 2 ) 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 another 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. In 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 slower. New 

 combinations are formed, and the same amount of heat 



