48 



AIR AND FIRE 



48. Burning and Oxidation. Many metals besides 

 mercury unite with the oxygen of the air. Thus, iron 

 rusts at the ordinary temperature. Hot, melted lead, if 

 stirred so that a fresh surface of it is kept exposed to the 

 air, is gradually changed to lead oxide, a yellow powder. 

 Tin is changed in the same way to tin oxide, a white pow- 

 der. Zinc and magnesium burn with bright flames to give 

 their oxides. Powdered magnesium is burned to give the 

 light for "flash light" photographs. If we weigh a 

 quantity of any one of these metals, and then weigh the 



oxide formed, we always notice a 

 gain in weight; this is due to the 

 oxygen that is taken up (Fig. 40) . 

 The uniting of a substance with 

 oxygen is called the oxidation of 

 the substance, and the substance 

 is said to be oxidized. Burning 

 is also called combustion, and a 

 substance that can burn in the 

 air is called a combustible sub- 

 stance. But burning is not differ- 

 ent from other oxidation: it is oxidation which is so rapid 

 that heat and light are given off. The phenomenon of 

 burning was mysterious for so long a time because the 

 matter of a burning body seems to be destroyed. This is 

 because most of our common combustibles, such as wax, 

 coal, wood, and paper, give invisible gases when they burn 

 (Fig. 41). Of course these escaped unnoticed (c/. 38). 



FIG. 40. 



A burning body takes up oxygen 

 from the air, and gains weight. 



49. Flames. There is a difference in the way in 

 which substances burn: some have a large, bright flame; 



