LIVING MATTER 17 



dioxide. Carbon is a black solid best known in the form 

 of charcoal. 



When oxygen unites with a substance, the substance 

 is said to burn. When wood burns oxygen from the air 

 unites with two substances in the wood, carbon and 

 hydrogen, a colorless gas. The union of oxygen with 

 carbon produces carbon dioxide; with hydrogen, water. 

 The substances in wood that do not unite with oxygen 

 are left unburned in the form of ashes. 



When a substance burns its weight is increased by the 

 weight of the oxygen that unites with it. It may be hard 

 to realize that an invisible gas has weight and that a 

 thing weighs more after it is burned than before, but the 

 fact is easily proved. When magnesium wire is burned, 

 the resulting product is a white powder called magne- 

 sium oxide. The weight of the powder is greater than 

 that of the original piece of wire by the amount of oxy- 

 gen which has united with it. 



Characteristics of Hydrogen, Carbon Dioxide, Oxy- 

 gen. — If a burning splinter is thrust successively into 

 three jars containing hydrogen, carbon dioxide and oxy- 

 gen its behavior in the three cases is markedly different. 

 In the jar of hydrogen, it goes out, but the hydrogen 

 itself begins to burn at the mouth of the jar with a blue 

 flame. The heat of the burning splinter raises the 

 hydrogen to the kindling temperature and it unites with 

 oxygen from the air. The water that is formed appears 

 in drops on the side of the jar. 



In the jar of oxygen the splinter burns for a moment 

 much more freely than it does in the air; then it goes 

 out. In the carbon dioxide, it instantly goes out. 



