FIRE AND ITS USES 149 



together in intimate associations or molecules like sugar. Oxygen 

 atoms have a strong attraction for carbon atoms, uniting vigor- 

 ously to form carbon dioxide. Similarly, oxygen and sulphur 

 unite to form sulphur dioxide, and phosphorus and oxygen unite. 

 These atoms rush together with such energy that the molecules 

 are set into rapid vibration. So heat is generated together with 

 light, and we say the substance burns. A burning substance as 

 usually understood is one whose atoms are uniting with oxygen 

 so rapidly as to produce heat and light. Oxidation, the union of 

 a substance with oxygen, may go on slowly and no heat or light 

 be noticeable. When iron rusts, it is uniting with oxygen, but 

 slowly. Other substances may unite chemically so rapidly as to 

 produce heat and light. Thus, if powdered antimony is sprinkled 

 into chlorine gas there is so rapid a union of chlorine and antimony 

 to produce antimony chloride that heat and light are produced. 

 We might say the antimony burns in an atmosphere of chlorine. 



It is a simple matter to generate oxygen and to collect it in 

 quantity (see Field and Laboratory Guide in Physical Nature- 

 Study^ p. 60). When a splinter of wood is lighted and allowed 

 to burn a moment, then the flame is blown out, leaving a glowing 

 ember, and this is stuck into a jar of oxygen, the splinter bursts 

 into flame again. A bit of sulphur when lighted burns sluggishly 

 in the air, which is only about one-fifth oxygen, but introduced 

 into a jar of oxygen it burns freely with a bright light. Iron 

 picture wire, which does not burn at all in air, burns vigorously 

 in oxygen, throwing off showers of sparks. 



The explanation of the process of burning is now so simple 

 that the child may get a reasonably clear notion of it. Yet it 

 quite mystified our great-great-grandparents. In their day the 

 four elementary things were earth, air, water, and fire. Every- 

 thing was made of these mixed in varying combinations and 

 proportions. True, the notion of atoms had occurred to the old 

 Greek philosophers, but it had been a shrewd guess rather than 

 a scientific theory based on anything like adequate evidence. 

 Even this was lost sight of during those dark ages that followed 



