COMBUSTION AND OXIDATION 73 



is that one proceeds more rapidly than the other. When 

 metals rust, or wood decays, the total amount of heat devel- 

 oped is the same as if the substance had been burned more 

 rapidly. If plenty of oxygen is available, it may unite with 

 other substances so rapidly as to raise their temperature to a 

 point where they glow, or it may even cause them to change 

 to gases and burst into flame. All flames are burning gases 

 of some kind. A flame, however, does not necessarily pro- 

 duce much light. Light results only when substances are 

 heated 'very hot or to the condition which is described as white 

 heat. The calcium light often used in stereopticons is pro- 

 duced by heating a pencil of lime white-hot in the flame from 

 burning hydrogen. The bright light from the Welsbach gas 

 light is due to the mantle being heated to incandescence by 

 the burning gas within it. In ordinary gas jets and in the 

 flame from wood, coal, oil, and the like, the light is due to 

 very hot particles of unburned carbon. When the burning is 

 so managed that all the carbon is oxidized, as in the bunsen 

 burner, very little light results. In pure oxygen glowing char- 

 coal bursts into flame and even iron and other metals burn 

 readily. If it were not for the fact that the oxygen of the air 

 is diluted with four times its volume of nitrogen, building a 

 fire in the stove would probably result in a disastrous conflagra- 

 tion (105). 



69. Kindling Temperature. In oxidation, as in other chem- 

 ical reactions, a certain amount of heat is found necessary to 

 induce the change. Most substances will not burn at ordinary 

 temperatures, but must first be heated to their kindling tem- 

 perature. This temperature is different for different sub- 

 stances. That of anthracite, for instance, is much higher 

 than that of wood. Some substances, such as phosphorus, 

 readily oxidize and burst into flame when exposed to the air at 

 ordinary temperature. In the laboratory, phosphorus has to 

 be kept under water, oil, or similar substances to protect it 



