74 EXPERIMENTAL GENERAL SCIENCE 



from the air. When a substance bursts into flame of its own 

 accord, the process is called spontaneous combustion. It occa- 

 sionally happens that the heat generated by piles of dead 

 leaves, oily rags, damp straw, manure piles, and the like, in- 

 stead of passing off into the air, accumulates in the material 

 until the kindling temperature is reached, when the material 

 begins to burn by spontaneous combustion. Numerous mys- 

 terious fires have been traced to this cause. Spontaneous 

 combustion often occurs in hay-mows, especially if the hay 

 was not thoroughly dried before storing. Even piles of fine 

 coal may take fire in this way. Thorough ventilation does 

 much to avert the danger from such fires. 



70. Explosions. When oxygen unites very rapidly with 

 another substance, especially in a closed space, an explosion 

 is the result. Unexpected explosions occurring in valuable 

 material may be most destructive, but when controlled, ex- 

 plosions are sources of much useful power. In general, the 

 more thoroughly the air is mixed with the substance, the 

 greater the explosion is likely to be. Gases, therefore, are 

 more explosive than liquids. If a burning match be thrust 

 into kerosene oil it will be put out, but the gas from kerosene 

 explodes at once if the flame reaches it. Air filled with fine 

 dust from flour, coal, starch, wood, or other substances may 

 cause explosions. In the gas engine, the power is produced 

 by a series of explosions in the cylinders. Gunpowder and 

 other high explosives consist of a mixture of substances which 

 oxidize almost instantly, the oxygen for this purpose being 

 supplied by some of the ingredients in the mixture. By means 

 of the gases produced, projectiles may be fired from guns, 

 buildings and other structures blown up, and rocks torn to 

 pieces (66). 



71. Products of Combustion. Nearly all combustible 

 materials contain both carbon and hydrogen, and these, unit- 

 ing with oxygen, produce on the one hand carbon dioxide (C02), 



