great quantity of heat is given out, which suddenly raises the temperature of the mixture. Combustion is, therefore, the rapid chemical combination of two or more substances with the production of heat and light.
11. Examples of Combustion, — The most common examples of combustion are found in the union of wood, coal, paper, or other combustible matter with the oxygen of the air. Certain substances, such as sodium and water, or oxygen and phosphorus, have so intense an affinity for each other that they will unite spontaneously under ordinary conditions of temperature and pressure. A small piece of sodium dropped into water unites with the water with such intensity as to blaze while floating on its surface. Phosphorus unites with the oxygen of the air slowly at ordinary temperatures, producing a hardly perceptible warmth, but if a little piece of phosphorus is warmed, even by rubbing it, it will burst instantly into flame. If a bit of phosphorus is dropped into a jar of pure oxygen, it will ignite spontaneously.
12. Products of Combustion. — The products of combustion are usually, but not always, gases or vapors. Consequently, if one of the original substances is a solid or liquid, such as wood, coal, oil, or the like, the products of the combustion thereof will greatly exceed in volume the original substance. The tremendous expansive force of a gunpowder explosion is due to this fact.
Liquids never burn as liquids. To produce combustion, they must first be vaporized and their vapors mixed with air in suitable proportions. This process requires the addition of heat, which may be secured by absorption or by radiation from surrounding objects, and which will come mainly from the combustion itself after that has been started. Liquids that volatilize at ordinary temperatures are highly dangerous if their vapors are inflammable, and should be handled with extreme care. If, like kerosene and the heavier oils, they do not volatilize at ordinary temperatures, they are not so dangerous. Alcohol and gasoline both give off inflammable vapors at ordinary temperatures.
