13. When the affinity of two substances is not so great
as to make them spontaneously combustible, it is necessary
to start combustion by bringing some portion of the substances to the temperature of combustion. In the case of gases or combustible vapors, the heat for ignition may be
derived from heated metals, from a flame, or from an electric
spark. The electric spark, although very small, is of a very
high temperature, and is amply sufficient for the purpose,
provided that the proportions of the two combustibles or of
the combustible and the oxygen adjacent to the spark are
approximately those required for chemical union. A strong
spark, however, will cause a more rapid spread of the flame
than a weak spark.
14. When carbon and oxygen combine, they form carbon dioxide, CO2; when hydrogen and oxygen combine, they form water, H2O. These are called the products of combustion. When, as is ordinarily the case, the oxygen is obtained from the air, the nitrogen of the air passes into the gas-engine cylinder along with the oxygen. It takes no part in the combustion and passes off through the exhaust with the carbon dioxide.
All the fuel oils and gases, including illuminating gas, are chemical compounds of hydrogen and carbon, and are called hydrocarbons. When burned in oxygen, they split up, the hydrogen and carbon uniting separately with the oxygen in the air and producing water vapor and carbon dioxide, respectively. It is this water vapor that causes the condensation on the windows of a cold room in which many gas jets or oil lamps have been burning for some time.
AIR REQUIRED FOR COMBUSTION
15. It has been shown that carbon dioxide is composed by weight of 12 parts of carbon to 32 of oxygen. Hence, to burn 1 pound of C requires 3212 = 223 pounds of O. If the O is taken from the air, it will take 223 ÷ .23 = 11.6 pounds of air to supply the 223 pounds of O. This is because only
