Gasoline is one of the liquefied lighter fractions obtained from the distillation of petroleum, coal, natural gas, and oil-bearing shales and sands. Most gasoline today comes from petroleum. Only 125 years ago, before kerosene lamps were replaced by electricity. gasoline was not even a byproduct. Many refiners dumped gasoline into the streams of Pennsylvania or poured it out onto the ground to burn. Then came the automobile, and gasoline became the mam product, and kerosene the byproduct. With the increased demand for gasoline came changes in refining, and the 42-gallon barrel of crude oil that in 1918 produced only 10.6 gallons of gasoline in 1952 produced 17.8 gallons.
   The technology of the production of gasoline is in such rapid development that few generalizations long remain valid. In general, gasoline is produced by either distillation or conversion of petroleum. Conver­siĆ³n has branched off into thermal cracking, catalytic cracking, and polymerization, which is the reverse of cracking. "Natural gasoline" is a product "squeezed" out of natural gas. It is not in itself a natural gasoline, but it is mixed with the regular product to be sold in service stations.
   The octane number of a gasoline indicates the percentage of a certain type of antiknock gasoline, called iso-octane, that has been mixed into the product sold. Iso-octane is almost knockproof in a high-compression gasoline engine, while heptane, the old-style gasoline, is prone to cause knocks. Knocking is uncontrolled combustion and is therefore wasteful. In the 1920's engineers found that antiknocking properties could also be put into gasoline by the addition of tetraethyl lead.