The next product which is driven off from the crude oil after all of the kerosene has been removed is a somewhat heavy and discoloured, but free flowing oil, known as “gas oil.” Gas oil is seldom sold at retail and the general public has very little knowledge of it. Its main use is for the manufacture of city gas, auxiliary to coal, the products of which form the base of city gas. The next product after the gas oil and the last important product of crude petroleum is the lubricating oil distillate, which is known as “paraffine distillate” for the reason that it contains the paraffine wax. With the exception of the gas oil, which by reason of the uses to which it is put does not usually require any further treatment, the products thus roughly separated from the crude petroleum each need not only further separation, but actual chemical purification to prepare them for the market. Considering these products in the order in which they are derived from the crude petroleum, the gasoline or naphtha fraction is often subjected to a second distillation by which it is further “sifted” into light, intermediate and heavy naphthas. It is customary to conduct this second distillation process by steam heat instead of by fire, since the gasoline or naphtha fraction boils at such a low temperature that it is unnecessary to resort to a furnace and furthermore, the quality of the product is thought to be better if the second distillation is conducted with steam. Following this second distillation the naphtha or gasoline is subjected to chemical purification which involves treatment with sulphuric acid, with sodium hydrate, sodium plumate and filtration through Fuller’s Earth--a species of clay which has been found to have not only a mechanical but probably also a chemical purifying and decolourizing action. There is a considerable variation in the purification or refining method employed by the different refiners, but the foregoing treatments are the principal ones now in vogue. The marketable products produced from the crude gasoline or naphtha distillate by this re-distillation and purification process are principally as stated--light naphthas, intermediate naphthas and heavy naphthas. The light naphthas range from petroleum ether, an exceedingly sweet-smelling and volatile liquid to aviation gasoline, especially suitable for use in aeroplane motors under extreme conditions of temperature and power development. The intermediate naphtha is the ordinary gasoline of commerce, principally used as fuel for automobile engines. The heavy naphtha is that often sold under the name of benzine, cleaners’ naphtha, solvent naphtha or varnish makers’ and painters’ naphtha. As these names indicate, the heavy naphtha is principally used in the manufacture of paints and varnishes, for dry-cleaning and as a solvent in the chemical industries.