Naphtha is derived from the Persian word nafatu, 'to exude,' and was originally applied to liquid hydrocarbons which exude from the ground in the neighbourhood of the Caspian Sea; in like manner it was applied to the natural oils found more or less plentifully in nearly all countries of the world, and also to the oil distilled from Bog-head mineral in Scotland. But the inconvenience and danger of classing all these oils indiscriminately as naphthas became apparent after the Scotch paraffin and the American petroleum refined oils began to be used for domestic illumination. The word naphtha is still used in a very general and vague sense, and has no specific application either scientifically or commercially to any particular liquid; but since the more general application of the words paraffin and petroleum to mineral oils the sense in which the word naphtha is used has been narrowed considerably. The various British Petroleum Acts since 1862 have also aided in the interest of public safety in emphasising the wise distinction now made between the heavier and safe hydrocarbon oils on the one hand, and the volatile and unsafe hydrocarbon spirits or naphthas on the other.
Commercially, naphtha is now understood to apply to the inflammable distillates of crude mineral oils and coal-tar. For trade convenience the volatile distillates of petroleum and shale oil are known respectively as petroleum spirit and shale spirit, to distinguish each from the other, and both from coal-tar naphtha. The term naphtha also embraces distillates of india-rubber, bones, peat, and wood, the last of these being known as wood-spirit or methyl alcohol. A few words with regard to each of these naphthas may serve to indicate more particularly the nature and method of production, and also the uses to which they are applied.
Petroleum spirit is obtained from crude petroleum in the process of refinement by distillation. The first or lightest portion of the oil which passes over from the still, being highly inflammable, is not allowed to mix with the burning oil, but is run into a separate or naphtha tank. American crude petroleum yields from 15 to 20 per cent. of crude naphtha, which in some of the refineries is separated into gasoline, sp. gr. '640 to '650; benzine, sp. gr. '670 to '710; and benzoline or deodorised spirit, sp. gr. '710 to '730. Russian crude petroleum yields a comparatively small proportion of naphtha, about 5 or 6 per cent., which is separated into light benzine and heavy benzine, varying in sp. gr. from '730 to '775. Shale spirit is a product of the crude oil distilled from shale, which is one of the important mining and chemical industries of Scotland. This crude oil contains 4 to 5 per cent. of naphtha, having a sp. gr. of '715 to '740; but some of the shale-oil works produce a small quantity of gasoline with a sp. gr. of '640 to '680. Coal-tar naphtha is distilled from the tar obtained from coal in gas-works. The production of tar is 10 to 12 gallons per ton of coal put through the retorts. This tar on distillation yields from 5 to 20 per cent. of naphtha according to the quality of coal used. Gas-tar from Newcastle coal gives only 5 per cent. of naphtha, while the tar from some cannel coals yields as much as 20 per cent. Coal-tar naphtha has a sp. gr. varying from '850 to '950, and is thus much heavier than the naphthas obtained from crude mineral oils. Coal-tar naphtha may be fractionated into a variety of hydrocarbons with boiling-points ranging from 175° to 350°; but the two of the greatest commercial importance are benzole and ordinary naphtha. Caoutchinc is a naphtha obtained by the destructive distillation of caoutchouc or india-rubber. It also may be fractionated into a number of hydrocarbons of different densities and boiling-points. Bone-naphtha is obtained by the distillation of bones in the manufacture of animal charcoal. It is known also as bone-oil, or Dippel's animal oil. Owing to some neutral or nitrogenous substance as yet unknown, it possesses a peculiarly offensive smell, and until some easy means is discovered of removing this very objectionable feature bone-oil can never become of much use as a naphtha. The crude naphthas obtained from these various sources are all refined or purified by similar processes—viz. simple redistillation by means of steam, as in America, for the lightest fractions; but for the heavier spirits a treatment with sulphuric acid and then with caustic soda, and a subsequent washing with water are necessary previous to redistillation.
The uses to which in the industrial arts the different qualities of naphtha are applied are very numerous. The lighter spirits, such as benzole and benzine, being solvents of grease and oil, are used for detergent purposes. Benzoline was for some years burned pretty generally by the poorer classes in cheap benzoline or sponge lamps; but its use in this way has happily been greatly restricted, if not quite superseded, by the low price at which petroleum and paraffin oils have for some years been obtainable. The light naphthas are also used for extracting the perfumes of flowers and plants, and the oil from various seeds. Gasoline is employed exclusively for carburetting air-gas. The bulk of the coal-tar naphtha, and much of the shale and petroleum spirit, are employed as solvents in the manufacture of india-rubber and gutta-percha goods. They are also solvents of wax, and fatty and resinous bodies generally, and are so used in refining the best qualities of paraffin wax. Large quantities are consumed in naphtha, torch, and other flaring lamps for outside use. They are also used as a substitute for turpentine in the preparation of paints; and in Scotland the solvent action of shale spirit is turned to account in the preparation of an anti-damp or stone- and timber-preserving fluid called Alexinoton. A considerable percentage of paraffin wax is dissolved and held in solution by the spirit; and if this liquid be applied to freestone, brick, or wood, it passes into the pores of the material, and the spirit rapidly evaporating leaves the wax permanently in the stone or wood, so that water cannot be absorbed by it. While for these various purposes all the naphthas produced in Britain find a ready market, in America and Russia petroleum spirit is made in such quantities that it is impossible to find profitable outlets for it all; and although large quantities in many petroleum refineries are consumed as fuel under the stills, yet much of the crude naphtha has to be burned in waste pits to get rid of it.