=Classes of oils.= When crude petroleum is distilled, it gives off in succession various substances and gradually thickens until it leaves a solid residue, which may be largely either paraffin wax or asphalt. The two main classes of oils are determined by the nature of this solid residual. The products given off are natural gas and then liquid hydrocarbons of various kinds, which evaporate in the order of their lightness. Petroleum is thus a mixture or mutual solution of different liquids, gases, and solids. Nearly one-fifth of the domestic consumption of crude petroleum is burned directly as fuel, and four-fifths are refined. The several principal primary products of refinement are gasoline, kerosene, fuel oil, and lubricating oil; but these may be broken up into other substances, each the starting point of further refinements, with the result that present commercial practice yields several hundred substances of commercial value. With increasing chemical and technical knowledge these products are being multiplied. The rapidly increasing demand for gasoline has led to the use of processes which extract a large proportion of this substance from the raw material, by "cracking" or breaking up other substances; but while, under the stress of necessity, there is possibility of slight modification of the proportions of principal substances extracted from the crude oil, it is not possible to change these proportions essentially. It is, therefore, a problem to adjust relative demands to supplies of the different products. The domestic demand for gasoline is greater than the supply. On the other hand, the demand for kerosene, which must be produced at the same time, is much less than the domestic supply. Hence the importance of maintaining export markets for kerosene. The nature or grade of the oil of various fields is an important matter in considering reserves for the future. Perhaps half of the United States reserves consist of the asphalt-base oils of the California and certain of the Gulf fields, which yield comparatively small amounts of gasoline and other valuable light products, though they are very satisfactory for fuel purposes. Similarly the large reserve tonnages of oil in Mexico and the Caribbean countries, in Peru, and probably in Russia, are essentially of the heavier, lower grade oils. The oils of the Mid-Continental and eastern fields of the United States, of Ontario, of the Dutch East Indies, of Burma, and of Persia and Mesopotamia are reported to be largely of the paraffin base type, which, because of its larger yield of gasoline and light oils, is at present considerably more valuable. These generalizations are of course subject to qualifications, in that the oils of a given region may vary considerably, and that some oils are intermediate in character, containing both asphalt and paraffin wax.