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.

=Conservation of oil.= The rapid increase in demand for oil as compared
with discovery of new sources is leading naturally to a more intensive
study of the conservational aspects of the industry. This is a complex
and difficult subject which we shall not take up in detail, but we may
point out some of the phases of the problem which are receiving especial
attention.

[Illustration: FIG. 10. Chart showing the relative values of
the principal petroleum products manufactured in the United States from
1899 to 1914. After Gilbert and Pogue. Note the decreasing importance of
kerosene in sustaining the cost of refining, and the necessity of
exports for maintaining a balanced outlet of products. Data from Story
B. Ladd, Petroleum Refining. Census of Manufactures: 1914, Bureau of
Census, Washington, 1917, p. 10.]

About 50 per cent of the oil in the porous strata, of oil pools is
ordinarily not recovered, because it clings to the rock. Efforts are
being made along various lines to increase the percentage of
recovery,--as, for instance, in preventing infiltration of wate