SPECIAL PUBLICATION NO. 27


element Any of a class of substances that cannot be separated into
simpler substances by chemical means. Elements are the building blocks
from which all chemical compounds are formed.

enstatite A common rock-forming mineral of the orthopyroxene
group: MgSi03. It is isomorphous with hypersthene and may contain a
little iron replacing the magnesium. Enstatite varies from grayish white to
yellowish, olive green and brown. It is an important primary constituent
of intermediate and basic igneous rocks.

ester A compound produced by the reaction between an acid and an
alcohol with the elimination of a molecule of water.

estuary (a) The seaward end or the widened funnel-shaped tidal
mouth of a river valley where freshwater mixes with and measurably
dilutes seawater and where tidal effects are evident; e.g. a tidal river, or a
partially enclosed coastal body of water where the tide meets the current
of a stream; (b) A portion of an ocean, as a firth or an arm of the sea,
affected by freshwater; e.g. the Baltic Sea; (c) A drowned river mouth
formed by the subsidence of land near the coast or by the drowning of
the lower portion of a nonglaciated valley due to the rise of sea level.

ethane A colorless, odorless, water-insoluble, gaseous paraffin
hydrocarbon, formula C2H6, which occurs in natural gas or can be pro-
duced as a by-product in the cracking of petroleum.

ethanol (alcohol) A colorless, volatile, flammable liquid, C2H5OH, pro-
duced by fermentation of certain carbohydrates, used chiefly as a sol-
vent, and in organic synthesis, beverages, medicine, colognes and anti-
freeze.

ethyl acetate A volatile, flammable liquid, CH3COOC2H5, used as sol-
vent for paints and lacquers.

eutrophication The process by which waters become more
eutrophic; the artificial or natural enrichment of a lake by an influx of
nutrients required for the growth of aquatic plants such as algae that are
vital for fish and animal life.

evapotranspiration Loss of water from a land area through transpira-
tion of plants and evaporation from the soil. Also, the volume of water
lost through evapotranspiration.

fen A waterlogged, spongy groundmass containing alkaline, decay-
ing vegetation characterized by reeds or peat. It sometimes occurs in the
sinkholes of karst regions. Cf: bog.