insects, so called from a peculiarity in
their wings, the superior being coriaceous at the base and
membranous at the apex, [Greek: hemisy], _hemisu_, half, and [Greek:
pteron], _pteron_, wing.

HORNBLENDE. A simple mineral of a dark green or black color, which
enters largely into the composition of several varieties of the
Trap-Rocks.

HORNSTONE. A siliceous mineral substance, sometimes approaching
nearly to flint, or common quartz. It has a conchoidal fracture, and
is infusible, which distinguishes it from compact felspar.

HUMERUS. The bone of the upper arm.

HYDROPHYTES. Plants which grow in water. _Etym._, [Greek: hydor],
_hydor_, water, and [Greek: phyton], _phyton_, plant.

HYPOGENE ROCKS. Those rocks which are _nether-formed_, or which have
not assumed their present form and structure at the surface, such as
granite, gneiss, &c. The term, which includes both the plutonic and
metamorphic rocks, is substituted for _primary_, because some
members of both these classes, such as granite and gneiss, are
posterior to many secondary or fossiliferous rocks. _Etym._, [Greek:
hypo], _hypo_, under, and [Greek: ginomai], _ginomai_, to be formed
or produced.

ICEBERG. Great masses of ice, often the size of hills, which float
in the polar and adjacent seas. _Etym._, ice, and _berg_, German for
hill.

ICHTHYOSAURUS. A gigantic fossil marine reptile, allied in part of
its structure to a fish. _Etym._, [Greek: ichthus], _ichthus_, a
fish, and [Greek: saura], _saura_, a lizard.

IGNEOUS ROCKS. All rocks, such as lava, trap, and granite, known or
supposed to have been melted by volcanic heat.

INCANDESCENT. White hot--having a more intense degree of heat than
red heat.

INDUCTION. A consequence, inference, or general principle drawn from
a number of particular facts or phenomena. The inductive philosophy,
says Mr. Whewell, has been rightly described as a science which
ascends from particular facts to general principles, and then
descends again from these general principles to particular
applications.

INFUSORY ANIMALCULES. Minute living creatures found in many
_infusions_; and the term _infusori_ has been given to all such
animalcules, whether found in infusions or in stagnant water,
vinegar, &c.

INSPISSATED. Thickened. _Etym._, _spissus_, thick.

INVERTEBRATED ANIMALS.