form of a crystal ; thus one of a " prismatic " habit is of the general form of a prism, that is, mom or less elongated.
Hemihedral.—Having only half the full number of crystal faces developed.
Hemimorphic.—Having the opposite ends of the crystal terminated by dissimilar faces.
Halogen Compound.—A chemical compound of a base with chlorine, bromine, iodine or fluorine.
High Potential.—Having a great electrical pressure or electromotive force.
Hydrous.—Containing water.
Hydro-thermal Mineral. -One whose formation is due to the action of heated water, usually acting under great pressure.
Hypogene Minerals.—Minerals arising by forces acting from within the earth outwards.
Idiomorphic.—Having
the form proper to the particular substance. Most minerals in
crystalline rocks that are idiomorphic have crystallised out from the
rock magma at an early stage in its consolidation.
Interference of Light.- A change in light due to its waves being altered.
Interpenetration Twins.- -Twinned
crystals appearing to have much of their substance in common, but with
the solid and dihedral angles of both members fully developed.
Intrusive Sheet.—A mass of volcanic rock lying in a flat bed, surrounded by, and often replacing, other rocks.
Isomorphous.—Having
the same form. Chemical substances and minerals which are isomorphous
are not only similar in crystalline form but also in composition, and
molecules of isomorphous substances are often found to replace one
another. The Felspar group affords a good example.
Isotropic.—Substances
which have but one index of refraction of light. All'cubic crystals and
all colloid substances that transmit light are isotropic.
Lenticular.—Having the shape of a bi-convex lens.
