Carat.—The unit of weight for gems ; from êøáôýç, one
of the vetches, the seeds of which were used as weights in ancient
times, on account of their uniform size. It is equivalent to 3.16
grains, about -2055 gram.
Coefficient of Expansion.—The increment by which a substance of unit size increases its size (at 0" C.) for an increase of one degree of temperature.
Colloid.—A
substance having no crystalline structure ; on account of difficulty of
absolutely determining this a colloid is now often regarded as a
substance having a molecular weight greater than a certain figure.
Conductivity of Heat.—The
conductivity of a substance is the amount of heat that will pass
through a unit area of that substance of unit thickness in unit time,
when the difference of temperature of the surfaces is 1" C.
Contact Twins.—Crystals showing a form as if a single crystal had been divided in two halves and one half rotated through 180".
Crystal.—A
substance of definite chemical composition having a definite internal
molecular arrangement and a definite external form, bounded by plane
surfaces, systematically arranged and meeting in angles of fixed and
definite value.
Crystalline.—Showing the internal structure of a crystal but not the external form.
D line.—A brilliant double line seen in the yellow portion of the spectrum, characteristic of the metal sodium.
Decrepitation.—A crackling of a substance when heated, due to the sudden separation of particles.
Dendritic.—In treedike forms.
Detritus.—Material resulting from the breaking down of a rock from natural causes.
Diffraction Grating.—A
piece of smooth glass or metal ruled with fine lines of equal width and
equidistant from one another so as to form a series of minute
rectangles. The lines may be up to 43,000 to an inch, usually about
30,000. Such a grating modifies light so as to produce a spectrum.
