PAINTING MATERIALS

tridinic, in which all axes are of unequal
length and all inclined toward each other
(copper sulphate, CuS04*5H20); hexagonal,
in which three equal axes in the same plane
intersect at angles of 60° and a fourth is at
right angles to all of these (quartz, 8162).

Deliquescence, the property of certain sub-
stances to become moist or liquid when ex-
posed to the air; as sodium hydroxide and
calcium chloride. It is a property of solid
substances whose hydrates have lower
aqueous vapor tension than the air.

Density, the weight of matter per unit volume
of a substance; it is conveniently expressed
as the number of grams per cubic centimeter
(see Specific Gravity).

Dicotyledon, having two cotyledons; a plant
whose seed develops two leaves on germinat-
ingj as the bean seed.

Diffraction, the bending of light as it strikes
the edge of an opaque body.

Double Bond, in chemistry, a method of
representing a certain type of unsaturation
in organic compounds where two single
valence bonds connect two atoms, as
in HsC = CEb; same as ethylenic linkage.
The position of the double bond in. the
carbon-to-carbon chain is a point of activity
in the molecule (see Unsaturated).

Efflorescence, the property of certain sub-
stances, particularly crystalline salt hy-
drates, to lose water when exposed to the
air and to crumble to a powder (*.£., wash-
ing soda, NajjCOs-ioBbO); a property of
solid substances in which hydrates have
greater aqueous vapor tension than the air.

Element (see Chemical Element).

Empirical Formula, in chemistry, a formula
which shows the number and variety of
atoms in a molecule but does not indicate
the way in which the atoms are linked to-
gether; e.g., the empirical formula of verdi-
gris [Cu(C2H302)2 • 2Cu(OH)2] is CmQHioOs
(see Structural Formula).

Emulsifying Agent, a material which reduces
interfacial tension between immiscible
liquids to aid in the formation of an emul-
sion, as soap.

Enzyme, a catalytic substance produced by
living cells which has a specific action in
causing the decomposition or synthesis of
compounds into new ones; e.g., the yeast
enzyme, zymase, which splits sugar into
alcohol and carbon dioxide.
Essential Oil, one of a group of volatile oils of
characteristic odors, distinguished from the
fatty oils by their volatility, non-greasiness,

and non-saponifying properties. They fall
into two main classes: (i) those existing in
plants as such, the odoriferous constituents
of leaves, flowers, or woods like oil of
turpentine, peppermint, and lemon; and (2)
those developed from plant constituents by
enzymic action or heat, like oil of bitter
almonds and creosote oil, respectively.
Essential oils may include as constituents
hydrocarbons like pinene, alcohols like
menthol, phenols like thymol, aldehydes
like geranial, ketones like camphor, acids
like hydrocyanic acid, and esters like
methyl benzoate.
Ester, an organic compound formed from an
alcohol and an organic acid by elimination
of water; hence, sometimes the name
* ethereal salt'; e.g., ethyl acetate (CHs-
COOC2Hs), formed by the action of acetic
acid upon ethyl alcohol.
Esterfication, the formation of an ester from
an alcohol and an organic acid, with the
aid of dehydrating or catalytic agents.
Ethylenic Linkage, in organic compounds, an
unsaturated linkage or double bond, as
contained in the substance, ethylene,
H2C = CH2 (see Double Bond).
Fat, a solid or liquid oil consisting of the
glyceryl esters of the higher fatty acids, as
tristearin (stearin) which is contained in
beef fat, and tripalmitin, contained in
vegetable fats and oils such as palm oil.
Fibril, a small fibre or filament.
Film, a thin, usually continuous layer or skin
of any substance. It may be more or less
homogeneous in structure like a varnish
film or heterogeneous like a paint film.
Flash Point, the lowest temperature, under
specified conditions, at which the vapors of
a liquid can be momentarily ignited; a con-
stant for many organic liquids.
Fluorescence, a kind of luminescence which
is the property of certain substances of
absorbing light of one wave-length and
emitting or radiating it as another wave-
length, usually greater. It is thought to be
caused by the return of electrons displaced
by the exciting radiation to a more stable
position. Fluorescence radiation is the im-
mediate result of, and takes place only dur-
ing, the absorption of radiation from some
other source. Ultra-violet light and x-rays
are the most commonly used exciting radia-
tions for producing fluorescence.
Fraction, in distillation, those liquids which
distill at a certain temperature or tempera-
ture range. Fractionate, to separate into