332

painting materials

angle of refraction; n ~ sin //sin r. The
refractive index (n) of water is 1.33; gyp-
sum, 1.52; zinc oxide, 2.00.
Roentgen Rays, same as x-rays (see X-Rays);
a form of radiant energy discovered by
Wilhelm Konrad Rontgen in 1895.
Salt, one of the group of substances that re-
sults from the reaction between acids and
bases; the product, in addition to water,
formed by the neutralization of an acid by
a base; e.g., sodium chloride (common salt)
formed by the action of hydrochloric acid
on sodium hydroxide.
Saponification, the conversion of an ester into
an alcohol and an acid by hydrolysis or into
an alcohol and an acid salt by means of an
alkali. It is the process by which soap is
made by action of alkali on vegetable or
animal fats and oils.
Saponification Number or Value, the quantity
of potassium hydroxide (in milligrams) re-
quired to saponify one gram of fat or oil;
the measure of the amount of true fat or
fatty acid in a substance.
Saturation, the complete satisfaction of the
valence bonds in a molecule; also the com-
plete or maximum absorption of a substance
by a solvent.
Single Bond, a single linkage or valency be-
tween atoms.
Slake, (i) to slack or loosen; (2) the addition
of water to quicklime to form calcium
hydroxide.
Sol, a colloidal solution or the liquid phase of
a colloidal solution; a colloidal system in
which matter is dispersed in a liquid dis-
persion medium and in which the dispersed
particles show independent movement, as
Brownian movement (see Gel).
Solution, the combination of a solid, liquid, or
gaseous substance (called the * solute ') and
a liquid (called the ' solvent *) to form a
homogeneous mixture from which the dis-
solved substance can be recovered un-
changed by evaporation and crystallization
or by other physical processes.
Specific Gravity, the ratio of the density of a
substance to the density of some other
substance chosen as standard. In the case of
solids and liquids, the standard is usually
water and, if metric units are selected,
specific gravity is equal to the density
(grams per cubic centimeter); <?.£,, s.g. of
water is i.oo; gypsum, 1.36; vermilion, 8.09.
Spectrum, a variously colored band of light
showing in succession the rainbow colors or

isolated lines or bands of color when light
is refracted by a prism or diffracted by a
grating.
Spectrogram, a photographic plate, film, or
print on  which  a spectrum  is  recorded,
together with a comparison spectrum.
Stria, a minute groove or channel or a narrow
line or band, as of color; one of a series of
parallel lines or grooves.
Structural Formula,  a representation on a
plane surface of the atomic arrangement of
a molecule, as that of benzene:
H
A
h-c/ vh
H-6	i-H
V
i
Sublimate, a solid substance which, on heat-
ing, passes into the vapor state and, on
cooling, returns to the solid state without
passing through the liquid state in either
direction; e.g., mercuric chloride, iodine,
and red mercuric sulphide.
Supernatant Liquid, the liquid standing above
a sediment or precipitate.
Surface Film, the thin, transparent film,
usually of varnish, spread as a protective
coating over the surface of a painting.
Surface Tension, the contractile surface force
of a liquid by which it tends to assume a
spherical form and to present the least
possible surface; its value is measured in
dynes on an instrument called a tensiometer.
Tack or Tackiness, stickiness, as the condition
of a surface of partially dried varnish.
Technically Pure (t.p.)> pure enough for tech-
nical or industrial uses but not pure enough
for analytical or pharmaceutical purposes.
Temper, to mix in proper proportions; to
compound or blend; to soften or mollify; to
combine with a liquid medium; to make a
paint or to make a material brushable; to
harden, as of metals, by heating and rapid
cooling.
Thermoplastic, capable of being softened and
• made to flow by heat and pressure; a term
commonly applied to artificial resins and
plastics which are resoftened by heating.
Thermosetting, a term applied to artificial
resins and plastics which are molded and