glossary

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set by heat and pressure but which do not
return to the plastic state on reheating.
Tincture, a dilute extract of a drug or chem-
ical, usually a plant principle in alcohol.
Tinting Strength, the ability of a coloring
material like dye or pigment to impart
color; same as * tinctorial power.*
Tooth, the roughened or absorbent quality of
a surface which favors the application and
adhesion of paint coatings.
Top Tone (or Mass Tone), the full strength
and color of a pigment or paint when
viewed by reflected light (see also Under-
tone).
Tufa, a sedimentary rock composed of silica
or calcium carbonate deposited from waters
of lakes, rivers, and springs.
Ultra-Violet (or Ultra-Violet Rays), that por-
tion of the invisible spectrum that lies
beyond the violet or on the shorter wave-
length side of the visible spectrum; that
portion of the light spectrum between
4000 A units and lao A units.
Undertone, the color of a pigment or paint
when viewed by transmitted light or when
spread thinly over, or mixed with, much
white.
Unsaturated, (i) designating an organic com-
pound having double or triple bonds or
linkages between carbon atoms; e.g.> ethyl-
ene, H_2C = CH_2, or acetylene, HC m CH;
(a) said of a solution which is capable of dis-
solving more solute.
U.S.P., letters commonly affixed to the name
of a material indicating that it conforms in
grade to the specifications of the United
States Pharmacopoeia and that it is ap-
proved for use hi medicinal preparations;

it does not necessarily mean, however, that
the material is chemically pure.
Valence, a value or number which expresses
the capacity of an atom to combine with
other atoms in definite proportions; it is
measured with the combining capacity of
hydrogen taken as a unit; e.g_f> chlorine in
HC1 (hydrochloric acid) is monovalent;
oxygen in HsO (water) is divalent; and
carbon in CHU (methane) is tetravalent.
Vapor Pressure, the pressure at which a liquid
and its vapor are in equilibrium at a definite
temperature. If the v.p. reaches the pre-
vailing atmospheric pressure (i atmos-
phere), the liquid boils. V.p. is usually ex-
pressed in millimeters of mercury; e.g., v.p.
at 20° C. for water is 17.5 mm.; for alcohol,
44 mm.; and for acetone, 178 mm.
Viscosity, the internal friction of a fluid which
influences its rate of flow or causes it to
exhibit slight resistance to change of form;
the state of being glutinous or sticky or
resistant to flow.
Volatile, designating a substance that evapo-
rates rapidly.
Water of Crystallization (Water of Eydration),
water that is combined with certain crystal-
line salts in definite proportions by weight
and which may be completely removed by
heating; e.g._9 CaSCX-aH^O (calcium sul-
phate dihydrate) and CuSO^EhO (copper
sulphate pentahydrate).
X-Rays, a group of invisible light rays of
extremely short wave-length, ranging from
0.06 to 20 Angstrom units, produced in an
exhausted tube (called an * x-ray tube ') by
fast-moving cathode rays impinging upon
a metal surface.