pesticides affect only the target pest. Persistent pesticides are those that remain in the envi-
ronment for a long time.
                                             Dye
      Dye is any substance, natural or synthetic, used to color various materials, especially
textiles, leather, and food. Natural dyes are so called because they are obtained from plants
(e. g. , alizarin, catechu, indigo, and logwood), from animals (e. g. , cochineal, kermes,
and Tyrian purple), and from certain naturally occurring minerals (e. g. , ocher and Prussian
blue). They have been almost entirely replaced in modern dyeing by synthetic dyes. Most of
these are prepared from coal tar, being formed from an aromatic hydrocarbon such as ben-
zene, from which indigo is derived, or anthracene, which yields alizarinz. Although some
 materials, e. g. , silk and wool, can be colored simply by being dipped in the dye (the dyes so
used are consequently called direct dyes), others, including cotton, commonly require the
use of a mordant. Alizarin is a mordant dye and the color it gives depends upon the mordant
used. Dyes are classified also as acidic or basic according to the medium required in the dyeing
process. A vat dye, e. g. , indigo, is so called from the method of its application; it is first
treated chemically so that it becomes soluble and is then used for coloring materials bathed in
a vat. When the materials become impregnated with the dye, they are removed and dried in
air, the indigo reverting to its original, insoluble form. The process by which a dye becomes
"attached" to the material it colors is not definitely known. One theory holds that a chemical
reaction takes place between the dye and the treated fiber; another proposes that the dye is
absorbed by the fiber.
                                          Pigment
      Pigment is a substance that imparts color to other materials. In paint, the pigment is a
powdered substance which, when mixed in the liquid vehicle, imparts color to a painted sur-
face. The pigments used in paints are nearly all metallic compounds, but organic compounds
are also used. Most black pigments are organic, e. g. , bone black (animal black or charcoal)
and lampblack. Some of the metallic pigments occur naturally. The brilliant and beautiful col-
oring of the rock and soil in some parts of the world. Yellow ocher, sienna, and umber are
oxides of iron. Litharge is a yellow oxide of lead. Red lead is also an oxide of this metal. Lead
chromate, or chrome yellow, is an important yellow pigment. White lead, or basic lead car-
bonate, is a pigment long in use; it is rendered more durable by mixture with zinc ox-
ide. Cadmium yellow is a sulfide of cadmium. Ultramarine is an important blue pigment, as is
Prussian blue (ferric ferrocyanide). Green pigment is produced by mixing Prussian blue and
chrome yellow. Vermilion (mercuric sulfide) is red. Pigments occur in plant and animal
bodies. The bright colors of plants, for example, are the result of the presence of such sub-
stances as chlorophyll (green) and xanthophyll (yellow), both of which are also found in
some animals. Among others are carotene, the yellow of carrots and certain other vegetables,
and anthocyanin, which imparts blue, red, and purple to flowers4. Blood receives its color
from the hemoglobin in the red corpuscles. Coloration of human skin is caused by the pres-
ence of pigments.
                                          Reagent
     A reagent or reactant is a material used to start a chemical reaction. For example hydro-
chloric' acid is the chemical reagent that would cause calcium carbonate to release carbon di-
oxide. Similarly, but less obvious, hydrochloric acid is the chemical reagent that reacts with
zinc to produce hydrogen, even though in this case the hydrogen comes from the acid and not
the metal. To classify any of the chemicals involved in a chemical reaction as the "reagent" is
thus largely a matter of convention or perspective.
     In another use of the term, when purchasing or preparing chemicals, "reagent" de-

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