al supply
houses. Dealers in distilled water are located in many communities.

Tap water should not be used in the preparation of the solution
because it generally contains chemicals which will partially
neutralize the silver nitrate.

It is suggested that the solution be placed in a glass or enamelware
tray approximately 18 by 12 by 5 inches for use, a size used in
photographic development. Treatment with this solution is called
"silvering." The specimen is immersed in the solution so that the
surfaces are completely moistened, then taken out, placed between
blotters to remove the excess solution, and dried. The drying is
readily accomplished with an electric hair dryer. Blotters may be
dried and used several times before discarding. It is not necessary to
work in a dark room. Work in an illuminated room but not in direct
sunlight. Soaking the specimen in the solution does not aid
development and is actually undesirable as it requires a longer drying
time. The specimen should be reasonably dry before exposing to the
light, otherwise the latent prints may be developed while the paper
is still wet, thus necessitating drying in subdued light to prevent
darkening.

Development of the latent impressions occurs rapidly when the specimen
is exposed to a blue or violet light source. A 1,000-watt blue or
daylight photographer's lamp, a mercury arc (most ultraviolet lamps
are of this type), or carbon arc is excellent for the purpose (fig.
422). If a weaker light is used, a stronger mixture of the solution
should be prepared. For instance, if a 300-watt bulb is used, the
10-percent solution would be preferable. Direct sunlight will cause
the latent impressions to appear very rapidly and if several specimens
are exposed at once it is not possible for a single operator to
properly control the development. Sunlight coming through a window
pane will serve for development. Where fingerprints containing sodium
chloride (normally exuded from the sweat pores in the ridges) have
been deposited, the silver chloride formed will darken against the
background.

[Illustration: 422. Developing silver nitrate prints using 1,000-watt
bulb reflector.]

As soon as the ridge detail of the prints is clearly visible, the
paper should be removed from the light. Continued exposure will darken
the paper and the contrast will be lost.

Paper so treated should be kept in darkness; that is, in a heavy
envelope or drawer until ready to photograph.