pt in an
airtight container when not being used.

The development of latent impressions with silver nitrate is dependent
on the fact that the sodium chloride (the same substance as common
table salt) present in the perspiration which forms the ridges in most
latent impressions reacts with the silver nitrate solution to form
silver chloride. Silver chloride is white but is unstable on exposure
to light and breaks down into its components, silver and chlorine. The
ridges of the fingerprints developed in this manner appear
reddish-brown against the background. Immersion in the silver nitrate
solution will wash traces of fat and oil from the paper;
consequently, it is necessary to fume the specimen for latents of such
a nature prior to treatment with silver nitrate.

Once the requisite equipment is assembled, the steps in the process
are these:

Dip the specimen in the solution, blot and dry it, expose to light,
and photograph latents when contrast is good.

Chemically standardized solutions are not required for the successful
application of this process. It has been determined through long
practice that a 3-percent solution of silver nitrate is adequate for
the purpose, although concentrations up to 10 percent are sometimes
used. A solution of approximately 3 percent may be prepared by
dissolving 4 ounces of silver nitrate in 1 gallon of distilled water.
Smaller quantities of 3-percent solution are made by using the
components in the same proportion. For instance, one quart of water
will require 1 ounce of the crystals. For a 10-percent solution, use
13-1/3 ounces of crystals per gallon.

An alcohol solution may be preferred. This is prepared by mixing 4
ounces of silver nitrate crystals, 4 ounces of distilled water, and 1
gallon of grain alcohol, 190 proof. The alcoholic solution dries
faster, and when treating paper bearing writing in ink, it is less
likely to cause the ink to run. On the other hand, the alcoholic
solution is much more expensive and there is some loss by evaporation
while in use.

The solutions may be used several times before losing their strength
and when not in use should be kept in brown bottles in cupboards to
retard deterioration. If the strength of the solution is doubtful, the
operator should attempt to develop test latent impressions before
proceeding on evidence.

Silver nitrate crystals and distilled water in small amounts are
obtainable from druggists or in large amounts from chemic