emical agents,
acids incompletely removed, or the surplus of which has been saturated
by an alkali, or by the help of alkaline substances. The change of the
color to red indicates an acid substance; an alkali would turn the
reddened litmus paper to blue, and the mauve and Georgina test-papers
to green.

Take a sheet of test-paper of the same dimensions as the document to
be examined, moisten it, and cover it underneath with a sheet of
Swedish filter-paper. These two sheets together (the filter-paper
underneath) are then applied to the document which has been moistened
already. The whole is then laid between two quires of paper, covered
by a weighted board, and left in this condition for about an hour. At
the end of this time examine the test-paper to see if it has partly or
altogether changed color. This examination finished, put the
test-paper in contact with distilled water, to be afterwards removed
and tried by appropriate tests to discover the nature of the alkali or
acid present.

Silver nitrate is also used to discover whether the paper has been
washed with chlorine or chlorites. A paper in that way becomes acid.
The chlorine changes to hydrochloric acid, which dissolves in the
water with which the suspected document or paper is moistened, and at
the contact of silver nitrate little spots of silver chloride appear.

There are various other tests such as gallo-tannic acid or infusion
of nutgalls prepared a short time before application and may be used
with advantage to restore writings that have been removed by washing.
Place the document or paper on a sheet of white paper and moisten the
whole of its surface with a paint brush dipped in the reagent, taking
care not to rub it or strongly press it. When the surface is well
impregnated allow the solution to act for an hour, and at the end of
this time examine the document again. Then moisten it a second time
and the following day, examine the results. Repeat the moistening
several times if necessary, for it often takes some time to make the
traces of writing reappear.

Chevallier and Lassaigne experimented together on the effect produced
by the vapor of iodine on the surface of the papers or documents upon
which the alteration of writing was suspected. Take a bottle with a
wide mouth from ten to eleven centimeters in height, and the opening
from five to six centimeters in width. This last is covered by a disk
of unpolished glass. Into the bottom of this vessel i