character is not always the same, and the acid bleaches the thinner
layer sooner than the thicker.

The determination of the age of a written paper is a problem difficult
of solution. According to F. Carré the age can be approximately
determined if the characters written in iron ink are pressed in a
copying press and a commercial hydrochloric acid diluted with eleven
parts of water is substituted for water; or, if the written characters
are treated for some time with this diluted acid.

The explanation is that the ink changes in time, its organic substance
disappears little by little, and leaves behind an iron compound, which
in part is not attacked even by acids.

An unsized paper is impregnated with the described diluted acid,
copied with the press, and a copy from writing eight or ten years old
can be obtained as easily as one by means of water from a writing one
day old.

A writing thirty years old gives, by this method, a copy hardly
legible, and one over sixty years old, a copy hardly visible. In order
to protect the paper against the action of the acid, it should be
drawn through ammoniacal water.

To determine the exact age of writings by the ink is not easy. The
approximate age may be determined with some degree of certainty. If
ink-writings are but a few days old, it is easy to distinguish them
from other writing years old. But to tell by the ink which of two
writings is the older, when one is but two months and the other two
years, is, as a rule, impossible.