thick, heavy, and glossy, in character, and flow sluggishly from the
pen. Few of these become much darker by standing. In this class will
be found the copying inks and those in which a large quantity of gums
or similar thickening agents are used.

Other inks are pale, limpid, and flow easily from the pen, and this
class usually shows a notable darkening by exposure to sunlight and
air. It will be unnecessary here to refer more particularly to the
intermediate varieties or to discuss their various composition.

It should be, remembered here that in the last twenty years, or since
the introduction into general commerce of aniline colors, which
Hofmann discovered in 1856, these latter have been employed more and
more in writing fluids; not only in mixtures of which they are the
principal ingredients, but to a greater or less degree in all inks.
Their presence, even in small quantity, in the gallo-tannate of iron
and logwood inks can be generally detected by an iridescent and
semi-metallic luster.

To assist in determining the ages of writings by one and the same ink,
it is to be observed that the older the writing the less soluble it is
in dilute ammonia. If the writing be lightly touched with a brush
dipped in ten-per-cent ammonia, the later writing will always give up
more or less soluble matter to the ammonia before the earlier. In case
of inks of different kinds this test is not serviceable, for
characters written in logwood ink, for instance, will always give up
their soluble material sooner than nutgall inks, even if the last
named be later applied. To estimate the age of writing from the amount
of bleaching in a given time by hydrochloric or oxalic acid is very
precarious, because the thickness of the ink film in a written