Another portion of the same infusion of galls had solution of isinglass
added to it until it no longer produced a precipitate; by employing the
sulphate of iron, a black compound was produced, which, although paler
than that formed from the entire fluid, appeared to be a perfect and
durable ink. Lastly, a portion of the infusion of galls was kept for
some time at the boiling temperature, by means of which a part of its
contents became insoluble; this was removed by filtration, when, by the
addition of the sulphate of iron, a very perfect and durable ink was
produced. In the above three processes I conceive that a considerable
part of the mucilage, the tan, and the extract, were respectively
removed from the infusion, while the greater part of the gallic acid
would be left in solution.
The three causes of deterioration in ink, the moulding, the precipitation
of the black matter, and the loss of colour, as they are distinct
operations, so we may presume that they depend on the operation of
different proximate principles. It is probable that the moulding more
particularly depends on the mucilage; and the precipitation on the
extract, from the property which extractive matter possesses of forming
insoluble compounds with metallic oxides.
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