As to the operation of the tan,
from its affinity for metallic salts, we may conjecture, that, in the
first instance, it forms a triple compound with the gallic acid and
the iron; and that, in consequence of the decomposition of the tan,
this compound is afterwards destroyed. Owing to the difficulty, if not
impossibility, of entirely depriving the infusion of galls of any one
of its ingredients, without, in some degree, affecting the others, I was
not able to obtain any results which can be regarded as decisive; but
the general result of my experiments favours the above opinion, and
leads me to conclude, that, in proportion as ink consists merely of the
gallate of iron, it is less liable to decomposition, or to experience
any kind of change.
The experiments to which I have alluded above, consisted in forming a
standard infusion by macerating the powder of galls in five times its
weight in water, and comparing this with other infusions, which had
either been suffered to mould, from which the tan had been extracted
by gelatine, or which had been kept for some time at the boiling
temperature; and by adding to each of these respectively, both the
recent solution of the sulphate of iron, and a solution of it, which
had been exposed for some time to the atmosphere.
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