he instances in which green vitriol has been added, a black
precipitate is produced, it indicates an astringent earthy compound, in
which there are few mucilaginous particles. The more the colour verges
to black, the more of this acid and mucilaginous substance will be found
in it.

The mixture of alum with a tingeing decoction shows by the coloured
precipitate that ensues from it, on the one hand, the colour it yields,
and on the other hand, by the precipitate dissolving either partly or
entirely in a strong alkaline lixivium, whether or not some of the earth
of alum has been precipitated together with the colouring particles.
Such substances as these must not, in general, be boiled with alum,
although this latter ingredient may be very properly used in the
preparation of the stuff.

When a tingeing decoction is precipitated by an alkaline lixivium, and
the precipitate is not redissolved by any acid, for the most part
neither one nor the other of these saline substances ought to be used,
but the neutral salts will be greatly preferable. In all these
observations that are made with respect to the precipitation effected by
means of different saline substances, attention must be paid at the same
time to the change of colour which ensues, in order to discover whether
the colour brightens, or entirely changes.

When the colour of a decoction is darkened by the above-mentioned
additions without becoming turbid, it shows that the colouring matter is
more concentrated and inspissated. When the colour is brightened, a
greater degree of solution and attenuation has taken place in the
colouring matter in consequence of the addition. If the colour becomes
clearer, and after a little time some of the tingeing substance is
separated, it shows that part of the colour is developed, but that
another part has been set loose from its combination by the saline
substance.

But if the colouring matter is separated in great abundance by the
saline addition, (the colour being brightened at the same time,) it may
be considered as a sign that the colouring substance is entirely
separated from the decoction, and that only an inconsiderable part, of a
gummy nature, remains behind united with the additaments, which is in a
very diluted state.--This is an effect of the solution of tin, as also
sometimes of the pure acids.

If, indeed, a portion of the colouring substance be separated by a
saline addition, but the rest of the colouring decocti