on becomes
not-withstanding darker, it shows that the rest of the colouring
particles have been more concentrated, and hence have acquired a greater
power of tingeing. With regard to the proportion of the addition, the
following circumstances may serve by way of guide:

When the colour of a decoction is darkened by the addition, without any
precipitate being produced, no detriment can easily arise from using a
redundancy of it, because the colour will not be further darkened by it.
But if the colour be required to be brighter, the trial must first be
made, which is the proportion by which the colour is darkened the most,
and then less of it must be employed.

When the colour of a decoction is brightened by an addition without a
precipitation ensuing, this addition can never be used in a larger
quantity without hurting the colouring particles; because the colouring
particles would be made too light, and almost entirely destroyed.--Such
is the consequence of too large an addition of the solution of tin or of
a pure acid.

When the addition produces a brighter colour, and part only of the
colouring substance is separated without a further addition occasioning
a fresh separation, somewhat more of it than what is wanted may be added
to produce the requisite shading; because experience shows that, by this
means, a greater quantity of tingeing particles is united with the
woolly fibres of the cloth, and is capable of being, as it were,
concentrated in them: for which purpose, however, these barks must be
boiled down. This effect is chiefly observed with sal ammoniac and wine
vinegar.

When by an addition which causes a separation of the colouring substance
the colour becomes brighter in proportion the more there is used of it,
it must be employed in a moderate quantity only; because otherwise, more
and more of the colouring substance will be separated, and its tingeing
power diminished. But when a colour is rendered dark at first by an
addition, and afterwards, upon more of the same substance being added,
becomes brighter, and this in proportion to the quantity that is added,
it will be found that the darkening power has its determined limits; and
that, for producing the requisite degree of darkness, neither too much
nor too little must be taken.

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To the before-mentioned principles also, the different proofs bear a
reference, by which the fixity and durability of the colour with which a
stuff has been dyed may be