tried. Of these, some may be called natural,
other artificial. The natural proof consists in exposing the dyed stuff
to the air, sun, and rain. If the colour is not changed by this exposure
in twelve or fourteen days, it may be considered as genuine; but if it
is, the contrary is allowed. This proof, however, is not adapted to
every colour; because some of them resist it, and yet will fade in
consequence of the application of certain acids; others, on the
contrary, that can not resist the natural proof remain unchanged by the
latter. Colours, therefore, may be arranged in three classes; and to
each of these a particular kind of artificial proof allotted. The first
class is tried with alum, the second with soap, and the third with
tartar.

For the proof with alum: Half an ounce of this is dissolved in one pound
of boiling water in an earthenware vessel; into this is put, for
instance, a drachm of yarn or worsted, or a piece of cloth of about two
fingers breadth; this is suffered to boil for the space of five minutes,
and is then washed in clean water. In this manner are tried crimson,
scarlet, flesh-colour, violet, ponceau, peach-blossom colour, different
shades of blue, and other colours bordring upon these.

For the proof with soap: Two drachms of this substance are boiled in a
pint of water, and the small piece of dyed stuff that is to be tried is
put into it, and likewise suffered to boil for the space of five
minutes. With this all sorts of yellow, green, madder-red, cinnamon, and
similar colours, are tried.

In the same manner is made the proof with tartar; only this should be
previously pounded very small, in order that it may be more easily
dissolved. With this all colours bordering upon the fawn are tried.

From the above we discover that the art of applying and fixing colours
in dyeing depends on the chemical affinity between the cloth and the
dyeing principle: and accordingly as this is more or less strong, so is
the facility with which the substance is coloured, and on this the
deepness of the dye depends: for frequently one kind of cloth will be
found to receive no colour at all, whilst another will receive from the
same composition a deep tinge. Cotton, for instance, receives scarcely
any tinge from the same bath that will dye woollen a deep scarlet. Wool
is that which appears to have the strongest affinity to colouring
matter; next to it is silk; then linen; and cotton the weakest, and is
therefore the