proofs bear a reference, by which the fixity and durability of the colour with which a stuff has been dyed may be 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 most difficult of all to dye perfectly.  Thus, if a piece of linen cloth be dipped into a solution of madder, it will come out just tinged with the colour; but if a piece of the same be previously dipped into a solution of alum or copperas, and dried previously to being dipped in the madder, the alum will become so far impregnated with the colouring principle, that