D Y I 
wifhed-for colour In perfection, we mud procure a mor¬ 
dant and a colouring matter of fuch a nature, that when 
combined together they (hall poffefs the wiflied-for colour 
in perfection. It is evident, too, that a great variety of 
colours may be produced with a fingle dye fluff, provided 
\ve can change the mordant fufficiently. 
Every thing which tends to weaken the affinity be¬ 
tween the mordant and the cloth, or between the mor¬ 
dant and the colouring matter, and every thing which 
tends in any way to alter the nature of the mordant, 
mud injure the permanency of the dye : becaufe, when¬ 
ever the mordant is dedroyed, there is no longer any 
thing to caufe the dye duff to adhere; and when its 
nature is altered, the colour of the dye mud alter at the 
fame time. All the obfervations, then, which can be 
made, concerning the nature of colouring matters, and 
the changes to which they are fubjedt, apply equally to 
mordants. Thefe fubdances, indeed, are f'carcely liable 
themfelves to any alteration. They are of a much more 
fimplc nature, in general, than dye-duffs; and therefore 
not nearly fo liable to decompofition. But when the 
colouring matter itfelf is altered, it comes to the fame 
thing : its affinity for the-mordant being now' dedroyed, 
there is nothing to retain it. As the permanency of a 
dye depends upon the degree of affinity between the 
mordant and the colouring matter, it is clear that a dye 
may want ■ permanency, even though it refid the oxy- 
muriatic acid, and all the other faline teds propofed by 
chemids. Thefe fubdances may happen to have very 
little aCtion on the dye duff, and therefore may not affeCt 
it; yet it may foon difappear, in confequence of its 
want of affinity for the mordant. 
A very ingenious abftradt of this theory is given by 
Berthollet: According to the attraction which the 
colouring particles have for wool, filk, cotton, and linen, 
they unite more or lefs readily, and more or lefs inti¬ 
mately with each of thofe fubdances ; and thence arifes 
the fird caufe of variation in the procedes employed, 
according to the nature of the duff, and of the colouring 
fubftance. By the attraction which the colouring par¬ 
ticles have for argillaceous earth and metallic calces, 
they form compounds with thefe fubdances, in which 
their colour is more or lefs modified, becomes more 
fixed, and more difficultly affeCted by external agents, 
than before. This compound being formed of principles, 
which have feparately the power of uniting with vege¬ 
table fubdances, and more efpecially with animal fub¬ 
dances, preferves this property, and forms a triple com¬ 
pound with the duff; and the colour which has been 
again modified by the formation of this triple union, 
acquires a greater degree of fixity, and-of indeftruCtibility, 
by external agents. The colouring panicles have fre¬ 
quently fo great an attraction for clay and metallic calces, 
that they feparate them from acids which held them in 
folution, and fall down with them ; but the attraction of 
the ftuff is fometimes neceffi-ry, in order to produce this 
feparation. The metallic calces which combine with 
the colouring particles, modify their colours, not only 
by their own, but alfo by adting upon their compofition 
by means of their vital air. The change which the 
colouring particles thereby fuffer, is fimilar to that 
occafioned by the air, which injures all colours more or 
lefs. Of the two principles which compofe the air of 
the atmofphere, it is only the vital air, or oxygenous 
gas, which acts upon the colouring particles : it com¬ 
bines with , them, weakens their colour, and renders it 
aler ; but its aCtion is principally exerted on the 
ydrogen, or inflammable air, which enters into their 
compofition, and it then forms water. This effect ought 
to be coplidered as a true combuftion, whereby the 
charcoal which enters into the compofition of the colour¬ 
ing particles becomes predominant, and the colour 
commonly changes to yellow, fawn colour, or brown ; 
of the injured part, by uniting with what remains of the 
original colour, produces other appearances. Light 
N G. I47 
favours the combination of the colouring particles, 
which frequently cannot take place without its aid; 
and it is thus that it contributes to the deftruCtion of 
colours. Heat promotes it alfo, but lefs powerfully than 
light, unlefs it has a certain degree of intenfity. To a 
fimilar combuftion are to be attributed the effects of the 
pale nitrous acid, of the oxygenated or aerated muriatic 
acid, and even' of the iulphuric acid, when they make 
the colour of the fubflances upon which they aCt pafs 
to a yellow, and even to a black. The efieCts of com¬ 
buftion may however be concealed by the vital air com¬ 
bining with the colouring particles, without adting par¬ 
ticularly on the inflammable air. Colours are therefore 
more or lefs permanent, or more or lefs fixed, according 
to the greater or lefs difpofition of the colouring par¬ 
ticles to undergo combuftion, and to allow it to go on 
to a more or lefs advanced ftage. Some fubflances are 
alfo capable of adting on the colour of (tuffs by a fupe- 
riority of attradtion, or by a folvent power; and in this 
confifts the action of acids, alkalis, and foap. A fmalL 
quantity of thefe agents, however, may fometimes form 
fupra-compounds with the ftuff, and thereby change its 
colour. Metallic calces produce, in the colouring par¬ 
ticles with which they unite, a degree of combuftion, 
proportioned to the quantity of vital air which can be 
taken from them by thefe particles. The colours which 
the compounds of metallic calces and colouring parti¬ 
cles aflume, then, are the product of the colour peculiar 
to the colouring particles, and of that peculiar to the 
metallic calx ; but the colouring particles and metallic 
calces mu ft be confidered in that ftate to which they 
have been reduced by the diminution of vital air in the 
calx, and the diminution of hydrogen in the colouring 
particles. 
Hence it follows : Firft, That the metallic calces to 
which the vital air is only (lightly attached, are not fit 
to ferve as connecting mediums for the colouring par¬ 
ticles, becaufe they produce in them too great a degree 
of combuftion ; fuch are the calces of filver, gold, and 
mercury. Second, That the calces which undergo con- 
fiderable changes of colour, by giving off more or lefs of 
their vital air, are alfo bad intermediums, efpecially for 
light (hades, becaufe they produce changeable colours ; 
fuch are the calces of copper, of lead, and of bifmuth. 
Third, That the calces which ftrongly retain their vital 
air, and fuffer very little change of colour by the lofs 
of a part of it, are beft fitted to anfvver this purpofe : 
fuch is particularly the calx of tin, which quits its 
menftnmm eafily, which has a ftrong attraction for the 
colouring particles, and which affords them a bails which 
is very white, and proper for giving a brightnefs to 
their (hades, without altering them by the admixture of 
another colour. The calx of zinc polfelfes fome of 
thefe qualities. 
In order to account for the colours which refult from 
the union of the colouring particles with the bails which 
a mordant gives them, we mull attend to the proportion 
in which the colouring particles unite to that bafis: 
thus the folution of tin, which produces a very copious, 
precipitate wuh a folution of colouring particles, and 
thereby proves that the calx of tin enters in a large 
proportion into the precipitate, has a much greater 
influence on the colour of the precipitate, by the white-, 
nefs of its bafis, than the folution of zinc or that of 
alum, which generally produce much lefs copious pre¬ 
cipitates. The precipitates produced by thefe two 1 aft 
fubflances retain very nearly the natural tint of the 
colouring particles. We mull then diftinguiffi, in the 
action of mordants, the combinations that may take 
place by their means between the colouring particles, 
the fluff, and the intermedium; the proportions of the 
colouring fnbflance and intermedium ; the modifications, 
of colour, which may arife from the mixture of the 
colour of the colouring particles, and of that of the 
bafis to which they are united; and finally, the changes 
3 which. 
