DYEING. 
stances which hive an attraction for the 
stutF, and the decompositions wliich may re- 
sult from that union ; but the exsiccation 
should be slow, that the substances may 
not be separated before their mutual at- 
tractions have produced their elfect. 
To judge of the most advantageous man- 
ner of employing mordants, v/e must first 
pay attention to the combinations which 
may be produced, either by the action of 
the substances which compose them, or by 
that of the colouring matter and the stuff ; 
secondly, to the circumstances which may 
concur in bringing about these combina- 
tions more or less quickly, or in rendering 
tliem more or less complete ; thirdly, to 
the action that the liquor in which tire 
stuff is immersed may have, either on its 
colour or texture ; and in order to foresee 
M'hat that may be, it is necessary to know 
the proportions of the principles wliich en- 
ter into the composition of the mordant, 
and what will be left in an uncombined 
state in the liquor. 
Of the action of air and light on colours. 
The action of atmospheric air on colours 
is chiefly owing to the oxygen it contains ; 
this Berthollet has shewn in some cases -to 
have similar effects to a slight combustion ; 
as when the air renders a substance yellow, 
fawn-coloured, or brown, which he sup- 
poses it does by the oxygen combining with 
die hydrogen of the stuff, and leaving the 
charcoal predominant, which then commu- 
nicates its own colour to it. The action of 
the air in bleaching he supposes to be 
caused by the combination of its oxygen 
with the colouring matter of the stuff, which 
renders it soluble in alkaline lixivia; which 
for this reason should always be used alter- 
nately, with exposure to the air. 
The changes which occur in the colours 
produced by the union of colouring matter 
with metallic oxides, are effects coniponnd- 
ed of the change that takes place in the co- 
louring matter, and of that which the me- 
tallic oxide undergoes. 
Tire light of the sun considerably accele- 
rates the alteration of colours; this, accord- 
ing to Berthollet, it effects by favouring 
the combination of oxygen, and by the 
combustion thereby produced. Mr. Sen- 
nebier (who Iras published many interesting 
observations on the effect of light on co- 
lours), on the contrary, attributes these ef- 
fects to a direct combination of light with 
the substances; but Berthollet established 
his opinion by a number of accurate ex- 
periments, which give it a decided supe* 
riority. 
Colouring substances resist the action of 
the air more or less, according as they ai'e 
more or less disposed to unite with oxygen, 
and thereby to suffer more or less quickly 
a smaller or greater degree of combustion : 
light favours this effect ; but the colouring 
matter, in its separate state, is much more 
prone to this combustion than when united 
to a substance, such as alumen, which may 
either defend it by its own power of resist- 
ing combustion, or, by attracting it strongly, 
weaken its action on other substances, 
which is the chief effect of mordants ■. and 
this compound acquires greater durability 
when it is capable of combining intimately 
with the stuff. Tlius the colouring matter 
of cochineal dissolves easily in water, and 
its colour is quickly changed by the air ; 
but when united to the oxide of tin, it be- 
comes much brighter and almost insoluble 
in water, though it is still easily affected 
by the air, and by oxygenated mar ine acid : 
it resists the action of these better however 
when it has formed a triple compound with 
a woollen stuff. 
Oxygen may unite in a small proportion 
with some colouring substances, without 
weakening their colour, or changing it to 
another: thus indigo, which becomes green 
by uniting with an alkali, with lime, or a 
metallic oxide, resumes its colour, and quits 
those substances, when it recovers a small 
portion of the oxygen which it bad lost. 
The liquor of the whelk, employed to dye 
purple, is naturally yellowish ; but when ex- 
posed to the air, and more especially in the 
sunshine, it quickly passes through various 
shades, and at length assumes that colour 
so precious in the eyes of the ancients. 
It may be considered as a general fact 
that colours become brighter by their union 
with a small portion of oxygen ; for this rea- 
son it is found necessary to air stuffs, when 
they come out of the bath, and sometimes 
even to take them out of it from time to 
time, expressly for this purpose ; but in 
some cases the quantity of oxygen, which 
thus becoming fixed, contributes to the 
brightness of the colour is very inconsider- 
able, and its deterioration soon commences. 
The action of the air affects not only the 
colouring matter and the stuffs, but also 
metallic oxide, when they are employed as 
intermedes, because the oxides are deprived 
at first by the colouring matter of part of 
their oxygen, and absorb it afterwards from 
the air. Those oxides then, whose colonr 
