DYEING. 
unites with a stuff, it undergoes new modi- 
fications. Thus, the properties of the co- 
louring particles of cochineal are modified, 
by being combined with the oxide of tin ; 
and those of the substance thence resulting 
are again modified by their union with the 
wool or silk ; and all the.se modifications are 
analogous to what is observed in other che- 
mical combinations. 
Of Mordants. 
The title of mordant is applied to those 
substances which serve as intermedes be- 
tween the colouring particles and the stuff 
to be dyed, either for the purpose of facili- 
tating, or of modifying their combination ; 
and by their means coloiu's are varied, 
bi'ightened, made to strike, and rendered 
more durable. 
Was it possible to procure a sufficient 
number of colouring matters having a strong 
affinity to cloth, to answer all the purposes 
of dyeing, that art would be exceedingly 
simple and easy. But except indigo, there 
is scarcely a dye stuff which yields of itself 
a good colour, sufficiently permanent to de- 
serve the name of a dye. This difficulty is 
obviated by employing an intermediate sub- 
stance, which has a strong affinity both for 
the stuff and the colouring matter, and this 
is tlie principal purpose for which the mor- 
dant is used. 
A mordant is not always a simple agent ; 
new combinations are sometimes formed 
by the ingredients which compose it ; so 
that the compounds resulting from the mix- 
ture, and not the simple substances that 
compose it, are tlie immediate agents, 
which produce the effect. 
Sometimes the mordant is mixed with 
the colouring particles, sometimes the stuff 
is impregnated with it, and on other occa- 
sions both those modes are united ; and fi- 
nally stuffs are dyed successively with li- 
quors containing different substances, the 
last of which only can act on that with 
which the stuff is impregnated. 
The principal substances employed as 
mordants are aluminous salts, lime, metallic 
oxides, some astringent substances, and ani- 
mal matters. 
Formerly sulphate of alumen w'as the 
only species used as a mordant in dyeing ; 
but of late years acetite of alumen has been 
introduced with excellent effect, particu- 
larly for cottons or linens, whose attraction 
to the alumen being weak, they require it 
to be applied, combined with a substance 
to which it has not so strong an union as it 
has to sulphuric acid ; and its attraction to 
acetous acid is found to be sufficiently infe- 
rior to that for the cotton or linen, that it 
readily quits the acetous acid to combine 
with them. 
Acetite of alumen is prepared by pour- 
ing acetite of lead into a solution of alum, 
in the proportion of one part of the acetite 
of lead to three of the alum in weight, a 
sixteenth of potash, and as much of pow- 
dered chalk are also added. In this mix- 
ture the sulphuric acid combines with the 
lead, and is precipitated; and the alumen 
or base of the alum, combines w'ith the ace- 
tous acid as it parts from the lead, and forms 
acetite of alumen ; the chalk and potash 
serve to saturate the excess of acid. 
The final effect of ahiinining, in whatever 
way performed, and whatever chemical 
changes may have taken place in it, consists 
in the combination of alumen with the stuff ; 
this union is probably imperfect, and the 
acids but partially separated at first, but 
becomes complete when the stuff is after- 
wards impregnated with the colouring sub- 
stance. 
The attraction of alumen for animal sub- 
stances may be shewn by direct experi- 
ment ; for if a solution of alum is mixed 
with a solution of glue, on adding an alkali, 
the glue is precipitated in combination with 
the alumen. 
The attraction of alumen for most colour- 
ing substances may also be proved by ex- 
periment. If a solution of a colouring sub- 
stance be mixed with a solution of alum, 
and an alkali be added, which decomposes 
the alum, the colouring matter wall be pre- 
cipitated combined with the alum, and the 
liquor will remain clear. The matter pre- 
cipitated is called a lake. In this experi- 
ment too much alkali must not be added, 
because alkalis are capable of dissolving 
most lakes. 
No direct experiment has yet slievra that 
alumen attracts any vegetable substances, 
except colouring matters : its attraction to 
them seems much w'eaker than that which 
it has for animal substances ; hence the ace- 
tite of alumen is a better mordant than 
alum, for linen and cotton, as has been ob- 
served ; and upon this depend the different 
means employed to increase the fixidity of 
the colouring particles in dyeing these sub- 
stances. 
Lime is the only earth besides alum which 
is employed in dyeing : the affinity of lime 
for cloth is sufficiently strong ; it is however 
