DYEING. 
found to answer the purpose of a mordant 
less perfectly than alumen, on account of 
the colour, which is not so good. It is em- 
ployed either in the state of lime-water, or 
of that of sulphate of lime dissolved in wa- 
ter. 
Metallic oxides have so great an attrac- 
tion for many colouring substances, that 
they quit the acids in which they were dis- 
solved, and are precipitated in combination 
with them ; they have also the property of 
uniting with animal substances ; it is there- 
fore natural that they should serve as a bond 
of union between the colouring particles 
and animal substances ; but besides the at- 
traction of the oxides for colouring sub- 
stances, and for animal matter, their solu- 
tions in acids possess qualities which render 
them more or less fit to act as mordants : 
thus those oxides which easily part with 
tlieir acids, such as that of tin, are capable 
of combining with animal substances with- 
out the aid of colouring particles : it is suf- 
ficient to impregnate wool or silk with a 
solution of tin, although they be afterwards 
carefully washed, which is not the case with 
other metallic solutions. 
Some metallic substances afford in com- 
bination only a white and colourless basis ; 
and some by the admixture of their own co- 
lour modify that which is proper to the co- 
louring particles ; but in many metallic ox- 
ides, the colour varies according to the pro- 
portion of the oxygen they contain, and the 
quantity of this is easily liable to change. 
IJpcn these circumstances their properties 
in dyeing principally depend. 
The attraction of metallic oxides for sub- 
stances of vegetable origin is much weaker 
than for animal substances, and we are even 
ignorant whether they are capable of con- 
tracting a real union with them or not ; me- 
tallic solutions are therefore ill fitted to 
serve as mordants for colours in linen or 
cotton, except iron, the oxide of wliich 
unites firmly with vegetable substances, as 
is shewn by iron moulds, which are ovving 
to a real combination of this oxide. A¥hen 
the colouring particles have precipitated a 
metallic oxide from its menstruum, the su- 
pernatent liquor contains the disengaged 
acid, which is commonly capable of dissolv- 
ing a portion of the compound of colouring 
substance and oxide, so that the liquor re- 
mains, coloured ; but sometimes tlie whole 
of the colouring matter is precipitated, 
when tlie proportions have been accurately 
adjusted: this precipitation is facilitated, 
and rendered more complete by the pre- 
sence of the stuff, which assists by the ten- 
dency it has to unite with the compound of 
oxide and colouring matter. 
Uncombined metallic oxides have also a 
very evident action on many colouring sub- 
stances, when boiled with them, and mo- 
dify their colour ; the oxide of tin in parti- 
cular increases the brightness and fixidity 
of several. 
The compounds of oxides and colouring 
substances may be compared to many other 
chemical compounds, which are insoluble 
when the principles of which they are form- 
ed are properly proportioned-, but which 
are capable of being super-saturated by au 
excess of one of the principles, and thence 
of becoming soluble. Thus a metallic oxide 
united with a colouring substance in excess, 
will produce a liquor, the colour of w'hich 
will be modified by tlie oxide ; whereas, 
when the colouring matter is not in excess, 
the compound will be insoluble or nearly 
so : these effects are very evident in the 
combinaiion of iron with the astringent 
principle. 
Some other saline substances, as well as 
the metallic salts, are also employed as 
mordants. The neutral salts, sal ammoniac, 
nitre, and particularly sea-salt, act as mor- 
dants and modify colours, but it is difficult 
to ascertain the manner in which they act : 
salts with calcareous bases also modify co- 
lours ; but as these modifications are nearly 
similar to those which would be produced 
by the addition of a small quantity of lime, 
it is probable they are decomposed, and 
that a little of the lime enters into combi- 
nation with the colouring particles and the 
stuff. 
Astringent substances are often employ- 
ed as mordants. Tan, or the astringent 
principle, having a strong affinity for cloth, 
and for colouring substances, is found very 
useful for this purpose. It is commonly 
prepared by infusing nut galls in water ; tlie 
cloth is immersed in this solution, and al- 
lowed to remain till it is sufficiently im- 
pregnated with the tan. Sumach, which 
consists of the shoots of the thus coriaria (a 
shrub that grows in the southern parts of 
Europe), is often used and prepared in the 
same way as nut-galls. 
Animal substances are sometimes used as 
mordants; in the process for dyeing the 
Turkey red, the cotton stuff is impregnated 
with animal oil; and it is probable linen 
and cotton would take other colours better 
after some similar preparation. 
Exsiccation favours the union of the sub- 
