DYEING. 
colour. Hellot published a methodical de- 
scription of tlie processes used in dyeing 
wool, which even now is the best treatise 
we have on the subject. Macquer has 
given an exact description of the processes 
employed in dyeing silk ; he has made us 
acquainted with the combinations of the 
colouring principle of Prussian blue ; he 
has endeavoured to make an application of 
it to the art of dyeing, and has given us a 
process for communicating the most bril- 
liant colours to silk by means of cochineal. 
He intended to publish a general treatise 
on the art of dyeing, of which he' gave the 
prospectus in 1782 ; but his death, which 
took place in 1784, prevented the execution 
of any part of the work. 
Berthollet succeeded Macquer ; his trea- 
tise on dyeing is one of the best epitomes 
on the subject ; and chemistry, and the arts 
of dyeing, and of bleaching, have been 
much indebted to his labours. 
In his theory of dyeing, he refers all the 
combinations produced in the formation 
of colours to the laws of chemical attrac- 
tion ; and all the changes, which the colour- 
ing particles undergo, to the conjunction 
of the elements of the new combination. 
The first effect of the attraction, he con- 
siders as analogous to the formation of neu- 
tral salts ; the second to combustion, pu- 
trefaction, and many other operations of 
nature. 
Besides the authors mentioned, Chaptal, 
D’Apligny, D’Ambourney, and Hauffman, 
in France, have published treatises on the 
art of dyeing, which have much contributed 
to its improvement. In Sweden, Scheffer 
alone has written on the subject; his work 
is accompanied with notes by the cele- 
brated Bergman. In Geiinany experiments 
in different processes of dyeing have been 
published by Beckmann, Poerner, Vogler, 
and Franchevilie. In England two very 
valuable essays on dyeing, by Delaval and 
by Henry, have appeared; to which may 
be added, the excellent treatise on the phi- 
losophy of permanent colours, by Dr. 
Bancroft. 
Of the Attractions of colouring Substances. 
A variety of theories have been produced 
by ingenious men to account for the effects 
of dyeing. Bergman seems to be the first 
who referred them entirely to chemical 
principles ; and this opinion is so conso- 
nant to reason, that it is now universally 
adopted. 
Dyeing then is to be considered merely 
as a chemical process : but in order that 
it may succeed, it is necessary that the co- 
louring matters should be dissolved in some 
fluid, (for in their solid state no attraction 
takes place between them and the stuff,) 
and that their attraction to the fluid should 
be less than that to the stuff. Besides the 
colouring matters being brought within the 
proper distance for attraction by this means, 
they are also caused to apply themselves 
more equally, as every part of the stuff has 
thus an opportunity of attracting to itself 
the proper quantity of colouring matter. 
Tlie stuff receives the dye better in pro- 
portion to the degree of affinity whicli the 
colouring matter has to it and to the solvent 
relatively, for if its attraction to the stuff 
is much more than to the solvent, the staff 
receives the dye too rapidly, and it will be 
scarcely possible to prevent its being un- 
equal : but if, on the other hand, its at- 
traction to the solvent is too great, the stuff 
will either not take the dye at all, or it will 
take it very slowly and faintly. Wool has 
a stronger attraction for colouring matters 
than silk, silk than cotton, and this latter 
a stronger than linen. Hence it is neces- 
sary to use solvents for the dyes of the stuffs 
last mentioned, which have a weaker affi- 
nity for them than for those used in dyeing 
wool. 
The essential circumstances in dyeing 
are to ascertain the affinities of the colour- 
ing substance ; first, to the solvents ; se- 
condly, to those substances which modify 
its colour, increase its brilliancy, and 
strengthen its union with the stuff ; thirdly, 
to the different agents, which may, change 
the colour, and principally to air and 
light. 
The colouring matters possess chemical 
properties, that distinguish them from all 
others; they have attractions peculiar to 
themselves, by means of which they unite 
with acids, alkalies, metallic oxides, and 
some earths, particularly alumen. They 
frequently precipitate oxides and alumine 
from the acids, which held them in solu- 
tion ; at other times they unite with salts, 
and form supra-compounds, which combine 
with the wool, silk, cotton, or linen : and 
with these their union is rendered much 
more close by means of alumine, or a me- 
tallic oxide, than it would be without their 
intervention. 
The qualities of the uncombined colour- 
ing particles are modified when they unite 
with any substance ; and if this compound 
