DYEING. 
varies in proportion to their quantity of 
oxygen, cause changes of colour in the stuff 
in this'inanner. 
Thus the blue given to wool, by sulphate 
of copper and logwood, soon changes into a 
green by-the action of the air ; because the 
copper, which is blue, when combined with 
a small portion of oxygen, becomes green, 
by its union with a larger quantity. 
Colouring matter, in a state of combina- 
tion with most substances, is less liable to 
be changed by the air, than when uncom- 
bined ; but there are some exceptions ; for 
alkalies produce a contrary effect : they 
darken, the colours to which they are added, 
and are found - by experiment to promote 
the absorption of air, and in proportion as 
this takes place, the colours in' general be- 
come more and more brown. This is con- 
sonant to the effect they produce on other 
substances, such as sulphur, for they favour 
the absorption of air, because they have a 
strong attraction for the substance which is 
the result of that absorption. 
Of the differences between Wool, Silk, Cot- 
ton, and Linen, and the operations by 
which they are prepared for Dyeing. 
Wool and silk are animal substances, 
cotton and linen are vegetable productions. 
Animal substances have a greater disposi- 
tion to combine with other substances, than 
those of vegetable origin ; hence they are 
more liable to be destroyed by different 
agents, and are more disposed to combine 
with colouring particles. Bertliollet ac- 
counts for these properties by their princi- 
ples being more disposed to assume a gase- 
ous form, and having less cohesive force 
among themselves. Thus the pure, or caus- 
tic alkalies destroy animal substances, be- 
cause they combine with them, and thereby 
lose their causticity. For this cause ani- 
mal substances cannot bear leys, and alka- 
lies should be used with great caution in -the 
processes for dyeing them ; whereas no dan- 
ger is to be apprehended from the use of 
alkalies with vegetable substances. Nitric 
and sulphuric acids have also considerable 
action on animal substances. 
Silk appears to bear some resemblance to 
vegetable substances, by being less dis- 
posed to combme with colouring particles, 
and by resisting the action of alkalies and 
acids more powerfully ; but thougli the ac- 
tion of these substances on sUk is weaker 
than upon wool, they should still be em- 
ployed with great caution, because the 
brightness of colour in silk appears to de- 
pend upon the smoothness of its surface, 
which siiould therefore be preserved unim- 
paired. 
Cotton withstands the action of acids 
better than dax or hemp, and is difficultly 
destroyed even by the nitric acid. 
Of Wool. 
Wool is naturally covered with a kind of 
grease called suint, which preserves it from 
moths, so that it is not scoured until it is 
about to be dyed or spun. In order to 
scour wool it is put for about a quarter of 
an hour into a kettle containing a sufficient 
quantity of water, mixed witli a fourth part 
of putrid urine, - heated to such a degree as 
the hand can just bear, and it is stirred 
from time to time with sticks ; it is then ta- 
ken out and put to drain. It is next carried 
in a large basket to a stream of running 
water, where it is moved about until the 
grease is entirely separated, and no longer 
renders the water turbid ; it is then taken 
out, and again left to drain. It sometimes 
loses in this operation more than a fifth of 
its weight. The scouring should be care- 
fully performed, because the wool is tliere- 
by better fitted to receive the dye. In this 
process the volatile alkali of the mane unites 
with tlie grease, and forms a Idnd of soap 
soluble in water. 
When wool is dyed in the fleece, its fila- 
ments being separate absorb a larger quan- 
tity of the colouring mattery for the same 
reason woollen yarn takes up more than 
cloth, but cloths themselves vary consider- 
ably in this respect, according to their de- 
gree of fineness, or the closeness of their 
texture. The wool dyed in the fleece is 
chiefly intended to form cloths of mixed 
colours. 
For most colours wool requires to be pre- 
pared by being boiled with saline sub- 
stances, principally with alum and tartar. 
For some dyes wool does not require this 
preparation, and then it must be well 
washed in warm water, and wrung out, or 
l.-ft to drain. 
The asperity of the surface of the fila- 
ments of wool, and their disposition to 
acquire a progressive motion towards their 
roots, forms an obstacle to the spinning 
of wool, which is removed by impregnating 
it with oil. This oil must be discharged 
previous to the stuff, formed of tlie wool, 
being dyed. For. this purpose it is carried 
to the fulling mill, where it is beaten witli 
large beetles in a trough of water, in which 
