4.4.6 
BLEACHING. 
the fibre. It is exposed to the gas for 12 to 24| by distillation after neutralizing the muriatic 
hours, to the liquid acid 24 to 48 hours. 
harshness derived from sulphuring is removed by 
soaking the wool in a warm and weak bath of 
potash-soap, wringing, and drying. To impart a 
more brilliant white, the bleached wool is some- 
times dipped into chalk-water. 
Wool is sometimes bleached in the fleece, and 
sometimes in the yarn. Both yarn and cloth are 
cleansed in a manner similar to the above by al- 
kali or soap, in order to remove grease and glu- 
tinous matter derived during spinning and weay- 
ing. Cloths may be cleansed in the wash-stocks, 
with fuller’s earth, or a soap containing it. 
Suk.—The bleaching of silk is of a simple na- 
ture. This fibre is coated with a gelatinous sub- 
stance, a little wax, oil, and generally a yellow 
colouring matter ; and is therefore essentially dif- 
ferent from gum, as it is usually called. The 
gluey material, often constituting 24 per cent. of 
the weight of the silk, is soluble in water, scarcely 
in boiling alcohol, precipitable by tincture of 
galls, putrefies like animal matter, and its odour, 
when heated, indicates the presence of nitrogen. 
The yellow colouring substance, constituting 
nearly 2 per cent. of the silk, is of a resinous 
character, insoluble in water, very soluble in al- 
cohol, scarcely soluble in cold, more in warm 
alkali, and still more in a hot solution of soap, is 
decolourized by chlorine, sulphurous acid, and in 
a few days by the atmosphere. 
The coating imparts a stiffness and elasticity 
to silk, which must be removed in order to impart 
to it its requisite softness and lustre. Although 
alkali has been proposed and employed to remove 
the coating, it is found that scouring by soap, 
used long since, cannot be well superseded. From 
30 to 40 lbs. of soap to 100 lbs. of silk are usually 
required. The greater part of the soap is dis- 
solved in boiling water, and when cooled a little, 
the silk is hung over and partly immersed in the 
solution, which is maintained below a boiling 
heat, until the immersed part has attained its 
requisite whiteness and softness, when the other 
portion is next introduced in like manner. After 
being wrung out it is next put into bags, which 
| are introduced into a weaker solution of soap, in 
which they are boiled for an hour or more. For 
silks designed to receive dark dyes 20 to 25 lbs. 
soap will be sufficient. 
Silks designed to be of a pure white are sub- 
jected to the action of sulphurous acid by expos- 
ing them in a sulphur chamber to the fumes of 
burning sulphur, while still moist with soap- 
water. After sulphuring, they are washed, and 
passed through a warm soap-bath, in order to re- 
move the sulphurous odour and restore their pli- 
ancy. Silks designed for blonds, gauzes, &c., are 
merely stripped of their yellowish colour by steep- 
ing them for two days in alcohol with sz as much 
muriatic acid; a process proposed by Beaume, 
which can be conducted economically, since a 
large proportion of the alcohol may be recovered 
The ; acid by chalk or limestone. 
Alcohol removes 
colour, wax, &c., without touching the gelatine. 
Straw—A careful culture insures a requisite 
degree of fineness and firmness in the material, 
but for most purposes the colour must be dimin- 
ished or removed. This may be done by chlorine, 
sulphurous acid, alkali, or atmospheric agents, 
but a violent process injures the fibrous texture. 
It may be steeped in pure fresh water for several 
weeks, exposed to the air, and then sulphured. 
According to Kurrer, it maybe perfectly whitened 
by repeated steeping in boiling water and very 
weak alkali, which removes all soluble matter, 
then treated alternately with very dilute solu- 
tions of chloride of lime and sulphurous acid 
vapour, finally washed and dried in the sun. The 
process is tedious, but is said to remove the 
varnish which makes the natural straw brit- 
tle, and to render the fibre brilliant, white, and 
pliant. 
It is even more difficult to explain the bleach- 
ing process by means of sulphurous acid than that 
by chlorine. It is generally assumed that the 
acid combines directly with the colour, without | 
either giving or receiving oxygen, and forms a 
colourless or slightly coloured compound with 
it; for by the action of alkali or a stronger acid, 
the original colour is restored ; and hence, also, 
the colour reappears on sulphured goods in the 
lapse of time by the gradual dissipation of sul- 
phurous acid. The action of alkali in the above 
operations with wool, silk, and straw, depends 
simply on the solubility of the colouring or other 
matters in the alkaline solution. 
IV. DiscHaraes. 
There are many operations practised by the 
calico-printer, which may be termed bleaching, 
although they consist only in a topical removal 
or discharge of colour, Thus chromic acid, or 
bichromate of potassa, or the chlorides of tin are 
employed to discharge colours, but their action 
has not been minutely studied. It is probable 
that chromic acid yields up oxygen, destroying 
the colour by oxidation, and that the tin-bases 
form with the colouring matters compounds pos- 
sessing little or no colour. But their action is 
different on different colours, and the protoxide 
of tin decomposes some colours by abstracting 
oxygen. In short, wherever we destroy colour, 
with the view of obtaining a white basis or 
ground, it is, strictly speaking, a bleaching oper- 
ation. Thus a liquid may be decolourized by 
passing it through animal black, which absorbs 
the colouring matter. Some oily substances may 
be whitened by sulphuric acid, and some again 
by nitric acid, the former carbonizing, the latter 
oxidizing the colouring substance. 
BLEACHING POWDER. The most impor- 
tant of the bleaching salts. Its manufacture de- 
pends on the absorption of chlorine gas by slacked 
lime, and consists: 1st, of the preparation of 
