The Chemistry of Hat Manufacturing | Page 3

of cellulose (cotton, linen, etc.). It is certain
that it is a kind of cellulose, but still not identical with true cellulose.
All animal fibres, when burnt, emit a peculiar empyreumatic odour
resembling that from burnt feathers, an odour which no vegetable fibre
under like circumstances emits. Hence a good test is to burn a piece of
the fibre in a lamp flame, and notice the odour. All vegetable fibres are
easily tendered, or rendered rotten, by the action of even dilute mineral
acids; with the additional action of steam, the effect is much more rapid,
as also if the fibre is allowed to dry with the acid upon or in it. Animal
fibres are not nearly so sensitive under these conditions. But whereas
caustic alkalis have not much effect on vegetable fibres, if kept out of
contact with the air, the animal fibres are very quickly attacked.
Superheated steam alone has but little effect on cotton or vegetable
fibres, but it would fuse or melt wool. Based on these differences,
methods have been devised and patented for treating mixed woollen
and cotton tissues--(1) with hydrochloric acid gas, or moistening with
dilute hydrochloric acid and steaming, to remove all the cotton fibre; or
(2) with a jet of superheated steam, under a pressure of 5 atmospheres
(75 lb. per square inch), when the woollen fibre is simply melted out of
the tissue, and sinks to the bottom of the vessel, a vegetable tissue
remaining (Heddebault). If we write on paper with dilute sulphuric acid,
and dry and then heat the place written upon, the cellulose is destroyed
and charred, and we get black writing produced. The principle involved
is the same as in the separation of cotton from mixed woollen and
cotton goods by means of sulphuric acid or vitriol. The fabric
containing cotton, or let us say cellulose particles, is treated with dilute
vitriol, pressed or squeezed, and then roughly dried. That cellulose then
becomes mere dust, and is simply beaten out of the intact woollen
texture. The cellulose is, in a pure state, a white powder, of specific
gravity 1·5, i.e. one and a half times as heavy as water, and is quite

insoluble in such solvents as water, alcohol, ether; but it does dissolve
in a solution of hydrated oxide of copper in ammonia. On adding acids
to the cupric-ammonium solution, the cellulose is reprecipitated in the
form of a gelatinous mass. Cotton and linen are scarcely dissolved at all
by a solution of basic zinc chloride.
[Footnote 1: J.S.C.I. = Journal of the Society of Chemical Industry.]
[Illustration: FIG. 1.]
[Illustration: FIG. 2.]
[Illustration: FIG. 3.]
[Illustration: FIG. 4.]
Silk.--We now pass on to the animal fibres, and of these we must first
consider silk. This is one of the most perfect substances for use in the
textile arts. A silk fibre may be considered as a kind of rod of solidified
flexible gum, secreted in and exuded from glands placed on the side of
the body of the silk-worm. In Fig. 4 are shown the forms of the silk
fibre, in which there are no central cavities or axial bores as in cotton
and flax, and no signs of any cellular structure or external markings,
but a comparatively smooth, glassy surface. There is, however, a
longitudinal groove of more or less depth. The fibre is semi-transparent,
the beautiful pearly lustre being due to the smoothness of the outer
layer and its reflection of the light. In the silk fibre there are two
distinct parts: first, the central portion, or, as we may regard it, the true
fibre, chemically termed fibroïn; and secondly, an envelope composed
of a substance or substances, chemically termed sericin, and often
"silk-glue" or "silk-gum." Both the latter and fibroïn are composed of
carbon, hydrogen, nitrogen, and oxygen. Here there is thus one element
more than in the vegetable fibres previously referred to, namely,
nitrogen; and this nitrogen is contained in all the animal fibres. The
outer envelope of silk-glue or sericin can be dissolved off the inner
fibroïn fibre by means of hot water, or warm water with a little soap.
Warm dilute (that is, weak) acids, such as sulphuric acid, etc., also
dissolve this silk-glue, and can be used like soap solutions for

ungumming silk. Dilute nitric acid only slightly attacks silk, and
colours it yellow; it would not so colour vegetable fibres, and this
forms a good test to distinguish silk from a vegetable fibre. Cold strong
acetic acid, so-called glacial acetic acid, removes the yellowish
colouring matter from raw silk without dissolving the sericin or
silk-gum. By heating under pressure with acetic acid, however, silk is
completely dissolved. Silk is also dissolved by strong sulphuric acid,
forming a brown thick liquid. If we add water to this thick liquid, a