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