Fig. 3.
has a bamboo-like, or jointed appearance; its tubes are not flattened, nor are they twisted. Flax belongs to a class called the bast fibres, a name given to certain fibres obtained from the inner bark of different plants. Jute also is a bast fibre. The finer qualities of it look like flax, but, as we shall see, it is not chemically identical with cotton, as linen or flax is. Another vegetable fibre, termed "cotton-silk," from its beautiful, lustrous, silky appearance, has excited some attention, because it grows freely in the German colony called the Camaroons, and also on the Gold Coast. This fibre, under the microscope, differs entirely in appearance from both cotton and flax fibres. Its fibres resemble straight and thin, smooth, transparent, almost glassy tubes, with large axial bores; in fact, if wetted in water you can see the water and air bubbles in the tubes under the microscope. A more detailed account of "cotton-silk" appears in a paper read by me before the Society of Chemical Industry in 1886 (see J.S.C.I., 1886, vol. v. p. 642). Now the substance of the cotton, linen or flax, as well as that of the cotton-silk fibres, is termed, chemically, cellulose. Raw cotton consists of cellulose with about 5 per cent. of impurities. This cellulose is a chemical compound of carbon, hydrogen, and oxygen, and, according to the relative proportions of these constituents, it has had the chemical formula C6H10O5 assigned to it. Each letter stands for an atom of each constituent named, and the numerals tell us the number of the constituent atoms in the whole compound atom of cellulose. This cellulose is closely allied in composition to starch, dextrin, and a form of sugar called glucose. It is possible to convert cotton rags into this form of sugar—glucose—by treating first with strong vitriol or sulphuric acid, and then boiling with dilute acid for a long time. Before we leave these vegetable or cellulose fibres, I will give you a means of testing them, so as to enable you to distinguish them broadly from the animal fibres, amongst which are silk, wool, fur, and hair. A good general test to distinguish a vegetable and an animal fibre is the following, which is known as Molisch's test: To a very small quantity, about 0·01 gram, of the well-washed cotton fibre, 1 c.c. of water is added, then two to three drops of a 15 to 20 per cent. solution of [Greek: alpha]-naphthol in alcohol, and finally an excess of concentrated sulphuric acid; on agitating, a deep violet colour is developed. By using thymol in place of the [Greek: alpha]-naphthol, a red or scarlet colour is produced. If the fibre were one of an animal nature, merely a yellow or greenish-yellow coloured solution would result. I told you, however, that jute is not chemically identical with cotton and linen. The substance of its fibre has been termed "bastose" by Cross and Bevan, who have investigated it. It is not identical with ordinary cellulose, for if we take a little of the jute, soak it in dilute acid, then in chloride of lime or hypochlorite of soda, and finally pass it through a bath of sulphite of soda, a beautiful crimson colour develops upon it, not developed in the case 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.
[1] J.S.C.I. = Journal of the Society of Chemical Industry.
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 clear solution is obtained, and then on adding tannic acid the fibroïn is precipitated. Strong caustic potash or soda dissolves silk; more easily if warm. Dilute caustic alkalis, if sufficiently dilute, will dissolve off the sericin and leave the inner fibre of fibroïn; but they are not so good for ungumming silk as soap solutions are, as the fibre after treatment with them is deficient in whiteness and brilliancy. Silk dissolves completely in hot basic zinc chloride solution, and also in an alkaline solution of copper and glycerin, which solutions do not dissolve vegetable fibres or wool. Chlorine and bleaching-powder solutions soon attack and destroy silk, and so another and milder agent, namely, sulphurous acid, is used to bleach this fibre. Silk is easily dyed by the aniline and coal-tar colours, and with beautiful effect, but it has little attraction for the mineral colours.
Wool.—Next to silk as an animal fibre we come to wool and different varieties of fur and hair covering certain classes of animals, such as sheep, goats, rabbits, and hares. Generally, and without going at all deeply into the subject, we may say that wool differs from fur and hair, of which we may regard it as a variety, by being usually more elastic, flexible, and curly, and because it possesses certain features of surface structure which confer upon it the property of being more easily matted together than fur and hair are. We must first shortly consider the manner of growth of hair without spending too much time on this part of the subject. The accompanying figure (see Fig. 5) shows a section of the skin with a hair or wool fibre rooted in it. Here we may see that the ground work, if we may so term it, is four-fold in structure. Proceeding downwards, we have—(first) the outer skin, scarf-skin or cuticle; (second) a second layer or skin called the rete mucosum, forming the epidermis; (third) papillary layer; (fourth) the corium layer, forming the dermis. The peculiar, globular, cellular masses below in the corium are called adipose cells, and these throw off perspiration or moisture, which is carried away to the surface by the glands shown (called sudoriparous glands), which, as is seen, pass independently off to the surface. Other glands terminate under the skin in the hair follicles, which follicles or hair sockets contain or enclose the hair roots. These glands terminating in the hair follicles secrete an oily substance, which bathes and lubricates as well as nourishes the hair. With respect to the origin of the hair or wool fibre, this is formed inside the follicle by the exuding therefrom of a plastic liquid or lymph; this latter gradually becomes granular, and is then formed into cells, which, as the growth proceeds, are elongated into fibres, which form the central portion of the hair. Just as with the trunk of a tree, we have an outer dense portion, the bark, an inner less dense and more cellular layer, and an inmost