4. Flat fibres, as cotton, and those obtained by the action of caustic ley upon wood.
5. Imperfect substances, as the pulp obtained from sawdust. In this class may also be included the fibre of the so-called "mechanical wood pulp."
Determination of Cellulose. For the determination of cellulose in wood and other vegetable fibres to be used in paper-making Müller recommends the following processes:[5] 5 grammes weight of the finely-divided substance is boiled four or five times in water, using 100 cubic centimètres[6] each time. The residue is then dried at 100° C. (212° Fahr.), weighed, and exhausted with a mixture of equal measures of benzine and strong alcohol, to remove fat, wax, resin, &c. The residue is again dried and boiled several times in water, to every 100 c.c. of which 1 c.c. of strong ammonia has been added. This treatment removes colouring matter and pectous[7] substances. The residue is further bruised in a mortar if necessary, and is then treated in a closed bottle with 250 c.c. of water, and 20 c.c. of bromine water containing 4 c.c. of bromine to the litre.[8] In the case of the purer bark-fibres, such as flax and hemp, the yellow colour of the liquid only slowly disappears, but with straw and woods decolorisation occurs in a few minutes, and when this takes place more bromine water is added, this being repeated until the yellow colour remains, and bromine can be detected in the liquid after twelve hours. The liquid is then filtered, and the residue washed with water and heated to boiling with a litre of water containing 5 c.c. of strong ammonia. The liquid and tissue are usually coloured brown by this treatment. The undissolved matter is filtered off, washed, and again treated with bromine water. When the action seems complete the residue is again heated with ammoniacal water. This second treatment is sufficient with the purer fibres, but the operation must be repeated as often as the residue imparts a brownish tint to the alkaline liquid. The cellulose is thus obtained as a pure white body; it is washed with water, and then with boiling alcohol, after which it may be dried at 100° C. (212° Fahr.) and weighed.
Recognition of Vegetable Fibres by the Microscope.—From Mr. Allen's admirable and useful work on "Commercial Organic Analysis"[9] we make the following extracts, but must refer the reader to the work named for fuller information upon this important consideration of the subject. In examining fibres under the microscope, it is recommended that the tissues should be cut up with sharp scissors, placed on a glass slide, moistened with water, and covered with a piece of thin glass. Under these conditions:—
Filaments of Cotton appear as transparent tubes, flattened and twisted round their axes, and tapering off to a closed point at each end. A section of the filament somewhat resembles the figure 8, the tube, originally cylindrical, having collapsed most in the middle, forming semi-tubes on each side, which give the fibre, when viewed in certain lights, the appearance of a flat ribbon, with the hem of the border at each edge. The twisted, or corkscrew form of the dried filament of cotton distinguishes it from all other vegetable fibres, and is characteristic of the matured pod, M. Bauer having found that the fibres of the unripe seed are simply untwisted cylindrical tubes, which never twist afterwards if separated from the plant. The matured fibres always collapse in the middle as described, and undergo no change in this respect when passing through all the various operations to which cotton is subject, from spinning to its conversion into pulp for paper-making.
Linen, or Flax Fibre, under the microscope, appears as hollow tubes, open at both ends, the fibres being smooth, and the inner tube very narrow, and joints, or septa,[10] appear at intervals, but are not furnished with hairy appendages as is the case with hemp. When flax fibre is immersed in a boiling solution of equal parts of caustic potash and water for about a minute, then removed and pressed between folds of filter-paper, it assumes a dark yellow colour, whilst cotton under the same treatment remains white or becomes very bright yellow. When flax, or a tissue made from it, is immersed in oil, and then well pressed to remove excess of the liquid, it remains translucent, while cotton, under the same conditions, becomes opaque.
New Zealand Flax (Phormium tenax) may be distinguished from ordinary flax or hemp by a reddish colour produced on immersing it first in a strong chlorine water, and then in ammonia. In machine-dressed New Zealand flax the bundles are translucent and irregularly covered with tissue; spiral fibres can be detected in the bundles, but less numerous than in Sizal. In Maori-prepared phormium the bundles are almost wholly free from tissue, while there are no spiral fibres.
Hemp Fibre resembles flax, and exhibits small hairy appendages at the joints. In Manilla hemp the bundles are oval, nearly opaque, and surrounded by a considerable quantity of dried-up cellular tissue composed of rectangular cells. The bundles are smooth, very few detached ultimate fibres are seen, and no spiral tissue.
Sizal, or Sisal Hemp (Agave Americana), forms oval fibrous bundles surrounded by cellular tissue, a few smooth ultimate fibres projecting from the bundles; is more translucent than Manilla, and a large quantity of spiral fibres are mixed up in the bundles.