To distinguish the difference between linen, cotton, wool, and .silk, examine the fibres under the microscope with a moderately low power. It will be found that the linen or flax fibres consist of transparent tubes, sometimes marked with lines and having very small central canals (see A in the illustration). The cotton fibres consist of straight or twisted flattened tubes with very large central canals and quite transparent (see B). The wool fibres are very variable, but consist of a number of plates or scales built up to form a tube, and the inner tube is usually more or less coloured in the natural wool (see C ). The silk fibre is usually very small and perfectly smooth (see D). The action of chemical agents upon the fibres depends upon their composition. Flax and cotton are nearly pure cellulose. By the action of moderately strong acids, the fibre is somewhat attacked, and the result is a parchment-like product: by long-continued action of strong sulphuric acid, cellulose is converted into dextrine, and by dilution with water and boiling it finally becomes glucose (a kind of sugar). Strong nitric acid converts cotton into nitro-cellulose or gun-cotton. Weak alkalies do not affect cotton or flax; strong alkalies toughen the fibre and shrink it, forming mercerised cotton.
Wool fibre has a composition similar to skin, horns, and feathers, and is composed of nitrogenous material called keratin, but contains sulphur also. Dilute acids do not affect wool; strong nitric acid and other acids destroy it, the former first rendering it yellow. Alkalies render wool very tender; strong alkalies used hot dissolve wool completely. Silk contains fibroin, gelatine, wax, albumin, etc. Concentrated acids destroy silk, but dilute acids do not affect it much; simply boiling with water removes the gelatine or sericin, which amounts to about 20 per cent. Weak alkalies impair the silk, and strong alkalies easily dissolve the silk entirely.
Magnified Fibres of Linen, Cotton, Wool, and Silk.