英文科學讀本 第五冊·Lesson 58 Coverings Of Mammals—Wool

Lesson 58 Coverings Of Mammals—Wool

The woolly fleece of the sheep is another modification of the common covering—hair. It has been used for making textile fabrics from very early ages, and for this purpose it is obtained from the animal, not only after its death, but also by shearing during its lifetime.

If we examine a handful of wool, we shall find it more or less wavy in appearance, while at the same time it is light and flexible, softer than ordinary hair, and very elastic.

Our lesson on heat showed us wool as a non-conductor. We place our hand on some wool, and we at once feel a sensation of warmth. Why does the wool feel warm? It is not because this substance is warmer in itself than any other we may handle. The thermometer, as you are aware, would register exactly the same temperature wrapped in wool as in a cold-feeling linen handkerchief. But the wool, being a non-conductor, does not rob the hand of its heat as conducting bodies do, and for this reason it feels warm, to the touch.

The thick, yet light fleece of non-conducting wool is Nature's admirable provision for this one of her creatures— the sheep, which is destined for the colder parts of the temperate zones, where all the year round, in all weather, it spends night and day in the open air, its favorite haunts, even there, being the bleak and breezy uplands of the country.

If a fiber of wool is examined with the aid of a good microscope, rows of scales will be seen covering it, and overlapping one another, something like the scales of a snake's skin, except that the scales themselves project outwards, and thus give the fiber a more or less serrated appearance. This scaly nature of its fibers is one of the most important characteristics of wool. It is one, moreover, which is not met with in any other textile material; the fibers of flax and cotton are smooth.

Even wool itself varies in the nature and abundance of its scales, and it is this which constitutes the different equalities of the material.

In a specimen of Saxony or fine Australian wool, the fibers are always found to be rather short, and very curly or wavy in character. This class of wool is known as short-staple. If one of its fibers were examined under the microscope, it would show a profusion of scales. In some of the finest kinds there are no less than 2500 to the inch, and the fibers have a very serrated or saw-like appearance.

In the English wool the fibers are longer, coarser, and less wavy. It is generally described as long-staple wool. A fiber of this long-staple wool would show under the microscope a much smaller number of scales and serrations than the short-staple variety.

The word staple refers to the individual fibers of the wool. Hence long or short staple means simply long or short fiber.

Its properties as a soft, light, flexible, non-conducting substance make wool a valuable material for clothing purposes. It is made into a great variety of fabrics, but the particular kind of fabric depends entirely upon the profusion, or otherwise, of its scales.

Think for a moment of those highly serrated fibers of the short-staple wool, with their 2500 scales to the inch. Imagine a number of such fibers crossing each other in opposite directions, and pressed close together. What would happen?

The projecting scales would catch one in the other, and hold fast. Take a piece of flannel and a piece of broadcloth, and examine them side by side. Both are made of wool. The flannel may be easily separated, thread by thread, and fiber by fiber; the cloth is a closely-matted felt, which it is very difficult to pull asunder.

This difference is entirely due to the scarcity of scaly projections in the fibers of the flannel material, and to their profusion in the material of the cloth.

It is this one difference which causes wool to yield to the manufacturer two distinct classes of products. The long-staple wool, possessing few projecting scales, yields flannels, blankets, moreens, merinos, alpacas, serges, and all kinds of worsted goods. The short-staple wool, with its abundance of projecting scales, is admirably adapted to felting purposes, and is made into broadcloth, kerseymere, and a great variety of materials, known under the collective name of woollen goods.