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Jean Henri Fabre


T HE culture of the silkworm having been explained by Uncle Paul in one of his previous talks, he now confined himself chiefly to the structure of the cocoon and the unwinding of the delicate silk thread composing it.

"The cocoon of the silkworm," he began, "is composed of two envelops: an outer one of very coarse gauze, and an inner one of very fine fabric. This latter is the cocoon properly so called, and from it alone is obtained the silk thread so highly valued in manufacture and commerce, whereas the other, owing to its irregular structure, cannot be unwound and furnishes only an inferior grade of silk suitable for carding.

"The outer envelop is fastened by some of its threads to the little twigs amid which the worm has taken its position, and forms merely a sort of scaffolding or openwork hammock wherein the worm seeks seclusion and establishes itself for the serious and delicate task of spinning its inner envelop. When, accordingly, the hammock is ready the worm fixes its hind feet in the threads and proceeds to raise and bend its body, carrying its head from one side to the other and emitting from its spinneret as it does so a tiny thread which, by its sticky quality, immediately adheres to the points touched. Without change of position the caterpillar thus lays one thickness of its web over that portion of the enclosure which it faces. Then it turns to another part and carpets that in the same manner. After the entire enclosure has thus been lined, other layers are added, to the number of five or six or even more. In fact, the process goes on until the store of silk-making material is exhausted and the thickness of the wall is sufficient for the security of the future chrysalis.

"From the way the caterpillar works you will see that the thread of silk is not wound in circles, as it is in a ball of cotton, but is arranged in a series of zigzags, back and forth, and to right and left. Yet in spite of these abrupt changes in direction and notwithstanding the length of the thread—from three hundred to five hundred meters—there is never any break in its continuity. The silkworm gives it forth uninterruptedly without suspending for a moment the work of its spinneret until the cocoon is finished. This cocoon has an average weight of a decigram and a half, and it would take only fifteen or twenty kilograms of the silk thread to extend ten thousand leagues, or once around the earth.

"Examined under the microscope, the thread is seen to be an exceedingly fine tube, flattened and with an irregular surface, and composed of three distinct concentric layers, of which the innermost one is pure silk. Over this is laid a varnish that resists the action of warm water, but dissolves in a weak alkaline solution. Finally, on the outside there is a gummy coating which serves to bind the zigzag courses firmly together and thus to make of them a substantial envelop.

"As soon as the caterpillars have completed their tasks, the cocoons are gathered from the sprigs of heather. A few of these cocoons, selected from those that show the best condition, are set aside and left for the completion of the metamorphosis. The resulting butterflies furnish the eggs or 'seeds' whence, next year, will come the new litter of worms. The rest of the cocoons are immediately subjected to the action of very hot steam, which kills the chrysalis in each just when the tender flesh is beginning slowly to take form. Without this precaution the butterfly would break through the cocoon, which, no longer capable of being unwound, because of its broken strands, would lose all its value.

"The cocoons are unwound in workrooms fitted up for the purpose. First the cocoons are put into a pan of boiling water to dissolve the gum which holds together the several courses of thread. An operator equipped with a small broom of heather twigs stirs the cocoons in the water in order to find and seize the end of the thread, which is then attached to a reel in motion. Under the tension thus exerted by the machine, the thread of silk unwinds while the cocoon jumps up and down in the warm water like a ball of worsted when you pull at the loose end of the yarn. In the heart of the unwound cocoon there remains a chrysalis, inert, killed by the steam.

"Since a single strand would not be strong enough for the purpose of weaving, it is usual to unwind all at once a number of cocoons, from three to fifteen and even more, according to the thickness of the fabric for which the silk is destined; and these united strands are used later as one thread in the weaving machines.

"As it comes from the pan the raw silk of the cocoon is found to have shed its coating of gum, which has become dissolved in the hot water; but it is still coated with its natural varnish, which gives it its firmness, its elasticity, its color, often of a golden yellow. In this state it is called raw silk and has a yellow or white appearance according to the color of the cocoons from which it came. In order to take on the dye that is to enhance its brilliance and add to its value, the silk must first be cleansed of its varnish by a gentle washing in a solution of lye and soap in warm water. This process causes it to lose about a quarter of its weight and to become of a beautiful white, whatever may have been its original color. After this purifying process it is called washed silk or finished silk. Finally, if perfect whiteness is desired, the silk is exposed to the action of sulphur, as I will explain to you when we come to the subject of wool.

"Cocoons that have been punctured by the butterfly, together with all scraps and remnants that cannot be disentangled and straightened out, are carded and thus reduced to a sort of fluff known as floss-silk, which is spun on the distaff or the spinning-wheel very much as wool is treated; but even with the utmost pains the thread thus obtained never has the beautiful regularity and the soft fineness of that which is furnished by unwinding the cocoon. It is used for fabrics of inferior quality, for stockings, shoe-laces, and corset-laces.

"The silkworm and the tree that feeds it, the mulberry, are indigenous to China, where silk-weaving has been practised for some four or five thousand years. To-day, when the highly prized caterpillar is dying out in our part of the world, China and its neighbor Japan are called upon to furnish healthy silkworm eggs. Silk-culture was introduced into Europe from Asia in the year 555 by two monks who came to Constantinople with mulberry plants and silkworm eggs concealed in a hollow cane; for it was strictly forbidden to disseminate abroad an industry that yielded such immense riches."