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oysters, mussels, &c. These shells have all a fine pearly lustrous internal surface; so much so, that it is said by Dampier, in his Voyage Round the World, that “the inside of the shell is more glorious than the pearl itself.” This internal coat is the nacre of the chemist; and upon analysing pearls, we find that they consist of alternate layers of very fine membrane, and layers of this nacre, regularly spread over each other like the coats of an onion. These pearls are found either adhering firmly to the inside of the shell, or lying loose in the very substance of the animal itself, commonly in its thickest and most fleshy part. According to the situation in which they are found, so are they more or less valuable, and require a different explanation to account satisfactorily for their growth.

In all cases, it appears that the ultimate cause of the animal's forming this beautiful substance is to get rid of a source of irritation. Sometimes this happens to be a grain of sand, or some such small foreign body, which has insinuated itself between the mantle of the oyster and the shell, and which, proving a great annoyance, the animal covers with a smooth coat of membrane, over which it spreads a layer of nacre. At other times, it is caused by some enemy of the inhabitant : of the shell perforating it from the outside to get within reach of its prey. With a plug of this same matter, the oyster immediately fills up the opening made, and shutting out the intruder, balks it of its nefarious design. In both these cases, we find the pearl usually adhering to the internal surface of the shell. The best, however, and the most valuable specimens, are

mantle that the calcareous matter is exuded which forms the shell, in those species which possess such a protection : its particles are held together by a sort of glue or gelatine, which exists in much larger proportions in some species than in others. In very hard and brittle shells, if the calcareous matter be removed by the action of an acid, the animal matter that remains appears in the form of separate flakes; but in many other shells thus treated, the animal portion retains its form after the removal of the lime, and there are few in which the (so-called) shell consists only of a substance like horn, without any intermixture of calcareous particles. Such a substance appears to be formed by the young animal before the true shell is secreted; and it is also the first that appears when the animal is repairing the effects of an injury to the old one. It is this that constitutes what is called the epidermis of shells-a covering possessed in their natural state by all that are not enveloped in a fold of the mantle. The shell is most solid and massive in those species which lead an inactive life, and is usually light and thin, or altogether deficient in those whose powers of locomotion are greater. Its thickness often varies greatly among different individuals of the same species, according to the roughness or tranquillity of the waters they inhabit. These explanations may be of use to the inquiring reader who will take the trouble to compare such animals as the common naked slug, the garden helix or snail, the mussel, cockle, oyster, the large strombus used as a mantel-piece ornament, the mother-of-pearl shell in the window of the curiosity dealer, and the elegant paper nautilus, all of which are members of the class Mollusca.

generally found in the body itself of the animal; and the source of irritation here is proved, according to the observations of Sir Everard Home, who has paid great attention to this subject, to be an ovum or egg of the animal, which, instead of becoming ripe, proves abortive, and is not thrown out by the mother along with the others, but remains behind in the capsule, in which the ova are originally contained. This capsule, being still supplied with blood - vessels from the parent animal, goes on increasing in size for another year, and then receives a covering of nacre, the same as the animal spreads over the internal surface of the shell. This discovery was rather pompously announced by Sir Everard some years ago, when he stated, “ If I can prove that this, the richest jewel in a monarch's crown, which cannot be imitated by any art of man, either in beauty of form or brilliancy of lustre, is the abortive egg of an oyster enveloped in its own nacre, who will not be struck with wonder and astonishment!”

We are certainly indebted to the learned baronet for calling the attention of scientific men to this subject; but long before that time Sandius had made known the same fact, and gives as his authority for the statement the testimony of an eye-witness, “Henricus Arnoldi, an ingenious and veracious Dane.” In a letter which he sent to the Royal Society of London, dated 1st December 1673, he says, “ Pearl shells in Norway do breed in sweet waters; their shells are like mussels, but larger; the fish is like an oyster, it produces clusters of eggs; these, when ripe, are cast out, and become like those that cast them; but sometimes it appears that one or two of these eggs stick fast to the side of the matrix, and are not voided with the rest. These are fed by the oyster against her will, and they do grow, according to the length of time, into pearls of different bigness, and do imprint a mark both on fish and shell by the situation conform to its figure." Sir Everard Home does not appear to have been aware of this statement of Sandius at the time he first made his discovery of this curious fact; but was led to it when investigating the mode of breeding of the fresh-water mussel, by generally finding in the ovarium round hard bodies, too small to be noticed by the naked eye, having exactly the appearance of seed-pearls, as they are called. Sometimes be found these bodies connected with the surface of the shell, in contact with the membrane covering it. In further examining into the structure of pearls, he ascertained that all split pearls upon which he could lay his hands universally possessed a small central cell, which surprised him by its extreme brightness of polish; and in comparing the size of this cell with that of the ovum when ready to drop off from its pedicle, he found it sufficiently large to enclose it. He came thus to the conclusion that these abortive eggs are the commencement or nuclei of the pearl. Being once formed, the animal continues to increase its size by the addition of fresh

coats, adding, it is said, a fresh layer every year. It is extremely probable, however, that its presence being still a source of irritation to the creature, the nacral covering is more rapidly deposited upon the pearl than upon the shell itself. Those pearls found in the substance of the animal are generally round (a), but occasionally we find them of a pyramidal form, the pedicle by which the egg is attached appearing to A have received a coat of nacre as well as itself (b). People conversant with the pearl -fishery assert that they do not appear till the animal has reached its fourth year, and that it takes from seven to nine years for the oyster to reach maturity.

The true pearl is remarkable, as is well known, for its beautiful lustre-a lustre which cannot altogether be given to artificial ones. According to Sir Everard Home, this peculiar lustre arises from the central cell, which is lined with a highly-polished coat of nacre ; and the substance of the pearl itself being diaphanous, the rays of light easily pervade it. Previous to Sir Everard's theory, it was supposed by opticians that the peculiar splendour was the effect of light reflected from the external surface. They took for granted that pearls were solid bodies, denied them to be diaphanous, and therefore, considering the subject mathematically, they contended that their brilliancy must be produced by the reflection from the nacral surface. In the Edinburgh Encyclopædia, we are told by Sir David Brewster that the fine pearly lustre and iridescence of the inside of the pearl-oyster arises from the circumstance, that we find in all “ mother-ofpearl a grooved structure upon its surface, resembling very closely the delicate texture of the skin at the top of an infant's finger, or the minute corrugations which are often seen on surfaces covered with varnish or with oil paint." Similar appearances, we are told, are to be seen in the structure of pearls. “The direction of the grooves," says Sir David, " is in every case at right angles to the line joining the common image and the coloured image; hence in irregularly-formed mother-of-pearl, where the grooves are often circular, and have every possible direction, the coloured images appear irregularly scattered round the ordinary image. In the real pearl these coloured images are crowded into a small space round the common image, partly on account of the spherical form of the pearl ; and the various hues are thus blended into a white unformed light, which gives to this substance its high value as an ornament." Pearls, however, at least the most valuable, are not perfectly solid, and are certainly translucent. In fact, in a split pearl we find the transparency to be considerable. “Upon taking a split pearl,” says Sir Everard Home," and putting a candle behind the cell, the surface of the pearl became immediately illuminated; and upon

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mounting one with coloured foil behind the cell, and by putting

a candle behind the foil, the outer convex surface became universally of a beautiful pink colour." If we take a split pearl and set it in a ring with the divided surface outwards, and look at this through a magnifying glass, this central cell becomes very conspicuous, and the different layers of which the pearl is composed are also beautifully displayed, as may be seen in the accom

panying sketch.

ARTIFICIAL PEARLS. It is the brilliancy above described that distinguishes the real from the factitious pearl-a lustre which no art can altogether give, though often attempted with considerable success. The Romans, who valued pearls so highly, do not seem to have been aware of any method of manufacturing them; but soon after their time, attempts were made to create them by artificial means. Philostratus, in his life of Apollonius, informs us that in the first centuries of the Christian era, the Arabians on the shores of the Red Sea adopted a plan by which they were able to form pearls at pleasure. “The Arabs," he says, “ first poured oil upon the sea, which it is well known has the effect of calming the agitation of the waves, and consequently rendering the water more transparent at the bottom. They then dived in those spots where they knew the fish were to be found, and enticed them to open their shells by rubbing them with some kind of ointment as a bait; which, having effected, they pricked them with a sharp instrument, having first placed near them a vessel hollowed out in various places into the form of pearls, into which moulds the liquor which flowed from the wound was received, and there hardened into the shape, colour, and consistence of the native gems." The method thus described is sufficiently apocryphal to induce even the most credulous to withhold his belief as to the exact mode adopted. That, however, some attempts were then made to form pearls in a somewhat similar manner, is extremely probable, as we know that the Chinese have long been famous for a similar artifice. Two or three different methods have been described, by which that ingenious people caused them to be produced within the pearl-producing shells.

One method is, by taking a small portion of the substance of the shell, and turning it in a lathe, into hemispheres of different sizes. These small hemispheres they introduce through the shell of the oyster, with the convex surface toward the animal. This prominent part proving a source of irritation to the creature, consequently soon gets covered with a coat of nacre, and a fresh coat is added every year. Half pearls are thus formed in a few years; and these, when set, will readily pass off undiscovered by an inexperienced eye. Another method is said to be " by opening the shell very carefully, and scraping off a small portion of the internal surface of the shell. In its place is inserted a spherical piece of mother-of-pearl, about the size of a small grain of shot. This serves as a nucleus, on which is deposited the pearly matter, and in time forms pearls." In the British Museum there is a fine specimen of a fresh-water shell from China, nearly allied to the fresh-water mussel, containing several very fine regular-shaped semi-orbicular pearls of most beautiful water. There are several fragments also of the same shell, with similar pearls upon them; "and on the attentive examination," says Mr Gray, “ of one of these, which was cracked across, I observed it to be formed of a thick coat, consisting of several concentric plates formed over a piece of mother-of-pearl, roughly filed into a plano-convex form, like the top of a mother-of-pearl button. On examining the other pearls, they all appeared to be formed on the same plan. In one or two places where the pearl had been destroyed, or cut out, there was left in the inside of the shell a circular cavity with a flat base, about the depth, or rather less than the thickness of the coat that covered the pearls, which distinctly proves that these pieces of mother-of-pearl must have been introduced when the shells were younger and thinner; and the only manner that they could have been placed in this part of the shell, must be by the introduction of them between the leaf of the mantle and internal coat of shell; for they could not have been put in through a hole in the shell, as there was not the slightest appearance of any injury near the situation of the pearls on the outer coat." Mr Gray describes another pearl found in the same species of shell from China, and which is deposited in the Museum of the Royal College of Surgeons of London. Upon attentively examining it, he found that this pearl was formed upon a small piece of silver wire, which had been introduced between the mantle of the animal (while yet alive) and the shell.

In 1748, the celebrated naturalist Linnæus had his attention turned to this subject. He may have remarked the formation of pearls in shells, the outer coat of which had been perforated by some marine worm, and the hole caused by which had been plugged up as already mentioned; or he might have acquired some knowledge of the Chinese methods of making pearls, and turned it to his own account. In a letter to his friend Haller, he says, “At length I have ascertained the manner in which pearls originate and grow in shells; and I am able to produce, in any mother-of-pearl shell that can be held in the hand, in the course of five or six years, a pearl as large as the seed of a common Vetch." This is believed to have been accomplished by his puncturing the shell with a pointed flexible wire, the end of which

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