at 



501 



times ere science had determined their real 

 nature, various amusing hypotheses to ac- 

 count for their existence. Pliny, the cele- 

 brated Roman naturalist, gravely tells us 

 that the oyster which produces pearls does 

 so from feeding upon heavenly dew. Our 

 own early writers entertained the same 

 notion ; and Boethius, speaking of the pearl- 

 mussel of the Scottish rivers, remarks, that 

 ' these mussels, early in the morning, when 

 the sky is clear and temperate, open their 

 mouths a little above the water, and most 

 greedily swallow the dew of heaven ; and 

 after the measure and quantity of the dew 

 which they swallow, they conceive and breed 

 the pearl. These mussels,' he continues, 

 4 are so exceedingly qiiick of touch and 

 hearing, that, however faint the noise that 

 may be made on the bank beside them, or 

 however small the stone that may be thrown 

 into the water, they sink at once to the 

 bottom, knowing well in what estimation 

 the fruit of their womb is to all people." In 

 the East, the belief is equally common that 

 these precious gems are 



' Rain from the sky, 

 Which turns into pearls as it falls in the sea.' 



But, alas for poesy and romance 1 the sci- 

 ence of chemistry which has, with its 

 sledge-hammer of matter-of-fact, converted 

 the all-glorious diamond into vulgar char- 

 coal has also pronounced the precious 

 pearl to .be composed of ' concentric layers 

 of membrane and carbonate of lime ! ' Ad- 

 mitting its composition, the question still 

 remains as to the cause of a substance so 

 dissimilar in appearance to the shell in 

 which it exists, and why it should be pre- 

 sent in some shells, and absent in others. 



44 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 irrita- 

 tion. 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 ani- 

 mal 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 ihis same matter, the 

 oyster immediately fills up the opening ! 

 made, and shutting out the intruder, balks I 

 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, I 

 are 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 re- 

 mains 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. 



"Sir Everard Home does not appear to 

 have been aware that Sandius, as long ago 

 as 1C73, communicated the same fact to the 

 Royal Society of London ; 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 ex- 

 actly the appearance of seed-pearls, as they 

 are called. Sometimes he 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 uni- 

 versally 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 suffi- 

 ciently large to enclose it. Heycame 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 irri- 

 tation 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, but occasionally we find them of a 

 pyramidal form, the pedicle by which the 

 egg is attached appearing to have received a 

 coat of nacre as well as itself. People con- 

 versant witli 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 mathema- 

 tically, they contended that their brilliancy 

 must be produced by the reflection from the 

 nacral surface. In the Edinburgh Encyclo- 

 paedia, we are told by Sir Pavid 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-of-pearl a grooved structure upon 

 its surface, resembling very closely the de- 

 licate 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. 4 The direc- 



