580 Pearls and Pearl Fisheries. [June, 
by ordinary acids, and serves as a protection to the carbonate of 
lime of which the true shell is composed. This carbonate of 
lime may be deposited in prisms, as of arragonite, in amorphous 
or fibrous layers, or as “ mother-of-pearl.” In all cases it con- 
tains more or less animal matter which binds it more firmly to- 
gether. In the case of the pearly or iridescent shell, which is all 
that it is necessary for us to consider at present, the pearly lus- 
ter is caused by the action upon light of the minute layers of 
which the nacre is composed. These layers are microscopically 
corrugated, and their edges meet the rays of light and partly de- 
compose them, as do the drops in a rainbow, producing the play 
of varied colors. This has been proved in two ways, first, by 
digesting mother-of-pearl in acid until all the lime is dissolved. 
A pellucid membrane, representing the animal matter, still 
remains, and if undisturbed still shows the iridescence. But if 
pressed flat so as to remove the corrugations, it also loses its 
pearliness. Again by means ofa diamond splinter, and an engine 
invented by Mr. Barton of the British mint, similar corrugations 
have been engraved on the polished surface of a steel button with 
the result of producing the pearly play of colors; a proces 
which would have proved commercially valuable could the en- 
graved buttons have been kept from tarnishing. 
There are also fine superficial lines on pearly surfaces which 
may add to the effect. These run in various directions, are rio of 
an inch apart, according to Dr. Carpenter, and may be due to the 
minute cilia with which the mantle is provided. 
Pearls are concretions in the tissues, of the same material as 
that which composes the shell layers, and are usually due to the 
presence, in the secreting tissue, of some irritating particle of 
parasite, much as in the tissues of an animal a Trichina becomes 
covered with a limy cyst. In the mollusk, however, the layers a 
constantly added to until the pearl reaches a considerable he 
When it becomes so large that the valves of the shell cann 
close, the mollusk soon dies and the pearl may be washed bape 
and lost. If the pearl escapes from the tissues during the ™ 
the animal, it may become cemented to the inside of the Se 
pseudo-pearls may be formed by the mantle over projections pom 
the inside of the valves. Concretions similar to pearls, but ! n 
less, are formed in many mollusks, as is frequently 
‘the common oyster on our tables. 
