NACRE AND PEARLS. 217 
usually iridescent. Most of the bivalves are supplied with it, 
although it generally changes its tint with different species, 
passing through all shades of blue to deep violet. The nacre 
of the Haliotis Iris is a sparkling emerald green, shot with 
purple. The mouth of the Zurbo argyrostomus presents the hue 
of silver, while that of the Turbo chrysostomus is golden. But the 
pintadine secretes nacre of a pure white, and remarkably thick ; 
and it owes its beauty to the play of colours which makes its 
surface iridescent. This iridescence is due to the interference of the 
light reflected from the edges of the plates of nacre. This fact 
may be proved by pressing a piece of white wax upon the surface 
of the mother-of-pearl, when it will be found that the wax exhibits 
the same iridescence, that is, the wax has taken the impression of 
the innumerable layers, and as their edges reflect the rays of light, 
the waves of light are placed in a position to interfere, some 
of them being obliterated ; thus the white light is destroyed, and 
coloured light is the result. A work on light must be consulted 
for a full explanation of this phenomenon; suffice it to say that the 
colours of a soap-bubble, and all such thin films, are due to this 
“interference.” For practical purposes the nacre is separated from 
its bed by a steel instrument, or the exterior of the shell is dissolved 
away by acids, and thus the precious substance is obtained. 
Pearls, those solidified drops of dew, as the Orientals poetically 
call them, are the product of the organ which secretes the nacre 
when the mollusk is in a diseased or irritated state. Instead of 
depositing the substance on the internal surface of the valves, for 
some reason or other it accumulates in little globules, either within 
the fleshy matter of the creature, or on the surface of the shell; 
usually a small grain of sand, or other foreign substance, is found 
as the nucleus of the pearls. It would seem that this particle 
irritated the animal; and in order to reduce the annoyance it 
coated it with nacre to render it smooth; layer by layer the 
beautiful deposit enlarged the pearl. 
The Chinese and other Eastern nations are said to turn this 
mode in which the mollusk relieves itself of an irritating body to 
practical account, by introducing into the live creature glass beads, 
or designs in metal ; in course of time the distressed mollusk covers 
