432 
PLIITY'S l^ATUEAL HISTORY. 
[Book IX. 
der^° as well, then it becomes alarmed, and closing the shell in 
an instant, produces what is known as a physema,^^ or pearl- 
bubble, filled with air, and bearing a resemblance to a pearl, 
but in appearance only, as it is quite empty, and devoid of 
body ; these bubbles are formed by the abortion of the shell- 
fish. Those which are produced in a perfectly healthy state 
consist of numerous layers, so that they may be looked upon, 
not inappropriately, as similar in conformation to the callosities 
on the body of an animal ; and they should therefore be cleaned 
by experienced hands. It is wonderful, however, that they 
should be influenced thus pleasurably by the state of the hea- 
vens, seeing that by the action of the sun the pearls are turned 
of a red colour, and lose all their whiteness, just like the human 
body. Hence it is that those which keep their whiteness the 
best are the pelagise, or main-sea pearls, which lie at too 
great a depth to be reached by the sun's rays ; and yet these 
even turn yellow with age, grow dull and wrinkled, and it 
is only in their youth that they possess that brilliancy which 
is so highly esteemed in them. When old, too, the coat grows 
thick, and they adhere to the shell,^^ from which they can 
only be separated with the assistance of a file.^^ Those pearls 
which have one surface flat and the other spherical, opposite 
to the plane side, are for that reason called tympania,^^ or tam- 
bour-pearls. I have seen pearls still adhering to the shell ; 
for which reason the shells were used as boxes for unguents. 
In addition to these facts, we may remark that the pearl is 
soft^^ in the water, but that it grows hard the instant it is 
taken out. 
20 Isidonis of Charax, in his description of Parthia, commended by 
Athenaeus, B. iii., says, on the other hand, that the fish are aided in bring- 
ing forth, by rain and thunder. 
21 From the Greek (pvarifjia, " air-bubble." 
22 It sometimes happens, Cuvier says, that the secretion which forms the 
mother-of-pearl makes tubercles in the interior of the shell, which are the 
pearls adhering to the shell here spoken of. 
23 Persius alludes to this in Sat. ii. 1. 66. " Haec baccam conchas 
rasisse " to file the pearl away from its shell." 
2* From this passage we learn that the " tympana," or hand-drums of 
the ancients, were often of a semiglobular shape, like the kettle-drums of 
the present day. 
25 Cuvier remarks that this is not the fact : the concretions are perfectly 
hard before the animal leaves the water. 
