432 PLTKT'S NATURAL HISTOKT. [Book IX. 



der 20 as well, then it becomes alarmed, and closing the shell in 

 an instant, produces what is known as a physema, 21 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, 22 from which they can 

 only be separated with the assistance of a file. 23 Those pearls 

 which have one surface flat and the other spherical, opposite 

 to the plane side, are for that reason called tympania, 24 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 25 in the water, but that it grows hard the instant it is 

 taken out. 



20 Isidorus of Charax, in his description of Parthia, commended by 

 Athenseus, B. iii., says, on the other hand, that the fish are aided in bring- 

 ing forth, by rain and thunder. 



21 From the Greek ^vo-rj/ia, " 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. " Hsec baccam conchae 

 rasisse ;" " to file the pearl away from its shell." 



24 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. 



