If, again, the fish is satiated in a reasonable time, then the pearl produced increases rapidly in size. If it should happen to lighten at the time, the animal shuts its shell, and the pearl is diminished in size in proportion to the fast that the animal has to endure: but if, in addition to this, it should thunder[2615] as well, then it becomes alarmed, and closing the shell in an instant, produces what is known as a physema,[2616] 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 heavens, 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 pelagiæ, 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,[2617] from which they can only be separated with the assistance of a file.[2618] Those pearls which have one surface flat and the other spherical, opposite to the plane side, are for that reason called tympania,[2619] or tambour-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[2620] in the water, but that it grows hard the instant it is taken out. CHAP. 55.—HOW PEARLS ARE FOUND.