Pearls differ in one essential respect from other precious gems in that they require no aid to enhance their beauty. They need only to be found, and the less they are handled the more perfect do they appear. Unlike diamonds, pearls were known to Greeks and Romans, while the area over which they are found comprises a large portion of the globe, extending from China to Mexico and from Scotland to Egypt. A certain pearl of astonishing magnitude formed the chief treasure of ancient Persia, while every one is familiar with the persistent myth of Cleopatra's ear-ring and the cup of vinegar. People for centuries have wondered over the insane extravagance of the draught; but they might have spared their wonder, for no acid which the human stomach can bear is powerful enough to dissolve a pearl. The various notions relative to the origin of pearls have done credit to the fertility of man's imagination. Some writers have affirmed that they were the product of "ocean dew," whatever that may be, and were accordingly affected by atmospheric conditions. Thus they were large and muddy during the season of the monsoon, becoming clear and lustrous again in hot dry weather, while thunder and lightning had a fatal effect upon them. These ideas were prevalent in the Ceylon fisheries, which at one time were most prolific in their precious crop. Another idea was even still more quaint. According to it, the oyster was looked upon as affecting the habits of the feathered tribe. The pearl was an egg which the oyster laid after the manner of hens. Modern science, more exact if less imaginative, has decided that the pearl is due to an accident, and an inconvenient accident which frequently befalls the parent oyster. A grain of sand, or some such minute foreign substance, gets within the jealous valves of the mollusk and causes great irritation to the soft body of the pulpy inhabitant. Accordingly it endeavors to render the presence of the intruder less irksome by coating it with exudations from its own body. In other words the grain of sand is "scratchy," so the oyster smooths it over. Why, then, after once coating the objectionable grain of sand and thus making it a comfortable lodger, the oyster should go on for years adding layer after layer of pearl-substance remains is truly a mystery. But such is its habitual practice, and to this apparently aimless perseverance we owe the existence of pearls.