Pearls are affected by acids and fetid gases, and may be calcined on exposure to heat. Their solubility in vinegar was referred to by the Roman architect Vitruvius (“De Architectura,” L. viii. c. 3) and also by Pausanias, a Greek geographer in the second century (“Hellados Periegesis,” L. viii, c. 18); but it seems that there could be little foundation for Pliny’s well-known anecdote in which Cleopatra is credited with dissolving a magnificent pearl in vinegar and drinking it—“the ransom of a kingdom at a draught”—to the health of her lover Antony.[73] It is no more easy to dissolve a pearl in vinegar than it is to dissolve a pearl-button—for the composition is similar, and one may easily experiment for himself as to the difficulty in doing this. Not only does it take many days to dissolve in cold vinegar the mineral elements of a pearl of fair size, but even with boiling vinegar it requires several hours to extract the mineral matter from one four or five grains in weight, the acid penetrating to the interior very slowly. And in neither case can the pearl be made to disappear, for even after the carbonate of lime has dissolved, the organic matrix of animal matter—which is insoluble in vinegar—retains almost the identical shape, size, and appearance as before. If the pearl is first pulverized, it becomes readily soluble in vinegar, and might be thus drunk as a lover’s potion, but it would scarcely prove a _bonne bouche_. Pearls assume an almost infinite variety of forms, due largely to the shapes of the nuclei, and also to their positions within the mollusk. The most usual—and, fortunately, also the most valuable—is the spherical, resulting from a very minute or a round body as a nucleus and the uniform addition of nacre on all sides. Of course, spherical pearls can result only where they are quite free from other hard substances; consequently they originate only in the soft parts of the mollusk and not by the fixation of some nucleus to the interior surface of the shell. The perfectly spherical pearls range in weight from a small fraction of a grain to three hundred grains or more, but it is very, very rare that one of choice luster weighs more than one hundred grains. The largest of which we have any specific information was that among the French crown jewels as early as the time of Napoleon, an egg-shaped pearl, weighing 337 grains. The largest pearl known to Pliny in the first century A.D. weighed “half a Roman ounce and one scruple over,” or 234½ grains Troy. These very large ones, weighing in excess of one hundred grains, are called “paragons.” The small pearls—weighing less than half a grain each—are known as “seed-pearls.” The very small ones, weighing less than 1⁄25 of a grain, are called “dust-pearls.” These are too small to be of economic value as ornaments.