gravity ranging from 2·65 to 2·69-2·84-2·89 in the case of pink pearls—which is appreciably less than that of aragonite, 2·94: the hardness is about the same, namely, 3½ to 4 on Mohs’s scale. That the arrangement of the mineral layers is approximately parallel is evinced by the distinctness of the shadow-edges shown on examination with the refractometer. Pearls require very careful handling, both because they are comparatively soft and therefore apt to be scratched, and because they are chemically affected by acids, and even by the perspiration from the skin. Acids attack only the calcium carbonate, not the organic matter; the well-known story therefore of Cleopatra dissolving a valuable pearl in vinegar, which is moreover, too weak an acid to effect the solution quickly, must not be accepted too literally. Pearls are not cut like stones, and therefore as soon as the precious bloom has once gone, nothing can be done to revive it. Attempts are sometimes made in the case of valuable pearls to remove the dull skin and lay bare another iridescent layer underneath, but the operation is exceedingly delicate. Even with the best of care pearls must in process of time perish owing to the decay of the organic constituent. Pearls that have been discovered in ancient tombs crumbled to dust at a touch, and those formerly in ancient rings have vanished or only remain as a brown powder, while the garnets or other stones set with them are little the worse for the centuries that have passed by. The largest known pearl was at one time in the famous collection belonging to the banker, Henry Philip Hope. Cylindrical in form, with a slight swelling at one end, it measures 50 mm. (2 inches) in length, and 115 mm. (4½ inches) in circumference about the thicker, and 83 mm. (3¼ inches) about the thinner end, and weighs 454 carats. About three-quarters of it is white in colour with a fine ‘orient,’ and the remainder is bronze in tint. It is valued at upwards of £12,000. A large pearl, 300 carats in weight, is in the imperial crown of the Emperor of Austria, and another, pear-shaped, is in the possession of the Shah of Persia. A beautiful white India pearl, a perfect sphere in shape, and 28 carats in weight, is in the Museum of Zosima in Moscow; it is known as ‘La Pellegrina.’ The ‘Great Southern Cross,’ which consists of nine large pearls naturally joined together in the shape of a cross, was discovered in an oyster fished up in 1886 off the beds of Western Australia. The collection of jewels in the famous Green Vaults at Dresden contains a number of pearls of curious shapes.