AERUGO. - Rust of a peculiar kind increases the price of brass coins, being an ornament imparted by nature alone, which the utmost rivalship of arts has not yet been able successfully to imitate. There is, indeed, some particular earth that communicates to the metal in question a coating and a colour, which in its hue of blue jasper, or turquois, sometimes even excels the gem of that name. The crimson or ruby, which adheres to other coins, is a sign of genuineness. Others are covered with a natural vernis, or varnish, of shining and splendid violet or purple, leaving far behind, in point of brilliancy and of exquisite smoothness, that brass out of which statues are cast - a quality which never fails to be recognised by those possessing the most ordinary acquaintance with numismatics, inasmuch as it greatly surpasses the colour so easily obtained from vinegar and ammoniac. The true aerugo is in general decidedly green, and at the same time forms a very thin covering, insinuating itself over the surface of the coin in a most delicate manner, without obliterating anything; somewhat in the way of an enamel. This, however, as already observed, solely applies to brass coins; for viror and rust corrode silver coins, and for that reason it is proper to rub it off from them, with juice of lemons. - Jobert, Science de Med. i. p. 335.