Ancient mercury barometer mounted on a mahogany table signed Cesare Vaselli from the early 19th century. This ancient measuring instrument consists of a glass tube filled with mercury and turned with the open side downwards inside a glass bulb containing other mercury. The mercury column tends to descend into the bulb leaving the void behind it: by measuring the height of the column the atmospheric pressure can be calculated. Mercury was already known in ancient times to the Chinese and Hindus; it was found in Egyptian tombs dated around 1500 BC. from 500 BC. begins its use as an amalgam for other metals. The ancient Greeks used it as a pigment for paintings, and the Romans used it in cosmetics. According to medieval alchemists it was the material from which everything else had been formed, and they thought that once hardened it would turn into gold.