All metals more or less oxidise or corrode when exposed to a damp atmosphere or corroding fumes. And if the oxide so formed is dissolvable in water or other liquid, or readily detaches itself from the metal, as in the case of iron, then rapid deterioration takes place. Although iron has many distinct advantages over other metals in the way of strength, working properties, and cheapness, yet it is the worst of the ordinary metals in offering resistance to the action of air and moisture when exposed to the atmosphere without some protective coating. Copper, lead, and zinc are all quickly acted upon by damp air, or if the atmosphere contains sulphur, carbonic acid, or other fumes, the metals very soon tarnish. The thin film of oxide or scale so formed, however, in the case of these metals holds tenaciously to the metal, and, consequently, acts in a very effective manner as a protecting skin for the metal underneath. Aluminium, it has been said, is not affected by the atmosphere, but this is not true. Probably what happens is the instant formation of a transparent film of oxide.. Again, sheet aluminium will not stand continued exposure in a damp atmosphere, as a heavy oxide forms on its surface, and if the sheet be thin the metal becomes very brittle. To protect the surfaces of metals from corroding influences, many methods are in vogue, such as galvanising, tinning, electro-plating, dipping, lacquering, enamelling, japanning, painting, oxidising, and, for special purposes, metals may be coloured by bronzing, steeling, gilding, etc.