A. Because the vapour of the air oxidizes its surface, and tarnishes it.
Q. Why do silver tea-pots and spoons tarnish more quickly than silver ore or bullion?
A. Because alloy (of some baser metal) is used to make it more hard and lasting; and this alloy oxidizes more quickly than silver itself.
Q. Why does German silver turn a dingy yellow in a few hours?
A. German silver has a great affinity for oxygen, and shows its oxidation by a sickly yellow tarnish, instead of rust.
Q. If quicksilver (or mercury) is tarnished like copper and lead,—Why does it preserve its brilliancy in barometers and thermometers?
A. Because air is excluded from it, and no moisture comes in contact with it to oxidize (or tarnish it).
Q. Is gold affected by the atmosphere?
A. Not readily: gold will never combine with oxygen of itself, (or without aid).
Q. Which of the metals is capable of resisting oxidation altogether?