In order to explain the cause of these remarkable effects, we must
notice a method which has been long known, though never explained, of
deciphering the inscriptions on worn out coins. This is done by merely
placing the coin upon a hot iron: an oxidation takes place over the
whole surface of the coin, the film of oxide changing its tint with the
intensity or continuance of the heat. The parts, however, where the
letters of the inscription had existed, oxidate at a different rate from
the surrounding parts, so that these letters exhibit their shape, and
become legible in consequence of the film of oxide which covers them
having a different thickness, and therefore reflecting a different tint
from that of the adjacent parts. The tints thus developed sometimes pass
through many orders of brilliant colours, particularly _pink_ and
_green_, and settle in a bronze, and sometimes a black tint, resting
upon the inscription alone. In some cases the tint left on the trace of
the letters is so very faint that it can just be seen, and may be
entirely removed by a slight rub of the finger.
When the experiment is often repeated with the same coin, and the
oxidations successively removed after each experiment, the film of oxide
continues to diminish, and at last ceases to make its appearance.
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