such a coin is laid upon the
red hot iron, the letters and figures become oxidated, and the film of
oxide radiating more powerfully than the rest of the coin will be more
luminous than the rest of the coin, and the illegible inscription may
be now distinctly read to the great surprise of the observer, who had
examined the blank surface of the coin previous to its being placed upon
the hot iron.

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. It
recovers the property however, in the course of time. When the coin is put
upon the hot iron, and consequently when the oxidation is the greatest, a
considerable smoke arises from the coin, and this diminishes like the film
of oxide by frequent repetition. A coin which had ceased to emit this
smoke, smoked slightly after having been exposed twelve hours to the air.
I have found from numerous trials that it is always the raised parts of
the coin, and in modern coins the elevated ledge round the inscription,
that becomes first oxidated. In an English shilling of 1816 this ledge
exhibited a brilliant yellow tint before it appeared on any other part
of the coin.

If we use an uniform and homogeneous disc of silver that has never been
hammered or compr