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 compressed, its surface will oxidate equally, provided all
its parts are equally heated. In the process of converting this disc
into a coin, the _sunk_ parts have obviously been _most compressed_
by the prominent parts of the die, and the _elevated_ parts _least
compressed_, the metal being in the latter left as it were in its
natural state. The raised letters and figures on a coin have therefore
less density than the other parts, and these parts oxiditate sooner or
at a lower temperature.
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