(_From Sir David Brewster's Letters on Natural Magic_.)
Among the numerous experiments with which science astonishes and
sometimes even strikes terror into the ignorant, there is none more
calculated to produce this effect than that of displaying to the eye
in absolute darkness the legend or inscription upon a coin. To do this,
take a silver coin, (I have always used an old one,) and after polishing
the surface as much as possible, make the parts of it which are raised
rough by the action of an acid, the parts not raised, or those which
are to be rendered darkest, retaining their polish. If the coin thus
prepared is placed upon a mass of red hot iron, and removed into a dark
room, the inscription upon it will become less luminous than the rest,
so that it may be distinctly read by the spectator. The mass of red hot
iron should be concealed from the observer's eye, both for the purpose
of rendering the eye fitter for observing the effect, and of removing
all doubt that the inscription is really read in the dark, that is,
without receiving any light, direct or reflected, from any other body.
If, in place of polishing the depressed parts, and roughening its raised
parts, we make the raised parts polished, and roughen the depressed
parts, the inscription will now be less luminous than the depressed
parts, and we shall still be able to read it, from its being as it were
written in black letters on a white ground. The first time I made this
experiment, without being aware of what would be the result, I used a
French shilling of Louis XV. and I was not a little surprised to observe
upon its surface in black letters the inscription BENEDICTUM SIT NOMEN
DEI.
The most surprising form of this experiment is when we use a coin from
which the inscription has been either wholly obliterated, or obliterated
in such a degree as to be illegible. When 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 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. When the letters of the legend are worn off by
friction, the parts immediately below them have also less density than
the surrounding metal, and the site as it were of the letters therefore
receive from heat a degree of oxidation, and a colour different from
that of the surrounding surface. Hence we obtain an explanation of the
revival of the invisible letters by oxidation.
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