You are well aware that chemical
preparations exist, and have existed time out of mind, by means of
which it is possible to write on either paper or vellum, so that the
characters shall become visible only when subjected to the action of
fire. Zaire, digested in aqua regia, and diluted with four times its
weight of water, is sometimes employed; a green tint results. The
regulus of cobalt, dissolved in spirit of nitre, gives a red. These
colors disappear at longer or shorter intervals after the material
written on cools, but again become apparent upon the re-application of
heat.
"I now scrutinized the death's-head with care. Its outer edges --the
edges of the drawing nearest the edge of the vellum --were far more
distinct than the others. It was clear that the action of the
caloric had been imperfect or unequal. I immediately kindled a fire,
and subjected every portion of the parchment to a glowing heat. At
first, the only effect was the strengthening of the faint lines in the
skull; but, on persevering in the experiment, there became visible, at
the corner of the slip, diagonally opposite to the spot in which the
death's-head was delineated, the figure of what I at first supposed to
be a goat.
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