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fall listlessly between your knees, and in close proximity to the fire.
At one moment I thought the blaze had caught it, and was about to
caution you, but, before I could speak, you had withdrawn it, and were
engaged in its examination. When I considered all these particulars, I
doubted not for a moment that heat had been the agent in bringing to
light, upon the parchment, the skull which I saw designed upon it. You
are well aware that chemical preparations exist, and have existed time
out of mind, by means of which it is possible to write upon either paper
or vellum, so that the characters shall become visible only when
subjected to the action of fire. Zaffre, 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 upon 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, upon 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
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