_scarabaeus_, there was no skull apparent on the
parchment. When I had completed the drawing I gave it to you, and
observed you narrowly until you returned it. _You_, therefore, did not
design the skull, and no one else was present to do it. Then it was not
done by human agency. And nevertheless it was done.

"At this stage of my reflections I endeavored to remember, and _did_
remember, with entire distinctness, every incident which occurred about
the period in question. The weather was chilly (O rare and happy
accident!), and a fire was blazing on the hearth. I was heated with
exercise and sat near the table. You, however, had drawn a chair close
to the chimney. Just as I placed the parchment in your hand, and as you
were in the act of inspecting it, Wolf, the Newfoundland, entered, and
leaped upon your shoulders. With your left hand you caressed him and
kept him off, while your right, holding the parchment, was permitted to
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caution you, but, before I could speak, you had withdrawn it, and were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, on the parchment, the skull which I saw designed on 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 on 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
reapplication 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 opposit