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 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. A closer scrutiny, however, satisfied me that it was
intended for a kid."
"Ha! ha!" said I, "to be sure I have no right to laugh at you--a
million and a half of money is too serious a matter for mirth--but you
are not about to establish a third link in your chain: you will not
find any especial connection between your pirates and a goat; pirates,
you know, have nothing to do with goats; they appertain to the farming
interest.
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