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