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 on cools, but again become apparent upon
the re-application of heat.
It´s
very common that the invisible ink appears in mystery novels for many
and different uses. Zaffre
is cobalt oxide but I have some doubts about whether cobalt produces
a red colour. It would be more usual, as the first part of the text
says, a green colour or a blue one. Aqua regia is very well-known for
all Chemistry students. It´s a mixture of nitric acid and
hydrochloric acid in proportion 1:3. So,
with such a composition, you can easily imagine that it can corrode
and dissolve everything in its way.
