If we give this name to any fluid, which when written with, will remain
invisible till after a certain operation, such liquids were known in
very early periods. Among the methods, with which Ovid teaches young
women to deceive their guardians, when they write to their lovers[260],
he mentions that of writing with new milk, and of making the writing
legible by coal-dust or soot. Ausonius proposes the same means to
Paulinus[261]; but his commentators seem not to have fully understood
his meaning; for _favilla_ is not to be explained by _favilla non
modice calida_, as Vinetus has explained it, but by _fuligo_. That
word is often employed by the poets in the same sense. As a proof of
it, Columella, speaking of the method, not altogether ineffectual,
and even still used, of preserving plants from insects by soot, calls
it _nigra favilla_; and afterwards, when mentioning the same method,
free from poetical fetters, he says _fuliginem quæ supra focos tectis
inhæret_[262]. It may be easily perceived, that instead of milk any
other colourless and glutinous juice might be employed, as it would
equally hold fast the black powder strewed over it. Pliny, therefore,
recommends the milky sap of certain plants for the like purpose[263].
There are several metallic solutions perfectly colourless, or, at
least, without any strong tint, which being used for writing, the
letters will not appear until the paper be washed over with another
colourless solution, or exposed to the vapour of it; but among all
these there is none which excites more astonishment, than that
which consists of a solution of lead in acetic acid, and which by
sulphuretted hydrogen gas becomes black, even at a considerable
distance. This ink, which may be employed by conjurers, proves the
subtlety of this gas, and the porosity of bodies; as the change or
colouring takes place, even when the writing is placed on the other
side of a thin wall.
This effect presented itself perhaps accidentally to some chemist; but
the discovery is not of great antiquity. Wecker, who compiled his book
De Secretis from Porta, Cardan, and several old writers, and printed it
for the first time in 1582, and gave a third edition in 1592, must have
been unacquainted with it; else he certainly would not have omitted it
in the fourteenth book, where he mentions all the methods of secret
writing. Neither would it have been unnoticed by Caneparius, whose book
De Atramentis was printed at Venice, for the first time, in 1619.
The first person who, as far as I have been able to learn, gave a
receipt for preparing this ink, was Peter Borel, in Historiarum et
Observationum Medico-physic. Centuriæ quatuor. In this work, which
was printed for the first time in 1653, and a second time in 1657, at
Paris, and of which there were several editions afterwards, the author
calls it a magnetic water, which acts at a distance[264]. After the
occult qualities of the schoolmen were exploded, it was customary to
ascribe phænomena, the causes of which were unknown, and particularly
those the causes of which seemed to operate without any visible agency,
to magnetic effluvia; as the tourmaline was at first considered to be
a kind of magnet. Others concealed their ignorance under what they
called sympathy, and in latter times attraction and electricity have
been employed for the like purpose. Borel, who made it his business
to collect new observations that were kept secret, learned the method
of preparing this magnetic water from an ingenious apothecary of
Montpelier, and in return taught him some other secrets. Otto Tachen,
a German chemist[265], afterwards thought of the same experiment,
which he explains much better, without the assistance of magnetism
or sympathy. The receipt for making these liquids, under the name of
sympathetic ink, I find first given by Le Mort[266], and that name has
been still retained[267].
Another remarkable kind of sympathetic ink is that prepared from
cobalt, the writing of which disappears in the cold, but appears again
of a beautiful green colour, as often as one chooses, after being
exposed to a moderate degree of heat.
The invention of this ink is generally ascribed to a Frenchman named
Hellot. He was, indeed, the first person who, after trying experiments
with it, made it publicly known, but he was not the inventor; and he
himself acknowledges that a German artist of Stolberg first showed him
a reddish salt, which, when exposed to heat, became blue, and which he
assured him was made out of Schneeberg cobalt, with aqua regia[268].
This account induced Hellot to prepare salts and ink from various
minerals impregnated with cobalt; but A. Gesner proved, long after,
that this ink is produced by cobalt only, and not by marcasite[269].
When Hellot’s experiments were made known in Germany, it was affirmed
that Professor H. F. Teichmeyer, at Jena, had prepared the same ink
six years before, and shown it to his scholars, in the course of his
lectures, under the name of sympathetic ink[270]. It appears, however,
that it was invented, even before Teichmeyer, in the beginning of the
last century by a German lady. This is confirmed by Pot, who says that
the authoress of a book printed in 1705, which he quotes under the
unintelligible title of D. J. W. in clave, had given a proper receipt
for preparing the above-mentioned red salt, and the ink produced by
it[271]. If it be true that Theophrastus Paracelsus, by means of this
invention, could represent a garden in winter, it must be undoubtedly
older[272].
[In consequence of the progress of modern chemistry and the discovery
of a vast number of new chemical compounds, sympathetic inks may be
made in an almost endless number and variety. The principal may be
classed in the following manner:--1, _such as when dried upon paper
being invisible, on moistening with another liquid become again
evident_: of this kind there are a vast number; among which we may
mention a solution of a soluble salt of lead, or bismuth, for writing,
and a solution of sulphuretted hydrogen for washing over; the writing
then appears black; or green vitriol for writing and prussiate of
potash for washing over, when the writing becomes blue[273]; 2, _such
as are rendered evident by being sifted over with some powder_, as
the milk with soot described above; 3, _those which become visible
by heat_, such as characters in dilute sulphuric acid, lemon-juice,
solutions of the nitrate and chloride of cobalt, and of chloride of
copper; the two former become black or brown, the latter are rendered
green, the colouring disappearing subsequently when allowed to cool
in a moist place. Amusing pictures are sometimes made with these
sympathetic inks, particularly those composed of cobalt; for if a
landscape be drawn to represent winter, the vegetation being covered
with a solution of cobalt, on holding the paper to the fire, all those
portions covered with the solution appear of a bright green, and thus
completely change the character of the scene.]