SYMPATHETIC INKS
The so-called _sympathetic inks_, by which is understood inks that give a
writing that is invisible, or nearly so, until it has been acted upon by
the air or treated with a special reagent, have been put to many ingenious
uses by the criminal.
Some five years ago an innocent-looking individual called at the
laboratory of one of the leading consulting chemists in London, and asked
whether he could be supplied with a writing fluid that would give writing
which would fade away in a short time, and also with another ink that
would produce words that would be invisible for some time and then appear.
He gave as his reason for requiring these that he wanted to amuse a small
boy.
The sequel was seen a few weeks later when the same plausible individual
was arrested for swindling on the race-course. He had made tempting bets
on certain horses, the names of which he had written on slips of paper,
and had handed these slips to those who had accepted his wagers.
In a short time the name of the horse on each slip of paper gradually
faded away while the name of another horse slowly appeared in its place.
One man to whom one of these slips had been given, having been warned by
another victim, hurried away to the police station, and was in time to let
the superintendent see the name of an "outsider" replace that of one of
the favourites upon which he had laid his money.
This appears to have been the last detected attempt to use a sympathetic
ink upon the race-course. A disappearing ink frequently used for this
purpose is a weak solution of starch containing a slight trace of iodine,
the effect of which is to produce a faint blue colour. On exposure to the
air the colour of writing done with such a fluid soon fades away.
Fugitive dye-stuffs have also been employed as disappearing inks, and some
of these, such as quinoline blue, give characters that rapidly disappear
when exposed to sunlight.
An ink that is invisible for some time is a solution of silver nitrate in
ammonia, which gradually becomes black when acted upon by air and light.
Or certain dye-stuffs such as magenta, that have been treated with a
bleaching reagent in just sufficient quantity to decolorise them fulfil
the same purpose, the original colour gradually reappearing as the oxygen
of the air acts upon the pigment.
The earliest inks that were rendered visible by chemical reagents were
believed to act by magnetism.
Thus in a medical book of the seventeenth century, written by Brossonius,
a "magnetic fluid" is described made from "arseniated liver of sulphur,"
which only became visible when looked at with the "eyes of affection."
This appears to have been nothing more mysterious than an ink of lead
acetate, the characters written with which could be rendered visible by
exposing them to the vapour of sulphuretted hydrogen.
Inks of this kind were also mentioned, in 1669, by Otto Tachen, who
referred to them as _aquæ magnetice e longinquo agentes_, but pointed out
that there was nothing magnetic in their action. The term _sympathetic
ink_ appears to have originated with Le Mort, who applied it to the lead
acetate ink, and later on the name was extended to all secret inks.
The best known sympathetic inks consist of solutions of cobalt salts, the
writing done with which changes on heating from a nearly invisible pink to
blue. This peculiarity of cobalt to form two series of salts containing
different amounts of water was discovered in 1715 by Waiz.
Other compounds that may be used as sympathetic writing fluids include
tannin, which forms ordinary ink on the addition of iron sulphate; cobalt
nitrate which becomes blue on adding oxalic acid, and gold chloride which
gives a purple colour with tin chloride.
Some thirty years ago a patent was taken out by Kromer for the use of a
sympathetic ink in detecting any tampering with envelopes. The two dried
constituents of the ink, say tannin and iron sulphate, are separated by
the adhesive gum upon the envelope, so that should steam be applied to
open the letter, the two substances come into contact, and form an ink,
which leaves a stain upon the paper.
Printing inks based upon these principles are used in preparing the
groundwork of cheques, so that any attempt to remove the writing from the
cheque by means of chemical agents will be betrayed by the change of
colour upon the body of the paper.
The value of sympathetic inks in detecting an offender was strikingly
shown in the recent Sutton libel case, in which a woman was found guilty
of sending offensive cards through the post.
The story is a very remarkable one. For many months during 1908 and the
early part of 1909, there was an epidemic of anonymous post cards in
Sutton, many people receiving them and no one being able to trace their
origin.
Among other people who received these cards was a Mrs. Tugwell, and in
some of them it was stated that she and another woman were "not fit
members" of a certain congregation. Suspicion fell upon the housekeeper of
the Roman Catholic priest, Annie Dewey, and mainly on the evidence of a
handwriting expert, she was committed for trial at the Assizes.
The writing on the libellous cards was undoubtedly extremely like that of
Miss Dewey, and, as events subsequently proved, was a very skilful
imitation of it by someone who wished to throw suspicion upon her. When
the Assizes came on, no evidence was offered by the prosecution and the
case was dismissed. The libels still continued, however, and Mrs. Tugwell
having received more libellous cards, her husband again took the matter
up, and Miss Dewey was once more committed for trial in March, 1910. The
trial was a very exhaustive one, but no convincing evidence was brought
against the accused, who was therefore acquitted.
In the meantime a number of suspicious circumstances pointed to the
conclusion that Mrs. Tugwell herself was the author of the libellous cards
and letters, and that she had also apparently written those that she had
received through the post.
In order to obtain proof of this the police, by arrangement with the
postal authorities, marked a large number of stamps with a sympathetic
ink, that would not become visible until it had been treated with another
reagent.
Instructions were given to the postmaster that these marked stamps were to
be supplied to none but members of the Tugwell family.
In April two more libellous post cards were sent to Canon Cafferata, a
Roman Catholic priest, and the stamps upon these cards were two of those
marked with the invisible ink.
The house of the Tugwells was now watched by the police, and one evening
when Mrs. Tugwell was seen coming out, the pillar-box close at hand was
immediately cleared of all its letters by an official. Mrs. Tugwell then
put two letters in the box, both of which contained foul libels. One of
these was addressed to a friend of hers and the other to herself.
The handwriting on both these letters was an imitation of that of Miss
Dewey. A warrant was now issued for the arrest of Mrs. Tugwell, and when
her house was searched, envelopes having the same watermark as that of the
envelopes containing the libellous letters were discovered. There were
also found some French books containing the French phrases used in the
letters, and several pieces of blotting paper upon which were words and
phrases occurring in libellous letters.