CHAPTER X
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.