The Poisoned Pen | Page 9

own story, you took them out of the safe and left
them in the sunlight all day. The process that had been started earlier in
ordinary light, slowly, was now quickly completed. In other words,
there was writing which would soon fade away on one side of the paper

and writing which was invisible but would soon appear on the other.
"For instance, quinoline rapidly disappears in sunlight. Starch with a
slight trace of iodine writes a light blue, which disappears in air. It was
something like that used in the Thurston letter. Then, too, silver nitrate
dissolved in ammonia gradually turns black as it is acted on by light
and air. Or magenta treated with a bleaching-agent in just sufficient
quantity to decolourise it is invisible when used for writing. But the
original colour reappears as the oxygen of the air acts upon the pigment.
I haven't a doubt but that my analyses of the inks are correct and on one
side quinoline was used and on the other nitrate of silver. This explains
the inexplicable disappearance of evidence incriminating one person,
Thurston, and the sudden appearance of evidence incriminating another,
Dr. Dixon. Sympathetic ink also accounts for the curious circumstance
that the Lytton letter was folded up with the writing apparently outside.
It was outside and unseen until the sunlight brought it out and
destroyed the other, inside, writing - a change, I suspect, that was
intended for the police to see after it was completed, not for the defence
to witness as it was taking place."
We looked at each other aghast. Thurston was nervously opening and
shutting his lips and moistening them as if he wanted to say something
but could not find the words.
"Lastly," went on Craig, utterly regardless of Thurston's frantic efforts
to speak, "we come to the note that was discovered so queerly
crumpled up in the jar of ammonia on Vera Lytton's dressing-table. I
have here a cylindrical glass jar in which I place some sal-ammoniac
and quicklime. I will wet it and heat it a little. That produces the
pungent gas of ammonia.
"On one side of this third piece of paper I myself write with this
mercurous nitrate solution. You see, I leave no mark on the paper as I
write. I fold it up and drop it into the jar - and in a few seconds
withdraw it. Here is a very quick way of producing something like the
slow result of sunlight with silver nitrate. The fumes of ammonia have
formed the precipitate of black mercurous nitrate, a very distinct black
writing which is almost indelible. That is what is technically called

invisible rather than sympathetic ink."
We leaned over to read what he had written. It was the same as the note
incriminating Dixon:
This will cure your headache. Dr. DIXON.
A servant entered with a telegram from New York. Scarcely stopping
in his exposure, Kennedy tore it open, read it hastily, stuffed it into his
pocket, and went on.
"Here in this fourth bottle I have an acid solution of iron chloride,
diluted until the writing is invisible when dry," he hurried on. "I will
just make a few scratches on this fourth sheet of paper - so. It leaves no
mark. But it has the remarkable property of becoming red in vapour of
sulpho-cyanide. Here is a long-necked flask of the gas, made by
sulphuric acid acting on potassium sulphocyanide. Keep back, Dr.
Waterworth, for it would be very dangerous for you to get even a whiff
of this in your condition. Ah! See - the scratches I made on the paper
are red."
Then hardly giving us more than a moment to let the fact impress itself
on our minds, he seized the piece of paper and dashed it into the jar of
ammonia. When he withdrew it, it was just a plain sheet of white paper
again. The red marks which the gas in the flask had brought out of
nothingness had been effaced by the ammonia. They had gone and left
no trace.
"In this way I can alternately make the marks appear and disappear by
using the sulpho-cyanide and the ammonia. Whoever wrote this note
with Dr. Dixon's name on it must have had the doctor's reply to the
Thurston letter containing the words, 'This will not cure your headache.'
He carefully traced the words, holding the genuine note up to the light
with a piece of paper over it, leaving out the word 'not' and using only
such words as he needed. This note was then destroyed.
"But he forgot that after he had brought out the red writing by the use
of the sulpho-cyanide, and though he could count on Vera Lytton's

placing the note in the jar of ammonia and