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The Poisoned Pen by Arthur B. (Arthur Benjamin) Reeve
page 26 of 387 (06%)
"It is like the start of the substituted letter, and the other is
like the missing note," gasped Leland in a daze.

"Yes," said Kennedy quickly. "Leland, no one entered your office.
No one stole the Thurston note. No one substituted the Lytton
letter. According to your 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."
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