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 decolorise it is invisible when used
for writing. But the original color 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 chance, 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."