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
vapor of sulpho-cyanide. Here is a long-necked flask of the gas, made
by sulphuric acid acting on potassium sulpho-cyanide. 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.