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