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 hence obliterating the
writing, while at the same time the invisible writing in the mercurous
nitrate involving Dr. Dixon's name would be brought out by the ammonia
indelibly on the other side of the note - he forgot" - Kennedy was
now speaking eagerly and loudly - "that the sulpho-cyanide vapours
could always be made to bring back to accuse him the words that the
ammonia had blotted out.
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