The fumes of
ammonia have formed the precipitate of black mercurous nitrate, a
very distinct black writing which is almost indelible. That is what
is technically called invisible rather than sympathetic ink."
We leaned over to read what he had written. It was the same as the
note incriminating Dixon:
This will cure your headache.
Dr. DIXON.
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 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.
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