I will wet it and heat it a little. That produces the
pungent gas of ammonia.
"On one side of this third piece of paper I myself write with this
mercurous nitrate solution. You see, I leave no mark on the paper as
I write. I fold it up and drop it into the jar--and in a few seconds
withdraw it. Here is a very quick way of producing something like the
slow result of sunlight with silver nitrate. 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
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.
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