We looked at each other aghast. Thurston was nervously opening
and shutting his lips and moistening them as if he wanted to say
something but could not find the words.

"Lastly," went on Craig, utterly regardless of Thurston's frantic
efforts to speak, "we come to the note that was discovered so
queerly crumpled up in the jar of ammonia on Vera Lytton's
dressing-table. I have here a cylindrical glass jar in which
I place some sal-ammoniac and quicklime. 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