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.

* * * * *