"Hullo!" came--"is that Dr. Stuart?"

"Yes. That you, Inspector? I have just remembered something which I
should have observed in the first place if I had been really wide-awake.
The envelope--you know the one I mean?--the one bearing the number, 30,
has been sealed with a Chinese coin, known as _cash_. I have just
recognized the fact and thought it wise to let you know at once."

"Are you sure?" asked Dunbar.

"Certain. If you care to call at my place later to-day I can show you
some _cash_. Bring the envelope with you and you will see that the
coins correspond to the impression in the wax. The inscriptions vary
in different provinces, but the form of all _cash_ is the same."

"Very good. Thanks for letting me know at once. It seems to establish
a link with China, don't you think?"

"It does, but it merely adds to the mystery."

Coming out of the call-box, Stuart proceeded home, but made one or
two professional visits before he actually returned to the house. He
now remembered having left his particular _cash_ piece (which he
usually carried) in his dispensary, which satisfactorily accounted
for his failure to find the coin in his waistcoat pocket. He had
broken the cork of a flask, and in the absence of another of correct
size had manufactured a temporary stopper with a small cork to the top
of which he had fixed the Chinese coin with a drawing-pin. His purpose
served he had left the extemporised stopper lying somewhere in the
dispensary.

Stuart's dispensary was merely a curtained recess at one end of the
waiting-room and shortly after entering the house he had occasion to
visit it. Lying upon a shelf among flasks and bottles was the Chinese
coin with the cork still attached. He took it up in order to study the
inscription. Then:

"Have I cultivated somnambulism!" he muttered.

Fragments of black sealing-wax adhered to the coin!

Incredulous and half fearful he peered at it closely. He remembered
that the impression upon the wax sealing the mysterious envelope had
had a circular depression in the centre. It had been made by the head
of the drawing-pin!

He found himself staring at the shelf immediately above that upon
which the coin had lain. A stick of black sealing-wax used for sealing
medicine was thrust in beside a bundle of long envelopes in which he
was accustomed to post his Infirmary reports!

One hand raised to his head, Stuart stood endeavouring to marshal his
ideas into some sane order.