Gold!" Mitchington, whose work hitherto had not led him into the mysteries of detective enterprise, nodded delightedly.
"Good!" he said.

"Rare idea! I should never have thought of it! And--what do you make out of that, now ?" "Nothing," replied Jettison.

"But--a good deal out of what I've learned since that bit of a discovery.

Now, put it to yourself--whoever it was that paid Collishaw that fifty pounds in gold did it with a motive.

More than one motive, to be exact--but we'll stick to one, to begin with.