He must resort to pressure.

With the potion and the man in his possession, he must force the secret from Basterga; force it by threats or promises or aught that would weigh with a man who lay helpless and in a dungeon.

It would not be difficult to get the truth in that way: not at all difficult.

It seemed, indeed, as if Providence--and Fabri and Petitot and Baudichon--had arranged to put the man in his power _ad hoc_.
He hugged this thought to him, and grew so enamoured of it that he wondered that he had not had the courage to seize Basterga in the beginning.

He had allowed himself to be disturbed by phantoms; there lay the truth.