Instead of obtaining his liberty, he was sent to the Bass, a very high rock surrounded by the sea; at this time converted into a state prison, and full of the unhappy Covenanters, He there remained in great misery, loaded with irons, till the year 1677, when it was resolved, by some new examples, to strike a fresh terror into the persecuted but still obstinate enthusiasts.
Mitchel was then brought before a court of judicature, and put upon his trial for an attempt to assassinate an archbishop and a privy counsellor.

His former confession was pleaded against him, and was proved by the testimony of the duke of Lauderdale, lord commissioner, Lord Hatton his brother, the earl of Rothes, and the primate himself.
Mitchel, besides maintaining that the privy council was no court of judicature, and that a confession before them was not judicial, asserted that he had been engaged to make that confession by a solemn promise of pardon.

The four privy counsellors denied upon with that any such promise had ever been given.

The prisoner then desired that the council books might be produced in court, and even offered a copy of that day's proceedings to be read; but the privy counsellors maintained, that, after they had made oath, no further proof could be admitted, and that the books of council contained the king's secrets, which were on no account to be divulged.

They were not probably aware, when they swore, that the clerk having engrossed the promise of pardon in the narrative of Mitchel's confession, the whole minute had been signed by the chancellor, and that the proofs of their perjury were by that means committed to record.