'That's true,' she murmured absently. 'But this text was not inscribed by the Greeks or Romans.' She was bent over the volume, adding a tincture to the surface of the parchment, across which I could see lettering, which appeared to be Latin, or perhaps Italian, inscribed in black. Her hair was whipped by the breeze and the door slammed shut. 'It was written much later than that.'

'At Constantinople?'

'Not at Constantinople either. Would you open the door, please? Cyanide becomes toxic when it vaporises. Next I tried a deliquescent sal ammoniac,' she continued, adding another drop. 'I made a solution by heating ammonium chloride and trapping the gas in oil of vitriol. I thought that the iron could be recovered if the tannin could not. The iron in the ink would have corroded over time, but I hoped to restore its colour if possible. But that method also failed. The erasure seems to have been made almost too well. You can understand that the process has been most time-consuming. It's taken several weeks altogether. Quite a number of successive washes.'

'Which is why I was hired,' I muttered. I was feeling ill now, I could barely stand. 'As a decoy. A pawn.'

'You created a diversion.' Another drop was added. I stumbled towards the window, bumping into her chair. Alethea, bent over the volume, seemed not to notice. 'You bought me several weeks of precious time,' she said. 'You see, not everything that I told you in Pulteney House was a lie. There is indeed a buyer for the parchment, someone willing to pay a handsome sum. But there are also those-our new Secretary of State is one-who wish to take it without paying. I believe his men paid you a visit the other night.'

I knocked the telescope from its tripod as I swung the casement wide. A pawn. A diversion. That's what I had been-nothing more. My head reeled as it had done in the crypt of the Rolls Chapel. She began describing in the same absent tone the whole grotesque ruse-the cipher, the graffiti, the curios in the coffee-house, the volume of Agrippa, the auction catalogue. All planted for me to find. All intended to lead me further and further away from Pontifex Hall and The Labyrinth of the World. And to lead others astray as well. For why should she have sent her letters through the General Letter Office unless she wished them to be opened by the agents of Sir Valentine Musgrave?

'But there are others involved,' she was saying in a distracted voice. 'Agents of powers even more treacherous than those of the Secretary of State. They too had to be led astray. Secret knowledge can be a dangerous thing. In the end even my father wanted to destroy the parchment. It was a curse, he said. Too many people already had died for it.'

I was barely listening now. Overcome by nausea I thrust my head between the mullions and sucked at the cool air. The rain was sibilant against the brickwork, and above my head the gutters roared. I could see beneath me the pointed roof of the pediment sluicing yet more rain. Then my spectacles blurred, and when I wiped them with my handkerchief I thought I glimpsed a coach beyond the stone arch, far in the distance-something barely visible as it moved through the dense foliage and rising mist. But then I was startled by an exclamation from behind me. I turned round to see Alethea holding the book aloft. Between two rows of black lettering another line, smudged and indistinct, had appeared in bright blue.

'At last,' she said. 'The reagents are beginning to take effect.'

'What is it?' The blue characters, a series of figures and letters, dipped and swam before my eyes. Again my anger began to dissipate and I found myself intrigued. 'The Hermetic text?'

'No,' she replied. 'A different one. One copied by Sir Ambrose.'

'Sir Ambrose made the palimpsest?' I could feel my hairline dampening with sweat. I sank into the chair, trembling, bewildered by the turn of events.

She nodded and once more the dropper hovered above the page. 'He was the one who copied the text and then effaced it. You see, he had already discovered two palimpsests in Constantinople. One was an Aristotelian text, the other a commentary on Homer by Aristophanes of Byzantium. Both were concealed behind parchments of the Gospels, but the old lettering had begun seeping back. It's called "ghosting", as if the former text had returned to haunt its successor. He realised soon enough how it would be the perfect disguise.'

'Disguise?'

'Yes. To hide one text within another.' More blue characters had appeared on the page, bleeding into it like ink across blotting paper, though from where I sat I could read none of them. 'It was the perfect way to smuggle a text. Especially if the reinscription was considered valueless.'

'What do you mean? Smuggle a text from where?'

From among the contents of the Imperial Library in Prague, she gradually explained as she continued her work, bent over the table as if performing delicate surgery. This had been in the year 1620, at the outset of the warring between the Protestants and Catholics. Frederick had been elected King of Bohemia one year earlier, a Protestant on a Catholic throne, and so his followers across Europe had suddenly gained access to the contents of the magnificent library assembled by the Emperor Rudolf. The nuncios and ambassadors scuttling back to Rome and her allies among the princes of the Catholic League were alarmed at this turn of events, because a library is always, like an arsenal, a locus of power. After all, had not Alexander the Great planned a library at Nineveh that he claimed would be as much an instrument of his rule as his Macedonian armies? Or when one of Aristotle's other students, Demetrius Philareus, became counsellor to Ptolemy I, monarch of Egypt, what did he advise the King to do but collect together all of the books that he could on kingship and the exercise of power? So the idea of Rudolf's great collection in the hands of Rosicrucians, Cabalists, Hussites, Giordanisti-heretics who for years had been undermining the power not only of the Habsburgs but of the Pope as well-set the tocsins ringing all over Europe. Thus as the armies of the Catholic League marched on Prague in the summer and autumn of 1620, one of their foremost aims, Alethea claimed, was the recovery-and the suppression-of the library.

'Dozens of heretical books were held in the collection,' she continued, 'copies of them had been burned in Rome and placed on the Index. Now the floodgates were about to burst. No sooner had Frederick arrived from Heidelberg than scholars from all over the Empire began their pilgrimages to Prague. The cardinals in the Sant'Uffizio realised they would soon lose control over who was allowed to read what book or manuscript. Knowledge would have been disseminated from Prague in a great explosion, fostering sectaries and revolutionaries both within Rome and without, creating still more heresies, still more books for the bonfires and the Index. The library in Prague had become a Pandora's box out of which, in the eyes of Rome, a swarm of evils was about to fly.'

I was sitting beside the window, letting the breeze cool my brow. The rain was falling harder than ever. The ceiling in the corridor had begun to leak and the vials and cuvettes were chiming together on the table. Heretical books? I scratched at my beard, trying to think.

'What manner of evil?' I asked when she fell silent, bent over the parchment. 'A new Hermetic text that the Holy Office wished to suppress?'

She shook her head. 'The Church no longer had anything to fear from the writings of Hermes Trismegistus. You of all people must know that. In 1614 the antiquity of the texts had been challenged by Isaac Casaubon, who proved beyond a doubt that they were forgeries of a later date. In the end, of course, Casaubon, for all his brilliance, turned his magnificent guns upon himself. With his book he hoped to refute the papists, Cardinal Baronius in particular. But instead he merely succeeded in destroying one of their greatest enemies.'