I refused to budge. 'Upstairs?'

'Yes. To the laboratory. You see, Mr. Inchbold, that is where you will find it. In the laboratory.'

'Find what?'

'Lock the doors, Phineas.' She had turned round and begun climbing, lifting her skirts and swaying up the steps. 'Allow no one inside. Mr. Inchbold and I have matters to discuss.'

'Find what?' I was bellowing again, feeling the anger rise inside me. Somehow I had been wrong-footed. Yet again I had lost my advantage. 'What are you talking about?'

'The object of your search, Mr. Inchbold. The parchment.' She was climbing still, ascending the great marble helix. Once more her voice echoed in the vast well. 'Come,' she repeated, turning to beckon me. 'After so many troubles do you not wish to see The Labyrinth of the World?'

Borax, sulphur, green vitriol, potash… My eyes roved over the legends inscribed on the vials and bottles littered among the bubble-shaped still-heads with their coiled glass tubes. Yellowish chemicals, green ones, white, rust, sky-blue. The stink was even stronger and more tart than I remembered. My membranes prickled and my eyes began to water. Oil of vitriol, aqua fortis, plumbago, sal ammoniac…

Reaching for my handkerchief I paused in mid-gesture. Sal ammoniac? I glanced again at the vial, at the colourless crystals, remembering the recipes for sympathetic ink, for inks that, like those made with sal ammoniac, could only be read if the page was heated by a flame. I felt a soft thrill of excitement briefly rouse itself; I also felt dizzy, as if my fever were returning.

'Ammonium chloride,' explained Alethea, catching my gaze. She was standing beside me, breathing audibly on account of our climb. 'Essential to alchemical transformations. The Arabs made it from a mixture of urine, sea salt and chimney soot. The first mention of it is found in the Book of the Secret of Creation, a work that the Muhammadans in Baghdad attributed to Hermes Trismegistus.'

I nodded dumbly, remembering my researches of a week or two earlier. But by now I had spotted something else in the room, the vial marked 'potassium cyanide', which was sitting three-quarters empty on the table before the open casement. Beside it stood the telescope, still on its tripod, pointed at the heavens. The copies of Galileo and Ortelius had been removed and replaced by another volume, a slimmer one half-buried in the detritus of the laboratory, a score of pages bound in a cover of tooled leather.

'The laboratory belonged to my father,' Alethea explained as she crossed to the table. 'He built it in the undercroft, where he conducted many of his experiments. But I've moved what little equipment remains into this room.' She paused briefly to lean across the table and pick up the vial of potassium cyanide. 'I required better ventilation for my purposes.'

I watched nervously as she unstoppered the poison. I was still trembling from my outburst in the atrium. I was also embarrassed. It had all been so unlike me. I wondered briefly if I ought to apologise-and then had to bank down yet another wave of anger and self-pity.

She had set the vial back on the table and begun rummaging among the other objects. She seemed to be shifting into and out of focus, so I lifted my spectacles from the bridge of my nose and wiped at my eyes with the handkerchief, which came away smudged with blood. When I replaced the spectacles she was turning round, the leather-bound volume-a volume bound in the style known as arabesco or arabesque-in her hands.

'Here, Mr. Inchbold.' She extended the book. 'You find it at last. The Labyrinth of the World.'

I made no move to accept the volume. By now I was wary of her talent for drawing the wool over my eyes; for making me feel like an awkward schoolboy. I would not be made a fool of again, I told myself. Besides, at this point I was more interested in that tiny bottle of poison, which I seemed to remember had been fuller before. Once again I considered the stories about the fine ladies of Paris and Rome poisoning their husbands. But then I felt her eyes searching mine and so asked, grudgingly, where she found it.

'I didn't find it anywhere,' she replied, 'because it was never lost in the first place. Not in the way that you understand. It's been at Pontifex Hall all the while. It's been here in the house, carefully hidden, for forty years.'

'It's been in your possession all this time? You mean to say that you hired me to locate a book that-'

'Yes and no,' she interrupted, opening the front cover. 'The parchment has been in my possession, that much is true. But matters are not quite so simple as that. Please…' She motioned me forward. The bitter scent of almonds had added itself to the mélange of smells. In the poor light I could see the ex-libris embossed on the volume's inside cover: Littera Scripta Manet. 'Stand over here, if you please. You're just in time to see the last wash.'

'The last wash?' Once again I didn't budge, only watched as she took up the vial again and sprinkled a measure of crystals into a solution of what appeared to be water.

'Yes.' She was unstoppering another bottle. 'It's in palimpsest. Do you know what that means? The parchment has been reinscribed, so the writing must be recovered by chemical means. The process is a most delicate one. Also highly dangerous. But I believe I've finally discovered the proper reagents. I made potassium cyanide by adding sal ammoniac to a mixture of plumbago and potash. The process is described in the work of a Chinese alchemist.'

I crept forward, made curious almost despite myself. I had heard stories of palimpsests, those ancient documents that had been discovered in monastic libraries and suchlike: old texts effaced from parchments on to which new ones had been inscribed. Greek and Latin scribes were known to recycle parchment whenever they ran short, erasing one text by soaking the leaves in milk and then scrubbing at the ink with a pumice-stone before reinscribing the surface, now blank, with a new one, so that one text lay dormant and hidden between the lines of another. But nothing disappears for ever. Over the centuries, because of atmospheric conditions or various chemical reactions, the effaced text sometimes returns, barely legible, to deliver its forgotten message between the interstices of the new script. So it was that a number of ancient books had been occulted and then discovered, centuries later: the frolics of Petronius interrupting the earnest Stoicism of Epictetus, or priapeia insinuating their bawdy verses between the Pauline Epistles. Littera scripta manet, I thought: the written word abides, even under erasure.

I was leaning forward, squinting at the cockled page. Alethea had opened the window even wider and was now prising the lid from another vial, this one marked 'green vitriol'. So was that, I wondered, how Sir Ambrose had come upon The Labyrinth of the World? Between the lines of another text? I was intrigued. What bookseller has not dreamed of finding a palimpsest, some text that for a millennium has been lost to the world?

'I tried an Aleppo gall at first.' She was carefully mixing the solution. I coughed gently into my handkerchief. The bitter smell had grown even stronger. 'The tannin should have bitten deeply into the parchment even after the gum arabic was dissolved. I thought a tincture of crushed gall might bring it back to the surface but…'

'Tannin?' I was trying to recall what I knew about ink, which was hardly anything at all. 'But the ink will be made from carbon, will it not? From a mixture of lampblack or charcoal? That was how the Greeks and Romans made their ink, after all. So an oak gall will be of little use if you wish to-'