And so I scoured the apothecaries’ shops and tapped their lore. I sought learned men and wise women. I pored over Flamel’s book until I could recite it word perfect and draw the figures in my sleep. I teased meanings from his riddles, mined the pictures until I struck new seams of understanding. I melted, alloyed, quenched and boiled. I learned more of the ways of metals than I would have in seven years in Konrad Schmidt’s shop. With many errors and missteps, I followed Flamel’s progress.

Along the way I made some curious discoveries. I burned copper oxide, reduced it with litharge and produced a liquid that was black as sin, yet dry to the touch in a very short time. Another time I alloyed lead, antimony and tin to create a wondrous new metal that melted easily over a flame, yet hardened like steel as soon as it cooled. When I showed it to Tristan he only grunted and asked if it brought us closer to the Stone.

It was not a happy time. When fatigue or Tristan’s petty cruelties drove me close to tears, I cursed my fate and despaired. What evil drove me on? I had spent ten years curing myself of my immoderate desires, years of agonies and mortification that drove me at last to the river ooze. In Basle I had been happy with a cell and a pen, a faithful servant to the ambitions of worthier men. A chance encounter and a single sentence in a book had undone it all. I felt as if I was stumbling through a dark tunnel, with an enormous burden crushing my back and chains dragging around my ankles.

But I was making progress. Gold turned black, then bronze, then a cloudy grey, then wine-red as I found ways to tinge it according to Flamel’s scheme. Silver resisted me longer, but after weeks of frustration it too yielded. At last, one night deep in November, I lifted my grinding mortar and, trembling, beheld a reddish powder the colour of cedar bark.

I dabbed a few grains on my fingertip and held them up to the lamp. It was very fine, like dust, sweet-smelling but dry as salt to the touch. There was terrifyingly little. All my weeks of labour had reduced to not much more than a thimbleful of the stuff.

I covered the bowl with an upturned jar, took the lamp and went to fetch Tristan. The house was dark, its filth disguised. As the costs of our experiments mounted, Tristan had dismissed the servants one by one until we were alone in our squalor. It made the cavernous house even more frightening. Rats played among the cobwebs just beyond the reach of my lamp; terrible creatures stalked me from the tapestries hanging on the walls. Once I knocked over a wooden stool and almost died of terror. My whole body was sunk in grim exhaustion, yet at the same time I thrilled to the wonder of the moment.

I found Tristan in his bed. A scrawny prostitute lay sprawled over him. Both were naked and half-asleep; I could see the scabs of flea bites down the backs of her legs, and something moving in her hair that looked like lice. Evidently the servants were not the only economy Tristan had made.

He propped himself up on his elbow. The prostitute rolled off him, revealing a meagre pair of breasts and a great deal of hair.

‘Have you come to join us after all?’ leered Tristan. ‘I have it.’

He pushed the whore aside and leaped out of bed, kicking over a glass of wine on the floor. He grabbed his father’s sword, which rested on a shelf in its scabbard. ‘Are you sure?’

‘There is one way to prove it.’

We returned to the tower, under the gaze of Flamel’s inscrutable figures. Black infinity yawned above us. Working in silence, I heaped up the powder on a fold of paper. I had wanted to hold some back in case the first projection did not work, but there was so little I did not dare spare a grain. I twisted it shut and sealed it with wax. A silver mirror lay on the bench from when I had tried to trap the sun’s rays: I glimpsed my reflection in it and trembled. My skin was grey, my hair thin and my eyes sunk beyond sight. The skin on my hands was pink and shiny, smooth as a baby’s from all the burns I had suffered in my haste. A splash of vitriol had seared a crescent scar into my cheek.

Tristan brought out an egg-shaped vase made of blown glass. He filled it with powdered lead which he measured in a balance, then sprinkled it with a few drops of quicksilver. Then he fitted a crystal plug to its end and burned the edges with a taper. While he did that, I shovelled coals into the furnace and worked the bellows. I watched the colours of the fire change, from red to orange to a brilliant white too painful to behold. When I saw that, I knew we were ready.

I grasped the glass egg with a pair of iron tongs and thrust it into the centre of the fire. Tristan rested his arm on my shoulder and leaned forward to look. Though the night was cold, we were both soaked in sweat.

‘How long will it take?’

‘We will know when the moment comes,’ I told him.

We stood there and watched, our bodies pressed together so close our sweat mingled into one. I hardly noticed. Steam began to curl out of the metal in the vase. The lead softened, melted and bubbled, drinking up the quicksilver.

I pulled away from Tristan and squeezed the bellows, building up the fire to new fury. The heat seared my face; smoke billowed into the tower. Tristan stumbled away with his hands over his eyes, but I stayed rooted in front of the furnace.

Something flashed in the glass and I knew the time had come. I reached in and knocked out the crystal plug with a poker, then lifted the paper twist in the tongs and flung it in. It dropped through the opening, fell onto the boiling metal in the base and burst into flame. It was the purest, whitest flame I ever saw, like sunlight on snow. And as it burned I saw an aura, an iridescent halo that filled the vase with colours. The rainbow.

I cried out to Tristan. He must have seen it too for he ran to my side. Together, we dragged the vase out of the fire and stood it upright on the floor. Tristan drew his sword, raised it and struck. The glass egg cracked in two and fell apart. Through the smoke and sweat that stung our eyes, we stared down at what we had created.

XXIII

New York City

Going back to the apartment was worse than he’d expected. A uniformed officer gave them plastic gloves and elasticated bags like shower caps to wear over their shoes, then lifted the tape that barricaded the door. Nick and Seth ducked under it into the living room.

The last time he’d seen it, Nick realised, had been on Buzz. He looked at Bret’s desk, then back where the video camera had pointed, trying to figure out where the killer had stood. Bret’s computer was gone, as was the chair he’d been tied to. So, thankfully, was the body, though there were stains on the carpet that might have been Bret, once. How long did you have to preserve the evidence at a crime scene?

He went into his own bedroom. The police must have been in here, too: it was tidy enough but disarranged, a product jammed back in its packaging. He made to sit on the bed and thought better of it. His whole body crawled with the fear that he would contaminate anything he touched. He crouched in front of the bedside table and pulled out the drawer. A leather travel wallet, a graduation present from his parents, lay buried at the back under the usual debris of aftershaves, paperbacks and condoms. He took the passport out, then slipped the wallet into his jacket pocket. You never knew what might prove useful. A gold eagle with a sheaf of arrows in its claws glared at him from the passport cover.

‘Nick?’

He pushed the drawer shut and turned around, trying not to look guilty. Seth was standing in the door, the policeman beyond watching over his shoulder. Had they seen him? Did the wallet make a bulge?

‘Got it.’ His voice sounded lifeless in the still apartment. He tossed the passport to Seth. ‘Let’s go.’