The jar came on a Wednesday, in the pocket of a large man's coat, wrapped in a clean white handkerchief.

Marco did not hand it to her at the door. He carried it up the narrow stair at the back of the house, one tread at a time, as if every board were a potential accident, and set it on the washstand in the slant-roofed room under the eaves where the evening sun came in through the window in a long copper bar. He unfolded the handkerchief. A small squat glass jar stood in the middle of it with a screw lid sealed in black wax. On the wax was a thumbprint, and on the label, in square pencil, a date. 11 November. Pier Four, annex floor, under stair.

"Gavin scraped it himself," said Marco. "On the morning after the fire, before the fire service had finished with the ground. I do not know why he thought of it. He says that Mr. Bellandi once told him that a good watcher takes a sample of everything." A pause. "It has been in the library safe since."

"You didn't open it."

"No, miss. It was not a thing I was told to do." He folded the handkerchief with his big careful fingers. "I am to say that you may have whatever else you ask for, and that it is no trouble, and that no one will inquire."

"Thank you."

"I'll be at the corner," said Marco, and went away down the stair.

The lab was quiet at night in the last week of August. It was the quiet of a great building with its lights turned down, a long soft humming under the floor, the click of a relay, the drip of a tap in a far corner of the basement. Dr. Pryce had changed the locks on the fourteenth of July, on a curt note to the porters that nobody had questioned, and had given Sienna the only key, which she wore on a string round her neck beneath her blouse like a girl with a medal. Pavel sat on the bench in the corridor, outside the door, with his paperback open on his knee and his ears out on stalks. He had not turned a page since eight.

There were three of them in the little room under the stairs.

Nell had come, of course. She had said that she would not be dissuaded, and had stood in the door of the sample room on the first night with her arms crossed and her chin set until Sienna gave up. She did not know what they were looking for. She did not ask. She sat on a high stool beside the sink in her cartoon scrubs, labeling tubes, and when anyone needed anything she was on her feet before it was said.

Helena stood at the second bench, in a white coat she had borrowed from a locker and had not had time to let down, with her sleeves rolled and her sleek dark hair pinned back from her face. She had asked on the first evening whether she might be of use. It was a thing she had not said to anyone for three years. Sienna had looked at her, and at the tall straight figure with its folded hands and its look of mortal courtesy, and had said, simply, that she would be glad of a pharmacist.

"A pharmacist," said Helena.

"You can get into the dispensary. I can't, not at this hour." She handed her a list. "And you know what's on the shelves."

"I know where every bottle in this hospital has been for six years," said Helena Roux. A small dry smile moved at the corner of her mouth. "It is the only thing about me that Dario has never been able to forge."

Sienna put the ash through the instrument at ten past eleven.

She had prepared it as carefully as she had ever prepared anything. She had broken the seal with a scalpel, and weighed out a quarter of a gram on the pan, and put it in a clean glass tube with a drop of dilute acid and a measure of solvent, and she had shaken it for a minute and a half and let it stand. She loaded it into the rack. It took the machine forty minutes, and she sat on her stool with her hands between her knees and did not watch the screen.

It was Helena who saw it first.

"Sienna," she said, very quietly.

The trace was coming up on the monitor, line by line, pale green on a black ground. It was a long, ugly, crowded trace, full of the carbon and the char and the ten thousand small ruins of a burned warehouse. She had been looking for one thing in it. She had looked for it for an hour before she let herself believe, and there it was. Far to the left of the main body, in the place where there should have been nothing but noise, a small narrow patient peak stood up from the baseline like a finger. She knew it. She would have known it in the dark. She had stood in front of it in October, with her hands flat on the steel, when she lifted a swab from the etched band of a crystal flute and saw it climb out of the screen.

It was not quite the same. It was a cousin. The shape was a little coarser, a little less pure, with a shoulder on the right where the lab-made thing in the glass had been clean. But the family was written in it as plainly as a name in a Bible. It carried the same signature, the same quiet metal at the center, wrapped in the same kind of carbon.

"Tin," said Sienna.

She heard her own voice go thin. She laid her hand flat against her mouth.

"It's tin. An organic salt of tin. It's the stuff that's in the grooves of the glass." She was reading it off the screen, quickly, as the figures came. "It's in the ash. It's in the ash of the warehouse, in the place where the fire was hottest, and there's a great deal of it. It didn't burn. They don't burn." A breath that was very nearly a laugh. "God. It was sitting in the wreckage the whole time, a heap of it, in a drum with the lid off."

Nell had stopped labeling. She looked from one face to the other. "Is that bad?" she said.

"It's the best thing that's happened to me in a month."

"It doesn't look like it."

"No." Sienna was laughing. It was a dreadful helpless little sound, high and cracked, and she could not stop it. She put her forehead down on her folded arms, on the cold steel, and her shoulders shook. "Oh, it's so simple. It's so stupid. It was written on the first page of the book and I never read it."

"Sienna," said Helena.

"Tinplate!"

It rang off the whitewashed walls. Nell flinched. Helena put out a hand and laid it on her shoulder, lightly, with a doctor's reflexive caution, and Sienna lifted a wet and furious and radiant face.

"He's called Tin," she said. "Isn't he? Rusk said it, he kept saying it, as if it were a thing that breathed. Tinplate would not have cared for it. Tinplate is not here. I looked at that word for a year as if it were a cargo, a sheet of steel dipped in something. Matteo told me that it was the stuff of cans. He was right. It's tin." She drew a long breath. "It's a joke. It's the stupidest, most childish joke in the world. A man with a nickname that means a cheap bright metal pretending to be silver, and he has built his whole murder out of the metal."

Helena's face had gone white. She had taken her hand from Sienna's shoulder and was holding it against her own breastbone, unconsciously, like a person feeling for a pulse. "Aurelio called him that," she said, in a whisper. "When he was very drunk, once, at the end. I was in the room. He laughed, and said, there goes my little Tin. I thought it was an endearment."

The tap dripped in the far corner of the basement. The great hospital hummed above them in the dark.

"It's not a cargo," said Sienna. "It's both. It's a man who has made himself a signature." She turned back to the screen. "Give me the ledger. The green one. The one from the library."

It was in her bag. It was the ledger that Matteo had once opened for her on a winter morning, in a long room with the gold light lying across a table, and shown her how a manifest is a promise and the book is where the promise meets the money. She turned the pages to a place where she had pressed a slip of paper. Carrow, the fourth of May. A hundred and twenty drums of industrial lubricant. Eleven kilos heavier.

"Eleven kilos," she said. "Per drum."

"Is that a great deal?" said Nell, in a small voice.

"It's thirteen hundred kilos of something that isn't oil." She did the sum aloud, on the back of a form. "Three lots over eleven months, consigned to Vance Maritime Trust, with no hazard class on the label. It's a bulk stabilizer. The kind of thing they put in plastic pipe to keep it from going brittle in the sun. A respectable, dull, industrial salt of tin, by the ton, from a firm in Carrow that sells it to anyone with a letterhead." She tapped the page. "And a very quiet man with a very good bench could take it from there."

"Take it where?" said Nell.

"To a flute," said Helena.

It was said very softly, in the voice of a woman reading out a verdict. Sienna looked at her. The tall woman stood in the blue light of the monitor with her hand still on her breastbone and the expression of someone who has just seen, in a mirror, a face she knew.

"I should have known," said Helena. "The label on that jar in my annex said stannous. It was the same family. I should have known, in April, from the look of it. It was a poor sort of lie. It was the thing a man says when he knows he will be believed."

"You were being flattered."

"I was being fed," said Helena Roux, "on small kindnesses, by an expert."

Marco came at midnight on the Thursday, and she told him all of it.

She told it in the back kitchen on Tern Street, at the oilcloth, with the lamp turned low and her father's slippers going to and fro overhead. She laid out the ash trace, and the ledger, and the three small lots, and the eleven kilos, and Marco stood at the far end of the table with his hat in his hands, and heard it to the end without moving a muscle of his big square face. When she had finished, he was silent for a long moment.

"There is a ship," he said.

She waited.

"The Carrow Star. A little coaster out of the northern port, with a cargo of what the manifest calls dock supplies." He turned his hat once. "It was in Gavin's cousin's report on Monday. It is due on the tenth of September, at Pier Four, which is now, as you know, a thing that is not ours. The lease belongs to a company called Kittiwake. It will land, Miss Hale, at the pier of Mr. Rusk's friend, on the Wednesday before the rite, and it will be taken to a shed with no manifest and no foreman."

"And you can't touch it."

"I cannot touch it," said Marco, "because I am a Bellandi man, and it is not a Bellandi pier. But there is a customs officer in the northern port who does not owe the house of Bellandi a farthing and has not been in anyone's book in his life. There is a harbormaster who dislikes Mr. Rusk. And there is," and here the great stoic face did something faintly like a smile, "an Inspector of the Port, who has had a letter."

"Hollis."

"He has agreed to a meeting on Monday. I do not know what he will say. Mr. Bellandi did not tell me what was in it. He said only that it was an honest letter." Marco paused. "If we can say to a decent man that a drum has been opened at the quay, with the stamp of Vance Maritime Trust upon it and the trace of what is in the glass in the ash of the burned shed, I think that he will go to look."

"It's the thing that I can prove." She said it slowly. "The method. Not the man. But if I can show that the stuff in the crystal came off that ship, and that the ship came to that pier with that name on the paper, then there's a line from the flute to the quay." She drew breath. "And from the quay to a man who signed for the freight."

"Mr. Vance does not sign for freight, miss," said Marco. "He has a clerk."

"Then we find the clerk."

"Yes," said Marco, gently. "I thought that you would say that."

She worked on the other half of it for nine nights.

It was the part that she had not told anyone she was doing. She had said, in the kitchen, with Helena's key on the oilcloth, that she meant to go to the rite with a chemistry set. It had been a bitter little joke, made at midnight. It had stayed in her mind for three weeks and grown, as a seed grows in the dark, into something that was not a joke at all.

A poison that needs an acid to wake it is a poison with a door. She had known that since the night of the lemons. A sour thing, poured into a glass, and a trace of the quiet metal in the groove, and the two meet in a mouth. But a door has two sides. If one could stand on the inside of it, if one could take something beforehand that would hold the quiet metal as a hand holds a bird, and keep it from waking in the blood, then the glass would be a glass and the sip would be a sip, and a bitter drink at a ceremony would be only a bitter drink. It was not a thing that she could prove. It was a hope, and it rested on a bench.

She began with the cultured cells.

The department kept them in a small steel incubator in the corner of the cold room, in rows of flat plastic dishes, each one a pale pink well of living stuff that had been alive for a hundred generations and would die on command. She loaded forty of them, in four rows of ten. She put the lab-made compound from the flute on all but the control row, and the drop of acid on the third, and she waited. Twenty minutes. At the end, in the second row, in the third, in the fourth, the cells lay in ruins. The control row bloomed.

"Right," said Sienna.

She went to the dispensary, with Helena, at a quarter past two in the morning, and they came back with a tray. It was not a dramatic tray. A pharmacy tray never is. Three small brown bottles, a box of ampoules, a roll of labels. They were the common things that a hospital keeps for the poisoning of clumsy painters and careless gardeners and children who eat the wrong berry, the agents that grip a heavy metal in the blood and carry it out in the water of the body. She had read about them in a very dry book. She had never in her life expected to bless them.

She tried the first. She added it to the dish before the acid, a drop to a well, in the proportion that Helena wrote out on a card in her upright medical hand. She waited. At the end of twenty minutes, she lifted the lid of the dish.

A third of the cells were alive.

It was not enough. It was a little more than nothing. She stood looking at the pale pink wells with a feeling in her chest of enormous effort and a very small reward, and Nell, at her elbow, drew a long breath and said, "That's good, isn't it?"

"It's a third."

"It's a third more than none."

She laughed. It surprised her. She went to the second bottle. It was a different kind of grip, a gentler one, from a different family, with a slower action, and she tried that, and a third of the dish survived, and she tried the two together, in the half measure that Helena said would not offend the liver, and she stood back, with her hands behind her, and watched the clock.

At the end of twenty minutes, nearly two-thirds of the dish were alive.

She did not say anything for a moment. She picked up the dish and held it to the light, and the cells lay in their pink bath like a thin soft carpet, still and whole. She looked at them for a long time. Then she put it down very carefully, and took off her gloves, and said to the two women beside her, in a level voice, "It slows it."

"How much?" said Helena.

"A great deal. If it's taken beforehand, with food, at a measure a person can bear. It wouldn't stop it. It wouldn't make a person safe." She was looking at the dish. "But it might turn a minute into an hour. And a person who has an hour can be given the rest."

"You would have to be certain of the hour."

"Yes."

"You would have to know the very moment."

"I know it," said Sienna.

She said it so quietly that neither of them answered. Helena looked at her across the bench, with the gray eyes very wide and very bright. Nell, on her stool, sat still, with a tube in her hand.

She slept in the lab on the Thursday night, on the camp bed behind the cold room, and came out at six in the morning to find her father's friend the porter in the corridor with a message in a hand she knew.

It was not a message. It was a box.

It stood in the middle of the floor outside the door of the little room, where Pavel had put it down, with the air of a man who has been entrusted with a wedding cake. It was a long flat case, covered in blue leather gone soft with age, with a small brass lock and a hinge of the same metal, and she knew it as one knows the face of a person one has not seen for a very long time. It was the blue box from the library cabinet. It was the one that had come to the lab in the middle of a night, with a man to carry it who would not let it out of his hand, and now it came to her in the early light on a porter's trolley, with nobody at all beside it.

"Mr. Marco brought it at five," said Pavel. He was pink to the ears. "He didn't come in. He said that there was a note."

There was a note, tucked under the strap in a square pencil hand.

He said that there was nothing to write. M.

She stood with it in her fingers for a very long time. She did not at first understand. And then she did, and it went through her like warm water through cold hands, and she pressed her fist against her mouth. He had sent the box. He had sent the thing that she needed, without being asked, because he had read her question on a scrap of blue notebook paper, and had thought about it, and had understood what the question was for. And he had put no word of his own. He had not so much as signed it with his name, only the pencil letter that Marco had added in obedience to some instruction that she could hear in every syllable. There was nothing to write. It was the only message in the world that she could not have borne to receive in a longer form.

She carried it into the little room and set it on the bench.

She opened the clasp. The lid went back with the soft sigh of an old hinge, and there they lay on their bed of gray satin, side by side. The two tall slender flutes with the spiral of tiny diamond facets climbing each stem, and the thin band of etched leaves at the rim. She had held one in her hand on a September night, in a top room, with the lamplight turning the wine to gold. Beside them, in a little niche cut in the satin, stood a small stoppered cruet of heavy old glass, with a thread of gilt round the neck, and a folded paper under it.

She took the paper out. It was not in his hand. It was in a spidery eighteenth-century copperplate that had gone brown with age, and it was headed in a large careful script, The Order of Saint Ilaria. A book of the ceremony, six pages, stitched together with a bit of faded silk. She sat down on the stool, and began to read.

She read it all, slowly, in the cold bright light of the morning, while the great hospital woke above her and a trolley went rattling by in the corridor.

It was a very old ceremony, and a simple one. It had been invented, so the book said, in the year of the great fire, by the first Matteo Bellandi, who had married a poor girl from the quay and wished the house to remember what a house costs. At the hour appointed, in the presence of the assembled kin, the bride was to be led to the head of the table. A cup was to be set before her. It was to be of the old crystal, the cup of the bride of the first Matteo, and it was to be filled by the senior of the house with a measure of verjuice. She was to drink it first, before the husband, before the guests, before the toast. It was bitter, said the book. It was meant to be. The bride was to taste, in a single draught, what the house had cost, so that she might never say afterward that she had not been told.

Verjuice.

She turned the cruet in her hand. It was a heavy thing, thick with the grime of a century, and the liquid in it was pale green and slightly cloudy, with a smell that caught the back of her throat. She took out the stopper. It was the sour juice of the unripe grape, as old as cooking, the sharpest acid in any kitchen in the country. She laid a strip of paper against it. It went the red of a hot coal.

"Oh," said Sienna, very softly.

She sat still with the cruet in one hand and the book in the other. She did not need to work it out. She had it entire, as one has a sum when one has stared at the numbers long enough. A glass that would be taken from a cabinet and cleaned with kindness, by a man who had done it a hundred times. A bride who would be handed the cup in front of a hundred and forty witnesses, and asked to drink, as no bride of the house had ever been asked to refuse. A sour thing, with a ceremony around it so old that no one would think to test it, and a plain unanswerable reason for every drop. It was the most elegant instrument in the world. He had not needed to bring anything with him. The house had made it for him, a hundred and forty years ago, and polished it, and kept it in a cedar room against the day.

He had not chosen the fourteenth for the dark. He had chosen it for the cup.

She thought of two long glasses on a sideboard, and a white ribbon, and a man's hand lifting a bottle out of the ice. She thought of a man on his knees on a rug who had wished her a very long life and been afraid to say what he knew. She thought of a quiet pink-faced man with an orange, and of a sixty-six-year-old woman in a paper crown, and of an old man in a basement room who had held out a cup of tea with a pinch of salt on the saucer. There were so many ways in which she might be asked to drink.

She put the stopper back. She put the book down on the bench, and sat with both hands pressed flat against the steel, and let herself look at the plain bare fact, as she had learned to look at every other.

She was going to go.

It was not the chemistry. It was not that she alone of all the people in the world knew the door and the key, though she did, and it was true, and it was the thing she had told Helena on an oilcloth at midnight. It was something else, and she made herself say it, in her own mind, in plain words, with her eyes on the blue leather lid.

She wanted to be in the room.

She wanted to be in the room where he was. She wanted to walk up a long hall in a family gown with a hundred and forty strangers watching, and take a cup from an old man's hands, and look across the table at a man with dark hair falling over his forehead and his heart standing in his eyes. She wanted him to see her come. She wanted to stand there, on the date and at the hour and in the place where a woman had once died, and not be afraid. She wanted it so much that it frightened her more than the glass.

It was a terrible thing to find at the bottom of a calculation. She had come down so carefully, step by step, through the cold tidy figures, looking for a reason that she could put in a report. And at the bottom, where the figures ended, there was only a girl in a lab coat, sitting on a stool at six in the morning, with her face in her hands, who had not stopped loving a man for one hour since she took off a ring.

She took out the blue notebook. She did not write a number. She wrote the date at the top of a clean page, and the word Verjuice, and under it, a single line.

I am not going because I must. I am going because I can.

She looked at it for a long time. She did not cross it out.

In the corridor, outside the door, Pavel turned a page of his paperback, for the first time in nine hours, and a very large man's voice said at the end of the passage, in a low and rueful growl, that he would put the kettle on.