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“Do you recognize it?”

“I’m afraid not,” says Vidocq.

“Me neither,” Allegra says.

“Do you mind if I run some tests?” says Vidocq.

“Please do.”

He takes the knife to his worktable, sets it on an iron disc the size of a dinner plate, selects a green bottle from a jumble of similar bottles at the back of his table. He gives it a shake and unstoppers it. I leave my coffee and go over.

“What is that?”

Allegra stands on his other side.

“My own invention. A personal amalgam of quicksilver, sulfur, and other rarer elements I’ve gathered in my travels.”

“What’s it going to do?”

“It reveals the history and composition of any object. Its true nature. Let’s see what it tells us about your knife.”

He puts an eyedropper into the bottle and suctions up a small potion of shimmering silvery metal. Holding the tip over the knife, he lets three drops fall.

The mercury slides down the length of the blade, making it look soft and liquid. A few seconds later, it begins to sizzle like someone frying an egg with a blowtorch.

I lean in for a better look.

“Is it supposed to do that?”

“Not necessarily,” says Vidocq.

Smoke rises from the boiling metal. It shudders. Turns yellow, then deepens to black. The mercury cracks like a broken roadbed, silver veins of the knife blade visible beneath the charred metal crust. A few seconds later, the black fades and the mercury turns back to its original shimmering form, flowing off the tip of the blade. When it falls on the worktable, it spreads and burns a poker-­chip-­size hole in the wooden surface, sending up a ribbon of gray smoke.

Like me, Allegra leans in to watch.

Vidocq pushes us both back.

“Don’t inhale the vapors,” he says.

The smoke stinks. I go to a window and open it.

“I’m guessing that hasn’t happened before.”

“What did we just see?” says Allegra.

Vidocq rubs his chin with the knuckle of his thumb.

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nbsp; “I don’t know. It’s never reacted so violently before.”

I reach for the knife and Vidocq pushes my hand away.

“I wouldn’t do that,” he says.

He takes a dark, ragged chamois from a drawer and wipes down the whole knife, holding it in a set of heavy pliers that look like they came from a yard sale at Hannibal Lecter’s. I point at the chamois.

“What is that?”