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Steris opens the chapter on the basement stairs at Ladrian Mansion, reflecting on how long it has taken her to realise that other people do not, in fact, share the constant hovering anxiety she does. They simply don't carry it — they live somewhere between blessed contentment and frightening ignorance. As the others race down toward the wreckage of Wax's safe-box experiment, she wants to call them back, hold them safe, forbid them from putting themselves at risk; she also knows by now that her instinct to do so runs to the extreme. She loves them, and that is exactly the problem. She has to recognise the line between *useful preparation* and *over-control* — and today she gets to be useful.

She makes them send VenDell down first — a fall, after all, won't hurt a kandra. Wax goes next, in case the stairs collapse. Steris stops everyone at the bottom and distributes cloth masks from her handbag, including one for Allik, whose wooden mask won't filter air. Wax is the only one who smiles as he puts his on, and not for the first time she lets herself feel how much it matters that he finds her preparations endearing rather than persnickety. He asks what is on her watch list for explosions. Acids on the floor that may mix to chlorine gas, glass shards, secondary detonations from exposed harmonium. He confirms he had been working with hydrochloric and hypochlorous acids. Marasi hears *chlorine gas*; VenDell grabs her arm — kandra have a thing about acids — and they retreat upstairs to let the lab ventilate. When they come back down they wear the masks without complaint and let her test the air with a kit before they tread on the broken glass.

The damage is total. The door of the harmonium safe box, which had survived previous detonations of up to three ounces, has been launched across the room and embedded into the far concrete wall. The steel box itself is mangled. Half a gram of harmonium did this. Steris notes wryly that she'll need to put in an order for a new spectroscope, a centrifuge, more flasks, and *new walls*. Wax, gloved at her insistence, scrapes a fine black powder off the broken safe and dismisses it as remnants of iron from the equipment — not atium or lerasium — and she pockets the shavings in specimen pouches anyway. He retrieves a still-smouldering smear of harmonium from the back wall of the box. The metal had not divided. The experiment, in that sense, had failed.

VenDell disagrees. Politely, gravely, he says Wax has accomplished something far more dangerous. He leads them through the cosmere physics: everything is one of three essences — matter, energy, or Investiture. Matter is built of axi, the smallest particles known. Energy is heat and light and the electricity now running through the room's surviving ceiling lamps. Investiture is the essence of the gods, the stuff Wax pulls from the Spiritual Realm when he burns metal. And the three are fundamentally one substance. They can convert into one another. Energy becoming Investiture is what Feruchemy is. Investiture becoming matter is what harmonium is. And matter becoming energy is what just happened in the safe box. The amount of energy locked inside even a tiny piece of matter is incredible; Wax released only a fraction of it from a tenth of an ounce, and the lab is gone. Harmony, VenDell tells them, is *deeply* frightened by the destructive potential. VenDell's own translucent pallor as he says it goes well past anxiety. He excuses himself, white-faced, to take this back to Harmony.

Wax looks to Steris for the worst case. To her, the worst case is obvious: that the Set knows. The dying man in the cavern raved about *returning ash to the Basin*; what if their plan is bombs? If the Set has discovered the interaction, it was likely by accident — through an experiment like this one — and there will be a record. Unexplained explosions in the Outer Cities, buried in broadsheet backpages. Wax takes the thought further: an explosive like this in one of the old Ashmounts — could they be restarted? *That sounds appropriately terrifying,* Steris says, and a deep nausea moves through her at having never thought of it before. First things first: she will pull broadsheets from across the Basin to the penthouse, and with Marasi's credentials they can crack the constabulary records too.

She steps past Allik — who has discovered fragments of his chocolate biscuits splattered against a wall and is sweeping up the glass — to talk to Marasi at the wrecked safe-box door. She has been enjoying her sister more, these recent years. Their father, embarrassed by Marasi's illegitimacy and now happily retired to a country estate, never made affection between his daughters easy; Steris has always worried that the wound he opened would scar Marasi inward instead of toward him. Marasi is staring at the door with a brooding expression. *The world is changing so fast,* Marasi finally says. Electric lights, airships, a god from another world, and now an explosive a pinch of which can level a room. She wonders aloud why she still spends her time on ordinary murder cases when there are people out there moving the fates of planets. *That's why you hunt the Set,* Steris tells her. *Why you've devoted so much when most of the precinct thinks you're going overboard.* Marasi chuckles. *Runs in the family.*

Steris is about to add something when Wax blows himself up. It is a much smaller detonation than the first — only enough to throw him to the ground — and she runs over to find him dazed but largely uninjured. His Versuli vest is ripped and charred; the apron she had insisted he wear has spared the worst. He sits up, taking her arm, and admits: he was trying to gather the smouldering harmonium off the back wall of the safe box, and it had reacted to something — air, leftover liquid. *You mentioned something about secondary explosions?* *It was on my list.* He has a sneeze, then smiles to reassure her. She does not let him see how worried she is, because she vowed when she married him that she would never become an obstacle to who he wanted to be. She had decided long ago that being shot at beside him would always be less terrifying than sitting at home wondering whether he was. He pats her hand, levers himself up, and proposes they take some tea and talk this over.