“Oxy-gen? What’s that?” Earnshaw asked, suspicious.

“It’s a gas in the air you breathe. Discovered in 1774 by Joseph Priestley. Necessary to support your life, and also, in a pure form, combustible.”

Daniel remembered why he’d studied art and history at university and not science.

“If you do not have oxygen, a fire cannot burn. It’s why when you put a jar over a candle, the flame goes out.” Rhynwick spread his hands, as if asking them to come along this simple journey of logic with him.

“I’ve heard of mills exploding before, but I thought machinery caused it. Something catching fire in the works,” Ed said.

“Though machinery can catch fire, explosions in mills are almost always caused by flour dust in the air igniting. It only takes a pinch of flour per cubic foot of air to combust. A particle of flour burns instantly, igniting the ones around it, flashing quickly. That’s why this fire did not burn long. There cannot have been too much flour in the air, because the blast was fairly contained. It went outward from here, fast and hard enough to kill the two men who caused it, though they wouldn’t have known it. The men were shoveling, which stirred flour into the air. A single spark from a shovel hitting the stone wall would have been enough to set things in motion.”

Removing a handkerchief from his pocket, Rhynwick wiped his hands, the inquisitive light dimming in his eyes, as if, now that he’d solved the puzzle, he was no longer interested.

“So it was an accident?” Earnshaw asked. “What about the threats from the Luddite anarchists? Anyway, there is adequate ventilation in the mill to avoid such explosions.”

“That may be so, but there are no windows in this passageway, and that is where the flour was stirred into the air. The doors to the offices were closed when the blast occurred.”

“How do you know that?” Daniel asked. Not that he doubted the young man, just that he wanted to know.

“The glass in the door. If the door had been open, the blast would have broken the glass, and it would all be lying along the wall. However, the glass is strewn across the room. I’m surprised the latch held when the explosion occurred. Before you ask”—he held up his hands—“if the door had blown open, there would be a gouge in the wall where the knob hit.” He inclined his head toward both offices. “I would have expected more paperwork in these rooms.” A pair of lines formed between his black brows, and his head lifted like a dog’s on a new scent.

“There was. We’ve removed it for examination.”

“I will need all of it back,” Earnshaw dictated. “How am I to report the state of her father’s affairs to Miss Montgomery if you have removed all the documents? Those belong to Miss Montgomery, and I shall have to insist you return them at once.”