The memory of crossing the River Thames—my coughing fit, the woman feeding her baby, Beatrice—seized me at once. “All night and into this morning,” I breathed. “It requires more than just harvesting the beetles. They must be roasted over the fire and ground.” I pointed to the mortar bowl and pestle across the small room; the bowl was as wide across as the woman’s bodice. “I ground them up in that basin over there.”

The lady, whose name I still did not know, lifted the jar of powder and shifted it in the light. “Do I simply drop the powder into a bit of food or drink? Is it really so simple?”

I crossed my ankles and leaned back in my chair. “You asked for something to incite lust. Cantharides are meant, foremost, to arouse. Blood will rush into the loins, and overtake—” I paused, aware that Eliza continued to listen closely. I turned to her. “This is not for your ears. Might you consider stepping into the storage room?”

But the woman placed her hand over mine and shook her head. “It is my powder, is it not? Go on. Let the girl learn.”

Sighing, I continued, “The swelling of the groin is insatiable. This arousal will continue for some time, then will be accompanied by abdominal pain and mouth blisters. I suggest you brew something dark—a molasses liqueur, perhaps—and drop in the powder, then give it a good stir.” I hesitated, choosing my words carefully. “A quarter of the jar, and he will not survive the night. Half of it, and he will not survive the hour.”

There was a long pause as the lady considered this, and the only noise came from the clock ticking by the door and the snapping of the fire. I remained still, my previous unease of the woman’s visit returning with a vengeance. She mindlessly touched the thin wedding band adorning her hand, her gaze locked on the low flame behind me, fire dancing in her eyes.

She lifted her chin. “I cannot kill him. I cannot have a child if I kill him.”

At once, I feared I had not properly explained the danger of the powder. My voice began to shake. “I assure you, this is a deadly poison. You cannot safely administer a nonfatal amount—”

She raised her hand to stop me. “You misunderstand me. I do, indeed, seek a deadly poison. I only mean to say, it is not him I want to kill. It is her.”

Her. I flinched at this final word; there was nothing more I needed to know.