Summary. It was Fifi, the girl lent up from Cruikshank Manor to cover Zane's fever, and she meant the dose for Aziz, not for Xerxes at all. Here is how I came to it.

The Reckoning. The poison was never in dispute. Dr Quill's own bottle came out of his bag stoppered and near full, and the house bottle on the passage shelf was found empty and set back label-outward, tidy as a woman leaves a thing she means no one to notice she has touched. That told me the hand was a household hand, or one let into the household that night — and only one such hand was there. Zane lay abed at Cruikshank with a fever, the housekeeper and two of her people swearing to it; his absence is what put Fifi in that passage at all, the only outside pair of hands among people who have served at Hartfield House twenty years and more. I looked at who had access before I looked at who had cause, and the access narrowed to one woman inside a quarter of an hour.

The rest fell out from the cups rather than the company. Two cups, one pattern, one tray, carried together to the landing and parted there in a lamp someone had turned down low — the housemaid swore she'd left it bright, and I believed her, because a lamp turned down is a small thing a frightened girl does without thinking much of why. Xerxes drank his black. Aziz took his sweet, from a basin left dry and a spoon left clean on the tray afterward, which meant the cup meant for him was never finished at the landing — meant he who dressed it went straight back down, in a hurry, before the sweetening was done. A woman bound only for the scullery, and not meaning to dress Aziz's cup at all that night, dresses instead the cup she takes for his by its place on the tray, in a passage gone dark by her own hand. She poisoned the cup she believed was Aziz's. In the dark, by its place and nothing more, it was Xerxes's.

As for why any of the house might wish the man dead, the easy answers do not survive an hour's asking. The Colonel was pressing the dining-room for greater tonnage in the Rangoon venture the whole evening, with Cousin R to vouch for every minute of it; a man does not lobby for more of a partnership he means to end by murder, and besides he never left that room. Nan sat the drawing-room with Lady Lavinia from half eight to ten, neither out of the other's sight, and she had neither Xerxes's secret to guard nor his money to want. Effie was on the front steps with Captain Frew under the footman's eye from nine until the alarm was raised, and in any case Xerxes had lent her money twice that very week without so much as a condition attached — he was her open purse, not her obstacle, and the sapphire she is said to covet lay untouched on the body. Quill, who by the house's own reckoning could poison a man as neatly as any physician in Cairnholm, was at cards with Gaunt from a quarter to nine, and it was Quill himself who pointed out the dose was far beyond anything he'd ever prescribed — no poisoner of his skill announces his own crime from his own prescription pad.

What was left was smaller and sadder than a sapphire or a shipping line. The burnt note in the grate told the rest: a refusal, kind and final and put down in writing, after two refusals before it that the household had treated as a standing joke. And on the very night the refusal reached her, Xerxes sat Aziz at his right hand before the whole table and said twice, for all to hear, whose place that was. A woman three times refused does not always grieve the man who refused her. Sometimes she goes looking for the one she holds to blame for it. Fifi went to the tray meaning harm to Aziz, in a passage she had darkened herself, and sent the dose instead to a man who had never done her any wrong beyond kindness and a clear conscience. I make it the third this twelvemonth, by my count, and I am sorry to say I shall likely be back at Hartfield House before the year is out. The charge is murder, and it is laid against her.

Fifi, once refused in writing, laid the cups with care,
While Xerxes drank the fatal one and perished in his chair.
The bottle wore its label, and the landing lamp burned low,
As though a household murder were a minor sort of show.
We shall be more attentive, if attention can be found,
And leave our solemn promise on the mantel with the sound.