Summary. It was Nan Pringle, the poor relation of Cruikshank Manor, and the poison was never meant for Lady Lavinia at all; here is how I came to it.

The Reckoning. The cup that killed her was set plain, and Lavinia never took her tea so — she asked it plain that day only on the spirits' say-so, sugar being wrong, she was told, for Mercury in retrograde. The Reverend Gaunt, who does take his tea plain and always has, was left the sugared cup, and made his sour face at sweetness he never wanted. A poisoner arranges her cups to a habit, not to a whim, and it was Gaunt's habit that was served that afternoon, not Lavinia's. The plain cup was meant for him. Lavinia only walked into it, on the spirits' bad advice.

The pouring was Nan's alone. She sits nearest the tray, always has, and it is a mark of this house that no one troubles to watch her hands at it — she is furniture to them, the poor cousin who keeps the sugar bowl filled and is thanked for nothing. Dr Quill, who took his own port and no tea that day, examined the body and named it plainly: a deliberate hand, not any tonic gone wrong, not any mishap of the sitting. That much was never in doubt. The doubt was whose hand.

I put the Colonel by first, and he falls easiest — held the whole while in the hall by Effie Strange's talk, never near the tray until the outcry brought him running. Effie herself is cleared on the same ledger; she kept him there, and never touched cup nor tray. Wat Frew came late and was kept waiting in the morning-room by Mrs Gow's own account, admitted only after the cup was drunk. Xerxes Hartfield was over papers in the Colonel's study, Mrs Gow again at his elbow with her ledgers, and came in only at the noise. Cousin R sat apart at the window and took no tea at all — no hand in the pouring, and nothing in the composing of the room that answers to him. Quill I set aside almost as soon as I took it up; the man who poisons the cup does not then stand over the body and call it poison for the coroner's jury. And Gaunt himself I cleared last of the principals, for the plainest of reasons: he drank the sugared cup and lived. No man poisons the cup that is set for himself.

That left the tray, and the hand that filled it. Nan had no want of money in her — she wants nothing, being given nothing, which is its own grievance, though not the one that moved her. What moved her was smaller and nearer than any of the household's tangled matters of purses and confidences and Xerxes's fortune. Gaunt had been at her again that very day, as he is always at her, pressing his half-sister's poor relation to make her match and marry Xerxes and be done with being a burden underfoot. She had borne it a long while. She could not bear it further. The plain cup was dosed for the plain-tea drinker who would not leave off nagging her into a marriage she did not want, and it found Lavinia's throat only because Lavinia trusted the spirits over her own habit that one afternoon, and asked for the wrong cup.

I had near forgot one thing, and it settles the matter past arguing: it is Nan who fills the sugar bowl each week, and Nan who would have known, better than any soul at that table, that Gaunt takes his tea plain and always has. She did not misjudge whose cup was whose. She judged Lavinia's, and lost the wager to the almanac. The charge is laid against her — the third this twelvemonth, by my count, and the year not out.

At Nan's steady hand by the pot burning bright,
Sweet Lavinia took up her cup for the night,
The sugar forbidden by phases of moon,
She drank down the draught that would bring her to soon,
The moral, we're certain, lies plain as a bell,
Yet none will take warning from Lavinia's knell.