Summary. It was Effie, and for money owed to a chemist rather than for taste or grievance; here is how I came to it.

The Reckoning. The method was drug-then-flourish, and once seen it cannot be unseen. Nan's milk was set out untouched at ten and drunk half-clouded with laudanum by twenty past eleven; the window in which it might have been dosed runs between those two hours, and only one person in the house was unaccounted for across the whole of it. Effie left the drawing-room on the pretext of a shawl and was gone long enough to have fetched three. The Colonel's medicine press is unlocked, and everybody in that house knows it. She emptied his bottle into the cup, waited on the milk to do its work, and then — because she is a woman who admires other people's manners and borrows them without quite earning them — she composed the body in Cousin R's fashion, folded hands, straightened cap, a sprig of heather laid across the lap. She got it a shade wrong, as a copyist will: the heather cut-ends outward, the chair turned to the door instead of the window. Cousin R noticed the error before I did, and named it without feeling, which is his way. He had, God help him, given the household its first reading for free — his own remark that Nan had achieved a perfect absence sent the case looking at him for the better part of a day. But he was in the library with Dr Quill from nine until the alarm, neither man stirring, and a craftsman does not usually botch his own trademark.

The others fell away by the clock and by the ledger, in about equal measure. Fifi was under Mrs Gow's eye in the scullery and on the back stair the whole evening, never near the morning-room in the dosing hour, and had nothing against Nan besides — Nan's threatened disclosure concerned Effie's accounts, not the twins. Zane was off the premises entirely, sent to the Cowgate at half past nine and not seen again until quarter past eleven, the lodge-keeper and the housekeeper both able to swear to his going and his return; and whatever hold Nan once had over him regarding Lady Lavinia, she had told him plainly on the Tuesday she meant never to speak of it, which took the knife out of his hand before the milk was ever poured. Lavinia herself was with the Colonel in the small parlour from ten until eleven, by his own word, and had neither claim on the will nor quarrel with the dead woman. Quill was in the library the whole evening and had not been upstairs at all; he is, besides, the physician who had refused Effie a bottle that same week rather than supplied one, and he was the one who named the poison outright over the body instead of letting it pass for a quiet end — not the behaviour of a guilty man.

The chemist's tally under the fender is the thing I nearly overlooked, tucked away as it was and made out to no name, but it wants only the reading. It fell from Effie when she stooped to set the wiped bottle square on the mantel, and it is dated that week, for a quantity no honest household orders. Her debt at Barrow's had been called; the Colonel's will was to be fixed on Monday; and Nan, going through the household books in his absence, had found those very debts written plain across the tradesmen's accounts and meant to lay them before him on his return. Nan was not merely the rival for his favour. She was the hand that would have opened the ledger and closed Effie's chances along with it, and she had said as much to Mrs Gow only the week before. That is motive enough, and grubbier than the drawing-room's first guess, which is generally how these things turn out. It is the third such matter I have closed out of that house and its neighbour this twelvemonth, by my count, and I confess I had hoped for a quieter autumn. Effie Strange will answer for the poisoning of Nan Pringle; the charge is laid, and the file may close.

When all's said and counted, the deed it was done,
and Effie the answer, when answer was none.
Poor Nan lies quiet, a candle, a knife,
the tidiest exit from Cairnholm and life.
The lesson is here, should we trouble to learn —
but the cards, being cards, will be dealt a fresh turn.