But through shrewd questioning of the servants in the house he ascertains that the husband was taken violently ill after supper and that no guests were present at the meal.
An analysis of the sediment in the husband's coffee cup establishes the presence of arsenic.

It must be inferred that the wife's cup contained none of the poison, for she developed no symptoms of poisoning after the meal.
The servants declare that the wife invariably made the after-dinner coffee in a percolator that stood on the sideboard.

On the night in question, she had boiled the coffee, but none of the servants had seen her draw it from the percolator or serve it in the cups.

But all of them assert that for a year or more it had been the wife's custom to do the serving, so it is a fair inference that the husband did not leave his seat at the table to help himself to coffee, on the occasion of his fatal illness.

No one but the wife being in the room with him, and it having been ascertained that she purchased the arsenic, hers was the exclusive opportunity to drop it into the cup--and the evidence against her is complete.
A case of this nature is not established by the deductive methods of a Lecocq, but by the patient labor of a score or a half score of detectives intelligently guided by their chief.