Your son Johnnie might be blamed for carelessness, nothing more."
"I - I don't know what you mean," gasped Dinsmead.
"I think you do." Mortimer took up a second teacup and filled a second test-tube.
He fixed a red label to one and a blue label to the other. "The red-labeled one," he said, "contains tea from your daughter Charlotte's cup, the other from your daughter Magdalen's.
I am prepared to swear that in the first I shall find four or five times the amount of arsenic than in the latter."
"You are mad!" said Dinsmead.
"Oh, dear me, no. I am nothing of the kind.
You told me today, Mr Dinsmead, that Magdalen was not your own daughter.
You lied to me. Magdalen is your daughter.
Charlotte was the child you adopted, the child who was so like her mother that when I held a miniature of that mother in my hand today I mistook it for one of Charlotte herself.
You wanted your own daughter to inherit the fortune, and since it might be impossible to keep Charlotte out of sight, and someone who knew her mother might have realized the truth of the resemblance, you decided on, well - sufficient white arsenic at the bottom of a teacup."
Mrs Dinsmead gave a sudden high cackle, rocking herself to and fro in violent hysterics.
"Tea," she squeaked, "that's what he said, tea, not lemonade."
"Hold your tongue, can't you?" roared her husband wrathfully.
Mortimer saw Charlotte looking at him, wide-eyed, wondering, across the table.
Then he felt a hand on his arm, and Magdalen dragged him out of earshot.
"Those," she pointed at the phials -
"Daddy. You won't -"
Mortimer laid his hand on her shoulder.
"My child," he said, "you don't believe in the past.
I do.
I believe in the atmosphere of this house.
If he had not come to this particular house, perhaps - I say perhaps - your father might not have conceived the plan he did.
I will keep these two test-tubes to safeguard Charlotte now and in the future.
Apart from that, I shall do nothing - in gratitude, if you will, to the hand that wrote S.O.S."