“Cyanide of potassium?”
Markham gave a slight start.
“Yes—or so the doctor thinks. There was a bitter-almond odor on her lips.” He shot his head forward angrily. “But if you knew——”
“Oh, I wouldn’t have stopped it in any case,” interrupted Vance. “I discharged my wholly mythical duty to the State when I warned the Sergeant. However, I didn’t know at the time. Von Blon just gave me the information. When I told him what had happened I asked him if he had ever lost any other poisons—you see, I couldn’t imagine any one planning so devilish and hazardous an exploit as the Greene murders without preparing for the eventuality of failure. He told me he’d missed a tablet of cyanide from his dark-room about three months ago. And when I jogged his memory he recalled that Ada had been poking round there and asking questions a few days before. The one cyanide tablet was probably all she dared take at the time; so she kept it for herself in case of an emergency.”[29]
“What I want to know, Mr. Vance,” said Heath, “is how she worked this scheme. Was there any one else in on the deal?”
“No, Sergeant. Ada planned and executed every part of it.”
“But how, in God’s name——?”
Vance held up his hand.
“It’s all very simple, Sergeant—once you have the key. What misled us was the fiendish cleverness and audacity of the plot. But there’s no longer any need to speculate about it. I have a printed and bound explanation of everything that happened. And it’s not a fictional or speculative explanation. It’s actual criminal history, garnered and recorded by the greatest expert on the subject the world has yet known—Doctor Hans Gross, of Vienna.”
He rose and took up his coat.