“I wouldn’t say that. I just—don’t know. Anthony was in the habit of taking arsenic. His wife got it for him. One day, by mistake, he takes far too much. Was the mistake his or his wife’s? Nobody could tell, and the jury very properly gave her the benefit of the doubt. That’s all quite right and I’m not finding fault with it. All the same—I’d like to know.”

Captain Haydock transferred his attention to his pipe once more.

“Well,” he said comfortably. “It’s none of our business.”

“I’m not so sure. . . .”

“But surely—”

“Listen to me a minute. This man, Merrowdene—in his laboratory this evening, fiddling round with tests—you remember—”

“Yes. He mentioned Marsh’s test for arsenic. Said you would know all about it—it was in your line—and chuckled. He wouldn’t have said that if he’d thought for one moment—”

Evans interrupted him.

“You mean he wouldn’t have said that if he knew. They’ve been married how long—six years you told me? I bet you anything he has no idea his wife is the once notorious Mrs. Anthony.”

“And he will certainly not know it from me,” said Captain Haydock stiffly.

Evans paid no attention, but went on:

“You interrupted me just now. After Marsh’s test, Merrowdene heated a substance in a test tube, the metallic residue he dissolved in water and then precipitated it by adding silver nitrate. That was a test for chlorates. A neat unassuming little test. But I chanced to read these words in a book that stood open on the table:

H2SO4 decomposes chlorates with evolution of CL4O2. If heated, violent explosions occur; the mixture ought therefore to be kept cool and only very small quantities used.”

Haydock stared at his friend.

“Well, what about it?”

“Just this. In my profession we’ve got tests too—tests for murder. There’s adding up the facts—weighing them, dissecting the residue when you’ve allowed for prejudice and the general inaccuracy of witnesses. But there’s another test of murder—one that is fairly accurate, but rather—dangerous! A murderer is seldom content with one crime. Give him time, and a lack of suspicion, and he’ll commit another. You catch a man—has he murdered his wife or hasn’t he?—perhaps the case isn’t very black against him. Look into his past—if you find that he’s had several wives—and that they’ve all died shall we say—rather curiously?—then you know! I’m not speaking legally, you understand. I’m speaking of moral certainty. Once you know, you can go ahead looking for evidence.”

“Well?”