“Indol?” repeated Pitts.

“Is,” explained Kennedy, “a chemical compound—­one of the toxins secreted by intestinal bacteria and responsible for many of the symptoms of senility.  It used to be thought that large doses of indol might be consumed with little or no effect on normal man, but now we know that headache, insomnia, confusion, irritability, decreased activity of the cells, and intoxication are possible from it.  Comparatively small doses over a long time produce changes in organs that lead to serious results.

“It is,” went on Kennedy, as the full horror of the thing sank into our minds, “the indol-and phenol-producing bacteria which are the undesirable citizens of the body, while the lactic-acid producing germs check the production of indol and phenol.  In my tests here to-day, I injected four one-hundredths of a grain of indol into a guinea-pig.  The animal had sclerosis or hardening of the aorta.  The liver, kidneys, and supra-renals were affected, and there was a hardening of the brain.  In short, there were all the symptoms of old age.”

We sat aghast.  Indol!  What black magic was this?  Who put it in the food?

“It is present,” continued Craig, “in much larger quantities than all the Metchnikoff germs could neutralise.  What the chef was ordered to put into the food to benefit you, Mr. Pitts, was rendered valueless, and a deadly poison was added by what another--”

Minna Pitts had been clutching for support at the arms of her chair as Kennedy proceeded.  She now threw herself at the feet of Emery Pitts,

“Forgive me,” she sobbed.  “I can stand it no longer.  I had tried to keep this thing about Thornton from you.  I have tried to make you happy and well—­oh—­tried so hard, so faithfully.  Yet that old skeleton of my past which I thought was buried would not stay buried.  I have bought Thornton off again and again, with money—­my money—­only to find him threatening again.  But about this other thing, this poison, I am as innocent, and I believe Thornton is as—­”

Craig laid a gentle hand on her lips.  She rose wildly and faced him in passionate appeal.

“Who—­who is this Thornton?” demanded Emery Pitts.

Quickly, delicately, sparing her as much as he could, Craig hurried over our experiences.

“He is in the next room,” Craig went on, then facing Pitts added:  “With you alive, Emery Pitts, this blackmail of your wife might have gone on, although there was always the danger that you might hear of it—­and do as I see you have already done—­forgive, and plan to right the unfortunate mistake.  But with you dead, this Thornton, or rather some one using him, might take away from Minna Pitts her whole interest in your estate, at a word.  The law, or your heirs at law, would never forgive as you would.”

Pitts, long poisoned by the subtle microbic poison, stared at Kennedy as if dazed.

“Who was caught in your kitchen, Mr. Pitts, and, to escape detection, killed your faithful chef and covered his own traces so cleverly?” rapped out Kennedy.  “Who would have known the new process of healing wounds?  Who knew about the fatal properties of indol?  Who was willing to forego a one-hundred-thousand-dollar prize in order to gain a fortune of many hundreds of thousands?”