Mackay drove us to the laboratory in his little car and it was dark and we were dinnerless when we arrived. Knowing Kennedy's habits, I sent out for sandwiches and started in to make strong coffee upon an electric percolator. The aroma tingled in my nostrils, reminding me that I was genuinely hungry. The district attorney, too, seemed more or less similarly disposed.

As for Kennedy, he was interested in nothing but the problem before him. He had been strangely quiet on the way, growing more and more impatient and nervous, as though the element of time had entered into the case, as though haste were suddenly imperative. Once the lights were on in the laboratory he hurried about his various preparations. The food samples he laid out, but he gave them no attention. The blood smears and stomach contents he put aside for future reference. His attack was upon the drop or two of liquid adhering to the stem of the broken champagne glass.

The entire chemical procedure seemed to be incomprehensible to Mackay and he was fascinated, so that he had considerable trouble at times keeping out of the way of Kennedy's elbow. Kennedy first washed the stem out carefully with a few drops of distilled water, then he studied the resulting solution. One after another he tried the things that occurred to him, making tests wholly unproductive of results. Slowly the laboratory table became littered completely with chemicals and apparatus of all sorts, a veritable arsenal of glass.

The sandwiches arrived, but Kennedy refused to drop his investigation for a moment. I did succeed in making him take a cup of strong coffee, and that was all. Over in a corner Mackay and I did full justice to the food, finishing the hot and welcome coffee and then refilling the percolator and starting it on the making of a second brew. The hours lengthened, and when Mackay grew tired of watching with intense admiration he joined me in the patient consumption of innumerable cigarettes.

Kennedy was filled with the joy of discovery. I noticed that he did not stop even for the solace of tobacco. It seemed to me that at times his nostrils dilated exactly like those of a hound on the scent. Finally he held up a test tube and turned to us.

"What is it?" I asked. "Some other poison as rare and little known as the snake venom?"

"No—something much more curious. In the stem of the glass I find the toxin of the Bacillus botulinus."

"Germs?" Mackay inquired.

Kennedy shook his head. "Not germs, but the pure toxin, the poison secreted by this bacillus."

"What does it do?" was my question.

"Well," thoughtfully, "botulism may be ranked easily among the most serious diseases known to medical science. It is hard to understand why it is not a great deal more common. It is one of the most dangerous kinds of food poisoning."

"Then the apple juice they used for the wine was bad, spoiled?"

"No, not that. Werner was the only one stricken. Somebody put the pure toxin in his glass. It was, as I suspected, deliberate murder, as in the case of Miss Lamar. Bacillus botulinus produces a toxin that is extremely virulent. Hardly more than a ten-thousandth of a cubic centimeter would kill a guinea pig. This was botulin itself, the pure toxin, an alkaloid just like that which is formed in meat and other food products in cases of botulism. The idea might also have been to make the death seem natural—due solely to bad food."

"Do you suppose it was used because it was quick and was colorless, so as not to be noticed in the glass?" I hazarded.

Kennedy paced up and down the laboratory several times in thought. "To me, Walter, this is another indication of the satanic cleverness of the unknown criminal in the case. First Miss Lamar is to be killed. For that purpose something was sought, probably, which could not be traced easily to the perpetrator. In snake venom an agent was employed which may be said to be almost ideal for the grim business of murder. It is extremely difficult to identify in its results, it is comparatively unknown, yet it is swift in action and to be obtained with fair ease.

"Differing from most poisons, it may be inflicted through a prick so slight as to be almost unnoticed by the victim. The scheme of fixing the needle in the curtain was so simple and yet so effective that the guilty person need never have feared its discovery under ordinary circumstances, or its association with the girl's death, if some one stumbled upon it accidentally. The idea of returning for the death-dealing point was only one of the many details of a precautionary measure upon which we have stumbled. Had I found it the next morning I would have been unable, in all probability, to identify it as belonging to or as obtained by any of our suspects.

"You must realize, Walter, that with all the scientific aids I have been able to bring to bear we possess almost no direct evidence. There are no fingerprints, no cigarette stubs, no array of personal, intimate clues of any sort to this criminal. These are the threads which lead the detective to his quarry in fiction and on the stage. Here we lack even the faintest description of the man, or woman if that is her sex. It is murder from a distance, planned with almost meticulous care, executed coolly and without feeling or scruple.

"After the death of Miss Lamar I was not so sure but that the selection of the snake venom was simply the inspiration of a perverted brain, the evolution of the detailed method of killing her—an outgrowth of someone's familiarity with studio life in general, with the script of 'The Black Terror' in particular. Now I realize that we are face to face with the studied handiwork of a skilled criminal. These two deaths may be his—or her—first departure into the realm of crime. But potentially we have a super-villain.

"I make that statement because of the manner of Werner's demise. It is evident that the director stumbled on a clue to the murderer. If my first hypothesis had been correct, if the use of snake venom and the unlucky thirteenth scene had been largely a matter of blind chance in the selection of poison and method, then we might have expected Werner to be struck down in some dark street, or perhaps decoyed to his death—at the best, inoculated with the same crotalin which had killed Miss Lamar.

"But let us analyze the method used in slaying the director. If he had been blackjacked there would be the clue of the weapon, always likely to turn up, the chance of witnesses, and also the likelihood in an extreme case that Werner might not die at once, but might talk and give a description of his assailant, or even survive. Much the same objections—from the criminal's standpoint—obtain in nearly all the accepted modes of killing a man. Even the use of venom a second time possesses the disadvantage of a certain alertness against the very thing on the part of the victim. Werner was a dope fiend, fully aware of the potency of a tiny skin puncture. I'll wager he was on constant guard against any sort of scratch.

"On the other hand, the few drops of toxin in the glass possessed every advantage from the unknown's standpoint. It was invisible, and as sure in its action as the venom. Also it was as rare and as difficult to trace. For, remember this. Botulism is food poisoning. If I had not found the stem of that glass it would be absolutely impossible to show that Werner died from anything on earth but bad food. That is why I do not even take time to analyze the stomach contents. That is why I say we are confronted by an archscoundrel of highest intelligence and downright cleverness. More"—Kennedy paused for emphasis—"I realize now the presence of a grim, invisible menace. It has just now been driven home to me. The botulin, with its deadly paralyzing power, sealed Werner's tongue even while he tried to tell me what he knew."

Mackay was tremendously impressed by Kennedy's explanation. "Does this mean," he asked, "that the guilty man or woman is some outsider? Those we have figured as possible suspects would hardly have this detailed knowledge of poisons."

"There are two possibilities," Kennedy answered. "The real person behind the two murders may have employed some one else to carry out the actual killing, a hypothesis I do not take seriously, or"—again he paused—"this may be a case of some one with intelligence starting out upon his career of crime intelligently by reading up on his subject. It is as simple to learn how to use crotalin or botulin toxin or any number of hundreds of deadly substances as it is to obtain the majority of them. In fact, if people generally understood the ease with which whole communities could be wiped out, and grasped that it could be done so as to leave virtually no clue to the author of the horror, they might not sleep as soundly at night as they do. The saving grace is that the average criminal is often clever, but almost never truly scientific. Unfortunately, we have to combat one who possesses the latter quality to a high degree."

"What is the invisible menace of which you spoke, Craig?" I inquired.

"The possibility of another murder before we can apprehend the guilty person or gain the evidence we need."

"Good heavens!" I imagine I blanched. "You mean—"

"Werner was struck down, apparently, for no reason but that he had guessed the identity of the villain. There is a second man in the company who has certain suspicions and is acting upon them. If he is on the right trail, by any chance—" Kennedy shrugged his shoulders soberly.

"Shirley?"

"Exactly! And there is still another possibility."

"What is that?"

"Here in this laboratory I have blood spots made on the portieres at the house of Phelps by the man who removed the needle, probably the unknown himself, possibly his—or her—agent. In any case it is a clue and—THE ONLY DIRECT AND INFALLIBLE CLUE IN EXISTENCE TO THE CRIMINAL! Also I have the evidence of the snake venom and of the botulin toxin here. Sooner or later the person who killed Werner because he suspected things will wake up to the fact that we possess tangible proof against him."

I grew pale. "You mean, then, that you may be attacked yourself? That even I—"

Kennedy smiled, unafraid. But from the expression in his eyes I knew that he took the thought of our possible danger very seriously.