“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—­”