“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.

XXIV

THE INVISIBLE MENACE

Mackay and I exchanged glances.  Kennedy busied himself putting away some of the more important bits of evidence in the case, placing the tiny tubes of solution, the blood smears, and other items together in a cabinet at the farther corner of the laboratory.  The vast bulk of his paraphernalia, the array of glass and chemicals and instruments, he left on the table for the morning.  Then he faced us again, with a smile.

“Suppose you start up the percolator once more, Walter!” He took a cigar and lighted it from the match I struck.  “I believe I’ve earned another cup of coffee,” he added.

Mackay had been fidgeting considerably since Kennedy’s explanation of the possible danger to Shirley, as well as to ourselves or even to others.

“Isn’t there something we can do, Kennedy?” he exclaimed, suddenly.  “Is it necessary to sit back and wait for this unknown to strike again?”

“Ordinarily,” Kennedy replied, “on a case like this it has been my custom to permit the guilty parties to betray themselves, as they will do inevitably—­especially when I call to my aid the recent discoveries of science for the detection and measurement of fine and almost imperceptible shades of emotion.  But now that I realize the presence of this menace I shall become a detective of action; in fact, I shall not stop at any course to hurry matters.  The very first thing in the morning I shall go to the studio and I want you and Jameson along.  I”—­his eyes twinkled; it was the excitement at the prospect—­“I may need considerable help in getting the evidence I wish.”

“Which is—?” It was I who interposed the question.