Hurriedly we drove across town to the City Hospital, where we had no difficulty in being admitted and finding, in a ward, on a white cot, the wounded guard.  Though his wound was one that should not have bothered him much, it had, as Marlowe said, puffed up angrily and in a most peculiar manner.  He was in great pain with it and was plainly in a bad way.

Though he questioned the man, Craig did not get anything out of him except that the shot had come from a cruiser which had been hanging about and was much faster than the patrol boat.  The nurse and a young intern seemed inclined to be reticent, as though we might imply that the mail’s condition reflected on the care he had received, which they were at pains to convince us had been perfect.

Puzzled himself, Craig did not say much, but as he pondered the case, shook his head gravely to himself and finally walked out of the hospital abstractedly.

“We have almost an hour before we are to meet Marlowe at the yard,” he considered, as we came to the car.  “I think I’ll go up to the laboratory first.”

In the quiet of his own workshop, Kennedy carefully examined again the peculiar grooves on the bullet.  He was about to scrape it, but paused.  Instead, he filled a tube with a soapy solution, placed the bullet in it, and let it stand.  Next he did the same with the pieces of glass from the envelope.

Then he opened a drawer and from a number of capillary pipettes selected a plain capillary tube of glass.  He held it in the flame of a burner until it was red hot.  Then carefully he drew out one end of the tube until it was hair fine.  Again he heated the other end, but this time he let the end alone, except that he allowed it to bend by gravity, then cool.  It now had a siphon curve.  Another tube he treated in the same way.

By this time he was ready to proceed with what he had in mind.  He took a glass slide and on it placed a drop from each of the tubes containing the bullet and the glass.  That done, he placed the bent, larger end of the capillary tubes in turn on each of the drops on the slide.  The liquid ascended the tubes by capillary attraction and siphoned over the curve, running as he turned the tubes up to the finely pointed ends.

Next in a watch glass he placed some caustic soda and in another some pyrogallic acid, from each of which he took just a drop, as he had done before, inclining the tubes to let the fluid gravitate to the throttle end.  Finally in the flame he sealed both the tip and butt of the tubes.

“There’s a bubble of air in there,” he remarked.  “The acid and the soda will absorb the oxygen from it.  Then I can tell whether I’m right.  By the way, we’ll have to hurry if we’re to be on time to meet Marlowe in the yard,” he announced, glancing at his watch as he placed the tubes in his little electric incubator.

We were a little late as the chauffeur pulled in at the executive offices at the gate of the shipyard, and Marlowe was waiting impatiently for us.  Evidently he wanted action, but Kennedy said nothing yet of what he suspected and appeared now to be interested only in the yard.