“It’s as noticeable as that?” asked Kennedy.  “And did she notice it?”

“I’m sure of it,” replied Norton confidently.  “She couldn’t help it.  Besides, after he left her and went into the dining-room himself she and Alfonso seemed to be discussing something.  I’m sure it was that.”

Kennedy said nothing, except to thank Norton and compliment him on his powers of observation.  Norton took the praise with evident satisfaction, and after a moment excused himself, saying that he had some work to do over in the Museum.

He had no sooner gone than Kennedy took from a drawer a little packet of powder and an atomizer full of liquid, which he dropped into his pocket.

“I think the Prince Edward Albert will be the scene of our operations, to-night, Walter,” he announced, reaching for his hat.

He seemed to be in a hurry and it was not many minutes before we entered.  As he passed the dining-room he glanced in.  There was Whitney, not half through a leisurely dinner.  Neither of the de Moches seemed to be downstairs.

Kennedy sauntered over to the desk and looked over the register.  We already knew that Whitney and the Senora had suites on the eighth floor, on opposite sides and at opposite ends of the hall.  The de Moche suite was under the number 810.  That of Whitney was 825.

“Is either 823 or 827 vacant?” asked Kennedy as the clerk came over to us.

He turned to look over his list.  “Yes, 827 is vacant,” he found.

“I’d like to have it,” said Kennedy, making some excuse about our luggage being delayed, as he paid for it for the night.

“Front!” called the clerk, and a moment later we found ourselves in the elevator riding up.

The halls were deserted at that time in the evening except for a belated theatre-goer, and in a few minutes there would ensue a period in which there was likely to be no one about.

We entered the room next to Whitney’s without being observed by any one of whom we cared.  The boy left us, and it was a simple matter after that to open a rather heavy door that communicated between the two suites and was not protected by a Yale lock.

Instead of switching on the lights, Kennedy first looked about carefully until he was assured that there was no one there.  It seemed to me to be an unnecessary caution, for we knew Whitney was down-stairs and would probably be there a long time.  But he seemed to think it necessary.  Positive that we were alone, he made a hasty survey of the rooms.  Then he seemed to select as a starting-point a table in one corner of the sitting-room on which lay a humidor and a heavy metal box for cigarettes.

Quickly he sprinkled on the floor, from the hall door to the table on which the case of cigarettes lay, some of the powder which I had seen him wrap up in the laboratory before we left.  Then, with the atomizer, he sprayed over it something that had a pungent, familiar odour—­walking backwards from the hall door to the table, as he sprayed.