Kennedy glanced at it only casually, as if he had fully expected the incident to turn out as it did.

“Not unopened, Miss Kendall,” he commented.  “We have already had a little scientific letter-opening.  This was a case of scientific letter-sealing.  That was a specially prepared envelope.”

He reached down into his desk and pulled out another, sealed it carefully, dried it, then held it over a steaming pan of water until the gum was softened and it could be opened again.  On the back were smudges just like those on the letter that had been returned.

“On the thin line of gum on the flap of the envelope,” he explained, “I have placed first a coating of tannin, over which is the gum.  Then on the part of the envelope to which the flap adheres when it is sealed I placed some iron sulphate.  When I sealed the envelope so carefully I brought the two together separated only by the thin film of gum.  Now when steam is applied to soften the gum, the usual method of the letter-opener, the tannin and the sulphate are brought together.  They run and leave these blots or dark smudges.  So, you see, someone has been found at the Montmartre, even if it is not Betty Blackwell herself, who has interest enough in the case to open a letter to her before handing it back to the postman.  That shows us that we are on the right trail at least, even if it does not tell us who is at the end of the trail.  Here’s another thing; This ‘Marie’ is a new one.  We must find out about her.”

“At the Futurist Tea Room at four this afternoon, when she meets our good friend, young Dr. Harris,” reminded Clare.  “Between cabarets and tea rooms I don’t know whether this is work or play.”

“It’s work, all right,” smiled Kennedy, adding, “at least it would be if it weren’t lightened by your help.”

It was the middle of the afternoon when Craig and I left the laboratory to keep our appointment with Miss Kendall at the Futurist Tea Room, where we hoped to find Dr. Harris’s friend “Marie,” who seemed to want to see him so badly.

A long line of touring and town cars as well as taxicabs bore eloquent testimony not only to the popularity of this tea room and cabaret, but to the growth of afternoon dancing.  One never realizes how large a leisure class there is in the city until after a visit to anything from a baseball game to a matinee—­and a dance.  People seemed literally to be flocking to the Futurist.  They seemed to like its congeniality, its tone, its “atmosphere.”

As we left our hats to the tender mercies of the “boys” who had the checking concession we could see that the place was rapidly filling up.

“If we are to get a table that we want here, we’d better get it now,” remarked Kennedy, slipping the inevitable piece of change to the head waiter.  “If we sit over there in that sort of little bower we can see when Miss Kendall arrives and we shall not be so conspicuous ourselves, either.”