We had planned a little vacation for ourselves, but the planning came to naught.  The next night we spent on a sleeper.  That in itself is work to me.

It all came about through a hurried message from Murray Denison, president of the Federal Radium Corporation.  Nothing would do but that he should take both Kennedy and myself with him post-haste to Pittsburgh at the first news of what had immediately been called “the great radium robbery.”

Of course the newspapers were full of it.  The very novelty of an ultra-modern cracksman going off with something worth upward of a couple of hundred thousand dollars—­and all contained in a few platinum tubes which could be tucked away in a vest pocket—­had something about it powerfully appealing to the imagination.

“Most ingenious, but, you see, the trouble with that safe is that it was built to keep radium in—­not cracksmen out,” remarked Kennedy, when Denison had rushed us from the train to take a look at the little safe in the works of the Corporation.

“Breaking into such a safe as this,” added Kennedy, after a cursory examination, “is simple enough, after all.”

It was, however, a remarkably ingenious contrivance, about three feet in height and of a weight of perhaps a ton and a half, and all to house something weighing only a few grains.

“But,” Denison hastened to explain, “we had to protect the radium not only against burglars, but, so to speak, against itself.  Radium emanations pass through steel and experiments have shown that the best metal to contain them is lead.  So, the difficulty was solved by making a steel outer case enclosing an inside leaden shell three inches thick.”

Kennedy had been toying thoughtfully with the door.

“Then the door, too, had to be contrived so as to prevent any escape of the emanations through joints.  It is lathe turned and circular, a ‘dead fit.’  By means of a special contrivance any slight looseness caused by wear and tear of closing can be adjusted.  And another feature.  That is the appliance for preventing the loss of emanation when the door is opened.  Two valves have been inserted into the door and before it is opened tubes with mercury are passed through which collect and store the emanation.”

“All very nice for the radium,” remarked Craig cheerfully.  “But the fellow had only to use an electric drill and the gram or more of radium was his.”

“I know that—­now,” ruefully persisted Denison.  “But the safe was designed for us specially.  The fellow got into it and got away, as far as I can see, without leaving a clue.”

“Except one, of course,” interrupted Kennedy quickly.

Denison looked at him a moment keenly, then nodded and said, “Yes--you are right.  You mean one which he must bear on himself?”

“Exactly.  You can’t carry a gram or more of radium bromide long with impunity.  The man to look for is one who in a few days will have somewhere on his body a radium burn which will take months to heal.  The very thing he stole is a veritable Frankenstein’s monster bent on the destruction of the thief himself!”