The lights flashed on and Dr. Bird rose to his feet, pulling up after
him the wilted figure of a middle-aged man.

"Gentlemen," said the doctor in ringing tones, "allow me to present to
you Professor James Kirkwood of the faculty of the Richton University,
formerly known as James Collier of the Bureau of Standards, and robber
of the First National Bank."

Detective-Captain Sturtevant jumped to his feet and cast a searching
glance at the captive.

"He's the man all right," he cried. "Hang on to him until I get a wagon
here!"

"Oh, shut up!" said Carnes. "He's under federal arrest just now, charged
with the possession of narcotics. When we are through with him, you can
have him if you want him."

"How did you get that picture, Doctor?" cried the cashier. "I watched
that cage every minute during the morning and I'll swear that man never
entered and stole that money as the picture shows, unless he managed to
make himself invisible."

* * * * *

"You're closer to the truth than you suspect, Mr. Rogers," said Dr.
Bird. "It is not quite a matter of invisibility, but something pretty
close to it. It is a matter of catalysts."

"What kind of cats?" asked the cashier.

"Not cats, Mr. Rogers, catalysts. Catalysts is the name of a chemical
reaction consisting essentially of a decomposition and a new combination
effected by means of a catalyst which acts on the compound bodies in
question, but which goes through the reaction itself unchanged. There
are a great many of them which are used in the arts and in
manufacturing, and while their action is not always clearly understood,
the results are well known and can be banked on.

"One of the commonest instances of the use of a catalyst is the use of
sponge platinum in the manufacture of sulphuric acid. I will not burden
you with the details of the 'contact' process, as it is known, but the
combination is effected by means of finely divided platinum which is
neither changed, consumed or wasted during the process. While there are
a number of other catalysts known, for instance iron in reactions in
which metallic magnesium is concerned, the commonest are the metals of
the platinum group.

"Less is known of the action of catalysts in the organic reactions, but
it has been the subject of intensive study by Dr. Knolles of the Bureau
of Standards for several years. His studies of the effects of different
colored lights, that is, rays of different w