de and the
outside surfaces.

"That," he explained to me at length as he worked, "is what is known as
a Berkefeld filter, a little porous cup, made of porcelain. The minute
meshes of this filter catch and hold bacteria as if in the meshes of a
microscopic sieve, just like an ordinary water filter. It is so fine
that it holds back even the tiny bacillus fluorescens liquefaciens which
are used to test it. These bacilli measure only from a half to one and
one-and-a-half micromillimeters in diameter. In other words 130,000
germs of half a micromillimeter would be necessary to make an inch."

"What has it been used for?" I ventured.

"I can't say, yet," he returned, and I did not pursue the inquiry,
knowing Kennedy's aversion to being questioned when he was not yet sure
of his facts.

It was the next day when the post-office inspectors, the police and
others who had been co-operating had settled on the raid not only of Dr.
Loeb's but of all the medical quacks who were fleecing the credulous of
the city out of hundreds of thousands of dollars a year by one of the
most cruel swindles that have ever been devised.

For the time, Kennedy dropped his investigations in the laboratory and
we went down to O'Hanlon's office, where a thick batch of warrants, just
signed, had been received.

Quickly O'Hanlon disposed his forces so that in all parts of the town
they might swoop down at once and gather in the medical harpies. Dr.
Loeb's stood first on the list of those which O'Hanlon decided to handle
himself.

"By the way," mentioned O'Hanlon as we hurried uptown to be ready in
time, "I had a letter from Darius Moreton this morning threatening me
with all kinds of trouble unless we let up on Dr. Loeb. It's pretty hard
to keep a big investigation like this secret, but I think we've planned
a little surprise for this morning."

With the post-office inspector we climbed into a patrol wagon with a
detail of police who were to make a general round-up of the places on
Forty-second Street.

As the wagon backed up to the curb in front of the building in which
Loeb's office was, the policemen hopped out and hurried into the
building before a crowd could collect. Unceremoniously they rushed
through the outer office, headed by O'Hanlon.

Quickly though the raid was executed, it could not be done without some
warning commotion. As we entered the front door of the office, we could
just catch a glimpse of a man retreating through a back d