e floated down to the office, which happened to be unoccupied at the
time. The medical ghost helped himself to a cigarette from a trayful on
the mantel-piece, and lighting it, he seated himself in an armchair, and
puffed away with evident enjoyment. I noticed the smoke, which he
inhaled continually, oozed from all parts of his body.

[Illustration: THE SMOKE CONTINUALLY OOZED FROM ALL PARTS OF HIS BODY.]

"My dear fellow," he said impressively, "you must understand that all
diseases are caused by germs--microscopic bugs and plants, you know,
many of them so small that they are invisible to an ordinary microscope,
or, if seen at all, are not recognized. There are thousands and
thousands of them, and each and every one has its mission in life, and
preys upon and destroys other germs. Now, the human body is constantly
getting a lot of germs inside of it which do not belong there. Some are
taken in by the lungs, while floating in the air; some by the stomach,
by the food and drink; some by the skin, etc.

"These germs are met by their natural enemies which live in man's
blood--his body-guard, as it were--and are destroyed. But if the
attacking army is very large, or from some reason the home army has been
weakened and decimated, then the invaders flourish, establish themselves
and wax powerful and strong, and the man becomes what is called 'sick.'

"Come," he said, rising abruptly, and throwing the unconsumed end of
his cigarette into the fireplace. "Come with me to the laboratory, and I
will show you in about two minutes more than I could explain if I talked
for years, and a great deal more satisfactorily."

We floated down to the laboratory, and the ghost took from the shelf a
wide-mouthed bottle and held it up to the light.

"Here," he said, "we have a culture. You, of course, understand how the
germs of disease are cultivated for experimental use. It is needless for
me to explain to you that certain media are used for these cultures,
such as milk, beef-broth, etc.

"Here we have the germ of diphtheria, here of tuberculosis, here of
typhoid fever, etc. That little short jar over yonder contains some
cholera bacilli, which have been lately sent here. Now look at this
typhoid germ. If we took a drop of healthy blood and put some of these
typhoid germs in it, how they would wiggle! but if the drop of blood was
from a typhoid patient, they won't wiggle very long, as you know. See
this blunt-headed chap which we have to