om
Switzerland.

"Mammalian blood," pronounced the chemist, "probably human--rather a
curious thing about it, too."

"What's that?" asked the Professor.

"Why," his friend answered, "I was able to identify the distinctive
bacillus----" He named the rare bacillus of an unusual and obscure
disease. And this disease was that from which the Professor's cousin had
died.

The professor was a man interested in all phenomena. In other
circumstances he would have observed keenly that which now occurred,
when the hair of his head underwent a curious involuntary stiffening and
bristling process that in popular but sufficiently accurate terms, might
be described as "standing on end." But at the moment he was in no state
for scientific observations.

He got out of the house somehow. He said he did not feel well, and his
friend, the chemist, agreed that his holiday in Switzerland did not seem
to have done him much good.

The Professor went straight home and shut himself up in his study. It
was a fine room, ranged all round with books. On the shelves nearest to
his hand stood volumes on mathematics, the theory of mathematics, the
study of mathematics, pure mathematics, applied mathematics. But there
was not any one of these books that told him anything about such a thing
as this. Though, it is true, there were many references in them, here
and there, to _X_, the unknown quantity.

The Professor took his pen and wrote a large _X_ upon the sheet of paper
in front of him.

"An unknown quantity!" he muttered. "An unknown--quantity!"

The days passed peacefully. Nothing was out of the ordinary except that
the Professor developed an odd trick of continually glancing at his
right hand. He washed it a good deal, too. But the first of the month
was not yet.

On the last day of the month he told his housekeeper that he was feeling
a little unwell. She was not surprised, for she had thought him looking
ill for some time past. He told her he would probably spend the next day
in bed for a thorough rest, and she agreed that that would be a very
good idea. When he was in his own room and had undressed, he bandaged
his right hand with care, tying it up carefully and thoroughly with
three or four of his large linen handkerchiefs.

"Whatever comes, shall now show," he said to himself.

He stayed in bed accordingly the next day. His housekeeper was a little
uneasy about him. He ate nothing and his eyes were strangely bright and
feverish