It seemed to him his demonstration had been
really brilliant, and then, also, he was already planning out with great
success the details of the scheme for his great work. He was making an excellent breakfast, for his appetite was always good,
and, needing some more cream, he rang the bell. The maid appeared, he
showed her the empty jug, and as she took it she dropped it with a
sudden cry, smashing it to pieces on the floor. Very pale, she stammered
out:

"Beg pardon, sir, your hand--there is blood upon your hand." In fact, on the Professor's right hand there showed a drop of blood,
perceptibly larger this time than before. The Professor stared at it
stupidly. He was sure it had not been there a moment before, and he
noticed by the heading of the newspaper at the side of his plate that
this was the first of the month. With a hasty movement of his napkin he wiped the drop of blood away. The
maid, still apologising, began to pick up the pieces of the jug she had
broken; but the Professor had no further appetite for his breakfast. He
silenced her with a gesture, and, leaving a piece of toast half-eaten on
his plate, he got up and went into his study. All this was trivial, absurd even. Yet somehow it disturbed him. He got
out a magnifying glass and examined his hand under it. There was nothing
to account for the presence of the drop of blood he and the maid had
seen. It occurred to him that he might have cut himself in shaving; but
when he looked in the mirror he could find no trace of even the
slightest wound. He decided that, though he had not been aware of it, his nerves must be
a little out of order. That was disconcerting. He had not taken his
nerves into consideration for the simple reason that he had never known
that he possessed any. He made up his mind to treat himself to a holiday
in Switzerland. One or two difficult ascents might brace him up a bit. Three days later he was in Switzerland, and a few days later again he
was on the summit of a minor but still difficult peak. It had been an
exhilarating climb, and he had enjoyed it. He said something laughingly
to the head guide to the effect that climbing was good sport and a fine
test for the nerves. The head guide agreed, and added politely that if
the nerves of monsieur the Professor had shown signs of failing on the
lower glacier, for example, they might all have been in difficulties. The Professor thrilled with pleasure at the head guide's implied praise. He was glad to know on such good authority that his nerves were all
right, and the incidents that had driven him there began to fade in his
memory. Nevertheless, he found himself watching the calendar with a certain
interest, and when he woke on the morning of the first day of the next
month he glanced quickly at his right hand. There was nothing there. He dressed and spent, as he had planned, a quiet day, busy with his
correspondence. His spirits rose as the day passed. He was still
watchful, but more confident; and, after dinner, though he had meant to
go straight to his room, he agreed to join in a suggested game of
bridge. They were cutting for partners when one of the ladies who was to
take part in the game dropped with a little cry the card she had just
lifted. "Oh, there is blood upon your hand," she cried, "on your right hand,
Professor!" Upon the Professor's right hand there showed now a drop of blood, larger
still then those other three had been. Yet the very moment before it had
not been there. The Professor put down his cards without a word, and
left the room, going straight upstairs. The drop of blood was still standing on his hand. He soaked it up
carefully with some cotton-wool he had, and was not surprised to find
beneath no sign or trace of any cut or wound. The cotton-wool he made up
carefully into a parcel and addressed it to an analytical chemist he
knew, inclosing with it a short note. He rang the bell, sent the parcel to the post, and then he got out pen
and paper and set himself to solve this problem, as in his life he had
solved so many others. Only this time it seemed somehow as though the data were insufficient. Idly his pen traced upon the paper in front of him a large _X_, the sign
of the unknown quantity. But how, in this case, to find out what was the unknown quantity? His
hand, his firm and steady hand, shook so that he could no longer hold
his pen. He rang the bell again and ordered a stiff whisky-and-soda. He
was a man of almost ascetic habits, but to-night he felt that he needed
some stimulant. Neither did he sleep very well. The next day he returned to England. Almost at once he went to see his
friend, the analytical chemist, to whom he had sent the parcel from
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.