Once there, the lodger courteously bade his landlady good-night, and proceeded upstairs to his own apartments.

Mrs. Bunting returned to the kitchen.  Again she lighted the stove; but she felt unnerved, afraid of she knew not what.  As she was cooking the cheese, she tried to concentrate her mind on what she was doing, and on the whole she succeeded.  But another part of her mind seemed to be working independently, asking her insistent questions.

The place seemed to her alive with alien presences, and once she caught herself listening—­which was absurd, for, of course, she could not hope to hear what Mr. Sleuth was doing two, if not three, flights upstairs.  She wondered in what the lodger’s experiments consisted.  It was odd that she had never been able to discover what it was he really did with that big gas-stove.  All she knew was that he used a very high degree of heat.

CHAPTER XV

The Buntings went to bed early that night.  But Mrs. Bunting made up her mind to keep awake.  She was set upon knowing at what hour of the night the lodger would come down into her kitchen to carry through his experiment, and, above all, she was anxious to know how long he would stay there.

But she had had a long and a very anxious day, and presently she fell asleep.

The church clock hard by struck two, and, suddenly Mrs. Bunting awoke.  She felt put out, sharply annoyed with herself.  How could she have dropped off like that?  Mr. Sleuth must have been down and up again hours ago!

Then, gradually, she became aware that there was a faint acrid odour in the room.  Elusive, intangible, it yet seemed to encompass her and the snoring man by her side, almost as a vapour might have done.

Mrs. Bunting sat up in bed and sniffed; and then, in spite of the cold, she quietly crept out of her nice, warm bedclothes, and crawled along to the bottom of the bed.  When there, Mr. Sleuth’s landlady did a very curious thing; she leaned over the brass rail and put her face close to the hinge of the door giving into the hall.  Yes, it was from here that this strange, horrible odor was coming; the smell must be very strong in the passage.

As, shivering, she crept back under the bedclothes, she longed to give her sleeping husband a good shake, and in fancy she heard herself saying, “Bunting, get up!  There’s something strange and dreadful going on downstairs which we ought to know about.”

But as she lay there, by her husband’s side, listening with painful intentness for the slightest sound, she knew very well that she would do nothing of the sort.

What if the lodger did make a certain amount of mess—­a certain amount of smell—­in her nice clean kitchen?  Was he not—­was he not an almost perfect lodger?  If they did anything to upset him, where could they ever hope to get another like him?

Three o’clock struck before Mrs. Bunting heard slow, heavy steps creaking up the kitchen stairs.  But Mr. Sleuth did not go straight up to his own quarters, as she had expected him to do.  Instead, he went to the front door, and, opening it, put on the chain.  Then he came past her door, and she thought—­but could not be sure—­that he sat down on the stairs.