“Must be locked from the other side,” said Tom.  “Come on out the way we came in.  Gracious!  Isn’t that awful stuff that is burning?” he added, for the vapor now filled the room completely.

In sudden alarm the four boys turned back toward the folding doors through which they had entered the dining-room.  To their consternation, the doors were tightly shut.

“Who shut these?” asked Dick as he tried to open one of the doors.

“I didn’t,” said Sam.

“Neither did I,” added Tom.

“Nobody touched the doors!” ejaculated Stanley.  “It must be some of the ghost’s work.”

“Nonsense!” answered Dick sharply.  “Somebody shut the doors—­and locked ’em,” he added after trying both.  “Hi, you!” he called.  “Open these doors, and be quick about it!”

“Thou fool, to come here!” exclaimed a hollow voice from the other side of the doors.

“It’s the ghost!  I said it was!” said Stanley,

“It’s somebody fooling us,” answered Tom.  “Open the door, or we’ll smash it down!” he added in a loud voice.

Instead of a reply there came a weird groan and then the rattle of some heavy chains.  Stanley turned pale and began to tremble, but the Rovers were not much impressed.

“We don’t believe in ghosts, so you might as well let us out!” cried Dick.  “That stuff you set on fire is smothering us!”

At this there was a murmur from the next room, but what was said the prisoners did not know.

“Come on, let us get out of a window!” cried Tom.  His head was commencing to swim, and he could hardly see.

“Tha—­that’s it,” murmured Sam.  “Say, I’m—­I’m—­going—­” He did not finish, but sank to the floor in a heap.

“Sam has been overcome!” cried Dick in horror.

“Oh, if only we hadn’t come here!” groaned Stanley.  “I—­the window—­I—­am—­smothering!” He took another step forward and then fell.  Dick tried to pick him up, but went down also, with his brain in a whirl and strange lights flashing before his closed eyes.

Tom was the last to be overcome.  He reached a window, only to find it tightly locked.  He smashed the glass, but could not open the blinds.  Then he went down; but before he closed his eyes he saw the door to the kitchen open and several masked faces appeared.  He tried to say something, but the words would not come, and then all became a terrible dark blank around him.

For about half a minute after Tom went down nothing was done.  Then the door to the kitchen was thrown wide open and four figures appeared.  All wore sheets and masks.

“You are sure it won’t kill any of them, Parwick?” asked a voice that sounded like Jerry Koswell’s, and which was far from steady.

“Yes, I’m sure,” answered the voice of a stranger.  “But we don’t want to leave them in this room too long.  Take ’em below.”

“If we get found out—­” said another, and one could readily recognize Flockley’s voice.