"No; it's gone already!" cried Jack, who, even in the midst of
danger and excitement, seemed to remain calm and still to have
his appreciation of it joke.

"Come on!" cried Mark as he scrambled to his feet. "We must get
out of here, Jack!"

"What's the use now? It's all over."

There was a tinkling sound, as fragments of the broken test tube,
the bell-jar and other things began falling about the room.

Mark was fumbling at the door of the laboratory, seeking to
escape.

"Come on back," said Jack. "It's all over. There's no more
danger. We'll try it again."

Just then one of the pile of books, that had been blown on an
upper shelf, came down, landing on Mark's head.

"No danger?" cried Mark, trembling from excitement. "No danger?
What do you call that?" and he pointed to the books at his feet,
while he rubbed his head ruefully.

"Well, there aren't any more," observed Jack, with a look upward.

Just then the door opened, and an elderly gentleman, wearing
spectacles, entered the laboratory. He seemed much excited.