"Ralph!"
The voice sounded in the boy's ears like the chiming of a far-away bell. Lying prone on the floor of the tunnel, overcome by the foul gases, he had been unconscious, he did not know for how long, when he felt his shoulders roughly shaken and Walt Phelps' voice in his ear.
His head ached terribly, and he felt weak and dizzy, but he struggled to reply.
"Oh, Walt, what is it? What has happened?"
"Why, we've all been knocked out, I guess," said Walt; "but the gas must be escaping, now, for although my head still feels as if a boiler factory was at work in it, I can think and feel."
The professor's voice now struck in as he recovered consciousness.
"Boys!" he exclaimed. "Are you there?"
"Yes, yes, professor; do you feel strong enough to move?"
"I think so. It is important that we should get out of here at once. I imagine that the gas must have become so distributed by this time that it has lost its harmful effect, but we must get to the open air."