"It would be extinguished instantly," remarked John.
"That gives me an idea," asserted George. "Why not take a lamp, and carry it ahead of us, about three feet from the ground, and whenever that goes out, it will show there is gas there?"
"That is a good observation; but I am afraid it would be very dangerous to do that."
"Dangerous? I thought you said that the carbonic gas would put out the light?"
"So it will; but if three parts of air should be added to one part of the gas it will make an explosive mixture,—that is, a mixture which will burn, as there has been enough oxygen added to support combustion."
"In what way could enough air mix with the gas to make it burn?"
"By stirring it; the movement of the body might make an admixture just above the surface of the gas, which would burn, and that might mean a catastrophe for us."
"Then we are certainly stopped at this cave."
"Not by any means," rejoined John, and he saw the boys' faces smiling again. "We must make a safety lamp."