not with my hands."

"I cannot see what difference that makes."

"If you had touched the dog with your hands it would have brought your
face down near the floor of the cave, and the gas is at the bottom of
the cave only."

"Why should it be there and not all over?"

"Because it is much heavier than the air we breathe, and remains at the
bottom, just like water. If you recall, this part is lower than the
corridor through which we came, so that it could not run out. I have
always observed that in all the other caves the floors within were
higher than the entrance, and in such cases there is no liability of
getting poisonous gases."

"But how are we to make the investigation, under the circumstances?"

"We must remove the gas."

"How can that be done?"

"Several ways are open; one would be to tap the floor and drain the gas
out, which would be difficult to do with our resources. Another plan
would be to force in a lot of air, so as to render the gas inert, or we
might put in enough air to make it burn, and consume it."

"Why, will it burn?"

"Most assuredly; all it needs is enough air; but I am afraid this plan
will not be a very safe one for us. If the floor of the cave is not at
any place more than four feet below the entrance, we can go about
safely, but in such case we must move about with the utmost caution, so
as not to get too much of the gas in the system."

"I am afraid it will be a difficult thing to go in unless we know
absolutely where the low places are, or unless we survey the bottom of
the cave," said George, brightening up at the idea.

"What would happen if we threw a light into the bottom where the gas
is?"

"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,