[Ill.u.s.tration: "_The old man pointed to the rocky wall_" [See p. 152]]

"I might add," continued John, "that when the courses are not regular it is called _broken_ ashlar; when stones of less than one foot in breadth are used it is called _small_ ashlar; if the wall is backed by rubble, or inferior work it is called _b.a.s.t.a.r.d_ ashlar. Then every kind of surface has a particular name, like the _random-tooled_, where the tool marks are shown in all directions; _rusticated_ when only the joined edges are trimmed up; _prison-rustic_ when it is pitted with deep holes; _herring-bone_ when it is tooled in rows of opposite directions to each other; and _nigged_ when finished up with a pointed hammer."

Within an hour the stones were removed and put aside, and then Ephraim was treated to another surprise when he was made acquainted with the little electric flash lights which John exhibited. With these they entered the cave.

All savage tribes have some sort of animals, as pets, and dogs are the most frequent. This was the case among these people. The dogs were with the party, and, as usual, ahead of the procession. Two of them went ahead on a scouting expedition, while John and the boys, with their flash lights followed.

After they had gone, probably two hundred feet, there was a slight descent apparent in the floor of the cave, and ahead were the two dogs stretched out, lifeless.

George ran ahead, as he noticed them, and John shouted out: "Do not touch them!"

He stopped, and looked back, and then slowly walked up to the animals.

John requested the party to halt, and he went forward, and put his foot on one of the dogs. "We must go back," he said.

"Are they dead?" asked Harry, as he came forward.

"Why not take them out and see what the trouble is?" inquired Harry.

"No need for that," responded John. "I know what the trouble is."

"Is there any danger in the cave?"

"Yes."

Ephraim and the natives were now alarmed. It will be remembered that the universal belief among the natives is, that to go into these caves unbidden, means death. True, John had shown the fallacy of this on several occasions, but here was positive evidence that death had visited the dogs, and this might be the fate of those who attempted to go on.

But the most alarming thing was the fact that John himself was the one who said there was danger, and that they must return. He did not venture to make an explanation until they were out of the cave.

"There is carbonic acid in the cave, and as it is a deadly poison we cannot go in until it is removed."

"That seems singular," responded George. "I went in as far as the dogs, and it didn't affect me."

"But you did not reach down to touch the dog."

"I saw you touch the dog, and it didn't seem to affect you."

"I touched it with my feet and 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 circ.u.mstances?"

"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 a.s.suredly; 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," a.s.serted 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."

"Do you mean a lamp that will not explode the gas, although it has enough oxygen to 'support combustion,' as you say?"

"Exactly. Have you ever heard of the Humphrey Davy lamp? Well, that was invented to meet the very condition found here."

"Tell us about it."

"In 1816 Davy discovered that a flame would not pa.s.s through a fine wire gauze, while conducting some experiments. It occurred to him that use could be made of this discovery by surrounding the flame of a lamp with gauze, and the well-known lamp was built on that principle."

"What I am curious to know is, that if it will not burn the gas, will it go out if it gets down in the gas?"

[Ill.u.s.tration: _Fig. 13. Types of Safety Lamps._]

"Yes; because enough air, or oxygen must go through the mesh to support combustion of the flame itself. If it does not get enough it is smothered."

"Then why not make a lamp of that kind?"

"But where can we get enough gauze to make a cylinder big enough to go around a lamp?"

John laughed at the serious predicament, which expressed itself in the faces of the boys. "That is true," he said; "but if we can get a small piece of tin, we can punch it full of fine holes, and probably make that answer."