"Not always; chemists have found out that man and animals absorb oxygen and expel nitrogen, in order to live; and that plants take in and live on nitrogen, and give out oxygen. They further learned that certain germs make nitrogen, just the same as we found that certain germs made carbonic gas in the dough; so that the United States Department of Agriculture, through its chemical division, concluded to set the germs to work, and the department will now send a box containing millions of the tiny creatures to any farmer who applies for them."

"When they get them, how do they set them to work?"

"The germs are thrown into a tub of water, and the seed, like corn, is put into the water and allowed to remain for a certain length of time.

When the seed is taken out, more or less of the germs remains on each kernel, and when it is put into the ground the germs keep on working, making nitrogen which the growing plant absorbs. It is wonderful to see the effect in a field where one row has these germ-infected seeds, and the other rows are not so treated."

It was now May, and the weather was slightly cooler, but there was neither snow nor frost. North of the equator it was growing warmer, because the winter had pa.s.sed. Here the summer had gone, and winter was coming on. From every indication they were not in a cold climate.

"Why do you think we shall not have any snow?" was Harry's inquiry.

"I notice too many trees, as well as shrubs and flowers, which could not live if we had frosts or freezing weather. Many of the trees about here do not shed their leaves, and the kind of animals which we now know exist here are sufficient evidence that we need not fear cold weather."

For more than a week the boys and the Professor put in their time prospecting in the hills and in carting various ores and mineralogical samples to their workshop.

The pelts which were on hand needed curing and besides there were also four yak pelts which had to be tanned, as shoe leather was badly needed.

The hide originally dehaired was long ago ready for tanning, as well as the later ones.

"What shall we use for the tanning process?"

"The bark of certain trees must be procured, so if you can find either oak, hemlock, birch or beech trees, we can probably make a tanning compound which will serve our purpose."

"In what way will the bark of those trees tan the leather?"

"All the barks named contain what is called tannic acid. Other elements also are used, such as gallic acid, alum, sulphate of iron, and copper, salt, and other agents."

"What are the chemicals for?"

[Ill.u.s.tration: _Fig. 13. Tanning Vat._]

"The tannic acid or the chemicals act on the skins, or, rather on the gelatin, glutin and alb.u.men in the skins, and thus harden the texture and preserve it. Where tannin is not used and only the chemicals are employed, it is called 'tawing' the leather, instead of 'tanning.'"

"Well, we can get the bark; I know where there are several oak trees, and also a number of beech trees."

"Then gather the bark by all means, and by the way, if you can find gall nuts we could use them to advantage."

"We don't know what they are."

"Then, if you can get some sumac, we can use that."

"Yes, I know; the kind with the long, red leaves."

"That is what I mean."

"Do the gall nuts have tannic acid?"

"Yes; but princ.i.p.ally gallic acid, but gallic acid will also tan the skins so as to make leather. The princ.i.p.al use of the gall nut is for making ink."

"What kind of tree does the gall nut grow on?"

"On any kind of tree or bush."

"That is rather odd."

"It is not the fruit of any tree, but is produced by the gall fly, which punctures almost any kind of tree or shrub. In this puncture the insect lays its eggs, and the tree in trying to treat the wound covers up the egg, and the sap, flowing from the tree, forms a sort of nut which finally hardens and produces a most bitter substance deposited by the fly. The nut is about the size of a marble, and must be gathered before the larva is hatched out. It is the most valuable nut in the world."

The necessary bark for the tanning process was ready within the next two days, and a tank prepared in which the hides were laid spread out, with the bark between them and covered with water.

"In our conversations, Professor, you have the habit in describing plants, and especially the leaves to call them by certain names. It would help us if we knew just what you meant by the different names you give them."

"That is a wise suggestion, because it has been said that the basis of knowledge, or of true science, is correct definitions.[1] What is meant by that is this: We should both have an understanding of the term used to describe a thing. In our talks I have tried to avoid the use of what is called technical terms, but it is difficult to describe some things without using such terms, and I have for some time thought of making a list of the things we are talking about, and defining them, so you can at any time go there and look up the definition."

[Footnote 1: Each of these books has the Professor's definitions on the last pages.]

[Ill.u.s.tration: _Fig. 14. Serrate. Fig. 15. Bi-serrate._]

"If you would make drawings of the different kinds of leaves and give their names we could hang them up and could look at them any time."

"The leaf is the proper part to commence on, because it is the most important thing of every plant, or tree, or shrub."

"What, more important than the fruit or the flower, or the nut that grows on the tree?"

"That is just what I mean. When you smell the rich red flower of the rose, or look at the pure white petals of the lily, or the sweet-smelling blossoms of the orange or the jasmine, you are simply seeing or smelling leaves. The fruit itself, whether in the form of an apple, or a berry or a nut, is simply a form of leaf, a perfected form of the plant, or bush, or tree. Originally all these fruits, flowers and nuts were but leaves in an undeveloped state."

"I never heard of such a thing before."

"It is a subject treated of in botanical knowledge which is called Morphology, and the object is to show that every fruit and flower was developed, in accordance with a well-known law, from the particular shape or form of the leaf. We can go into that branch of the subject later on. What we now want is to know something about the shape of the leaf, so we can have a starting point. There are two particular things about leaves; one has reference to the shape of the leaf, and the other to the way in which the edges are formed. To simplify the explanation, the drawings which I make pertain only to the edges. That will be sufficient for one time.

"Look at Fig. 14. The edges are like the teeth of a saw. This is called the serrate leaf. The rose and the common nettle have such leaves.

"Fig. 15 shows a leaf with a saw tooth edge wherein the teeth themselves have a lot of little saw teeth, as in the nettle-leaved bell-flower, and this is called bi-serrate.

"Fig. 16 has very large, sharp teeth, not pointing in any particular direction, like the oak leaf. This is called the dentate, or tooth.

"Fig. 17 has rounded projections instead of angular teeth, and is called crenate. Ground ivy and horseradish have such leaves.

"When we make drawings of the shapes of the leaves that will take us along another step, and thus enable us to find out just what kind of tree or plant we are talking about."

[Ill.u.s.tration: _Fig. 16. Dentate. Fig. 17. Crenate._]

The following day Harry proposed a trip over to the cave which George had discovered at the time he found the big air pocket that shot water and spray out into the ocean.

"I am interested," he said, "in seeing the air pocket George spoke about, and we might as well take our vacation to-day."

"As the distance is not great we need not bother about taking our luncheon along." And off they started, with the Professor bringing up the rear.

The course was first to Observation Hill, where they had erected a new and a larger pole than the one which had so mysteriously disappeared two months before. On every such visit it was the duty of the one who made the trip to scan the horizon in every direction.

It must not be inferred that because the boys were engaged in work which was all-absorbing that they had no thoughts of home, and had given up all hopes of a final rescue. If they could only let the people at home know they were alive and happy--that is, in learning the secrets of nature and in the exciting exploring trips, they would be satisfied.