"I don't see that you particularly need anything."

"Why not?"

"If you let the dough stand in a temperature of between 90 and 120 degrees for a certain time, fermentation will take place, and it can then be baked."

"But why should it ferment?"

"Bread raising is merely fermentation. All flour is largely composed of starch. The high temperature, of 100 degrees or over, causes the starch to turn first into sugar, then into alcohol and carbonic acid, and the gases thus formed force their way up through the dough, causing it to swell, as you have often noticed."

[Ill.u.s.tration: _Fig. 8. Imperfectly leavened. Perfectly Leavened.

SAMPLES OF BREAD_]

Without further instructions the boys began the making of bread. Shortly afterwards the Professor appeared laughing immoderately.

"Come and see the Baby."

The boys were out in an instant. The Baby was in the storeroom adjoining, and discovered the honey pot. It was a "sight." He sat there, both hands and arms covered with honey, blinking innocently, and licking his fingers and arms with the greatest joy imaginable.

"You little rascal, you are getting too fat now," was George's greeting; but Baby didn't mind. He knew George by this time.

The bread raised, but it, too, was a "sight." It was full of holes and at some places the bread had no appearance of having "come up," which is kitchen parlance for unraised bread.

"What is the matter with it, Harry?"

"Did you work it before you put it into the oven?"

"I forgot that."

When the Professor saw the sample he divined the trouble at once.

"Of course, you have to work it, for the reason that 'working'

distributes the gases through the ma.s.s. I think you made the mistake in working it and then putting it into the oven immediately."

"How long should it stand after working?"

"That depends on the amount of carbonic gas which is developed. When it first raises the gas forces its way through the dough irregularly, and by then working it the gas is broken up and distributed evenly, so that if the ma.s.s is allowed to stand after the second working every part of it will be leavened. When it is then put into the oven, the heat at first causes a more rapid expansion, or raising, of the dough, and as the heat increases, fermentation is stopped, and the baking process sets the dough. The result is tiny little holes throughout the bread, where the gases were."

"But why do they use yeast if it can be done without?"

"Because it makes the raising process easier, and more positive."

"Is it the carbonic acid which makes some bread sour?"

"Yes; sour bread results if the fermentation is continued too long."

[Ill.u.s.tration: _Fig. 9. Air Pocket._]

It was George's custom each day to watch the movements of the yaks, because it was through them that they learned of the barley field which was such a source of usefulness to them. One day while out on an expedition of this kind, he wandered down to the rock cliffs, probably five hundred feet west of Observation Hill, this hill, it will be remembered, being close to the landing place when they were cast on the island. The sea was heavy and the tide coming in. He could not help reflecting, and his home, his parents, and his beautiful life there came up to his inward vision. The dreary pounding sea made him homesick, and for the first time he burst into tears. But George was a brave boy. He knew that crying was useless, and felt a little ashamed of himself.

His reflections were not long, however. To his left he saw a peculiar sight. At every inrushing wave there was a report like a cannon shot, followed by a tremendous stream and spray of water, which was shot out to sea high up above the waves.

This was an extraordinary sight to him, and unexplainable. The story was related to the Professor that evening.

"That was an air pocket in the rocks."

"What is an air pocket?"

"From your description it is probably a large cave, so situated in the wall of the cliff, that at a certain period the waves will entirely close the mouth. When the wave dashes up against the cliff and closes the mouth of the cave, the water tries to enter the cave. In doing so air is compressed in the pocket, and when the wave again starts to go out to sea, and the pressure is partly taken away, the compressed air explodes, so to say, and shoots out the water into a spray, and also causes the noise you heard."

"How much can air be compressed?"

"It is not known definitely how far. It has been compressed to less than one-eight-hundredth of its bulk. It is the most elastic substance known."

"Isn't water compressible?"

"No; if it had been compressible you would not have had that exhibition at the air pocket."

CHAPTER V

AN EXCITING HUNT

"What is that rocking?" cried Harry, jumping out of his couch, one night.

The Professor was awake and had noticed it.

"Probably an earthquake."

The rocking continued for several minutes, and then gradually subsided.

They boys were so excited that sleep was out of the question, for the time, besides the shaking might again recur at any moment.

"Do you think there is any danger, Professor?"

"It is impossible to say what will happen when these symptoms in the earth's crust take place."

"Are there not some instruments which indicate the extent and possible dangers of the quakes?"

"There is an instrument called the seismograph, which records the vibratory movements of the earth, and also locates the distances at which the shocks are from the observer, but there is nothing to indicate what the extent and probable dangers are."

"Is it true that the interior of the earth is in a liquid state?"

"Such has been the theory for many years; but it is now believed to be a solid--a body with a density five and a half times greater than water."

"If that is the case, why is it that the molten metal flows out of the volcanoes?"