"And how can we get the air out?" said Rollo.

"We can't," said his father, "get it all out; but we can get a part of it out by shutting the door quick. The door will carry with it a part of the air that was in the closet, and then the outside air will be spouted in, through the key-hole of the other door. Only we can't see it, as we can the water."

"No," said Rollo; "but I can put my hand there, and feel it."

"A better way," said his father, "would be to hold a lamp opposite to the key-hole, and see if it blows the flame."

Rollo tried the experiment, in the way his father had described. He went into the closet with the lamp. He held the lamp opposite to the key-hole, and pretty near to it, and then he asked Nathan to shut the other door suddenly. Nathan, who was standing all ready by the other door, which was about half open, put his two hands against it, and pushed it to, with all his strength, producing a great concussion.

"O Nathan," said his father, "you need not be quite so violent as that."

"It succeeded, father, it succeeded," said Rollo.

"I'm glad it succeeded," said his father; "but Nathan need not have shut the door with so much force."

"I wanted to drive out all the air," said Nathan.

"I'll show you how to do it," said his father.

Rollo's father accordingly arose, and came to the closet door. He opened the door wide, and then explained to the boys, that the beginning of the movement of the door, when it was wide open, did not drive out any air.

"For," said he, "there is so large a s.p.a.ce between the edge of the door and the wall, that the air that is put in motion by the movement of the door, can pa.s.s directly round the edge, back into the closet again. It is only when the door is almost shut, when the edge of it comes close to the casing all around, that the movement of the door drives the air out."

Then he took hold of the latch of the door, and put it almost to, very gently. He turned the latch so as to prevent its snapping against the catch, and then pushed it suddenly into its place three or four times, opening the door only a very little way every time.

"Now," said he, "hold the lamp at the key-hole, and watch the flame, while I shut the door two or three times in this way."

Rollo did so, Nathan standing all the time by his side. They observed that the flame of the lamp was driven into the room every time the door was shut; proving that, every time a little of the air was driven out by the door, a little puff rushed in at the key-hole.

"Let us stop up the key-hole," said Rollo, "and then it can't get in."

"Yes," said his father, "there are a great many little crevices all around the closet, where the air can come in."

"Couldn't we stop those up too?" said Rollo.

"No," said his father, "not so as to make the closet air-tight. For, if the crevices could all be stopped exactly, the air would come in through the very wood itself."

"How?" said Rollo.

"Why, there are little pores in wood, that is, little channels that the sap flowed in when the wood was growing, and the air can pa.s.s through these."

Here Rollo's father observed that Rollo was looking very intently at the table; and he asked him what he was doing: he said he was trying to find some of the pores.

"You can't see them there," said his father. "St. Domingo mahogany is a very hard and close-grained kind of wood. If it was summer, and you could dig down and get a small piece of the root of the great elm-tree in the yard, you could see the pores and channels there."

After some more conversation on this subject, Rollo asked his father if he could not think of some other experiments for them to try. His father said that he did not just then think of any experiment, but that, if Rollo and Nathan would come and sit down by the fire, he would give them some information on the subject. Rollo's mother said that she should like to hear too. They accordingly waited until she was ready, and then, when all were seated, Mr. Holiday began thus:--

"Air is in many respects much like water."

"Yes," interrupted Rollo, "just like water, only thinner, because, you see----"

"You must not interrupt me," said his father, "unless to ask some question, which is necessary to understand what I say. It is entirely irregular for a pupil, instead of listening to his teacher, to interrupt, in order to tell something that he knows himself."

Rollo's father smiled, as he said this, but Rollo looked rather ashamed.

Then his father proceeded:--

"There is one very remarkable difference between them. Water is not compressible by force; but air is."

"What is the meaning of _compressible_?" said Nathan.

"Compressible things," said his father, "are those that can be compressed, that is, pressed together, so as to take up less room than they did before. Sponge is compressible. A pillow is compressible. But iron is not compressible, and water is not compressible."

"I should think it was," said Nathan; "it is very soft."

"It is very _yielding_," replied his father, "when you press it, but it is not pressed into any smaller s.p.a.ce. It only moves away. If you have a tumbler half full of water, and press a ball down into it, you could not crowd the water into any smaller s.p.a.ce than it occupied at first; but, as fast as the ball went down, the water would come up around the sides of the ball."

"But suppose," said Rollo, "that the ball was just big enough to fit the tumbler all around; then the water could not come up."

"And then," said his father, "you could not crowd the ball down."

"Could not a _very_ strong man?" said Nathan.

"No," replied his father, "the water cannot be sensibly compressed. But now, if the tumbler contained only air, and if a ball were to be put in at the top, just large enough to fit the tumbler exactly, and if a strong man were to crowd it down with all his strength, he would, perhaps, compress the air into half the s.p.a.ce which it occupied before."

"Perhaps the tumbler would break," said Nathan.

"Yes," replied his father, "and the tumbler will answer only for a supposition; but for a real experiment it would be best to have a cylinder of iron."

"What is a cylinder?" said Nathan.

"An iron vessel, shaped like a tumbler, only as large at the bottom as it is at the top, would be a cylinder. Now, if there was a cylinder of iron, with the inside turned perfectly true, and a bra.s.s piston fitted to it----"

"What is a piston?" said Nathan.

"A piston," said his father, "is a sort of stopper, exactly fitted to the inside of a cylinder, so as to slide up and down. It is made to fit perfectly, and then it is oiled, so as to go up and down without much friction, that is, hard rubbing. There is a sort of stem coming up from the middle of the piston, called the piston rod, which is to draw up the piston, and to press it down by.

"Now," continued his father, "if a strong man had a cylinder like this, with a piston fitted to it, and a strong handle across the top of the piston rod, perhaps he might press the air into one half the s.p.a.ce which it occupied before. That is, if the cylinder was full of air when he put the piston in, perhaps he could get the piston down half way to the bottom. Then the air would be twice as _dense_ as it was before; that is, there would be twice as much of it in the same s.p.a.ce as there was before. It would be twice as compact and heavy. This is called _condensing_ air. The philosophers have ingenious instruments for condensing air.

"If, however, a man condenses air in this way, by crowding down a piston, he does not begin the condensation when the piston begins to descend. The air is condensed a great deal before he begins. All the air around us is condensed."

"How comes it condensed?" said Rollo.

"Why, you recollect that, when you bored a hole through the board in the bottom of your dam, the water spouted out."

"No, father," said Rollo, "we pulled the plug out; Jonas bored the hole."

"Well," said his father, "the water spouted out."

"Yes," said Rollo.