"Yes," replied his father. "Now, the property I am going to tell you about this evening is this--that, if the man lets go of the piston rod, the air that is condensed into the bottom of the cylinder, will spring up, and force the piston up again. This property is called _elasticity_. It is sometimes called the _expansive force_ of the air. For it is a force tending to expand the air, that is, to swell it out into its original dimensions. This is another great difference between air and water. "Now, as all the air around us," continued Rollo's father, "is pressed down very heavily, and is condensed a great deal, it is all the time endeavoring to expand; and it would expand, were it not that the great burden of the air above it keeps it condensed. But water is not compressed, and has no tendency to expand. The water of Rollo's dam, for instance, had all the weight of the atmosphere resting upon it, but it did not compress it at all, and so it did not tend to expand. "And now," said his father, "I cannot perform any experiment, to show you that air tends strongly to expand or swell out into a great space, while water does not; but I can make a supposition, which will illustrate it. Suppose we had a large, but very thin, glass bottle, filled with water, and put down upon the floor in the middle of this room. Suppose, also, that we had another bottle, of the same size and shape, filled with air, and we put that down upon the floor by the side of the other; both bottles being stopped very tight. Now, if we could by any means suddenly take away all the air from the room, so that there should be nothing around the bottles, then the bottle of water would remain just as it is, for the glass would have nothing to support but the weight of the water, and it would be strong enough for that. But the bottle of air would fly all to pieces; for that would not rest quietly, like the water, satisfied with the space which it already has, and only pressing with its own weight upon the sides of the glass; but it would immediately expand with so much force as to break the thin glass all to pieces." "Would it!" exclaimed Rollo and Nathan together. "And would it make a loud noise?" "Yes," replied their father, "I presume it would make a loud explosion; that is, if the air in the room around it could by any means be all at once and suddenly removed. "And so you must remember," he continued, "that there are two very remarkable differences between air and water. Air may be condensed by pressure, and, as it exists all around us, is greatly condensed by the pressure of the air above, and it may be compressed more. And air is expansive, while water is not. Whenever the pressure upon it is removed, it suddenly expands, or spreads out in all directions." "O dear me!" said Nathan, with a sigh. "What is the matter?" said his father. "Why, I can't understand it very well."