stay flat, because the lead has no elasticity. If an india-rubber ball be dropped on the pavement, it will be flattened very much more than the lead ball; but the elasticity of the substance is so great that the particles all resume their old positions with such force as to bounce the ball upward to a considerable height, and when the ball is examined, no sign of the flattening is seen.

Some substances may be squeezed or compressed into very much smaller bulk; such substances are said to be compressible. If, upon being released, they return to their former bulk, they are of course elastic. All substances are to some extent compressible. Under great pressure, the ether is crowded out from between the atoms, and the substance becomes more solid or dense. The real matter in substances is so little, compared with the ether between the atoms, that it has been stated that the whole earth, if the atoms, were pressed into actual contact, and made perfectly dense, would not fill a square foot of space. Such a statement seems hard to believe, but will serve to bring our minds to a sense of the porousness of all matter.

A cubic inch of lead can be compressed into one-tenth of that bulk, and it will remain so, and be more dense or solid. A room-full of air can be compressed into a thimble, but will immediately expand to its full volume when released, owing to its great elasticity.

We know very little of the real cause of natural forces. We know that some great force tends to draw the atoms of some matter closely together, and we call that force cohesion. We know that some great force tends to force the atoms of some matter apart, and we call that force repulsion. While we know little of the cause of these forces, we may, by close observation, become familiar with their general laws of action, and it is a knowledge of these general laws of nature which forms the basis of science and of all practical operations with matter.

In many substances, as has been stated, some natural force causes one atom to draw closely and powerfully toward other atoms.

There is also another natural force which tends to draw all atoms and all substances, in fact everything, toward the centre of the earth. This force we call gravity. Cohesion draws the atoms composing a piece of iron closely together with great force, and gravitation draws the piece of iron powerfully toward the centre of the earth. It seems as if all pieces of iron, and all other pieces of matter, were simple atoms constituting a great universe which the force of gravitation was seeking to bring to the centre of the earth. If we lift a weight it falls, and if it could fall far enough, its line of travel would pass through the earth's centre. A plumb line suspended gives us the direction to the earth's centre. Two

plumb lines five miles apart, would each point to the earth's centre, and as the earth is round, these two lines would not be parallel. They would be further apart at the top than at the bottom, and would cross at the centre of the earth. A very long building, whose walls are perfectly plumb, is wider at the top than at the bottom. A little study will show that this tapering building with the large end upward is the most secure form. The stones composing it tend to fall to the earth's centre, and as the walls are built in that direction, it follows that each stone is supported by other stones placed directly under and in the real path of the force of gravity.

Gravity acts with a different degree of force on different substances. Those substances which are attracted powerfully we call heavy substances, and those which are attracted but slightly we call light substances. In comparing heavy and light bodies we speak of their weight.

In referring to the size of bodies, science uses the term mass, which means the same as bulk. A large thing is spoken of as a thing of great

mass.

In studying the action of gravity we must consider it as acting on substances of equal mass. Thus it would not be proper in investigating the laws of gravity to experiment on one cubic foot of lead and two cubic feet of iron. The mass of the iron would then be twice the mass of the

lead, and we could determine but little as to the laws of gravity.

We should deal with equal masses and study closely the different effects of gravity on similar masses of different substances. As before stated, when the atoms of a substance are packed closely together we say that substance is dense. If a substance is very dense, we can readily see that there is more matter than ether in it; while if it is very porous there may be very little matter in a large mass. Gravity acts on substances in proportion to their densities.

Lead is a very dense material; gravity acts on it powerfully and we say it is heavy. Cork is very porous; gravity acts on it slightly and we say it is light.

A cubic foot of cork weighs fifteen pounds, and a cubic foot of lead weighs 700 pounds. We say that lead is more than forty-six times as heavy as cork. There is so much difference in the density of these two substances that we can easily see it. We see great pores in the cork, and thus know that the particles composing it are not packed closely together as they are in lead. Still the mass of the lead and the cork is the same, for we are speaking of a cubic foot of each. There is the same difference in nature between all substances as we see in these two substances, but the degree of difference is not the same. Thus cast iron is very much denser and

heavier than cork, but is lighter and more porous than lead.

If we should compress a cubic foot of cork with sufficient force we might make it very dense, and reduce its mass so that forty-six cubic feet would occupy only one cubic foot of space. If we should do this the cork would become as dense as lead, and its weight would be the same.

Gravity acts on all matter. There is no substance which does not have weight. If we take an evenly-balanced pair of scales and place a cubic foot of lead in one pan and a cubic foot of cork in the other pan, the lead will be drawn downward by gravity; the cork will also be drawn downward by gravity, but the superior attraction for the dense lead so far overbalances the attraction for the porous cork that the cork rises and the lead falls. Iron dropped into water sinks and crowds water upward, while iron dropped in quicksilver is held upward and floats on the heavier quicksilver.

Almost all substances will drop through the light air, but common gas is lighter than air, and floats upon it. The air is the heaviest, and is attracted the most by gravity. All these substances, however light and airy, have some weight. We look upon the air as a something having no weight, because we are always comparing it with substances very much heavier. Air has weight, and a bottle full of it will weighi