tion of volume arises from the particles of one of the fluids insinuating themselves between those of the other; but it is clear that where a particle of alcohol is, there a particle of water can not be. In like manner when a nail is driven into a board, the particles of the latter are thrust aside and compressed to make room for those of the former.

Inertia.

8. INERTIA is the tendency which a body has to maintain its state of rest or motion. If a body is at rest it has no power to set itself in motion, or if it is in motion it has no power to change either its rate of motion or the direction in which it is moving. Hence, if a body is at rest, it will remain at rest, or if in motion, it will move on uniformly in a straight line until acted upon by some force.

The reason why we do not see bodies continue to move on uniformly in straight lines, when set in motion, is that they are continually acted upon by forces which change their state of motion. Thus, a ball thrown from the hand, besides meeting with the resistance of the air, is continually drawn downwards by the attraction of the earth, till at last it is brought to rest.

Many familiar phenomena are explained by the principle of inertia. For example, when a vehicle in motion is suddenly arrested, loose articles in it are thrown to the front, because they tend to keep the motion which they had acquired. When a man in running strikes his foot against an obstacle, the inertia of the upper part of his body carries it forward, and he falls to the ground. For the same reason, when a man jumps from a car in motion, he will be in danger of falling in the direction of the moving car. It is inertia which renders accidents upon railroads so terrible. When from any cause the locomotive is suddenly arrested, the inertia of the entire train acts to pile the cars together in one general wreck. It is the inertia of the hammer that enables it to overcome the resistance

Give examples of apparent penetrability. (8.) What is Inertia? Illustrate. Why do we not see bodies conform to the law of inertia? Give examples of the principle of inertia.

which the wood offers to the entering nail; and in driving piles, the principal effect is due to the inertia of the descending ram.

Porosity.

9. POROSITY is the degree of separation between the mole

cles of a body The intervals between the molecules are called pores. When these intervals are very great, the body is said to be porous, as in steam, air, and gases. When the intervals are very small, the body is said to be dense, as in gold, platinum, and mercury. Pores must not be confounded with cells, as in sponge, light bread, and the like.

All bodies are more or less porous.

The following experiment shows the porosity of leather. A long glass tube (Fig. 1) is surmounted by a brass cup, with a thick leather bottom, fitting the tube air-tight. The lower end of the tube terminates in a brass cap, which is attached to a machine for exhausting the air from the tube, called an air-pump.

If a quantity of mercury is

(9.) What is Porosity? When are bodies porous? When dense? Explain the

experiment showing the porosity of leather.

poured into the upper cup, and the air exhausted from the tube, the mercury, being pressed down by the external air, is seen falling through the leather in small drops like rain.

Gold was shown to be porous by some Florentine philosophers in the following manner. A hollow sphere of gold was filled with water and tightly closed, after which it was subjected to great pressure. The water was seen to issue from the globe and form on its surface like dew. The experiment has since been repeated with other metals, and with like results.

Gases are shown to be porous by their enormous reduction in volume when compressed; if a gas be introduced into a jar, it will spread by its expansive force and completely fill the vessel; if a second gas be introduced into the same vessel, it likewise expands and fills the vessel as though the first gas did not exist. This proves that the molecules of the second gas arrange themselves in the pores of the first.

The property of porosity finds an important application in the process of filtering, that is, in separating foreign particles from liquids.

Explain the Florentine experiment. What are filters?

Fig. 2 represents a filter for purifying water; it is simply a box divided into two parts by a partition of porous stone, A. The water to be filtered is placed in the upper part, from which it passes slowly into the lower part through the pores of the stone. In one corner of the box is a tube, a, which permits the air to escape as the lower part of the box fills with water. The purified water is drawn off by means of a faucet near the bottom of the box.

Fig. 3 represents a filter used by chemists. It consists of a pocket of some porous material, as felt, for example, suspended by cords. The substance to be filtered is poured into the pocket, from which the liquid escapes slowly through the pores, leaving the solid parts behind.

Filters are also formed by layers of powdered charcoal, or finely ground quartz, through the pores of which the liquids pass. It is to a natural filtration through sand that many kinds of spring water owe their purity.

It is in consequence of porosity, that burning coals covered up with ashes continue to burn slowly. The air which is necessary to combustion penetrates through the pores of the ashes, in sufficient quantity to keep the fire from being entirely extinguished.

Finally, it is in consequence of their porosity, that many kinds of wood absorb moisture from the air, and tend to swell and crack; this difficulty is remedied by applying oils and varnishes, which close the pores and exclude the moisture.

Divisibility.

10. DIVISIBILITY is that property by virtue of which a body may be divided into parts. All bodies are capable of subdivision, and in many cases the parts that may be obtained are of almost inconceivable minuteness.

The following examples serve to show the extreme smallness of the molecules of matter. A single grain of carmine imparts a sensible color to a gallon of water; this gallon of water may be separated into a million of drops, and if we suppose each drop to contain ten particles of carmine, which is a low estimate, we shall have

Explain the water filter. Explain the chemist's filter. Other applications of porosity. (10.) What is Divisibility? Give examples of divisibility by solution.

divided the grain of carmine into ten million of molecules, each of which is visible to the naked eye.

The microscope reveals to us, in certain vegetable infusions, animalculæ so small that several hundred of them can swim in a drop of water that adheres to the point of a needle. These little animals are capable of motion, and even of preying upon each other; they therefore possess organs of motion, digestion, and the like. How minute, then, must be the molecules which go to make up these organs.

A grain of musk is capable of diffusing its odor through an apartment for years, with scarcely an appreciable diminution of its weight. This shows that the molecules of musk continually given off to replenish the odor, are of inconceivable smallness.

The blood of animals consists of minute red globules swimming in a serous fluid; these globules are so small that a drop of human blood, no larger than the head of a small pin, contains at least 50,000 of them. In many animals these globules are still smaller; in the musk deer, for example, a single drop of blood of the size of a pin's head contains at least a million of them.

Compressibility.

11. COMPRESSIBILITY is the property of being reduced to a smaller space by pressure. This property is a consequence of porosity, and the change of bulk comes from the particles being brought nearer together by the pressure. Sponge, india-rubber, cork, and elder pith, are examples of compressible bodies; they may be sensibly diminished in volume by the pressure of the fingers. Gases are, however, the best examples of compressible bodies.

Fig. 4 represents an apparatus by means of which the compressibility of gases may be shown. It consists of a tube of glass, with metallic caps, completely closed at its lower end. An air-tight piston is introduced at the upper end, and on being pushed down we see the inclosed air reduced to the half, fourth, and even the hundredth part of its original bulk.

Examples of minute animals. Examples of odoriferous bodies. Blood globules. (11.) What is Compressibility? Examples. Explain the experiment.