First: _Cohesion_. This exists between molecules which are of the same kind, as for instance, iron. Cohesion of the particles is very strong in solids, much weaker in liquids, and scarcely exists at all between the particles in gases.

Second: _Adhesion_ is that property which exists between the surfaces of bodies in contact. If two flat surfaces are pressed together, as for instance, two perfectly smooth and flat pieces of lead, they will adhere. If, for instance, oil should be put on the surfaces, before putting them together, they would adhere so firmly that it would be difficult to pull them apart.

Third: _Affinity_. This is another peculiarity about materials. Thus, while cohesion binds together the molecules of water, it is chemical affinity which unites two elements, like hydrogen and oxygen, of which water is composed.

POROSITY.--All matter has little hollows or s.p.a.ces between the molecules. You know what this is in the case of a sponge, or pumice stone. Certain metals have the pores so small that it is difficult to see them except with a very powerful gla.s.s. Under great pressure water can be forced through the pores of metals, as has been done in the case of gold. Water also is porous, but the s.p.a.ces between the molecules are very small.

COMPRESSIBILITY.--It follows from the foregoing statement, that if there are little interstices between the molecules, the various bodies can be compressed together. This can be done in varying degrees with all solids, but liquids, generally, have little compressibility. Gases are readily reduced in volume by compression.

ELASTICITY.--This is a property by virtue of which a body resumes its original form when compressed. India rubber, ivory and gla.s.s are examples of elasticity; whereas, lead and clay do not possess this property. Air is the most elastic of all substances.

INERTIA.--This is a property of matter by virtue of which it cannot of itself change its state of motion or of rest.

Newton's first law of motion is, in substance, that matter at rest will eternally remain at rest, and matter in motion will forever continue in motion, unless acted on by some external force.

A rider is carried over the head of a horse when the latter suddenly stops. This ill.u.s.trates the inertia of movement. A stone at rest will always remain in that condition unless moved by some force. That shows the inertia of rest.

MOMENTUM.--This is the term to designate the quant.i.ty of motion in a body. This quant.i.ty varies and is dependent on the ma.s.s, together with the velocity. A fly wheel is a good example. It continues to move after the impelling force ceases; and a metal wheel has greater momentum than a wooden wheel at the same speed, owing to its greater ma.s.s.

If, however, the wooden wheel is speeded up sufficiently it may have the same momentum as the metal one.

WEIGHT.--All substances have what is called _weight_. This means that everything is attracted toward the earth by the force of gravity.

Gravity, however, is different from weight. All substances attract each other; not only in the direction of the center of the earth, but laterally, as well.

Weight, therefore, has reference to the pull of an object toward the earth; and gravity to that influence which all matter has for each other independently of the direction.

CENTRIPETAL FORCE.--This attraction of the earth, which gives articles the property of weight, is termed centripetal force--that is, the drawing in of a body.

CENTRIFUGAL FORCE.--The direct opposite of centripetal, is centrifugal force, which tends to throw outwardly. Dirt flying from a rapidly moving wheel ill.u.s.trates this.

CAPILLARY ATTRACTION.--There is a peculiar property in liquids, which deserves attention, and should be understood, and that is the name given to the tendency of liquids to rise in fine tubes.

It is stated that water will always find its level. While this is true, we have an instance where, owing to the presence of a solid, made in a peculiar form, causes the liquid, within, to rise up far beyond the level of the water.

This may be ill.u.s.trated by three tubes of different internal diameters.

The liquid rises up higher in the second than in the first, and still higher in the third than in the second. The smaller the tube the greater the height of the liquid.

This is called _capillary attraction_, the word capillary meaning a hair. The phenomena is best observed when seen in tubes which are as fine as hairs. The liquid has an affinity for the metal, and creeps up the inside, and the distance it will thus move depends on the size of the tube.

THE SAP OF TREES.--The sap of trees goes upwardly, not because the tree is alive, but due to this property in the contact of liquids with a solid. It is exactly on the same principle that if the end of a piece of blotting paper is immersed in water, the latter will creep up and spread over the entire surface of the sheet.

In like manner, oil moves upwardly in a wick, and will keep on doing so, until the lighted wick is extinguished, when the flow ceases. When it is again lighted the oil again flows, as before.

If it were not for this principle of capillary attraction, it would be difficult to form a bubble of air in a spirit level. You can readily see how the liquid at each end of the air bubble rounds it off, as though it tried to surround it.

SOUND.--Sound is caused by vibration, and it would be impossible to convey it without an elastic medium of some kind.

_Acoustics_ is a branch of physics which treats of sounds. It is distinguished from music which has reference to the particular kinds.

_Sounds_ are distinguished from _noises_. The latter are discordant and abrupt vibrations, whereas the former are regular and continuous.

SOUND MEDIUMS.--Gases, vapors, liquids and solids transmit vibrations, but liquids and solids propagate with greater velocity than gases.

VIBRATION.--A vibration is the moving to and fro of the molecules in a body, and the greater their movement the more intense is the sound. The intensity of the sound is affected by the density of the atmosphere, and the movement of the winds also changes its power of transmission.

Sound is also made more intense if a sonorous body is near its source.

This is taken advantage of in musical instruments, where a sounding-board is used, as in the case of the piano, and in the violin, which has a thin sh.e.l.l as a body for holding the strings.

Another curious thing is shown in the speaking tube, where the sound waves are confined, so that they are carried along in one line, and as they are not interfered with will transmit the vibrations to great distances.

VELOCITY OF SOUND.--The temperature of the air has also an effect on the rate of transmission, but for general purposes a temperature of 62 degrees has been taken as the standard. The movement is shown to be about 50 miles in 4 minutes, or at the rate of 1,120 feet per second.

In water, however, the speed is four times greater; and in iron nearly fifteen times greater. Soft earth is a poor conductor, while rock and solid earth convey very readily. Placing the ear on a railway track will give the vibrations of a moving train miles before it can be heard through the air.

SOUND REFLECTIONS.--Sound waves move outwardly from the object in the form of wave-like rings, but those concentric rings, as they are called, may be interrupted at various points by obstacles. When that is the case the sound is buffeted back, producing what is called echoes.

RESONANCE.--Materials have a quality that produces a very useful result, called _resonance_, and it is one of the things that gives added effect to a speaker's voice in a hall, where there is a constant succession of echoes. A wall distant from the speaker about 55 feet, produces an almost instantaneous reflection of the sound, and at double that measurement the effect is still stronger. When the distance is too short for the reflecting sound to be heard, we have _resonance_. It enriches the sound of the voice, and gives a finer quality to musical instruments.

ECHOES.--When sounds are heard after the originals are emitted they tend to confusion, and the quality of resonance is lost. There are places where echoes are repeated many times. In the chateau of Simonetta, Italy, a sound will be repeated thirty times.

SPEAKING TRUMPET.--This instrument is an example of the use of reflection. It is merely a bell-shaped, or flaring body, the large end of which is directed to the audience. The voice talking into the small end is directed forwardly, and is reflected from the sides, and its resonance also enables the vibrations to carry farther than without the use of the solid part of the instrument.

The ear trumpet is an ill.u.s.tration of a sound-collecting device, the waves being brought together by reflection.

THE STETHOSCOPE.--This is an instrument used by physicians, and it is so delicate that the movements of the organs of the body can be heard with great distinctness. It merely collects the vibrations, and transmits them to the ears by the small tubes which are connected with the collecting bell.

THE VITASCOPE.--Numerous instruments have been devised to determine the rate of vibration of different materials and structures, the most important being the _vitascope_, which has a revolvable cylinder, blackened with soot, and this being rotated at a certain speed, the stylus, which is attached to the vibrating body, in contact with the cylinder, will show the number per second, as well as the particular character of each oscillation.

THE PHONAUTOGRAPH.--This instrument is used to register the vibration of wind instruments, as well as the human voice, and the particular forms of the vibrations are traced on a cylinder, the tracing stylus being attached to a thin vibrating membrane which is affected by the voice or instrument.

THE PHONOGRAPH.--This instrument is the outgrowth of the stylus forms of the apparatus described, but in this case the stylus, or needle, is fixed to a metallic diaphragm, and its point makes an impression on suitable material placed on the outside of a revolvable cylinder or disc.

Light.-Light is the agent which excites the sensation of vision in the eye. Various theories have been advanced by scientists to account for the phenomenon, and the two most noted views are the _corpuscular_, promulgated by Sir Isaac Newton, and the _undulatory_, enunciated by Huygens and Euler.

The _corpuscular_ theory conceives that light is a substance of exceedingly light particles which are shot forth with immense velocity.

The _undulatory_ theory, now generally accepted, maintains that light is carried by vibrations in ether. Ether is a subtle elastic medium which fills all s.p.a.ce.

_Luminous_ bodies are those like the sun, which emit light. Rays may _diverge_, that is, spread out; _converge_, or point toward each other; or they may be _parallel_ with each other.

VELOCITY OF LIGHT.--Light moves at the rate of about 186,000 miles a second. As the sun is about 94,000,000 miles from the earth, it takes 8-1/2 minutes for the light of the sun to reach us.

REFLECTION.--One of the most important things connected with light is that of reflection. It is that quality which is utilized in telescopes, microscopes, mirrors, heliograph signaling and other like apparatus and uses. The underlying principle is, that a ray is reflected, or thrown back from a mirror at the same angle as that which produces the light.

When the rays of the sun, which are, of course, parallel, strike a concave mirror, the reflecting rays are converged; and when the rays strike a convex mirror they diverge. In this way the principle is employed in reflecting telescopes.

REFRACTION.--This is the peculiar action of light in pa.s.sing through substances. If a ray pa.s.ses through water at an angle to the surface the ray will bend downwardly in pa.s.sing through, and then again pa.s.s on in a straight line. This will be noticed if a pencil is stood in a gla.s.s of water at an angle, when it will appear bent.

Refraction is that which enables light to be divided up, or a.n.a.lyzed. In this way white light from the sun is shown to be composed of seven princ.i.p.al colors.