A workman is able to select the right metals because he knows that each has some peculiar property which is best adapted for his particular use. These with their meaning will now be explained.

Elasticity.—This exists in metals in three distinct ways: First, in the form of traction. Hang a weight on a wire and it will stretch a certain amount. When the weight is removed the wire shrinks back to its original length.

Second: If the weight on the wire is rotated, so as to twist it, and the hand is taken from the weight, it will untwist itself, and go back to its original position. This is called torsion.

Third: A piece of metal may be coiled up like a watch spring, or bent like a carriage spring, and it will yield when pressure is applied. This is called flexure.

Certain kinds of steel have these qualities in a high degree.

Tenacity.—This is a term used to express the resistance which the body opposes to the separation of its parts. It is determined by formingp. 80 the metal into a wire, and hanging on weights, to find how much will be required to break it. If we have two wires, the first with a transverse area only one-quarter that of the second, and the first breaks at 25 pounds, while the second breaks at 50 pounds, the tenacity of the first is twice as great as that of the second.

To the boy who understands simple ratio in mathematics, the problem would be like this:

25 × 4 : 50 × 1, or as 2 : 1.

The Most Tenacious Metal.—Steel has the greatest tenacity of all metals, and lead the least. In proportion to weight, however, there are many substances which have this property in a higher degree. Cotton fibers will support millions of times their own weight.

There is one peculiar thing, that tenacity varies with the form of the body. A solid cylindrical body has a greater strength than a square one of the same size; and a hollow cylinder more tenacity than a solid one. This principle is well known in the bones of animals, in the feathers of birds, and in the stems of many plants.

In almost every metal tenacity diminishes as the temperature increases.

Ductility.—This is a property whereby a metal may be drawn out to form a wire. Some metals,p. 81 like cast iron, have absolutely no ductility. The metal which possesses this property to the highest degree, is platinum. Wires of this metal have been drawn out so fine that over 30,000 of them laid side by side would measure only one inch across, and a mile of such wire would weigh only a grain, or one seven-thousandth of a pound.

Malleability.—This is considered a modification of ductility. Any metal which can be beaten out, as with a hammer, or flattened into sheets with rollers, is considered malleable. Gold possesses this property to the highest degree. It has been beaten into leaves one three-hundred-thousandth of an inch thick.

Hardness.—This is the resistance which bodies offer to being scratched by others. As an example, the diamond has the capacity to scratch all, but cannot be scratched by any other.

Alloys.—Alloys, that is a combination of two or more metals, are harder than the pure metals, and for this reason jewelry, and coins, are usually alloyed.

The resistance of a body to compression does not depend upon its hardness. Strike a diamond with a hammer and it flies to pieces, but wood does not. One is brittle and the other is tough.

The machinist can utilize this property by understanding that velocity enables a soft materialp. 82 to cut a harder one. Thus, a wrought iron disc rotating rapidly, will cut such hard substances as agate or quartz.

Resistance.—All metals offer more or less resistance to the flow of an electric current. Silver offers the least resistance, and German silver the greatest. Temperature also affects the flow. It passes more easily over a cold than a warm conductor.

Persistence.—All metals on receiving heat, will retain it for a certain length of time, and will finally cool down to the temperature of the surrounding atmosphere. Some, like aluminum, retain it for a long time; others, as iron, will give it off quickly.

Conductivity.—All metals will conduct heat and cold, as well as electricity. If one end of a metal bar is heated, the heat creeps along to the other end until it has the same temperature throughout. This is called equalization.

If a heated bar is placed in contact with another, the effect is to increase the temperature of the cold bar and lower that of the warm bar. This is called reciprocity.

Molecular Forces.—Molecular attraction is a force which acts in such a way as to bring all the particles of a body together. It acts in threep. 83 ways, dependent on the particular conditions which exist.

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 spaces 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 glass. Under great pressure water can be forced through the pores of metals, as has been done inp. 84 the case of gold. Water also is porous, but the spaces 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 glass 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 illustrates 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 thep. 85 quantity of motion in a body. This quantity varies and is dependent on the mass, 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 mass.

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 top. 86 throw outwardly. Dirt flying from a rapidly moving wheel illustrates 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 illustrated 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 immersedp. 87 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 movementp. 88 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 shell 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-likep. 89 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 thep. 90 vibrations to carry farther than without the use of the solid part of the instrument.

The ear trumpet is an illustration 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 dep. 91scribed, 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 space.

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 81/2 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, microp. 92scopes, 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 passing through substances. If a ray passes through water at an angle to the surface the ray will bend downwardly in passing through, and then again pass on in a straight line. This will be noticed if a pencil is stood in a glass of water at an angle, when it will appear bent.

Refraction is that which enables light to be divided up, or analyzed. In this way white light from the sun is shown to be composed of seven principal colors.

Colors.—If the light is passed through a prism, which is a triangularly shaped piece of glass, the rays on emerging will diverge from each other, and when they fall on a wall or screen the colors red, orange, yellow, green, blue, indigo and violet are shown

The reason for this is that the ray in passing through the prism has the different colors in it refract at different angles, the violet bending more than the red.

The Spectroscope.—The ability to make what is thus called a spectrum, brought forth one of the most wonderful instruments ever devised by man. If any metal, or material, is fused, or put in such a condition that a ray of light can be obtained from it, and this light is passed through a prism, it will be found that each substance has its own peculiar divisions and arrangements of colors.

In this way substances are determined by what is called spectrum analysis, and it is by means of this instrument that the composition of the sun, and the planets and fixed stars are determined.

The Rainbow.—The rainbow is one of the effects of refraction, as the light, striking the little globular particles of water suspended in the air, produces a breaking up of the white light into its component colors, and the sky serves as a background for viewing the analysis thus made.

Heat.—It is now conclusively proven, that heat, like light, magnetism and electricity, is merely a mode of motion.

The mechanical theory of heat may be shown by rubbing together several bodies. Heat expandsp. 94 all substances, except ice, and in expanding develops an enormous force.

Expansion.—In like manner liquids expand with heat. The power of mercury in expanding may be understood when it is stated that a pressure of 10,000 pounds would be required to prevent the expansion of mercury, when heated simply 10 degrees.

Gases also expand. While water, and the different solids, all have their particular units of expansion, it is not so with gases, as all have the same coefficient

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