Gold, silver, and lead are examples of elements, and water, alcohol, cider, sand, and marble are complex substances, or compounds, as we are apt to call them. Everything, no matter what its size or shape or character, is formed from the various combinations into molecules of a few simple atoms, of which there exist about eighty known different kinds. But few of the eighty known elements play an important part in our everyday life. The elements in which we are most interested are given in the following table, and the symbols by which they are known are placed in columns to the right:

Oxygen

O

Copper

Cu

Phosphorus

P

Hydrogen

H

Iodine

I

Pota.s.sium

K

Carbon

C

Iron

Fe

Silver

Ag

Aluminium Al

Lead

Pb

Sodium

Na

Calcium

Ca

Nickel

Ni

Sulphur

S

Chlorine

Cl

Nitrogen

N

Tin

Sn

We have seen in an earlier experiment that twice as much hydrogen as oxygen can be obtained from water. Two atoms of the element hydrogen unite with one atom of the element oxygen to make one molecule of water. In symbols we express this H_2O. A group of symbols, such as this, expressing a molecule of a compound is called a _formula_. NaCl is the formula for sodium chloride, which is the chemical name of common salt.

CHAPTER X

LIGHT

98. What Light Does for Us. Heat keeps us warm, cooks our food, drives our engines, and in a thousand ways makes life comfortable and pleasant, but what should we do without light? How many of us could be happy even though warm and well fed if we were forced to live in the dark where the sunbeams never flickered, where the shadows never stole across the floor, and where the soft twilight could not tell us that the day was done? Heat and light are the two most important physical factors in life; we cannot say which is the more necessary, because in the extreme cold or arctic regions man cannot live, and in the dark places where the light never penetrates man sickens and dies. Both heat and light are essential to life, and each has its own part to play in the varied existence of man and plant and animal.

Light enables us to see the world around us, makes the beautiful colors of the trees and flowers, enables us to read, is essential to the taking of photographs, gives us our moving pictures and our magic lanterns, produces the exquisite tints of stained-gla.s.s windows, and brings us the joy of the rainbow. We do not always realize that light is beneficial, because sometimes it fades our clothing and our carpets, and burns our skin and makes it sore. But we shall see that even these apparently harmful effects of light are in reality of great value in man's constant battle against disease.

99. The Candle. Natural heat and light are furnished by the sun, but the absence of the sun during the evening makes artificial light necessary, and even during the day artificial light is needed in buildings whose structure excludes the natural light of the sun.

Artificial light is furnished by electricity, by gas, by oil in lamps, and in numerous other ways. Until modern times candles were the main source of light, and indeed to-day the intensity, or power, of any light is measured in candle power units, just as length is measured in yards; for example, an average gas jet gives a 10 candle power light, or is ten times as bright as a candle; an ordinary incandescent electric light gives a 16 candle power light, or furnishes sixteen times as much light as a candle. Very strong large oil lamps can at times yield a light of 60 candle power, while the large arc lamps which flash out on the street corners are said to furnish 1200 times as much light as a single candle. Naturally all candles do not give the same amount of light, nor are all candles alike in size. The candles which decorate our tea tables are of wax, while those which serve for general use are of paraffin and tallow.

[Ill.u.s.tration: FIG. 57.--A photograph at _a_ receives four times as much light as when held at _b_.]

100. Fading Illumination. The farther we move from a light, the less strong, or intense, is the illumination which reaches us; the light of the street lamp on the corner fades and becomes dim before the middle of the block is reached, so that we look eagerly for the next lamp.

The light diminishes in brightness much more rapidly than we realize, as the following simple experiment will show. Let a single candle (Fig. 57) serve as our light, and at a distance of one foot from the candle place a photograph. In this position the photograph receives a definite amount of light from the candle and has a certain brightness.

If now we place a similar photograph directly behind the first photograph and at a distance of two feet from the candle, the second photograph receives no light because the first one cuts off all the light. If, however, the first photograph is removed, the light which fell on it pa.s.ses outward and spreads itself over a larger area, until at the distance of the second photograph the light spreads itself over four times as large an area as formerly. At this distance, then, the illumination on the second photograph is only one fourth as strong as it was on a similar photograph held at a distance of one foot from the candle.

The photograph or object placed at a distance of one foot from a light is well illuminated; if it is placed at a distance of two feet, the illumination is only one fourth as strong, and if the object is placed three feet away, the illumination is only one ninth as strong. This fact should make us have thought and care in the use of our eyes. We think we are sixteen times as well off with our incandescent lights as our ancestors were with simple candles, but we must reflect that our ancestors kept the candle near them, "at their elbow," so to speak, while we sit at some distance from the light and unconcernedly read and sew.

As an object recedes from a light the illumination which it receives diminishes rapidly, for the strength of the illumination is inversely proportional to the square of distance of the object from the light.

Our ancestors with a candle at a distance of one foot from a book were as well off as we are with an incandescent light four feet away.

101. Money Value of Light. Light is bought and sold almost as readily as are the products of farm and dairy; many factories, churches, and apartments pay a definite sum for electric light of a standard strength, and naturally full value is desired. An instrument for measuring the strength of a light is called a photometer, and there are many different varieties, just as there are varieties of scales which measure household articles. One light-measuring scale depends upon the law that the intensity of illumination decreases with the square of the distance of the object from the light. Suppose we wish to measure the strength of the electric light bulbs in our homes, in order to see whether we are getting the specified illumination. In front of a screen place a black rod (Fig. 58) which is illuminated by two different lights; namely, a standard candle and an incandescent bulb whose strength is to be measured. Two shadows of the rod will fall on the screen, one caused by the candle and the other caused by the incandescent light. The shadow due to the latter source is not so dark as that due to the candle. Now let the incandescent light be moved away from the screen until the two shadows are of equal darkness. If the incandescent light is four times as far away from the screen as the candle, and the shadows are equal, we know, by Section 100, that its strength is sixteen candle power. If the incandescent light is four times as far away from the screen as the candle is, its power must be sixteen times as great, and we know the company is furnishing the standard amount of light for a sixteen candle power electric bulb. If, however, the bulb must be moved nearer to the rod in order that the two shadows may be similar then the light given by the bulb is less than sixteen candle power, and less than that due the consumer.

[Ill.u.s.tration: FIG. 58.--The two shadows are equally dark.]

102. How Light Travels. We never expect to see around a corner, and if we wish to see through pinholes in three separate pieces of cardboard, we place the cardboards so that the three holes are in a straight line. When sunlight enters a dark room through a small opening, the dust particles dancing in the sun show a straight ray. If a hole is made in a card, and the card is held in front of a light, the card casts a shadow, in the center of which is a bright spot. The light, the hole, and the bright spot are all in the same straight line. These simple observations lead us to think that light travels in a straight line.

[Ill.u.s.tration: FIG. 59.--The candle cannot be seen unless the three pinholes are in a strait line.]

We can always tell the direction from which light comes, either by the shadow cast or by the bright spot formed when an opening occurs in the opaque object casting the shadow. If the shadow of a tree falls towards the west, we know the sun must be in the cast; if a bright spot is on the floor, we can easily locate the light whose rays stream through an opening and form the bright spot. We know that light travels in a straight line, and following the path of the beam which comes to our eyes, we are sure to locate the light.

103. Good and Bad Mirrors. As we walk along the street, we frequently see ourselves reflected in the shop windows, in polished metal signboards, in the metal tr.i.m.m.i.n.gs of wagons and automobiles; but in mirrors we get the best image of ourselves. We resent the image given by a piece of tin, because the reflection is distorted and does not picture us as we really are; a rough surface does not give a fair representation; if we want a true image of ourselves, we must use a smooth surface like a mirror as a reflector. If the water in a pond is absolutely still, we get a clear, true image of the trees, but if there are ripples on the surface, the reflection is blurred and distorted. A metal roof reflects so much light that the eyes are dazzled by it, and a whitewashed fence injures the eyes because of the glare which comes from the reflected light. Neither of these could be called mirrors, however, because although they reflect light, they reflect it so irregularly that not even a suggestion of an image can be obtained.

Most of us are sufficiently familiar with mirrors to know that the image is a duplicate of ourselves with regard to size, shape, color, and expression, but that it appears to be back of the mirror, while we are actually in front of the mirror. The image appears not only behind the mirror, but it is also exactly as far back of the mirror as we are in front of it; if we approach the mirror, the image also draws nearer; if we withdraw, it likewise recedes.

104. The Path of Light. If a mirror or any other polished surface is held in the path of a sunbeam, some of the light is reflected, and by rotating the mirror the reflected sunbeam may be made to take any path. School children amuse themselves by reflecting sunbeams from a mirror into their companions' faces. If the companion moves his head in order to avoid the reflected beam, his tormentor moves or inclines the mirror and flashes the beam back to his victim's face.

If a mirror is held so that a ray of light strikes it in a perpendicular direction, the light is reflected backward along the path by which it came. If, however, the light makes an angle with the mirror, its direction is changed, and it leaves the mirror along a new path. By observation we learn that when a beam strikes the mirror and makes an angle of 30 with the perpendicular, the beam is reflected in such a way that its new path also makes an angle of 30 with the perpendicular. If the sunbeam strikes the mirror at an angle of 32 with the perpendicular, the path of the reflected ray also makes an angle of 32 with the perpendicular. The ray (_AC_, Fig. 60) which falls upon the mirror is called the incident ray, and the angle which the incident ray (_AC_) makes with the perpendicular (_BC_) to the mirror, at the point where the ray strikes the mirror, is called the angle of incidence. The angle formed by the reflected ray (_CD_) and this same perpendicular is called the angle of reflection. Observation and experiment have taught us that light is always reflected in such a way that the angle of reflection equals the angle of incidence. Light is not the only ill.u.s.tration we have of the law of reflection. Every child who bounces a ball makes use of this law, but he uses it unconsciously. If an elastic ball is thrown perpendicularly against the floor, it returns to the sender; if it is thrown against the floor at an angle (Fig. 61), it rebounds in the opposite direction, but always in such a way that the angle of reflection equals the angle of incidence.

[Ill.u.s.tration: FIG. 60.--The ray _AC_ is reflected as _CD_.]

[Ill.u.s.tration: FIG. 61.--A bouncing ball ill.u.s.trates the law of reflection.]

105. Why the Image seems to be behind the Mirror. If a candle is placed in front of a mirror, as in Figure 62, one of the rays of light which leaves the candle will fall upon the mirror as _AB_ and will be reflected as _BC_ (in such a way that the angle of reflection equals the angle of incidence). If an observer stands at _C_, he will think that the point _A_ of the candle is somewhere along the line _CB_ extended. Such a supposition would be justified from Section 102. But the candle sends out light in all directions; one ray therefore will strike the mirror as _AD_ and will be reflected as _DE_, and an observer at _E_ will think that the point _A_ of the candle is somewhere along the line _ED_. In order that both observers may be correct, that is, in order that the light may seem to be in both these directions, the image of the point _A_ must seem to be at the intersection of the two lines. In a similar manner it can be shown that every point of the image of the candle seems to be behind the mirror.

[Ill.u.s.tration: FIG. 62.--The image is a duplicate of the object, but appears to be behind the mirror.]

It can be shown by experiment that the distance of the image behind the mirror is equal to the distance of the object in front of the mirror.

106. Why Objects are Visible. If the beam of light falls upon a sheet of paper, or upon a photograph, instead of upon a smooth polished surface, no definite reflected ray will be seen, but a glare will be produced by the scattering of the beam of light. The surface of the paper or photograph is rough, and as a result, it scatters the beam in every direction. It is hard for us to realize that a smooth sheet of paper is by no means so smooth as it looks. It is rough compared with a polished mirror. The law of reflection always holds, however, no matter what the reflecting surface is,--the angle of reflection always equals the angle of incidence. In a smooth body the reflected beams are all parallel; in a rough body, the reflected beams are inclined to each other in all sorts of ways, and no two beams leave the paper in exactly the same direction.

[Ill.u.s.tration: FIG. 63.--The surface of the paper, although smooth in appearance, is in reality rough, and scatters the light in every direction.]

Hot coals, red-hot stoves, gas flames, and candles shine by their own light, and are self-luminous. Objects like chairs, tables, carpets, have no light within themselves and are visible only when they receive light from a luminous source and reflect that light. We know that these objects are not self-luminous, because they are not visible at night unless a lamp or gas is burning. When light from any luminous object falls upon books, desks, or dishes, it meets rough surfaces, and hence undergoes diffuse reflection, and is scattered irregularly in all directions. No matter where the eye is, some reflected rays enter it, and the various objects are clearly seen.

CHAPTER XI

REFRACTION

107. Bent Rays of Light. A straw in a gla.s.s of lemonade seems to be broken at the surface of the liquid, the handle of a teaspoon in a cup of water appears broken, and objects seen through a gla.s.s of water may seem distorted and changed in size. When light pa.s.ses from air into water, or from any transparent substance into another of different density, its direction is changed, and it emerges along an entirely new path (Fig. 64). We know that light rays pa.s.s through gla.s.s, because we can see through the window panes and through our spectacles; we know that light rays pa.s.s through water, because we can see through a gla.s.s of clear water; on the other hand, light rays cannot pa.s.s through wood, leather, metal, etc.

[Ill.u.s.tration: FIG. 64.--A straw or stick in water seems broken.]

Whenever light meets a transparent substance obliquely, some of it is reflected, undergoing a change in its direction; and some of it pa.s.ses onward through the medium, but the latter portion pa.s.ses onward along a new path. The ray _RO_ (Fig. 65) pa.s.ses obliquely through the air to the surface of the water, but, on entering the water, it is bent or refracted and takes the new path _OS_. The angle _AOR_ is called the angle of incidence. The angle _POS_ is called the angle of refraction.

[Ill.u.s.tration: FIG. 65.--When the ray _RO_ enters the water, its path changes to _OS_.]

The angle of refraction is the angle formed by the refracted ray and the perpendicular to the surface at the point where the light strikes it.

When light pa.s.ses from air into water or gla.s.s, the refracted ray is bent toward the perpendicular, so that the angle of refraction is smaller than the angle of incidence. When a ray of light pa.s.ses from water or gla.s.s into air, the refracted ray is bent away from the perpendicular so that the angle of refraction is greater than the angle of incidence.

The bending or deviation of light in its pa.s.sage from one substance to another is called refraction.

108. How Refraction Deceives us. Refraction is the source of many illusions; bent rays of light make objects appear where they really are not. A fish at _A_ (Fig. 66) seems to be at _B_. The end of the stick in Figure 64 seems to be nearer the surface of the water than it really is.

[Ill.u.s.tration: FIG. 66.--A fish at _A_ seems to be at _B_.]

The light from the sun, moon, and stars can reach us only by pa.s.sing through the atmosphere, but in Section 76, we learned that the atmosphere varies in density from level to level; hence all the light which travels through the atmosphere is constantly deviated from its original path, and before the light reaches the eye it has undergone many changes in direction. Now we learned in Section 102, that the direction of the rays of light as they enter the eye determines the direction in which an object is seen; hence the sun, moon, and stars seem to be along the lines which enter the eye, although in reality they are not.

109. Uses of Refraction. If it were not for refraction, or the deviation of light in its pa.s.sage from medium to medium, the wonders and beauties of the magic lantern and the camera would be unknown to us; sun, moon, and stars could not be made to yield up their distant secrets to us in photographs; the comfort and help of spectacles would be lacking, spectacles which have helped unfold to many the rare beauties of nature, such as a clear view of clouds and sunset, of humming bee and flying bird. Books with their wealth of entertainment and information would be sealed to a large part of mankind, if gla.s.ses did not a.s.sist weak eyes.

By refraction the magnifying gla.s.s reveals objects hidden because of their minuteness, and enlarges for our careful contemplation objects otherwise barely visible. The watchmaker, una.s.sisted by the magnifying gla.s.s, could not detect the tiny grains of dust or sand which clog the delicate wheels of our watches. The merchant, with his lens, examines the separate threads of woolen and silk fabrics to determine the strength and value of the material. The physician, with his invaluable microscope, counts the number of infinitesimal corpuscles in the blood and bases his prescription on that count; he examines the sputum of a patient to determine whether tuberculosis wastes the system. The bacteriologist with the same instrument scrutinizes the drinking water and learns whether the dangerous typhoid germs are present. The future of medicine will depend somewhat upon the additional secrets which man is able to force from nature through the use of powerful lenses, because as lenses have, in the past, been the means of revealing disease germs, so in the future more powerful lenses may serve to bring to light germs yet unknown. How refraction accomplishes these results will be explained in the following Sections.