Where Do Emeralds Come From?

In ancient times, the various gems were distinguished only by their colours. The name ruby was given to all precious stones of a red hue. All green stones were called emeralds. All those of blue were called sapphires.

Later on it was seen that some of the gems were harder than others and endured longer. So it came about that the value of a gem depended not only on its colour, brilliancy, and rarity, but also on its hardness.

All the gems are called precious stones. But strictly speaking, "precious" is used only for the four most valuable stones—the dia­mond, the ruby, the emerald, and the sapphire. The other valuable stones are called semiprecious.

The emerald is one of the softest of the precious gems. It is a variety of beryl. In its perfect state an emerald is a rich, clear green. But flawless stones are seldom found and for that reason are very expensive.

In ancient times, all the emeralds came from the mines of Egypt. These mines are still worked today but yield only a small quantity of the precious stones. The finest emeralds are at present obtained from the mines near Bogota, Colombia. There are also emerald de­posits in the Ural Mountains of Russia and in the Salzburg Alps. A few fine emeralds have been found in North Carolina. And more re­cent discoveries of emeralds have been made in the Transvaal in South Africa.

There are many legends about emeralds. One of them is that the Holy Grail—the cup from which Christ drank at the Last Supper—was carved from a huge emerald. The emerald was believed to have medical power, too. It was supposed to have the power of curing the “falling sickness" (epilepsy). Another old legend about the emerald was that it endowed its owner with the gift of foretelling the future. It was said that by looking into its green depths one could see the things that were to come to pass!

The largest known uncut emerald is five centimetres long and five centimetres in diameter.

Diamonds are the result of a process that took place in nature. Millions of years ago, the earth was gradually becoming cooler. At that time, there existed beneath the ground a mass of hot liquid rock. This mass was subjected to extreme heat and pressure. As a result of this, molecules of carbon became packed together in dense, clear crystals. A diamond is simply a crystal of pure carbon.

When a diamond is found in "rough" form, its outside appearance is rather dull. Now man takes over to make it into the sparkling gem we all know about. Most diamonds are sawed in two, and each half is shaped and cut into a round diamond called a brilliant.

Then little faces, or facets, are cut into the diamond. The aver­age brilliant is cut into fifty-eight facets, or even more. These facets make a diamond sparkle.

The reason for this is that a diamond has a very high refractive power. This means that when light enters it, the diamond bends the light more than other substances do. The light, instead of passing through the diamond, is bent so that it is reflected back into the stone. So a greater amount of light is returned to our eyes when we look at a diamond and it looks more brilliant. The diamond also breaks up the light into its different colours, which gives a diamond its "fire".

Did you know that diamonds were not worn as personal orna­ments until 1430, when a Frenchwoman called Agnes Sorel started the custom? From then on the custom spread.

Why Does a Diamond Sparkle?

Suppose diamonds were not as rare as they are. Suppose they didn't cost much and almost everyone could have them. Would dia­monds still be valuable?

Two things would probably make people still want to have dia­monds. One is that a diamond is the hardest substance known to man, so diamonds would still be very useful in industry. The second is that diamonds would still be beautiful, and so people would still enjoy looking at them.

Diamonds are the result of a process that took place in nature. Millions of years ago, the earth was gradually becoming cooler. At that time, there existed beneath the ground a mass of hot liquid rock. This mass was subjected to extreme heat and pressure. As a result of this, molecules of carbon became packed together in dense, clear crystals. A diamond is simply a crystal of pure carbon.

When a diamond is found in "rough" form, its outside appearance is rather dull. Now man takes over to make it into the sparkling gem we all know about. Most diamonds are sawed in two, and each half is shaped and cut into a round diamond called a brilliant.

Then little faces, or facets, are cut into the diamond. The aver­age brilliant is cut into fifty-eight facets, or even more. These facets make a diamond sparkle.

The reason for this is that a diamond has a very high refractive power. This means that when light enters it, the diamond bends the light more than other substances do. The light, instead of passing through the diamond, is bent so that it is reflected back into the stone. So a greater amount of light is returned to our eyes when we look at a diamond and it looks more brilliant. The diamond also breaks up the light into its different colours, which gives a diamond its "fire".

Did you know that diamonds were not worn as personal orna­ments until 1430, when a Frenchwoman called Agnes Sorel started the custom? From then on the custom spread.

What Is Neon?

When we walk down the main street of a city at night, we see all sorts of coloured lights on stores and advertising signs. We think of them as neon lights.

But the fact is that not all of them are lights made by glowing neon gas. Other gases, such as helium, argon, krypton, and xenon, are also used in lights. Each gas gives out a different-coloured light when electricity is sent through it,

The colour of the light given out will vary, depending on such things as the temperature, pressure, and electric voltage. Neon gives out a red-orange light; argon gives out a reddish-blue light; the light from helium is white, yellow, or sometimes violet; from krypton it is yellow, green, or pale violet; and from xenon it is either blue or blue-green.

By passing electricity through neon gas, the atoms are made to give off light. What happens is that the energy of the electric cur­rent knocks electrons off some of the atoms of neon. When these electrons rejoin the neon atoms, energy in the form of light is given off.

All the gases we have mentioned here form a family of elements called the noble gases. Sometimes they are called rare gases because they are fairly scarce. All of these gases are relatively inactive chem­ically. This means they do not burn, and they form no chemical com­pounds under normal conditions.

The chief source of these gases is ordinary air (except for helium, which is obtained from natural gas). The gases are mixed together in the air with oxygen, nitrogen, carbon dioxide, and other substances.

To get the noble gases, the air is separated into its elements, and the gases are removed one at a time. This is done by chilling air to a very low temperature so that it turns into a liquid. The liquid air is piped into tall towers and heated.

As each gas reaches its boiling point, it boils off from the liquid air as a gas.

What Is a Fuel ?

A fuel is a material that is burned in order to get heat and light, and also to generate power. The process of burning, or combustion, is a chemical reaction. A material combines with oxygen from the air and gives off energy. The energy is given off in the form of heat and light.

The energy in fuels came originally from the sun. The plants from which fuels come trap energy from the sun's rays and use it to build their tissues. Burning wood and charcoal releases energy that has been stored up by plants in this way. When we burn coal or oil, we use energy stored up by plants that lived millions of years ago.

There are many different types of fuels, and actually anything that burns can be called a fuel. But the most common fuels are wood, coal, natural gas, and petrol.

 Fuels can also be classified as solid, liquid, or gaseous. Or they can be classified according to their origin—natural, chemical, or metal based.

Wood was one of the first fuels used by man, and was his most important one for many centuries. It was the easiest to get, and the cheapest. But during the sixteenth century, wood started to become scarce in Europe, and coal began to replace it.

Coal contains a high percentage of carbon. Carbon is the most important part of most fuels. Fuels with a high percentage of carbon burn evenly and with a hot flame. Hard coal, or anthracite, has a higher percentage of carbon than other types of coal, and so makes less smoke and ash.

 The most important liquid fuels come from petroleum. They include kerosene, petrol, and heating oils.

What Is Lignite?

One of the most important searches being carried on by scientists is for new sources of energy. It's not just that they want to find cheaper and more efficient sources of energy to do the work of the world, it's also because some of the natural sources we now have will eventually be used up!

Have you ever seen an abandoned coal mine? At one time coal was being taken from there to feed huge furnaces and to heat build­ings. But now there isn't enough coal left to make it worth mining.

Coal is still, however, the most important solid fuel man has. In fact, it furnishes the United States with about half of its fuel.

Let's consider the different kinds of coal. Obviously, a process that took millions upon millions of years didn't create exactly the same kind of product everywhere.

Peat, for example, is the youngest of all coals. This means the vegetation from which it came was buried a shorter time than that of other coals. Peat has the least heat value of all solid fuels.

Lignite is sometimes called brown coal. It is a little older than peat and has more heating value.

Bituminous coal has the highest heating value of all solid fuels. Finally, there is anthracite coal, which is the hardest of all coals and the oldest in nature.

What Is Peat?

Peat is not coal. It might be called a step in the process of mak­ing coal.

Coal itself is made of the remains of ancient trees and plants that grew in swampy jungles in warm, moist climates hundreds of mil­lions of years ago. These trees and plants fell into the swamp waters. Bacteria changed some parts of the wood into gases that escaped, leav­ing behind a black mixture, mostly carbon. In time the pressure from mud and sand above squeezed out most of the liquid, leaving behind a pasty mass that slowly hardened into coal.

This process, from beginning to end, took thousands of years. But the first stages of that process of making coal can actually be seen going on today. In the Great Dismal Swamp of Virginia and North Carolina and in thousands of swamps of the northern states of the U.S.A. and Canada, peat is being made.

In these swamps, plants are gradually decaying in a process that leaves most of the carbon in place. A few years of such action pro­duces a brown, matted mass of twigs, branches, and leaves. This is known as peat. When the water is drained from such a swamp, the peat can be cut into blocks, set out to dry, and then burned as fuel.

Drying is important because peat in the ground may be three- fourths water. In Ireland, where peat is plentiful and the higher forms of coal are expensive, more than half of the farms depend entirely on peat for fuel.

The other forms of coal are developments from peat. If peat is allowed to remain where it forms, it gradually changes into lignite, or brown coal. It is more solid than coal, but still soft enough to crumble when shipped long distances.

The next form of coal is bituminous, or soft coal. It is formed from lignite by chemical change and pressure in the earth over thou­sands of years. This is the most important member of the coal family. It burns easily and is abundant.

If bituminous coal remains in the earth and is subjected to enough pressure, it gradually changes to hard coal or anthracite. It burns with very little smoke and for a longer time than bituminous coal.

What Is Gum?

Gum has a whole variety of interesting uses—many of them more important than providing you with something to chew on! And, of course, there are a great many different kinds of gums.

One gum, called gum arabic, is used in making candy, medicines, and mucilage, and in the manufacture of silks. Like most of the true gums, it comes from the plant as a thick, sticky liquid, which hardens when exposed to air and dissolves in water.

Gum arabic is produced by several varieties of acacia trees, which grow in Africa, Australia, and Asia. It is sold in the form of clear yellow or reddish lumps. Some gums, instead of dissolving in water, absorb it and make a soft, gluey, gelatine-like mass. One such gum is called gum dragon, and is used in making cough medicines and as a sizing in cloth.

Another important gum is cherry gum, which is used in stiffening straw for the manufacture of straw hats and other articles. Other common gums are plum, peach, spruce, and chicle.

Chicle is produced by the naseberry and other trees and is used in making ordinary chewing gum. All these gums are used in the man­ufacture of ink, cloth, paper, and medicines. Some of the finest gums are used in medical and research laboratories for work with cultures.

Most gums are gathered in the dry seasons and brought to the markets in the form of nodules or "tears". Chemically, gums consist of an acid nucleus combined with sugar molecules.

In addition to the true gums we have described above, there are gum resins. They are also produced by plants, but they differ from true gums because they contain resin, and therefore will not dissolve completely in water.

Two famous gum resins are frankincense and myrrh. They are very fragrant and are used in making perfume and incense.

What Is the Tallest Tree?

Trees are green plants. They have roots, stems, leaves, and seeds, just as other green plants do. Trees are also the oldest of all green plants. Some of the sequoia trees of the north-western United States are more than four thousand years old. This means they wete fully grown trees long before Columbus discovered America!

And trees are the biggest of all green plants. The tallest trees known to exist on earth are the giant redwoods of California. There is one tree there, growing in the Humboldt National Forest, that is believed to be the tallest tree in the world. It is called the Founder's Tree and is over 110 metres high!

Some authorities believe that long ago the eucalyptus tree of Australia may have been as tall as these California redwoods, but those growing today average fifteen metres shorter. Two other kinds of trees that come close to the redwood are the Douglas fir and the sequoia, some of which have grown over 91 metres tall.

The roots of a tree not only take water and mineral materials from the soil, but also hold the plant firmly in the ground. And with very large trees, it takes quite a bit of root system to hold the tree firmly. Did you know that the roots of trees take up nearly as much room under the ground as their tops or crowns do above?

Scientists have learned to tell the age of trees by counting rings in the wood. There are rings in the cross-section of most kinds of tree trunks. New wood is formed each year in a layer outside the old wood and beneath the bark. It is this layer that becomes the ring. Each ring in the wood of the trunk of a tree represents one year of the tree's life. The trunk gets bigger and bigger around as new rings or layers are added.

A tree adds rings not only to its trunk but to its branches and twigs, too. Some new length is added to the tip ends of the twigs and branches each year. Because growth in height takes place only at the tips of branches, the limbs of trees never move farther from the ground than they were in the beginning!

Do Cacti Have Leaves?

A cactus (plural: cacti) is able to exist under extreme conditions because it is a plant that has adapted itself to those conditions.

Cacti have the same basic structures and processes as other plants. But the work that is done by leaves in most other plants is done by the stems and branches of the cacti. In fact, the absence of leaves and the presence of spine-covered branches and stems enable them to survive in hot, dry regions.

The leaves of other plants are thin structures and are filled with pores through which the plant breathes. During the cell-making process carried on by the plants, water is given off to the air through these pores.

A cactus plant must guard every drop of water. So the work of the leaves is taken over by the stems and branches. Their thick skins have very few pores, and the water in the cactus is retained.

The roots of cacti are spread out, close to the surface of the ground. That's why cacti can quickly absorb water from the earth after a rainfall. This water, which is taken in through the roots, is stored in the spongy or hollow stems of a cactus. The outer layer of the plant is thick and waxy, and this also prevents the escape of water.

The outer skin of a cactus is ribbed. Some cacti have ribs that fold and expand like an accordion. They expand as they fill up with water and fold together as the water in the stem is used up.

There are some members of the cactus familyihat do have leaves, such as the lemon vine of the West Indies. But in most cacti the leaves have developed into spines, needles, or hairs. These help protect the cacti from animals that would otherwise eat them, since they may be the only green plants in the area.

How Do New Flowers Grow ?

Every living thing has some means of reproducing itself. In flow-ers, the process takes place as follows:

A typical flower has four main parts. There is usually a green outer cup made up of leaflike sepals. Within the sepals are the petals. Within the petals are the reproductive organs necessary for producing seeds.

 In the very centre of the flower are one or more pistils. Around the pistils is a ring of stamens. The pistil is the female part of the flower. The bottom of it is enlarged and is called the ovary. Inside the ovary are little round ovules, which later become seeds. But they become seeds only if they are fertilized by the contents of a pollen grain.

Pollen grains are produced by the stamens, the male organs of the flower. If seeds are to form, the pollen grains must go through the top of the pistil and reach the ovules at the bottom. The top of the pistil is called the stigma.

Pollen grains first fall on the stigma. They absorb moisture there from the sugary liquid on the surface. Then they swell and grow. The grain pushes down and becomes a tube. The tube keeps growing down through the stalk of the pistil, then through the wall of the ovary, and then into an ovule.

The contents of the tube then empty into the ovule and fertilize it. Many pollen tubes may grow down to an ovary at the same time. Each tube will enter and fertilize a single ovule.

Only pollen from the same kind of plant will grow tubes and reach the ovules. The part of the stamen that produces pollen is called the anther. The transfer of pollen from anther to stigma is called pollination. If it takes place in the same flower, it is called self-polli­nation. If the pollen goes to a flower on a different plant of the same kind, it is called cross-pollination.

Cross-pollination of flowers is done by the wind, by insects, by birds, and by certain animals. Later on, the seeds that develop also have to be carried to a place where they can take root and grow into flowers.

What Is a Spore?

A flowering plant makes a new plant by means of a seed. Plants that don't have flowers make a new plant by means of a spore.

A spore is a one-celled organism. It is invisible to the eye and can only be seen under a microscope. There are spores in the air all around us. That's why when food is left exposed, and moulds and mildews form on it, we know where they came from. Some types of spores that were in the air settled on the food and began to grow. 

Some of the plants that reproduce by means of spores are mush-rooms, ferns, and mosses. The algae that live in water also produce spores.

 A plant carries its spores in cases that are called sporangia. in a mushroom, the sporangium is inside the gills beneath the mushroom cap. In mosses, the spores are carried in a capsule at the top of the stalk.

When the spore case is ripe, it opens, and the ripe spores are released. Since they are finer than dust, the wind scatters them far and wide. In the case of water plants, such as the algae, the spores can actually swim away. They have tiny tails called cilia. These spores are called zoospores, and when the ripe case opens, the zoospores swim away quickly. After a short time they come to rest and lose their tails. Then they begin to grow into new plants.

Some spores reproduce by cell division. They grow by pushing out a germ tube through a thin place in the cell wall. The germ tube branches into a mass of threads out of which the new plant grows. This is called asexual reproduction, because differentiated male and female cells are not needed for reproduction to take place.

Other spores are specialized male and female cells. In order to start a new plant, one male and one female cell must join to form a fertilized egg. Some plants alternate in the kind of spores they produce, asexual in one generation and sexual spores in the next. 

Why Do Plants Have Roots?

A plant needs roots for two chief reasons: as a means of anchor­age or support, and to absorb water and mineral salts from the soil.

The roots of most plants grow in the soil. They don't "just sit there", but seem to reach out in the soil to help the plant grow. By elongating near their tips, roots are always coming in contact with new portions of soil.

Thousands of tiny root hairs project from the surface of the young root and absorb materials from the soil. That's why when a young root is pulled from the soil, soil particles often cling to the root hairs.

Some plants have taproots. A taproot is a large, single root, much larger than any of the branch roots. Other plants have not one large root, but several roots of approximately equal size. These form what is called a fibrous root system.

Grasses have fibrous root systems. Soil in which there are many fibrous roots is protected in this way from erosion. In other plants, most of the roots grow from stems as, for example, the geranium.

As roots grow older, some of them store large quantities of sugar and starch. Beet and sweet potatoes are examples of this. A sweet potato is a root, but an Irish potato, with its eycs, is a stem.

Not all plants have roots that grow in the soil. Some tropical orchids that grow on trees have spongy roots that grow in the air and absorb moisture. Both the English ivy and poison ivy cling to walls or trees by means of tiny aerial roots.

Some plants have special roots that develop from the stem above the ground and grow down into the soil, forming props. A few roots, such as the sweet potato, form buds that grow into leafy branches and can be used to the plant .

What is Paprika?

True pepper is made from the pepper plant, which has the scientific name of Piper nigrum. But a great many other kinds of pepper are obtained from plants of entirely different families.

For example, there are the red peppers, or chillies. They belong to the genus capsicum. There are also cayenne peppers and tabasco peppers. Still ,another kind, bell peppers, are called pimientos when canned in oil. Pimientos is the Spanish name for the pepper plant. And finally there is paprika, which is a red pepper produced from the bell pepper. When these are ripe, they are red and hot, but are milder than many other kinds. That's why paprika can be used more freely than other kinds of pepper.

Pepper is considered to be the most important of all the spices in the world. After salt, it is the seasoning most used for food. In ancient times, and during the Middle Ages, only the rich could afford to use pepper. It had to be carried by caravan from the Far East, and this made it so expensive that a kilo of pepper was considered a fitting present for a king!

In some ways pepper was like gold. People could pay taxes with pepper, and it was given as tribute to rulers by their subjects. When an army conquered an enemy and soldiers were given a share of the spoils, pepper would be one of the great rewards they would receive.

The Portuguese were so anxious to find a way of getting pepper at lower cost that they tried to find a sea route to India. After they found the way around the Cape of Good Hope, the cost of pepper in Europe dropped a great deal. Today, of course, pepper costs so little that we don't even think twice when we buy it.

Pepper comes from the fruit or seeds of a climbing shrub. Black pepper is made by picking unripe berries and drying them until black. White pepper is made by removing the outer coat before grinding.

What Are the Sizes of the Planets?

A planet is very different from a star. A star is a huge ball of hot gases that gives off heat and light. A planet is a much smaller body that shines by reflected light.

Let's start with the planet nearest to the sun and move outward. The first one is Mercury. Mercury's diameter is 2,900 miles—about the width of the Atlantic Ocean. So it's only a fraction of the earth's size.

The next planet we meet is Venus. It is very nearly the same size as the earth. Its diameter is 7,600 miles, while that of the earth is 7,913 miles. By the way, an odd fact about Venus is that it rotates backward; that is, from east to west. The next planet is our earth, and then comes Mars.

Mars shines in the sky with a reddish colour. It has a diameter of 4,200 miles, a little more than half that of the earth. Parts of its surface look like the moon, with large craters apparently formed by meteorites, but there are also giant volcanoes and valleys which prove there could be geological activity on the planet.

Jupiter, the next planet, is far away from the sun. It takes about 11.9 years to complete one orbit. Jupiter is the largest of the planets. It has a diameter of 88,700 miles, nearly 11 times the diameter of the earth.

Saturn, the next planet, is another giant. It has a diameter of 75,100 miles, which is about nine times that of the earth. An unusual thing about Saturn is the group of flat rings that circle it. These rings are made up of billions of tiny particles.

Uranus, the next planet, is much larger than the earth. It has a diameter of 32,000 miles. Uranus is tilted over on its side. Its axis is tilted over at an angle of 98 degrees. (The earth tilts at an angle of 23'/2 degrees.) Neptune, the next planet, is 30,000 miles in diameter. And finally, the last known planet, Pluto, is believed to have a diameter of about 1,865 miles. It is so far from the sun that the sun appears to it as only a bright star in the sky.

Why Do All the Planets Look Different?

The reason each of the planets looks different to us is that each one seems to be made up of different substances. Even though they are all planets revolving around the sun and part of the solar system, their composition varies.

We actually know very little about what the planets are made of, and this is one of the questions man hopes to answer with the space explorations that have taken place and those that are being planned for the future.

Let's take a brief look at each of the other planets and see what is known of their make-up. Mercury is a small, rocky world. It has some dark areas and many craters on its surface, but has no atmosphere or water of any kind.

Venus is a white globe with some hazy markings. It is completely covered by a layer of white clouds, which are not made of water vapour but of concentrated sulphuric acid. Underneath its clouds, Venus's atmosphere is made largely of unbreathable carbon-dioxide gas. This traps heat from the sun like a blanket, so temperatures at the planet's surface reach nearly 500°C, and there is scarcely any water on Venus.

Mars is known as the red planet because of the colour of its deserts. It is half the size of earth and has a thin atmosphere of carbon-dioxide in which clouds form. No definite signs of life have been found on Mars, perhaps because it is very cold there.

Jupiter appears as a yellowish globe with colourful bands of clouds swirling around it and a giant red spot clearly visible in the clouds. It is agiant ball of liquid, mostly hydrogen and helium, becoming denser and denser towards the planet's centre.

Saturn is mostly liquid hydrogen with a set of brilliant rings around it. These are made of countless particles all moving in orbits around the planet like miniature moons.Uranus has rings around it too, though these are much darker than Saturn's. Neptune is a dim greenish object, and Pluto is the smallest planet in the solar system, smaller even than our own moon. It has an eccentric orbit that sometimes brings it closer to the sun than Neptune.

Exploration of space by satellites and probes is helping scientists learn more about the other members of the solar system.

How Many Galaxies Are There?

Scattered throughout the universe are vast islands of stars, called galaxies. Our sun is a star in the Milky Way galaxy. This is our galaxy, too, and it has billions of stars in it. It takes light about one hundred thousand years, to go from one end of our galaxy to the other (and light travels about six million million miles in one year !)

Astronomers have discovered through the use of telescopes that there are millions of galaxies beyond our own. There are basically three types of galaxies that we know about. Those that have a spiral shape (like our own Milky Way) are called spiral galaxies. The one nearest to us is about two million light-years away. It is the great spiral galaxy in the constellation Andromeda.

About 17 per cent of the brightest galaxies that have been ob­served are called elliptical galaxies. (An ellipse is like a stretched-out circle.) These galaxies contain mostly stars and seem to have little or no dust and gas.

Some galaxies are called irregular galaxies because they appear to have no special shape. These galaxies contain stars, dust, and gas. The two galaxies nearest the Milky Way are irregular galaxies.

There are also some small galaxies that are called dwarf galaxies. The smallest are only a few hundred light-years across and contain only a few thousand stars. There may be more dwarf galaxies in the universe than larger ones.

Galaxies are separated from one another by hundreds of thousands of light-years. They usually occur in groups or clusters containing from several tens to many thousands of galaxies.

The most distant clusters of galaxies so far observed lie billions of light-years from our Milky Way. And there are galaxies so far away. from us that it is almost impossible for us to imagine the vastness of the distance. So the answer to the question of how many galaxies there are in the universe will probably always remain a mystery.

What Are UFO's?

The popular name for them is "flying saucers". UFO stands for "unidentified flying object".

Do they really exist? Many books have been written about them and thousands of people claim they have seen them; some even claim they have photographed them. And no matter what scientific investiga­tions reveal, there will still be people who believe they exist.

Studies of saucer reports show that UFO's are very different from one another. Some people report having seen flat saucers; others see saucers shaped like spheres, cigars, or doughnuts.

The colours of saucers seem to be as different as their sizes. Saucers of nearly all colours have been reported. Some seem to change colour as they are being watched.

Saucers have been seen to move in every direction and at nearly every speed. They can turn at right angles, move straight up or straight down, or travel in a zigzag path. They can hang motionless in the air, and make either a hissing noise or a roar.

When the United States Air Force started to investigate the re­ports about flying saucers, it discovered that people weren't "imagin­ing" what they saw. Everyone who reported a flying saucer had seen something. But what?

In some cases, the "something" was actually a weather balloon. In other cases, it was a satellite, a cloud, a meteor, a star, a bird, a comet, a planet, or fireworks. It was also what are called sun dogs. These are images of the sun reflected through ice crystals. Many flying- saucer stories have been traced to fireballs, which are formed by lightning.

If saucers were really spaceships, there would be a certain pat­tern in the reports about them. But there is no such pattern. The rea­son is that people are not seeing spaceships but many other things. So scientists believe that there is no evidence that we are being visited, watched, or invaded by intelligent beings from other worlds.

About 17 per cent of the brightest galaxies that have been ob­served are called elliptical galaxies. (An ellipse is like a stretched-out circle.) These galaxies contain mostly stars and seem to have little or no dust and gas.

Some galaxies are called irregular galaxies because they appear to have no special shape. These galaxies contain stars, dust, and gas. The two galaxies nearest the Milky Way are irregular galaxies.

There are also some small galaxies that are called dwarf galaxies. The smallest are only a few hundred light-years across and contain only a few thousand stars. There may be more dwarf galaxies in the universe than larger ones.

Galaxies are separated from one another by hundreds of thousands of light-years. They usually occur in groups or clusters containing from several tens to many thousands of galaxies.

The most distant clusters of galaxies so far observed lie billions of light-years from our Milky Way. And there are galaxies so far away. from us that it is almost impossible for us to imagine the vastness of the distance. So the answer to the question of how many galaxies there are in the universe will probably always remain a mystery.

What Is Radiation?

Looking at it very simply, radiation is the sending out of waves of energy. You have known about it since you were a baby—though you didn't know what it was. When you held your hand in front of a hot stove or radiator, or a light bulb, you felt radiant heat. When you sat in the warm sun, a type of radiation called ultraviolet rays was striking your skin.

All these are examples of electromagnetic radiation. The other major type of radiation is called radioactive radiation, and it comes from either radioactive material or nuclear reactions. In radioactive radiation, particles, as well as waves of energy, are given off.

Since electromagnetic radiation is the sending out of waves of energy, we should know something about those waves. The distance between the waves is called the wavelength. The number of waves passing a given point each second is the frequency. And when all the waves within a certain range of wavelengths are grouped together, we call them the spectrum.

The group with the shortest wavelength is the X-ray spectrum. Next comes the ultraviolet spectrum. Then comes the visible-light spec­trum; we can. see these waves. The waves get still longer, and we can no longer see them. This is the infrared spectrum. Even longer waves (Hertzian waves) are used for radio, television, and radar.

What produces all these waves? In some cases machines are re­quired; in others they are produced naturally. Naturally made waves come from the sun. To produce any radiation requires energy. In the case of the sun, atomic energy is produced by a reaction called fusion. In the case of X-rays, a target must be bombarded with particles.

Radioactive radiation is the process of change or decay that cer­tain elements undergo. Such elements are radioactive. They radiate particles (and waves, too) as the nuclei of their atoms break up.