People, animals, trees, bacteria, rocks, the oceans, the air, and the stars and planets are all different forms of what scientists call matter. If we put it more simply, matter will be that which occupies space. We have no difficulty in recognizing certain forms of matter. We can see and touch solids, like wood, iron, and marble, and liquids, like water, gasoline, and milk. We know that muscular effort is required to move these materials and that they can be weighed. It would be easier to identify gases as matter, if many gases were not invisible. Yet if you blow against the palm of your hand, you can feel the onrush of the gases contained in your lungs. You will find, too, that a container weighs more when it is full of air – a mixture of gases – than when the air has been removed by means of a vacuum pump.
It is clear enough, then, that matter occupies space and has weight. But what, exactly, is matter? We know a vast number of different materials – different kinds of matter. Concrete, iron, milk, illuminating gas, cosmetics, butter, blood, a cat's fur, a man's teeth – have these anything in common?
The problem seems to become more complicated than ever when we consider that many apparently pure substances are not so pure as we had imagined. From a distance a slab of concrete appears to be a uniform material of white or greyish colour. However, if we examine it closer, we will find that it is made up of several kinds of matter. There are particles of gravel or crushed stone imbedded in cement. A panful of seawater looks like a simple, uniform substance. Yet if the water evaporates, various salts will be left behind, including sodium chloride, or common table salt. Suppose we now pass an electric current through molten sodium chloride. We break it down into the metal called sodium and the gas called chlorine.
This breaking down of apparently simple substances into other substances and then breaking down these others into still others might seem to lead nowhere. But if we continue the process long enough, we will find that all matter consists of about one hundred pure substances which cannot be decomposed into anything simpler by ordinary methods of analysis. We call such substances elements. Elements are made up of identical atoms. Atoms are, in turn, made up of smaller particles. Atoms cannot be separated by ordinary chemical means, however.
Here are some familiar elements:
Under ordinary conditions many of these elements are solids – copper, gold, iron, and lead, for example. Others are liquids – bromine, mercury. Still others are gases – oxygen, nitrogen. The elements of which matter consists are distributed throughout the universe.
The oceans are composed of water (made up of the elements hydrogen and oxygen) and various salts. Air is almost entirely a mixture of oxygen and nitrogen, with a few other elements in small quantities. Astronomers have succeeded in identifying in the sun and the other stars a great number of elements that we know on the earth. They have not found any that are not now known to exist on the earth. A number of meteorites have crashed through our atmosphere and have landed on the earth. More than fifty of the elements known on earth have been identified in these visitors from outer space.