can be always falling without being stopped by something else, as a falling ball is stopped by the ground, will be explained hereafter. Another difference between the Earth and a falling ball is, that what the Earth moves in is not the air; for the air is really part of the Earth, and goes along with it. What the Earth moves in is not known. It is commonly called ether (the Greek word for air), because it is thought to resemble air in some respects; but the chief reason for believing that there is any such thing as this ether is that the light of the sun, moon, and stars takes time to come from them to us: and the easiest explanation of this is that the light comes to us through ether as sound comes through air, or as waves come along the water from the sea to the shore. Ether is a name which is also given to certain chemical liquids; but this use of the word has nothing to do with the other.

4. We know more of what the ether contains than of what it is. Many great globes, more or less like the Earth, are moving about in the ether. Some of them are known to be vastly larger than the Earth; others, again, are smaller. Besides these, the ether contains, as we have reason to think, a great deal of material resembling that of which the Earth is made, but scattered about instead of being collected together in masses. It is all in motion, however, so far as is known. All the knowledge we can get of these various contents of the ether belongs to the science of astronomy,—which also includes so much study of the Earth itself as is useful in making comparisons between it and other objects. For this reason, astronomy has much in common with geography, geology, chemistry, mechanics, optics, and other sciences. But the direct purpose of astronomy is the study of what lies outside of the Earth, and at a great distance from it. Every object of this kind is called a celestial object; what is part of the Earth, on the other hand, is called terrestrial.

CHAPTER II.

PROPERTIES OF MATTER.

5. EVERY thing which we perceive by means of our senses is called matter, when it is spoken of as one thing. But when we speak of the separate things which our senses make known to us, we call them material objects, or sometimes bodies, instead of matter. Material objects are either solids, liquids, or gases; and the same thing may be at different times solid, liquid, or gaseous, according to the circumstances in which it is placed. The most familiar example of this is furnished by water, which is often seen in the solid form of ice or snow, and is known to exist as a gas, although its shape cannot be seen when it is in that state; for when we see watery vapor, the water is in the form of little liquid drops, such as those which make up fog or mist. As for clouds, they may be made either of crystals of snow or ice, or of drops of water which are kept up for a long time by the air, just as dust is kept up by it. But besides the liquid water floating in the air, there is also a great deal of water forming part of the atmosphere itself, which is no more to be seen than are the other gases of the atmosphere, such as nitrogen. Steam, too, is a gas while it continues hot enough to remain invisible. We see, by the way in which water changes from one state to another, that what makes any material object solid, liquid, or gaseous, is partly its amount of heat, and partly other causes, which are to be learned by the study of natural philosophy and chemistry.

6. Solids, liquids, and gases may all be so hot as to shine, or give out light of their own. They are then said to be incandescent. An incandescent object may be on fire, as we say; but burning and incandescence do not always go together. A piece of lime, for instance, shines very brightly

when it is exposed to a great heat; but it does not burn: for what is meant by burning is permanent change as well as brightness and heat; and when the lime is cool again, it is still lime, as it was before. Melted metals also are often incandescent, and yet burn very little, if at all. In fact, few metals are commonly considered as combustible. We seldom see an incandescent gas, unless it is burning, because the particles of a gas are usually driven away by heat to some cooler place. Every flame is caused by burning gas; but most of the light of a flame is generally due to incandescent solid particles in it, not to the incandescence of the gas itself. An incandescent gas, whether burning or not burning, usually gives out much less light than an equally hot solid or liquid.

7. When any object shines by light of its own, and yet is not very hot, we call it phosphorescent instead of incandescent. Phosphorescence is not so usual a kind of shining as incandescence is; and when a celestial object shines by light of its own, we naturally suppose it to be incandescent, unless we can discover some proof that it is only phosphorescent. Incandescent and phosphorescent objects are sometimes called sources of light, or self-luminous bodies.

8. Many objects shine very brightly, and yet are neither incandescent nor phosphorescent. If a piece of polished glass or metal, for example, is held in the sunlight, it looks very bright when we see it from that particular place towards which most of the light that falls on it is reflected. All objects, so far as is known, reflect some light, although some reflect very little; and it is by means of this reflected light that we see most of the terrestrial and many of the celestial objects which are visible to us. Even gases reflect a little light. Some gases, like chlorine, have a distinct color, which enables us to see them; and the mixture of gases which makes up the Earth's atmosphere, or the air, as we commonly call it, has a blue tint, which can be seen when we look through many miles of it at a time. But if the floating solid and liquid matter contained in the air were all removed

from it, it would probably reflect very little light, and would scarcely look blue. This blueness of our atmosphere occasions the appearance which we call the sky, as will be shown hereafter.

9. Any material object may be hot or cold, luminous or dark; and probably, too, may exist under certain circumstances either as a solid, a liquid, or a gas. But there are other differences between material objects, which at all times distinguish them from each other. A piece of iron, for instance, differs in some respects from a piece of lead, whether the metals are solid or liquid, incandescent or dark and cold. Differences of this sort are called chemical differences. These chemical differences are accompanied by differences in the kind of light which incandescent or phosphorescent objects emit, or which other bodies reflect, so that if a celestial object cannot be otherwise examined, we can still find out something about its chemical properties by means of the kind of light which it sends us.

10. If we take bodies which have the same chemical properties, and are in the same mechanical condition, that is, bodies which are equally warm, and as much alike as may be in every way which our senses can recognize, except that they may be of different sizes, we then find that the largest is also the heaviest. By this we mean that it requires more exertion to lift a large piece of lead, for instance, than a small one; and that when it is lifted it can be made by proper means to do more work than the small piece while it is coming back to the level from which it was lifted. For example, the wheels of a large clock require a large weight to make them turn. This gives us a notion that the weight of a body depends on the quantity of matter which there is in it; so that it is supposed that a piece of lead has more matter in it, or has a greater mass, as we say, than an equally large piece of wood. But we should have no reason for believing this if we did not know that when we melt some lead and make two bullets of different sizes from it, so that neither has any hollow place in it, or can be shown to be less compact than the other, the

larger is always the heavier; and that the same general principle applies to wood as well as to lead, and to everything else that we know of as well as to lead and wood. It is still easier to show that it applies to liquids and gases than that it applies to solids, because the particles of liquids and gases can move among each other so freely that there is apt to be less difference between different parts of a liquid or gas than between different parts of a solid. If we cut two pieces of wood. from the same log, one of them may have a closer grain than the other, so that we can see a reason why it should weigh more, although it is no larger, than the other piece. But one pint of water is almost exactly like another taken from the same bucket. We should expect, therefore, to find, as we actually do, that if equal measures of a certain liquid or gas are weighed under exactly similar circumstances, one is as heavy as the other. Hence, when we find that a bottle full of quicksilver is very much heavier or harder to move than a similar bottle of water, we naturally suppose that the reason must be that quicksilver is denser than water; that is, that the particles of the quicksilver are closer together than those of the water, so that more of them are contained in the same space. However, this supposition is not necessary to enable us to study the simpler portions of natural philosophy and astronomy. When two material objects are said to be equal in mass, what is meant is that it will take as much force to give one of them a particular movement as is needed to move the other just in the same manner and just as far. If we say that one object has more density than another, we mean that any portion of the denser object will have more mass than a portion equal to it in bulk of the other object, or that equal masses of the two objects will differ in bulk.

11. Every material object has some mass and density; but how dense it is, compared with others of the same bulk, depends on several circumstances. Such circumstances are, first, chemical constitution; a ball of lead has more mass than an equally large iron ball. Secondly, the density of a body depends on its compactness; a loaf of bread is larger