than the flour and water it contains, which would weigh as much in any other form as they do in the form of bread. Thirdly, density depends on temperature, and, generally speaking, decreases as the temperature rises; a gallon of hot water is lighter than a gallon of cold water. Fourthly, the state of aggregation of the particles of a body affects its density; that is, when the body is a solid, it has not the same density as when it is a liquid, and it has still another density when it is a gas. On a wet day in winter, for instance, water often exists about us in all three forms, of ice, moisture, and invisible vapor; in all three forms it may have the same temperature, so far as the thermometer can show it, and it is the same thing chemically in all three forms. Moreover, we cannot see that the particles of a compact piece of ice, or of the vapor in the air, are not as close together as those of the liquid water. Still, the ice is not so dense as the water, and is denser than the vapor. It usually happens that a solid is denser than the liquid form of the same substance, but water is a well-known exception to this rule.

12. Facts like these have occasioned many notions to spring up about the way in which particles of matter are combined together in material objects, and the way in which matter is affected by heat. But a knowledge of the facts alone is sufficient to enable us to understand the principles of astronomy.

13. What has just been stated about material objects may be summed up as follows:

All matter has some chemical constitution, mass, and density, and may be divided into parts: it may exist in different states of aggregation, that is, as solid, liquid, and gaseous matter; it may have more or less heat, and may be a source of light, or may only reflect light received from other sources.

14. Matter has other important properties, such as those on which depend the facts called electrical and magnetic phenomena. These properties have undoubtedly much to do with many events which come under the notice of astronomers; but so little is yet known of the nature of

their connection with such events, that it will be enough for us merely to notice their existence.

15. It may be thought that if matter is whatever our senses show us, then light and heat, for example, must be matter. This was, in fact, an old opinion, now given up. But, strictly speaking, we are informed by our senses, not of light and heat by themselves, but of bright and hot bodies. In the same way, we do not see colors by themselves, but colored objects. However, color, like many other words, is used in a variety of ways, which may easily puzzle people who are not good reasoners. The full meaning of such words can only be learned by learning all the facts to which they relate; and the student who understands this will not be likely to become confused by the imperfections of language, the use of which is the only way men have of telling others what they have observed.

CHAPTER III.

THE MATERIAL UNIVERSE.

16. EVERY material object is a part of the material universe, which is often simply called the universe. The difference between the universe and matter in its widest sense is that, in speaking of the universe, we think of the separate shapes and properties of the objects of which it is composed, but take no notice of these shapes and properties when we speak of matter. The extent of the universe is entirely unknown; but, as far as we are concerned with it, it is limited by the capacity of our senses. That is, the universe which we are to attend to in studying astronomy comprises every material object of which men know any thing, and no others.

17. There are many questions about the universe with which people sometimes amuse and sometimes perplex themselves, but which are not properly part of any science at all. There are others which are logical or mathematical, but not strictly astronomical questions. Still others are really parts of astronomy, but require so much knowledge of mathematics and logic that very few astronomers are capable of studying them; and other people, of course, cannot form any rational opinion upon them at all, or learn what is thought about them by those who can study them. It is curious that these questions, which can be comprehended only by a few unusually learned men, and which even those who can comprehend them cannot answer, are often supposed to be so simple that anybody may set himself to making guesses about them, and writing down his guesses with the notion that he is making discoveries in astronomy. Inquiries into the past history of the universe are often made by mere guess-work of this kind. It is natural that people should ask how the universe came to be what it now is; and the question may some day be

partly answered.

But at present no distinct answer can be given; and even to understand the reasons which have led some distinguished astronomers to form opinions upon this subject, is out of the power of any one who has not thoroughly studied what is called theoretical astronomy. The less people can know, however, the more they seem disposed to conjecture; and one opinion or another about the origin of the universe, or some part of it, is maintained by many who have not the means of forming any sensible judgments at all upon such a question. In considering the history of astronomy, we shall have occasion to notice such opinions as deserve respect with regard to the condition of the universe before it assumed that structure which observation has shown it to have; but nothing is as yet generally admitted by competent judges to be known of this early condition.

18. There are interesting questions relating to the space which contains the universe; but they are too difficult for study, except by well-trained minds, and are not strictly astronomical, but rather geometrical questions. We occasionally see them discussed in a weak and unscientific way, which leads to no useful result. Of this kind are such questions as the possibility that the number of the stars is infinite in the strict sense of that word. In studying astronomy, as has been said, we must confine our attention to what can be actually observed.

19. One very common inquiry deserves some notice in a book of this kind, although it is one of that class of questions which have been described as not belonging to any science. Its object is to decide whether other parts of the universe than the Earth are inhabited; and whole books have been written upon this subject. It is a good subject to write about, because, as it is commonly handled, it has hardly any thing to do with facts, and a great deal with the sense in which people please to take certain words; and for the same reason it is not to be considered any part of science. Nobody can say exactly how much like a man a creature living elsewhere than on the Earth must be before it is fit to be called an inhabi

tant of whatever place it occupies. To make the question plainer, suppose we alter it, and ask only whether a man could continue alive if he were suddenly removed from the Earth and set down somewhere else. It is not at all likely that he could, because we know that men are easily killed by what seem comparatively slight changes in the condition of things about them. Exposure to a climate to which they are unaccustomed, for instance, is enough to kill many people; and a climate which belonged to a place entirely removed from the Earth would not be apt to be as much like terrestrial climates as they are like each other. However, most of what has been written on this subject, so far as it has any distinct object, seems to aim at deciding whether any thing is known about the parts of the universe beyond the Earth which will enable us to judge whether we could live in any of them. So far as we can tell, the prospect is rather discouraging to any person who should wish to emigrate from the Earth with his present body, and should find means of doing so. We can see that there are some differences between the Earth and other places, and we do not know how many other differences there may be which we cannot see. But whatever we find out, we can always imagine something not yet found out, which would make it either possible or impossible, whichever we wish to believe, for us to live in any place we have not yet visited. Astronomical discoveries, then, will do very little to put an end to such a dispute.

20. Let us try one other way of putting the question. Are there any rational animals living in the universe except those upon the Earth? If astronomers could see any such animals, and find out the nature of their minds by their actions, the question could be treated as a matter of fact. The Moon is the nearest place in which such animals can be looked for; and an animal would have to be two or three thousand feet round before we could see it on the Moon plainly enough to tell what it was with any instruments we have at present. We are not likely, then, to make discoveries of this kind for some time to come. Meanwhile, some people argue as if animals