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the Battle of Worcester.— Jonathan Swift : 1667-1745.
From History of the Rebellion...257 The Spider and the Bee. From
Letter to Lord Chesterfield.......
.276 The Siege.–From Ivanhoe........298
Oliver Goldsmith: 1728-1774.
Citizen of the World ..............289 The Most Honourable.-From
Physics or PHYSICAL SCIENCE, from the Greek word physis, nature, was the name originally applied to the whole of man's knowledge regarding Nature or the material universe. The universe is composed of an immense variety of materials or objects, many of which bear a general resemblance to each other. This resemblance has led to a classification of these materials into three Kingdoms—the ANIMAL, the VEGETABLE, and the MINERAL ; and the minute description of these classes constitutes the science of NATURAL HISTORY. Besides this reseinblance between particular objects, it has been found that certain motions or changes, called phenomena (Gr., 'appearances '], are continually going on among the objects themselves. And as, for convenience, the materials of the universe have been arranged under separate classes, there is likewise a classification of the phenomena to which they are subject. There is one class of phenomena which are always accompanied by a very decided change in the bodies themselves: the science which treats of these phenomena is called CHEMISTRY. A second class, caused by the action of Life, is included under the science of PHYSIOLOGY. A third class is formed of those phenomena which are caused by the action of Mind: these belong to the science of MENTAL PHILOSOPHY. The fourth and last class of phenomena includes those which are neither accompanied by any essential change in the object, nor caused by the action of life or of mind, but which are due simply to the properties of bodies as bodies, and are common to all objects, animate or inanimate. It is to the knowledge of this last class of phenomena that the name of PHYSICS, PHYSICAL SCIENCE, or NATURAL PHILOSOPHY is now properly applied.
Properties of Matter. Under the name of Matter is included everything that we become acquainted with by means of our senses. Farther on, Matter will be treated of under the forms, Solid, Liquid, and Gaseous; but there are certain properties, common to all kinds of matter, which must first be described.
1. Extension or Magnitude.—Extension or magnitude is the property of matter which implies that it is extended or occupies room or space. Bodies are extended in three directions, or have three dimensions or measures-length, breadth, and depth. Width is another term used for breadth ; and for depth we often use height, and sometimes thickness. By these three dimensions the shape of a body is determined. If we think of a stone, it may be round or square ; or if of a mountain, it may be high or low; but it will have some shape. When we speak of the size of anything, we can do so only by comparing it with something else, the size of which we do know; for example, a boy describes something to his friend as being as big as his fist, or as big as his head. For the sake of convenience, standards of measurement have been fixed upon to be used by all. For length, the inch is the standard ; and we say a thing is so many inches or so many feet long. For measuring a surface, which has both length and breadth, the standard is a small square an inch long and an inch broad, called a square inch ; and a surface is said to contain so many square inches, or so many square feet. Lastly, for solids, which have length, breadth, and depth, the standard is a small cube, each side of which is a square inch ; and the bulk or volume of any quantity of matter is said to be so many cubic inches or so many cubic feet.
2. Impenetrability.—The word impenetrability must have a peculiar meaning here, for there is no material so hard that it could not be penetrated or pierced, if proper instruments were used. Impenetrability, as applied to matter, means simply that two bodies cannot be in the same place at the same time. A nail can be driven into wood, but it is impossible that there can be wood in the very space occupied by the nail : the particles of the wood are merely forced more closely together, in order to make room for it. This property of matter is obvious in regard to solid bodies, but it is not so obvious with regard to fluids. Common air offers so little obstruction to our movements, that we are apt to forget that it is a real material body. That it is so, can be shewn by many simple illustrations. When a bladder is filled with air, it is impossible to press the sides together without bursting the bladder. If a tumbler be put, mouth downwards, into a vessel full of water, the water does not fill the tumbler completely, because it is prevented from doing so by the air in the tumbler. This property of air is taken advantage of in the diving-bell. That a liquid cannot occupy the same space with any other body, is clear from the fact, that if anything be put into a vessel full of water, the water will flow over, so as to make room for the body put in.
3. Divisibility.—The nature of matter is such that it can be divided to an extent far beyond the limits perceptible to the senses. A grain of gold, the bulk of which is one five-thousandth part of a cubic inch, can be beaten out so as to cover 57 square inches. The leaf thus formed is so thin that a pile an inch thick would contain 282,000 leaves. The microscope has revealed the existence of animals, a million of which would not occupy more space than a grain of sand. Yet these animalcules, as they are called, have members and organs, and display all the appearances of vitality. How shall we conceive the smallness of the tubes or vessels in which their fluids circulate, and the minuteness of the particles of matter composing these tubes and fluids ! It must not, however, be supposed that there is no limit to the divisibility of matter. On the contrary, there are many reasons for believing that there is a limit somewhere; and that there are ultimate particles of a determinate size and shape, incapable of further subdivision. These assumed particles are called atoms [Gr. atomos, from a, not, temno, to cut].
4. Cohesion.—Cohesion is the property by which the particles of matter stick together and form masses or bodies. Without this force to bind its particles together, matter would only exist in the shape of sand or powder. There is another kind of attraction, called the Attraction of Gravitation, by which one body acts upon every other body at any distance, however great ; but cohesion acts only when the particles are in contact, or when the distances between them are imperceptible. Thus, when a stone is broken, the fragments cannot be made to adhere, although placed together again ; in other words, the cohesion which existed between the particles before will not operate after they have been separated. Tho degree of cohesion in all bodies, however, is not the same ; and this gives rise to what are called states of aggregation of particles, of which there are three—the solid, the liquid, and the gaseous.
When the air has been entirely drawn out of a vessel by an air-pump, and a small quantity of gas introduced, the gas does not remain of the same bulk, but spreads itself throughout the whole vessel. This proves that there is in the gas itself a force repelling the particles with a power sufficient to overcome their own weight. This force was formerly called Repulsion, but it is now known that it is not an essential property, but the effect of heat, which is a form of motion among the particles. In addition to the cohesive force which binds the particles of matter together, there is thus another which repels them from each other. When the cohesion is greater than the repulsion, the body is firm and