LESSONS

IN

ELEMENTARY PHYSICS.

INTRODUCTION.

1. Definition of Physics.-As we look around on the universe in which we dwell, we are struck with a variety of objects outside of ourselves and independent of us. Some of these we see, some we hear, others we touch, or taste, or smell, while many appeal to various senses at once.

When quite young we begin to reason upon these impressions, and the constant recurrence of phenomena in a certain order gives us a well-grounded expectation that in future the same order will be observed. As night approaches, the sun appears to sink below the horizon, and so to vanish from our sight; and yet, from past experience, we have the most perfect confidence that he will reappear on the morrow. But while all classes of men in every age acquire from the necessities of life a certain knowledge of the laws which regulate the phenomena around them, this knowledge is nevertheless most superficial and imperfect. A child knows that a stone will fall to the ground, but it required a Newton to discover the law of gravitation.

It is only within the last three centuries that men have

seriously set themselves to the task of acquiring a knowledge of the laws of Nature, and even now we know but a small part of these laws. Nevertheless, a great deal has been gained to the human race in that which has already been done, and the subject forms a study as elevating as it is instructive. These lessons are intended to serve as an introduction to this branch of knowledge which is called Physics.

2. Various Aggregations of Matter.-The student should first endeavour to realize the magnitude of the Universe or Cosmos; and although questions of size and distance belong more particularly to astronomy, yet the results of this science may with propriety be imported into the introduction of a work on Physics.

In a clear night we see stretching across the heavens a faintly luminous band, called the milky way or galaxy. When viewed by the telescope, it is found to consist of innumerable stars, which are massed so closely together in this particular part of the heavens as to give the appearance of a gigantic whole or substance (of which the grains or particles are individual stars), occupying a particular region This whole is probably the largest whole in the

of space. universe.

A ray of light, moving at the rate of nearly 200,000 miles a second, would take at least many years to move across the diameter of the milky way.

Now, each of the stars of this galaxy is an intensely hot and very large globe in size comparable to our Sun, which is in reality a star of average size. Many of these stars have, no doubt, associated with and circulating round them a number of bodies smaller than themselves. The sun has a number of satellites of this kind, of which our own earth is one. The sun and his satellites together form the solar system, and in like manner we may imagine each star to represent a system.

Descending now from the larger masses of the universe to our own earth, we meet with substances of various kinds, and it becomes the office of the chemist to resolve these into

their components. some fifty or sixty elements, united together in various ways. Let us take, for example, a piece of table salt or chloride of sodium, and imagine that we have the power of subdividing it without limit. We have reason to think that, if we continued the process of subdivision long enough, we should at last reach a limit which could not be overpassed without altering the nature of the substance; or, in other words, we should at last reach the smallest body capable of possessing the properties of salt. This we term

He finds that all bodies are made up of

a molecule.

If we still continue the subdivision, we separate the compound molecule of salt into its two components, sodium and chlorine, forming elementary atoms which we do not imagine to be capable of further subdivision by any means at our disposal.

Thus, in the large or cosmical scale, we have, in the first place, clusters of starry systems; secondly, individual systems; thirdly, individual components of these systems: while in the small scale we have substances, molecules, and

atoms.

3. Porosity.--Now, just as in the starry firmament there are vacant spaces between the various individual stars, so in the small scale there are probably vacant spaces between the various molecules of a body; and just as there are vacant spaces between the various components of the solar system, so there are probably vacant spaces between the various atoms that go to form the compound molecule. In other words, bodies are porous, but we must distinguish between two kinds of pores,—namely, physical pores, which exist in bodies with no apparent want of continuity, their existence being rendered evident by the contraction of such bodies when exposed to cold, and sensible or visible pores, which form actual cavities capable of being seen by the microscope, or made evident in some other way.

The skin of the human body is a very good example of a substance possessing sensible pores, and a piece of blottingpaper or sponge is another.

4. Three States of Matter.-Very many of the substances with which we are acquainted are capable of appearing before us in three different states. There is first of all the solid state, in which a body has a definite form, and endeavours to retain it; secondly, there is the liquid state, in which the body requires to be kept in a vessel, and adapts itself so as always to have its surface horizontal; and there is, thirdly, the gaseous state, in which the body cannot be held in an open vessel, but must be shut in on all sides, and always fills the vessel in which it is held. Both liquids and gases possess extreme mobility, in contradistinction to the rigidity of a solid; while a gas again is distinguished from a liquid by its incapacity of remaining in an open vessel, and having a surface.

Earth, a rock, a mountain, a table, a chair, are examples of solids; water and wine are examples of liquids; while the atmospheric air is a very good example of a gas.

5. Motion. Having now described the various aggregations and kinds of matter, something may be said about motion.

We can only conceive of relative motion, for when a body is in motion we can only know the fact by reference to some other body which is not moving with it. Thus we know that planets are in motion because we see them continually changing their positions among the fixed stars. We know, too, that our earth is in rapid motion round the sun; and yet in a calm day, although this rapid motion of the earth as a whole is going on, there is no motion of the various parts of the terrestrial landscape among themselves.

Thus, despite the rapid motion of the whole, there may be a profound repose of the various parts. On the other hand, a body may be at rest as a whole, and yet there may be violent motions of its various parts among themselves. Let us take, for instance, any substance apparently at rest, say a block of stone. Although there is no appearance of motion in this substance, yet we have very strong reasons for supposing that its various molecules are in rapid motion of some sort among themselves, so minute and so rapid that

we should not perceive it, even if we used a microscope of very great power.

In fine, no substance in the universe is at rest; the particles of all bodies are in rapid motion backwards and forwards, and the bodies themselves in rapid motion through

space.

6. Force.-Let us now take a group of bodies at rest with regard to one another; this state of rest can only be changed by force. Thus, for instance, suppose we fire a gun, the previous state of rest of the bullet has now been changed by the force of the gunpowder into one of rapid motion. Or take a railway train at rest; the train is set into rapid motion through the force derived from the engine which draws it.

But as it needs force to produce motion, so does it equally need force to destroy it; the bullet from the gun will ultimately have its motion destroyed by the resistance from some hard substance against which it strikes, and the railway train will have its motion stopped by the friction caused by the break. A thing which is difficult to move is difficult to stop, and a thing which is easy to move is easy to stop, the reason being that it requires an equal and opposite application of force to set a body in motion, and to bring it again to rest.

We have various kinds of force in Nature, the most prominent being the force of gravitation. It is in virtue of this force that a body falls to the ground, and it is in virtue of this same force that the earth moves round the sun. If the attraction of gravitation were to cease, the earth would continue to move at a uniform rate in a straight line, and soon leave the sun behind it, while we in turn should be able to separate ourselves from the Earth.

On the small scale we have the force of cohesion, in virtue of which the molecules of a body keep together. If this force were taken away, everything would be reduced into small particles, and scattered about.

Again there is the force of chemical attraction, in virtue of which two different atoms cling together to form a