INTRODUCTION

AIM AND METHOD OF PHYSICAL SCIENCES.

1. Physical Sciences.-The study of nature includes two great divisions, natural and physical sciences. The former includes those that involve the complex phenomena of life, while the latter are concerned with the investigation of the fundamental phenomena of matter. Chemistry and Physics are the underlying physical sciences.

Chemistry is distinguished from Physics in that it is chiefly concerned with the phenomena that result when different kinds of matter are brought together and enter into combination. It deals largely with the qualities in which one kind of matter differs from another. Physics, on the other hand, is concerned with the properties common to all kinds of matter and those changes of form and state which matter undergoes without being changed in kind, as well as such general phenomena as sound, heat, light, electricity, and magnetism. There are, however, many points where these sciences merge into each other, and the domain of physical chemistry lies largely in this border land.

2. The Aim of Physical Science.-It is the aim of physical science so to systematize our knowledge of the material world that all its phenomena shall be seen as special instances under a few far-reaching and more inclusive generalizations called laws. And when a given phenomenon is analyzed in this way into separate parts or phases each of which is but a special case under some general law, the phenomenon is said to be explained.

In seeking an explanation we determine the causes of the phenomenon in question; that is, the essential circumstances or those circumstances without which the given event does not occur; and then we seek to determine the effect of each of these circumstances separately, and exhibit, if possible, each such effect as a special instance under some general law.

For example, the complex motion of a ball struck by a bat

is found to be dependent on the motion given to it by the blow of the bat, on the presence of the earth, and on air resistance.

We first try and determine how a body moves when set free in the presence of the earth without any initial blow or impulse and in a vacuum. We find in this way an unvarying rule of motion that applies to all bodies of whatever size or shape, and we call it the law of falling bodies. That part of the motion of the ball which depends only on the nearness of the earth is but a special instance under this law. Now, making allowance for the motion due to the earth, we seek to determine that part of the motion due to the initial blow, and here again we find that the actual motion seems to be exactly according to a general rule which is found to hold whenever an impulsive force acts on a mass. And finally we investigate the effect of air resistance, determining how it affects a body at rest and how it modifies the motion of a body moving through it, and here again certain general rules are found which apply not only to the special case under consideration, but to all cases of bodies moving through air. When the effects of all three circumstances are taken into account, the motion is found to be exactly accounted for, and is then said to be explained.

Leverrier, and Adams, in analyzing the motion of the planet Uranus, found that after taking account of all the known circumstances, such as the attractions of the sun and other planets upon it, there still remained a part of its motion which was not accounted for, and assuming it to be due to an unknown planet they computed its position and mass, and thus the planet Neptune was discovered.

But in analyzing our problem we may go deeper and show that the motion of the ball near the earth is such as would result from a force urging the two bodies together, and we may then discover that it is merely a special instance of the law that all bodies are influenced by forces urging them together or, in other words, that all bodies attract each other. When we can show also that the forces between the air and the moving body are due to the motion given to the air and so are simply particular exhibitions of the general rule which holds whenever matter is set in motion, we feel that a still higher degree of understanding is reached.

By such a process all the complex facts of nature are assigned their places in an orderly system. But a limit is soon reached beyond which the mind cannot go, because thinking is conditioned by experience, and even in its profoundest theories and speculations the mind must employ those conceptions which it has obtained from the world about it.

3. Experiment.-Physics is an experimental science, its generalizations rest solely upon experiment, and although reasoning upon established facts has often led to the discovery of new truths of great importance, the final appeal must always be to experiment. If the deduction is thus disproved, it appears either that the reasoning was wrong or that there are certain elements entering into the problem that were neglected. In seeking for the causes of such discrepancies new truths have often been discovered.

An experiment is a combination of circumstances brought about for the purpose of testing the truth of some deduction or for the discovery of new effects. The usual course of an experimental inquiry is to modify the circumstances one by one, noting the corresponding effect until the influence of each is thoroughly understood.

4. Necessary Assumptions. In every experimental science it is assumed that the same causes always produce the same effects and that the position of the event as a whole in either time or space only affects the absolute time and position of the result, provided there is no change in the relative time or space relations of the various circumstances involved. For example, if all the other circumstances are the same, a stone will fall in exactly the same manner next week as it does to-day, or if the solar system be changing its place in space no change in the manner of the stone's falling will take place from that cause alone.

Experience up to this time has justified these assumptions, and without them progress in physical science would be impossible.

FUNDAMENTAL CONCEPTIONS.

5. Force. Our ideas of force are derived primarily from muscular effort. It requires an effort to lift a weight, to throw