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WHEN we begin the study of physics in school, we already have a certain amount of information concerning physical phenomena. We know, for instance, that water can be changed into ice or into steam; we have seen the colors of the rainbow; we know that an unsupported body will fall. It is the purpose of the study of physics to add to this information and to put our knowledge into orderly arrangement.

1. Physical and Chemical Changes. — The physical phenomena, the laws of which we are to investigate, are the phenomena of matter and energy which may occur without changing the identity of a substance. These phenomena are caused by various physical forces, and include very many physical changes. Whenever a change takes place in matter without destroying the identity of the substance, it is known as a physical change. The fall of a stone thrown into the air, the changing of water into steam, the shrinking of a board while seasoning, the attraction between a magnet and a nail, are all physical phenomena, and the changes that take place are physical changes. If, however, the board is burned, or the nail is eaten by acids, the identity of the substance is destroyed, and the change is a chemical change.

2. Matter is that which occupies space and may be perceived by one or more of the senses. There are various kinds of matter, called substances, such as wood, stone, water, air, etc., while bodies are composed of definite volumes of these substances.

3. Atoms and Molecules. The fact that bodies can be compressed gave rise to the belief that the space they occupy is not filled entirely by the matter of which they are composed, and that its particles do not really touch one another. These particles are called molecules, and they are the smallest parts into which a body can be divided without destroying the substance as such. If the forces which keep the molecule intact are overcome, the molecule may be broken up into atoms, which are understood to be the smallest quantities of matter that can enter into combination. The name electron has been given to particles of matter smaller than the atom, which act as carriers of negative electricity.

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4. Size of Molecules. - Molecules are so small that they cannot be seen by microscopes of the highest power. Lord Kelvin (Sir William Thomson), however, calculated their size in some substances, and from a study of the thickness of the film in soap bubbles he found that if a globe of water the size of a football were magnified to the size of the earth, the molecules would occupy spaces intermediate in size between small shot and footballs.

5. States of Matter. In the air we breathe, the water we drink, and the bread we eat we have examples of the three different forms or states that matter can assume; namely, gaseous, liquid, and solid. A solid is a body which,

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at ordinary temperatures and under slight pressures, does not change its shape. If the shape is changed under these conditions, the body is a fluid. Fluids may be divided into two classes. Those that retain a definite surface on being poured into a vessel are liquids, while those that have a tendency to expand indefinitely are gases.

Rigid solid

Soft solid.

Bodies form an almost continuous gradation from the most rigid solid to the most tenuous gas, and the above classification may be extended as in the following table:




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Viscous liquid

Mobile liquid.


Liquid in minute separate particles mixed with gas Fog or cloud



Fluids flow; it is commonly supposed that solids never do. This is not strictly true, since it has been shown that under certain pressures solid bodies also flow. The state of matter is largely determined by conditions of temperature and pressure. A stick of sealing wax fastened at one end so that it will stand horizontally and having a two-pound weight attached to the other end will become permanently bent in a short time; an asphalt pavement on a sloping street will flow down hill during a hot day; and a bullet placed upon a cake of shoemaker's wax, resting upon two corks in a dish of water, will in a few months pass entirely through the wax, while the corks will pass upward into it. All these are examples of what are called solid bodies, yet under the proper conditions they are seen to flow.

6. Kinetic Theory of Matter. According to the kinetic theory of the structure of matter, the molecules of all bodies

are in rapid vibration and the three states which matter assumes may be considered as resulting from the kind of motion of the molecules and their relative velocities.

In solids the motion of the molecule is restricted to a limited space, and although it is in constant vibration, its position with respect to the other molecules of the body is relatively fixed. Hence the shape of a rigid solid, under normal conditions of pressure and temperature, is unchanged.

In liquids the molecule is free to move in any direction. This means that the molecules of a liquid will glide over one another, and that the liquid will take the shape of any vessel into which it is poured.

In gases the molecule has a high velocity, moving in straight-line path until it comes in contact with some other molecule or with the walls of the containing vessel. On account of this high molecular velocity a gas cannot be kept in an open vessel, and however small the quantity of gas, it will always fill any vessel in which it is confined. The quantity of gas in a closed vessel determines the pressure it will exert upon the walls.

Some substances assume all three states through a change of temperature alone, as water, which may be solid (ice), liquid (water) and gaseous (invisible vapor). Others require a change of pressure as well as a change of temperature.

7. Experiments. An experiment is a question put to nature, and the results obtained by it are her answer. If the conditions are the same, it is found that to a given question nature always makes the same answer, and thus we learn that the order of nature is constant; or that she has definite laws by which she works. A careful study of the experiments described in the text for class demonstrations, attended by

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practical work in the laboratory, will show the student in how great variety the questions can be asked, and will show that every changed condition has its own effect.

It is by means of experiments that physicists have discovered the laws of physics. In interpreting the results of various experiments an hypothesis is formed to explain these results. When the hypothesis is found to account satisfactorily for all the observed facts, it becomes an accepted theory; when this theory is established so firmly that it cannot be overthrown, it is the expression of physical law.

8. Physics is that science of matter and energy which treats of the laws that express the relation between physical phenomena and their causes. In order to study and verify these laws the student makes use of experiments in which various changes in the conditions may be made and their results noted.

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