Excerpt from Physics for College Students
Reality of Physical Phenomena. - In studying the facts of nature we assume the objective reality of physical phenomena and the existence of external objects apart from the mind of the observer. While we become acquainted with the physical universe solely through our senses, it will probably be admitted that every form of matter, such as pebble, a drop of dew, and the oxygen of the air, has objective existence. The supreme test of physical reality is the fact that the material world remains unchanged in quantity in whatever wy it is measured. From this point of view, only two classes of things or entities are found in the physical world, - - matter and energy.
Matter - The ultimate nature and structure of matter are not known with and degree of certainty. The discovery of radium, and its spontaneous disintegration into substances of simpler constitution, such as helium, and of the generation of heat by its disintegration, have profoundly modifies the older views of the constitution of matter. Matter is often defined as something occupying space, but it is much better described by its properties.
A limited portion of matter is a body, and different kinds of matter, having properties, are called substances.
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