Carhart Henry S. Physics for College Students

Boston, Chicago: Allyn and Bacon, 1910. — 623 p.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 a 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 way it is measured. From this point of view, only two classes of things or entities are found in the physical world, matter and energy.