Page:Elementary Text-book of Physics (Anthony, 1897).djvu/15



1. Divisions of Natural Science.—Everything which can affect our senses we call matter. Any limited portion of matter, however great or small, is called a body. All bodies, together with their unceasing changes, constitute Nature.

Natural Science makes us acquainted with the properties of bodies, and with the changes, or phenomena, which result from their mutual actions. It is therefore conveniently divided into two principal sections,—Natural History and Natural Philosophy.

The former describes natural objects, classifies them according to their resemblances, and, by the aid of Natural Philosophy, points out the laws of their production and development. The latter is concerned with the laws which are exhibited in the mutual action of bodies on each other.

These mutual actions are of two kinds: those which leave the essential properties of bodies unaltered, and those which effect a complete change of properties, resulting in loss of identity. Changes of the first kind are called physical changes; those of the second kind are called chemical changes. Natural Philosophy has, therefore, two subdivisions,—Physics and Chemistry.

Physics deals with all those phenomena of matter which are not directly related to chemical changes. Astronomy is thus a branch of Physics, yet it is usually excluded from works like the present on account of its special character.

lt is not possible, however, to draw sharp lines of demarcation between the variisus departments of Natural Science, for the