Page:Popular Science Monthly Volume 65.djvu/56

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HE domain of physics is coextensive with the whole range of phenomena of the material world, but the science of physics, as commonly understood, is restricted to a much smaller field whose boundaries were perhaps first marked by setting off certain groups of phenomena for special study.

In this way there arose five significant branches of physical science: (1) Astronomy, in which are treated the facts and phenomena observed in connection with the heavenly bodies. One peculiarity of astronomical phenomena is worth noting, namely, that they are entirely beyond human control and can not be made the subject of experiment. (2) Chemistry, which treats the relations of different kinds of matter one to another, and those phenomena which accompany material changes, i. e., alteration in the composition of substances. (3) Biology, which deals with vital phenomena; current, as in physiology, or past, as in paleontology. (4) Meteorology, in which are grouped phenomena peculiar to the earth's atmosphere and incapable of repetition at will. (5) All the remaining natural phenomena form the subject of physics, which may be said to treat of mechanics, i. e., the motion and interaction of bodies upon one another, and of those groups of phenomena commonly designated as heat, light, sound and electricity. These conventional divisions of science involve other differences not always clearly apprehended, but important alike to the student and the teacher.

While one may not say that one branch of knowledge is more worthy than another, one set of facts may be more precise or scientific than another, whatever may be the meaning attached to the word. Exactly where to draw the line between science and knowledge has been the subject of some dispute. For the purpose of the present discussion we will adopt as the definition of a science, the precise knowledge of a body of facts accurately verified and erected into a logical system. In this definition, only those branches of learning are intentionally excluded from the rank of science in which the knowledge is either ill defined, or uncertain, or unsystematized, though just what classes of historical investigation or of psychological speculation fail to meet the requirement are of no moment, as we are at present concerned only with branches of physical science. It is obvious that the branches of science thus defined may differ considerably in the precision with which the facts