PHYSICS may be defined as the science of energy, of the transformations of energy and of its relation to matter. This is an im perfect definition in that, first, a comprehensive understanding of the meaning of energy involves a complete knowledge of physics; and second, the science of energy, in its broadest sense, com prehends all ecience, while the content of physics is generally limited to the scope of the topics which will be outlined below. In deed, in its earlier history and under its early title of natural philosophy, physics did encom pass a much wider scope; but by a process of exclusion of certain branches as they developed into specialized sciences, such as chemistry, astronomy, metallurgy, meteorology and ge ology, it has been limited to its present scope; and about 1870 its present name of physics took the place of the older title. This process of separation is still going on : as, for example, certain parts of electricity, that have heretofore belonged to physics, are gradually being de veloped into the separate science of electrical engineering, as distinguished from the practice or art of electrical engineering. At the same time, investigation and discovery are adding new fields to the science of physics. In its applications and its inter-relationships, physics lies at the foundation of all of the experimental sciences and still assists their modern develop ment, to a lesser extent only than does mathe matics. Thus the interconnections between physics and the other sciences, particularly chemistry, are so intimate that it is impossible to draw an exact line of separation, so that in these borderlands between the sciences have been built up new sciences which partake of the nature of both and which have consequently been named physical chemistry or chemical physics, meteorological physics, physical as tronomy or astrophysics, etc.
Modern physics is an inductive science. Like the other sciences, it is based upon a belief in the of natures; upon the belief that the same cause, operating under the same conditions, will result in the same effects at any time. In order to ascertain the causal rela
tions of physical occurrences, or phenomena, observations are made of what conditions seem to affect the phenomenon under study and ex periments are performed in which the condi tions are under the control of the observer, so that they may be varied separately, in order to determine which conditions are essential and which are accidental. Then exact measure ments are made to determine the quantitative relation between the essential conditions and the effect resulting. This quantitative relation, rigidly established by exhaustive experiments and with its limitations clearly specified, is known as a physical low. For example, it is found that for a constant mass of gas, at a constant temperature, the product of its pres sure by its corresponding volume is a constant for most gases within a moderate range of pres sures; and this relation is known as Boyle's Law. It may be called an empirical law, in that it represents a relationship between two physical quantities, the explanation of which it does not give. A study of many similar or related phenomena suggests a generalization or hypothesis as the explanation of them all. This hypothesis, tested, modified and perfected by extensive and experimentation along lines suggested by it and extending to all known phenomena to which it may have any relation and strengthened by its use in suggesting and predicting new phenomena, becomes a physical theory and may later attain to the dignity of a general physical law. Thus a further study of the relations between the pressure and vol ume of a as and the other factors (tempera ture, chemical structure, etc.) which might be related to it, led to the kinetic hypothesis of gases and this hypothesis, suitably modified, having been found to be in agreement with the phenomena it was called upon to explain and to be still further supported by other phenomena (such as, for example, the Brownian movements) has become the kinetic or dynamic theory of gases.