Physics, as the science of energy, has its unification in the principle of the conservation of energy. The statement of this, in the words of Maxwell, is : (The total energy of any ma terial system is a quantity which can neither be increased nor diminished by any action between the parts of the system, though it may be trans formed into any of the forms of which energy is susceptible" The importance of this princi ple and its influence in unifying and in de veloping physics cannot be overestimated. Just as the development of the principle of conserva tion of matter unified chemistry and put into the hands of the chemist a rule for measuring his facts and for checking his theories, so the principle of conservation of energy has become the basic law connecting all physical phenomena, for testing the accuracy of physical experi ments and for checking, modifying and extend ing physical theory. Obviously, a great deal depends upon our definition of energy and the great importance of the conservation principle lies in the fact that there is so simply definable a physical function as mechanical energy which does remain invariant in a °closed') system; and furthermore, that it is possible to express quantitatively all other physical manifestations of nature in terms of this invariant energy function. In the paragraphs which follow, at tempt is made to outline the content of physics as it exists at the present writing. It should be again emphasized here that this content can not be fixed for all time; that it is changing, undergoing development, essentially dynamic in its nature. It is at present generally divided into the following topics, or similar ones: me chanics and (mechanical) properties of matter and °physical)) mechanics), sound, heat, light and magnetism and electricity. This divi sion is to a great extent arbitrary, made for con venience in study and the modern science is careful to recognize it as such and to recognize the unity of relationship among all its parts.
Mechanics (or "pure' mechanics) is essen tially a branch of mathematics, based on cer tain axioms derived from physics, but its basic importance in all branches of physical science makes it one of the first topics included in a study of physics. It is that branch of science which deals with motion, with forces, with mat ter in so far as it is affected by motion and forces i.e., mass and configuration, and with their inter-relations. It is generally divided into kinematics, which deals with motion by itself ; and dynamics, of which statics deals with forces alone, under conditions of balance, or equilibrium, and kinetics deals with mat ter in motion, as the result of the action of forces, and with forces as producing motion. Hydrostatics and hydraulics should be men tioned as important branches or extensions of this part of the science, although often in cluded in the next topic.
Properties of matter (aphysica) mechanics) comprises the general mechanical properties of matter in each of its three physical states—solid, liquid and gaseous, and the laws that express the causal relations found to exist among them. It includes also all of the distinctive data and physical constants obtained by experimenta tion with different kinds of material. Density, gravitation, buoyancy, gas laws, elasticity, vis cosity, diffusion, osmosis and surface tension are some of the important sub-topics of this subject. Other divisions of the subject are often made, in order to present some particular phase or view-point: as, for example, division into • masse mechanics and (molecular" me chanics.
Although wave motion is, in its mathemati cal development, properly a branch of ((pure) mechanics, it is generally included with, or placed just before, the subject of sound, since in sound waves and in the vibrations of sound ing bodies (stretched strings, rods, bells, air columns, etc.), there exist the most perfect and
tangible examples of such motion. And, aside from a study of the physical characteristics of audible sounds, particularly musical sounds and their physical relations in the musical scale and in musical composition, and quite recently, vowel sounds and other sounds occurring in articulate speech, the study of sound is the study of a typical wave motion,. whose characteristics (re Election, refraction, interference, etc.), may he easily demonstrated on a convenient scale. The practical importance of the applications of the physics of sound, in the design of sound-pro during and reproducing instruments, and in the design of auditoriums and similar public halls, has attained to considerable recognition and emphasis in recent years. Descriptions of the mode of production of vocal sounds, and of the mechanism of audition, is usually included in a general treatise on physics, although they now more properly belong to the respective of physiology and of the new (experimental) psychology.
The sub-topics of heat, given in the usual order, are thermometry, thermal ex pansion, calorimetry, change of state, transfer ence of heat and thermodynamics. Ther mometry covers the usual empiric methods of temperature measurement; the idea of the Kelvin or thermodynamic (absolute) tempera ture scale is left to the proper section of thermodynamics. Thermal expansion includes the experimental determination of the empirical relations existing between physical dimension and temperature for different materials, and the principles and constants thus determined are of considerable importance to the design of all kinds of machinery and construction, and are of vital importance in physical mensuration and chronometry.
Calorimetry and changes of state deal with measurements of quantities of heat Although arbitrary units of heat quantity (calory, British thermal unit, etc.), are most frequently used, the establishment by Joule and others of the exact equivalence between heat quantity and mechanical energy has made the use of energy units (erg, joule, etc.), increasingly more com mon. The experimental work consists largely in the determination of specific heats (latent), heats of changes of state (fusion, vaporization, etc.), and of solution, combustion, etc.; and in the study of the physical phenomena which ac company change of state, such as change of volume, of specific heat, of color, etc., and of the effects of all the possible external factors which may affect or change the conditions of change of state. The values of these physical quantities and relationship for the pure ele ments and for their compounds has assumed importance mportance in chemical theory and prac tice that their determination has become an im portant part of the borderland science, physical chemistry. The study of change of state has made valuable use of thermodynamic reasoning, and among the important developments result ing in recent years should be mentioned the ob tainment of nearly all of the normally gaseous elements and compounds in liquid form, and in many cases in sold form and the production of extremely low temperatures by their means. Transfer of heat may be accomplished by three different methods — conduction, convection and radiation: the first two methods depend for their operation upon intervening material media and are properly a transfer of heat energy as such; while the third is a type of electromagnetic radiation, and is more completely treated under that heading.