Investigations of radioactivity and the observations based upon the passage of electric currents through gases have recently modified our views with respect to the atomic theory, but these points will be dealt with in the chapter dealing with radiation. Questions regarding the eventual loss of energy in matter are best studied in gases. A considerable number of important investigations are now being carried on in Europe with the view of tracing the interchanges of molecular energies in gas molecules. Maxwell and other investigators found long ago that the motion of molecules cannot go on perpetually. The energy of motion will in time be frittered away by friction, air resistance, collisions with other molecules, vibrations set up by collisions, and other molecular movements. It has been found that the energy which is dissipated by air resistance is transformed into energy in the air. That which is lost by collisions is converted into internal vibrations within each molecule. The question now arises as to what effects are exerted on a gas. It involves the effects of the communicated internal molecular vibrations and their transference of energy to the surrounding medium. What is known as the Quantum dynamic theory has been proposed to account for this phenomena. Quantum dynamics appear to be distinct from the Newtonian. Carnot and Clausius discovered that the motive power of heat is independent of the agents brought into play for its realization. The motive power of a waterfall depends, for example, on its height and on the quantity of water falling within a given time. Clausius stated the Carnot idea in mechanical terms by saying: That in a series of transformations, in which the final is identical with the initial stage, it is impossible for heat to pass from a colder to a warmer body unless some other accessory phenomenon occurs at the same time. A heat motor, which, after a series of transformations, returns to its initial state, can only supply work, or power, if there exist two sources of heat, and if a certain quantity of heat is given to one of the sources which can never be the hotter of the two. The output of a reversible machine working between two given temperatures is greater than that of any nonreversible engine, and it is the same for all reversible machines working between these two temperatures. Clausius showed that this principle conduces to the definition of an absolute scale of temperature and there is another factor assisting in restoring physical equilibrium which he termed entropy. It is a variable which, like pressure or volume, serves concurrently with another variable to define the state of a body. These discoveries of Carnot and Clausius showed the impossibility of finding a source of perpetual motion and helped to solve many of the difficulties in securing efficiency from internal combustion engines. Industrial, as well as scientific results of immense importance have developed from these principles.