coveries in reference to its action in the physical world explain the motions and relations of the heavenly bodies, but it is affirmed confidently that the theory of universal motion or the kinetic theory removes more difficulties in the explanation of the phenomena of the universe than either the astronomical or the atomic theory. In passing, it may be mentioned that this theory of universal motion was taught not only by Heraclitus, but by Anaxagoras of Athens, who saw that with his well-nigh innumerable germs he could not solve the problems of the universe apart from motion, and therefore introduced into his system of philosophy, the nous, or intelligence, to give the first push to matter and impart to it the movement which he believed to be universal and without which life could not exist.
The fourth of the abstract theories employed to explain the universe is the physical theory or the theory of energy. By this theory we understand that whatever is or has come to be has been caused by the force or energy there is in matter. The astronomical theory fails to account satisfactorily for molecular activity and chemical affinity: the kinetic theory is set aside because it is based on dualism or the existence of matter and ether. What is true in these theories is preserved and the difficulties found in the application of each one of them are avoided by the use of the general term energy, a term under which, according to Young, all that is known in science may be expressed. Energy or force is that something in nature which can do work and be stated in terms of horse power. We speak of latent energy, of energy created by friction, by the fall of water, by steam, and, although we are unable to define it, we describe it as something everywhere present and able, when properly harnessed or directed, to do a certain amount of work.
While refusing to speak of energy as a property of matter and denying that it is matter in any true sense of the word, it is affirmed that its amount in the material world can neither be increased nor diminished, that while from a heated object a certain portion of heat departs in cooling, a fact which is described as entropy, the sum total of energy, whether in the form of heat or of some other force, remains the same. This truth is set forth by Helmholtz as the conservation of force, a doctrine which, while admitting that a particular form of energy may be changed into another form, affirms that the energy itself is neither destroyed nor diminished by the transformations through which it may pass. Energy is valueless unless capable of transformation. It is good for nothing till it is made usable. Hence the study of energy has been more constantly directed toward discovering methods by means of which its power may be employed for the welfare of mankind, than toward ascertaining its nature. We know that energy may be changed in form, that it now appears as heat, now as light, now as electricity, now as magnetism. Nor are these the only