Heat is evolved which raises steam in the boiler, and the steam actuates an engine, and the latter exhibits measurable kinetic energy. Where did this come from? It was potential in the coal, we say, though no method known to physics enables us to prove this by mere inspection of the coal. We must cause the latter to undergo some transformation. But by rigid methods we can estimate very exactly the potential energy of the coal, and we can calculate the kinetic energy equivalent to this. Yet again we find that the kinetic energy of the steam-engine is only a fraction of that which calculation shows us is the equivalent of the kinetic energy of the coal. What becomes of the balance? We can be quite certain that it has been dissipated in friction, radiation, loss of heat by conduction, loss of heat in the condenser, and so on, although we cannot prove this rigidly by experimental methods. Think of the universe as an isolated system. It contains an invariable quantity of energy. This energy may be that of bodies in motion--suns, planets, cosmic dust, molecules, etc.--when it is kinetic energy; or it may be the energy of electric charges at rest or in motion; or any one of the many kinds of potential energy. It may pass through numerous transformations--the chemical potential energy of coal may be transformed into the kinetic energy of water molecules (steam at high temperature), and this into the kinetic energy of the revolving armature of a dynamo, and this again into the energy of moving electrons (the current of electricity in the circuit of the dynamo), and then again into the energy of ethereal vibration (light, heat, X-rays, or other electro-magnetic waves), and these again into mechanical or kinetic energy, and so on. When we say that we can control energy we say that we can produce these transformations; we can cause things to happen, we bring becoming into being. In this sense energy is causality. But while the sum-total of energy in the universe remains constant, the sum of causality continually diminishes. Energy is the power, or condition, of producing _diversity_, but while energy can suffer no diminution of quantity, diversity tends continually to decrease. In the last two sentences we state, in one way, the second law of thermodynamics--in some respects the most fundamental result of our experience in the physical investigation of the universe. In its most technical form, as enunciated by Clausius, this law states that the value of a certain mathematical function, called _entropy_,[8] tends continually towards a maximum, when it is applied to the universe as a whole. When we say the “universe,” we mean all that comes within our power of physical investigation. Let us now see what this statement means. [8] See appendix, p. 369. Entropy is a shadowy kind of concept, difficult to grasp. But again we may point out that the reader who would extend the notion of mechanism into life simply _must_ grasp it.