68 The soluble double sugars (maltose, milk sugar, and cane sugar) are reduced to the simple sugars (dextrose and levulose). Furthermore the action on the proteids does not stop with the production of peptones and proteoses, but these in turn are still further reduced. 69 Energy, which is defined as _the ability to do work_, or _to cause motion_, exists in two general types, or forms, known as kinetic energy and as potential energy. _Kinetic_ energy is energy at work, or energy in the act of producing motion; while _potential_ energy is reserve, or stored, energy. All moving bodies have kinetic energy, and all stationary bodies which have within them the _capability_ of causing motion possess potential energy. A bent bow, a piece of stretched rubber, a suspended weight, the water above a mill dam, all have the capability of causing motion and all have potential energy. Examples of kinetic energy are found in the movements of machinery, in steam and electricity, in winds, and in currents of water. Kinetic is the active, and potential the inactive, form of energy. 70 As the atoms of hydrogen and oxygen that make up the molecules of water separate, they unite with atoms of their own kind—the hydrogen with hydrogen and the oxygen with oxygen atoms. Since these combinations are weaker than those of the water molecules, energy is required to bring about the change. But when hydrogen burns in the oxygen, the change is from a weaker to a stronger combination. The stored-up energy is then given up or becomes active. 71 In the evaporation of water, the energy of the sun is stored with reference to the force of gravity. In evaporating, water rises as a gas, or vapor, above the earth’s surface, but on condensing into a liquid, it falls as rain. It then finds its way through streams back to the ocean. All water above the sea level is in such a position that gravity can act on it to cause motion, and it possesses, on this account, potential or stored-up energy. It is because of this energy that rapids and waterfalls are such important sources of power.