we know one cannot be converted into another. They may be united in countless combinations, but each is itself not only indestructible but unchangeable. Why this is so is an interesting subject of speculation. We do not positively know.
That energy is also something which cannot be created or destroyed is not so generally recognized. Transformations of energy from one form to another are constantly occurring before our very eyes; and yet we seldom stop to think what the conservation of energy means in any given case. Energy itself is often defined as that which has the capacity for doing work, and work is done when force or resistance is overcome. A hod carrier does work when, overcoming the force of gravity upon his body and his hod of brick, he climbs to the top of a ladder; and the work done is a measure of the energy expended. Energy stored up in his body has been transferred to the brick in their elevated position, and if they are allowed to fall to the ground their energy is turned into heat, developed by their impact upon the ground. Again, work is done by a windmill in pumping water up into an elevated reservoir, and the so-called 'potential' energy which the water possesses in its elevated position has all been transferred to the water from the wind which drove the mill. If the water be allowed to flow down to the ground again through a water motor the latter could drive machinery and so do work; and the work it could do plus the heat produced by friction would exactly equal the work done in pumping the water up to its elevated position. Thus is the energy conserved, and not destroyed. More or less of it is dissipated by friction, and lost, so far as useful effect may go. But it all remains in existence, somewhere.
Again, coal is burned under the boiler of a steam engine. Heat is produced, steam is generated, the engine does work. The coal possessed a store of energy, potentially. That is, the coal had the capacity of uniting with the oxygen of the air and setting free a store of energy. This energy, potential or latent in the coal, becomes kinetic and evident in the heat of the boiler and the work of the engine. Moreover, the work done by the engine added to the heat given off by the boiler and engine is exactly equal to the total store of energy possessed by the coal. And if from a store of energy, either in the body of a man or horse, or in a pile of wood or coal, a certain portion is expended in doing work, the amount remaining is exactly the difference between that expended and the original amount. In short, energy can be measured, stored up and expended, just as truly as merchandise or money.
Thus the conservation of energy means that energy cannot be created or destroyed; but it may be transferred from one body to another or transformed from one form to another. Heat may be converted into work and work into heat. The chemical energy of a zinc rod may be expended to generate an electric current, and the latter