It exists, however, in a variety of forms, which may be either active or passive.  In the active state it takes some form of motion.  The various forces which we recognize in nature—­heat, light, electricity, chemism, etc.—­are simply forms of motion, and thus forms of this energy.  These various types of energy, being only expressions of the universal energy, are convertible into each other in such a way that when one disappears another appears.  A cannon ball flying through the air exhibits energy of motion; but it strikes an obstacle and stops.  The motion has apparently stopped, but an examination shows that this is not the case.  The cannon ball and the object it strikes have been heated, and thus the motion of the ball has simply been transformed into a different form of motion, which we call heat.  Or, again, the heat set free under the locomotive boiler is converted by machinery into the motion of the locomotive.  By still different mechanism it may be converted into electric force.  All forms of motion are readily convertible into each other, and each form in which energy appears is only a phase of the total energy of nature.

A second condition of energy is energy at rest, or potential energy.  A stone on the roof of a house is at rest, but by virtue of its position it has a certain amount of potential energy, since, if dislodged, it will fall to the ground, and thus develop energy of motion.  Moreover, it required to raise the stone to the roof the expenditure of an amount of energy exactly equal to that which will reappear if the stone is allowed to fall to the ground.  So in a chemical molecule, like fat, there is a store of potential energy which may be made active by simply breaking the molecule to pieces and setting it free.  This occurs when the fat burns and the energy is liberated as heat.  But it required at some time the expenditure of an equal amount of energy to make the molecule.  When the molecule of fat was built in the plant which produced it, there was used in its construction an amount of solar energy exactly equivalent to the energy which may be liberated by breaking the molecule to pieces.  The total sum of the active and potential energy in the universe is thus at all times the same.

This magnificent conception has become the cornerstone of modern science.  As soon as conceived it brought at once within its grasp all forms of energy in nature.  It is primarily a physical doctrine, and has been developed chiefly in connection with the physical sciences.  But it shows at once a possible connection between living and non-living nature.  The living organism also exhibits motion and heat, and, if the doctrine of the conservation of energy be true, this energy must be correlated with other forms of energy.  Here is a suggestion that the same laws control the living and the non-living world; and a suspicion that if we can find a natural explanation of the burning of a piece of coal and the motion of a locomotive, so, too, we may find a natural explanation of the motion of a living machine.