NATURAL PHILOSOPHY. 185 



power developed. The modification of Garnet's theory to adapt it to this 

 truth was alluded to; and the great distinction which it leads to between 

 reversible and not reversible transformations of motive power was only men- 

 tioned. 



To facilitate further statements regarding transformations of motive power, 

 certain terms, introduced to designate various forms under which it is mani- 

 fested, were explained. Any piece of matter, or any group of bodies, how- 

 ever connected, which either is in motion, or can get into motion, without 

 external assistance, has what is called mechanical energy. The energy of 

 motion may be called either "dynamical energy," or "actual energy." The 

 energy of the material system at rest, in virtue of which it can get into 

 motion, is called "potential energy," or, generally, motive power possessed 

 among different pieces of matter, in virtue of their relative positions, is called 

 potential energy. To show the use of these terms, and explain the ideas of a 

 store of energy, and of conversions and transformations of energy, various 

 illustrations were adduced. A stone at a height, or an elevated reservoir of 

 water, has potential energy. If the stone be let fall, its potential energy is 

 converted into actual energy during its descent, exists entirely as the actual 

 energy of its own motion at the instant before it strikes, and is transformed 

 into heat at the moment of coming to rest on the ground. If the water flow 

 down by a gradual channel, its potential energy is gradually converted into 

 heat by fluid friction, and the fluid becomes warmer by a degree Fahr. fbr 

 every 772 feet of the descent. There is potential, and there is dynamical 

 energy, between the earth and the sun. There is most potential energy and 

 least actual energy in July, when they are at their greatest distance asunder, 

 and when their relative motion is slowest. There is least potential energy 

 and most dynamical energy in January, when they are at their least distance, 

 and when then' relative motion is most rapid. The gain of dynamical energy 

 from the one time to the other is equal to the loss of potential energy. 



Potential energy of gravitation is possessed by every two pieces of matter 

 at a distance from one another; but there is also potential energy in the 

 mutual action of contiguous particles in a spring when bent, or in an elastic 

 cord when stretched. 



There is potential energy of electric force in any distribution of electricity, 

 or among any group of electrified bodies. There is potential energy of mag- 

 netic force between the different parts of a steel magnet, or between different 

 steel magnets, or between a magnet and a body of any substance of either 

 paramagnetic or diamagnetic inductive capacity. There is potential energy 

 of chemical force between any two substances which have what is called 

 affinity for one another, for instance, between fuel and oxygen, between 

 food and oxygen, between zinc in a galvanic battery and oxygen. There is 

 a potential energy of chemical force among the different ingredients of gun- 

 powder or gun cotton. There is potential energy of what may be called 

 chemical force, among the particles of soft phosphorus, which is spent in the 

 allotropic transformation into red phosphorus; and among the particles of 

 prismatically crystallized sulphur, which is spent when the substance assumes 

 the octahedral crystallization. 



