202 Professor W, Thomson, on the Origin [Feb. 29, 



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. for every 772 feet 

 of the descent. There is potential energy, 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 their 

 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 poten- 

 tial 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 magnetic 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 dia- 

 magnetic inductive capacity. There is potential energy of chemical 

 force between any two substances which have what is called afl^inity 

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

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

 There is potential energy of chemical force among the different 

 ingredients of gunpowder 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. 



To make chemical combination take place without generating 

 its equivalent of heat, all that is necessary is to resist the chemical 

 force operating in the combination, and take up its effect in some 

 other form of energy than heat. In a series of admirable researches 

 on the agency of electricity in transformations of energy,* Joule 



* On the Production of Ileat by Voltaic Electricity," communicated to the 

 Koyal Society Dec. 17, 1840, {see Proceedings of that date,) and published Phil. 

 3i«(/. Oct. 1841. [On 



