24 Prof. Oliver Lodge on 



o 



happens in every atomic encounter, whether it be with the 

 walls of the vessel or with another free molecule, then, so 

 long as the air remains compressed, there is merely a storage 

 of energy, each molecule possessing and retaining its own 

 share. But directly an opening is made, or a communication 

 established with an empty vessel, it is equivalent to the 

 removal of a portion of wall, and some of the atoms, with 

 their energy, pass along without obstruction into a new 

 region of space. 



In one sense each may still be said to retain its original 

 energy ; but in order to regard it thus we must be capable of 

 dealing with the atoms individually. If the atoms escaping 

 from confinement are lost in the vastness of space, their energy 

 is unavailable, and their pressure, on which entirely depends 

 the potential energy of a compressed mass of gas, is nil. If 

 they escape into another equal vessel, then only the first 

 individuals can enter freely without obstruction, all the others 

 experience encounters on the way, as if the hole in the wall 

 were gradually being restored ; and though at the end of the 

 operation, when everything has finally settled down, each 

 atom may be said to possess its original energy, yet the 

 pressure, on which depends the potential energy of the com- 

 pressed gas, has been halved, and the other half is unavailable 

 or has been dissipated. 



Plainly the idea of potential energy belongs to the tempo- 

 rary order of ideas, to which the dissipation of energy and 

 the second law of thermodynamics belong, and is appropriate 

 to the present period when we have not yet learnt how to 

 deal with molecules individually. 



The kinetic energy of a set of gas molecules is only 

 really available when it is combined with momentum or 

 angular momentum, i. e. when the motion of a majority of 

 molecules has the same sign, when there are more plus terms 

 than minus in either their translational or their angular 

 velocity ; in other words, when there is a wind or cyclone "*. 

 A stream of particles can be utilised, as by a windmill or 

 a Pelton water-wheel, and that is an example of available 

 kinetic energy. But usually the energy of fluids can only be 



* It may be worth while to point out that the reason why momentum 

 is necessarily preserved in cases of impact, while translatory energy is 

 usually not conserved, is because momentum depends on the first power 

 of velocity, and therefore is unconcerned with vibrations ; while every kind 

 of vibration, whether it be of sound or of heat, enters into the sum total 

 of a thing depending on an even power of velocity, like energy, whose 

 translational value is therefore to that extent diminished by the pro- 

 duction of vibratory disturbances, 



