Recent Theories of Electricity. 199 



mass to be a fundamental unit/ The failure of the mechan- 

 istic school has arisen from the attempt to explain the 

 nature of matter, the cause of its forces, and the properties 

 of atoms. However we may try to reason away the belief 

 in the objective reality of matter, our minds persistently 

 cling to the advantage, and even necessity, of such a postu- 

 late ; and we consciously or unconsciously endow any sub- 

 stitute of it with all the properties of matter, excepting its 

 name. 



Energy therefore, although it is an inalienable property 

 of matter, will remain a derived unit, the total quantity of 

 which is a constant. It is customary and convenient to 

 divide energy into two classes — kinetic and potential. The 

 measure of potential energy is usually taken to be the 

 product of the attractive force of two masses into a power of 

 their distance apart. In the majority of problems we can go 

 no further, but in certain cases, as for instance the pressure 

 of gases, we may express a portion of the energy of the 

 whole mass as due to the kinetic energy of small, or mole- 

 cular, portions of it. But the internal energy of a gas must 

 still be considered as strictly potential and incapable of 

 further explanation. Kinetic energy is mass into a function 

 of velocity and its formula is T — mcp^v). There is gradually 

 arising a tendency, which is well founded, to distinguish two 

 kinds of kinetic energy. The first is that produced by -the 

 momentum of a moving body, the vis viva whose measure is 

 lj'2mv 2 ; the other is the kinetic energy, which originates 

 in one body and is transferred through space, apparently 

 vacuous, to another body — in other words 5 the radiant energy 

 known as heat, light, and electricity. 



It has been the persistent attempt of physicists for centuries 

 to explain this radiant energy by mechanical analogies. And 

 this effort has fastened on the science an interminable series 

 of impossible fictitious aethers and mechanical atoms. The 

 most indefatigable labours of the greatest minds have been 

 spent to imagine an atom, which would serve satisfactorily 

 as a source and, at the same time, as a receptacle of radiant 

 energy, and an sether which would transfer it. Not one of 

 these models has been even partially adequate : the course 

 of the development has been steadily from the simple to the 

 complex, from the concrete to the abstract, from the physical 

 to the metaphysical, until the most recent atom is a complex 

 more intricate than a stellar cosmogony whose parts are an 

 entity called electricity, and the aether is an abstraction 

 devoid of any mechanical attributes. Out of all this con- 

 troversy we have gained the following facts: — Heat, light, 



