456 On the Conservation of Force. [1857. 



required for friction ; we do not know how, but we search to 

 know, not being willing to admit that the electric force can 

 arise out of nothing. The two electricities are developed in 

 equal proportions ; and having appeared, we may dispose 

 variously of the influence of one upon successive portions of 

 the other, causing many changes in relation, yet never able to 

 make the sum of the force of one kind in the least degree ex- 

 ceed or come short of the sum of the other. In that necessity 

 of equality, we see another direct proof of the conservation of 

 force, in the midst of a thousand changes that require to be 

 developed in their principles before we can consider this part 

 of science as even moderately known to us. 



One assumption with regard to electricity is, that there is 

 an electric fluid rendered evident by excitement in plus and 

 minus proportions. Another assumption is, that there are two 

 fluids of electricity, each particle of each repelling all particles 

 like itself, and attracting all particles of the other kind always, 

 and with a force proportionate to the inverse square of the di- 

 stance, being so far analogous to the definition of gravity. 

 This hypothesis is antagonistic to the law of the conservation of 

 force, and open to all the objections that have been, or may 

 be, made against the ordinary definition of gravity. Another 

 assumption is, that each particle of the two electricities has a 

 given amount of power, and can only attract contrary particles 

 with the sum of that amount, acting upon each of two with 

 only half the power it could in like circumstances exert upon 

 one. But various as are the assumptions, the conservation of 

 force (though wanting in the second) is, I think, intended to 

 be included in all. I might repeat the same observations 

 nearly in regard to magnetism, whether it be assumed as a 

 fluid, or two fluids or electric currents, whether the external 

 action be supposed to be action at a distance, or dependent on 

 an external condition and lines of force still all are intended 

 to admit the conservation of power as a principle to which the 

 phenomena are subject. 



The principles of physical knowledge are now so far de- 

 veloped as to enable us not merely to define or describe the 

 known t but to state reasonable expectations regarding the 

 unknown ; and I think the principle of conservation of force 

 may greatly aid experimental philosophers in that duty to 



