NATURAL PHILOSOPHY. 187 



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 developed as to en- 

 able us not merely to define or describe the known, but to state reasonable 

 expectations regarding the unknown ; and I think the principle of the con- 

 servation of force may greatiy aid experimental philosophers in that duty to 

 science, which consists in the enunciation of problems to be solved. It will 

 lead us, in any case where the force remaining unchanged in form is altered 

 in direction only, to look for the new disposition of the force ; as in the cases 

 of magnetism, static electricity, and perhaps gravity, and to ascertain that as 

 a whole it remains unchanged in amount or, if the original force disap- 

 pear, either altogether or in part, it will lead us to look for the new condition 

 or form of force which should result, and to develop its equivalency to the 

 force that has disappeared. Likewise, when force is developed, it will cause 

 us to consider the previously existing equivalent to the force so appear- 

 ing ; and many such cases there are in chemical action. When force dis- 

 appears, as in the electric or magnetic induction after more or less discharge, 

 or that of gravity with an increasing distance, it will suggest a research as to 

 whether the equivalent change is one Avithin the apparently acting bodies, or 

 one external (in part) to them. It will also raise up inquiry as to the nature 

 of the internal or external state, both before the change and after. If sup- 

 posed to be external, it will suggest the necessity of a physical process, by 

 which the power is communicated from body to body ; and in the case of 

 external action, will lead to the inquiry whether, in any case, there can be 

 truly action at a distance, or whether the ether, or some other medium, is not 

 necessarily present. 



We are not permitted as yet to see the nature of the source of physical 

 power, but we are allowed to see much of the consistency existing amongst 

 the various forms in which it is presented to us. Thus, if, in static electri- 

 city, we consider an act of induction, we can perceive the consistency of all 

 other like acts of induction with it. If we then take an electric current, and 

 compare it with this inductive effect, we see their relation and consistency. 

 In the same manner we have arrived at a knowledge of the consistency of 

 magnetism with electricity, and also of chemical action and of heat with all 

 the former ; and if we see not the consistency between gravitation with any 

 of these forms of force, I am strongly of the mind that it is because of our 

 ignorance only. How imperfect would our idea of an electric current now 

 be, if we were to leave out of sight its origin, its static and dynamic induc- 

 tion, its magnetic influence, its chemical and heating effects ; or our idea of 



