3. _Induction of Force._--The inference of _Mechanical Forces_ from phenomena has been so abundantly practised, that it is perfectly familiar among scientific inquirers. From the time of Newton, it has been the most common aim of mathematicians; and a persuasion has grown up among them, that mechanical forces,--attraction and repulsion,--are the only modes of action of the particles of bodies which we shall ultimately have to consider. I have attempted to show that this mode of conception is inadequate to the purposes of sound philosophy;--that the Particles of crystals, and the Elements of chemical compounds, must be supposed to be combined in some other way than by mere mechanical attraction and repulsion. Dr. Faraday has gone further in shaking the usual conceptions of the force exerted, in well-known cases. Among the most noted and conspicuous instances of attraction and repulsion exerted at a distance, were those which take place between electrized bodies. But the eminent electrician just mentioned has endeavoured to establish, by experiments of which it is very difficult to elude the weight, that the action in these cases does not take place at a distance, but is the result of a chain of intermediate particles connected at every point by forces of another kind. 4. _Induction of Polarity._--The forces to which Dr. Faraday ascribes the action in these cases are _Polar Forces_[57\3]. We have already endeavoured to explain the Idea of Polar Forces; which implies[58\3] that at every point forces exactly equal act in opposite directions; and thus, in the greater part of their course, neutralize and conceal each other; while at the extremities of the line, being by some cause liberated, they are manifested, still equal and opposite. And the criterion by which this polar character of forces is recognized, is implied in the reasoning of Faraday, on the question of one or two electricities, of which we {250} formerly spoke[59\3]. The maxim is this:--that in the action of polar forces, along with every manifestation of force or property, there exists a corresponding and simultaneous manifestation of an equal and opposite force or property. [Note 57\3: _Researches_, 12th series.] [Note 58\3: B. v. c. i.] [Note 59\3: Book v. c. i.]