When we now consider how various, how delicate, and how detached from each other, are the pheno mena this theory embraces; with what exactness also, it represents them, and follows, in a manner, all the windings of experiment, we must be convin ced that it is one of the best established in physics, and that it bestows on the real existence of the two electric fluids the highest degree of probability, if not an absolute certainty. But what is not less va luable for science, it teaches us to fix, by exact de finitions, the true meaning which we must attach to certain elements of the electrical phenomena, which are too often vaguely enunciated, or even confound ed, with others; although the knowledge of each of them, individually, is indispensable to form a correct and general idea of the phenomena.
The first of these elements is the species, vitreous or resinous, of the electricity which exists at the sur face of an electrified body, and at every point of this surface, This is determined by touching it with the proof plane, and presenting this to the needle of the electroscope, already charged with a known species of electricity.
The second element is the quantity of this electri city accumulated on every point, or, what comes to the same thing, the thickness of the electric stratum. This we still measure by touching the body with the proof plane, and communicating the electricity ac quired by this contact to the fixed ball of the electric balance, the moveable one having been previously charged with electricity of the same nature. The force of torsion necessary to balance the electric re action communicated by the plane to the fixed ball, is at equal distances proportional to the quantity of electricity which it possesses, or, what is the same thing, to the thickness of the electric stratum on the element of the surface which it has touched.
The third element which it is of importance to consider in the phenomena, is the attractive or re pulsive action exerted by each element of the elec tric stratum upon a particle of the fluid situated at its exterior surface or beyond this surface. This at traction or repulsion is directly proportional to the thickness o the electric stratum on the superficial element which attracts or repels, and is inversely proportional to the square of the distance which se parates this element from the point attracted or re pelled.
In fine, the last element to be considered, and which is a consequence of the preceding ones, is the pressure which the electricity exerts against the ex ternal air in each ce point of the surfa of the electri fied body. The intensity of this pressure is propor
tional to the square of the thickness of the electric stratum.
By adhering strictly to these denominations, there will be no risk of falling into error from vague con siderations ; and if we also keep in mind the deve lopement of electricity by influence at a distance, we shall then find no difficulty in explaining all the elec tric phenomena.
To place this truth in its full light, we shall apply it to some general phenomena which, view ed in this manner, can be conceived with perfect clearness, but which, otherwise, do not admit but of vague and embarrassed explications. These phenomena consist in the motions which elec trified bodies assume, or tend to assume, when they are placed in presence of each other, and in which they appear as if they really acted upon each other by attraction or by repulsion. But it is extreme ly difficult to conceive the cause of these movements, when we consider that, according to the experi ments, the attraction and repulsion are only exert ed between the electric principles themselves, with out the material substance of the body, provided it be a conductor, having any influence on their distri bution or their displacement. We cannot hence ad , mit, that the particles of the electric principles, what ever they may be, really attract or repel the mate rial particles of the bodies. It is absolutely neces sary, therefore, that the attractive and repulsive ac tions of these principles, whatever they are, be trans mitted indirectly to the material bodies, by some me chanism which it is of extreme importance to dis cover, as it is the true key to these phenomena. But we will see that this mechanism consists in the reac tion produced by the resistance which the air and non-conducting bodies in general oppose to the pas sage of electricity.
For the sake of greater simplicity, we may first confine ourselves to the consideration of two electri fied spheres A and B ; the one A flied, the other B moveable. Three cases may arise which it is neces sary to discuss separately.
In the first case, the elettric particles are fixed upon the bodies A and B, by the unknown force which produces the nen-conductibility. Unable to quit these bodies, they divide with them the motions which their reciprocal action tends to impress upon themselves.