Just as the rigidity of the ether is of a purely electric character, and is not felt mechanically--since mechanically it is perfectly fluid,--so its density is likewise of an electromagnetic character, and again is not felt mechanically, because it cannot be moved by mechanical means. It is by far the most stationary body in existence; though it is endowed with high intrinsic energy of local movement, analogous to turbulence, conferring on it gyrostatic properties. Optically, its rigidity and density are both felt, since optical disturbances are essentially electromotive. Matter loads the ether optically, in accordance with the recognised fraction (μ²-1) / μ²; and this loading, being part and parcel of the _matter_, of course travels with it. It is the only part amenable to mechanical force. The mechanical density of matter is a very small portion of the etherial density; whereas the optical or electrical density of matter--being really that of ether affected by the intrinsic or constitutional electricity of matter--is not so small. The relative optical virtual density of the ether inside matter is measured by μ²; but it may be really a defect of elasticity, at least in non-magnetic materials. Electrical and optical effects depend upon _e_. Mechanical or inertia effects depend upon e². Electric charges can load the ether optically, quite appreciably; but as regards mechanical loading, the densest matter known is trivial and gossamer-like compared with the unmodified ether in the same space. _Massiveness of the Ether deduced from Electrical Principles._ Each electron, moving like a sphere through a fluid, has a certain mass associated with it; dependent on its size, and, at very high speeds, on its velocity also. If we treat the electron merely as a sphere moving through a perfect liquid, its behaviour is exactly as if its mass were increased by half that of the fluid displaced and the surrounding fluid were annihilated. Ether being incompressible, the density of fluid inside and outside an electron must be the same. So, dealing with it in this simplest fashion, the resultant inertia is half as great again as that of the volume of fluid corresponding to the electron: that is to say the effective mass is 2πρα³, where ρ is the uniform density. If an electron is of some other shape than a sphere, then the numerical part is modified, but remains of the same order of magnitude, so long as there are no sharp edges. * * * * * If, however, we consider the moving electron as generating circular lines of magnetic induction, by reason of some rotational property of the ether, and if we attribute all the magnetic inertia to the magnetic whirl thus caused round its path,--provisionally treating this whirl as an actual circulation of fluid excited by the locomotion,--then we shall proceed thus:--