motion are finely mixed together, so that on a large scale the mass is homogeneous, having within any sensible volume an equal amount of vortex-motion in all directions. To a fluid having such a type of motion he gave the name vortex-sponge.
Five years later, FitzGerald[1] discussed the suitability of the vortex-sponge as a model of the aether. Since vorticity in a perfect fluid cannot be created or destroyed, the modification of the system which is to be analogous to an electric field must be a polarized state of the vortex motion, and light must be represented by a communication of this polarized motion from one part of the medium to another. Many distinct types of polarization may readily be imagined: for instance, if the turbulent motion were constituted of vortex-rings, these might be in motion parallel to definite lines or planes; or if it were. constituted of long vortex filaments, the filaments might be bent spirally about axes parallel to a given direction. The energy of any polarized state of vortex-motion would be greater than that of the unpolarized state; so that if the motion of matter had the effect of reducing the polarization, there would be forces tending to produce that motion. Since the forces due to a small vortex vary inversely as a high power of the distance from it, it seems probable that in the case of two infinite planes, separated by a region of polarized vortex-motion, the forces due to the polarization between the planes would depend on the polarization, but not on the mutual distance of the planes—a property which characteristic of plane distributions whose elements attract according to the Newtonian law.
It is possible to conceive polarized forms of vortex-motion which are steady so far as the interior of the medium is concerned, but which tend to yield up their energy in producing motion of its boundary—a property parallel to that of the aether, which, though itself in equilibrium, tends to move objects immersed in it.
In the same year Hicks[2] discussed the possibility of trans-
