axis is in the
direction of the magnetic force, forms part of the phenomenon. This
angular velocity cannot be any portion of the medium of sensible
dimensions rotating as a whole. We must, therefore, conceive the
rotation to be that of very small portions of the medium, each rotating
on its own axis.

This is the hypothesis of molecular vortices. The displacements of the
medium during the propagation of light will produce a disturbance of the
vortices, and the vortices, when so disturbed, may react on the medium
so as to affect the propagation of the ray. The theory proposed is of a
provisional kind, resting as it does on unproved hypotheses relating to
the nature of molecular vortices, and the mode in which they are
affected by the displacement of the medium.

_IV.--Action at a Distance_

There appears to be some prejudice, or _a priori_ objection, against the
hypothesis of a medium in which the phenomena of radiation of light and
heat, and the electric actions at a distance, take place. It is true
that at one time those who speculated as to the cause of physical
phenomena were in the habit of accounting for each kind of action at a
distance by means of a special aethereal fluid, whose function and
property it was to produce these actions. They filled all space three
and four times over with aethers of different kinds, the properties of
which consisted merely to "save appearances," so that more rational
inquirers were willing to accept not only Newton's definite law of
attraction at a distance, but even the dogma of Cotes that action at a
distance is one of the primary properties of matter, and that no
explanation can be more intelligible than this fact. Hence the
undulatory theory of light has met with much opposition, directed not
against its failure to explain the phenomena, but against its assumption
of the existence of a medium in which light is propagated.

The mathematical expression for electro-dynamic action led, in the mind
of Gauss, to the conviction that a theory of the propagation of electric
action would in time be found to be the very keystone of
electro-dynamics. Now, we are unable to conceive of propagation in time,
except either as the flight of a material substance through space or as
the propagation of a condition of motion or stress in a medium already
existing in space.

In the theory of Neumann, the mathematical conception called potential,
which we are unable to conceive as a material s