generations remained practically
unnoticed. The philosophers of the first half of our century seem
to have despaired of explaining gravitation, though Faraday long
experimented in the hope of establishing a relation between gravitation
and electricity or magnetism. But not long after the middle of
the century, when a new science of dynamics was claiming paramount
importance, and physicists were striving to express all tangible
phenomena intenus of matter in motion, the theory of Le Sage was
revived and given a large measure of attention. It seemed to have at
least the merit of explaining the facts without conflicting with any
known mechanical law, which was more than could be said of any other
guess at the question that had ever been made.

More recently, however, another explanation has been found which also
meets this condition. It is a conception based, like most other physical
speculations of the last generation, upon the hypothesis of the vortex
atom, and was suggested, no doubt, by those speculations which consider
electricity and magnetism to be conditions of strain or twist in
the substance of the universal ether. In a word, it supposes that
gravitation also is a form of strain in this ether--a strain that may be
likened to a suction which the vortex atom is supposed to exert on the
ether in which it lies. According to this view, gravitation is not
a push from without, but a pull from within; not due to exterior
influences, but an inherent and indissoluble property of matter itself.
The conception has the further merit of correlating gravitation with
electricity, magnetism, and light, as a condition of that strange
ethereal ocean of which modern physics takes so much account. But
here, again, clearly, we are but heaping hypothesis upon hypothesis,
as before. Still, an hypothesis that violates no known law and has the
warrant of philosophical probability is always worthy of a hearing. But
we must not forget that it is hypothesis only, not conclusive theory.

The same caution applies, manifestly, to all the other speculations
which have the vortex atom, so to say, for their foundation-stone. Thus
Professors Stewart and Tait's inferences as to the destructibility
of matter, based on the supposition that the ether is not quite
frictionless; Professor Dolbear's suggestions as to the creation of
matter through the development of new ether ripples, and the same
thinker's speculations as to an upper limit of temperatur