was
the conception of Descartes, and was perhaps the true idea of Newton
himself. Newton points out, in many passages, that the laws he had
discovered were independent of the hypotheses that could be formed on
the way in which universal attraction was produced, but that with
sufficient experiments the true cause of this attraction might one day
be reached. In the preface to the second edition of the Optics he
writes: "To prove that I have not considered weight as a universal
property of bodies, I have added a question as to its cause,
preferring this form of question because my interpretation does not
entirely satisfy me in the absence of experiment"; and he puts the
question in this shape: "Is not this medium (the ether) more rarefied
in the interior of dense bodies like the sun, the planets, the comets,
than in the empty spaces which separate them? Passing from these
bodies to great distances, does it not become continually denser, and
in that way does it not produce the weight of these great bodies with
regard to each other and of their parts with regard to these bodies,
each body tending to leave the most dense for the most rarefied
parts?"

Evidently this view is incomplete, but we may endeavour to state it
precisely. If we admit that this medium, the properties of which would
explain the attraction, is the same as the luminous ether, we may
first ask ourselves whether the action of gravitation is itself also
due to oscillations. Some authors have endeavoured to found a theory
on this hypothesis, but we are immediately brought face to face with
very serious difficulties. Gravity appears, in fact, to present quite
exceptional characteristics. No agent, not even those which depend
upon the ether, such as light and electricity, has any influence on
its action or its direction. All bodies are, so to speak, absolutely
transparent to universal attraction, and no experiment has succeeded
in demonstrating that its propagation is not instantaneous. From
various astronomical observations, Laplace concluded that its
velocity, in any case, must exceed fifty million times that of light.
It is subject neither to reflection nor to refraction; it is
independent of the structure of bodies; and not only is it
inexhaustible, but also (as is pointed out, according to M. Hannequin,
by an English scholar, James Croll) the distribution of the effects of
the attracting force of a mass over the manifold particles which may
successively e