For a long time, philosophers who rejected the idea that ponderability
is a primary and essential quality of all bodies have sought to reduce
their weight to pressures exercised in a very subtle fluid. This 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.
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