why of
gravitation--that he can no more explain why a stone tossed into the
air falls back to earth than can the boy who tosses the stone. But while
this statement puts in a nutshell the scientific status of explanations
of gravitation, yet it is not in human nature that speculative
scientists should refrain from the effort to explain it. Such efforts
have been made; yet, on the whole, they are surprisingly few in number;
indeed, there are but two that need claim our attention here, and one
of these has hardly more than historical interest. One of these is the
so-called ultramundane-corpuscle hypothesis of Le Sage; the other is
based on the vortex theory of matter.

The theory of Le Sage assumes that the entire universe is filled with
infinitely minute particles flying in right lines in every direction
with inconceivable rapidity. Every mass of tangible matter in the
universe is incessantly bombarded by these particles, but any two
non-contiguous masses (whether separated by an infinitesimal space or by
the limits of the universe) are mutually shielded by one another from a
certain number of the particles, and thus impelled towards one another
by the excess of bombardment on their opposite sides. What applies to
two masses applies also, of course, to any number of masses--in short,
to all the matter in the universe. To make the hypothesis workable, so
to say, it is necessary to assume that the "ultramundane" particles are
possessed of absolute elasticity, so that they rebound from one another
on collision without loss of speed. It is also necessary to assume that
all tangible matter has to an almost unthinkable degree a sievelike
texture, so that the vast proportion of the coercive particles pass
entirely through the body of any mass they encounter--a star or world,
for example--without really touching any part of its actual substance.
This assumption is necessary because gravitation takes no account of
mere corporeal bulk, but only of mass or ultimate solidarity. Thus a
very bulky object may be so closely meshed that it retards
relatively few of the corpuscles, and hence gravitates with relative
feebleness--or, to adopt a more familiar mode of expression, is light in
weight.

This is certainly heaping hypotheses together in a reckless way, and
it is perhaps not surprising that Le Sage's conception did not at first
arouse any very great amount of interest. It was put forward about
a century ago, but for two or three