bethought him of the frictionless medium which
physicists had now begun to accept--the all-pervading ether. What
if vortex rings were started in this ether, must they not have the
properties which the vortex rings in air had exhibited--inertia,
attraction, elasticity? And are not these the properties of ordinary
tangible matter? Is it not probable, then, that what we call matter
consists merely of aggregations of infinitesimal vortex rings in the
ether?

Thus the vortex theory of atoms took form in Lord Kelvin's mind, and its
expression gave the world what many philosophers of our time regard as
the most plausible conception of the constitution of matter hitherto
formulated. It is only a theory, to be sure; its author would be the
last person to claim finality for it. "It is only a dream," Lord Kelvin
said to me, in referring to it not long ago. But it has a basis in
mathematical calculation and in analogical experiment such as no other
theory of matter can lay claim to, and it has a unifying or monistic
tendency that makes it, for the philosophical mind, little less than
fascinating. True or false, it is the definitive theory of matter of the
twentieth century.

Quite aside from the question of the exact constitution of the ultimate
particles of matter, questions as to the distribution of such particles,
their mutual relations, properties, and actions, came in for a full
share of attention during the nineteenth century, though the foundations
for the modern speculations were furnished in a previous epoch. The most
popular eighteenth-century speculation as to the ultimate constitution
of matter was that of the learned Italian priest, Roger Joseph
Boscovich, published in 1758, in his Theoria Philosophiae Naturalis.
"In this theory," according to an early commentator, "the whole mass of
which the bodies of the universe are composed is supposed to consist
of an exceedingly great yet finite number of simple, indivisible,
inextended atoms. These atoms are endued by the Creator with REPULSIVE
and ATTRACTIVE forces, which vary according to the distance. At very
small distances the particles of matter repel each other; and this
repulsive force increases beyond all limits as the distances are
diminished, and will consequently forever prevent actual contact. When
the particles of matter are removed to sensible distances, the repulsive
is exchanged for an attractive force, which decreases in inverse ratio
with the squares of the