r
without or with the rotational quality corresponding to that of the true
luminiferous ether in the magnetic fluid--in short, do all by the
perforated solids with circulations through them that we saw we could do by
means of linked gyrostats. But something that we cannot do by linked
gyrostats we can do by the perforated bodies with fluid circulation: we can
make a model gas. The mutual action at a distance, repulsive or attractive
according to the mutual aspect of the two bodies when passing within
collisional distance[1] of one another, suffices to produce the change of
direction of motion in collision, which essentially constitutes the
foundation of the kinetic theory of gases, and which, as we have seen
before, may as well be due to attraction as to repulsion, so far as we know
from any investigation hitherto made in this theory.

[Footnote 1: According to this view, there is no precise distance, or
definite condition respecting the distance, between two molecules, at which
apparently they come to be in collision, or when receding from one another
they cease to be in collision. It is convenient, however, in the kinetic
theory of gases, to adopt arbitrarily a precise definition of collision,
according to which two bodies or particles mutually acting at a distance
may be said to be in collision when their mutual action exceeds some
definite arbitrarily assigned limit, as, for example, when the radius of
curvature of the path of either body is less than a stated fraction (one
one-hundredth, for instance) of the distance between them.]

There remains, however, as we have seen before, the difficulty of providing
for the case of actual impacts between the solids, which must be done by
giving them massless spring buffers or, which amounts to the same thing,
attributing to them repulsive forces sufficiently powerful at very short
distances to absolutely prevent impacts between solid and solid; unless we
adopt the equally repugnant idea of infinitely small perforated solids,
with infinitely great fluid circulations through them. Were it not for this
fundamental difficulty, the hydro-kinetic model gas would be exceedingly
interesting; and, though we could scarcely adopt it as conceivably a true
representation of what gases really are, it might still have some
importance as a model configuration of solid and liquid matter, by which
without elasticity the elasticity of true gas might be represented.

But lastly, since the hyd