whereby a certain amount of energy is required to produce a
certain change of motion. In addition to this, any two atoms could
attract or repel each other with a force depending upon their distance
apart. The Law which regulates these forces for all distances greater
than 1/1000 of an inch is an attraction varying inversely as the square
of the distance, and a repulsion for less distances.

We have, therefore, to suppose that in place of the hard atom, there is
merely a geometrical point which can exert attractive or repulsive
forces to, or from, the central point. So far as external particles are
concerned, they would behave just the same as a hard atom would do. This
conception was largely entertained in recent times by Faraday. It is
more a mathematical explanation than a physical one, but has been found
convenient in explaining what takes place in the interior of bodies in
their three states, namely: solid, liquid, and gaseous.

_Lord Kelvin's Vortex Atom._--Another hypothesis which has been
suggested for the constitution of an atom, is that known as the Vortex
Atom, which received its birth at the hands of Lord Kelvin. The
underlying principle of this Vortex Atom is, that matter may be entirely
due to the rotating parts of a fundamental medium, the Aether, which
fills all space.

The properties of vortex motion were first mathematically calculated by
Helmholtz, but it was left to Sir Wm. Thompson, now Lord Kelvin, to give
a physical idea of the Vortex Atom.

Before entering further into the question of the Vortex Atom, it may be
as well to explain how vortex smoke rings may be made.

[Illustration: Fig: 3.]

All that is required is a wooden box, about one cubic foot in size, with
a round hole perforated in one of the sides, and the opposite side
covered with a piece of linen in place of the wooden side. The bottom of
the box should then be covered with some strong solution of ammonia, and
some hydrochloric acid poured into a saucer and put into the box. The
combination of these two will cause thick clouds to form in the box, and
if the linen is sharply tapped by the hand, a ring of this cloud will be
forced through the hole on the opposite side of the box. The ring so
formed will be circular in shape, and will go sailing through the room
in which it is made.

When the hole is circular, the rings will be found circular also, but if
the hole is square, then the rings will be irregular in shape. One
remarkable