ence is one in the structure
of the atomic elements. If one imagines them to be vortex-rings, they
may differ in size, thickness, and rate of rotation; either of these
might make all the observed difference between the elements, including
their density. In the second way, density implies compactness of
molecules. Thus if a cubic foot of air be compressed until it occupies
but half a cubic foot, each cubic inch will have twice as many molecules
in it as at first. The amount of air per unit volume will have been
doubled, the weight will have been doubled, the amount of matter as
determined by its weight will have been doubled, and consequently we say
its density has been doubled.

If a bullet or a piece of iron be hammered, the molecules are compacted
closer together, and a greater number can be got into a cubic inch when
so condensed. In this sense, then, density means the number of molecules
in a unit of space, a cubic inch or cubic centimeter. There is implied
in this latter case that the molecules do not occupy all the available
space, that they may have varying degrees of closeness; in other words,
matter is discontinuous, and therefore there may be degrees in density.

THE ETHER HAS DENSITY.

It is common to have the degree of density of the ether spoken of in the
same way, and for the same reason, that its elasticity is spoken of. The
rate of transmission of a physical disturbance, as of a pressure or a
wave-motion in matter, is conditioned by its degree of density; that is,
the amount of matter per cubic inch as determined by its weight; the
greater the density the slower the rate. So if rate of speed and
elasticity be known, the density may be computed. In this way the
density of the ether has been deduced by noting the velocity of light.
The enormous velocity is supposed to prove that its density is very
small, even when compared with hydrogen. This is stated to be about
equal to that of the air at the height of two hundred and ten miles
above the surface of the earth, where the air molecules are so few that
a molecule might travel for 60,000,000 miles without coming in collision
with another molecule. In air of ordinary density, a molecule can on the
average move no further than about the two-hundred-and-fifty-thousandth
of an inch without such collision. It is plain the density of the ether
is so far removed from the density of anything we can measure, that it
is hardly comparable with such things. If, in