rent type from that belonging to matter, and cannot be defined in
any such terms as are employed for matter.

If one considers gravitative phenomena, the difficulty is enormously
increased. The orbit of a planet is never an exact ellipse,
on account of the perturbations produced by the planetary
attractions--perturbations which depend upon the direction and distance
of the attracting bodies. These, however, are so well known that slight
deviations are easily noticed. If gravitative attraction took any such
appreciable time to go from one astronomical body to another as does
light, it would make very considerable differences in the paths of the
planets and the earth. Indeed, if the velocity of gravitation were less
than a million times greater than that of light, its effects would have
been discovered long ago. It is therefore considered that the velocity
of gravitation cannot be less than 186000,000000 miles per second. How
much greater it may be no one can guess. Seeing that gravitation is
ether-pressure, it does not seem probable that its velocity can be
infinite. However that may be, the ability of the ether to transmit
pressure and various disturbances, evidently depends upon properties so
different from those that enable matter to transmit disturbances that
they deserve to be called by different names. To speak of the elasticity
of the ether may serve to express the fact that energy may be
transmitted at a finite rate in it, but it can only mislead one's
thinking if he imagines the process to be similar to energy transmission
in a mass of matter. The two processes are incomparable. No other word
has been suggested, and perhaps it is not needful for most scientific
purposes that another should be adopted, but the inappropriateness of
the one word for the different phenomena has long been felt.

14. MATTER HAS DENSITY.

This quality is exhibited in two ways in matter. In the first, the
different elements in their atomic form have different masses or atomic
weights. An atom of oxygen weighs sixteen times as much as an atom of
hydrogen; that is, it has sixteen times as much matter, as determined by
weight, as the hydrogen atom has, or it takes sixteen times as many
hydrogen atoms to make a pound as it takes of oxygen atoms. This is
generally expressed by saying that oxygen has sixteen times the density
of hydrogen. In like manner, iron has fifty-six times the density, and
gold one hundred and ninety-six. The differ