of this medium must not be considered as a vague or
fanciful conception on the part of scientific men. Of its reality
most of them are as convinced as they are of the existence of the sun
and moon. The luminiferous aether has definite mechanical properties.
It is almost infinitely more attenuated than any known gas, but its
properties are those of a solid rather than of a gas. It resembles
jelly rather than air. This was not the first conception of the
aether, but it is that forced upon us by a more complete knowledge of
its phenomena. A body thus constituted may have its boundaries; but,
although the aether may not be co-extensive with space, it must at all
events extend as far as the most distant visible stars. In fact it is
the vehicle of their light, and without it they could not be seen.
This all-pervading substance takes up their molecular tremors, and
conveys them with inconceivable rapidity to our organs of vision. It
is the transported shiver of bodies countless millions of miles
distant, which translates itself in human consciousness into the
splendour of the firmament at night.

If the aether have a boundary, masses of ponderable matter might be
conceived to exist beyond it, but they could emit no light. Beyond
the aether dark suns might burn; there, under proper conditions,
combustion might be carried on; fuel might consume unseen, and metals
be fused in invisible fires. A body, moreover, once heated there,
would continue for ever heated; a sun or planet once molten, would
continue for ever molten. For, the loss of heat being simply the
abstraction of molecular motion by the aether, where this medium is
absent no cooling could occur. A sentient being on approaching a
heated body in this region, would be conscious of no augmentation of
temperature. The gradations of warmth dependent on the laws of
radiation would not exist, and actual contact would first reveal the
heat of an extra ethereal sun.

Imagine a paddle-wheel placed in water and caused to rotate. From it,
as a centre, waves would issue in all directions, and a wader as he
approached the place of disturbance would be met by stronger and
stronger waves. This gradual augmentation of the impression made upon
the wader is exactly analogous to the augmentation of light when we
approach a luminous source. In the one case, however, the coarse
common nerves of the body suffice; for the other we must have the
finer optic nerve. But suppose t