motion. Hence, we have a _distinction_ applicable to the difference of
momentum of luminous and calorific rays. The velocity of a wave of sound
through the atmosphere, is the same for the deep-toned thunder and the
shrillest whistle,--being dependent on the density of the medium, and
not on the source from which it emanates. So it is in the ethereal
medium.

This view is in accordance with the experiments of M. Delaroche and
Melloni, on the transmission of light and heat through diaphanous
bodies--the more calorific rays feeling more and more the influence of
thickness, showing that more motion was imparted to the particles of the
diaphanous substance by the rays possessing more material momentum, and
still more when the temperature of the radiating body was low, evidently
analogous to the illustration we have cited. Light may therefore be
regarded as the effect of the vibration of atoms having little mass, and
as this mass increases, the rays become more calorific, and finally the
calorific effect is the only evidence of their existence; as towards the
extreme red end of the spectrum they cease to be visible, owing to their
inability to impart their vibrations to the optic nerve. This may also
influence the law of gravitation. In this we have also an explanation of
the dispersion of light. The rays proceeding from atoms of small mass
having less material momentum, are the most refrangible, and those
possessing greater material momentum, are the least refrangible; so that
instead of presenting a difficulty in the undulatory theory of light,
this dispersion is a necessary consequence of its first principles.

It is inferred from the experiments cited, and the facts ascertained by
them, viz.: that the velocity of light in water is less than its
velocity in air; that the density of the ether is greater in the first
case; but this by no means follows. We have advocated the idea, that the
ethereal medium is less dense within a refracting body than without. We
regard it as a fundamental principle. Taking the free ether of heaven;
the vibrations in the denser ether will no doubt be slowest; but within
a refracting body we must consider there is motion lost, or _light
absorbed_, and the time of the transmission is thus increased.

There has been a phenomenon observed in transits of Mercury and Venus
across the sun, of which no explanation has been rendered by
astronomers. When these planets are visible on the solar disc, the