i88 3 J 
The Velocity of Light, 
515 
so hid away in the general solid that we may well say light 
and even radiant heat are absolutely reflected by the solid 
earth ; they do not penetrate except as conducted heat, with 
vibrations whose axes are less than the mean radii of the 
molecules. 
Even liquids, the leading one again being water, are as 
often an obstacle as a medium for their propagation ; undu- 
lation and actual transport get confounded. The true 
medium for light and radiant heat, making them two in one, 
is vacuum, in which they are zero, distance, and matter in 
the form of gas, and foremost that great mass of gases we 
call the air, the atmosphere itself being — by its ascending 
vapour of water — a still more rapid, or the real, propagator, 
and by its water, in the form of vesicles, drops, solids, and 
vapour in absorption, an obstacle to the rapidity and effect- 
iveness of propagation of light and radiant heat. 
That relative quantities now to be used, and described in 
this causation in “ On some Properties of the Earth,” have 
been obtained without any regard to sound and light, but 
with reference to gravity and heat, is in favour of their cor- 
rectness and of my views. 
We have to consider the limited earth as an individual of 
a certain mass, density, shape, and inner distribution of 
force, i. e., matter ; and any planet, moon, comet, and 
meteor, and any star and nebula exchanging light and heat 
with the solar system, as another individual. Such indivi- 
dual is, in the vacuum of the heavens, a medium for the 
propagation of outside and inside light and heat, consisting 
itself of various media. And as the velocity of light is not 
the same in different media, it will, without time in vacuo, 
not aCt beyond vacuum in equal time on those individuals ; 
and even with regard to the same individual, indifferently 
from where it is generated, or reflected and re-transmitted, the 
time which elapses before it becomes sensible for the receiver 
will depend on the rectilinear interaction of the generator 
and receiver, or on their distance and change in distance. 
The ear is a terrestrian organ ; it finished with the velocity 
of absolute c.entrifugality of a particle of the earth : the eye 
is a solar terrestrian organ, and this velocity enters as a first 
term in the velocity of light ; it is the velocity of resistance 
to solar differentiation and induction, productive of light 
within the medium earth ; and as with the earth so with 
other bodies, and with any particle according to its micro- 
cosmos. 
Light cannot be generated where there is no atmosphere 
and no liquidity from which it is developed, no actual 
