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RADIATION. 173

cess which science does not even. tend to unravel, the tre-
mor of the nervous matter is converted into the conscious
impression of light.

Darkness might then be defined as ether at rest; light
as ether in motion. But in reality the ether is never at
rest, for in the absence of light-waves we have heat-waves
always speeding through it. In the spaces of the universe
both classes of undulations incessantly commingle. Here
the waves issuing from uncounted centres cross, coincide,
oppose, and pass through each other, without confusion or
ultimate extinction. The waves from the zenith do not
jostle out of existence those from the horizon, and every
star is seen across the entanglement of wave-motions pro-
duced by all other stars. It is the ceaseless thrill which
those distant orbs collectively create in the ether, which
constitutes what we call the temperature of space. As the
air of a room accommodates itself to the requirements of an
orchestra, transmitting each vibration of every pipe and
string, so does the inter-stellar ether accommodate itself to
the requirements of light and heat. Its waves mingle in
space without disorder, each being endowed with an in-
dividuality as indestructible as if it alone had disturbed the
universal repose.

All vagueness with regard to the use of the terms radia-
tion and absorption will now disappear. Radiation is the
communication of vibratory motion to the ether, and when
a body is said to be chilled by radiation, as for example the
grass of a meadow on a starlight night, the meaning is, that
the molecules of the grass have lost a portion of their mo-
tion, by imparting it to the medium in which theyr Vibrate.
On the other hand, the waves of ether once generated may
so strike against the molecules of a body exposed to their
action as to yield up their motion to the latter; and in this
transfer of the motion from the ether to the molecules con-
sists the absorption of radiant heat. All the phenomena

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