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 ever}- 

 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- 

 dividual^ 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 they 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 



