RADIATION. 25 



which the science of mechanics does not even tend to un- 

 ravel, the tremor 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 centers cross, coincide, 

 oppose, and pass through each other, without confusion or 

 ultimate extinction. Every star is seen across the entangle- 

 ment of wave-motions produced by all other stars. It is 

 the ceaseless thrill caused by those distant orbs collectively 

 in the ether, that constitutes what we call the " tempera- 

 ture of space." As the air of a room accommodates itself to 

 the requirements of an orchestra, transmitting each vibra- 

 tion 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 en- 

 dowed with an individuality as indestructible as if it alone 

 had disturbed the universal repose. 



All vagueness with regard to the use of the terms 

 " radiation " and " absorption " will now disappear. Radia- 

 tion 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 motion, by imparting it to the medium in 

 which they vibrate. On the other hand, the waves of 

 ether 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 consists the absorption of radiant heat. All the 

 phenomena of heat are in this way reducible to inter- 

 changes of motion; and it is purely as the recipients or 

 the donors of this motion, that we ourselves become con- 

 scious of the action of heat and cold. 



3. The Atomic Theory in reference to the Ether. 



The word " atoms "has been more than once employed 

 in this discourse. Chemists have taught us that all mat- 

 ter is reducible to certain elementary forms to which they 

 give this name. These atoms are endowed with powers of 



