RADIATION. 35 



merits of an orchestra, transmitting each vibration of 

 every pipe and string, so does the inter-stellar aether 

 accommodate itself to the requirements of light and 

 heat. Its waves mingle in space without disorder, 

 each being endowed with an individuality as inde- 

 structible 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. 

 Eadiation is the communication of vibratory motion to 

 the aether ; 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 im- 

 parting it to the medium in which they vibrate. On 

 the other hand, the waves of aether 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 aether to the molecules 

 consists the absorption of radiant heat. All the pheno- 

 mena of heat are in this way reducible to interchanges 

 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 dfther. 



The word ( atoms ' has been more than once em- 

 ployed in this discourse. Chemists have taught us that 

 all matter is reducible to certain elementary forms to 

 which they give this name. These atoms are endowed 

 with powers of mutual attraction, and under suitable 

 circumstances they coalesce to form compounds. Thus 

 oxygen and hydrogen are elements when separate, or 



D 2 



