RADIATION 37 



of colors, red, orange, yellow, green, blue, indigo, violet; 

 thirdly, of ultra-violet rays which, like the ultra-red ones, 

 are incompetent to excite vision, but which, unlike the 

 ultra- red rays, possess a very feeble heating power. In 

 consequence, however, of their chemical energy these ultra- 

 violet rays are of the utmost importance to the organic 

 world. 



2. Origin and Character of Radiation. The Ether 



When we see a platinum wire raised gradually to a 

 white heat, and emitting in succession all the colors of the 

 spectrum, we are simply conscious of a series of changes in 

 the condition of our own eyes. "We do not see the actions 

 in which these successive colors originate, but the mind 

 irresistibly infers that the appearance of the colors corre- 

 sponds to certain contemporaneous changes in the wire. 

 What is the nature of these changes? In virtue of what 

 condition does the wire radiate at all ? We must now 

 look from the wire, as a whole, to its constituent atoms. 

 Could we see those atoms, even before the electric current 

 has begun to act upon them, we should find them in a 

 state of vibration. In this vibration, indeed, consists such 

 warmth as the wire then possesses. Locke enunciated this 

 idea with great precision, and it has been placed beyond 

 the pale of doubt by the excellent quantitative researches 

 of Mr. Joule. "Heat," says Locke, "is a very brisk agi- 

 tation of the insensible parts of the object, which produce 

 in us that sensation from which we denominate the object 

 hot: so what in our sensations is heat in the object is noth- 

 ing but motion. 11 When the electric current, still feeble, 

 begins to pass through the wire, its first act is to intensify 

 the vibrations already existing, by causing the atoms to 



