EADIATION. 171 



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 seems placed beyond the 

 pale of doubt by the excellent quantitative researches of 

 Mr. Joule. "Heat," says Locke, "is a very brisk agitation 

 of the insensible parts of the object, which produce in us 

 that sensation from which we denominate the object hot : 

 so what in our sensation is heat in the object is nothing 

 but motion" 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 swing 

 through wider ranges. Technically speaking, the ampli- 

 tudes of the oscillations are increased. The current does 

 this, however, without altering the periods of the old vi- 

 brations, or the times in which they were executed. But 

 besides intensifying the old vibrations the current gener- 

 ates new and more rapid ones, and when a certain definite 

 rapidity has been attained the wire begins to glow. The 

 color first exhibited is red, which corresponds to the lowest 

 rate of vibration of which the eye is able to take cognizance. 

 By augmenting the strength of the electric current more 

 rapid vibrations are introduced, and orange rays appear. 

 A quicker rate of vibration produces yellow, a still quicker 

 green ; and by further augmenting the rapidity, we pass 

 through blue, indigo, and violet, to the extreme ultra-violet 

 rays. 



Such are the changes which science recognizes in the 

 wire itself, as concurrent with the visual changes taking 



