RADIATION. 33 



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 spec- 

 trum, 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 irre- 

 sistibly infers that the appearance of the colors corresponds 

 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 pre- 

 cision, 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 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 

 sensations 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 amplitudes of the 

 oscillations are increased. The current does this, however, 

 without altering the periods of the old vibrations, or the 

 times in which they were executed. But besides intensi- 

 fying the old vibrations the current generates 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 



