32 FRAGMENTS OF SCIENCE. 



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 colours 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 colours origin- 

 ate, but the mind irresistibly infers that the appearance 

 of the colours 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 vibra- 

 tion. 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 agita- 

 tion 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. 9 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 



