RADIATION. 23 
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, jconsists 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 onr 
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 
