38 The Realm of Nature CHAP. 



set the particles of bodies in vibration when they fall on 

 them ; they are invisible to the eye and are known as heat- 

 waves. The shortest of the light- waves (v} produce the 

 effect of violet light, longer ones (b} blue, still longer (g) 

 green, longer yet (j) yellow, and the longest that produce 

 any effect on the eye (r) red. Thus when one looks at a 

 glowing solid body through a spectroscope, an instrument 

 containing one or more prisms, the colours red, yellow, 

 green, blue, violet are seen ranged in a row as in the rain- 

 bow (Fig. 8, which gives a detailed view of the range vr 

 of Fig. 7), but the eye sees nothing of the short wave-length 

 rays beyond the violet, nor of the relatively long wave-length 

 rays beyond the red. Still longer waves can be detected 

 by their electro-magnetic action. In fact, all radiation is 

 essentially electro-magnetic. 



63. Radiation and Absorption. The different wave- 

 lengths of sound in air correspond to different musical notes, 

 the different wave-lengths of light in the ether to different 

 colours. The molecules of each of the elements vibrate 

 in a way of their own when set in motion, and produce 

 waves in the ether of one or more definite lengths only. 

 Sodium vapour, for example, when intensely heated sets up 

 only rays the wave-length of which is 4 3 Q o o * an mcn > 

 and these produce the sensation of yellow light in the 

 eye. A spectroscope sorting out the light from glowing 

 sodium shows only a strong double yellow line (D in 

 Fig. 8). The molecules of calcium vapour produce several 

 distinct kinds of quivering, originating rays corresponding 

 to definite colours of light. The same is true of all the 

 other elements ; the spectra of the radiant energy sent 

 out from them are distinctive in every case. But, as in 

 the case of sound, bodies absorb the same kind of radia- 

 tions as they emit. If a beam of white light, which 

 includes rays of all wave-lengths, is passed through sodium 

 vapour, the particles of sodium are set vibrating by the 

 waves -j-^jo^- of an inch in length, and the energy of these 

 waves is absorbed, so that when the beam is examined by 

 the spectroscope, and the rays are spread out side by side, 

 the peculiar double yellow ray is missing and in its place 



