THE SIGNIFICANCE OF SPECTROSCOPY 225 



placed parallel and side by side, each corresponding to radiation 

 of a particular wave-length. 



It would seem at first that a mere difference of wave-length, 

 or frequency, would be of slight importance, and that radiation 

 of all kinds would have approximately the same properties. 

 As a matter of fact, this is very far from being the truth. 

 The differences between radiation of different kinds are so much 

 more obvious than the resemblances, that more than once a 

 great deal of investigation has been necessary to determine 

 whether observed phenomena have been due to etherial radia- 

 tion or not. The longest waves that have yet been detected 

 are those employed in wireless telegraphy. Proceeding along 

 the spectrum in the direction of shorter wave-length, we come 

 next to the " infra-red " waves which, in most terrestrial sources 

 of radiation, have the maximum heating effect. Still further 

 down the scale we come to the visible spectrum, beginning 

 with red, and proceeding through orange, yellow, green, and 

 blue to violet — the colours of the rainbow. The visible spec- 

 trum is but a very small part of the whole range of known 

 radiation. Beyond the violet we get the " ultra-violet " 

 waves, which make no obvious effect on our senses, but are 

 readily detected by a photographic plate. Beyond these again 

 we get the extremely short X-rays and 7-rays from radio- 

 active substances. The range of wave-lengths now known 

 extends from over one thousand centimetres, for the wireless 

 telegraphywaves,to about one thousand-millionthofacentimetre, 

 for the 7-rays of radium, and it is possible that there is a con- 

 tinuous gradation from one of these extremes to the other. 



Let us now see what happens when a body — say an iron 

 poker — is gradually heated. Suppose that at first the poker 

 is at a constant temperature equal to that of its surroundings. 

 Then the radiation it emits will be of the same kinds, and will 

 be discharged at exactly the same rate, as that which it receives 

 from neighbouring bodies. Now let heat be supplied to it. 

 It will commence to radiate more quickly, and with our spec- 

 troscope we can analyse the radiation at different stages of the 

 heating. We shall find that, as the temperature rises, two things 

 happen. First, the quantity of radiation of every kind emitted 

 increases continuously ; second, shorter and shorter waves 

 make their appearance as the heating process goes on. The 

 first of these effects accounts for the increasing amount of heat 

 we receive when we place our hands near the poker. The 

 second explains why, after a time, the poker acquires a dull 

 red colour : it has reached a temperature at which it can pour 

 out radiation short enough to stimulate our sense of sight. 

 At a higher temperature, the shorter orange and yellow waves 

 appear and dominate the red, so that the poker successively 



