32 BIOLOGICAL EFFECTS OF RADIATION 



however, pass spontaneously into other states, intermediate between the 

 normal state and the one into which they were transferred by the absorp- 

 tion; they then emit photons of lesser energy and lesser frequency than 

 those which they absorbed. This is a more complex case of production 

 of light, the new kind being of different quality from the old. 



The former phenomenon is most striking in such a case as that of 

 sodium vapor irradiated by the yellow line X5896 A. The atoms are 

 transferred into an excited state which is "next" in energy to the normal 

 state, in the sense that there is no state of intermediate energy. It is 

 therefore impossible for them to quit their excited state in any other way 

 than by radiating the same kind of photon which they received (unless 

 they have interactions with other atoms or other photons, as I shall later 

 mention) ; and the emission of X5896 in all directions from a tubeful of 

 sodium vapor, into which a beam of such light is projected from a single 

 direction, can be spectacular. Another case in which this effect is very 

 strong is that of mercury vapor irradiated by the ultra-violet line X2536, 

 although here the excited state is not next (in the foregoing sense) to the 

 normal state, and the invisibility of the light deprives the phenomenon 

 of its spectacular quality. In general, one expects a conspicuous effect 

 of this kind when a vapor is irradiated by the first line {i.e., the 



o 



line of lowest frequency) of its absorption series ; 5896 A of sodium and 

 2536 A of mercury are of this class. These are known as cases of "reso- 

 nance radiation," the first fine of an absorption series being called the 

 "resonance line." 



The more complex phenomenon — the production, by light capable of 

 transferring atoms from the normal state N to an upper excited state A, of 

 photons due to transitions from A to other excited states B, C, D, . . . 

 intermediate (in energy) between N and A — can easily be demonstrated. 

 It is most striking with gases in which the atoms are joined into diatomic 

 molecules, on account of the extraordinary profusion of stationary states 

 having energies very close together and the consequent extraordinary 

 profusion of lines. Thus, if sodium vapor or iodine vapor (both of which 

 contain a large proportion of diatomic molecules Na2 or I2, as well as atoms 

 Na or I) be irradiated by a beam of monochromatic light of almost any 

 frequency whatever (in the visible or ultra-violet) the chances are very 

 good that this frequency will be capable of transferring the molecules into 

 some stationary state from which they will then be capable of making 

 transitions into dozens, nay even hundreds, of intermediate states. The 

 single irradiating frequency will then evoke a "resonance spectrum" 

 of dozens or hundreds of lines ; and another irradiating frequency differing 

 from the first one by only a few parts in a hundred thousand will evoke 

 another resonance spectrum of dozens or hundreds of totally different 

 hnes. 



