34 BIOLOGICAL EFFECTS OF RADIATION 



frequency NA will be proportional to the product of the numbers of both 

 kinds of photons. However, the intensity of such a line as would be 

 designated by the symbol AN, emitted by virtue of a spontaneous 

 transition occurring after a normal atom has been struck by a photon 

 of the frequency NA and not requiring the collaboration of a second 

 photon, will be doubled. This difference in the behavior of the two kinds 

 of lines has been observed. 



The "short time interval" of which I have just spoken is the "life" or 

 duration of the excited state — the state A, in this illustration. If we 

 could put a large number of atoms into the same excited state and 

 measure in each individual case the length of time elapsing before it leaves 

 this state by a spontaneous transition into some other state of lower 

 energy, we should almost certainly find that these time lengths are not 

 ah the same but are distributed according to the exponential law or 

 "law of chance." The mean time length or "mean life" can be measured, 

 or at least estimated, in a variety of ways, and for excited states such as 

 we have been considering it is of the order of 10~^ sec. — a very small value 

 indeed! The reader must not, however, infer that if he were to project a 

 beam of, say, the resonance line of sodium into sodiiun vapor, and were 

 to interrupt it with infinite suddenness at a certain instant, the vapor 

 would absolutely cease to emit photons of the resonance frequency 

 within a few times 10~^ sec. after that instant. The photons emitted by 

 some of the atoms are captured by others, absorbed, reemitted, some- 

 times absorbed and reemitted yet again and again; and this process of 

 handing around the energy from atom to atom may go on for so long that 

 the vapor in a tube of ordinary size is still emitting photons for several 

 hundred times 10~* sec. after the irradiation is discontinued. For the 

 same reason, it may be observed that when a sharply defined narrow beam 

 of light is sent into the vapor of which it is the resonance line, the vapor 

 emits similar photons not merely from the path of the beam but also from 

 some distance outside of the beam. 



Peculiar and important phenomena occur when a mixture of gases is 

 irradiated with light which one of them is capable of absorbing and 

 another is not. For definiteness consider the first of them to be dis- 

 covered, which happens when mercury and thallium vapors are mixed 

 and the mixture is illuminated with the resonance line of mercury, 

 2536 A. As the term "resonance line" signifies, the photons of this hght 

 are capable of transferring mercury atoms from the normal state A^ to a 

 certain excited state A, because their energy is equal to the energy 

 difference (4.88 ev) between the states N and A. Falhng on mercury 

 vapor, they cause the emission of identical photons; but falling on pure 

 thallium vapor they cause no emission at all, for thallium atoms have no 

 stationary state 4.88 ev above the normal, and therefore cannot absorb 



