30 BIOLOGICAL EFFECTS OF RADIATION 



Owing to the variety of the electron speeds represented in a discharge, 

 one must not expect to get emission spectra composed of a single lin.e or 

 even of a single series. When an atom has been "excited" to one of its 

 upper stationary states, it need not (sometimes indeed it cannot) return 

 direct by a single transition to its normal state. There is a tremendous 

 network of possible transitions between the many stationary states of 

 which an atom is capable, and in a discharge a great many of these 

 transitions are perceptibly represented, giving rise to spectra immensely 

 more rich and complicated than the absorption spectrum of the rarefied, 

 cool, and unexcited gas. With certain kinds of atoms spectroscopists 

 have found it necessary to produce the absorption spectrum (sometimes 

 a difficult matter, in the cases of metals of high boiling point) before they 

 could make much headway with the analysis of the emission spectrum. 



While the brilliance of the arc makes it often the most convenient 

 source of spectrum lines, this advantage is often bought by the sacrifice 

 of other desirable qualities, such as sharpness and narrowness of the lines. 

 Hotness tends to broaden a line, because the speed at w^hich an atom is 

 moving affects the wave-length of its radiation; density of gas tends to 

 broaden it, because when two atoms come close together, the electric 

 field of each perturbs the other and alters the energy values of its station- 

 ary states. The gas in an arc is hotter and (usually) denser than the gas 

 in a glow, and this produces the undesired effects. Indeed, a striking 

 and paradoxical effect is well known to occur, by virtue of which an arc 

 may fail to send out precisely those wave-lengths which one expects to be 

 the most prominent in its spectrum. Suppose a vapor having a strong 

 spectrum line corresponding to a transition between the normal state and 

 some excited state, the wave-length of this fine being given in the tables 

 as Xo. Form a glow discharge in a cool and rarefied sample of this 

 vapor; the line will be very narrow, extending, say, between two wave- 

 lengths \o — X and Xo -|- x, where x is only a few millionths of Xq. Form 

 an arc discharge in a dense sample of this vapor; the fine will be very 

 broad, extending, say, between wave-lengths \o — y and Xo -|- y, where y 

 may be hundreds of times as great as x. The light, however, will be 

 emitted from the "core" or central portion of the gas containing the arc, 

 and this core will be surrounded by cooler and relatively less excited gas. 

 Atoms in this region will absorb photons of the same wave-lengths as they 

 could emit, which is to say, photons of wave-lengths between \o — x 

 and Xq -|- x. The photons of the middle of the broadened line, those 

 having the wave-length assigned to the line in the tables, will be absorbed 

 before they escape from the region of the arc, while those of the "wings" 

 of the broadened fine will make their exit without impediment. The 

 result is the same as if the arc, instead of emitting the line Xo, emitted two 

 lines one on each side of it! This is very serious in cases where the light 



