450 nr.l.I. SYSTEM TECHMCAl. JOIRXAL 



k'lii volts is alili- to f)iii tlif atom into a p.iriicuiar excited state; the 

 atom emits radiation of wavelength 4o71 in returning to its normal 

 state. To get the atom to emit another sort of radiation, tlie electron 

 must possess 6.5 e<|iii\ ilciil \olis lo piii i( into another excited state. 

 Any excited state can Ik- reaclu-d if the ilecunn has 10 e(nii\alent 

 volts to pass over to tlie atom. 



In a gas sustaining an electrical discharge, the atoms ari' snhji-ct 

 to stimuli of such variegated force and type that the distinctions 

 between ilifTerent lines are not so clearly marked; but it can be seen 



Fig. 12 — Successive excitation of lines requiring electron-impacts of successively 

 greater violence to liring atoms into the neccssarv initial states. (Foote, Meggers, 



.\nil Mnlilcr, Pliili>xi>t>liinil'\Jiixi'zi>ie) 



that mild discharges faxnr lines for wliicii ihc iniii.il le\el is adjacent 

 or close to the normal levil, wliiie otiur lines ri(|uire .i more \iolent 

 stimulus. Furthermore, when a gas is steadiK' heated to higher and 

 higher temperatures, various lines of its spectrum apjiear in more or 

 less the order of the stationary states which are the initial states of 

 the transitions responsible for these lines. Accordingh' a "tempera- 

 ture classification" of spectrum lines has been dexeloped at Mount 

 Wilson Obserxatory and elsewhere, and is \aluable in deciphering 

 intricate s|)ectra. 



E IJf. Absorplion-Sprrlni 



An atom which will emit a fniiuency r when it is originall\ in a 

 state B and passi's over into a >late .1, will absorb light of the same 

 freciuenc\' if it is inili.illy in the state .1. This has the important 

 conse(|ueiice that the lines which a gas absorbs, when King at rest 

 and imexcited, are those which it emits in passing from an\' and e\er\' 

 other state itilo Ike normal slalc. The line^ einiiicil when .in .iloni 

 passes from one of its stationary slates into .mother which l,ill(.'r is 

 not the normal state, are not absorbed b\- thi' gas King quiesci'nl ,ind 

 undislurbi'd. l-"or this reason heliimi and neon and argon are (|uile 

 tr.insp.irent lo .ill visible lii;lii, .illlioiii;h llicy li,i\e many i-mission- 



