SOStR CONrEMPOh'.IKV .IDr.lXCIS l\ l-UYSICS rill 451 



liiu's in this rt-jjioii of llu- siHTtrum; for racli of tiu'st- liiu-s ntrrv- 

 s|xii)(ls to a transition into sonu- oIIut tlian tlit- normal slati- and tin- 

 linos which correspond to transitions into the norma! slate lie far 

 off in the nllrax iolet. Hut if such a gas is made the theatri' of a self- 

 snstainiiiK l-lectrii-al (lischar^;e, the other lines likewise are absorbed ■ 

 for the disch.irije pnts the atoms of the >;as tempor.iriK' hnt fre(|nenlly 

 into various almormal st.ites. This inci<lentally is one of the hits 

 of evidence that an atfun may sojourn for a finitely lon>{ lime in 

 another stationar>' state than the normal one. If the gas is heated, 

 the s;une effect occurs; for the violent collisions between atoms in a 

 hot gas occasionally bring atoms into excited states. 



By observing the absorption-spectrum of a quiescent gas one learns 

 which lines in the emission-spiectrum correspond to transitions into 

 the normal state — a valuable piece of information in the cases of 

 elements of whiili the spectr.i are complicali'd and obscure. 



F. /•). Spectra of Ionized Atoms 



In a \iolent electrical discharge, such as a spark, the gas emits 

 many lines which cannot be fitted into the system of series of the 

 usual spectrum of the gas. These may also be produced by bom- 

 barding the gas with electrons possessing more than enough energy 

 to ionize its atoms. They are belie\'cd to emanate from ioni/i-d 

 atoms, or from atoms deprived of one electron. The sped nun nl 

 ionizcfl-helium has been ver\- imjjortant in these pages. In very 

 violent sparks many more lines emerge, and these are associated with 

 atoms deprived of two, three, or even more electrons. 



The spectrum of the ionized atom of an element resembles, in its 

 system of series, and in more minute details, the spectrum of the 

 neutral atom of the element preceding it in the periodic system. 

 The spectrum of an atom deprived of n electrons resembles the spec- 

 trum of the neutral atom preceding it by n places in the periodic 

 system. This confirms the belief that the spectrum and the other 

 properties of an element are determined chiefly by the number of 

 electrons which its atom contains. 



E 111. X-ray Spectra 



The difference between the X-ray spectra to which we now come, 

 and the "optical" spectra which we have been discussing seemed 

 profound and vital in the era of very defective knowledge, but it 

 has faded steadily away with the dee[)ening of understanding. TweUe 

 or fifteen years ago the contrast was multiform and very sharp; for 



