456 BELL SYSTEM TECHNICAL JOURNAL 



excitation, this has ne\er been tloiie with metals or gases exposed to 

 X-ray freciiiencies. 



Atoms therefore do not absorb such X-ray frequencies as are 

 represented by the downward-pointing arrows in Fig. 13. They do 

 absorb such frequencies as would be represented by arrows drawn 

 from the very bottom of the diagram — a little below the le\'el marked 

 P — up to the various le\"els; and (it may seem, unexpectedly) they also 

 absorb frec)uencies somewhat higher than these. This however does 

 not mean that the atom may be put into an excited state of higher 

 energy than the K state, for instance; it means simply, as direct evi- 

 dence proves, that the extruded electron receives the extra energy and 

 goes away with it. Owing to this fact, the X-ray absorption-spectrum 

 consists not of sharp absorption-lines at the several fretjuencies cor- 

 responding to a transfer of the atom into the K-state, the Li-state, 

 and so forth, but of continuous bands commencing with sharp edges 

 at these frequencies, and Irailini; out gradually towards higher fre- 

 quencies. 



Another curious feature of the X-ray spectra is that transitions 

 from the various excited states of high energy-values, such as the 

 A'-stale and the L-states, direct K' into the normal state, ajiparentK- 

 do not occur. 



E 17. Bands pedra 



Band-spectra are the s[)eclra of molecules, — that is to say, of 

 clusters of two or more atoms, such as appear in certain gases. This 

 is proved by the fact that they are displayed by gases which are 

 known in other ways (gramme-molecular volume, specific heat) to 

 consist of molecules; by the fact that the band-spectrum of such a 

 gas disappears when the gas is heated to the point where its molecules 

 are dissociated into atoms; and by the general successfulness of the 

 quantitative theory based on the assumption that they are due to 

 molecules. Occasionalh' band-s])ectra are displayed by gases which 

 are not otherwise known to contain molecules, such as helium and 

 potassium; it is supposed that they are due to molecules too few 

 to be detected by the other acce|)ted methods. I'sually they arc 

 easy to distinguish at first glance from the optical spectra of atoms, 

 although there are exceptions, such as the band-spectrum of the 

 Indrogen molecule, l.ike the spectra we have di.scussed, they consist 

 (jf lines; the term "band-spectrum" describes the manner in which 

 these lines are grouped. Again like the spectra we have discussed, 

 they are analyzed according to Bohr's funtlamental principle, by 



