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PKOFESSOfi A. FOWLEE, F.R.S., ON 



under the action of gentle electric discharges, many substances 

 give spectra consisting of broad bands, or flutings, such bands 

 consisting of a multitude of very fine lines closely packed 

 together. At the higher temperature of the electric arc these 

 bands are replaced by lines which occupy quite different posi- 

 tions in the spectrum. 



Further modifications, involving the weakening of some lines 

 appearing in the flame or arc, the brightening of others, or 

 even the appearance of new lines, are often found when the 

 substance is submitted to the violent action of the condensed 

 electric discharge. 



Lines which are intensified, or only appear under spark 

 conditions, have been called " enhanced lines," and it is by the 

 study of such lines, initiated by Sir Norman Lockyer, that much 

 of the recent progress in the interpretation of solar and stellar 

 spectra has been due. 



We see, then, that the same substance may give widely 

 different spectra under different experimental conditions, but 

 the spectrum is nevertheless always the same under the same 

 conditions, and no two substances ever give the same spectrum. 

 This multiplicity of spectra might at first sight appear to be an 

 undesirable complication in spectrum analysis, but in reality it 

 enormously increases the interest of observations of the celestial 

 bodies, because it enables us to learn something of the physical 

 conditions which prevail as well as of their chemical constitu- 

 tion. 



We do not yet kuow the precise cause of the variations in 

 the spectrum of a substance, but it is generally believed that, 

 while band spectra are produced by the vibrations of molecules, 

 or of electrons which form part of molecular systems, line 

 spectra are only produced when the applied energy is sufficient 

 to break up the molecules into atoms. As to the change in the 

 line spectrum which is often observed on passing from the arc 

 to the spark spectrum, modern theories of atomic structure 

 suggest that further dissociation takes the form of the removal 

 of one or more electrons from each of the atoms involved. 

 Whatever the ultimate cause may be, we do know that the 

 change from bands to lines, and from ordinary flame to arc 

 lines, and again from arc to enhanced (spark) lines, accompanies 

 the application of greater energy to the molecules and atoms, 

 whether it be in the form of heat or electricity. 



So far, reference has been made to emission spectra only. 

 Kirchhoffs famous experiment of 1859 proved that a luminous 

 vapour has the property of absorbing precisely the same kind of 



