POTASSIUM, RUBIDIUM, CJESIUM, AND LITHIUM 563 



observed in the central portions of the scale. If a mixture of sodium 

 and potassium salts be now introduced into the flame, three lines 

 will be seen simultaneously namely, the red and pale violet lines of 

 potassium and the yellow line of sodium. In this manner it is possible, 

 by the aid of the spectroscope, to determine the relation between the 

 spectra of metals and known portions of the solar spectrum. The con- 

 tinuity of the latter is interrupted by dark lines (that is, by an absence 

 of light of a definite index of refraction), termed the Fraunhofer lines 

 of the solar spectrum. It has been shown by careful observations (by 

 Fraunhofer, Brewster, Foucault, Angstrom, Kirchhoff, Cornu, Lockyer, 

 Dewar, and others) that there exists an exact agreement between the 

 spectra of certain metals and certain of the Fraunhofer lines. Thus the 

 bright yellow sodium line exactly corresponds with the dark Fraun- 

 hofer line D of the solar spectrum. A similar agreement is observed 

 in the case of many other metals. This is not an approximate or chance 

 correlation. In fact, if a spectroscope having a large number of re- 

 fracting prisms and a high magnifying power be used, ib is seen 

 that the dark line D of the solar spectrum consists of an entire system 

 of closely adjacent but definitely situated fine and wide (sharp, distinct) 

 dark lines, 26 and an exactly similar group of bright lines is obtained 

 when the yellow sodium line is examined through the same apparatus, 

 so that each bright sodium line exactly corresponds with a dark line in 

 the solar spectrum. 26 bis This conformity of the bright lines formed by 

 sodium with the dark lines of the solar spectrum cannot be accidental. 

 This conclusion is further confirmed by the fact that the bright lines 

 of other metals correspond with dark lines of the solar spectrum. 

 Thus, for example, a series of sparks passing between the iron electrodes 

 of a Ruhmkorff coil gives 450 very distinct lines characterising this 

 metal. All these 450 bright lines, constituting the whole spectrum corre- 

 sponding with iron, are repeated, as Kirchhoff showed, in the solar 

 spectrum as dark Fraunhofer lines which occur in exactly the same situa- 

 tions as the bright lines in the iron spectrum, just as the sodium lines 

 correspond with the band D in the solar spectrum. Many observers 

 have in this manner studied the solar spectrum and the spectra of 

 different metals simultaneously, and discovered in the former lines which 



* 6 The two most distinct lines of D, or of sodium, have wave-lengths of 589-5 and 588*9 

 millionths of a millimeter, besides which fainter and fainter lines are seen whose wave- 

 lengths in millionths of a millimeter are 588'7 and 688-1, 616'0 and 615-4, 615'5 and 515'2, 

 498-8 and 498-2, &c., according to Liveing and Dewar. 



86 M " In the ordinary spectroscopes which are usually employed in chemical research, 

 one yellow band, which does not split up into thinner lines, is seen instead of the system 

 of sodium lines, owing to the small dispersive power of the prism and the width of the 

 Edit of the object tube. 



