554 PRINCIPLES OF CHEMISTRY 



the gas burner (for which purpose a platinum wire on whose end sodium 

 chloride is fused is fixed to the stand), then the flame is dyed a yellow 

 colour, and on looking through the spectroscope the observer will see a 

 bright yellow line falling upon the 50th division of the scale, which is 

 seen together with the spectrum in the telescope. No yellow lines 

 of other refractive index, nor any rays of any other colour, will be 

 seen, and, therefore, the spectrum corresponding with sodium com- 

 pounds consists of yellow rays of that index of refraction which belong 

 to the Frauenhofer (black) line D of the solar spectrum. If a potassium 

 salt be introduced into the flame instead of a sodium salt, then two 

 bands will be seen which are much feebler than the bright sodium band 

 namely, one red line near the Frauenhofer line A and another violet 

 line. Besides which, a pale, almost continuous, spectrum will be 

 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 simultaneously seen 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 Frauenhofer lines 

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

 Frauenhofer, 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 Frauenhofer lines. Thus the 

 bright yellow sodium line exactly corresponds with the dark Frauen- 

 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, then it 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. In the common spectroscopes which are usually 

 employed in chemical research, one yellow band, which does not split 



2ti The two most distinct lines of D, or of sodium, have wave lengths of 589'5 and 588'0 

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

 lengths in millionth parts of a millimeter are 588'7 and 588'1, 616'0 and 615'4, 515'5 and 

 515*2, 498'8 and 498'2, &c., according to Liveing and Dewar. Many strive to find a simple 

 relation, subject to a law, in the wave lengths, both of these pairs of lines and those 

 of other elements. 



