182 THE SUN — ITS CHEMICAL ANALYSIS. 



gives eight remarkable line?i, six rod, one orange, and one blue, and 

 the eye is able, by means of the spectrum, to perceive even six-rail- 

 lionths of a milligram of this metal in the air. Calcium, a metal 

 Avhich combines with oxygen to form lime, gives three lines, green, 

 red, and blue, which appear only in intense flames. Barium, the 

 metal of baryta, is distinguished by two green lines. Iron, which 

 produces very numerous lines, manganese, zinc, copper, gold, all the 

 metals, in a word, have been tried by MM. Bunsen and Kirchoff, and 

 they have carefdlly studied the lines which each of them exhibits in 

 the spectrum. These bright zones remain invariable, whatever may 

 be the composition of the salt, in which the metal is contained; they 

 are still the same when the metal is directly volatilized in the flame 

 or in passing a strong electric current between two metallic points 

 placed at some distance from one another. The optical properties 

 which we have thus indicated are, therefore, attributes of the simple 

 bodies themselves, and they may be observed at all times when 

 these latter are raised to a high temperature. 



This curious investigation of the bright lines of the artificial spec- 

 trum has an intimate and close connexion with the explanation of the 

 dark lines of the solar spectrum. This connexion, however, it is not 

 easy to discover at the first glance, and it even escaped so inge- 

 nious a physicist as M. Foucault. In 1849 M. Poucault announced 

 that he had observed the following fact: when we are examining in 

 a spectrum the yellow line of sodium, from being bright it becomes 

 dark if we vividly illuminate the source of artificial light in which the 

 sodium is in suspension. M. Foucault was observing the voltaic arch 

 which unites two points of carbon, and he saw a bright yellow line 

 produced on the spectrum, due to the presence of a compound of so- 

 dium reduced to incandescent vapor by the action of the current. 

 Now, when the luminous voltaic arch was traversed by the rays of 

 the solar light that line became dark. 



This strange phenomenon was neither explained nor generalized by 

 the French physicist. M, Kirchofl" was ignorant of it when, in 1859, 

 he commenced with M. Bunsen his series of fruitful experiments. He 

 showed that the bright line of sodium occupies in the series of ele- 

 mentary colors the place which, in the common solar spectrum, is 

 occupied by the black line w^hich Frauenhofer designated as the line 

 D. To borrow his own expressions, the line D is only the bright line 

 of sodium reversed, (it would surely be better to say extinguished.) 

 But how is this line to be extinguished in a flame which holds sodium 

 in suspension ? We have seen that it is by directing upon this flame 

 the rays of a flame still more vivid. If we observe the spectrum of 

 the sodium flame, we shall perceive at first the characteristic yellow 

 line; let us then allow the solar light to penetrate with increasing 

 intensity into this flame, and the yellow line will grow pale by de- 

 grees, and at length will become dark when the spectrum produced 

 by the sun shall have overpowered that of the artificial flame. What 

 occurs with sodium occurs with all the metals. M. Kirchoff has con- 

 verted the red line of lithium into the dark line just as he had done the 

 yellow line of sodium; the other metals likewise present, though with 



