Scientific Lectures. 137 



Kirchlioff was led to study a coincidence between the bright yellow 

 line given by incandescent sodium vapor and the solar line D, which 

 coincidence had already been noticed by Fraunhofer. Upon applying 

 a greater dispersive power, he noticed that the line D was a double 

 one ; but so also was the sodium line under these conditions. More- 

 over, each line of the one coincided exactly with each line of the 

 other. The suspicion became strong that it was sodium in the sun 

 which caused the D lines. He then extended this comparison to 

 other elements. He carefully measured sixty bright lines of the 

 spectrum of iron ; and found every one of these sixty lines to cor- 

 respond with a dark line in the solar spectrum. The overwhelming 

 probability of a common cause for both was forced upon him ; and, 

 by calculation, he ascertained that this probability was as one million- 

 million-million to one in its favor. By a series of accurate compari- 

 sons of this sort, it has been ascertained that fourteen of the common 

 elements known upon the earth exist in the sun, viz., sodium, iron, 

 calcium, barium, magnesium, manganese, titanium, chromium, nickel, 

 cobalt, hydrogen, aluminum, zinc and copper. One other question 

 remains to be answered. Granting that these coincidences in the 

 spectral lines prove a common cause for them, why are the lines in the 

 solar spectrum dark? In one of Kirchhoff 's experiments, he placed a 

 yellow sodium light between the sun and the slit of his instrument, 

 expecting thereby to neutralize, and perhaps to extinguish, the D lines. 

 But what was his surprise to see these lines become darker. He repeated 

 the experiment with the lime light and with a platinum wire heated by 

 the electric current ; the result was the same :. two dark lines appeared 

 in the yellow, corresponding to the D line. He had in fact succeeded 

 in imitating this part of the solar spectrum ; what wonder, then, that he 

 should generalize upon it and conclude that the dark solar lines had a 

 similar origin ? His discovery was an important one in physics ; it 

 established for light what others had done for heat, and extended the 

 law of exchange to another form of radiant energy. Kirchhoff's law 

 is simply this : A glowing gas absorbs rays of the same refrangibility 

 as those which it emits, i. e., every colored flame is opaque to rays of 

 its own color. The yellow sodium flame, in the experiment just 

 mentioned, permits the red, the blue, and the green to pass, but 

 retains the yellow ; a dark line appears, therefore, in its place in the 

 spectrum. It is now clear, says Kirchhoff, how the sun is constituted : 

 there is within it a solid or liquid nucleus, intensely heated, which, 

 were it seen alone, would give us white light and a continuous spec- 

 trum ; but the intense heat there present converts the substances 



