Clifton College Scientific Society. 91 



spectrum, and placed in front of the slit a Bunsen's flame, coloured 

 with the vapour of sodium. The dark lines D were immediately 

 changed into bright lines. He then wished to see how strong 

 the solar spectrum could be made without impairing the distinct- 

 ness of the sodium lines, and, for this purpose, he allowed a beam 

 of sunlight to fall on the slit, after passing through the sodium 

 flame. He was greatly astonished at finding the dark lines re- 

 appear as clear as possible. He then allowed the lime-light to 

 fall through a flame coloured with sodium, and he observed dark 

 lines in the spectrum in the position of the sodium lines. From 

 these experiments, and their results, he concluded that the sodium 

 flame absorbs rays of the same degree of refrangibility as those 

 it emits, whilst it is perfectly transparent for all other rays. If 

 we heat some sodium behind a sodium flame, the flame will appear 

 smoky, and will give the dark lines D in the spectrum. If, how- 

 ever, we ignite some lithium salt behind the flame, the red colour 

 will appear, and the spectrum will show both the sodium and the 

 lithium bright lines. Kirchhofi" then states that it appears likely 

 that glowing gases have the power of absorbing rays, especially of 

 the same degree of refrangibility as those they emit ; and so that 

 the bright lines can be changed into dark ones by allowing a suffi- 

 ciently intense light, giving a continuous spectrum, to pass through 

 it. Many other substances besides sodium have had their flames 

 reversed, by experimenting in exactly the same way. We may 

 compare to this power which certain substances have of absorbing 

 the same sort of rays which they emit, the phenomenon in acoustics 

 known as resonance, in which a certain column of air takes up 

 and reinforces a certain set of vibrations. The presence of these 

 dark lines in the solar spectrum is a positive proof of the existence 

 of various substances. It is almost an impossibility that they 

 should be caused by our atmosphere, because it could not contain 

 enough iron vapour to cause such distinct absorption lines in the 

 spectrum ; and, besides this, it is noticed that these lines do not 

 appreciably alter when the sun approaches the horizon. 



As soon as the existence of one terrestrial substance in the 

 sun's atmosphere was established, it seemed but natural that 

 other bodies should be present also. Experiments have been 

 made, and the following metals have been found to have their 

 dark representatives in the sunlight : — 



1. Sodium. 5. Iron. 9. Zinc. 13. Hydrogen. 



2. Calcium. 6. Chromium. 10. Strontium. 14. Manganese. 



3. Barium. 7. Nickel. 11. Cadmium. 15. Aluminium. 



4. Magnesium. 8. Copper. 12. Cobalt. 16. Titanium. 



The sun most probably, as Kirchhoff says, consists of a solid or 



