165. _Chemical Constitution of the Sun's Atmosphere._--To find whether any element which exists on the earth is present in the solar atmosphere, we have merely to ascertain whether the bright lines of its gaseous spectrum are matched by dark lines in the solar spectrum when the two spectra are placed side by side. In Fig. 183, we have in No. 1 a portion of the red end of the solar spectra, and in No. 2 the spectrum of sodium vapor, both as obtained in the same spectroscope by means of the comparison prism. It will be seen that the double sodium line is exactly matched by a double dark line of the solar spectrum: hence we conclude that sodium vapor is present in the sun's atmosphere. Fig. 184 shows the matching of a great number of the bright lines of iron vapor by dark lines in the solar spectrum. This matching of the iron lines establishes the fact that iron vapor is present in the solar atmosphere. [Illustration: Fig. 184.] The following table (given by Professor Young) contains a list of all the elements which have, up to the present time, been detected with certainty in the sun's atmosphere. It also gives the number of bright lines in the spectrum of each element, and the number of those lines which have been matched by dark lines in the solar spectrum:-- Elements. Bright Lines Reversed. Observer. Lines. 1. Iron 600 460 Kirchhoff. 2. Titanium 206 118 Thalen. 3. Calcium 89 75 Kirchhoff. 4. Manganese 75 57 Angström. 5. Nickel 51 33 Kirchhoff. 6. Cobalt 86 19 Thalen. 7. Chromium 71 18 Kirchhoff. 8. Barium 26 11 Kirchhoff. 9. Sodium 9 9 Kirchhoff. 10. Magnesium 7 7 Kirchhoff. 11. Copper? 15 7? Kirchhoff. 12. Hydrogen 5 5 Angström. 13. Palladium 29 5 Lockyer. 14. Vanadium 54 4 Lockyer. 15. Molybdenum 27 4 Lockyer. 16. Strontium 74 4 Lockyer. 17. Lead 41 3 Lockyer. 18. Uranium 21 3 Lockyer. 19. Aluminium 14 2 Angström. 20. Cerium 64 2 Lockyer. 21. Cadmium 20 2 Lockyer. 22. Oxygen a 42 12 ± bright H. Draper. Oxygen b 4 4? Schuster.