THE PRESIDENT'S ADDRESS. 315 



It is to the astro-chemical researches of M. M. Kirchhoff and 

 Bunsen that we owe the discovery of the coincidence of the 

 bright lines in the spectra of terrestrial substances with corres- 

 ponding dark lines in the solar spectrum. In 1859, soon after 

 Kirchhoff discovered that the bright intermittent lines in the 

 metallic spectra were converted into dark lines, by viewing them 

 through less intensely heated vapours of the same metals, he 

 had a very powerful spectroscope constructed, so arranged as to 

 enable him to observe the solar and metallic spectra simultane- 

 ously. While examining the two classes of spectra with this 

 instrument, he saw, to his astonishment, that the positions of 

 many of the dark solar lines were exhibited in coincidence with 

 those of the bright metallic lines. In this manner, he was able 

 to demonstrate conclusively that the bright lines of the vapours 

 of magnesium, copper, iron, and of several other terrestrial sub- 

 stances, were represented by certain dark absorption lines visible 

 in the solar spectrum, so far as could be distinguished by their 

 respective wave-lengths. For, if any metal or gas is heated to 

 a state of incandescence, or, in other words, made white-hot, and 

 the rays of light emitted from it are allowed to pass through less 

 heated vapours of the same metal or gas, a large portion of the 

 light is absorbed while passing through these vapours, and the 

 lines which would have appeared bright, had there been no 

 interference, become dark. On one occasion, while comparing 

 the spectrum of iron with that of the sun, Kirchhoff unexpectedly 

 perceived that dark solar lines occurred in positions coincident 

 with those of all the bright iron lines. Not only had each iron 

 line its dark representative in the solar spectrum, but the breadth 

 and intensity of the two sets of lines also agreed with each other, 

 the brightest iron lines corresponding to the darkest solar lines. 

 Thus for each of more than four hundred iron lines, a dark solar 

 line was seen to correspond. An idea at once flashed across the 

 mind of this distinguished philosopher, that to produce so per- 

 fect a coincidence, vapours of iron must be contained in the solar 

 photosphere. From this discovery of Kirchhoff have sprung 

 the long series of investigations which, in the hands of more 

 recent physical-astronomers, has produced results of the most 

 wonderful character, thus enlarging our knowledge of the cos- 

 mical origin of the universe to an extent, of which the first 



