136 Transactions of the American Institute. 



or liquid ; but inasmuch as each gas or vapor, when heated to incan- 

 descence, gives a spectrum peculiar to itself, it is clear that the exami- 

 nation of its spectrum may be the means of its identification. [To 

 show that the light emitted by incandescent vapors is colored, the 

 lecturer heated salts of several metals in the flame of a Bunsen 

 gas-burner. Potassium gave a lilac color, sodium a yellow, lithium a 

 crimson, barium a green, and so on. Gases may be rendered incan- 

 descent by inclosing them in tubes under diminished pressure, and 

 passing the electric spark through them. The word " Yale, " in letters 

 four feet long, made up of such tubes filled with different gases, was 

 illuminated by the spark of the large Ruhmkorff coil of the Stevens 

 Institute. The nitrogen emitted rose-colored light, chlorine a green, 

 carbonic acid, greenish white, carbondisulphide vapor, white etc. Pro- 

 fessor Barker then showed that the light emitted by the intensely heated 

 solid carbon points gave a continuous spectrum on the screen ; while 

 by placing volatile substances on the lower carbon, they were con- 

 verted into vapor by the heat, and this vapor being rendered incan- 

 descent gave a spectrum consisting of bright lines interrupted by 

 dark spaces. The spectra of the metals lithium, magnesium, mer- 

 cury, silver, copper, zinc and thalium, thus produced, were projected 

 on the screen, and attention was called to the fact that each spectrum 

 was peculiar and characteristic for the metal which gave it, however, 

 it was shown that the incandescent vapor of brass gave the spectra of 

 zinc and copper, of which it was composed, and that a mixture of the 

 chlorides of several metals gave, in this way, spectra of all the consti- 

 tuents present ; thus proving the identity of character possessed by an 

 element, even when it is in combination.] 



What, now, is the character of the spectrum afforded by sunlight, 

 and what results may be obtained by its study concerning the com- 

 position and physical condition of the sun ? If sunlight be received 

 upon the prism from a narrow, elongated opening, the resulting spec- 

 trum will be found to be crossed by a multitude of dark lines, parallel 

 to the axis of the prism. These lines were first observed by Wollaston 

 in 1802, though they were not accurately mapped until 1814, when 

 Fraunhofer fixed the position of 576 of them, and gave to the most 

 prominent of them, beginning at the red end, the letters of the 

 alphabet for names. Sir David Brewster subsequently mapped 2,000, 

 and recently this number has been increased to above 3,000, by the 

 researches of Angstrom and Kirchhoff. They probably number 

 10,000, at least. Various attempts have been made to account for 

 these dark lines in the solar spectrum. In 1860-61, Professor 



