NATURAL PHILOSOPHY. 1G7 



cate instrument, Prof. Kirchhoff made the following very remarkable 

 experiment, which is interesting as giving the key to the sokition of 

 the problem regarding the existence of sodium and other metals in 

 the sun : u In order to test in the most direct manner possible the 

 frequently asserted fact of the coincidence of the sodium lines with 

 the lines D, I obtained a tolerably bright solar spectrum, and brought 

 a flame colored by sodium vapor in "front of the slit. I then saw the 

 dark lines D change into bright ones. The flame of a Bunsen's lamp 

 threw the bright sodium lines upon the solar spectrum with unex- 

 pected brilliancy. In order to find out the extent to which the inten- 

 sity of the solar spectrum could be increased without impairing the 

 distinctness of the sodium lines, I allowed the full sunlight to shine 

 through the sodium flame, and to my astonishment I saw that the 

 dark lines D appeared with an extraordinary degree of clearness. I 

 then exchanged the sunlight for the Drumrnond's or oxyhydrogen 

 lime-light, which, like that of all incandescent solid or liquid bocKes, 

 gives a spectrum containing no dark lines. When this light was al- 

 lowed to fall through a suitable flame colored by common salt, dark 

 lines were seen in the spectrum in the position of the sodium lines. 

 The same phenomenon was observed if instead of the incandescent 

 lime a platinum wire was used, which being heated in a flame was 

 brought to a temperature near its melting-point by passing an elec- 

 tric current through it. The phenomenon in question is easily ex- 

 plained upon the supposition that the sodium flame absorbs rays of 

 the same degree of refrangibility as those it emits, whilst it is per- 

 fectly transparent for all other rays." Kirchliojf ; Researches, etc., 

 pp. 13, 14. 



Thus Kirchhoff succeeded in producing artificial sunlight, at least 

 as far as the formation of one of Fraunhofer's lines is concerned. 

 He proved that the yellow soda flame possesses this at first sight 

 anomalous property of absorbing just that kind of light which it 

 emits ; it is opaque to the yellow D light, but transparent to all other 

 kinds of light. Hence, if the yellow rays in the spectrum produced 

 by the Drummond's light in the above experiment are more intense 

 than those given off by the soda flame, we shall see in the yellow 

 part of the spectrum shadows or dark lines ; and if the difference of 

 intensity be very great, these shadows may by contrast appear per- 

 fectly black. This opacity of heated sodium vapor for the particular 

 kind of light which it is capable of giving off was strikingly exhibited 

 by Prof. Roscoe, in a lecture on Spectrum Analysis, lately delivered by 

 him in London at the Royal Institution. A glass tube, containing a 

 small quantity of metallic sodium, was rendered vacuous and then 

 closed. On heating the tube, the sodium rose in vapor, filling a 

 portion of the empty space. Viewed by ordinary white light, this 

 sodium vapor appeared perfectly colorless, but when seen by the 

 yellow light of a soda flame, the vapor cast a deep shadow on a white 

 screen, showing that it did not allow the yellow rays to pass through. 



This remarkable property of luminous gases to absorb the same 

 kind of light as they emit, is not without analogy in the cognate 

 science of acoustics. Sound is produced by the vibration of the 

 particles of gravitating matter, whilst light is supposed to be pro- 

 duced by a similar vibration of the particles of a non-gravitating 



