62 CHEMICAL PHYSICS. 



from an electric light, and if then a little sodium chloride be evapo- 

 rated in the flame of a Bunsen burner that has been placed between 

 the slit and the luminous flame, it will be seen that the spectrum is 

 no longer continuous, but that a black line intercepts it, a line hold- 

 ing precisely the position occupied by the yellow sodium line seen 

 on a dark background when the luminous flame is removed. In 

 repeating the experiment with the salts of various metals, we find 

 like conditions, viz., the discontinuous bright color spectra of indi- 

 vidual metals are converted into spectra showing black lines in the 

 continuous spectrum obtained from the luminous flame. (See spectra 

 on plate facing title page. 



From these facts we- may draw the conclusion that the vapors of 

 different substances absorb precisely the same rays that they are 

 capable of emitting. Such spectra are called absorption spectra, or 

 reversed spectra. 



Absorption spectra are also of interest because, if white sunlight 

 be examined with a spectroscope of sufficiently high power, it is found 

 not to be continuous, but to contain many hundreds of black lines, 

 called after their discoverer Frauenhofer lines. The more prominent 

 lines are designated by letters or numbers, as shown in the solar spec- 

 trum represented on the accompanying plate. On comparing these 

 lines in the sun spectrum with the positions of lines obtained by known 

 elements, such as iron, sodium, calcium, etc., it is found that they 

 correspond exactly to one another. The explanation given is this : 

 The main body of the sun consists of a highly luminous mass sur- 

 rounded by an atmosphere of cooler vapor. The luminous body 

 would give a continuous spectrum, but the rays passing through the 

 vapors of the sun's atmosphere are partly absorbed, and thus a large 

 number of black lines are produced. Absorption-spectra are, further- 

 more, of great interest, because when light passes through certain 

 liquids, such as blood or a solution of quinine sulphate, dark line 

 spectra are obtained, sufficiently characteristic to assist in the recog- 

 nition of these substances. 



From what has been said, we learn that we have three kinds of 

 spectra, viz., continuous spectra, produced by luminous solids or 

 liquids, and possibly by luminous gases tinder high pressure ; bright 

 line spectra, produced by luminous vapors ; and absorption spectra, 

 produced by light that has passed through certain media. 



The value of the spectroscope depends on the use made of it in 

 spectroscopic analysis, as both the bright line spectra and absorption 

 spectra enable us to determine the nature of many substances, and 



